Patent Publication Number: US-10315579-B2

Title: Vehicle aerodynamic apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a vehicle aerodynamic apparatus; to a guide vane assembly; to a guide vane and to a vehicle. 
     BACKGROUND 
     It is known to provide guide vanes on a front corner of vehicle. Guide vanes can be provided on a lorry or articulated vehicle above the wheel arch primarily to control soiling of the vehicle. These vanes control the airflow around the outside of the vane to increase the effective corner radius. A fixed guide vane can be provided on a car to control front lift. 
     It is against this backdrop that the present invention has been conceived. At least in certain embodiments, the present invention seeks to overcome or ameliorate at least some of the shortcomings associated with the prior art arrangements. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention relate to a vehicle aerodynamic apparatus; to a guide vane assembly; to a guide vane; and to a vehicle. 
     According to a further aspect of the present invention there is provided a vehicle comprising one or more guide vanes for guiding airflow along a side of the vehicle, wherein each guide vane is movable from a retracted position to a deployed position. At least in certain embodiments, each guide vane can be deployed to reduce aerodynamic drag on the vehicle. The guide vane(s) can be adapted to be deployed only when the vehicle is moving. The guide vane(s) can effectively modify the profile of the front corner of the vehicle. 
     The guide vane(s) according to certain aspects of the present invention can reduce drag by turning (guiding) airflow around the corner of the vehicle. In doing so, flow disturbances downstream of the guide vane, for example along the body side, can be reduced. At least in certain embodiments, the airflow downstream of each guide vane can also help to reduce the flow disturbance caused by the front wheels. The internal flow through the guide vane (i.e. the flow between the vehicle and the guide vane) can be controlled also to suppress disturbed flow emanating from a front wheel house. 
     A control unit could be provided to energize an actuator to deploy each guide vane when at least one predefined vehicle dynamic parameter has been satisfied. For example, the control unit can output a control signal to deploy said one or more guide vanes when the vehicle speed exceeds a predefined threshold. 
     The one or more guide vanes can each comprise a first outer guide surface. The first outer guide surface of each guide vane can define an outer surface of the vehicle when that guide vane is in said retracted position. In particular, when retracted, the first outer guide surface can be disposed substantially flush with a surrounding portion of the outer vehicle surface (the vehicle A-surface). A recess could be formed in the outer vehicle surface for receiving said guide vane. Alternatively, an aperture can be formed in the outer vehicle surface for receiving said guide vane. The aperture can match the shape of the associated guide vane. When the guide vane is retracted, the aperture can be closed by the guide vane so as to form a substantially continuous outer surface. 
     The one or more guide vanes can undergo translation and/or rotation when displaced from said retracted position to said deployed position. The one or more guide vanes can be displaced laterally when travelling from said retracted position to said deployed position. For example, each guide vane can be displaced substantially along a transverse axis and/or undergo rotation about a vertical axis. 
     Each guide vane can be fixedly mounted to an inner support panel. One or more struts can be provided to mount each guide vane to the inner support panel. The inner support panel can define a second outer guide surface. The inner support panel can locate in the aperture in the outer vehicle surface (vacated by the guide vane) when it is displaced to said deployed position. When the guide vane is in said deployed position, the second outer guide surface can define an outer surface of the vehicle. The second outer guide surface can be disposed substantially flush with a surrounding section of the vehicle outer surface to form a substantially continuous surface. The second outer guide surface can have the same profile as the first outer guide surface, or can have a different profile. The first and second outer guide surfaces can have the same or different surface finishes. 
     The offset between each guide vane and the associated inner support panel can be substantially constant. Alternatively, the offset can increase in a longitudinal direction towards the rear of the vehicle to reduce the speed of the air flow passing between the inner support panel and the guide vane. Alternatively, the offset can decrease in a longitudinal direction towards the rear of the vehicle to increase the speed of the airflow passing between the inner support panel and the guide vane. One or more longitudinally-extending channels can be defined between said inner support panel and each guide vane. 
     When each guide vane is in a deployed position, the guide vane forms a turning-vane operative to guide airflow around the front of the vehicle. The guide vane can be spaced apart from the inner support panel which can be configured to locate in the aperture in the outer vehicle surface. The first and second outer surfaces of each guide vane can alter the vehicle planform when the guide vane is deployed. At least in certain embodiments, the guide vane can provide a more aerodynamically favorable front corner planform curvature. 
     The one or more guide vanes can each be elongated in a vertical direction. The one or more guide vanes can each have a curved profile in planform, for example to define an aerofoil section. Each guide vane can be disposed in line with at least a portion of a wheel of the vehicle (typically a front wheel). Thus, there is at least a partial overlap in a vertical direction (along the Z-axis) between each guide vane and the front wheel of the vehicle. The one or more guide vanes can be disposed in front of a wheel of the vehicle to control the airflow past that wheel. 
     The vehicle can comprise first and second said guide vanes. The first and second guide vanes can be arranged to guide the airflow around respective left and right corners at the front of the vehicle. The first and second guide vanes can be disposed at the left and right sides of the vehicle. The one or more guide vanes can be disposed in a front bumper or skirt of the vehicle. 
     The vehicle can further comprise at least one planform-changing member movable between a retracted position and a deployed position. The at least one planform-changing member can be deployed to direct the incident airflow over an adjacent one of said guide vanes. 
     The planform-changing member can be disposed adjacent to the inner support panel as described herein. The planform-changing member can be adapted to align with the inner support panel. An outer surface of the planform-changing member and the second outer guide surface of the guide vane can align with each other when they are both in their respective deployed positions and/or retracted positions. The planform-changing member and the inner support panel can thereby define a substantially continuous outer surface of the vehicle when they are aligned with each other. Alternatively, or in addition, the outer surface of the planform-changing member can align with the first outer guide surface when the planform-changing member and the guide vane are in their retracted positions. 
     Alternatively, two or more of said planform-changing members can be provided. The planform-changing members can be arranged adjacent to each other. One or more of said planform-changing members can be deployed to modify the vehicle planform. The planform-changing members can be deployed simultaneously or sequentially. When deployed, the planform-changing members can be arranged to form a substantially continuous outer surface. The vehicle can comprise first and second planform-changing members. The first and second planform-changing members can be disposed on respective sides of the vehicle for controlling airflow along the sides of the vehicle. The first and second planform-changing members can be disposed at or proximal to the left and right front corners of the vehicle. 
     According to a further aspect of the present invention there is provided a vehicle comprising first and second planform-changing members each movable between a retracted position and a deployed position, wherein said first and second planform-changing members are deployable in a transverse direction to control airflow along the respective sides of the vehicle. The planform-changing members can be deployed to modify the effective aerodynamic profile of the front corner of the vehicle. 
     The planform-changing members can each comprise a lateral guide surface for guiding the incident airflow in a direction having a transverse component in relation to the vehicle. The position and/or orientation of the lateral guide surface can change when the planform-changing member is moved between said retracted position and said deployed position. 
     The at least one planform-changing member can be displaced to said deployed position to direct the incident airflow over an adjacent guide vane. 
     Each planform-changing member can translate and/or rotate forwards in relation to the vehicle when moving from said retracted position to said deployed position. 
     The vehicle described herein can further comprise first and second deployable sills arranged on respective left and right sides of the vehicle. This arrangement is believed to be patentable independently. 
     According to a still further aspect of the present invention there is provided a vehicle comprising: 
     a first sill and a second sill, the first and second sills being disposed on respective left and right sides of the vehicle; 
     the first and second sills each being movable between a retracted position and a deployed position; 
     wherein the first and second sills are displaced laterally outwardly when they move from said retracted position to said deployed position. The first and second sills can be displaced laterally downwardly when they move from said retracted position to said deployed position. 
     The first and second sills are longitudinally elongated and are disposed on each side of the vehicle between the front and rear wheels. When in their respective deployed positions, the first and second sills can promote flow reattachment downstream of the front wheels of the vehicle. At least in certain embodiments, this can improve the flow condition onto the rear wheels of the vehicle. The outflow from under the vehicle can be reduced, potentially improving the performance of a rear diffuser (if fitted). 
     A control unit can be provided for energizing an actuator to deploy each sill. The control unit can be configured to move said first and second sills from said retracted position to said deployed position only when said vehicle is moving, for example when the vehicle speed exceeds a predefined threshold. The control unit can be configured to retract said first and second sills when said vehicle is stationary or the speed decreases below a predefined threshold. 
     The first and second sills can be configured to function as a step to facilitate ingress to and/or egress from the vehicle. The control unit can be configured to deploy the first and second sills when the vehicle is stationary to form a step. 
     The vehicle described herein can comprise one or more rear guide vanes each disposed in a rear quarter of the vehicle for guiding the airflow expelled from a rear wheel of the vehicle inwardly to reduce the wake behind the vehicle. Each rear guide vane can be movable from a retracted position to a deployed position. The one or more rear guide vanes can each be displaced laterally and/or undergo rotation as they are displaced from said retracted position to said deployed position. The rear guide vanes can each comprise a rear guide surface which forms a substantially continuous surface of the vehicle when retracted. The rear guide vanes can provide a clearly defined boundary layer flow separation edge suitable for controlling the size of the vehicle wake. The rear guide vanes can also promote pressure recovery, thereby reducing drag. At least in certain embodiments, this can enhance lateral stability at low yaw angles, particularly for vehicles with large rear corner radii. 
     The rear guide vane can be elongated vertically. At least a portion of each rear guide vane can be disposed in line with a rear wheel of the vehicle. Thus, each rear guide can be arranged in an overlapping arrangement with the associated rear wheel of the vehicle. 
     According to a still further aspect of the present invention there is provided a vehicle comprising: 
     a first sill and a second sill, the first and second sills being disposed on respective left and right sides of the vehicle; 
     the first and second sills each being movable between a retracted position and a deployed position; 
     wherein the first and second sills are displaced laterally outwardly and/or downwardly when they move from said retracted position to said deployed position. 
     The apparatus herein is described with reference to a longitudinal axis (X), a transverse axis (Y) and a vertical axis (Z) of the vehicle. References herein to the front and rear of the vehicle are to rely on the conventional meaning of the terms. The term planform used herein refers to a profile (outline) of the apparatus when viewed in a plan elevation along the vertical axis. 
     Furthermore, the apparatus has been described herein with reference to a vehicle. The vehicle can be an automobile, a truck, a lorry, an articulated vehicle and so on. The aerodynamic apparatus described herein could be applied to a trailer to be towed behind a vehicle. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a front perspective view of a vehicle incorporating a deployable turning vane in accordance with an embodiment of the present invention; 
         FIG. 2  shows a front perspective view of the vehicle shown in  FIG. 1  with the turning vane in a deployed position; 
         FIGS. 3A-C  show perspective views of a vehicle incorporating a deployable profile-changing assembly in accordance with a further embodiment of the present invention; 
         FIG. 3D  shows a perspective of a vehicle incorporate a modified arrangement of the profile-changing assembly shown in  FIGS. 3A-C ; 
         FIGS. 4A-C  show plan views of a vehicle incorporating the profile-changing assembly in accordance with the further embodiment of the present invention; 
         FIGS. 5A and 5B  illustrate the airflow along a side of the vehicle incorporating the profile-changing assembly according to the further embodiment in a retracted position and in a deployed position; 
         FIGS. 6A-C  show perspective views of the rear quarter of a vehicle incorporating a deployable rear guide vane in according with a further embodiment of the present invention; 
         FIG. 7A  shows a side elevation of the vehicle shown in  FIGS. 6A-C  with the rear guide vane in a deployed configuration; 
         FIG. 7B  shows a plan view of the vehicle shown in  FIGS. 6A-C  with the rear guide vane in a deployed configuration; 
         FIGS. 8A and 8B  illustrate the airflow along a side of the vehicle incorporating the rear guide vane shown in  FIGS. 6A-C  respectively in retracted and deployed positions; 
         FIGS. 9A and 9B  show perspective views of a vehicle incorporating a deployable sill according to a further embodiment of the present invention; 
         FIG. 10A  shows a side elevation of a vehicle incorporating a pillar guide vane according to a further embodiment of the present invention; 
         FIG. 10B  shows a rear perspective view of the vehicle shown in  FIG. 10A ; 
         FIG. 10C  shows a plan view of the vehicle shown in  FIG. 10A ; 
         FIG. 11  shows an enlarged perspective view of a rear corner of the vehicle shown in  FIGS. 10A-C ; 
         FIGS. 12A and 12B  show a first vehicle model and a computational fluid dynamics simulation of the wake formed behind the model; 
         FIGS. 13A and 13B  show a second vehicle model and a computational fluid dynamics simulation of the wake formed behind the model; 
         FIGS. 14A and 14B  show a third vehicle model and a computational fluid dynamics simulation of the wake formed behind the model; 
         FIG. 15  shows a rear perspective view of the vehicle shown in  FIGS. 10A-C  incorporating a transverse aerodynamic member; 
         FIGS. 16A and 16B  show rear perspective views of a vehicle incorporating a deployable diffuser in accordance with a further embodiment of the present invention; 
         FIGS. 17A and 17B  show rear elevations of the vehicle shown in  FIGS. 16A and 16B ; and 
         FIGS. 18A and 18B  show side elevations of the vehicle shown in  FIGS. 16A and 16B . 
     
    
    
     DETAILED DESCRIPTION 
     A vehicle  1  comprising first and second deployable turning-vane assemblies  3  in accordance with an embodiment of the present invention is illustrated in  FIGS. 1 and 2 . The first and second turning-vane assemblies  3  are provided on the left and right sides respectively of the vehicle  1 . 
     The first and second turning-vane assemblies  3  are deployable outwardly to modify the planform of the front corners of the vehicle  1  to improve aerodynamic efficiency. The outer surface of each of the first and second turning-vane assemblies  3  is profiled to improve longitudinal airflow along the respective sides of the vehicle  1 . In the illustrated arrangement, the vehicle  1  is a motor vehicle having a saloon (sedan) configuration, but the first and second turning-vane assemblies  3  can be employed in other motor vehicle configurations. 
     The first and second turning-vane assemblies  3  are described herein with reference to a longitudinal axis (X), a transverse axis (Y) and a vertical axis (Z) of the vehicle  1 , as shown in  FIG. 1 . The vehicle  1  comprises a front bumper  5 , a rear bumper  7 , left and right longitudinal sills  9 , a bonnet  13 , a boot (trunk)  15 , left and right wings (fender)  17 , left and right quarter panels  21  left and right front wheels W 1 , W 2 , left and right rear wheels W 3 , W 4 , and four doors D 1 - 4 . The front wheels W 1 , W 2  are disposed in front wheel houses formed in the left and right wings  17 ; and the rear wheels W 3 ,W 4  are disposed in rear wheel houses formed in the left and right quarter panels  21 . It will be appreciated that the first and second turning-vane assemblies  3  could be implemented in other vehicle configurations, for example a vehicle  1  having two doors D 1 , D 2 . 
     The front bumper  5  extends across the front of the vehicle  1 . The lateral sections of the front bumper  5  curve around the sides of the vehicle  1  thereby defining left and right front corners of the vehicle  1 . The first and second turning-vane assemblies  3  are disposed on respective sides of the front bumper  5  at the front corners of the vehicle  1  (where the front of the vehicle  1  undergoes a transition into the vehicle body sides). The front bumper  5  defines an outer surface  25  (i.e. an “A” surface) which, when viewed from above along the vertical axis (Z), defines lateral curves at the front of the vehicle  1 . The outer surface  25  of the front bumper  5  has a continuous profile (i.e. the outer surface  25  is formed at least substantially without steps, gaps or other discontinuities) to promote the flow of air around the front of the vehicle  1 . 
     The first and second turning-vane assemblies  3  have the same general configuration, but only the first turning-vane assembly  3  will be described herein for the sake of brevity. The first turning-vane assembly  3  comprises a front guide vane  27  disposed at a vertical position on the vehicle  1  so as to be in line with at least a portion of the front wheel W 1 . In other words, there is at least a partial overlap in a vertical direction (along the Z-axis) of the front guide vane  27  and the associated front wheel W 1  (or the front wheel arch). The front guide vane  27  is vertically elongated and has a curved planform profile. The front guide vane  27  can be selectively displaced between a retracted position (as shown in  FIG. 1 ) and a deployed position (as shown in  FIG. 2 ). In the present embodiment, the front guide vane  27  undergoes lateral translation when it is moved between said retracted and deployed positions. In the present embodiment, the front guide vane  27  translates substantially along the transverse axis Y of the vehicle  1 . 
     At least in certain embodiments the first turning-vane assembly  3  is an active system and one or more actuators (not shown) are provided for displacing the front guide vane  27  between said retracted and deployed positions. The actuators could be electromechanical, pneumatic or hydraulic. In the present embodiment the first and second turning-vane assemblies  3  each comprises an electric drive coupled to a lead screw (not shown) for actuating the front guide vane  27 . A control unit (not shown) is provided for controlling operation of said one or more actuators. 
     The front guide vane  27  is mounted to a front support panel  29  by upper and lower longitudinal struts  31 ,  33 . The outer surface of the front guide vane  27  forms an outer guide surface  35 ; and an outer surface of the front support panel  29  forms an inner guide surface  37 . As shown in  FIG. 2 , three longitudinal channels  39 A-C are formed between the front guide vane  27  and the front support panel  29  by the longitudinal struts  31 ,  33 . The longitudinal channels  39 A-C each extend from a front (leading) edge of the front guide vane  27  to a rear (trailing) edge of the front guide vane  27 . A top plate  41  and a bottom plate  43  are provided on the front guide vane  27  to prevent the airflow within the longitudinal channels  39 A-C mixing with the airflow travelling over the outer guide surface  35 . The top and bottom plates  41 ,  43  could be omitted. It will be appreciated that the first and second turning-vane assemblies could have less than, or more than three longitudinal channels  39 A-C. 
     The outer guide surface  35  and the inner guide surface  37  can have substantially the same profile, or they can have different profiles. The cross-sectional area of each longitudinal channel  39 A-C is substantially uniform along its length. In alternate arrangements, the cross-sectional area of the longitudinal channels  39 A-C could increase towards the rear of the vehicle  1  to decelerate the airflow travelling between the front guide vane  27  and the front support panel  29 . Alternatively, the cross-sectional area of the longitudinal channels  39 A-C could decrease towards the rear of the vehicle  1  to accelerate the airflow travelling between the front guide vane  27  and the front support panel  29 . 
     The exit region of each longitudinal channel  39 A-C could be profiled to introduce a directional component into the airflow relative to the vehicle  1 , for example to direct the airflow laterally inwardly or outwardly. Indeed, the longitudinal channels  39 A-C could be profiled to direct the airflow upwardly or downwardly. These arrangements could, for example, facilitate tuning of the flow along the side of the vehicle  1 . In the present embodiment, the exit region of each longitudinal channel  39 A-C is configured to direct the airflow substantially parallel to the longitudinal axis Y of the vehicle  1 . 
     The control unit for controlling operation of the first and second turning-vane assemblies  3  can be configured to deploy the front guide vane  27  when the speed of the vehicle  1  exceeds a predefined threshold, such as 30 mph. When the vehicle speed is below the predefined threshold, the front guide vane  27  is retained in its retracted position. However, when the vehicle speed exceeds the predefined threshold, the front guide vane  27  is displaced laterally outwardly to its deployed position. Alternatively, the first and second turning-vane assemblies  3  could be configured to deploy the front guide vane  27  when a predefined vehicle dynamic mode is selected, for example a Track or Sport dynamic mode. The first and second turning-vane assemblies  3  could be configured actively to adjust the position of the front guide vane  27  in relation to the front bumper  5 , for example based on vehicle speed. 
     When the front guide vane  27  is in its retracted position, a leading edge  35   a  of the outer guide surface  35  forms a continuation of the outer surface  25  of the front bumper  5  and a rear (trailing) portion  35   b  of the outer guide surface  35  is disposed substantially in line with a side of the vehicle. Conversely, when the front guide vane  27  is in its deployed position, it is displaced laterally outwardly from a surrounding area of the front bumper  5  and the rear portion  35   b  of the outer guide surface  35  is displaced laterally outwardly from the side of the vehicle. The longitudinal channels  39 A-C are thereby introduced into the incident airflow when the front guide vane  27  is deployed. 
     In use, the longitudinal channels  39 A-C, the outer guide surface  35  and the inner guide surface  37  are operative to guide the lateral airflow at the front of the vehicle  1  around the front corner of the vehicle  1 . The front guide vane  27  controls airflow along the vehicle body side and, at least in certain embodiments, the flow downstream of the front guide vane  27  is modified to suppress the disturbed flow which would otherwise emanate from the respective front wheel houses. The front guide vane  27  can thereby reduce the flow disturbance caused by the rotation of the front wheels W 1 , W 2  resulting in reduced flow disturbances along the vehicle body sides. At least in certain embodiments, the front guide vane  27  can operate to reduce vehicle drag. 
     The first turning-vane assembly  3  according to the present embodiment has been described as undergoing lateral translation when it is displaced to said deployed position. Alternate deployment mechanisms could be adopted. The front guide vane  27  could be pivotally mounted and undergo a pivoting motion as it travels between said retracted and deployed positions. Alternatively, the front guide vane  27  could undergo a combination of rotation and translation movement as it travels between said retracted and deployed positions. 
     A vehicle  101  comprising first and second deployable planform-changing assemblies  103  in accordance with a further embodiment of the present invention is shown in  FIGS. 3A-C  and  4 A-C. The first and second planform-changing assemblies  103  are a development of the first and second turning-vane assemblies  3  according to the preceding embodiment. Like reference numerals will be used for like components, albeit incremented by 100 to aid clarity. 
     The first and second planform-changing assemblies  103  are provided on the left and right sides respectively of the vehicle  101 . The first and second planform-changing assemblies  103  have generally the same configuration and only the first planform-changing assembly  103  will be described herein for the sake of brevity. 
     The first planform-changing assembly  103  comprises a front sub-assembly  145  and a rear sub-assembly  147 . The front and rear sub-assemblies  145 ,  147  are positioned adjacent to each other with the front sub-assembly  145  disposed longitudinally in front of the rear sub-assembly  147 . The front sub-assembly  145  comprises a front profiling member  149  which defines a front guide surface  151  forming a section of the outer surface  125  of the front bumper  105 . The rear sub-assembly  147  comprises a front guide vane  127  which is broadly equivalent to the front guide vane  27  described herein with reference to the preceding embodiment. The front guide vane  127  is fixedly mounted to a front support panel  129  by a series of longitudinal struts. The outer surface of the front guide vane  127  forms an outer guide surface  135 ; and an outer surface of the front support panel  129  forms an inner guide surface  137 . The front profiling member  149  and the front guide vane  127  are movable independently of each other. The front guide vane  127  is disposed on the vehicle  101  such that there is at least a partial overlap in a vertical direction (along the Z-axis) of the front guide vane  127  and the associated front wheel W 1  (or the front wheel arch). 
     The front profiling member  149  is movable pivotally between a retracted position (as shown in  FIGS. 3A and 4A ) and a deployed position (as shown in  FIGS. 3B, 3C, 4B and 4C ). Furthermore, the front guide vane  127  is also movable between a retracted position (as shown in  FIGS. 3A, 3B, 4A and 4B ) and a deployed position (as shown in  FIGS. 3C and 4C ). The front guide vane  127  undergoes a combination of lateral translation (along the Y-axis) and pivoting motion (substantially about the Z-axis) as it transitions between said retracted and deployed positions. 
     When the front profiling member  149  and the front guide vane  127  are both in their retracted positions, the front guide surface  151  and the outer guide surface  135  form a substantially continuous exterior surface defining the outer surface  125  of the front bumper  105 . In this configuration, a rear (trailing) portion  135   b  of the outer guide surface  135  of the front guide vane  127  is arranged substantially in line with a side of the vehicle  101  (typically substantially parallel to the longitudinal axis X). 
     The front guide vane  127  is located behind the front profiling member  149  and a lap joint is formed to define a substantially continuous exterior surface. In the present embodiment, the front and rear sub-assemblies  145 ,  147  undergo sequential displacement to their respective deployed or retracted positions. When the front and rear sub-assemblies  145 ,  147  are deployed, the movement of the front profiling member  149  commences before that of the front guide vane  127 . Conversely, when the front and rear sub-assemblies  145 ,  147  are retracted, the movement of the front guide vane  127  commences before that of the front profiling member  149 . The front and rear sub-assemblies  145 ,  147  comprise actuators (not shown) to enable independent operation of the front guide vane  127  and the front profiling member  149 . 
     In the present embodiment the pivoting of the front profiling member  149  to its deployed position commences while the front guide vane  127  remains in its retracted position (as shown in  FIGS. 3B and 4B ). The front guide vane  127  is subsequently displaced to its deployed position by undergoing combined translation and pivoting movement. When deployed, the inner guide surface  137  is arranged to align with the front guide surface  151  to form a substantially continuous exterior surface. Thus, the inner guide surface  137  and the front guide surface  151  modify the planform of the front bumper  105  when they are deployed. The outer guide surface  135  of the front guide vane  127  is displaced laterally outwardly to project beyond the side of the vehicle to provide additional control of the incident airflow. 
     A control unit can be provided for controlling operation of the first and second deployable planform-changing assemblies  103 . The control unit can be configured to deploy the front first and second deployable planform-changing assemblies  103  when the speed of the vehicle  1  exceeds a predefined threshold, such as 30 mph. When the vehicle speed is below the predefined threshold, first and second deployable planform-changing assemblies  103  can be retained in their retracted positions. When the vehicle speed exceeds the predefined threshold, first and second deployable planform-changing assemblies  103  can be displaced to their respective deployed positions. Alternatively, the control unit could be configured to deploy the first and second deployable planform-changing assemblies  103  when a predefined vehicle dynamic mode is selected, for example a Track or Sport dynamic mode. The control unit could be configured actively to adjust the position of the first and second deployable planform-changing assemblies  103 , for example based on vehicle speed. 
     The operation of the second planform-changing assembly  103  will now be described with reference to  FIGS. 5A and 5B . 
     The airflow around the vehicle  101  with the second planform-changing assembly  103  retracted is illustrated in  FIG. 5A . The airflow tends to separate at the front of the vehicle due to the change in direction as the airflow undergoes a transition from a lateral flow direction to a longitudinal flow direction at the corner. The resulting airflow along the vehicle body side is dominated by vortices and turbulent flow. 
     The airflow around the vehicle  101  with the second planform-changing assembly  103  deployed is illustrated in  FIG. 5B . When both the front guide vane  127  and the front profiling member  149  are deployed, the outer surface  125  of the front bumper  105  forms a substantially continuous exterior surface. A region of increased suction is realized on the forward facing surfaces of the front guide vane  127  and the front profiling member  149 . As a consequence, the airflow remains attached to the surface of the vehicle  1  and is guided around the front corner by the front guide vane  127 . The area of flow separation is reduced allowing a reduction in the related energy losses. The velocity of the airflow closer to the vehicle body sides is increased compared to the scenario when the planform-changing assembly  103  is not deployed. The width of the wake downstream of the vehicle  1  is reduced resulting in a reduction in the drag of the vehicle. By controlling airflow around the vehicle  1 , the front planform-changing assembly  103  can help to reduce the deposition (total or rate) of surface contaminants on the vehicle body side, for example on the Body-In-White, cladding, moldings handles or lamp covers. 
     In an alternate arrangement, the front guide vane  127  could be movable to an intermediate position in which the outer guide surface  135  of the front guide vane  127  forms a continuation of the front profiling member  149  in its deployed position. In a further arrangement (illustrated in  FIG. 3D ), the inner guide surface  137  can be omitted such that the front guide vane  127  deploys to form a continuation of the front guide surface  151 . In both of these arrangements the planform of the front bumper  105  can be changed to control airflow along the side of the vehicle. This functionality is believed to be patentable in its own right. 
     It will be appreciated that the planform-changing assembly  103  described herein can be modified. For example, the front and rear sub-assemblies  145 ,  147  could be combined. The deployable mechanism could consist of a single sub-assembly, for example pivoting about an axis proximal its leading edge. A further alternative would be to implement more than two sections, for example to accommodate deployed configurations having a different profile. A still further alternative would be to use a morphing surface, for example a bi-stable surface. A yet further alternative would be to use a flexible material to define the outer surface  125  of the front bumper  105 . 
     A vehicle  201  comprising first and second rear fender assemblies  203  in accordance with a further embodiment of the present invention is illustrated in  FIGS. 6, 7 and 8 . At least in certain embodiments, the rear fender assemblies  203  are arranged to provide a surface to which the airflow can re-attach in order to reduce the wake downstream of the vehicle  201 . The first and second rear fender assemblies  203  in the present embodiment are selectively deployable, but they could have a fixed configuration. The description is with reference to a longitudinal axis (X), a transverse axis (Y) and a vertical axis (Z) of the vehicle  201 . 
     The vehicle  201  comprises a front bumper, a rear bumper  207 , left and right sills  209 , a bonnet  213 , a tailgate  215 , left and right wings (fender)  217 , left and right quarter panels  221 , left and right front wheels W 1 , W 2 , left and right rear wheels W 3 , W 4 , and four doors D 1 - 4 . The front wheels W 1 , W 2  are disposed in front wheel houses formed in the left and right wings  217 ; and the rear wheels W 3 , W 4  are disposed in rear wheel houses formed in the left and right quarter panels  221 . 
     The first and second rear fender assemblies  203  are disposed in the left and right quarter panels  221  respectively and are positioned behind the rear wheels W 3 , W 4 . Only the second rear fender assembly  203  (provided on the right-hand side of the vehicle  201 ) is visible in the Figures and, therefore, the second rear fender assembly  203  will be described herein. It will be appreciated that the first rear fender assembly  203  has a corresponding configuration, albeit disposed on the left-hand side of the vehicle  201 . 
     The second rear fender assembly  203  comprises a rear guide vane  227  which is deployable to provide a surface to which the airflow at the rear of the vehicle  1  can re-attach. The rear guide vane  227  can thereby guide the airflow at the rear of the vehicle  1  to reduce the size of the wake downstream of the vehicle  1 . The rear guide vane  227  has a front (leading) edge  227   a  and a back (trailing) edge  227   b . In the present embodiment, the front edge  227   a  is profiled to match a rear section of the wheel arch formed around the wheel housing. It will be appreciated, therefore, that the rear guide vane  227  could be profiled to include a section of the wheel arch trim. 
     The rear guide vane  227  is mounted to a rear support panel  229  by three longitudinal struts  231 ,  232 ,  233 . The rear fender assembly  203  is operable selectively to displace the rear guide vane  227  between a retracted position (as shown in  FIG. 6A ) and a deployed position (as shown in  FIG. 6C ). In the present embodiment, the rear guide vane  227  undergoes lateral translation along the transverse axis Y when it is moved between said retracted and deployed positions. An actuator is disposed in the vehicle  1  actively to displace the rear support panel  229  between said retracted position and said deployed position. 
     The rear guide vane  227  is spaced apart from the rear support panel  229 . The outer surface of the rear guide vane  227  forms a first outer surface in the form of an outer guide surface  235 ; and an outer surface of the rear support panel  229  forms a second outer surface in the form of an inner guide surface  237 . As shown in  FIG. 6C , two longitudinal channels  239 A,  239 B are formed between the rear guide vane  227  and the rear support panel  229  by the longitudinal struts  231 ,  232 ,  233 . The longitudinal channels  239 A,  239 B each extend from a front (leading) edge  227   a  of the rear guide vane  227  to a back (trailing) edge  227   b  of the rear guide vane  227 . A top end plate and a bottom end plate can optionally be provided on the rear guide vane  227 . When the rear guide vane  227  is deployed, the inner guide surface  237  forms a continuation of the surrounding region of the right quarter panel  221 . 
     The rear guide vane  227  is disposed behind the rear wheel W 3  to provide a surface to which the turbulent air expelled from the rear wheel housing can re-attach. To provide a suitable surface for re-attaching the turbulent airflow, the rear guide vane  227  is disposed below a waistline (beltline) of the vehicle (which defines a transition from the side body panels to the glasshouse). In the present embodiment, the rear guide vane  227  is disposed at a vertical height on the vehicle  1  such that it is positioned in line with at least a portion of the rear wheel W 3 . Thus, there is at least a partial overlap in a vertical direction (along the Z-axis) of the rear guide vane  227  and the associated rear wheel W 3  (or the rear wheel arch). When deployed, at least the leading edge  227   a  of the rear guide vane  227  extends into the turbulent airflow expelled from the rear wheel housing and/or the rear wheel W 3 . In use, the airflow emanating from the rear wheel housing re-attaches to the outer guide surface  235  of the rear guide vane  227 . The rear guide vane  227  is configured to guide the airflow at the rear of the vehicle  201  to reduce aerodynamic drag of the vehicle  1 . Specifically, the rear guide vane  227  is inclined at an angle relative to the longitudinal axis X to guide the local airflow inwardly towards a centre line of the vehicle  201 . 
     In the present embodiment, the rear guide vane  227  is arranged such that the outer guide surface  235  is oriented to guide the airflow inwardly (towards a centre line of the vehicle  201 ). The flow downstream of the rear guide vane  227  is thereby directed inwardly which can, at least in certain embodiments, provide a ‘boat tail’ rear planform for the vehicle, thereby reducing drag. The trailing edge  227   b  of the rear guide vane  227  also defines a consistent separation point for the attached airflow which provides uniform flow control across a wider range of conditions. This is in contrast to prior art arrangements in which the separation of the airflow around a curved surface (such as the rear corner of the vehicle) results in greater variability. It will be appreciated that, at least in certain embodiments, the first and second rear fender assemblies  203  are operative to reduce aerodynamic drag on the vehicle. This has particular application in vehicles having pronounced wheel arches, such as a sports utility vehicle (SUV) or an off-road vehicle. 
     The rear guide vane  227  has a deployed position in which it functions as a turning vane to guide the airflow inwardly behind the vehicle  201  to control the vehicle wake. The rear support panel  229  locates in the aperture left by the movement of the rear guide vane  227 . When deployed, the rear guide vane  227  delivers a more aerodynamically favorable rear quarter profile which can attach flow downstream of the rear wheel arch and also separate the airflow cleanly and predictably off the rear of the vehicle  201 . 
     The rear guide vane  227  could optionally travel forwards towards the wheel W 3  to locate within the turbulent airflow expelled laterally from the wheel housing. This arrangement would potentially reduce the lateral displacement required to position the rear guide vane  227  in relation to the wheel housing. The rear guide vanes  227  disposed on each side of the vehicle could be controlled independently, for example to compensate for changes due to rear wheel steering and/or crosswinds. 
     The channel formed between the rear guide vane  227  and the inner surface  237  and the A-surface can be uniform along its length or can be converging. If the channel converges towards the rear of the vehicle  201 , the resulting (relatively) high velocity air jet can be used to constrain/reduce the extent of the wake downstream of the vehicle  201 . Alternatively, a diverging channel may be used to reduce the drag associated with the rear guide vane  227 . 
     When the rear guide vane  227  is retracted, the outer guide surface  235  is disposed substantially flush with the surrounding surface of the rear quarter of the vehicle. Thus, the outer surface of the rear quarter has a substantially continuous exterior surface which is free from steps or discontinuities. The airflow around the vehicle  1  with the rear guide vane  227  retracted is illustrated in  FIG. 8A . The airflow travels along the vehicle body sides and separates at the rear of the vehicle  201  to form a wake downstream thereof. A reduction in the width of the vehicle  201  towards the rear can help to reduce the size of the wake to reduce aerodynamic drag. However, the rotation of the rear wheels W 3 , W 4  tends to pump air out of the rear wheel houses which does not re-attach to the downstream surfaces of the left and right quarter panels  221 . The air expelled from the wheel houses tends to be directed laterally outwardly which increases the wake of the vehicle  1 . 
     The airflow around the vehicle  1  with the rear guide vane  227  deployed is illustrated in  FIG. 8B . The rear guide vane  227  is displaced laterally outwardly of the right quarter panel  221  and rotates about a vertical axis (arranged substantially parallel to the vertical axis Z in the present embodiment). When deployed, the rear guide vane  227  is disposed within the path of the air expelled from the rear wheel houses. The expelled air flows over the rear guide vane  227  and re-attaches to the outer guide surface  235 . The rear guide vane  227  is oriented to guide the airflow inwardly towards a longitudinal centre-line of the vehicle  201 , as illustrated in  FIG. 8B . 
     The rear guide vane  227  can provide a clearly defined boundary layer flow separation edge suitable for controlling the size of the vehicle wake. The rear guide vane  227  can also promote pressure recovery, thereby reducing drag and potentially enhance lateral stability at low yaw angles, particularly for vehicles with large rear corner radii. The rear guide vane  227  can help to reduce the deposition (total or rate) of surface contaminants at the rear of the vehicle, for example on the rear surfaces, for example the Body In White, tail gate, lift gate, glazing cladding, moldings, handles or lamp covers. 
     The first and second rear fender assemblies  203  are believed to be patentable independently of the other arrangements described herein. However, the first and second rear fender assemblies  203  have particular application in combination with the turning-vane assemblies  3  and/or the planform-changing assemblies  103  described herein. The turning-vane assemblies  3 ; and the planform-changing assemblies  103  promote the linear flow of air along the vehicle body sides, thereby reducing the vehicle wake. The first and second rear fender assemblies  203  can also operate to reduce or control the wake behind the vehicle  201 . 
     The rear guide vane  227  has been described as undergoing translation in a lateral direction and rotation about a vertical axis as it is deployed. It will be appreciated that the rear guide vane  227  could undergo additional movements as it travels between said deployed and retracted positions. For example, the angular orientation of the rear guide vane  227  in relation to the longitudinal axis X of the vehicle  201  could alter as it is deployed. 
     A control unit can be provided for controlling operation of the first and second rear fender assemblies  203 . The control unit can, for example, be configured to deploy the rear guide vane  227  when the speed of the vehicle  1  exceeds a predefined threshold, such as 30 mph. Alternatively, the first and second rear fender assemblies  203  could be configured to deploy the rear guide vanes  227  when a predefined vehicle dynamic mode is activated, for example a Track or Sport dynamic mode. The first and second rear fender assemblies  203  could be configured actively to adjust the angular position and/or relative position of the associated rear guide vane  227 , for example based on vehicle speed. 
     A vehicle  301  incorporating left and right deployable sill assemblies  303  in accordance with a still further aspect of the present invention will now be described with reference to  FIGS. 9A and 9B . The vehicle  301  is a development of the vehicle  201  according to the preceding embodiment described herein. Accordingly, like reference numerals will be used for like components, albeit incremented by 100 to aid clarity. 
     The vehicle  301  comprises a front bumper  305 , a rear bumper  307 , left and right sills  309 , a bonnet  313 , a tailgate  315 , left and right wings (fenders)  317 , left and right quarter panels  321 , left and right front wheels W 1 , W 2 , left and right rear wheels W 3 , W 4 , and four doors D 1 - 4 . The front wheels W 1 , W 2  are disposed in front wheel houses formed in the left and right wings  317 ; and the rear wheels W 3 ,W 4  are disposed in rear wheel houses formed in the left and right quarter panels  321 . 
     The left and right sills  309  extend longitudinally between the front and rear wheels W 1 , W 3 ; W 2 , W 4  on the respective sides of the vehicle  301 . The left and right sills  309  each comprise a vertical section (sidewall)  353  and a lower transverse section  355 . The left and right sills  309  can be displaced between a retracted position (as shown in  FIG. 9A ) and a deployed position (as shown in  FIG. 9B ). When the left and right sills  309  are retracted, they nest around the vehicle body structure such that the outer surface of the vertical section  353  and the adjacent body structure align to form a substantially continuous exterior surface, such as the bottom of the doors D 1 -D 4 . When the left and right sills  309  are deployed, they undergo translation laterally outwardly and downwardly partially filling the longitudinal space between the front and rear wheels. 
     A control unit (not shown) is provided for controlling the deployment of the left and right sills  309 . The left and right sills  309  are intended to provide an active aerodynamic aid to reduce the drag of the vehicle  301  whilst it is moving. As such, the control unit is configured to displace the left and right sills  309  only when the vehicle is moving. The control unit can, for example, be configured to deploy the left and right sills  309  only when the vehicle  301  speed exceeds a predetermined threshold, for example 30 mph. Below this speed, or when the vehicle  301  is stationary, the control unit is configured to retract the left and right sills  309 . The control unit could optionally also be configured to deploy the left and right sills  309  when the vehicle is stationary to provide a step to facilitate ingress to and/or egress from the vehicle  301 . 
     At least in certain embodiments the left and right deployable sill assemblies  303  can help to control the airflow along respective sides of the vehicle  301 . For example, the left and right sills  309  can be deployed laterally to help reduce the outflow of air from under the vehicle  301 , thereby potentially improving the performance of a rear diffuser can be improved. Furthermore, the lateral displacement of the left and right sills  309  can promote flow re-attachment aft of the front wheels W 1 , W 2  through the provision of a suitable outboard surface. In particular, when the left and right sills  309  are deployed, an outer surface of the vertical section  353  is disposed in the turbulent airflow expelled from the front wheels W 1 , W 2  and/or the front wheel housings. This turbulent flow re-attaches to the outer surface and the flow conditions onto/over the rear wheels W 3 , W 4  can potentially be improved. The provision of deployable left and right sills  309  (rather than fixed sills) also allows the vehicle  301  to retain a waisted profile (i.e. a mid-section having a reduced cross-sectional area). 
     The left and right sills  309  can promote flow reattachment downstream of the front wheels W 1 , W 2  of the vehicle  301 , thereby improving the flow condition onto the rear wheels W 3 , W 4 . The outflow from under the vehicle  301  can also be reduced, potentially improving the performance of a rear diffuser (not shown). 
     The left and right deployable sill assemblies  303  described herein are believed to be patentable independently of the other arrangements described herein. However, it will be appreciated that the left and right deployable sill assemblies  303  can be combined with one or more of the other assemblies described herein. For example, the deployable sill assemblies  303  can be combined with the turning-vane assemblies  3  and/or the planform-changing assemblies  103  and/or the rear fender assemblies  203 . 
     A vehicle  401  incorporating left and right rear pillar ducts  457  in accordance with a still further aspect of the present invention will now be described with reference to  FIGS. 10 to 14 . The vehicle  401  is a development of the vehicle  201  described herein with reference to  FIG. 6 . Accordingly, like reference numerals will be used for like components, albeit incremented by 200 to aid clarity. 
     As shown in  FIG. 10A , the vehicle  401  comprises a rear bumper  407 , a bonnet  413 , a tailgate  415 , left and right wings (fenders)  417 , left and right rear quarter panels  421 , left and right front wheels W 1 , W 2 , left and right rear wheels W 3 , W 4 , and four doors D 1 - 4 . The front wheels W 1 , W 2  are disposed in front wheel houses formed in the left and right wings  417 ; and the rear wheels W 3 , W 4  are disposed in rear wheel houses formed in the left and right rear quarter panels  421 . 
     The vehicle body structure comprises an A-pillar (A), a B-pillar (B), a C-pillar (C) and a D-pillar (D). As shown in  FIG. 10B , left and right rear pillar vanes  459  are disposed at the rear of the vehicle  401  proximal to the D-pillars (D) on each side of the vehicle  401 . In the present embodiment, the D-pillars (D) are inset from the sides of the vehicle and the left and right rear pillar vanes  459  are each disposed laterally outwardly of the associated D-pillar (D) to form the left and right rear pillar ducts  457 . The left and right rear pillar vanes  459  are elongated in a vertical direction and extend between the top of the respective left and right rear quarter panels  421  and a roof  461  of the vehicle  401 . In the present embodiment, the left and right rear pillar vanes  459  extend vertically upwardly from the respective left and right rear quarter panels  421  to form a continuous exterior surface. An outer surface  463  of the D-pillar (D) defines an inner sidewall of the respective rear pillar duct  457  and an inner surface  465  of the rear pillar vane  459  defines an outer sidewall of the rear pillar duct  457 . In the present embodiment, the orientation of the left and right rear pillar vanes  459  is fixed in relation to the associated D-pillar (D), but alternate arrangements could comprise movably mounted rear pillar vanes  459 , for example to deploy in a lateral direction. The rear pillar vanes  459  are non-structural in the present embodiment, but they could be formed as a structural component of the vehicle body structure. For example, the rear pillar vanes  459  could form a structural component which reinforces the D-pillar; or the D-pillar and the rear pillar vane  459  could be combined on each side of the vehicle  401 . 
     In the present embodiment, the outer surface  463  of the D-pillar (D) and the inner surface  465  of the associated rear pillar vane  459  converge towards each other as they extend towards the rear of the vehicle  401 . Thus, the left and right rear pillar ducts  457  each have a converging section which, in use, accelerates the entrained airflow and a jet of air is expelled therefrom. The resulting jet of air is elongated in a vertical direction. The sides of the vehicle  401  may converge inwardly towards the roof  461  and the left and right rear pillar ducts  457  may follow this profile to form a pair of vertically-elongated jets of air which converge inwardly towards their upper ends (i.e. are inclined relative to a vertical axis Y of the vehicle  401 ). The airflow through the left and right rear pillar ducts  457  is illustrated in  FIGS. 10B and 10C  by dashed lines (AF) and arrowheads represent the speed of the airflow (a single arrowhead representing relatively slow airflow upstream of the rear pillar duct  457  and a double arrowhead representing relatively fast airflow downstream of the rear pillar duct  457 ). A variant could comprise a diverging duct (which opens outwardly towards the rear of the vehicle) to reduce the aerodynamic drag resulting from the rear pillar vanes  459 . 
     As shown in  FIG. 11 , the right rear pillar duct  457  comprises a duct inlet  467  and a duct outlet  469 . The duct inlet  467  lies flush with (or is inset from) a respective side of the vehicle  401  so as not to protrude into the airflow along the side of the vehicle. Thus, airflow attached to a side of the vehicle  401  (the glazing or ‘glasshouse’ in the present embodiment) is drawn into the rear pillar duct  457  disposed on that side of the vehicle  401 . The right rear pillar duct  457  is arranged such that the airflow exiting the duct outlet  469  comprises a lateral component directed inwardly towards a longitudinal centre line of the vehicle  401 , as illustrated in  FIG. 10C . The left rear pillar duct  457  has a corresponding arrangement, albeit a mirror image of the right rear pillar duct  457 . 
     The rear pillar ducts  457  are each arranged to entrain airflow travelling along a side of the vehicle  401  and direct it inwardly towards a longitudinal centre line of the vehicle, thereby reducing the width of the wake formed behind the vehicle  401 . A perspective view of the rear corner of the vehicle  401  is shown in  FIG. 11  to illustrate the angular orientation of the airflow exiting the duct outlet  469  and the resulting airflow at the rear of the vehicle  401 . The rear pillar duct  457  and/or the rear pillar vane  459  on each side of the vehicle  401  are configured such that, in use, the airflow exiting the respective duct outlets  469  is inclined inwardly at a first angle β relative to a longitudinal axis X of the vehicle  401 . The first angle β can be substantially uniform over the vertical extent of each rear pillar duct  457 . Alternatively, the first angle β can vary with vertical position on each rear pillar duct  457 . For example the first angle β can increase towards the top of each rear pillar duct  457  to increase the change in flow direction closer to the roof  461  of the vehicle  401 . A continuous change in the configuration of the rear pillar duct  457  and/or the rear pillar vane  459  can result in a progressive change in the first angle β, for example to increase the angle β towards the top of each rear pillar duct  457  to increase the turn-in effect on the airflow closer to the roof  461 . 
     The duct outlet  469  can have a substantially uniform width along its length. In an alternate arrangement, the width of the duct outlet  469  can vary. For example, the width of the duct outlet  469  can decrease towards the top of the respective left and right rear pillar ducts  457 . In use, this configuration can develop a faster (more energetic) jet of air proximal the roof  461  to promote interference with the rear corner vortices. The change in width of the duct outlet  469  can be continuous or could be stepped. 
     The left and right rear pillar ducts  457  are provided to control the formation of the wake behind the vehicle  401 . The left and right rear pillar ducts  457  each generate a vertically-elongated jet of air which is directed inwardly towards a centre line of the vehicle  401  to control the formation of the wake behind the vehicle  401 . The influence of the jet of air on the airflow downstream of the vehicle  401  (i.e. behind the vehicle  401  when it is travelling in a forwards direction) will now be described in more detail with reference to the fluid flow simulations shown in  FIGS. 12 to 14 . First, second and third virtual vehicle model (M 1 , M 2 , M 3 ) are shown in  FIGS. 12A, 13A and 14A  respectively further to illustrate the function of the left and right rear pillar ducts  457  to control the formation of the wake behind the vehicle. The wake in relation to each of the vehicle models M 1 , M 2 , M 3  is visualized in a horizontal rectangle R 1 , R 2 , R 3  by the flow patterns illustrated in  FIGS. 12B, 13B and 14B . Within the wake, the vortices generated off each side of the vehicle model M 1 , M 2 , M 3  close together as they travel downstream and a wake closure point (d) defines the rearmost extent of the near-wake. 
     The first vehicle model M 1 , shown in  FIG. 12A , has a bluff rear section (equivalent to a tailgate  415  arranged perpendicular to the sides of the vehicle  401 ). As shown in  FIG. 12B , the wake formed behind the first vehicle model M 1  has a first width (e 1 ). A first wake closure point (not shown) for the first vehicle model M 1  is located downstream of the modeled flow region shown in  FIG. 12B . 
     The second vehicle model M 2 , shown in  FIG. 13A , has a tapered rear section (equivalent to a boat tail planform) to allow the airflow to be drawn inwardly in a progressive manner as it flows around the rear of the second vehicle model M 2 . As shown in  FIG. 13B , the wake formed behind the second vehicle model M 2  has a second width (e 2 ). A second wake closure point (d 2 ) is also shown in  FIG. 13B . 
     The third vehicle model M 3 , shown in  FIG. 14A , comprises left and right rear pillar ducts  457  in accordance with the present embodiment. As shown in  FIG. 14B , the wake formed behind the third simulation model M 3  has a third width (e 3 ). A wake closure point (d 3 ) is also shown in  FIG. 14B . 
     The modeled wake closure point for the first vehicle model M 1  is further downstream than the second and third wake closure points (d 2 , d 3 ). Furthermore, the first width (e 1 ) of the wake behind the first vehicle model M 1  is larger than both the second and third widths (e 2 , e 3 ). As shown in  FIGS. 13B and 13C , the wake from the second and third vehicle models M 2 , M 3  are similar in size and profile. It will be appreciated, therefore, that the left and right rear pillar ducts  457  change the effective shape of the rear of the vehicle to provide a boat-tail effect. At least in certain embodiments, the left and right rear pillar ducts  457  can help to reduce aerodynamic drag as the vehicle  401  travels in a forwards direction. 
     By introducing a jet of air into the wake behind the vehicle  401 , the outer shear layers can be drawn inwardly providing a flow structure approaching that resulting from physical ‘boat tailing’. In particular, the wake closure point (d 3 ) moves closer to the vehicle rear and the wake width (e 3 ) reduces. At least in certain embodiments, this can lead to a pressure recovery on the rear surfaces (base) of the vehicle and a consequent reduction in aerodynamic drag. The left and right rear pillar ducts  457  can each generate a jet of air operatively to control lateral propagation of the turbulent airflow downstream of the vehicle (the vehicle wake). It will be appreciated that the left and right rear pillar ducts  457  are a passive (un-powered) ducting system that takes airflow attached to the vehicle roof, glasshouse, bodyside or bumper and ducts it into the rear wake. 
     A modified arrangement of the vehicle  401  according to the previous embodiment is shown in  FIG. 15 . The modified vehicle  401  comprises a roof spoiler  471  mounted to the roof  461 . Like reference numerals are used for like components in this modified arrangement. 
     The roof spoiler  471  is arranged to direct airflow downwardly at the rear of the roof  461 . A chord line S CL  is defined in a vertical plane through the roof spoiler  471 . The chord line S CL  extends from a leading edge of the roof spoiler  471  to a trailing edge thereof. The roof spoiler  471  is arranged such that the chord line S CL  is disposed at an angle of incidence α relative to the longitudinal axis X of the vehicle  401 . The angle of incidence α is positive along at least a portion of the width of the roof spoiler  471  such that the leading edge is disposed vertically above the trailing edge. The roof spoiler  471  is arranged to define a roof duct  473  having a roof duct inlet  474  and a roof duct outlet  475 . The roof duct  473  extends transversely and has a converging section towards the rear of the vehicle  401  to form a jet of air extended in a lateral direction. When the vehicle  401  is travelling in a forwards direction, the jet of air established by the roof spoiler  471  has a downwardly directed component. At least in certain embodiments, the jet of air can help to control the vertical extent of the wake formed downstream of the vehicle  401 . 
     The roof spoiler  471  can be arranged such that the angle of incidence α is uniform across its width. Alternatively, the roof spoiler  471  can be arranged such that the angle of incidence α α varies in a transverse direction. For example, the angle of incidence α can increase towards the lateral sides of the roof spoiler  471 . 
     Alternatively, or in addition, the height of the roof duct  473  can vary in a transverse direction. The height of the roof duct  473  can decrease in a laterally outwards direction such that, in use, a faster (more energetic) jet of air is established outboard. At least in certain embodiments, the resulting airflow at the outboard regions of the roof duct  473  would interfere with vortices that form at the upper outboard corners of the rear of the vehicle  401 . 
     The roof spoiler  471  could be used without the left and right rear pillar ducts  457 . However, the combination of the roof spoiler  471  and the left and right rear pillar ducts  457  can provide effective control of the wake downstream of the vehicle  401 . 
     The roof spoiler  471  in the present embodiment has a fixed configuration. However, this could be modified to implement a movable roof spoiler  471 . For example, the roof spoiler  471  could be movable between a retracted position and a deployed position. The roof spoiler  471  could travel along a rectilinear path, for example along one or more linear rails. Alternatively, the roof spoiler  471  could travel along an arcuate path as it is deployed. The angle of incidence α of the roof spoiler  471  could change as the roof spoiler  471  is deployed. A drive mechanism could be provided on one or both sides of the roof spoiler  471 . Alternatively, or in addition, a drive mechanism can be disposed in a centre spine formed in roof  461  of the vehicle  401 . The drive mechanism can comprise an actuator, for example an electric machine, arranged to drivingly rotate a lead screw. The roof spoiler  471  could have a slatted configuration, for example comprising a plurality of transverse spoiler elements which deploy to control the incident flow in a progressive manner. 
     A vehicle  501  incorporating a deployable diffuser assembly  575  according to a further embodiment is shown in  FIGS. 16 to 18 . The vehicle  501  is a development of the vehicle  201  described herein with reference to  FIG. 6 . Accordingly, like reference numerals will be used for like components, albeit incremented by 300 to aid clarity. 
     As shown in  FIG. 16A , the vehicle  501  comprises a rear bumper  507 , a bonnet  513 , a tailgate  515 , left and right wings (fenders)  517 , left and right quarter panels  521 , left and right front wheels W 1 , W 2 , left and right rear wheels W 3 , W 4 , and four doors D 1 - 4 . The front wheels W 1 , W 2  are disposed in front wheel houses formed in the left and right wings  517 ; and the rear wheels W 3 , W 4  are disposed in rear wheel houses formed in the left and right quarter panels  521 . 
     As shown in  FIG. 16B , the diffuser assembly  575  comprises a central diffuser  577 , a left lateral outboard diffuser  579  and a right lateral outboard diffuser  581 . The diffusers  577 ,  579 ,  581  are each movable from a retracted position (as shown in  FIG. 16A ) to one or more deployed positions (as shown in  FIG. 16B ). In their retracted positions, the diffusers  577 ,  579 ,  581  are displaced upwardly into a recess formed under the vehicle body within the rear bumper  507 . When they are deployed, the diffusers  577 ,  579 ,  581  are displaced downwardly into the region behind the rear wheels W 3 , W 4  below the rear bumper  507 . In the present embodiment, the diffusers  577 ,  579 ,  581  each undergo a combination of translation and rotation as they travel between said retracted and deployed positions. 
     The diffuser assembly  575  comprises separate actuator mechanisms (not shown) for drivingly displacing each diffuser  577 ,  579 ,  581  to said one or more deployed positions. An electronic control unit (not shown) is provided to control operation of the actuator mechanisms selectively to deploy the diffusers  577 ,  579 ,  581 . The actuator mechanisms can, for example, each comprise an electric machine for drivingly rotating a lead screw coupled to a linkage assembly. The linkage assembly can, for example, comprise a four-bar link assembly configured to provide the combined translational and rotational movement. 
     The lateral outboard diffusers  579 ,  581  each comprise a lateral diffusing panel  583  (shown in  FIGS. 17A and 17B ) for controlling the high speed airflow exiting from underneath the rear of the vehicle  501  and the wake from the rear wheels D 3 , D 4 . The central diffuser  577  comprises a central diffusing panel  585  (shown in  FIGS. 17A and 17B ). The left and right lateral outboard diffusers  579 ,  581  each comprise an inner side panel  587 , an outer side panel  589  and a lateral rear facing panel  591 . The inner side panels  587  are arranged adjacent to the lateral sides of the central diffuser  577  to control airflow at the sides of the central diffusing panel  585 . In particular, the inner side panels  587  function as endplates to limit airflow around the lateral edges of the central diffuser  577 . The outer side panels  589  are optional but can help to fill a void created behind the rear wheels W 3 , W 4  when the lateral outboard diffusers  579 ,  581  are deployed. The outer side panels  589  could be fixedly mounted to the lateral diffusing panels  583  or could be deployed separately. The outer side panels  589  could optionally define a separation edge to control separation of the airflow along the sides of the vehicle. The separation edge can be in the form of a discrete edge configured to cause the airflow to separate in a controlled manner. 
     The central diffuser  577  comprises a central rear facing panel  593 . When the vehicle is travelling in a forwards direction, the central rear facing panel  593  can operate to control airflow exiting at the trailing edge of the central diffusing panel  585 . The outer side panels  589 , the lateral rear facing panels  591 , and the central rear facing panel  593  hide the underside of the vehicle body from view when the diffusers  577 ,  579 ,  581  are deployed and can help to reduce aerodynamic drag. 
     A side elevation of the vehicle  501  with the diffusers  577 ,  579 ,  581  in a retracted position is shown in  FIG. 18A . The profile of the diffusers  577 ,  579 ,  581  in a deployed state is shown in  FIG. 18B  (with the central diffuser  577  shown as a dashed line). The left and right lateral outboard diffusers  579 ,  581  can be deployed together or independently of each other. The extent to which the left lateral outboard diffuser  579  and/or the right lateral outboard diffuser  581  are deployed could be actively controlled, either together or independently of each other. Furthermore, the central diffuser  577  can be deployed independently or in conjunction with said left lateral outboard diffuser  579  and/or the right lateral outboard diffuser  581 . As shown in  FIG. 18B , the left and right lateral outboard diffusers  579 ,  581  can be deployed to a deployed position projecting downwardly below the central diffuser  577 . In this configuration, the inner side panels  587  of the left and right lateral outboard diffusers  579 ,  581  function as end-plates for the bottom panel of the central diffuser  577 . The leakage of air around the lateral edges of the central bottom panel can be reduced or inhibited, thereby improving aerodynamic efficiency. 
     The lateral outboard diffusers  579 ,  581  can optionally also comprise a closure panel  595  for extending the effective depth of the rear of the wheel arch. The closure panel  595  extends transversely behind the rear wheels W 3 , W 4  and can be deployed with the lateral outboard diffusers  579 ,  581 . The closure panel  595  could be formed integrally with the lateral outboard diffusers  579 ,  581  or could be a separate component. The closure panel  595  can help control the airflow from the rear wheels W 3 , W 4  onto the lateral diffusing panels  583 . 
     The electronic control unit can control the deployment of one or more of the diffusers  577 ,  579 ,  581  based on vehicle dynamic parameters. For example, one or more of the diffusers  577 ,  579 ,  581  can be deployed when the vehicle speed exceeds a predefined threshold. The lateral outboard diffusers  579 ,  581  can be deployed together in conjunction with or independently of the central diffuser  577 . The diffuser  577 ,  579 ,  581  could also be deployed when the vehicle is stationary for aesthetic purposes. The electronic control unit can be configured selectively to retract the one or more of the diffusers  577 ,  579 ,  581  to provide improved ground clearance for the vehicle  501 . For example, the electronic control unit could be configured to retract the one or more of the diffusers  577 ,  579 ,  581  when an off-road driving mode is activated. The off-road driving mode could be activated by the driver or could be activated automatically. 
     At least in certain embodiments, the deployable diffuser assembly  575  is operative to reduce aerodynamic drag and/or to control lift. When deployed, the diffusers  577 ,  579 ,  581  help to control the transition between the airflow underneath the vehicle  501  (having a relatively high velocity) and the airflow behind the vehicle (having a relatively low velocity). The diffusers  577 ,  579 ,  581  thereby promote a progressive change in the speed of the airflow exiting from underneath the vehicle  501  and potentially also generate downforce. By controlling the airflow exiting from underneath the vehicle  501 , the diffusers  577 ,  579 ,  581  can help to control the wake formed behind the vehicle  501 . 
     At least in certain embodiments, the deployable diffuser assembly  575  is operative to reduce aerodynamic drag and/or to control lift. 
     The deployable diffuser assembly  575  can be combined with one or more of the other embodiments described herein. For example, the deployable diffuser assembly  575  can be combined with the deployable roof spoiler  471  incorporated into the embodiment illustrated in  FIG. 15  (with or without the rear pillar ducts  457 ). The diffuser assembly  575  and the roof spoiler  471  can be used in conjunction with each other to control the formation and/or propagation of the vehicle wake. For example, the roof spoiler  471  and one or more of the diffusers  577 ,  579 ,  581  can be deployed to control the depth of the wake formed behind the vehicle. The combination of the central diffuser  577  and the roof spoiler  471  may prove particularly effective in this respect. Thus, the central diffuser  577  and the rear spoiler  471  may be deployed in unison. The lateral outboard diffusers  579 ,  581  may help to control the lateral propagation of the vehicle wake. 
     It will be appreciated that various changes and modifications can be made to the embodiments described herein without departing from the spirit of the present application. 
     The embodiments described herein have particular application to road-going vehicles (also referred to as street legal or road legal vehicles) that are equipped and licensed for use on public roads. The vehicles may also have off-road capabilities. The embodiments described herein refer to a motor vehicle comprising four doors (excluding the tailgate or boot lid), but the vehicle could have a two door configuration (excluding the tailgate or boot). Furthermore, the embodiments described herein by way of example include a motor vehicle having a saloon (sedan) configuration and a sports utility vehicle. It will be appreciated that aspects of the present invention(s) could be applied to other vehicle configurations. For example, the vehicle could be an estate car (station wagon), hatch-back, coupe, off-road vehicle or a sports utility vehicle. Furthermore, the invention(s) described herein are not limited to motor vehicles. The vehicle can be an automobile, a truck, a lorry, an articulated vehicle and so on. 
     The present disclosure describes positioning adjacent panels to form a substantially continuous exterior surface. It will be appreciated that this is subject to usual manufacturing clearances and tolerances for exterior panels. A shut line (or cut line) is formed between adjacent panels where one (or both) of the panels is movable. The shut line comprises a clearance gap to accommodate relative movement of the panels. The outer surfaces of the panels on each side of the shut line are aligned with each other to form the substantially continuous exterior surface described herein. Thus, the composite exterior surface (defined by two or more panels) is substantially continuous insofar as it is free from steps or offsets at the interface between the panels. By way of example, the substantially continuous exterior surface can comprise a continuous curved surface (formed in 2-dimensions or 3-dimensions) and/or a continuous planar surface.