EACWE 5 Florence, Italy 19 th 23 rd July Keywords: Non-steady loads; Train; Traffic sign. ABSTRACT

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1 EACWE 5 Florence, Italy 9 th 3 rd July 9 Flying Sphere image Museo Ideale L. Da Vinci Experimental investigation on the force induced by a high speed vehicle passing by a traffic sign A. Sanz-Andres, C. Gragera, J. Meseguer, F. Zayas : Professor, Universidad Politécnica de Madrid (IDR /UPM ) E.T.S.I. Aeronauticos, 84 Madrid, Spain angel.sanz.andres@upm.es : Assistant Professor, Universidad de Extremadura Escuela de Ingenierías Industriales, 67Badajoz, Spain cgragera@unex.es 3: Professor, Universidad Politecnica de Madrid ( IDR /UPM ) E.T.S.I. Aeronauticos, 84 Madrid, Spain j.meseguer@upm.es 4: Professor, Universidad de Extremadura Escuela de Ingenierías Industriales, 67Badajoz, Spain fzayas@unex.es Keywords: Non-steady loads; Train; Traffic sign. ABSTRACT The aim of this paper is to introduce a new experimental facility developed and tuned with the purpose of carrying out tests to measure the forces produced by a moving vehicle on a fixed object, like e.g a traffic sign or a human body placed close to the track or road where the vehicle is moving along it at high speed. The force on an object placed close to the vehicle path have received a wide attention in the literature (Gerhardt & Krüger (998), Cali & Covert (), Quinn, Baker & Wright (), Johns & Dexter (998), Sanz-Andres & Santiago-Prowald (), Sanz-Andres et al (3)) both under experimental and theoretical points of view. Sanz-Andres et al (3) showed that the initial variation of the force (caused by the nose passing) can be explained by using a simple potential model, based on assumptions which were shown to be valid to this application. It should be pointed out that it is not the aim of this facility to study the effect of the slipstream or the wake generated by the vehicle (which should be largely influenced by the centrifugal force), it is only to study the initial phase (nose passing), when the potential flow can be assumed, to explore the range of validity of potential flow theoretical models. The idea is to avoid as much as possible the realisation of experiments on tracks or roads, and replace them with tests on a lab, which show some obvious advantages: comfort, convenience, repeatability, accessibility, affordability, just to mention a few. A relatively large number of tests has been already performed in order to develop the facility, to tune it, and check it for repeatability. The results are reasonably promising. The aim of this paper is to show some of the results obtained, concerning the passing of several types of objects by vehicles of different types. Contact person: st Author, Professor, Universidad Politécnica de Madrid (IDR /UPM ), E.T.S.I.Aeronauticos, 84 Madrid, Spain, telephone FAX angel.sanz.andres@upm.es

2 . INTRODUCTION The facility devised (Figs. and ) is based on a rotating arm (like the first aerodynamic test facilities used in old times, before wind tunnels were developed) kind of centrifuge. The centrifuge motion is powered by a electrical motor MAXON DC-RE 35 (9 W) with a gear box for velocity reduction. With this motor, the centrifuge is able to reach a maximum rotation speed of 6 rpm. The motor is servo controlled by using a tachometer dynamo. At the tip of the arm the vehicle model is firmly supported. The rotation of the arm creates the motion of the vehicle along a circular path, which can be assumed as a linear motion if short time intervals are considered. A switch mounted on a support is pulsed by the arm when it passes in front of the test object. This object, which experiences the force generated by the passing vehicle, transmits it to two ULC Interface load cells, featuring a.5 N maximum load. Both load cells are carefully mounted on a rigid structure, to obtain a first resonant frequency high enough to allow good measurements. One load cell measures the longitudinal force component and the other one the transversal force component (Fig 3). The load cells are connected to a power supply Freak EP-63, and the output signals are connected to a PC-based data acquisition system AT-MIO-6 E from National Instruments (with 6 analog channels, bits conversion, khz maximum sampling rate ADC). The measurement software developed is based on Labview. The force acting on the object is typically very small (some.3 N), asking for very sensitive load cells, and therefore, for special care in operation of the facility. The rotating arm is.5 m in length, and allows to obtain linear speed in the range to 7 m/s. The typical size of the object tested is some 4 cm wide and cm high, and the vehicle size is some. m. m cross section and.5 m length. A relatively large number of tests has been already performed in order to develop the facility, to tune it, and check it for repeatability. The results are reasonably promising. The aim of this paper is to show some of the results obtained, concerning the passing of several types of objects by vehicles of different types. The object tested were rectangular flat plates and cylinders of several dimensions, and two vehicles were considered: box-like (type C), and streamlined shape (A). The box-like vehicle is a parallelepiped box (, m, m.46 m side dimensions). The streamlined vehicle is an axisymmetric PVC model whose shape has been obtained from the shape of the body generated in a potential flow by a source placed in an otherwise uniform, steady flow (see Fig. 4).. EXPERIMENTAL RESULTS In each experimental run, the centrifuge rotating speed is set at some constant value. The values of the signals are registered and stored during a given number of centrifuge turns (some 9 turns), together with the data time, and the passing switch status. A typical evolution of the force measured with time during 3 turns is shown in Fig. 5a. A detail containing two consecutive passes (two turns) data is shown in Fig. 5b. Note that there is a constant level of force, F, due to a small tilt angle of the object support (offset force). This force is different in general for each load cell. The estimated value of the evolution of the force with time during a pass is obtained from the data of multiple passes, as plotted in Fig 5a, in the following way. The common reference to syncronize the registers obtained from all the passes is set at the passing time instant, given by the passing switch, considering 4 data before the pass and 6 data after the pass time instant, as shown in Fig.5c, where the origin of time has been taken at the passing time instant minus 4 sampling intervals.the individual force evolutions that show large shifts from the mean value are rejected, and then the average value is obtained (solid line in Fig. 5c). The values of the force F measured by the load cells are transformed to force coefficient c f, which is the force made dimensionless with the dynamic pressure relative to the vehicle, and the object front area, defined as in Sanz-Andres et al (3)

3 F F c f = ρu Lc H where ρ is the air density, U is the vehicle speed, L c is the characteristic length of the object (that is, the chord B for the plate, or the diameter R for the cylinder), and H is the object height. F is the offset force. In some cases, as proposed on Sanz-Andres et al (3), the configuration parameter is used, defined for plates as c s = BA b /d 3 where A b is the area of transversal final section of the object, and d is the distance between the object centre and the centre of the vehicle trajectory; and for cylinders as c r = RA b /d 3. For the final plotting of the results, the time t is transformed to a dimensionless time T = tu/d as suggested in Sanz-Andres et al (3). Observe that, for the definition of the dimensionless time, the characteristic length relevant to the variation of the force is the distance of the object to the vehicle trajectory, d. The set of tests performed has been summarized in table. The influence of several parameters has been studied: the height of the object (h), the distance from the object to the vehicle (d), the orientation of the plate with regard to the vehicle trajectory (o), and the porosity of the plate (p). In Figs 6 and 7 the variation of the longitudinal and the transversal force and of the force coefficients with time, at three different speeds, are shown. It can be observed that the force obtained at several speeds collapse (or gather together) in almost the same curve when plotted in coefficient form (Fig. 7) in the interval 4 < T <.3 during the pass of the vehicle nose. Therefore, the effect of scaling of speed as U is shown to be a reasonable one. This scaling effect has been checked to be valid in all these tests, by comparing the evolutions obtained at several speeds with the otherwise identical configurations. This fact supports the validity of the scaling laws derived by Sanz-Andres et al (3) from the unsteady potential flow model used for studying this phenomenon. The origin of the dimensionless time has been placed at the first negative peak for longitudinal force time variation and at the first zero crossing in the transversal force time variation. The potential peak value in longitudinal force coefficient (see Fig 7, left side, at T = ) is close to one, while the transversal peak (see Fig 7, right side, at T =.3) is almost twice as large. After passing the vehicle nose, the force variation continues showing the effect of the end of the vehicle (first negative peak in transversal force, first large positive peak in longitudinal force)) which could be explained based on potential flow grounds. This effect is combined with the effect of the elasticity of the object support (including the load cells themselves), until the start of the wake effect (irregular force variation which appears after the second positive peak in transversal force, and after the second negative peak in longitudinal force). Note that in these cases the most important effect is the pass of the nose and the vehicle s end, and the effect of the wake is smaller. The relatively short length of the vehicle and the circular confdition of the global motion of the centrifuge can have an influence in the development of a large wake behind the vehicle. However, we do not want to analyze this point further as we have not put our aim in this part of the flow. The effect of the plate height is shown in Fig.8 for the longitudinal and transversal forces. The average of the peak values measured at several passes has been used for the comparison. It can be shown that both force coefficients remain almost constant for plates whose height is smaller than.5 m. For larger plates, the force coefficients increase. The effect of plate orientation with regard to the vehicle trajectory is shown in Fig. 9. Note that the curve γ = 9º is similar to the case of the longitudinal force evolution with time, and the curve γ = º to the case of the transversal force evolution, and even are in quantitative agreement with the results in Fig.5 of Sanz-Andres et al (3). The effect of the object-vehicle distance for the box-like vehicle is shown in Fig., for the longitudinal and transversal forces, respectively. The effect on the longitudinal force is quite unusual, as the force first increases with the distance for small distances; however, the transversal force decreases with distance, as expected. The same effect has been studied for the case of the streamlined vehicle (source-like) and the results have been plotted in Figs.. In this case, the prediction of the potential theory, a decay as d -3 is readily shown for both the longitudinal and transversal forces. Therefore, the irregular behaviour of the longitudinal force variation that appears in the case of the box-like vehicle could be due to the detached flow and recirculation bubble that appears at the vehicle front face edges. Comparing values

4 in Figs. and, it can be shown that the force coefficients for the box-like vehicle are some times larger than for the streamlined one. The effect of plate porosity is shown in Fig.. The porosity is some 5%. The force in the case of a porous plate is quite reduced in comparison with the solid plate, nearly to a % in the case of the longitudinal force, and nearly to a 5% in the case of transversal force. Note the small influence of vehicle speed on the force coefficients, as expected. In the case that the object is a cylinder (Fig. 3) the effect of the distance d on the force coefficients shows some differences with regard to the case of a plate. The anomalous effect in the longitudinal force coefficient that appears in the case of the plate does not appear, although in order to fully checked it some more measurements, closer to the distance d =.3, should be performed. It can be shown that the effect of the vehicle pass on a cylinder measured in this facility (in Fig. 4) is similar to the effect considered in the standards, e.g. the variation shown in Fig. 7 UNE-EN 467-:3 can be compared with the variation of the transversal force coefficient c f-t as a function of the longitudinal force coefficient c f-l in Fig. 4. The influence of the vehicle-object distance d in the force coefficients on the cylinder induced by a streamlined object (Fig. 5) is similar, although smaller, than the influence induced by a Box-like vehicle (Fig. 3). The variation of the transversal force coefficient c f-t as a function of the longitudinal force coefficient c f-l, in the case of streamlined body passing by a cylinder object is shown in Fig. 6. In Fig. 7 the same plot is shown, although considering the effect of the distance using the configuration parameter c r abovementioned. Table. Test performed to study the influence of several parameters: the height of the object (h), the distance from the object to the vehicle (d), the orientation of the plate with regard to the vehicle trajectory (o), and the porosity of the plate (p). Additional tests, although no reported here has been performed on the streamlined vehicle supplemented with a cylindrical extension (ex). Vehicle Box (C) Streamlined (A) Object Plate (PL) PLC-h PLC-d PLA-o PLA-d PLA-p Cylinder (CL) CLC-d CLA-d CLA-ex CONCLUSIONS A new experimental facility has been developed and commissioned with the aim of carrying out tests to study the effect produced by a moving vehicle on a fixed object (e.g. traffic sign, human body, etc.) which is placed close to the vehicle trajectory (track or road). The setup, as it is based on a rotating arm (kind of centrifuge), allows us to consider the rotational motion as a linear one only over small values of the rotation angle, that is, when the distance covered by the vehicle along the experiment (let say the vehicle length) is small compared with the rotating arm length. Therefore, we consider the facility valid only to study the passing effect of the vehicle nose, the results concerning the effect of the end and the wake being more arguable. A number of preliminary tests have been performed, measuring the forces that two vehicles produce on two types of objects, which show the capabilities of the facility for the purpose. When possible, the experimental results have been compared to the available theoretical potential models. A number of problems had to be solved to make the facility work properly. One of these problems has been the measurement of quite small unsteady forces, which has imposed stringent requirements on both the measurement equipment and on the supports of the objects to be tested.

5 REFERENCES Gerhardt, H.J., Krüger, O. (998). Wind and train driven air movements in train stations, J. Wind Eng. Ind. Aerodyn , Cali, P.M., Covert, E.E. (). Experimental measurements of loads on an overhead highway sign structure by vehicle-induced gusts, J. Wind Eng. Ind. Aerodyn. 84, 87. Quinn, A.D., Baker, C.J., Wright, N.G. (). Wind and vehicle induced forces on flat plates. Part : vehicle induced force, J. Wind Eng. Ind. Aerodyn. 89, Johns, K.W., Dexter, R.J. (998). The development of fatigue design load ranges for cantilevered sign and signal support structures, J. Wind Eng. Ind. Aerodyn. 77&78, Sanz-Andres, A., Santiago-Prowald, J. (). Train-induced pressure on pedestrians, J. Wind Eng. Ind. Aerodyn. 9, 7 5. Sanz-Andres, A., Santiago-Prowald, J., Baker, C.J., Quinn, A.D. (3). Vehicle induced loads on traffic sign panels, J. Wind Eng. Ind. Aerodyn. 9, Figure : Sketch of the experimental set-up.

6 C V R RS LC LC OS OS Figure : Sketch of the experimental set-up. V: vehicle. R: rotating arm. RS: rotating arm support. C: cylinder. LC: Load cell. OS: Object support structure. Load cells Streamlined vehicle Test object Object support structure Figure 3: Sketch of the load cell set-up to measure the force acting on the test object. Y.77 m. m X.5 m.35 m Figure 4: Shape of the streamlined body. The origin of the reference frame is placed at the virtual position of the source.

7 a b -,39 -,45 c F [N] -,44 -,465 -,49 5 Ns 5 75 Figure. 5: Force components acting on the load cells, F, as a function of the time, t, or the number of samples, N s. Results of the test with the plate (perpendicular to the trajectory) and the box-like vehicle. Centrifuge rotation speed 3 rpm. a) data from the first 6 s of the test; b) detail of two selected turns; c) the data form the passes has been put together in the same time interval, by referring the data to a common time origin, which is taken as 4 samples before the pass time instant. Solid line: averaged longitudinal force. Dashed line: averaged transversal force. d =. m, h =.3 m. B =.5 m.

8 ,4,75, F [N],5,5 F [N] -, -,5 -, -,,, t [s] -,4 -, -,,, t [s] Figure 6: Variation with the time t [s] of the force acting on the plate F [N]. PLC-h test (vehicle: box-like; object: plate). Left: longitudinal force on a perpendicular plate. Right: transversal force on a parallel plate. The angular rotation speed of the centrifuge arm is 7 rpm (solid line);. rpm (dashed line); and 5 rpm (gray line). d =.45 m, h =.5 m. B =.5 m Cf 4 Cf T T Figure 7: Variation with the dimensionless time T = tu/d of the force coefficient c f acting on the plate. PLC-h test (vehicle: box-like; object: plate). Left: longitudinal force on a perpendicular plate. Right: transversal force on a parallel plate. The angular rotation speed of the centrifuge arm is 7 rpm (solid line);. rpm (dashed line); and 5 rpm (gray line). d =.45 m, h =.5 m. B =.5 m.,5,5,,,5,5,5,5 5,5,5,35 h [m] 5,5,5,35 h [m] Figure 8: Variation of the first peak in the force coefficient c fp with the height of the plate h, in the PLC-h test (vehicle: box-like; object: plate). Left: longitudinal force on a perpendicular plate; right: transversal force on a parallel plate. The distance vehicle-object is d =.45 m. B =.5 m.

9 Cf/cs,5,5 -,5 9º º 6º 3º PL A o -,5 - - T Figure 9: Variation with the dimensionless time T = tu/d of the ratio of the force coefficient c f to the configuration parameter c s, at several orientations of the plate (º, 3º, 6º, 9º are the angle between the chord of the plate and the vehicle trajectory). PLA-o test (vehicle: streamlined ; object: plate). The angular rotation speed of the centrifuge arm is 5 rpm. d =.35 m, h =.3 m. B =.5 m. 4 PL C d 4 PL C d 3 3,,,4,6,8,,,4,6,8,3 Figure : Variation of the first peak of force coefficient c fp with the distance between the object and the vehicle. PLC-d test (vehicle: box-like; object: plate). Left: longitudinal force; right: transversal force. h =.3 m. Solid line: polynomial fitting. B =.5 m. PL A d,3 PL A d,,,,,,,4,6,8 Figure : Variation of the first peak of force coefficient c fp with the distance between the object and the vehicle. PLA-d test (vehicle: Streamlined ; object: plate). Left: longitudinal force; right: transversal force. Solid line: polynomial fitting. h =.3 m. Solid line: polynomial fitting as d 3. B =.5 m.,,,4,6,8

10 3, PL C p 3, PL C p,,,5,75 3,5 3,75 4,5 v [m/s],5,75 3,5 3,75 4,5 v [m/s] Figure : Effect of the plate porosity. Variation of the first peak of force coefficient c fp with the speed of the vehicle U. Solid plate (rhombi); perforated plate (triangles). Test PLC-p (vehicle: box-like; object: plate). Left: longitudinal force; right: transversal force. d =.5 m h =.3 m. B =.5 m.,5 CL C d,5 CL C d _l,5 _t,5,5,9,3,7,5,9,3,7 Figure 3: Variation of the first peak of force coefficient c fp with the distance between the object and the vehicle. CLC-d test (vehicle: Box-like ; object: cylinder). Left: longitudinal force coefficient c fp_l ; right: transversal force coefficient c fp_t. h =.55 m. Solid line: polynomial fitting as d 3. R=.7 m. CL C d, Cf_t - -,5 - -,5 Cf_l -, Figure 4: Variation of the transversal force coefficient c fp_t with the longitudinal force coefficient c fp_l. CLC-d test (vehicle: Box-like ; object: cylinder). d =.7 m, h =.55 m. 7 rpm,solid line; rpm,dashed line ; 5 rpm,gray line); double dashed, mean value of the three. R=.7 m.

11 ,5 CL A d,5 CL A d _l,5 _t,5,,6,,4,,6,,4 Figure 5: Variation of the first peak of force coefficient with the distance between the object and the vehicle. CLA-d test (vehicle: Streamlined ; object: cylinder). Left: longitudinal peak force coefficient c fp_l ; right: transversal peak force coefficient c fp_t. h =.5 m. Solid line: polynomial fitting as d 3. R=.4 m. CL A d,7,4 Cf_t -,4 -,7 - -,5 Cf_l Figure 6: Variation of the transversal force coefficient c fp_t with the longitudinal force coefficient c fp_l. CLA-d test (vehicle: Streamlined ; object: cylinder). d =.5 m; h =.5 m; solid line, 7 rpm; dashed line, rpm; gray line, 5 rpm. R =.4 m. CL A d (N=) 4 Cf_t/cr Cf_l/cr - Figure 7: Variation of the transversal force coefficient c fp_t with the longitudinal force coefficient c fp_l, as ratios to the configuration parameter c r. CLA-d test (vehicle: Streamlined ; object: cylinder). d =.5 m; h =.5 m; solid line, 7 rpm; dashed line, rpm; gray line, 5 rpm.r =.4 m. -4

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