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Characterization of the streamwise vortices and near-wake dynamics in the turbulent flow around the
25° Ahmed body based on SPIV
Cyril Jermann, G. Pujals, Philippe Meliga, E. Serre, F. Gallaire
To cite this version:
Cyril Jermann, G. Pujals, Philippe Meliga, E. Serre, F. Gallaire. Characterization of the streamwise vortices and near-wake dynamics in the turbulent flow around the 25° Ahmed body based on SPIV.
3rd GDR Symposium “Flow Separation Control”, Ecole Centrale de Lille, 7th and 8th, Nov 2013, Lille, France. �hal-01309842�
Characterization of the streamwise vortices and near-wake dynamics in the turbulent flow around the25◦ Ahmed body based on SPIV
Cyril Jermann(1,2), Gr´egory Pujals(2), Philippe Meliga(1), Eric Serre(1)& Fran¸ois Gallaire(3)
(1)Laboratoire M2P2, UMR 7340 CNRS, Aix-Marseille Universit´e , Centrale Marseille, 13451 Marseille, France
(2)PSA Peugeot Citro¨en, Centre Technique de Velizy, Route de Gisy, 78943 V´elizy-Villacoublay Cedex, France
cyril.jermann@mpsa.com
(3)Laboratory of Fluid Mechanics and Instabilities - Ecole Polytechnique F´ed´erale de Lausanne, CH1015 Lausanne, Switzerland
Abstract :
Ahmed body is considered as a simplified road vehicle, appropriate to study typical swirling structures encountered in automotive aerodynamics, in particular the two counter-rotating longitudinal vortices produced from the side edges of the rear window which are responsible for a large part of the aerodynamic drag. Here, we use a moving Stereo-PIV facility to characterize experimentally the longitudinal vortices produced at high Reynolds number (Re= 2.8×106)by a25oslant angle, involving the formation of a marginal separation bubble over the slanted surface. A smoke visualization technique suggests the occurrence of a conical vortex breakdown in the near wake, the observed dynamics being reminiscent of that documented in the literature for diverging cylindrical tubes at high Reynolds number.
R´esum´e :
Le corps d’Ahmed est un mod`ele acad´emique de v´ehicule vou´e `a l’´etude des ´ecoulements de culot. Il produit des structures tourbillonnaires longitudinales typiques des ´ecoulements automobiles, `a savoir deux tourbillons contra-rotatifs issus des arˆetes lat´erales de la lunette arri`ere et contribuant fortement `a la train´ee totale du corps.
Nous pr´esentons une caract´erisation exp´erimentale de la dynamique moyenne de ces tourbillons `a haut nombre de Reynolds(Re= 2.8×106), pour un angle de25ocorrespondant `a la formation d’une recirculation marginale sur la lunette. D’autre part, l’utilisation de la canne `a fum´ee comme outil de visualisation des tourbillons longitudin- aux permet de mettre en ´evidence les signes d’un ´eclatement tourbillonnaire conique dans le sillage, les r´esultats observ´es ´etant similaires `a ceux obtenus dans les exp´eriences de r´eference men´ees `a grand nombre de Reynolds dans des tubes cylindriques divergents.
Mots-clefs :
Stereo-PIV ; Ahmed body ; Longitudinal vortices; Conical vortex breakdown 1 Introduction
The aerodynamic forces acting on road vehicles are governed by complex interactions between flow separations and the dynamic behavior of the vortex wake that require a more detailed knowledge to successfully design future cars. The Ahmed body is an academic test-case meant to reproduced a wide range of the flow features encountered in automotive aerodynamics. In the present study, we focus on the 25o slanted rear end, which is known to produce a fully three-dimensional, turbulent unsteady state consisting of a separation bubble over the slanted surface with highly energetic streamwise vortices issuing from the slant side edges, as observed experimentally by Lienhart et al. [1]. Since then, the25o configuration has received a lot of attention from experimentalists (Thacker [2], Lehugeur et al. [3], among others) and numericists
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Figure 1: Sketch of the generic car model used in the present study. The two dashed boxes indicate the location of the Stereo-PIV(y∗, z∗)planes measured during the experiment.
(Serre et al. [4], Minguez et al. [5], Krajnovic et al. [6] , among others) but there is still a lack of experimental data for the dynamics of the longitudinal vortices to be fully unraveled. Past studies have attempted to manipulate heuristically these vortices (Lehugeur et al. [3]) in view of reducing the drag, but optimized control methods remain out of reach as long as we will not have an accurate understanding of these vortices structure and dynamics. With the recent evolution of the measuring techniques, new approaches of the problem are yet possible. The present work aims at focusing on high resolution results to clarify the structure and dynamics of the longitudinal vortices through a detailed physical description.
2 Experiments and techniques 2.1 Description of the facility
We use the generic geometry originally described in Ahmed et al. [7] (1.044mmlong,390mm width,288mmhigh and50mmas support high) and focus on the25orear slant configuration with a sharp edge at the connection between the roof and the rear slant. The coordinate system is defined asxin the streamwise direction,znormal to the ground andyforming a direct trihe- dral. The axis system origin is taken at mid-width on the ground located on the separation line between the roof and the rear window, as located in Figure 1.
The experiments are performed in PSA Peugeot Citro¨en in-house facility. We use an open wind tunnel with a rectangular cross-section of2.1mhigh,5.2mwide and6mlong. The turbulence intensity in the free stream is1.3%. The main flow velocity is U0 = 40 m/s, which yields a Reynolds number of 2.8×106 based on the length of the geometry. The model is fixed on a 3m wide flat plate located0.5m above the floor, whose leading edged has been covered with a Naca airfoil profile, which enables controlling the boundary layer development without any suction device.
The test section is equipped with a moving Stereo-PIV system. It consists of a Quantal Big Sky Laser (dual pulse Nd : YAG) set outside the test section - on one side of the wind tunnel - and two DANTEC CCD sensors (FlowSense MkII 4mpx), covering both sides of the raised floor. All devices are fixed on a 1D DANTEC traverses system set parallel to the raised floor.
Moreover, they are synchronized with each other, hence offering the opportunity to acquire the velocity field along thexaxis in several iso-x planes without any modification of the cameras
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Figure 2: Partial visualization of experimental time-averaged streamlines colored by streamwise velocityU∗, obtained by linear interpolation of90 (y∗, z∗)Stereo-PIV planes.
and laser settings.
Camera angles are oriented40o and45orelative to the Ahmed body symmetry plane, with par- ticular attention paid to satisfy the Scheimpflug condition meant to ensure a satisfying sharpness on the whole image. The laser light sheet is set normal to the free stream to measure the velocity field through the longitudinal vortices. The test section is seeded with micro-sized droplets of olive oil to ensure goods reflection properties. The optical set-up was chosen to generate a sheet as thin as possible (roughly4mm) in order to avoid the particles to leave the laser sheet during the motion.
Thanks to the moving part of the facility, only one calibration is required to set the Stereo-PIV parameters, greatly simplifying acquisitions. Laser and camera are externally synchronized, the separation time∆t = 10µsbeing optimized to have a displacement of about one quarter of the interrogation window in the potential flow. This is appropriate in view of further investigating the 3D mean dynamics, as it allows exploring and acquiring several planes along the streamwise direction. The third component of velocity is obtained by combination of the two images from CCD cameras based on the stereoscopy principle.
Images are recorded using a105mmoptical lens with large aperture and a2048×2048pixels CCD camera. We use a16×16pixels interrogation window with a 50% overlap leading to 255×255vectors for each image. One thousand pairs of independent images with a sampling frequency of7hzare acquired to obtain a converged time-averaged velocity field. After Stereo- PIV processing, the typical physical dimensions of the velocity field are322 mm×262 mm resulting inδy = 1.6mm, δz = 1mmspatial resolution.
Two component classical PIV measurements were performed in the vertical plane of symmetry of the body, the comparison with representative SPIV velocity profiles being very good. The same agreement was also found with respect to the reference experiment of Lienhart et al. [1], hence demonstrating the reliability of the protocol.
In practice, we assume the mean flow to be symmetric. Consequently, we focus the acquisition only on one side of the model (passenger side) and use reflectional symmetry of the obtained fields to achieve the full representation. Since the aim of this study is to investigate longitudinal vortices, the acquisition field extends up to150mmof the ground, which means that we do not
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Figure 3: Experimental time-averaged vorticity magnitude over the rear window and in the wake. A mirror symmetry on they∗ = 0plane has been used to obtain the complete 3D view.
capture the lower spanwise structures in the near wake.
2.2 Characterization of the 3D base flow
The projected length of rear slant on x-axisLs and inlet velocity U0 are used to obtain non- dimensional values marked in the following with an asterisk. Several cross-flow planes(y∗, z∗) were acquired, 40planes along of the rear slant, and 50 planes in the near wake, yielding to respectively δx = 5 mm, δx = 10 mm as spatial resolution (Figure 1). This collection of time-averaged plane was then used to reconstruct the fully three-dimensional (3D) mean flow by linear interpolation of the data. We present in Figure 2 the so-reconstructed 3D streamlines colored by the streamwise velocity. Two distinct structures emerge, namely the longitudinal vortex originating from the side of the slanted surface, and the large separation at the base orga- nized under the form of a toroidal vortex. Both structures are clearly visible on Figure 3 which presents the evolution of the vorticity norm on the rear slant and in the wake up tox∗ = 3.5 together with the 3D separation bubble over the slanted surface.
The vortices are well defined over the rear slant, with the vortex intensity gradually increasing while the core follows a straight line trajectory. The(y∗, z∗)plane located immediately down- stream the base evidences the existence of the two vorticity sheets, one coming from the lateral side and one from the rear slant. We observe the rolling up of these vorticity sheets following a deviated path as reported by Thacker et al. [8], explained by interaction between the vorticity sheets of the trailing vortices and the wake structures. The streamwise vortices are no longer supplied by the rear slant, leading a loss of intensity in the near wake and a more drastic de- crease as fromx∗ = 1.89. It is worth noticing that this comes together with a dramatic drop in the streamwise velocity, as seen from Figure 2. Eventually, the trailing vortices loose their coherent behavior atx∗ = 2.29and finally diffuse themselves in the wake.
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Figure 4: Left : Contours of time-averaged streamwise vorticityωx∗ of the(y∗, z∗)SPIV plane located atx∗ = 0.95(ωx∗ < 0: dashed lines,ωx∗ > 0: solid lines) . Right: Iso-contours of Q- criterion identifying the three swirling structures over the rear window. The rotational direction is indicated by streamlines in white.
2.3 The longitudinal vortex as three swirling structures
The leftmost part of Figure 4 represents a cut in the streamwise vorticity atx∗ = 0.95with a focus on the global swirling structure. It is noteworthy there is no information at the center of the vortex core which is a consequence of no particles in this area due to the strongly centrifugal force. The principal swirling structure generates a secondary one which remains attached to the wall and counter-rotates relatively to the first one with a lower intensity, as reported initially by Spohn et al. [9] and now classically acknowledge for this type of configuration. More interestingly, a third swirling structure, co-rotating with the first one, appears as a resulting of a gearwheel mechanism produced intrinsically by the wall. Indeed when the longitudinal vortex leaves the rear slant, only the main swirling structure remains. For the sake of completeness we provide in the rightmost part of Figure 4 a 3D view of this peculiar topology by plotting iso- values of the Q-criterion together with associated streamlines in representative(y∗, z∗)planes.
The organization of the longitudinal vortex under the form of three distinct sub-structures is in a good agreement with the numerically results of Krajnovic et al. [10] but to the best of our knowledge, it had never been observed experimentally before.
3 Indication of vortex breakdown in the near wake
Vortex breakdown is a classical phenomenon for this class of flows involving vortices with axial flow, which consists of an abrupt change in the flow topology, namely from a so-called colum- nar solution characterized by negligible longitudinal gradients, small rotational velocities and large longitudinal velocities, to a breakdown solution exhibiting an internal stagnation point and characterized by large rotational velocities and small longitudinal velocities. The sharply decay of the vorticity and the suddenly decline of streamwise velocity suggest that the longitudinal vortex may undergo vortex breakdown in the near wake (aroundx∗ = 1.89).
This point is now further investigated using a classical smoke visualization technique. More specifically smoke is introduced through a small pipe end up to an injector, placed on the left
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Figure 5: Lateral view evidencing the longitudinal vortex with the help of smoke visualization on the 25o Ahmed body with U0 = 10 m/s as inlet-velocity. Left: instantaneous snapshot.
Right: time-averaged image of1200snapshots,10sas acquisition time.
side just before the separation line between the roof and rear slant. Thereby smoke lines are convected with flow, experience rolling up of the two vorticity sheets coming from lateral side and from the roof, indicating the position and dynamical behavior of vortical structure. Various inlet velocity were investigatedU0 = 5,10,12,15m/s as the smoke device did not offer the opportunity to go above U0 = 15 m/s. The smoke quantity is adjusted and adapted to the air-flow velocity with a mean droplet size around1µm. All data are recorded from the left side of the model with WVGA (800×480pixels) as video resolution at the frame rate of120f ps.
Results are presented in Figure 5, with an instantaneous snapshot on the left, and the time- averaged image obtained processing a physical time of10s (i.e 1200frames) on the right. In both cases, the longitudinal vortex exhibits very smooth variations while developing over the slanted surface and in the near wake, a behavior reminiscent of that of the columnar solution.
It then undergoes a sudden expansion at approximatelyx∗ = 1.5, where the smoke starts be- coming disordered. These observations are similar to the experiment in a slightly diverging cylindrical tube at high Reynolds number described by Sarpkaya et al. [11] and to the exper- iment on F-18 described by Mitchell et al. [12], hence suggesting the onset of the emergence of conical vortex breakdown in the near wake of Ahmed body 25o when the vortex can no longer sustain the adverse pressure gradient.This conical vortex breakdown persists for all inlet velocity tested, the only difference being that the extrapolated stagnation point is shifted down- stream as the velocity is increased from approximatelyx∗ = 1.3 (U0 = 5 m/s) to x∗ = 1.7 (U0 = 15m/s).
4 Conclusions
We performed a full characterization of the mean flow past the 25o Ahmed body at a high Reynolds number using a moving Stereo-PIV facility, which provides both visualization and measurements of the main flow features. The obtained database was used to achieve a full reconstruction of the 3D flow, and the accurate spatial resolution allowed unraveling the com- plex structure of the longitudinal vortex found to consist of three distinct structures. Additional smoke visualization was used to evidence a conical vortex breakdown-like behavior in the near wake, further efforts being currently devoted to achieving a more quantitative description of this
phenomenon.
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