This content is not included in your SAE MOBILUS subscription, or you are not logged in.
Experimental Investigation of the Bi-Stable Behavior in the Wake of a Notchback MIRA Model
ISSN: 0148-7191, e-ISSN: 2688-3627
Published April 02, 2019 by SAE International in United States
This content contains downloadable datasetsAnnotation ability available
This paper reports an experimental investigation of the wake flow behind a 1/12 scale notchback MIRA model at Re = UL/ν = 6.9×105 (where U is free-stream velocity, L the length of the model and ν viscosity). Focus is placed on the flow asymmetry over the backlight and decklid. Forty pressure taps are used to map the surface pressure distribution on the backlight and decklid, while the wake topology is investigated by means of 2D Particle Image Velocimetry. The analysis of the instantaneous pressure signals over the notch configuration clearly shows that the pressure presents a bi-stable behavior in the spanwise direction, characterized by the switches between two preferred values, which is not found in the vertical direction. Based on the barycenter of momentum deficit, the instantaneous velocity fields in the plane z/H = 0.8 (H height of the model) can be sorted into three patterns, including two bi-stable states and one switch state, corresponding to the result of the instantaneous pressure signals. The sequence of these asymmetric states is random. In order to enable a deep insight of the bi-stable states, conditional averaging on the pressure data and the unsteady wake flow based on PIV snapshots has been conducted to extract the two distinct states and switch state, and the average pressure contours of the two bi-stable states are almost reflection symmetrical and their average wakes are also nearly symmetrical, meaning this bi-stable behavior is statistically symmetric. However, this bi-stable wake dynamic is not found behind the trunk in the plane z/H = 0.44. Besides, this bi-stable phenomenon is of a long timescale with the order of 300L/U, which is hundreds of times the common vortex shedding.
CitationYan, G., Xia, C., Zhou, H., Zhu, H. et al., "Experimental Investigation of the Bi-Stable Behavior in the Wake of a Notchback MIRA Model," SAE Technical Paper 2019-01-0663, 2019, https://doi.org/10.4271/2019-01-0663.
Data Sets - Support Documents
|[Unnamed Dataset 1]|
|[Unnamed Dataset 2]|
- Carr, G., “Influence of Rear Body Shape on the Aerodynamic Characteristics of Saloon Cars,” MIRA Report 1974/2, 1974.
- Nouzawa, T., Haruna, S., Hiasa, K., Nakamura, T. et al., “Analysis of Wake Pattern for Reducing Aerodynamic Drag of Notchback Model,” SAE Technical Paper 900318, 1990, doi:10.4271/900318.
- Nouzawa, T., Hiasa, K., Nakamura, T., Kawamoto, A. et al., “Unsteady-Wake Analysis of the Aerodynamic Drag of a Notchback Model with Critical Afterbody Geometry,” SAE Technical Paper 920202, 1992, doi:10.4271/920202.
- Gilhome, B., Saunders, J., and Sheridan, J., “Time Averaged and Unsteady Near Wake Analysis of Cars,” SAE Technical Paper 2001-01-1040, 2001, doi:10.4271/2001- 01-1040.
- Jenkins, L., “An Experimental Investigation of the Flow Over the Rear End of a Notchback Automobile Configuration,” SAE Technical Paper 2000-01-0489, 2000, doi:10.4271/2000-01-0489.
- Lawson, N., Garry, K., and Faucompret, N., “An Investigation of the Flow Characteristics in the Bootdeck Region of a Scale Model Notchback Saloon Vehicle,” Journal of Automobile Engineering 221(6):739-754, 2006, doi:10.1243/09544070JAUTO155.
- Gaylard, A., Howell, J., and Garry, K., “Observation of Flow Asymmetry Over the Rear of Notchback Vehicles,” SAE Technical Paper 2007-01-0900, 2007, doi:10.4271/2007-01-0900.
- Okada, Y., Nouzawa, T., Nakamura, T., and Okamoto, S., “Flow Structures above the Trunk Deck of Sedan-Type Vehicles and Their Influence on High-Speed Vehicle Stability 1st Report: On-Road and Wind-Tunnel Studies on Unsteady Flow Characteristics that Stabilize Vehicle Behaviour,” SAE Technical Paper 2009-01-004, 2009, doi:10.4271/2009-01-0004.
- Sims-Williams, D., Marwood, D., and Sprot, A., “Links between Notchback Geometry, Aerodynamic Drag, Flow Asymmetry and Unsteady Wake Structure,” SAE Int. J. Passeng. Cars - Mech. Syst. 4(1):156-165, 2011, doi:10.4271/2011-01-0166.
- Wood, D., “The Effect of Rear Geometry Changes on the Notchback Flow Field,” Ph.D. thesis, Loughborough University, 2015.
- Mair, W. and Wilkin, S., “Asymmetric Distribution of Base Pressure on an Axisymmetric Body,” Aeronautical Journal 82:273-275, 1978.
- Fabre, D., Auguste, F., and Magnaudet, J., “Bifurcations and Symmetry Breaking in the Wake of Axisymmetric Bodies,” Physics of Fluids 051702, 2008, doi:10.1063/1.2909609.
- Herry, B., Keirsbulck, L., Labraga, L., and Paquet, J., “Flow Bistability Downstream of Three-Dimensional Double Backward Facing Steps at Zero-Degree Slideslip,” Journal of Fluids Engineering 133:054501, 2011, doi:10.1115/1.4004037.
- Grandemange, M. and Cadot, O., “Reflectional Symmetry Breaking of the Separated Flow over Three-Dimensional Bluff Bodies,” Physical Review 035302, 2012.
- Grandemange, M., Gohlke, M., and Cadot, O., “Turbulent Wake Past a Three-Dimensional Blunt Body. Part 1. Global Modes and Bi-Stability,” Journal of Fluid Mechanics 722:5-84, 2013, doi:10.1017/jfm.2013.83.
- Grandemange, M., Gohlke, M., and Cadot, O., “Bi-Stability in the Turbulent Wake Past Parallelepiped Bodies with Various Aspect Ratios and Wall Effects,” Physics of Fluids 095103, 2013, doi:10.1063/1.4820372.
- Volpe, R., Devinant, P., and Kourta, A., “Experimental Characterization of the Unsteady Natural Wake of the Full-Scale Square Back Ahmed Body: Flow Bi-Stability and Spectral Analysis,” Experiments in Fluids 56:-99, 2015, doi:10.1007/s00348-015-1972-0.
- Pavia, G., Passmore, M., and Sardu, C., “Evolution of the Bi-Stable Wake of a Square-Back Automotive Shape,” Experiment in Fluids 59:20, 2018, doi:10.1007/s00348-017-2473-0.