Patent Publication Number: US-2016223446-A1

Title: Generating Pressure Fluctuations In A Line

Description:
The invention concerns a method and a pressure modulator for generating pressure fluctuations in a fluid which is located in a line, as well as a pipeline arrangement with a line in which pressure fluctuations can be generated in a fluid present in the line. 
     In many application instances it is desirable to generate pressure fluctuations in a fluid which is present in a line. The term line here stands for a single line, a piping system of communicating lines, such as a pipe system or some other arrangement of hollow bodies in which a fluid is present, which is to be subjected to pressure fluctuations. 
     The invention is applicable to many fields of technology, e.g., the testing of medical stents or bursting pressure tests, and shall be explained below only as an example with the aid of a pipeline arrangement in the form of a test layout for medical stents. The generic term stent subsumes stents, grafts, combinations of these and similar objects. Before stents go into clinical use, they must be tested thoroughly. In particular, fatigue tests are conducted during which the stents are subjected to deformations. 
     A test layout for stents is known, e.g., from U.S. Pat. No. 5,670,708. A radial flexible line in which the stent is installed simulates a blood vessel. A fluid in the line simulates blood and pressure pulses in the fluid simulate the blood pressure fluctuations due to the beating heart of the patient. Thus, a radial expansion and contraction of the line and thus a cyclical deformation of the stent occurs. Pressure pulses in the fluid are generated by expansion and compression of a bellows. The stents are deformed, following the radial movement of the line, and are thus subjected to the fatigue test. 
     Stents are generally tested for a lifetime of ten years, which for an average human heart rate corresponds to around a total of 100 million pressure pulses. The drawback to the known arrangement is that the bellows is deformed during each pressure pulse, and thus exposed to a high constant loading and is itself subject to fatigue effects. 
     The problem which the invention proposes to solve is to indicate an improved method of generating pressure fluctuations in a line containing a fluid, as well as an improved pressure modulator and an improved pipeline arrangement. 
     In regard to the method, the problem is solved by a method according to patent claim  1  for generating pressure fluctuations in a fluid which is present in a line. The line is in particular a line system of a test layout for a medical stent, wherein the pipeline arrangement has a test section which can be expanded by an increasing of the fluid pressure. The stent is placed or can be placed in the test section. The method has the following steps: 
     The line is flowed through by the fluid from an inlet of the line to its outlet. At the inlet the fluid is delivered at a constant volume flow (volume per unit of time) into the inlet. At the outlet the volume flow of the fluid flowing from the outlet is throttled to varying degrees, dynamically over time. The dynamic throttling over time occurs here periodically, in particular. Thanks to the dynamic variation of the throttling over time, i.e., the different degrees of throttling of the outgoing fluid at different moments of time, dynamic pressure fluctuations over time are generated upstream in the line, i.e., a pressure variation in the fluid in the line which varies over the course of time—especially periodically. 
     Thus, the invention makes conscious use of the principle of the pressure surge, which is generally undesirable in pipeline systems. A pressure surge arises upstream in a line due to a downstream throttling of a fluid flow in the line, for example, with the help of a shutoff fitting. For a fluid in the form of a liquid, the effect of pressure surges is greater for the same throttling—that is, the pressure fluctuations are greater—than in the case of gaseous fluids, since liquids are less compressible than gases. The pressure surges are propagated from the site of the throttling, i.e., the shutoff fitting for example, upstream into the line. 
     Fluid in the aforementioned test for stents therefore means in particular a liquid, and a physiological salt solution is chosen in particular as the liquid. Thanks to using a liquid, the desired effect of the pressure surges is intensified as compared to gaseous fluids. The table salt solution is especially suitable for stent testing. The method according to the invention thanks to the use of the pressure surge principle affords the advantage that any suitable mechanical type throttling device can be used for the throttling of the fluid flow or for the variable throttling. Embodiments as a valve, sliding gate, impact plate and so forth are conceivable here. These devices generally make do with no elastically deformable elements and are therefore subject to significantly less wear, especially compared to the known devices with bellows. The method can thus be carried out with less wear than the know method. The delivery of the fluid to the inlet of the line can likewise be done with known low-wear devices, such as the use of a pulsation-free pump, such as a gear pump. Thanks to the reduced wear on the components involved, the method is especially advantageous in terms of reliability and availability. 
     In the test layout the expandable test section is cylindrical in particular, e.g., in the form of a flexible hose, such as a silicone hose. A stent can be introduced and held there. The stent is held in that the expandable test section holds a less flexible stent by a radially inward directed spring force and/or the stent itself is flexible in design and thus it is prestressed in the test section with radially outward spring action. In particular, the pipeline system with the exception of the test section is rigid in design, i.e., not expandable, so that only the test sections expand due to the pressure fluctuations in the pipeline system, since only these can yield under a pressure increase. The fluid in the line is maintained free of bubbles especially in the case of liquids, i.e., free of gas, for otherwise pressure falsifications will occur when the gas volume present in the bubbles is compressed and therefore the pressure fluctuations will not act solely on the test section, as desired. 
     Thanks to an increasing of the throttling or an increasing of the flank stiffness in the time plot of the throttling, i.e., a stronger or faster throttling of the volume flow, there is an intensified pressure increase upstream. Therefore, by changing the mentioned parameters of the throttling, pressure fluctuations and pressure variations of different degree can be set. For example, shutoff valves or other throttling fixtures can be used for the throttling of different degree, which reduce the available flow cross section for the fluid. 
     In a preferred embodiment, the fluid emerges as a fluid jet along a fluid path from an outlet opening downstream from the outlet into a clearance. For the throttling of different degrees, an impact body is introduced in the clearance in different relative positions to the fluid path therein and in the fluid jet. Thus, at least one portion of the fluid jet impinges on the impact body at least some of the time. The site of impinging of the fluid jet on the impact body, i.e., the part of the impact body struck by fluid in the form of the fluid jet, then forms an impact surface. In other words, the impact body is subjected at least temporarily to at least a portion of the fluid emerging from the outlet in the fluid jet. By impinging on the impact body, the fluid jet experiences a back flow, which results in the aforementioned pressure surge. The “variable” introducing of the impact body includes every variation, e.g., a variable spatial position or orientation of the impact body in the fluid path. It also includes a temporary complete removal of the impact body from the fluid jet, so that the fluid jet can emerge unhindered from the outlet for some of the time and not produce any back flow. For example, a linear back and forth movement of the impact body is conceivable, wherein the line of motion can be situated in any given orientation to the axis of the fluid jet, as long as a pressure surge effect is created. The variable introducing of the impact body in the fluid path can be done especially easily and free of wear. 
     Furthermore, the variable introducing is to be understood in the sense of a changing of the relative position of impact body and outlet opening relative to each other. In a fixed reference system, therefore, the impact surface and/or the fluid jet or the fluid path is moved in order to create different relative positions between impact body and fluid jet. The changing of the fluid path is done in particular by changing the outlet opening determining the fluid path by steering the fluid jet. The impact body can have a surface of any given shape. By a fluid jet is meant the pressurized outflow of the fluid upon exiting from the outlet opening, thus in particular not a pressure-less fluid draining away simply due to gravity, for example. What is decisive is that the introducing of the impact body into the fluid jet in fact causes a pressure surge effect upstream in the fluid or in the line. 
     In a preferred variant of this embodiment, for the throttling of different degrees a spacing and/or an angle of inclination of the impact surface to the outlet opening and/or a degree of overlap between the cross sectional area of the fluid jet and the projection surface of the impact surface on the cross sectional area—looking in the direction of the emerging fluid jet—is varied dynamically. Here again, the “change” is to be understood relatively, that is, in a fixed reference system the fluid jet is changed relative to the impact surface and/or the impact surface relative to the fluid jet. Spacing, angle of inclination and degree of overlap can likewise be realized by very simple and low-wear movements of impact body and/or fluid path. 
     In another preferred variant of the aforementioned embodiment, for the throttling to different degrees the impact body and the fluid path are moved relatively to each other such that each time different parts of the surface of the impact body form impact surfaces introduced variously into the fluid jet on account of the movement between different relative positions. In other words, at least one portion of the fluid jet thanks to the relative movement of impact body and fluid jet impinges on different segments of its surface, i.e., impact surfaces, on the impact body. This enables, for example, a dynamic movement of the impact body in the form of being led through the fluid jet. The impact body can then be outfitted with a surface profile or height profile, i.e., other points of impinging of the fluid on the impact body form other impact surfaces. For example, the impact body can be configured as a rotating perforated disk, wherein the fluid jet impinges alternately on certain regions of the impact body and/or passes through its perforations. Thus, the changing of the throttling can be done especially easily by structuring of the impact body in combination with its movement. 
     In one variant of the aforementioned embodiment the relative movement between impact body and fluid jet occurs by a rotation about an axis of rotation, wherein the impact body in particular is rotated. The rotation in this case can also be part of an overall movement, i.e., combined with other movements, such as linear ones. In particular, a rotational movement of the impact body can be realized in especially easy and low-wear manner by means of a rotary bearing. 
     In another variant of this embodiment the relative movement can be a translatory movement between fluid path and impact body. This can also be part of an overall movement, as explained above. Translatory movements are also possible with low wear. By changing the translatory position in particular the throttling can be adjusted so that, for example, a lower and upper pressure level is established for pressure fluctuations. The cyclical pressure fluctuation between the two levels is then accomplished or superimposed by a rotational movement. 
     In regard to the pressure modulator, the problem is solved by a pressure modulator according to patent claim  7 . The pressure modulator contains an outlet opening, which can be connected such to a line through which fluid is flowing that the outlet opening forms the exit end of the line through which—preferably all of—the fluid flowing through the line emerges. The outlet opening empties into a clearance. In operation, i.e., when fluid is flowing through the line, fluid emerges from the outlet into the clearance and forms a fluid jet in the clearance, which moves along a fluid path through the clearance. In other words, after emerging from the outlet opening, the fluid fills the fluid path in the form of the fluid jet. The pressure modulator furthermore has an impact body which can be introduced into the fluid path in different ways, dynamically over time. At least some of the time, i.e., when at least one part of the impact body is dipped into the fluid path, the impact body forms an impact surface, on which at least a portion of the fluid jet impinges during operation. In other words, the impact surface is that part of the impact body in the flow of fluid of the fluid jet. 
     Due to the impinging on the impact surface, the volume flow of the fluid is throttled to different degrees, dynamically over time. Thus, dynamic pressure fluctuations over time are created in the fluid upstream, e.g., in a line connected to the pressure modulator. The pressure modulator according to the invention is therefore especially suitable for connection to the outlet of a line or a piping system in order to bring about the throttling of the fluid to different degrees dynamically over time in the above-described method. In particular, the pressure modulator can be designed so that its only moving part is the impact body. As explained above, the impact body can be moved with especially low wear, so that the pressure modulator is especially suitable for the above-mentioned stent testing, requiring many millions of pressure fluctuations. Otherwise, the same remarks about the method according to the invention also apply to the pressure modulator and its operation. 
     In a preferred embodiment, the spacing and/or the angle of inclination of the impact surface to the outlet and/or the degree of overlap between the cross sectional area of the fluid path and the projection area of the impact surface on this cross sectional area is variable. 
     In another preferred embodiment, impact body and fluid path, especially the outlet opening determining the fluid path, can be adjusted or moved relative to each other so that each time different parts of the surface of the impact body form impact surfaces protruding variously into the fluid path on account of the adjustment or movement between different relative positions. 
     In one variant of this embodiment, impact body and fluid path, especially the outlet opening determining the fluid path, are rotatable relative to each other about an axis of rotation. Rotational movements can be performed with especially low wear and simple design, so that different parts of the surface of the impact body can be introduced variously as impact surfaces into the fluid path in especially advantageous manner. As an especially simple variant, only the impact body is rotated. 
     In another variant of this embodiment, alternatively or in addition the outlet opening determining the fluid path can be rotated eccentrically about an axis of rotation, especially one parallel to a jet direction of the fluid path. Eccentric movements are mechanically easy to perform and can be easily achieved in terms of fluid tightness. Thanks to the movement of the outlet opening, the fluid path also changes and thus, in particular for a position-invariant impact body, so does the site of impinging of the fluid jet on the impact body, i.e., the impact surface. By configuring the impact body with different impact surfaces in this respect, once again parameters of the pressure fluctuations can be changed. The position invariance of the impact body includes, e.g., a movement of the impact body in itself, such as the rotation of an impact body with rotational symmetry in contour, as a perforated disk. 
     In another preferred embodiment, impact body and fluid path, especially the outlet opening determining the fluid path, can be moved in translation relative to each other. 
     In another embodiment, the impact body has a surface topography, especially a height profile and/or interruptions, which is distributed over the impact body such that, when it is introduced variously into the fluid path, different segments of the surface topography lie alternately in the fluid path. A height profile is to be understood here in relation to the spacing of the surface of the impact body from the outlet opening. The surface topography extends for example along a curve on the surface of the impact body, wherein different impact surfaces are formed on the curve, against which the fluid jet impinges during operation. 
     In other words, the impact body is profiled or shaped such that, for example, recesses, elevations, or interruptions are introduced into the impact body. For example, the impact body is a disk with recesses, especially through openings, or a profile curve, i.e., a height profile, which is fashioned in the circumferential direction about an axis of rotation. This configuration is especially advisable in combination with embodiments in which the impact body is rotated about an axis parallel to the jet axis of the fluid jet. 
     For the mentioned embodiments of the pressure modulator and their benefits, the above remarks in connection with the method of the invention also apply. 
     In regard to the pipeline arrangement, the problem is solved by a pipeline arrangement according to patent claim  14  with a line having an inlet and an outlet. The line is configured to be flowed through with a fluid from the inlet to the outlet. In particular, the line is configured as a pipeline system of a test layout for a medical stent, wherein the pipeline system then has a test section. This can be expanded by a pressure fluctuation in the form of an increase in the pressure of the fluid. The stent can be placed in the test section. The pipeline arrangement has a delivery mechanism for the fluid, in order to deliver the fluid at the inlet at a constant rate to the inlet of the line. The pipeline arrangement furthermore has a throttle element for the fluid, in order to throttle the volume flow of the fluid emerging from the outlet to different degrees dynamically over time, especially periodically—especially at or in the immediate vicinity of the outlet, especially downstream. This serves to generate dynamic pressure fluctuations of the fluid upstream in the line by the dynamic change in the throttling over time. 
     The pipeline arrangement is therefore especially suitable to carry out the method according to the invention and thus especially as a particularly wear-free test layout for medical stents with the aforementioned benefits. The throttle element is, for example, a proportional valve or another throttle element which throttles the volume flow of the fluid by changing a cross sectional area for the fluid. This can be connected in series with the outlet, especially in an adjoining line piece downstream. 
     In one preferred embodiment, the delivery element is a pump designed to feed a fluid into the entrance at constant rate. Unlike a membrane pump, which would be subject to wear on the membrane during a stent fatigue test, a gear pump has especially low wear. 
     In another preferred embodiment, the throttle element is a pressure modulator connected in series with the outlet of the line downstream, such as was described above, and bringing with it the accordingly mentioned benefits. 
    
    
     
       For a further description of the invention, reference is made to the sample embodiments of the drawing. There are shown each time in schematic diagram: 
         FIG. 1  a pipeline arrangement or test layout for a stent 
         FIG. 2  a pressure modulator 
         FIG. 3  a detail of the pressure modulator of  FIG. 2  in cross section 
         FIG. 4  an alternative pressure modulator with perforated disk 
         FIG. 5  another alternative pressure modulator with eccentric adjustment of the outlet opening 
         FIG. 6  an alternative test layout for stents 
         FIG. 7  another alternative test layout for stents 
     
    
    
       FIG. 1  shows a line  2  with an inlet  4  and an outlet  6 . Through the line  2  there flows a fluid  8  from the inlet  4  to the outlet  6  in a flow direction according to the arrow  10 . A pipeline arrangement  12  contains, besides the line  2 , a delivery mechanism  14  for the fluid  8 , in order to deliver it at constant rate into the inlet  4  of the line  2 . Furthermore, the pipeline arrangement  12  contains a throttle element  16  for the fluid  8 . The throttle element  16  is connected in series to the outlet  6  and it throttles the volume flow of the entire fluid  8  emerging from the outlet  6 . The throttling is dynamic over time, i.e., of different intensity over the course of time, especially periodical. Thanks to the change in intensity of the throttling, dynamic pressure fluctuations Δp of the pressure p of the fluid  8  in the line  2  arise over the course of time upstream from the throttle element  16 , i.e., opposite the direction of the arrow  10 . 
     The pipeline arrangement  12  in an alternative embodiment is part of a test layout  17  for a medical stent  18 , not further represented in  FIG. 1 . The line  2  then has a test section  20 , in which the stent  18  is placed, namely, being prestressed radially outward with spring action. The test section  20  of the line  2  can be radially expanded by pressure fluctuations Δp in the form of a rise in pressure p in the fluid  8 , as indicated by the broken line in  FIG. 1 . The stent  18  then follows the expansion in the test section  20  and is mechanically deformed in this way. Upon drop in the pressure p, a contrary movement occurs. By cyclical pressure fluctuations Δp the stent  18  is therefore subjected to a fatigue testing. 
     The delivery mechanism  14  in the example is a pump  22  feeding the fluid  8  to the inlet  4 , being designed for a constant rate, especially a gear pump. The throttle element  16  in the example is a pressure modulator  24  connected to the outlet  6 . 
       FIG. 2  shows the pressure modulator  24  of  FIG. 1  in more detail. The pressure modulator contains an outlet opening  26 , which is fashioned in the example as an open end of a pipe piece  27 , which is connected to the line  2  with the flow of fluid  8  or to its outlet  6 . All of the fluid  8  flowing through the outlet  6  thus emerges from the outlet opening  26 . The outlet opening  26  empties into a clearance  28  of the pressure modulator  24 . In operation, i.e., when fluid  8  is flowing through the line  2 , the fluid emerges at the outlet opening  26  into the clearance  28  and in this process forms a fluid jet  30 , which moves along a three-dimensional, spatially extended fluid path  32  through the clearance  28 ; in  FIG. 2  the fluid path  32  is shown by broken line. An impact body  34  of the pressure modulator  24  can be introduced into and also removed from the fluid path  32  in a manner varying over time. In operation, i.e., when the fluid jet  30  is present, at least a portion of the fluid jet  30 , i.e., the fluid  8 , impinges on the impact body  34  whenever at least a portion of the impact body  34  protrudes into the fluid path  32 . The particular surface region of the impact body  34  which is struck by the fluid jet  30  at the moment then forms a respective impact surface  36   a - c  of the impact body  34 . 
       FIG. 2  shows four different relative positions R 1-4  between impact body  34  (one solid and two broken line) and fluid path  32  or fluid jet  30 . Each time, different impact surfaces  36   a - d  are formed on the impact body  34 . Thanks to the introducing of the impact body  34  dynamically varying over time into the fluid path  32  in or between the relative positions R 1-3  indicated, pressure surges of the pressure p in the fluid  8  occur during operation, which are propagated upstream into the line  2 , i.e., opposite the flow direction of the fluid  8 , i.e., opposite the direction of the arrow  10 , in the form of pressure fluctuations Δp. 
     Due to the translatory and/or rotary movement and/or slanting movement of the impact body  34 , the spacing a 1,2  between impact surface  36   a - d  and outlet opening  26  changes in a first variant—looking in the jet direction of the fluid jet  30  or along the fluid path  32 . Alternatively or in addition, the angle of inclination α between the impact surface  36   a - d  and the outlet opening  26  changes. 
       FIG. 3  shows, looking in the direction of arrow III in  FIG. 2 , i.e., in the jet direction, how the different introducing of the impact body  34  in different relative positions R 1-3  alternatively or additionally also changes a degree of overlap G=Q/A 1,2 . This describes the areal ratio between the cross sectional area Q of the fluid path  32  and the projection areas A 1,2  of the projections of the impact surfaces  36   a - d  on this cross sectional area Q or the corresponding cross sectional plane. In  FIG. 3  the different positions of the impact body  34  are shown solid and the broken line shows the resulting different impact surfaces  36   a - d  and their projection areas A 1,2  by different hatch marks. 
       FIG. 4  shows an alternative embodiment of a pressure modulator  24  with a housing  38 , in which a Pitot tube  40  is provided. The Pitot tube  40  is functionally similar to the additional pipe piece  27  of  FIG. 2 , being connected at the outlet  6  of the line  2  and ending for its part in the outlet opening  26 . The housing  38  encloses the clearance  28 , in which the impact body  34  is arranged. The fluid path  32  extends starting at the outlet opening  26  into the clearance  28 . The impact body  34  is realized here as a circular disk  44 , which is joined as a single piece to a shaft  42 . Shaft  42  and disk  44  can rotate about an axis of rotation  46 . A rotational drive occurs in a manner not explained more closely across a belt (not shown) and a belt pulley  48  or alternatively through other drive variants, not represented, such as a direct drive. The disk  44  has distributed around its circumference massive segments  50  and interruptions  52  in the form of through boreholes. Thanks to the rotation of the disk  44  about the axis of rotation  46 , the massive segments  50  or the interruptions  52  alternately arrive in the region of the fluid path  32 . If a massive segment  50  lies in the fluid path  32 , an impact surface  36  on the impact body  34  is formed here, as explained in  FIGS. 2 and 3  for the impact surfaces  36   a - c . If an interruption  52  lies in the region of the fluid path  32 , the fluid jet  30  experiences no noticeable resistance and can pass unhindered through the interruption  52 , without striking the impact body  34 . No impact surface  36  is present in that case. 
     The impact body  34 , i.e., its massive or solid region, is thus alternately brought into the fluid path  32  or not in dynamically alternating manner over time, as it rotates, and thereby forms impact surfaces  36  or not in dynamically alternating manner over time. This produces pressure surges varying in time in the fluid  8 , which are propagated contrary to the direction of the arrow  10  in the line  2  and result in pressure fluctuations Δp there. The fluid  8  emerging from the outlet opening  26  gathers free of pressure, after having struck or passed through the impact body  34 , in a collection chamber  54 , from which it can flow off through a drain  56  with no pressure and thus again arrive at the pump  22 , for example, in a circulation, from which it is again delivered into the line  2 . 
     The spacing a in the form of a gap between the rotating impact body  34  and the outlet opening  26  can be changed by moving the Pitot tube  40  with the aid of a motor drive unit  58  in or opposite the direction of the arrow  10  away from the disk  44  or toward it. The spacing a here determines the upper pressure level of the resulting pressure fluctuations Δp. A corresponding lower pressure level can be adjusted in that the effective cross section of the interruptions  52  can be changed with the aid of an aperture disk  60  which can rotate about the axis of rotation  46  relative to the disk  44 . In other words, two perforated disks are turned relative to each other about the axis  46  in order to reduce or enlarge the respective effective perforation cross section. Such an adjustment is done, e.g., only once or occasionally during setup or maintenance of the layout, but not during regular operation. The turning of the two disks relative to each other requires a standstill of the layout, since the disks are fixed in rotation relative to each other by a screw connection  53 , only hinted at in  FIG. 4  and not explained more closely, which has to be loosened to change the rotary position of the two disks and then secured again. In such a perforated disk, if the spacing between the perforations looking in the circumferential direction corresponds to roughly the perforation diameter, one gets a sine curve as the pressure variation. 
       FIG. 5  shows a cutout of another alternative embodiment of a pressure modulator  24 , which resembles the embodiment of  FIG. 4  in terms of construction and mode of operation in that a rotating disk  44  with interruptions  52  and massive, solid segments  50  (see  FIG. 4 , not visible in  FIG. 5 ) serves as the impact body  34  and is rotated about an axis of rotation  46  such that the interruptions  52  and massive segments  50  of the disk  44  alternately lie opposite the outlet opening  26  of the Pitot tube  40 . The adjustment of the spacing a is done in the aforementioned manner by a motor drive unit  58 , which here brings about the rotation of a guide body  62  about an axis of rotation  64 . Thanks to a screw engagement between the guide body  62  and a mating body  66  firmly disposed on the housing  38 , a displacement of the guide body  62  occurs in the axial direction of the axis of rotation  64 , carrying along the Pitot tube  40 . This adjustment option again serves to adjust the upper pressure level of the pressure fluctuations Δp. 
     The lower pressure level is set in alternative fashion in the embodiment of  FIG. 5 , namely, by rotation of the Pitot tube  40  about the axis of rotation  64  with the aid of the belt pulley  68 . To make an adjustment possible, the outlet opening  26  on the Pitot tube  40  is disposed eccentrically in regard to the axis of rotation  64 . Upon rotation of the Pitot tube  40 , there thus occurs a shifting of the position of the outlet opening  26  to the disk  44 , i.e., their radial spacing from the axis of rotation  46  is changed. In this way, the position of the fluid path  32  relative to the disk  44  and thus relative to its interruptions  52  or massive segments  50  also changes. The overlap between the cross section of the fluid path  32  and the interruptions  52  is changed. In this way, the size of the impact surfaces  36   a - c  created on the impact body  34  (not shown in  FIG. 5 ) also changes, as explained in  FIGS. 2 and 3 . As compared to the embodiment of  FIG. 4 , this has the benefit that the lower pressure level can also be adjusted during operation of the pressure modulator  24  free of interruption, by activating the belt pulley  68 . The cylindrically shaped Pitot tube  40  with eccentric feature is easily sealed against leakage of fluid  8 , for example, by traditional O-rings. 
       FIG. 6  shows a pipeline arrangement  12  as part of a test layout  17  for stents  18 . The pipeline arrangement  12  here contains several lines  2  for fluid  8 , including ones configured as a distributor rail  70  and a collector rail  72 , as well as ones in the form of several test sections  20 , each of which receives the flow from the distributor rail  70  and empties into the collector rail  72 . The test sections  20  in the example are artificial vessels  73 , such as silicone tubes, in which the stents  18  are inserted prestressed in radially outward direction with spring action. With the exception of the artificial vessels  73 , all the rest of the lines  2  are rigid in configuration, i.e., they do not expand upon pressure fluctuations Δp in the fluid  8 , so they are shape-stable. Only the artificial vessels expand, carrying along the stents  18 , thereby subjecting them to a fatigue testing by radial expansion and compression. To determine the diameter of the individual stents  18 , a meter  74  is indicated symbolically. In the test layout  17  there is furthermore hooked up a volume flow meter  76 , a thermometer  78  and a pressure gauge  80 —each of them for the fluid  8  and indicated symbolically. 
     The fluid  8  is pressurized only in the region upstream along the arrow  10  between the inlet  4  and the outlet  6  or the outlet opening  26 . Already in the clearance  28  or in the collection chamber  54  and in a supply tank  82  as well as in a return line  84 , the medium  8  is present without pressure. The delivery mechanism  14  therefore removes fluid  8  from the supply tank  82  in order to feed it at constant rate into the pipeline arrangement  12 . The supply tank  82  as well as a chamber  84 , enclosing the test layout  17 , can be heated. 
     The alternative test layout  17  of  FIG. 7  differs from the test layout  17  of  FIG. 6 : the distributor rail  70  can move toward or away from the collector rail  72 . Furthermore, additional rigid lines  2  are hooked up to the distributor rail  70  and the collector rail  72 , ending in respective plugs or sockets of quick couplings  86 . Between the respective quick couplings  86  the flexible artificial vessels  73  with stents  18  (not shown) can then be placed. The quick couplings  86  arranged on the collector rail  72  are again able to move separately toward or away from the collector rail  72 , in order to enable installing or removing individual artificial vessels or stents during operation. The meter  74  in this embodiment is movably disposed in order to measure different artificial vessels  73  or stents  18  as desired. The line connections between pressure modulator  24 , supply tank  82 , pump  22  and inlet  4  are only hinted at by broken lines. 
     LIST OF REFERENCE SYMBOLS 
     
         
           2  Line 
           4  Inlet 
           6  Outlet 
           8  Fluid 
           10  Arrow 
           12  Pipeline arrangement 
           14  Delivery mechanism 
           16  Throttle element 
           17  Test layout 
           18  Stent 
           20  Test section 
           22  Pump 
           24  Pressure modulator 
           26  Outlet opening 
           27  Pipe piece 
           28  Clearance 
           30  Fluid jet 
           32  Fluid path 
           34  Impact body 
           36 , 36   a - d  Impact surface 
           38  Housing 
           40  Pitot tube 
           42  Shaft 
           44  Disk 
           46  Axis of rotation 
           48  Belt pulley 
           50  Segment 
           52  Interruption 
           53  Screw connection 
           54  Collection chamber 
           56  Drain 
           58  Motor drive unit 
           60  Aperture disk 
           62  Guide body 
           64  Axis of rotation 
           66  Mating body 
           68  Belt pulley 
           70  Distributor rail 
           72  Collector rail 
           73  Vessel 
           74  Meter 
           76  Volume flow meter 
           78  Thermometer 
           80  Pressure gauge 
           82  Supply tank 
           84  Chamber 
           86  Quick coupling 
         Δp Pressure fluctuation 
         p Pressure 
         Q Cross sectional area 
         a,a 1,2  Spacing 
         R 1-4  Relative position 
         G Degree of overlap 
         α Angle of inclination 
         A 1,2  Area