Large-Scale Separated Vortex Generated in a Wake Flow of Ahmed’s Body
Unsteady wake structures around a bluff body, which has a slanted rear surface, are investigated experimentally with measuring the velocity and the pressure fields. Velocity distribution obtained by Particle Image Velocimetry showed that vortices are intermittently generated and occasionally it grows to a large-scale vortex which separates the flow from the slanted surface. From the pressure distribution, which is simultaneously measured with the velocity field, it is clarified that the large-scale separated vortex has considerable extent in the spanwise direction. This fact suggests that the re-attachment line of the downwash is completely separated from the slanted surface when the large-scale vortex is generated. From the time-frequency analysis, which is applied to the velocity fluctuation measured by hot-wires, the flow observed at points downstream of the body exhibits the periodic fluctuation, whereas the flow near the slanted surface does not involve the periodic fluctuations. Instead, the intermittent fluctuation is observed for the case when the downwash of the flow structure is separated from and re-attaches to the slanted surface. The intermittent fluctuation on the slanted surface is observed regardless of the existence of upwash, whereas the periodic fluctuation at the point downstream vanished when the upwash is absent.
KeywordsVehicle aerodynamics Flow transition Vortex generation Particle image velocimetry Pressure distribution
Authors are grateful to Mr. Y.Hashizume for the enlightening discussion with him.
Compliance with Ethical Standards
Conflict of interests
The authors declare that they have no conflict of interest.
- 1.Janssen, L.J., Hucho, W.H.: Aerodynamische entwicklung von vw golf und scirocco. ATZ 77, 1–5, 01 (1975)Google Scholar
- 2.Hucho, W.H., Janssen, L.J., Emmelmann, H.J.: The optimization of body details-a method for reducing the areodynamic drag of road vehicles. SAE Technical Paper, (760185), 02 (1976)Google Scholar
- 3.Morel, T.: Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, chapter The Effect of Base Slant on the Flow Pattern and Drag of Three-Dimensional Bodies with Blunt Ends, pp. 191–226. Springer (1978)Google Scholar
- 4.Ahmed, S.R., Ramm, G., Faltin, G.: Some salient feature of the time-averaged ground vehicle wake. SAE Technical Paper (840300) (1984)Google Scholar
- 5.Kohri, I., Yamanashi, T., Nasu, T., Hashizume, Y., Katoh, D.: Study on the transient behavior of the vortex structure behind Ahmed body. SAE Int. J. Passenger Cars - Mech. Syst. 7(2) (2014)Google Scholar
- 6.Johnson, S.A., Hourigan, K., Thompson, M.C.: Effect of aspect ratio on the wake structures of simplified automotive geometries. In: Summary Paper for 5th International Colloquium on Bluff Body Aerodynamics and Applications, pp. 557–560 (2004)Google Scholar
- 8.Spohn, A., Gilliėron, P.: Flow separations generated by a simplified geometry of an automotive vehicle. In: IUTAM Symposium Unsteady Separated Flows (2002)Google Scholar
- 9.Krajnović, S.: Large Eddy simulation of flows around ground vehicles and other bluff bodies. Phil. Trans. R. Soc. A, 367 (2009)Google Scholar
- 10.Kobayashi, Y., Kohri, I., Kasai, A., Nasu, T., Katoh, D., Hashizume, Y.: Numerical analysis on the transitional mechanism of the wake structure of the Ahmed body. In: SAE Technical Paper. SAE International, 04 (2016)Google Scholar
- 11.Sims-Williams, D.B.: Self-Excited Aerodynamic Unsteadiness Associated with Passenger Cars. PhD thesis, University of Durham (2001)Google Scholar
- 12.Sims-Williams, D., Duncan, B.D.: The Ahmed model unsteady wake: Experimental and computational analyses. SAE Technical Papers pages 2003–01–1315 (2003)Google Scholar
- 15.Nasu, T., Katoh, D., Kobayashi, Y., Kohri, I., Hashizume, Y.: Unsteady characteristics of the wake structure of Ahmed bluff body. In: ICJWSF-2015, 5th Int. Conf. on Jets, Wakes and Separated Flows, number 2148047 (2016)Google Scholar
- 17.Venning, J., Lo Jacono, D., Burton, D., Thompson, M., Sheridan, J.: The effect of aspect ratio on the wake of the Ahmed body. Exper. Fluids 56(126) (2015)Google Scholar
- 18.Krajnović, S., Davidson, L.: Large-Eddy simulation of the flow around simplified car model. In: 2004 SAE World Congress, SAE Paper, number 2004-01-0227 (2004)Google Scholar