Ahmed Body

NOTE: Consider building an ML model on this. Show something to mehdi.

S. R. Ahmed first described the Ahmed body in 1984 ¹1Unresolved citation key: ahmedSalientFeaturesTime1984.

The wake has three features. Recirculation region A forms where flow separates at the vertical back surface. Recirculation region B forms at the base. C-pillar vortices form where boundary-layer vorticity rolls over slant edges.

The wake varies with slant angle. Flow stays attached below 12 degrees with low, two-dimensional drag. Between 12 and 30 degrees, flow becomes three-dimensional. Vortices peak at 30 degrees, raising drag. Flow separates fully past 30 degrees. Drag drops and vortices weaken.

Aspect Ratio affects the wake. Wider bodies stop reattaching at 25 degrees. Critical angle drops as aspect ratio increases.

Description of the test case: The Ahmed body (Fig. 1) was first defined in experimental work by Ahmed [1]. Two configurations with slant angles of 25 degrees and 35 degrees serve as the test case. Becker, Lienhart, and Stoots [2,3] provided detailed LDA measurements in the LSTM low-speed wind tunnel. The tunnel has a cross-section of 1.87 x 1.4 square meters for width and height. The bulk velocity reaches 40 meters per second. The test section was 3/4 open with only a ground plate present. The distance between the body and the ground plate is 50 mm. Ahmed used a flow velocity of 60 meters per second in the original experiment [1]. The Reynolds number reached 4.29 million based on model length.

CFD cases

https://www.cfdsupport.com/ahmed-body-tcfd-benchmark.html

Experimental techniques

²2Unresolved citation key: lienhartFlowTurbulenceStructures shows oil streaks that visualize the flow on the surface.

³3Unresolved citation key: makelaExperimentalStudyFlow shows a great superimposition of CFD streamlines with actual experimental pictures.

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