Abstract:
A turbine comprising a first housing having a first shaft rotatably mounted in the housing; a plurality of first spaced apart discs having an outer diameter and mounted on the first shaft and rotatable therewith, each first disc having a radial inner end defining an inner opening, a radial outer end and a pair of opposed surfaces extending therebetween; a second housing having a second shaft rotatably mounted in the housing; and, a plurality of second spaced apart discs having an outer diameter and mounted on the second shaft and rotatable therewith, each second disc having a radial inner end defining an inner opening, a radial outer end and a pair of opposed surfaces extending therebetween, the outer diameter of at least some of the first spaced apart discs is less than the outer diameter of at least some of the second spaced apart discs.

Description:
FIELD OF THE INVENTION 
     This invention relates to an apparatus used to transmit motive force between a fluid and a plurality of spaced apart rotatable members. The apparatus may be used to transmit the motive force from a fluid to the spaced apart members or, alternately, from the spaced apart members to the fluid. 
     BACKGROUND OF THE INVENTION 
     Prandtl layer turbines were first described by Nikola Tesla in U.S. Pat. No. 1,061,206 (Tesla). For this reason, these turbines are sometimes referred to as “Tesla Turbines”. FIGS. 1 and 2 show the design for a prandtl layer turbine as disclosed in Tesla. As disclosed by Tesla, a prandtl layer turbine 10 comprises a plurality of discs 12 which are rotatably mounted in a housing 14. Housing 14 comprises ends 16 and ring 18 which extends longitudinally between ends 16. Discs 12 are spaced apart so as to transmit motive force between a fluid in housing 14 and rotating discs 12. 
     The discs 12, which are flat rigid members of a suitable diameter, are non-rotatably mounted on a shaft 20 by being keyed to shaft 20 and are spaced apart by means of washers 28. The discs have openings 22 adjacent to shaft 20 and spokes 24 which may be substantially straight. Longitudinally extending ring 18 has a diameter which is slightly larger than that of discs 12. Extending between opening 22 and the outer diameter of disc 12 is the motive force transfer region 26. 
     The transfer of motive force between rotating discs 12 and a fluid is described in Tesla at column 2, lines 30-49. According to this disclosure, fluid, by reason of its properties of adherence and viscosity, upon entering through inlets 30, and coming into contact with rotating discs 12, is taken hold of by the rotating discs and subjected to two forces, one acting tangentially in the direction of rotation and the other acting radially outwardly. The combined effect of these tangential and centrifugal forces is to propel the fluid with continuously increasing velocity in a spiral path until it reaches a suitable peripheral outlet from which it is ejected. 
     Conversely, Tesla also disclosed introducing pressurized fluid via pipes 34 to inlets 32. The introduction of the pressurized fluid would cause discs 12 to rotate with the fluid travelling in a spiral path, with continuously diminishing velocity, until it reached central opening 22 which is in communication with inlet 30. Motive force is transmitted by the pressurized fluid to discs 12 to cause discs 12 to rotate and, accordingly, shaft 20 to rotate thus providing a source of motive force. 
     Accordingly, the design described in Tesla may be used as a pump or as a motor. Such devices take advantage of the properties of a fluid when in contact with the rotating surface of the discs. If the discs are driven by the fluid, then as the fluid passes through the housing between the spaced apart discs, the movement of the fluid causes the discs to rotate thereby generating power which may be transmitted external to the housing via a shaft to provide motive force for various applications. Accordingly, such devices function as a motor. Conversely, if the fluid in the housing is essentially static, the rotation of the discs will cause the fluid in the housing to commence rotating in the same direction as the discs and to thus draw the fluid through the housing, thereby causing the apparatus to function as a pump or a fan. In this disclosure, all such devices, whether used as a motor or as a pump or fan, are referred to as “prandtl layer turbines” or “Tesla turbines”. 
     Various designs for prandtl layer turbines have been developed. These include those disclosed in U.S. Pat. No. 4,402,647 (Effenberger), U.S. Pat. No. 4,218,177 (Robel), U.S. Pat. No. 4,655,679 (Giacomel), U.S. Pat. No. 5,470,197 (Cafarelli) and U.S. Reissue Pat. No. 28,742 (Rafferty et al). Most of these disclosed improvements in the design of a Tesla turbine. However, despite these improvements, Tesla turbines have not been commonly used in commercial environment. 
     SUMMARY OF THE INVENTION 
     In accordance with the instant invention, there is provided a turbine comprising: 
     (a) a first housing having a first shaft rotatably mounted in the housing; 
     (b) a plurality of first spaced apart discs having an outer diameter and mounted on the first shaft and rotatable therewith, each first disc having a radial inner end defining an inner opening, a radial outer end and a pair of opposed surfaces extending therebetween; 
     (c) a second housing having a second shaft rotatably mounted in the housing; and, 
     (d) a plurality of second spaced apart discs having an outer diameter and mounted on the second shaft and rotatable therewith, each second disc having a radial inner end defining an inner opening, a radial outer end and a pair of opposed surfaces extending therebetween, the outer diameter of at least some of the first spaced apart discs is less than the outer diameter of at least some of the second spaced apart discs. 
     In accordance with the instant invention, there is also provided an apparatus comprising: 
     (a) a plurality of first spaced apart members rotatably mounted to transmit motive force between a fluid and the spaced apart members, the spaced apart members having an upstream end and a downstream end; and, 
     (b) a plurality of second spaced apart members rotatably mounted to transmit motive force between the fluid and the spaced apart members, the spaced apart members having an upstream end and a downstream end, 
     each of the first and second spaced apart members has a centre and an outer edge, and the distance between the centre and the outer edge of at least some of the first spaced apart members is less than the distance between the centre and the outer edge of at least some of the second spaced apart members. 
     In one embodiment, the first and second housings comprise a single housing, the first and second shafts comprise a single shaft. 
     In another embodiment, each of the first and second spaced apart discs has an outer diameter, and the outer diameter of each of the first spaced apart discs is less than the outer diameter of each of the second spaced apart discs. 
     In another embodiment, the first spaced apart discs rotate at a different speed to the second spaced apart discs and may rotate at a faster speed to the second spaced apart discs. 
     In another embodiment, the first spaced apart discs may be at least partially nested or fully nested within the second spaced apart discs or positioned in series with the second spaced apart discs. The first spaced apart discs may be positioned between the opposed ends of the second spaced apart members or downstream of the first spaced apart member of the second spaced apart members. In each case, the first spaced apart discs may be coaxially mounted with the first spaced apart discs. 
     In another embodiment, the first spaced apart discs are positioned adjacent the upstream end of the second spaced apart discs. 
     In accordance with the instant invention, there is also provided an apparatus comprising: 
     (a) a first means for transmitting motive force between a fluid and a first plurality of rotatable spaced apart members; and, 
     (b) a second separate means for transmitting motive force between a fluid and a second plurality of rotatable spaced apart members, 
     each rotatable member having a pair of opposed surfaces, the surface area of the opposed surfaces of at least some of the rotatable members of the first means being less than the surface area of the opposed surfaces of at least some of the second means. 
     The first means may be positioned at least partially internal of the second means or in series with the second means. 
     In use, the first and second spaced apart members may rotate at different speeds. 
     The surface area of the opposed surfaces of each of the first rotatable members may be less than the surface area of the opposed surfaces of each of the second rotatable members. 
     In accordance with the instant invention, there is also provided a method for transmitting motive force between a fluid and rotatable spaced apart members comprising: 
     (a) passing the fluid through a first plurality of first spaced apart members having an upstream end and a downstream end, each spaced apart member having a pair of opposed surfaces to form a boundary layer which passes over the opposed surfaces of the first spaced apart members; and, 
     (b) passing the fluid through a second plurality of second spaced apart members having an upstream end and a downstream end, each spaced apart member having a pair of opposed surfaces to form a boundary layer which passes over the opposed surfaces of the second spaced apart members, the surface area of the opposed surfaces of at least some of the spaced apart members of the first plurality being less than the surface area of the opposed surfaces of at least some of the spaced apart members of the second plurality. 
     In one embodiment, the fluid passes sequentially through the first plurality of first spaced apart members and then through the second plurality of second spaced apart members. 
     In another embodiment, a portion of the fluid passes through some of the second spaced apart members while another portion of the fluid passes through some of the second spaced apart members. 
     In another embodiment, the fluid passes radially outwardly from the first plurality of spaced apart members to the second plurality of spaced apart members. 
     In another embodiment, the spaced apart members rotate as the fluid passes therethrough and the method further comprises rotating the first spaced apart members at a greater rotational speed than the second spaced apart members. 
     In another embodiment, the spaced apart members rotate as the fluid passes therethrough and the method further comprises passing the fluid through the first and second spaced apart members to rotate the first spaced apart members at a greater rotational speed than the second spaced apart members. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the instant invention will be more fully and particularly understood in connection with the following description of the preferred embodiments of the invention in which: 
     FIG. 1 is a cross section along the line  1 — 1  in FIG. 2 of a prior art prandtl layer turbine; 
     FIG. 2 is a cross section along the line  2 — 2  in FIG. 1 of the prior art prandtl layer turbine of FIG. 1; 
     FIG. 3 is a top plan view of a disc according to a first preferred embodiment of the instant invention; 
     FIG. 4 a  is an side elevational view of the disc of FIG. 3; 
     FIGS. 4 b - 4   d  are enlargements of area A of FIG. 4 a;    
     FIG. 5 is a longitudinal cross section of a prandtl layer turbine according to a second preferred embodiment of the instant invention; 
     FIG. 6 is a schematic drawing of the spaced apart members of one of the prandtl layer turbine unit of FIG. 5; 
     FIG. 7 is a graph of suction and flow versus the ratio of the inner diameter of a spaced apart member to the outer diameter of the same spaced apart member; 
     FIG. 8 is a longitudinal cross section of a prandtl layer turbine according to a third preferred embodiment of the instant invention; 
     FIG. 9 is a longitudinal cross section of a prandtl layer turbine according to a fourth preferred embodiment of the instant invention; 
     FIG. 10 is a longitudinal cross section of a prandtl layer turbine according to a fifth preferred embodiment of the instant invention; 
     FIG. 11 is a longitudinal cross section of a prandtl layer turbine according to a sixth preferred embodiment of the instant invention; 
     FIG. 12 a  is a longitudinal cross section of a prandtl layer turbine according to a seventh preferred embodiment of the instant invention; 
     FIG. 12 b  is a cross section along the line  12 — 12  in FIG. 12 a;    
     FIG. 13 is a longitudinal cross section of a prandtl layer turbine according to an eighth preferred embodiment of the instant invention; 
     FIG. 14 is a longitudinal cross section of a prandtl layer turbine according to a ninth preferred embodiment of the instant invention; 
     FIG. 15 is an end view from upstream end  78  of the prandtl layer turbine of FIG. 14; 
     FIG. 16 is a longitudinal cross section of a prandtl layer turbine according to a tenth preferred embodiment of the instant invention; 
     FIG. 17 is an end view from upstream end  78  of the prandtl layer turbine of FIG. 16; 
     FIG. 18 is a perspective view of a prandtl layer turbine according to an eleventh preferred embodiment of the instant invention; 
     FIG. 19 is a further perspective view of the prandtl layer turbine of FIG. 18 wherein additional housing of the outlet is shown; 
     FIG. 20 is a perspective view of the longitudinally extending ring of a prandtl layer turbine according to an twelfth preferred embodiment of the instant invention; 
     FIG. 21 is a transverse cross section along the line  21 — 21  of a prandtl layer turbine having the longitudinally extending ring of FIG. 20 wherein the turbine has secondary cyclones in flow communication with the turbine outlets; 
     FIG. 22 is longitudinal section of a vacuum cleaner incorporating a prandtl layer turbine; 
     FIG. 23 is a longitudinal section of a mechanically coupled prandtl layer motor and a prandtl layer fan; 
     FIG. 24 is a perspective view of a windmill incorporating a prandtl layer turbine; 
     FIG. 25 is a cross section along the line  25 — 25  of the windmill of FIG. 24; and 
     FIG. 26 is a longitudinal cross section of the prandtl layer turbine according to a further preferred embodiment of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the instant invention, improvements to the design of prandtl layer turbines are disclosed. These improvements may be used in conjunction with any known designs of prandtl layer turbines. Without limiting the generality of the foregoing, housing  14  may be of any particular configuration and mode of manufacture. Further, the fluid inlet and fluid outlet ports may be of any particular configuration known in the art and may be positioned at any particular location on the housing which is known in the art. In addition, while discs  12  are shown herein as being relatively thin, flat members with a small gap  56  between the outer edge of the disc and the inner surface of ring  18 , it will be appreciated that they may be of any particular design known in the art. For example, they may be curved as disclosed in Effenberger and/or the distance between adjacent discs may vary radially outwardly from shaft  20 . Further, the perimeter of discs  12  need not be circular but may be of any other particular shape. Accordingly, discs  12  have also been referred to herein as “spaced apart members”. 
     Referring to FIGS. 3 and 4 a-d,  preferred embodiments for spaced apart members  12  are shown. As shown in FIG. 3, spaced apart members  12  have an inner edge  40  and an outer edge  42 . If spaced apart member  12  has a central circular opening  22 , then inner edge  40  defines the inner diameter of spaced apart member  12 . Further, if the periphery of spaced apart member  12  is circular, then outer edge  42  defines the outer diameter of spaced apart member  12 . 
     Spaced apart members  12  may extend at any angle form shaft  20  as is known in the art and preferably extend at a right angle from shaft  20 . Further, spaced apart member  12  may have any curvature known in the art and may be curved in the upstream or downstream direction (as defined by the fluid flow through housing  14 ). Preferably, spaced apart member  12  is planer so as to extend transversely outwardly from shaft  20 . In this specification, all such spaced apart members are referred to as extending transversely outwardly from longitudinally extending shaft  20 . 
     Each spaced apart member  12  has two opposed sides  44  and  46  which extend transversely outwardly from inner edge  40  to outer edge  42 . These surfaces define the motive force transfer region  26  of spaced apart members  12 . The spacing between adjacent spaced apart members  12  may be the same or may vary as is known in the art. 
     Without being limited by theory, as a fluid travels across motive force transfer region  26 , the difference in rotational speed between the fluid and spaced apart member  12  causes a boundary layer of fluid to form adjacent opposed surfaces  44 ,  46 . If the fluid is introduced through openings  22 , then the fluid will rotate in a spiral fashion from inner edge  40  outwardly towards outer edge  42 . At some intermediate point, the fluid will have sufficient momentum that it will separate from opposed surfaces  44 ,  46  (i.e. it will delaminate) and travel towards the fluid exit port. By thickening the boundary layer, for a given rotation of a spaced apart member  12 , additional motive force may be transferred between the rotating spaced apart member  12  and the fluid. Thus the efficiency of the motive force transfer between spaced apart members  12  and the fluid may be increased. 
     The boundary layer may be thickened for a particular opposed surface  44 ,  46  of a particular spaced apart member by providing an area on that spaced apart member  12  having an increased width (i.e. in the longitudinal direction) at at least one discrete location of the particular opposed surface  44 ,  46 . Preferably, a plurality of such areas of increased width are provided on each opposed surface  44 ,  46  of a particular spaced apart member  12 . Further, preferably such areas of increased width are provided on at least some, preferably a majority and most preferably all of spaced apart members of turbine  10 . 
     Referring to FIGS. 3 and 4, the discrete areas of increased width may be provided by having raised portions  48  which are positioned at any place on surface  44 ,  46 . As shown in FIG. 3, these may be positioned on the inner portion of spaced apart member  12  such as adjacent inner edge  40  or spaced some distance outwardly from inner edge  40 . Raised portion  48  preferably is positioned on the inner portion of spaced apart member  12 . Further, a series of raised portions  48  may be sequentially positioned outwardly on spaced apart member  12  so as to successively thicken the boundary layer as it encounters a plurality of raised areas  48 . 
     Raised portion  48  is a discontinuity or increased width in surface  44 ,  46  which the fluid encounters as it rotates around spaced apart member  12 . As the fluid passes over raised portion  48 , the boundary layer thickens. By passing the fluid over a series of raised portions, the boundary layer may be continuously thickened. This is advantageous as the thicker the boundary layer, the more energy is transferred between the rotating spaced apart members and the fluid. 
     Side  50  of raised portion  48  may extend generally perpendicular to surface  44 ,  46  (eg. raised portion  48  may be a generally square or rectangular protuberance as shown in FIG. 4 b ) at an obtuse angle alpha (eg. 102-122°) to surface  44 ,  46  (eg. raised portion  48  may be a generally triangular protuberance as shown in FIG. 4 c ), or a rounded member on surface  44 ,  46  (eg. raised portion  48  may be a generally hemispherical protuberance as shown in FIG. 4 c ). Raised portion  48  may be constructed as a point member so as to be positioned at a discrete location on surface  44 ,  46 . Alternately, it may extend for an indefinite length as shown in FIG.  3 . 
     Side  50  is preferably positioned such that the direction of travel of the fluid as it encounters side  50  is normal to side  50 . As the travels outwardly over surface  44 ,  46 , it will be subjected to both tangential and radial acceleration as shown by arrows T and R in FIG.  3 . Generally, these forces will cause the fluid to travel outwardly at an angle of about 40° to the radial as shown in FIG.  3 . By positioning side  50  at such an angle (eg. 30° to 50°), the direction of travel of the fluid as it encounters side  50  will be about 90°. 
     Raised portion  48  may have a vertical height from surface  44 ,  46  varying from about 0.5 to about 25, preferably from about 0.5 to about 10 and more preferably 0.5 to about 2 of the thickness of the boundary layer immediately upstream of raised portion  48 . 
     The boundary layer may be delaminated from a particular opposed surface  44 ,  46  of a particular spaced apart member  12 , or the delamination of the boundary layer from a particular opposed surface  44 ,  46  of a particular spaced apart member  12 , may be assisted by providing an area on that spaced apart member  12  having an increased width (i.e. in the longitudinal direction) at at least one discrete location of the particular opposed surface  44 ,  46 . Preferably, a plurality of such areas of increased width are provided on each opposed surface  44 ,  46  of a particular spaced apart member  12 . Further, preferably such areas of increased width are provided on at least some, preferably a majority and most preferably all of spaced apart members of turbine  10 . 
     Referring to FIGS. 3 and 4 a - 4   d,  such discrete areas of increased width may be provided by having raised portions  52  which are positioned on surface  44 ,  46 . As shown in FIG. 3, these may be positioned on the outer portion of spaced apart member  12  such as adjacent outer edge  42  or spaced some distance inwardly from outer edge  42 . 
     As the fluid travels over opposed surface  44 ,  46 , it encounters raised portion  52 . This results in, or assists in, the delamination of the boundary layer from opposed surface  44 ,  46 . If the fluid has not delaminated from opposed surface  44 ,  46  when it reaches outer edge  42  then the delamination process will absorb energy from the prandtl layer turbine thereby reducing the overall efficiency of the prandtl layer turbine. 
     Raised portions  52  may be positioned adjacent outer edge  42  or at an intermediate position inwardly thereof as shown in FIG.  3 . Further, as with raised portion  48 , raised portion  52  preferably has an upstream side  54  which is a marked discontinuity to opposed surface  44 ,  46 . As shown in FIG. 4 a,  side  54  extends longitudinally outwardly from surface  44 ,  46 . However, raised portions  52  may have the same shape as raised portions  48 . 
     As fluid travels radially outwardly between inner edge  40  and outer edge  42 , a boundary layer is produced (with or without raised portions  48 ) which thickens as the boundary layer moves radially outwardly from shaft  20 . Preferably, at least one raised portion  54  is positioned radially outwardly on opposed surface  44 ,  46 . Preferably, raised portion  52  may be positioned at any point on surface  44 ,  46  where it is desired to commence the delamination process. Typically, the fluid will commence to delaminate at a position where the fluid has a velocity of about 103 to about 105 mach. Accordingly, raised portion  52  is positioned adjacent such a position and preferably just upstream of where the fluid reaches about 103 mach. This velocity corresponds to the region where the boundary layer achieves fluid flow characteristics which but for raised portion  52  would cause the fluid to delaminate. 
     Raised portion  52  may have a vertical height from surface  44 ,  46  varying from about 1 to about 100, preferably from about 1 to about 25 and more preferably 1 to about 5 of the thickness of the boundary layer immediately upstream of raised portion  52 . 
     In another embodiment, any of the spaced apart members  12  may include both one or more raised areas  48  to assist in thickening the boundary layer and one or more raised areas  52  to assist in the delamination of the boundary layer. 
     In the specification, the word “fluid” is used to refer to both liquids and gases. In addition, due to the formation of a boundary layer adjacent opposed surfaces  44 ,  46 , the fluid may include solid material since the formation of the boundary layer results in a reduction of, or the prevention of, damage to the surface of spaced apart members  12  by abrasion or other mechanical action of the solid material. For this reason, spaced apart members  12  may be made from any materials known in the art including plastic, metal, such as stainless steel, composite material such as Kevlar™ and reinforced composite materials such as carbon fibre or metal mesh reinforced Kevlar™. 
     In a further preferred embodiment of the instant invention, one or more fan members  68 ,  70  may be provided to assist in the movement of air through the prandtl layer turbines (see for example FIG.  5 ). This figure also shows a further alternate embodiment in which two prandtl layer turbines units  64 ,  66 , each of which comprises a plurality of discs  12 , are provided in a single housing  14 . Each prandtl layer turbine unit  64 ,  66  is provided with an inlet  60  having a single outlet  62 . Discs  12  of each prandtl layer turbine  64 ,  66  are mounted on a common shaft  20 . This particular embodiment may advantageously be used to reduce the pressure drop through the prandtl layer turbine. For example, instead of directing all of the fluid at a set number of spaced apart members  12 , half of the fluid may be directed to one half of the spaced apart members (prandtl layer turbine unit  64 ) and the other half may be directed at another set of spaced apart members (prandtl layer turbine unit  66 ). Thus the mean path through the prandtl layer turbine is reduced by half resulting in a decrease in the pressure loss as the fluid passes through prandtl layer turbine  10 . In the embodiment of FIG. 5, the fluid feed is split in two upstream of housing  14  (not shown). Alternately, as shown in FIGS. 10 and 11, all of the fluid may be fed to a single inlet  60  which is positioned between prandtl layer turbine units  64 ,  66 . While in these embodiments a like number of similar spaced apart members  12  have been included in each prandtl layer turbine unit  64 ,  66 , each turbine unit  64 ,  66  may incorporate differing number of spaced apart members  12  and/or differently configured spaced apart members  12 . 
     It will be appreciated that discs  12  of prandtl layer turbine unit  64  may be mounted on a first shaft  20  and discs  12  of the second prandtl layer turbine unit  66  may be mounted on a separate shaft  20  (not shown). This alternate embodiment may be used if the two shaft are to be rotated at different speeds. This can be advantageous if the prandtl layer turbine is to be used to as a separator as discussed below. If spaced apart members  12  are of the same design, then the different rotational speed of spaced apart members  12  will impart different flow characteristics to the fluid and this may beneficially be used to separate the fluid (or particles entrained into the fluid) into different fluid streams, each of which has a different composition. 
     Fan member  68  may be of any particular construction that will transport, or will assist in transporting, fluid to opening  22  of spaced apart member  12 . Similarly, fan member  70  may be of any particular construction that will assist in the movement of fluid through unit  64 ,  66  and transport it, or assist in transporting it, to an outlet  62 . Fan member  68  acts to pressurize the fluid and to push it downstream to one or more of spaced apart members  12 . Conversely, fan member  70  acts to create a low pressure area to pull the fluid downstream, either through downstream spaced apart members  12  or through outlet  62 . Fan member  70  may optionally be positioned outside of the interior of ring  18  so as to draw the fluid from housing  14 . Such a fan member may be of any particular construction. 
     As shown by FIG. 5, a fan member  68  may be positioned immediately upstream of the first spaced apart member  12  of prandtl layer turbine unit  64 . It will also be appreciated as also shown in FIG. 5 that fan member  68  may be positioned upstream from upstream end  78  of prandtl layer combining at  66 . Fan member  68  has a plurality of blades  72  which are configured to direct fluid towards central opening  22  of the first spaced apart member  12 . Blades may be mounted on a hub so as to rotate around shaft  20 . Alternately, for example, fan  70  may be a squirrel cage fan or the like. As shown in FIG. 5, blades  72  are angled such that when fan member  68  rotates, fluid is directed under pressure at central opening  22 . 
     Fan member  68  may be non-rotationally mounted on shaft  20  so as to rotate with spaced apart members  12 . Alternately, fan member  68  may be mounted for rotation independent of the rotation of shaft  20 , such as by bearings  76  which engage ring  18  (as shown in dotted outline in FIG. 5) or fan member  68  may be driven by a motor if it is mounted on a different shaft (not shown). If the prandtl layer turbine is functioning as a pump, then if fan member  68  is non-rotationally mounted on shaft  20 , the rotation of shaft  20  will cause blades  72  to pressurize the fluid as it is introduced into the rotating spaced apart members. Alternately, if the prandtl layer turbine unit is to function as a motor, the movement of the fluid through housing  14  may be used to cause spaced apart members  12  to rotate and, accordingly, fan member  68  to rotate (if fan member  68  is freely rotatably mounted in housing  14 ). By pressurizing the fluid as it enters the spaced apart members with no other changes to spaced apart members  12 , the pressure at outlet  62  is increased. As the downstream pressure may be increased, then there is additional draw on the fluid which allows additional spaced apart members  12  to be added to the prandtl layer turbine unit  64 ,  66 . 
     Outlet fan members  70  may be mounted in the same manner as fan member  68 . For example, outlet fan  70  may be non-rotatably mounted on shaft  20 , or rotatably mounted in housing  14  independent of spaced apart member  12  such as by a bearing  76  (not shown). Blade  72  may be configured so as to direct fluid out of housing  14  through outlet  62 . If fan member  70  is outside housing  14 , then fan member is constructed so as to draw fluid from outlet  62  (not shown). By providing a source of decreased pressure at or adjacent outlet  62 , additional spaced apart members may be provided in a single prandtl layer turbine unit  64 ,  66 . Further, an increased amount of the fluid may travel towards downstream end  80  such that the amount of fluid which passes over each spaced apart member  12  will be more evenly distributed. 
     In another preferred embodiment of the instant invention, the surface area of motive force transfer region  26  of opposed surfaces  44 ,  46  varies between at least two immediately adjacent spaced apart members  12 . This may be achieved by varying one or both of the inner diameter and the outer diameter of spaced apart members  12 . 
     Preferably, for at least a portion of the spaced apart members  12  of a prandtl layer turbine unit  64 ,  66 , the distance between inner edge  40  and outer edge  42  of a spaced apart member  12  varies to that of a neighbouring spaced apart member  12 . More preferably, the distance between inner edge  40  and outer edge  42  of a spaced apart member  12  varies to that of a neighbouring spaced apart member  12  for all spaced apart members in a prandtl layer turbine unit  64 ,  66 . The distance between inner edge  40  and outer edge of  42  of spaced apart members  12  may increase in the downstream direction and preferably increases from upstream end  78  towards downstream end  80 . Alternately, the distance between inner edge  40  and outer edge of  42  of spaced apart members  12  may decrease in the downstream direction and preferably decreases from upstream end  78  towards downstream end  80 . 
     As shown in FIGS. 5 and 6, the size of central opening  22  of at least one of the discs of prandtl air turbine unit  64 ,  66  varies from the size of the central opening of the remaining spaced apart members  12  of that prandtl air turbine unit. 
     FIG. 6 is a schematic diagram, in flow order, of the top plan views of spaced apart members  12  of prandtl layer turbine unit  64 . As shown in this drawing, each spaced apart member has a centrally positioned shaft opening  74  for non-rotatably receiving shaft  20  (if shaft  20  has a square cross-section similar in size to that of shaft opening  74 ). It will be appreciated that spaced apart members  12  may be fixedly mounted to shaft  20  by any means known in the art. 
     In a more preferred embodiment, a major proportion of the spaced apart members have central openings  22  which are of varying sizes and, in a particularly preferred embodiment, the size of cental opening  22  varies amongst all of the spaced apart members of a prandtl layer turbine unit  64 ,  66 . An example of this construction is also shown in FIGS. 8 and 9. 
     As the size of central opening  22  increases, then the amount of fluid which may pass downstream through the cental opening  22  of a spaced apart member  12  increases. Accordingly, more fluid may be passed downstream to other spaced apart members where the fluid may be accelerated. The size of central opening  22  may decrease in size for at least a portion of the spaced apart members  12  between upstream end  78  and downstream end  80 . As shown in the embodiment of FIG. 8, the size of central opening  22  may continually decrease in size from upstream end  78  to downstream end  80 . 
     An advantage of this embodiment is that the amount of fluid which may pass through housing  14  per unit of time is increased. This is graphically represented in FIG. 7 wherein the relative amount of fluid which may flow per unit time through a prandtl layer turbine may be maximized by adjusting the ratio of the inner diameter of a spaced apart member  12  to its outer diameter. This ratio will vary from one prandtl layer turbine to another depending upon, inter alia, the speed of rotation of spaced apart members  12  when the turbine is in use, the spacing between adjacent spaced apart members. However, as the size of cental opening  22  increases, then, for a given size of a spaced apart member  12 , the surface area of motive force transfer region  26  of spaced apart member  12  is decreased. Accordingly, this limits the velocity which the fluid may achieve as it travels between inner edge  40  and outer edge  42  of a spaced apart member  12  on its way to outlet  62 . Thus, by increasing the amount of fluid which may flow through the prandtl layer turbine  10 , the amount of suction which may be exerted on the fluid at inlet  60  is decreased as is also shown in FIG.  7 . 
     The size of central opening  22  may increase in size for at least a portion of the spaced apart members  12  between upstream end  78  and downstream end  80 . As shown in FIG. 9, the size of cental opening  22  may continuously increase from upstream end  78  to downstream end  80 . Less fluid passes through each central opening  22  to the next spaced apart member  12  in the downstream direction. Accordingly, less fluid will be available to be accelerated by each successive spaced apart member  12  and accordingly each successive spaced apart member  12  may have a smaller motive force transfer area  26  to achieve the same acceleration of the fluid adjacent the opposed surface  44 ,  46  of the respective spaced apart member  12 . 
     In the embodiments of FIGS. 8 and 9, the size of openings  22  varies from one spaced apart member to the next so as to form, in total, a generally trumpet shaped path (either decreasing from upstream end  78  to downstream end  80  (FIG. 8) or increasing from upstream end  78  to downstream end  80  (FIG.  9 ). It will be appreciated that the amount of difference between the size of central openings  22  of any to adjacent spaced apart members  12  may vary by any desired amount. Further, the size of the openings may alternately increase and decrease from one end  78 ,  80  to the other end  78 ,  80 . 
     As shown in FIG. 5, more than one prandtl layer turbine unit  64 ,  66  may be provided in a housing  14 . Further, the size of central opening  22  of the spaced apart members  12  of any particular prandtl layer turbine unit  64 ,  66  may vary independent of the change of size of central openings  22  of the spaced apart members  12  of a different prandtl layer turbine  64 ,  66  in the same housing  14  (not shown). As shown in FIG. 5, the size of central opening  22  decreases from each upstream end  78  to each downstream end  80 . However, it will be appreciated that, if desired, for example, the size of central openings  22  may decrease in size from upstream end  78  to downstream end  80  of prandtl air turbine unit  64  while the size of central openings  22  may increase in size from upstream end  78  to downstream end  80  of prandtl layer turbine unit  66 . 
     FIGS. 10 and 11 show a further alternate embodiment wherein the size of cental openings  22  varies from end  78 ,  80  to the other end  78 , 80 . In this particular design, the fluid inlet is positioned centrally between two prandtl layer turbine units  64 ,  66 . In the embodiment of FIG. 10, the size of cental opening  22  increases from upstream end  78  to downstream end  80  thus producing a prandtl layer turbine  10  which has improved suction. This is particularly useful if the prandtl layer turbine is to be used as a pump or fan to move a fluid. 
     In the embodiment of FIG. 11, the size of central opening  22  decreases from upstream end  78  to downstream end  80  thus producing a prandtl layer turbine  10  that has improved fluid flow. This particular embodiment would be advantageous if the prandtl layer turbine end were used as a compressor or pump. 
     In the embodiments of FIGS. 5-9, each spaced apart member  12  is in the shape of a disc which has the same outer diameter. Further, the housing has a uniform diameter. Accordingly, for each spaced apart member  12 , space  56  (which extends from outer edge  42  of each spaced apart member  12  to the inner surface of longitudinally extending  18 ) has the same radial length. In a further alternate embodiment of this invention, the outer diameter of each spaced apart member  12  may vary from one end  78 ,  80  to the other end  78 ,  80  (see FIGS.  12  and  13 ). In such an embodiment, space  56  may have a differing radial length (see FIG. 12) or it may have the same radial length (see FIG.  13 ). If prandtl layer turbine  10  is to be used as a separator, the then space  56  preferably includes a portion  56   a  which is an area of reduced velocity fluid (eg. a dead air space) in which the separated material may settle out without being re-entrained in the fluid. For example, as shown in FIG. 12 b,  ring  18  has an elliptical portion so as to provide portion  56   a.    
     It will be appreciated that in either of these embodiments, the size of cental opening  22  may remain the same (as is shown in FIG. 13) or, alternately, cental opening  22  may vary in size. For example, as shown in FIG. 12, cental opening may increase in size from upstream end  78  to downstream end  80 . This particular embodiment is advantageous as it increases the negative pressure in housing  14  at downstream end  80 . and increases the fluid flow through prandtl layer turbine  10 . Alternately, the size of cental opening  22  may vary in any other manner, such as by decreasing in size from upstream end  78  to downstream end  80  (not shown). 
     In a further preferred embodiment of the instant invention, a plurality of prandtl layer turbine units  64 ,  66  may be provided wherein the surface area of the motive force transfer region  26  of the spaced apart members  12  of one prandtl layer turbine unit  64 ,  66  have is different to that of the spaced apart members  12  of another prandtl layer turbine unit  64 ,  66 . This may be achieved by the outer diameter of at least some of the spaced apart members  12  of a first prandtl layer turbine unit  64  having an outer diameter which is smaller than the outer diameter of at least some of the spaced apart members  12  of a second prandtl layer turbine unit  66 . In a preferred embodiment, all of the spaced apart members  12  of prandtl layer turbine unit  64  have an outer diameter which is smaller than the outer diameter of each of the spaced apart members  12  of prandtl layer turbine unit  66 . Examples of these embodiments are shown in FIGS. 14-17. It will be appreciated that more than two prandtl layer turbine units  64 ,  66  may be provided in any particular prandtl layer turbine  10 . Two have been shown in FIGS. 14-17 for simplicity of the drawings. 
     Referring to FIGS. 14 and 15, the spaced apart members  12  of prandtl layer turbine unit  64  have the same outer diameter and the spaced apart members  12  of prandtl layer turbine unit  66  have the same outer diameter. The outer diameter of the spaced apart members  12  of prandtl layer turbine unit  64  is smaller than the outer diameter of the spaced apart members  12  of prandtl layer turbine unit  66 . As discussed above with respect to FIGS. 5-13, the outer diameter and/or the inner diameter of the spaced apart members of one or both of prandtl layer turbine units  64 ,  66  may vary so that the surface area of motive force transfer area  26  may vary from one spaced apart member  12  to another spaced apart member  12  in one or both of prandtl layer turbine units  64 ,  66  (see for example FIG.  26 ). 
     As shown in FIG. 14, prandtl layer turbine unit  64  is provided in series with prandtl layer turbine unit  66 . Further, the spaced apart members  12  of prandtl layer turbine unit  64  are non-rotatably mounted on shaft  20 ′ and the spaced apart members  12  of prandtl layer turbine unit  66  are non-rotatably mounted on shaft  20 . It will be appreciated that prandtl layer turbine unit  64  may be provided in the same housing  14  as prandtl layer turbine unit  66  or, alternately, it may be provided in a separate housing which is an airflow communication with the housing of prandtl layer turbine unit  66 . Preferably, in such an embodiment, each prandtl layer turbine unit  64 ,  66  is mounted co-axially. Optionally, the spaced apart members of prandtl layer turbine units  64  and  66  may be non rotationally mounted on the same shaft  20  (see for example FIGS.  16  and  17 ). 
     Prandtl layer turbine unit  64  has inlet  60 ′ and is rotationally mounted on shaft  20 ′ whereas prandtl layer turbine unit  66  as an inlet  60  and is mounted for rotation on shaft  20 . Fluid passes through spaced apart members  12 ′ to outlet  62 ′ from where it is fed to inlet  60  such as via passage  61 . Thus the fluid introduced into prandtl layer turbine unit  66  may have an increased pressure. Passage  61  may extend in a spiral to introduce fluid tangentially to prandtl layer turbine units  66 . Thus the fluid introduced into prandtl layer turbine unit  66  may already have rotational momentum in the direction of rotation of spaced apart members  12 . 
     In a further preferred embodiment as shown in FIGS. 16 and 17, prandtl layer turbine unit  64  may be nested within prandtl layer turbine unit  66 . For ease of reference, in FIG. 16, the cental openings and motive force transfer regions of prandtl layer turbine unit  64  are denoted by reference numerals  22 ′ and  26 ′. The central opening and motive force transfer regions of the spaced apart members of prandtl layer turbine unit  66  are denoted by reference numerals  22  and  26 . The spaced apart members of prandtl layer turbine units  64  and  66  may be mounted on the same shaft  20  or the spaced apart members of each prandtl layer turbine unit  64 ,  66  may be mounted on its own shaft  20  (as shown in FIG.  14 ). 
     It will be appreciated that prandtl layer turbine unit  64  may be only partially nested within prandtl layer turbine  66 . For example, the upstream spaced apart members  12  of prandtl layer turbine unit  64  may be positioned upstream from the first spaced apart member  12  of prandtl layer turbine unit  66  (not shown). Further, prandtl layer turbine units  64 ,  66  need not have the same length. For example, as shown in FIG. 16, prandtl layer turbine unit  64  comprises four discs whereas prandtl layer turbine unit  66  comprises seven discs. In this embodiment, the prandtl layer turbine unit  64  commences at the same upstream position as prandtl layer turbine unit  66  but terminates at a position intermediate of prandtl layer turbine unit  66 . It will be appreciated that prandtl layer turbine unit  64  may extend conterminously for the same length as prandtl layer turbine unit  66 . Further, it may commence at a position downstream of the upstream end of prandtl layer turbine unit  66  and continue to an intermediate position of prandtl layer turbine unit  66  or it may terminate to or past the downstream end of prandtl layer turbine unit  66 . 
     In a further alternate preferred embodiment, as shown in FIG. 14, prandtl layer turbine unit  64  is rotationally mounted on shaft  20 ′ whereas prandtl layer turbine unit  66  is mounted for rotation on shaft  20 . For example, shaft  20 ′ may be rotationally mounted around shaft  20  by means of bearings  82  or other means known in the art. In this manner, spaced apart members  12  of prandtl layer turbine unit  64  may rotate at a different speed to spaced apart members  12  of prandtl layer turbine unit  66 . Preferably, prandtl layer turbine unit  64  (which has spaced apart members  12  having a smaller outer diameter) rotates at a faster speed than prandtl layer turbine unit  66 . For example, if a first prandtl layer turbine unit had discs having a two inch outer diameter, the prandtl layer turbine unit could rotate at speeds up to, eg., about 100,000 rpm. A second prandtl layer turbine unit having larger sized discs (eg. discs having an outer diameter from about 3 to 6 inches) could rotate at a slower speed (eg. about 35,000 rpm). Similarly, a third prandtl layer turbine unit which had discs having an even larger outer diameter (eg. from about 8 to about 12 inches) could rotate at an even slower speed (eg. about 20,000 rpm). In this way, the smaller discs could be used to pressurize the fluid which is subsequently introduced into a prandtl layer turbine unit having larger discs. By boosting the pressure of the fluid as it is introduced to the larger, slower rotating discs, the overall efficiency of the prandtl layer turbine  10  may be substantially increased. In particular, each stage may be designed to operate at its optimal flow or pressure range. Further, if the fluid is compressible. For example, the increase in the inlet pressure will increase the outlet pressure, and therefore the pressure throughout housing  14 . This increase in pressure, if sufficient, will compress the fluid (eg. a gas or a compressible fluid) in housing  14 . This increases the density of the fluid and the efficiency of the transfer of motive force between the fluid and the spaced apart members. 
     Referring to FIGS. 18 and 19, a further preferred embodiment of the instant invention is shown. Fluid outlet port  62  extends between a first end  84  and a second end  86 . Traditionally, in prandtl layer turbine units, outlet port  62  has extended along a straight line between first and second ends  84  and  86 . According to the preferred embodiment shown in FIGS. 18 and 19, second and  86  of fluid outlet port  62  is radially displaced around housing  14  from first end  84 . The portion of the fluid that passes downstream through opening  22  of a spaced apart member  12  will have some rotational momentum imparted to in even though it does not pass outwardly at that location adjacent that spaced apart member. Therefore, assuming that all spaced apart members are similar, the portion of the fluid which passes outwardly along the next spaced apart member will delaminate at a different position due to the rotational momentum imparted by its passage through opening  22  in the immediate upstream spaced apart member. Outlet  62  is preferably configure to have an opening in line with the direction of travel of the fluid as it delaminates and travels to ring  18 . Thus downstream portions of outlet  62  are preferably radially displaced along ring  18  in the direction of rotation of spaced apart members  12 . 
     Preferably, fluid outlet port  62  is curved and it may extend as a spiral along ring  18 . Preferably, the curvature or spiral extends in the same direction as the rotation of the spaced apart members  12 . The fluid flow in prandtl layer turbine  10  is generally represented by the arrow shown in FIG.  19 . As represented by this arrow, the fluid will travel in a spiral path outwardly across an opposed surface  44 ,  46  and then radially outwardly through fluid outlet port  62 . Fluid outlet port  62  preferably curves in the same direction as the direction of the rotation of the spaced apart members. 
     It will be appreciated that all of fluid outlet port  62  need not be curved as shown in FIGS. 18 and 19. For example, a portion of fluid outlet port  62  may be curved and the remainder may extend in a straight line as is known in the prior art. It will further be appreciated that while fluid outlet port  62  in FIG. 18 extends conterminously with spaced apart members  12 , first and second ends  84  and  86  need not coincide with the upstream and downstream ends of the spaced apart members  12 . In particular, fluid outlet port  62  may have any longitudinal length as is known in the art. 
     According to further preferred embodiment of the instant invention, a single prandtl layer turbine unit  64 ,  66  may have a plurality of outlets  62 . Each outlet  62  may be constructed in any manner known in the art or, alternately they may be constructed as disclosed herein. For example, they may extend in a spiral or curved fashion around ring  18  in the direction of rotation of spaced apart members  12  of a prandtl layer turbine unit  64 ,  66 . Referring to FIG. 20, the ring of a prandtl layer turbine  10  having a single prandtl layer turbine unit  64 ,  66  is shown. In this embodiment, two outlets,  90  and  92  are provided. Each outlet extends longitudinally along ring  18  from upstream end  78  of spaced apart members  12  to downstream end  80  of spaced apart members  12 . For ease of reference, spaced apart members  12  have not been shown in FIG.  20 . 
     Each outlet  90 ,  92  may be of any particular construction known in the art or taught herein. For example, each outlet  90 ,  92  may extend in a curve or spiral around ring  18 . Outlets  90 ,  92  may have the same degree of curvature or, alternately, the degree of curvature may vary to allow separation of a specific density and mass of particulate matter. For example, if prandtl layer turbine  10  is used for particle separation, particles having a different shape and/or mass will travel outwardly at different positions. The outlets are preferably positioned to receive such streams and thus their actual configuration will vary depending upon the particle separation characteristics of the turbine. 
     Each outlet  90 ,  92  may curve in the same direction (eg. the direction of rotation of spaced apart members  12 ). Alternately, they may curve in opposite directions or one or both may extend in a straight line as is known in the prior art. Further, a plurality of such outlets  90  may be provided. 
     It will be appreciated that in an alternate embodiment, each outlet  90 ,  92  may be a portion  56   a  wherein the separated particulate matter may settle out and be removed from housing  14  and an outlet  62  may be provided to receive the fluid from which the particulate material has been removed. 
     Assuming that the portion of a fluid which is introduced through a central opening  22  to a position adjacent an opposed surface  44 ,  46  has approximately the same momentum, and assuming that the fluid has portions of differing density, then the rotation of spaced apart member  12  will cause the portions of the fluid having differing densities to commence rotating around shaft  20  at differing rates. As the fluid travels outwardly between inner edge  40  and outer edge  42  during its travel around shaft  20 , the portions of the fluid having differing densities will tend to delaminate and travel outwardly towards ring  18  at different locations around ring  18 . Accordingly, in a preferred embodiment of this invention, a fluid outlet port is positioned to receive each portion of the fluid as it delaminates from the opposed surface. Accordingly, in the embodiment shown in FIG. 20, it is assumed that the fluid would contain two distinctive portions (eg. two elements having differing densities). Fluid outlet ports  90  and  92  are angularly displaced around ring  18  so as to each receive one of these portions. 
     If the fluid also contains a solid, then, due to aerodynamic effects, particles having the same density but differing sizes will tend to separate due to the centrifugal forces exerted upon the particles as they travel in the fluid from inner edge  40  to outer edge  42 . Accordingly, a prandtl layer turbine may also be utilized as a particle separator. For example, in the embodiment of FIG. 20, if the particles have the same density, then first outlet  90  may be positioned to receive particles having a first particle sized distribution and fluid outlet port  92  may be positioned to receive particles having a smaller particle size distribution. 
     The positioning of fluid outlet ports  90 ,  92  may be selected based upon several factors including the total mass and density of the fluid and/or particles to be separated, the amount of centrifugal force which is imparted to the fluid and any entrained particles by spaced apart members  12  (eg. the inner diameter of spaced apart members  12 , the outer diameter spaced apart members  12 , the longitudinal spacing between adjacent spaced apart members  12 , the disc thickness and the speed of rotation of spaced apart members  12 ). 
     In the embodiment of FIG. 20, outlets  90  and  92  may be in flow communication with any downstream apparatus which may be desired. Accordingly, each portion of the fluid may be passed downstream for different processing steps. 
     Referring to FIG. 21, two cyclones  94 ,  96  may be provided in flow communication with fluid outlet ports  90 ,  92 . For example, if the fluid includes particulate matter, fluid outlet port  90  may be positioned to receive particles having a first particle sized distribution. First cyclone  94  may be provided in fluid flow communication with first outlet port  90  for separating some or all of the particles from the fluid. Similarly, fluid outlet port  92  may be positioned to receive a portion of the fluid containing particles having a different particle sized distribution and second cyclone  96  may be provided to remove some or all of these particles from the fluid. 
     Generally, cyclones are effective to efficiently remove particles over a limited particle size distribution. By utilizing a prandtl layer turbine to provide streams having different particle size distributions, each of cyclones  94 ,  96  may be configured to efficiently separate the particles which will be received therein from the fluid. It will be appreciated that a plurality of such cyclones  94 ,  96  may be provided. Each cyclone  94 ,  96  may be of any particular design known in the art. Further, they may be the same or different. 
     It will be appreciated that while several improvements in prandtl layer turbines have been exemplified separately or together herein, that they may be used separately or combined in any permutation or combination. Accordingly, for example, the turbines, whether nested or in series, may have varying inner and/or outer diameters. Further, any of the prandtl layer turbines disclosed herein may have a curved or spiral outlet  62 . Further, if a central air inlet  60  is utilized as disclosed in FIGS. 10 and 11, two fluid outlet ports having the same or differing curvature may be employed or, alternately, all or a portion of each of the outlets  62  may extend in a straight line. It will further be appreciated that even if a series of nested turbines are utilized to pressurize the fluid, that an inlet fan member  68  may also be incorporated into the design. Further, any of the prandtl layer turbines disclosed herein may have an outlet fan member  70 . These and other combinations of the embodiments disclosed herein are all within the scope of this invention. 
     Prandtl layer turbines may be used in any application wherein a fluid must be moved. Further, a prandtl layer turbine may be used to convert pressure in a fluid to power available through the rotational movement of a shaft. 
     In one particular application, a prandtl layer turbine may accordingly be used to assist in separating two or more fluids from a fluid stream or in separating particulate matter from a fluid stream or to separate particulate matter carried in a fluid stream into fluid streams having different particle sized distributions or a combination thereof (FIGS.  20  and  21 ). 
     A further particular use of such a prandtl layer turbine may be as the sole particle separation device of a vacuum cleaner or, alternately, it may be used with other filtration mechanisms (eg. filters, filter bags, electrostatic precipitators and/or cyclones) which may be used in the vacuum cleaner art. 
     Referring to FIG. 22, a vacuum cleaner including a prandtl layer turbine is shown. In this embodiment, vacuum cleaner  100  includes a first stage cyclone  102  having an air feed passage  104  for conveying dirt laden air to tangential inlet  106 . First stage cyclone  102  may be of any particular design known in the industry. The air travels cyclonically downwardly through first stage cyclone  102  and then upwardly to annular space  108  where it exits first stage cyclone  102 . It will be appreciated by those skilled in the art that cyclone  102  may be of any particular orientation. Generally, a first stage cyclone may remove approximately 90% of the particulate matter in the entrained air. 
     The partially cleaned air exiting first stage cyclone  102  via annular space  108  may next be passed through a filter  110 . Filter  110  may be of any design known in the art. For example, it may comprise a mesh screen or other filter media known in the art. Alternately, or in addition, filter  110  may be an electrostatic filter (eg. an electrostatic precipitator). In such an embodiment, the electrostatic filter is preferably be designed to remove the smallest particulate matter from the entrained air (eg. up to 30 microns). In another embodiment, the air may be passed instead to one or most second cyclones. In a further alternate embodiment, the air may be passed before or after the one or more second cyclones through filter  110 . 
     The filtered air may then passes next into inlet  60  of prandtl layer turbine  10 . Depending upon the efficiency of the cyclone and the filter (if any) and the desired level of dirt removal, the prandtl layer turbine may be used to provide motive force to move the dirty air through the vacuum cleaner but not to itself provide any dirt separation function. The prandtl layer turbine is preferably positioned in series with the cyclone such that the air exiting the cyclone may travel in a generally straight line from the cyclone to the prandtl layer turbine. If the vacuum cleaner is an upright vacuum cleaner, then the prandtl layer turbine is preferably vertically disposed above the air outlet from the cyclone. If the vacuum cleaner is a canister vacuum cleaner, then the prandtl layer turbine is preferably horizontally disposed upstream of the air outlet from the cyclone. 
     Subsequent to its passage trough the prandtl layer turbine, the air may be passed through filter  110  and/or one or more second cyclones in any particular orders. Further, in any embodiment, prior to exiting the vacuum cleaner, the air may be passed through a HEPA™ filter. 
     In an alternate embodiment, the prandtl layer turbine may also function as a particle separator. For example, in the embodiment of FIG. 22, the prandtl layer turbine of FIG. 21 has been incorporated. Prandtl layer turbine  10  separates the particulate matter into two streams having different particle size distributions. These streams separately exit prandtl layer turbine  10  via outlets  90 ,  92  and are fed tangentially into cyclones  94 ,  96 . The cleaned air would then exits cyclones  94 ,  96  via clean air outlets  112 . This air may be further filtered if desired, used to cool the motor of the vacuum cleaner or exhausted from the vacuum cleaner in any manner known in the art. 
     It will be appreciated that these embodiments may also be used to separate solid material from any combination of fluids (i.e. from a gas stream, from a liquid stream or from a combined liquid and gas stream). Further, these embodiments may also be used to separate one fluid from another (eg. a gas from a liquid or two liquids having differing densities). 
     In a further particular application, two prandtl layer turbines may be used in conjunction whereby a first prandtl layer turbine is used as a motor and a second prandtl layer turbine is used as a fan/pump to move a fluid. The prandtl layer turbine which is used as a motor is drivingly connected to provide motive force to the second prandtl layer turbine. An example of such an embodiment is shown in FIG.  23 . In FIG. 23, reference numeral  10  ′ denotes the prandtl layer turbine which is used as a motor (the power producing prandtl layer turbine). Reference numeral  10  denotes the prandtl layer turbine which is used as a fan/pump (the fluid flow causing element). 
     Each prandtl layer turbine  10 ,  10 ′ may be of any particular construction known in the art or described herein. Further, each prandtl layer turbine  10 ,  10 ′ may be of the same construction (eg. number of discs, size of discs, shape of discs, spacing between discs, inner diameter of discs, outer diameter of discs and the like) or of different constructions. It will be appreciated that the configuration of each prandtl layer turbine  10 ,  10 ′ may be optimized for the different purpose for which it is employed. 
     A first fluid is introduced through inlet port  60 ′ into prandtl layer turbine  10 ′. The passage of fluid through prandtl layer turbine  10 ′ causes spaced apart members  12 ′ to rotate thus causing shaft  20  to rotate. The fluid exits prandtl layer turbine  10 ′ through, for example, outlet  62 ′ which may be of any particular construction known in the art or described herein. 
     The fluid introduced into prandtl layer turbine  10 ′ may be a pressurized fluid which will impart motive force to spaced apart members  12 ′. Alternately, or in addition, fluid  10  may be produced by the fluid expanding as it passes through prandtl layer turbine  10 ′. For example, if prandtl layer turbine  10 ′ has a substantial pressure drop, then another source of fluid for prandtl layer turbine  10 ′ may be a pressurized liquid which expands to a gas as it travels through prandtl layer turbine  10 ′ or a pressurized gas which expands as it travels through prandtl layer turbine  10 . The fluid may also be the combustion product of a fuel. The fuel may be combusted upstream of prandtl layer turbine  10 ′ or within prandtl layer turbine  10 ′. The combustion of the fluid will produce substantial quantities of gas which must travel through prandtl layer turbine  10 ′ to exit via outlet  62 ′. Another source of fluid for prandtl layer turbine  10 ′ may be harnessing natural fluid flows, such as ocean currents, ocean tides, the wind or the like. 
     As a result of the passage of a fluid through prandtl layer turbine  10 ′, motive force is obtained which may then be transmitted to prandtl layer turbine  10 . As shown in FIG. 23, spaced apart members  12  of prandtl layer turbine  10  are mounted on the same shaft  20  as spaced apart members  12 ′ of prandtl layer turbine  10 ′. However, it will be appreciated that prandtl layer turbine  10 ′, and  10  may be coupled together in any manner which would transmit the motive force produced in prandtl layer turbine  10 ′to the spaced apart members  12  of prandtl layer turbine  10 . For example, each series of spaced apart members  12 ,  12 ′ may be mounted on a separate shaft and the shafts may be coupled together by any mechanical means known in the art such that prandtl layer turbine  10 ′ is drivingly connected to prandtl layer turbine  10 . 
     Prandtl layer turbine  10  has an inlet  60  which is in fluid flow connection with a second fluid. The rotation of shaft  12  will cause spaced apart members  12  to rotate and to draw fluid through inlet  60  to outlet  62 . Accordingly, prandtl layer turbine  10 ′ may be used as a pump or a fan to cause a fluid to flow from inlet  60  to outlet  62 . Depending upon the power input via shaft  20  to prandtl layer turbine  10 , the fluid exiting prandtl layer turbine  10  via outlet  62  may be at a substantial elevated pressure. 
     Accordingly, prandtl layer turbine  10 ′ functions as a motor and may be powered by various means such as the combustion of fuel. Accordingly, prandtl layer turbine  10 ′ produces power which is harnessed and used in prandtl layer turbine  10  for various purposes. 
     Referring to FIGS. 24 and 25, a prandtl layer turbine which may be used to produce motive force from a naturally moving fluid (such as wind or an ocean current or a tide) is shown. In this embodiment, prandtl layer turbine  10  (which may be of any particular construction) is provided with a fluid inlet  124  (for receiving wind or water). The entry of the fluid through inlet port  124  causes spaced apart members  12  to rotate. In this embodiment, the fluid would travel radially inwardly along spaced apart members  12  from the outer edge  42  to inner edge  40 . The fluid would then travel downstream through central opening  22  to fluid outlet  126 . The rotation of spaced apart members  12  by the fluid would cause shaft  20  to rotate. Shaft  20  exits from prandtl layer turbine  10  and provides a source of rotational motive force which may be used in any desired application (eg. electrical generation and pumping water). 
     Prandtl layer turbine is preferably rotatably mounted so as to align inlet  124  with the direction of fluid flow so that the fluid is directed into prandtl layer turbine  10 . It will also be appreciated that inlet  124  may be configured (such as having a funnelled shape or the like) to capture fluid and direct it into spaced apart members  12 . In FIG. 24, prandtl layer turbine  10  is positioned vertically on support member  120 . It will be appreciated that prandtl layer  10  may also be horizontally mounted (or at any other desired angle). 
     Tail  122  may be provided on ring  18  and positioned so as to align inlet  124  with the fluid flow. Tail  122  may be constructed in any manner known in the art such that when the portion of the fluid which does not enter prandtl layer turbine  10  passes around ring  18 , tail  122  causes opening  124  to align with the direction of the fluid flow thereby assisting in maintaining opening  124  aligned with the fluid flow as the direction of fluid flow changes.