Abstract:
The measurement of volume flows or mass flows in the intake system of motor vehicle internal combustion engines plays a significant role in reducing pollutant emissions. Therefore, an ultrasonic flow meter for measuring a flow rate of a fluid flowing in a primary flow direction is described. The ultrasonic flow meter has at least two ultrasonic transducers, the ultrasonic transducers being capable of emitting and/or receiving ultrasonic waves at an angle α to the primary flow direction which is different from 90°. Furthermore, the ultrasonic flow meter has at least one guide element which is entirely or partially situated in the fluid. This guide element diverts at least one part of the flowing fluid in such a way that in the diversion, a velocity component is transferred to at least one part of the flowing fluid perpendicular to the primary flow direction. Guide vanes or displacers in particular are described as guide elements. In addition, turbulators may be provided on the guide elements, the turbulators generating a longitudinal fluid bed along the guide elements and thus causing the flow of the fluid to have a better contact with the guide elements when flowing around them. This reduces turbulences within the ultrasonic flow meter. Compared to the devices known from the related art, the ultrasonic flow meters described are distinguished by an improved signal-to-noise ratio and accordingly by a higher measuring precision.

Description:
FIELD OF THE INVENTION  
       [0001]     Ultrasonic flow meters are used in the automotive industry, in particular in the intake system of internal combustion engines, for measuring volume flow or mass flow. Typically ultrasonic transducers are used which are capable of both emitting ultrasonic waves into a fluid and receiving ultrasonic waves. The propagation time of ultrasonic signals which are transmitted from an emitter to a receiver is influenced by the flow of the fluid. It is possible to infer the flow velocity of the fluid from the degree of influence of the propagation time.  
       BACKGROUND INFORMATION  
       [0002]     British Published Patent Application No. 2 101 318 describes an ultrasonic flow meter in which two ultrasonic transducers are mounted on opposite sides of a pipe through which a fluid ( 112 ) flows. The transducers are situated slightly offset with respect to one another, so that ultrasonic waves emitted by one transducer and received by the second transducer propagate at an angle to the flow direction of the fluid which is different from 90°.  
         [0003]     In addition to the system described in British Published Patent Application No. 2 101 318, ultrasonic flow meters are also known in which ultrasonic waves emitted by an ultrasonic transducer are initially reflected one time or multiple times before they are received by a second ultrasonic transducer situated on the same side of the pipe through which the fluid flows as the first ultrasonic transducer. Such systems are described, for example, in European Published Patent Application No. 0 477 418, in British Published Patent Application No. 1 541 419 and in Japanese Published Patent Application No. 59100820. In European Published Patent Application No. 0 477 418 A1, a unit made up of two ultrasonic transducers and one reflector system is integrated into a coherent unit which may be installed in a measuring tube.  
         [0004]      FIG. 1  shows the operating principle of these measuring systems corresponding to the related art. A fluid  112 , for example, air, flows through a flow pipe  110  in an essentially laminar flow at a flow velocity v FL    114 . Two ultrasonic transducers  116  and  118  are mounted on opposite sides of flow pipe  110  in such a way that first ultrasonic transducer  116  is able to emit ultrasonic waves, which may be received by second ultrasonic transducer  118 , these ultrasonic waves propagating at a velocity v UL    120  at an angle α to flow velocity 114 which is different from 90°. In the system depicted here, the ultrasonic waves of ultrasonic transducer  116  propagate toward ultrasonic transducer  118  at a velocity v UL,1  which is higher than in an unmoving fluid  112  due to the motion of fluid  112  at velocity  114 . 
   v   UL,1   =v   UL   +v   FL ·cos α  (1)  
         [0005]     v UL  stands for the propagation velocity of the ultrasonic waves in an unmoving fluid. In contrast, if ultrasonic waves are emitted by ultrasonic transducer  118  and received by ultrasonic transducer  116 , these waves propagate at a velocity v UL,2  which is lower than propagation velocity v UL  in unmoving fluid  112 . 
 
 v   UL,2   =v   UL   −v   FL ·cos α  (2) 
 
         [0006]     Comparing a propagation time t 1  which a signal needs from ultrasonic transducer  116  to ultrasonic transducer  118  with a propagation time t 2  which an ultrasonic signal needs from ultrasonic transducer  118  to ultrasonic transducer  116  allows flow velocity v FL    114  of the fluid to be determined:  
               v   FL     =       L       2   ·   cos     ⁢           ⁢   α       ·     (       1     t   1       -     1     t   2         )               (   3   )             
 
         [0007]     A similar calculation of flow velocity v FL  may also be performed for reflection systems such as described in EP 0 477 418 A1, for example.  
         [0008]     The systems described in the related art, however, all have the problem that angle α in  FIG. 1  must be sufficiently small for a successful flow measurement, but at least substantially smaller than 90°. This results in the problem that it is not possible to fit the surfaces of ultrasonic transducers  116 ,  118  flush to the inside surface of flow pipe  110 . Protrusions  122  are thus formed in flow pipe  110  in the area of ultrasonic transducers  116 ,  118 , which result in turbulences and flow separations. These turbulences cause pressure fluctuations and may result in interfering signal contributions which are superimposed on the actual ultrasonic signals as noise.  
         [0009]     Another disadvantage of these turbulences and flow separations is that contaminants or particles such as dust, oil, or water droplets contained in the flowing medium tend to be deposited in the turbulence zones. One possible remedy is to insert wedge-shaped adaptor elements which fill up protrusions  122  of flow pipe  110  but are permeable to ultrasonic waves. However, the disadvantage here is that the layer thickness of the wedge-shaped adaptor elements varies over the cross section of an emitted ultrasound beam. This makes resonance adjustment for efficient ultrasound injection into the flowing medium difficult. Furthermore, such a construction responds sensitively to structure-borne noise injected into flow pipe  110 .  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention therefore provides an ultrasonic flow meter for measuring the volume flow and/or the mass flow of a fluid which may be used in particular in the intake system of a motor vehicle internal combustion engine.  
         [0011]     The present invention is based on an ultrasonic flow meter corresponding to the related art for measuring a flow velocity of a fluid flowing in a main flow direction. At least two ultrasonic transducers are used, it being possible for the ultrasonic transducers to emit ultrasonic waves into the flowing fluid at an angle to the primary flow direction which is different from 90° or receive ultrasonic waves. The core of the present invention is that at least one guide element is situated in the fluid, diverting at least one part of the flowing fluid, and that in the diversion, a velocity component perpendicular to the primary flow direction is transferred to at least one part of the flowing fluid.  
         [0012]     The present invention may be applied to both linear systems such as described, for example, in GB 2 101 318 A and reflection systems such as presented in EP 0 477 418 A1. EP 0 477 418 A1 describes how, for example, a reflection plate may be used as a guide vane for flow harmonization. Nonetheless, the present invention goes beyond this and uses at least one guide element, which is in particular able to divert the flow in such a way that the flow is optimally adapted to the protrusions described above which are caused by an ultrasonic transducer let into the wall of a flow pipe.  
         [0013]     The guide elements may be implemented in different ways. Guide elements tilted in relation to the primary flow direction of the fluid have proven to be advantageous in particular. The guide elements, the tilted guide vanes in particular, may be situated in the fluid in a lamellar pattern, for example.  
         [0014]     Furthermore, instead of guide vanes, it is advantageously possible to use displacers which, for example, locally narrow the flow cross section of the flow pipe.  
         [0015]     Furthermore, the described system may also be entirely or partly integrated into an insertion sensor which may be used in the flow pipe. In this connection, in particular, one or multiple ultrasonic transducers as well as an electronic control unit may be integrated into the insertion sensor for activating and/or reading out at least one ultrasonic transducer. The electronic control unit for reading out at least one ultrasonic transducer may, for example, contain electronics for preprocessing received signals. Appropriate electronic plug-and-socket connections may also be integrated for contacting the insertion sensor. Furthermore, at least one reflection element having a reflection surface may be integrated into the insertion sensor, making it possible to implement, for example, one of the reflection systems described above. Advantageously, the at least one reflection element is inserted into the flow pipe in such a way that fluid is able to flow on both sides of the reflection surface along the reflection body. This has the advantage in particular that if the reflection element is not in contact with the wall of the flow pipe, water droplets that may be contained in the flow are precipitated on the flow pipe as a wall film before flowing through the insertion sensor, the wall film then being able to flow through the flow pipe without wetting or contaminating the reflection surface and thus interfering with the reflection. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows a schematic diagram of a system for ultrasonic flow measuring.  
         [0017]      FIG. 2A  shows a system for ultrasonic flow measuring according to the related art having two diametrically opposed ultrasonic transducers without guide elements.  
         [0018]      FIG. 2B  shows a system for ultrasonic flow measuring having two diametrically opposed ultrasonic transducers having two guide vanes tilted against a primary flow direction of the fluid including a depiction of the separation areas.  
         [0019]      FIG. 3  shows a system for ultrasonic flow measuring having two ultrasonic transducers, a reflection surface and a guide vane tilted against the primary flow direction of the fluid.  
         [0020]      FIG. 4  shows an embodiment alternative to  FIG. 3  of a system integrated into an insertion sensor having two guide vanes tilted against the primary flow direction of the fluid.  
         [0021]      FIG. 5  shows an embodiment alternative to  FIG. 4  having a plurality of guide vanes situated in a lamellar pattern and tilted against the primary flow direction of the fluid.  
         [0022]      FIG. 6A  shows a side view of an embodiment alternative to  FIGS. 3 through 5  having complex guide elements.  
         [0023]      FIG. 6B  shows a sectional depiction of the system according to  FIG. 6A  having a section plane perpendicular to the primary flow direction of the fluid.  
         [0024]      FIG. 6C  shows a top view of a first embodiment of the complex guide elements according to  FIGS. 6A and 6B .  
         [0025]      FIG. 6D  shows a front view of the embodiment according to  FIG. 6C .  
         [0026]      FIG. 6E  shows a second embodiment of the complex guide elements according to  FIGS. 6A and 6B .  
         [0027]      FIG. 6F  shows a front view of the embodiment according to  FIG. 6E .  
         [0028]      FIG. 6G  shows a third embodiment of the complex guide elements according to  FIGS. 6A and 6B .  
         [0029]      FIG. 6H  shows a front view of the embodiment according to  FIG. 6G .  
         [0030]      FIG. 7  shows a system for ultrasonic flow measuring having two diametrically opposed ultrasonic transducers including a displacer. 
     
    
     DETAILED DESCRIPTION  
       [0031]     Similarly to  FIG. 1 ,  FIG. 2A  shows an additional system having two diametrically opposed ultrasonic transducers  116 ,  118  which, in a direction transverse to primary flow direction  210  (i.e., parallel to flow velocity  114  in  FIG. 1 ), are capable of exchanging ultrasonic waves at an angle to primary flow direction  210  which is different from 90°. As an example,  FIG. 2A  shows two flow lines  212  of the fluid. The flow of the fluid in the tube is predominantly free from separation areas.  
         [0032]     The two ultrasonic transducers  116 ,  118  send signals to one another, the propagation times of which, as described above, allow a calculation of flow velocity v FL    114 .  
         [0033]     As shown in  FIG. 2A , separation areas  214  are formed in the areas of the protrusions  122  upstrean of ultrasonic transducers  116  and  118 , within which laminar flow no longer prevails, but a turbulent flow prevails instead. One result of these turbulences is that the simple equation (1) (see above) for calculating the superimpositions of the velocities of ultrasonic wave v UL  and of flow velocity v FL  of the fluid no longer produce meaningful results because flow velocity v FL  of the fluid is no longer unambiguously defined in these separation areas  214  in particular. Furthermore, significant pressure fluctuations within the fluid occur in these separation areas  214 , as a result of which propagation velocity v UL  of the ultrasonic waves in fluid  112  is able to fluctuate strongly. On the whole, these separation areas  214  which are localized immediately upstream from ultrasonic transducers  116  and  118  including the eddies formed there cause a severe worsening of the signal-to-noise ratio and accordingly a reduction of measuring precision. In addition, the width of these separation areas  214  may fluctuate strongly with the temperature of the flowing fluid. Using the system shown in  FIG. 2A , it is only possible with great difficulty to obtain a drift-free and precise flow detection in an intake system of an internal combustion engine for the purpose of complying with stringent exhaust gas standards.  
         [0034]     In contrast, a modification according to the present invention of the measuring system of  FIG. 2A  is shown in  FIG. 2B . In this embodiment according to the present invention, two guide vanes  216  and  218  are used, each of which is tilted in relation to primary flow direction  210  of the fluid and which diverts parts  220  and  222  of the flow of the fluid in the direction of ultrasonic transducers  116  and  118 , respectively. This has the result that flow lines  212  in the vicinity of the wall of flow pipe  110  are more strongly adapted to the wall shape and protrusions  122  upstream from ultrasonic transducers  116  and  118 . As a result, the flow velocity of the fluid receives a component V S   FL    226  perpendicular to primary flow direction  210  of the fluid, in the direction of ultrasonic transducers  116  and  118 . As a result, separation areas  214  are strongly reduced in the area of protrusions  122 , as can be seen in  FIG. 2B . In this exemplary embodiment, separation areas  214  basically no longer project into wave area  224  which is necessary for an exchange of ultrasonic waves between ultrasonic transducers  116  and  118 . This strongly improves the measuring precision of flow velocity  114 . Such an improvement of measuring precision is not obtainable using the rectifier screens known from the related art, which are primarily elements parallel to primary flow direction  210 , for example, a reflector body (see, for example, EP 0 477 418 A1), which is used only for homogenizing the flow.  
         [0035]     Guide vanes  216  and  218  cause eddy areas  228  and  230 , respectively, to be formed downstream of these elements, which lie in wave area  224  of ultrasonic transducers  116  and  118 , it being possible for pressure fluctuations to occur within the eddy areas. However, these pressure fluctuations may be compensated by an integrative measuring characteristic of ultrasonic transducers  116  and  118  or a corresponding electronic control unit for reading out ultrasonic transducers  116  and  118 . For that reason, eddy areas  228  and  230  in wave area  224  which are generated by guide vanes  216  and  218  do not result in a reduction of measuring precision.  
         [0036]     The tilt angle of guide vanes  216  and  218  relative to primary flow direction  210  of the fluid were selected in such a way that the flow (i.e., flow lines  212 ) is in fact diverted slightly in the direction of ultrasonic transducers  116  and  118 ; however, it still has a sufficiently large component in the direction of propagation velocity v UL . The result of this compromise is that it is still possible to implement the measuring system described above (which is no longer usable at an angle α=90° between flow velocity v FL    114  and propagation velocity v UL    120 ) of the ultrasonic waves.  
         [0037]     As shown in  FIG. 2B , guide vanes  216  and  218  are designed as flat bodies having straight, parallel edges, which is adequate for most applications. As an alternative, guide vanes  216  and  218  may be designed rounded or pointed or aerodynamically favorable for a uniform flow guidance on the particular upstream or downstream edges. Furthermore, guide vanes  216  and  218  may be designed to be curved or arched for a more specific guidance of the fluid, it also being possible, for example, to vary a material thickness of guide vanes  216  and  218  over one length of guide vanes  216  and  218 , for example, similar to an airfoil of an airplane.  
         [0038]     As an alternative to the embodiment of the present invention shown in  FIG. 2B  having two diametrically opposed ultrasonic transducers  116  and  118 , embodiments having reflection systems (see above) may also be used according to the present invention. An example of such an embodiment is shown in  FIG. 3 . Two ultrasonic transducers  116  and  118  are again used in this case; however, in this exemplary embodiment, they are integrated on the same side and tilted with respect to one another in one wall of a flow pipe  110 . The tilting of ultrasonic transducers  116  and  118  again produces a protrusion  122 , in which it is possible for separation areas  214  (not shown) to form. In this exemplary embodiment, a space  310  is situated between ultrasonic transducers  116  and  118 , through which fluid  112  does not flow and which, for example, may be used for accommodating an electronic connecting device (for example, a plug-and-socket connection) and/or for placing an electronic control device for activating and/or reading out at least one of ultrasonic transducers  116 ,  118 . For example, it is possible for the signals generated by ultrasonic transducers  116  and  118  to be preprocessed in this electronic control unit. Complete processing of the signals is also possible.  
         [0039]     Furthermore, the system in  FIG. 3  has a reflection surface  312  on the wall of flow pipe  110 , which is capable of reflecting ultrasonic signals of ultrasonic transducers  116 ,  118 . For example, the wall material of flow pipe  110  may have adequate reflection properties for ultrasonic waves. In addition, however, the inside wall of the flow pipe may also be provided with an additional coating in the area of reflection surface  312 . Ultrasonic waves emitted by one of ultrasonic transducers  116 ,  118  are reflected on reflection surface  312  so that they may be received by the other ultrasonic transducer  116 ,  118 . More complex reflection systems are also possible in which, for example, it is possible for ultrasonic waves to reflect multiple times on different reflection surfaces  312  before they reach the other ultrasonic transducer  116 ,  118 . It is possible in this manner to increase, for example, the propagation time differences and accordingly the measuring precision.  
         [0040]     A guide vane  314  is again provided in this exemplary embodiment which deflects one part of the flow of fluid  112  toward ultrasonic transducers  116 ,  118  or toward protrusion  122 . Again, the formation of a separation area  214  (not shown here) upstream from ultrasonic transducers  116 ,  118  is prevented or reduced in the area of protrusion  122 , and thus the signal quality and the measuring precision of the ultrasonic flow measuring is considerably improved.  
         [0041]      FIG. 4  shows an alternative embodiment to the system in  FIG. 3  in which two guide vanes  410  and  412  tilted against primary flow direction  210  of fluid  112  flowing through a flow pipe  110  are used. In this example, guide vanes  410  and  412  are again designed as elongated flat bodies having two long parallel, straight side walls. Curved guide vanes could also be used as an alternative, which adapt better to the desired flow. In this exemplary embodiment, guide vane  410  takes over the task of providing a velocity component  226  to fluid  112  perpendicular to protrusion  122 , as a result of which the flow is better adapted to the wall shape of the system. Second guide vane  412  is used to adapt the flow again to primary flow direction  210  after flowing through ultrasonic flow meter  414 .  
         [0042]     Flow pipe  110  may have, for example, a rectangular or round or oval cross section and, for example, be designed as a cylinder. A reflection system is again used in this exemplary embodiment similarly to  FIG. 3 . In this case, however, the components of ultrasonic flow meter  414  are integrated into an insertion sensor  416 . Insertion sensor  416  includes both ultrasonic transducers  116  and  118 , space  310  lying between them, which, corresponding to the exemplary embodiment described in  FIG. 3 , for example, may be used for the integration of an electronic connecting device and/or an electronic control unit (as a result of which the electronic connecting device and the electronic control unit become components of insertion sensor  416 ) as well as a reflection element  418 , which is provided with a reflection surface  312 . Furthermore, insertion sensor  416  has a mount  420  which connects the individual components and aligns them with one another. In this exemplary embodiment, guide vanes  410  and  412  are also affixed by mount  420  of insertion sensor  416  and make up an integral component of insertion sensor  416 .  
         [0043]     Ultrasonic transducers  116  and  118  send ultrasonic waves to one another across wave area  224 , the ultrasonic waves which are emitted by one of ultrasonic transducers  116 ,  118  being reflected on reflection surface  312  before each reaches the other ultrasonic transducer  116 ,  118 . Reflection element  418  is situated in flow pipe  110  in such a way that it is at a distance from a wall of flow pipe  110  and fluid  112  is able to flow around it on both sides. This distance between flow pipe  110  and reflection element  418  makes it possible for water droplets or other contaminants which may be contained in the flow to precipitate onto the wall of flow pipe  110  as wall film  422  before flowing through ultrasonic flow meter  414 . This wall film  422  or the liquid contained in it may flow through flow pipe  110  without wetting reflection surface  312  and interfering with the reflection of the ultrasonic waves. In contrast to similar devices described in the related art (which, however have no guide vanes  410 ,  412 ), the device described including insertion sensor  416  has considerable advantages with respect to susceptibility to interference by liquids and contaminants.  
         [0044]     Guide vanes  410 ,  412  integrated into insertion sensor  416  cause separation area  424 ,  426  to be strongly reduced in the area of protrusion  122 . This is shown symbolically in  FIG. 4  by reference numerals  424  and  426 ,  424  denoting the separation area without the use of guide vanes  410 ,  412 , while reference numeral  426  denotes the separation area when guide vanes  410  and  412  are used. Interfering signals caused by separation area  424  and  426 , respectively, are thus considerably reduced by using guide vanes  410 ,  412 . It is not possible to achieve such a reduction through suitable flow guidance using the guide vanes for flow harmonization known from the related art alone, which have no adjustment angle in relation to primary flow direction  210 , i.e., in particular through shaping of the mounts of a reflection plate.  
         [0045]     Mount  420  of insertion sensor  416  may be designed in particular in such a way that it offers as little flow resistance as possible to the flow of fluid  112 . To that end, mount  420  may in particular be made up of a plurality of webs which are shaped so as to have as little flow resistance as possible. Similarly, mount  420  may be designed in such a way that reflection element  418  together with reflection surface  312  and mount  420  form a dish-shaped unit. This dish-shaped unit may, for example, also have a plurality of openings via which fluid  112  outside of insertion sensor  416  is in connection with fluid  112  within insertion sensor  416 . Reflection element  418  may be designed to be flat or curved, for example, for bundling of the ultrasonic waves.  
         [0046]     A preferred exemplary embodiment of an ultrasonic flow meter  414  designed as an insertion sensor  416  as an alternative to  FIG. 4  is shown in  FIG. 5 . Instead of the two guide vanes  410  and  412 , however, a plurality of guide vanes  510  and  512  are situated in a laminar pattern in this exemplary embodiment. Guide vanes  510  and  512 , respectively, are again tilted in relation to primary flow direction  210  of the fluid, the tilt angle being larger the closer guide vanes  510 ,  512  are placed to ultrasonic transducers  116 ,  118  or to the side of the wall of flow pipe  110 , on which ultrasonic transducers  116 ,  118  are situated. Guide vanes  510 ,  512  are thus approximately adapted to an idealized shape of flow lines  212  (not shown in  FIG. 5 ).  
         [0047]     In this exemplary embodiment each of guide vanes  510 ,  512  situated in a lamellar pattern is situated in a guide vane area  514  and  516 , respectively, guide vane area  514  being indicated by shading in  FIG. 5 . These guide vane areas  514  and  516  are designed in this example in such a way that none of guide vanes  510 ,  512  projects into wave area  224 , which would thus make it possible for them to interfere with the propagation of ultrasonic waves between ultrasonic transducers  116  and  118 . For this purpose, guide vanes  510 ,  512  have not only different tilt angles relative to primary flow direction  210  of fluid  112  but also have a length which increases as the distance from ultrasonic transducers  116  and  118  increases. Compared to the related art and compared to the exemplary embodiment shown in  FIG. 4 , guide vanes  510 ,  512  designed in this way make it possible to obtain a flow which is more uniform, turbulence-free and low in separation, strongly increasing the measuring precision of the device.  
         [0048]     Furthermore, reflection surface  312  in this exemplary embodiment is, as described above, provided with a coating. In particular, it is possible to use metallic or ceramic coatings that are good reflectors of ultrasonic waves. The reflection surface is advantageously designed to be flat and provided with a slight roughness. As described above, however, other embodiments are also possible. In selecting the reflection coating of reflection surface  312 , it should be noted in particular that the propagation rate of ultrasonic waves in the medium of the reflection coating as well as the density of the reflection coating should be as different as possible from the corresponding material parameters in the flowing medium in order to optimize the reflective effect. Furthermore, reflection angle α (see  FIG. 1 ) should be selected as a function of the material used for the reflection coating in such a way that as little of the sound energy as possible is converted into Rayleigh waves (surface waves).  
         [0049]     To compensate for the usually non-optimally bundled transmission or receive characteristics of ultrasonic transducers  116 ,  118 , it may be advantageous to provide reflection surface  312 , as described above, with a slight curvature. Furthermore, parts of mount  420 , reflection surface  312 , and guide vanes  510 ,  512  or layers additionally applied to these components may be made from materials whose surfaces suppress a reflection of ultrasonic waves in order to avoid or suppress interfering signal echoes.  
         [0050]     Additional embodiments of an ultrasonic flow meter  414  are shown in  FIGS. 6A through 6H  as alternatives to  FIGS. 3 through 5 . Again, two guide elements  610  and  612  are introduced into a flow pipe  110  and tilted in relation to primary flow direction  210  of fluid  112 , the two guide elements  610  and  612  in turn being positioned symmetrically to one another similarly to, for example, guide vanes  410  and  412  in  FIG. 4 .  FIG. 6A  shows a sectional depiction of ultrasonic flow meter  414  having a section plane parallel to primary flow direction  210 ;  FIG. 6B  shows a sectional depiction having a section plane perpendicular to primary flow direction  210 .  FIGS. 6C through 6H  show various embodiments of guide elements  610  and  612 , both as a top view ( FIGS. 6C, 6E ,  6 G) and as a front view ( FIGS. 6D, 6F ,  6 H). Guide elements  610 ,  612  are designed in such a way that backflows also flow through the flow pipe with as little formation of turbulence at the ultrasonic transducers as possible. For the mechanical attachment and stabilization of guide elements  610 ,  612 , mounting plates  614 ,  616 ,  618  may be placed parallel to the flow direction. These mounting plates  614 ,  616 ,  618  have the additional effect of calming the flow of fluid  112  through flow pipe  110 , thus reducing turbulences.  
         [0051]     Guide elements  610 ,  612  may be designed differently, for example, they may be adapted to flowing fluid  112 .  FIGS. 6C and 6D  show an embodiment in which guide element  610 ,  612  has a guide plate  620  having hills and valleys in profile (e.g., a “zigzag line”) instead of a flat guide vane. Hills  622  and valleys  624  extend parallel to the particular flow lines at the location of guide elements  610 ,  612 , thus producing “flow channels” which additionally stabilize the flow of fluid  112  through flow pipe  110  and prevent turbulences.  
         [0052]     As an alternative, as shown in  FIGS. 6E, 6F ,  6 G and  6 H, “turbulators”  626 ,  628  may also be provided on one side or on both sides of guide elements  610 ,  612 , it also being possible, for example, to situate the turbulators on a flat guide plate  620 . As shown in  FIGS. 6D and 6E , these turbulators  626 ,  628  may be, for example, situated at intervals and may be situated across primary flow direction  210 , for example, at the top and/or at the bottom of guide elements  610 ,  612 . These turbulators  626 ,  628  cause longitudinal eddy areas to form directly on the surface of guide elements  610 ,  612 . The result of this effect is that separation areas (see, for example, reference numeral  228 ,  230  in  FIG. 2B ) which form in primary flow direction  210  are strongly reduced downstream from guide elements  610 ,  612 . Overall, the formation of this longitudinal fluid bed around guide elements  610 ,  612  favors an essentially laminar flow guide elements  610 ,  612 . The pressure fluctuations or eddy areas in wave area  224  of the ultrasonic metering is thus reduced, resulting in an improvement of the signal-to-noise ratio of the ultrasonic flow metering, thus strongly improving the measuring precision of the measurement of the flow velocity. Turbulators  626 ,  628  have a rectangular (see  FIGS. 6E and 6F ) or triangular (see  FIGS. 6G and 6H ) or also a polygonal, rounded or lamellar design, for example.  
         [0053]      FIG. 7  shows another alternative embodiment of an ultrasonic flow meter  414  which, similarly, for example, to the embodiment shown in  FIG. 2A  and  FIG. 2B , has two diametrically opposed ultrasonic transducers  116 ,  118  which are capable of exchanging ultrasonic signals at an oblique (see  FIG. 1 ) angle α which is different from 90° to primary direction of flow  210  of a fluid flowing through a flow pipe  110 . Based on the positioning of ultrasonic transducers  116 ,  118  oblique to primary direction of flow  210 , protrusions  122  are again formed upstream from ultrasonic transducers  116 ,  118 . Within these protrusions  112  separation areas  214  are formed, within which eddies and accordingly pressure and velocity fluctuations occur (see above).  
         [0054]     In this exemplary embodiment, ultrasonic transducers  116 ,  118  each have a piezoceramic disc, which has a metallic coating on both sides and which is embedded in a vibration-damping plastic material. In the direction of flowing fluid  112 , the piezoceramic disc is surrounded by a material whose characteristic acoustic impedance is adjusted to a value between the corresponding values for the piezoceramic material and the flowing medium. A suitable selection of thickness and geometric shaping of this material layer produces a resonance amplification and optimized transmission and receive characteristics of ultrasonic transducers  116 ,  118 . Using an additional plastic material in relation to flow pipe  110  (or if integrated into an insertion sensor  416 , in relation to a mount  420  of insertion sensor  416 ), the vibration damping system configuration described above is affixed in flow pipe  110 . Electrical leads connected to a control and evaluation circuit are connected to the electrodes of the piezoceramic discs. The control and evaluation circuit and the electrical leads are not shown in  FIG. 7 . The unit made up of the piezoceramic discs and damping and impedance matching materials is denoted in  FIG. 7  as ultrasonic transducer  116  or  118 .  
         [0055]     The exemplary embodiment shown in  FIG. 7  eliminates the problem of the formation of separation areas  214  using, among other things, a displacer  710 . This displacer  710  may, for example, be held in the interior of flow pipe  110  using one or multiple (not shown in  FIG. 7 ) mounting plates  614  (see  FIG. 6A , for example). Multiple displacers  710  may also be used instead of one single displacer  710 . Displacer  710  locally reduces the cross section of flow pipe  110 . This displaces the flow of fluid  112  more strongly into protrusions  122  upstream of ultrasonic transducers  116  and  118 . This strongly reduces the size of separation areas  214  in these protrusions. After the fluid has flowed around the displacer, the cross section of flow pipe  110  is re-widened. As shown in  FIG. 7 , displacer  710  may be designed, for example, to be asymmetric in order to take into account the asymmetric configuration of ultrasonic transducers  116 ,  118 . Because protrusion  122  upstream from ultrasonic transducer  116  in primary flow direction  210  appears earlier than protrusion  122  upstream from ultrasonic transducer  118 , flow lines  212  in the upper area of flow pipe  110  must be deflected upwards in the direction of ultrasonic transducer  116  earlier than flow lines  212  in the lower area of flow pipe  110  which are diverted in the direction of ultrasonic transducer  118 . Accordingly, displacer  710  in narrowing area  712  is shaped asymmetrically. Similarly, the opposite end of displacer  710  placed in primary flow direction  210  in widening area  714  of the flow is also shaped asymmetrically so as not to project into wave area  224  oriented obliquely to primary flow direction  210 .  
         [0056]     It is possible for turbulences  716  to occur in widening area  714  of the flow, in which part  220  of the flow displaced by displacer  710  toward ultrasonic transducer  116  and part  22  of the flow displaced toward ultrasonic transducer  118  flow together again. By analogy to the exemplary embodiment shown in  FIG. 2B , these turbulences  716  are not, however, as detrimental to measuring the flow velocity to the same degree as turbulences in the area of separation areas  214 . This is in particular due to the fact that relatively wide ultrasound beams are used, causing wave area  224  to be relatively wide. As a result, an average is calculated over the velocity differences of fluid  112  within these turbulences  716  across the width of wave area  224 . In contrast, the amplitude of the pressure fluctuations locally on the surfaces of ultrasonic transducers  116 ,  118  is sharply reduced. Therefore, as a whole, the amplitude of the occurring signal interference diminishes strongly.  
         [0057]     Furthermore, an additional guide element embodied as guide vane  718  is used in the exemplary embodiment shown in  FIG. 7 . As shown in  FIG. 7 , this guide vane  718  has a curved shape which is adapted to the shape of flow lines  212  of fluid  112  in this area. Guide vane  718  is situated in primary flow direction  210  “downstream” from displacer  710  in order to take into account the fact that protrusion  122  upstream from ultrasonic transducer  118  in the primary flow direction is situated after protrusion  122  upstream of ultrasonic transducer  116 . As a result, in addition to the asymmetry of displacer  710  already described, the occurrence of turbulences is further reduced upstream from ultrasonic transducer  118 .  
         [0058]     Furthermore, in the embodiment shown in  FIG. 7 , additional turbulators  720  are affixed to displacer  710  in the narrowing area as are further turbulators  722  on the upstream side of guide vane  718  in relation to primary flow direction  210 . Similarly to the exemplary embodiments in  FIG. 6D  and  FIG. 6E , these turbulators  720 ,  722  may, for example, be designed as comb-like toothed structures. Similarly to the embodiment described in  FIG. 6D  and  FIG. 6E , longitudinal eddy areas then primarily form along displacer  710  or guide vane  718 . These result in an increased pulse exchange and accordingly reduce the subsequent separation areas, in particular turbulences  716  in widening area  714  downstream from displacer  710 . This may generally result in an improved laminar flow around displacer  710  or guide vane  718  and the flow is stabilized. Turbulences  716  within wave area  224  of the ultrasonic waves are also reduced and the measuring precision is improved accordingly.  
         [0059]     In  FIG. 7 , displacer  710  is used in combination with a measuring system in which ultrasonic transducers  116 ,  118  are located on diametrically opposed sides of flow pipe  110 .  
         [0060]     Similarly, it is possible, however, to use displacers  710  in multiple ways in flow pipe  110  as well as for reflection systems similarly to  FIG. 3 , for example.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           110  Flow pipe  
           112  Fluid  
           114  Flow rate  
           116  First ultrasonic transducer  
           118  Second ultrasonic transducer  
           120  Propagation rate of the ultrasonic waves  
           122  Protrusion  
           210  Primary flow direction  
           212  Flow lines  
           214  Separation areas  
           216  Guide vane  
           218  Guide vane  
           220  Part of the flow  
           222  Part of the flow  
           224  Wave area  
           226  Component of the fluid rate perpendicular to the primary flow direction  
           228  Eddy area  
           230  Eddy area  
           310  Installation space  
           312  Reflection surface  
           410  Guide vane  
           412  Guide vane  
           414  Ultrasonic flow meter  
           416  Insertion sensor  
           418  Reflection body  
           420  Mount  
           422  Wall film  
           424  Separation area without guide vane  
           426  Separation area with guide vane  
           510  Guide vanes situated in a lamellar pattern  
           512  Guide vanes situated in a lamellar pattern  
           514  Guide vane area  
           516  Guide vane area  
           610  Guide element  
           612  Guide element  
           614  Mounting plates  
           616  Mounting plates  
           618  Mounting plates  
           620  Guide plate  
           622  Hills  
           624  Valleys  
           626  Turbulators  
           628  Turbulators  
           710  Displacer  
           712  Narrowing area  
           714  Widening area  
           716  Turbulences  
           718  Guide vane  
           720  Turbulators  
           722  Turbulators