Experimental characterization of the unsteady natural wake of the full-scale square back Ahmed body: flow bi-stability and spectral analysis

Experimental characterization of the unsteady natural wake of the full-scale square back Ahmed... In recent years, the increasing interest in reducing the aerodynamic drag of vehicles, such as station wagons, minivans or buses, has led research to focus on the characterization of square back bluff geometries. In this paper, the results of an extensive experimental campaign on the full-scale well-known body of Ahmed et al. (1984) are presented, for two height-based Reynolds numbers, $$Re_{\rm H} = 5.1 \times 10^5$$ R e H = 5.1 × 10 5 and $$7.7 \times 10^5$$ 7.7 × 10 5 . Eighty-one measurement points were used to map the base pressure field, while the wake topology was investigated by means of a series of ten 2D Particle Image Velocimetry planes. These measurements clearly show that the wake presents a bi-stable behavior, characterized by a random succession of switches between two well-defined mutually symmetric configurations, confirming the results from Grandemange et al. (J Fluid Mech 722:51–84, 2013b. doi: 10.1017/jfm.2013.83 ) for the same model. For the presented results, the timescale of this phenomenon is of the order of $$800 \, V_{\infty} / H$$ 800 V ∞ / H . The sensitivity of the bi-stability to the yaw angle was also investigated, and considerations on how to take such a behavior into account in post-processing this kind of field are given. High-frequency measurements were also carried out with four piezoelectric transducers and a synchronized two-component hot-wire. The results show a low-frequency spectral activity: peaks at $$St_{\rm H} = 0.13$$ S t H = 0.13 and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at $$St_{\rm H} = 0.08$$ S t H = 0.08 was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal decomposition confirm the interpretation of the latter mode as the pumping of the recirculation bubble. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Experiments in Fluids Springer Journals

Experimental characterization of the unsteady natural wake of the full-scale square back Ahmed body: flow bi-stability and spectral analysis

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Publisher
Springer Berlin Heidelberg
Copyright
Copyright © 2015 by Springer-Verlag Berlin Heidelberg
Subject
Engineering; Engineering Fluid Dynamics; Fluid- and Aerodynamics; Engineering Thermodynamics, Heat and Mass Transfer
ISSN
0723-4864
eISSN
1432-1114
D.O.I.
10.1007/s00348-015-1972-0
Publisher site
See Article on Publisher Site

Abstract

In recent years, the increasing interest in reducing the aerodynamic drag of vehicles, such as station wagons, minivans or buses, has led research to focus on the characterization of square back bluff geometries. In this paper, the results of an extensive experimental campaign on the full-scale well-known body of Ahmed et al. (1984) are presented, for two height-based Reynolds numbers, $$Re_{\rm H} = 5.1 \times 10^5$$ R e H = 5.1 × 10 5 and $$7.7 \times 10^5$$ 7.7 × 10 5 . Eighty-one measurement points were used to map the base pressure field, while the wake topology was investigated by means of a series of ten 2D Particle Image Velocimetry planes. These measurements clearly show that the wake presents a bi-stable behavior, characterized by a random succession of switches between two well-defined mutually symmetric configurations, confirming the results from Grandemange et al. (J Fluid Mech 722:51–84, 2013b. doi: 10.1017/jfm.2013.83 ) for the same model. For the presented results, the timescale of this phenomenon is of the order of $$800 \, V_{\infty} / H$$ 800 V ∞ / H . The sensitivity of the bi-stability to the yaw angle was also investigated, and considerations on how to take such a behavior into account in post-processing this kind of field are given. High-frequency measurements were also carried out with four piezoelectric transducers and a synchronized two-component hot-wire. The results show a low-frequency spectral activity: peaks at $$St_{\rm H} = 0.13$$ S t H = 0.13 and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at $$St_{\rm H} = 0.08$$ S t H = 0.08 was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal decomposition confirm the interpretation of the latter mode as the pumping of the recirculation bubble.

Journal

Experiments in FluidsSpringer Journals

Published: May 6, 2015

References

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