Abstr:Ahmed body

Application Area 1: External Aerodynamics

Application Challenge AC1-05

Abstract

A basic ground vehicle type of bluff body is investigated. The body consists of three parts: a fore body, a mid section and a rear end.

Two experiments are available:

A first one (Exp1) is performed at DLR-Göttingen in a wind tunnel at a significant Reynolds number (4.9 million for 60 m/s). The model is mounted near the wall of the wind tunnel, such that a ground effect is present. However, a ground plate is placed between the model and the floor of the wind tunnel in order to minimize this effect. The angle of the rear end slope is adjustable, between 0 and 40° with a 5° step. More details are available for angles of 5, 12.5 and 30°. About 210 pressure locations are available on the fore body, 83 in the mid section and 450 on rear ends. Wall flow visualizations are available. Detailed wake surveys are performed with 10 holes probes. Drag measurements are provided.

A second, more recent experiment (Exp2) is provided by the Erlangen LSTM lab within the MOVA “Models for Vehicle Aerodynamics” European project (TU Delft, Univ. Of Manchester UMIST, LSTM, Electricité de France, AVL List, PSA Peugeot-Citroën). It concerns the same model as in the Ahmed study, running at 40m/s. Two-component LDV is used. Hot wire measurements are performed at 0.38 model length upstream of the model and in the boundary layer. Mean values and turbulence measurements (second and third order moments) are provided for 25° and 35° slant angle. 7,500 discrete positions are provided lying on 13 unique planes. Pressure measurements are performed on the rear part of the model.

The basic shape of the so-called “Ahmed body” contains all the important features of real road vehicles: a main body, followed by a fully separated region. Prediction of separated flows is one of the more difficult task of CFD. The parametric variation of the angle of the rear part permits the simulation of various configurations relevant to real car characteristics. The complex 3D wake structure is also a challenging problem for CFD. This data base also contains drag results that are essential to prediction for practical purposes. The influence of turbulence models as well as the influence of the mesh and numerical scheme can then be tested at several levels: wall pressure, wake structure and force drag.

The second set of experiments provides complementary results concerning the turbulent quantities that will be useful for a detailed analysis of turbulence models.

The Design and Assessment parameters (DOAP's) for judging a CFD simulation are for this case as follows:

The first DOAP is clearly the drag coefficient computed for several slant angles.

A second DOAP is the topology of the flow, particularly the wall streamlines. Comparisons between computations and flow pattern from visualizations on the slant part will be useful. The structure of the flow will also be analyzed from the 3D representations in order to visualize the vortices.

A third DOAP will be the static pressure distributions.

A fourth DOAP will be the mean velocity distributions at several locations around and downstream of the body.

Lastly, the turbulent quantities can be considered as a DOAP; this however is more a guide for modelling than for a direct validation of codes.

The Ahmed body configuration was a test case for 2 ERCOFTAC-IAHR workshops ( 2001 in Darmstadt and 2002 in Poitiers) and CFD results from 15 teams obtained for these workshops with various simple and more advanced RANS models and one LES are reported in the original test-case version produced in 2004. An update was provided in 2024 by F.R. Menter by adding LES results with accompanying documentation for the 25° slant- angle case based on the Menter et al (2024) paper.

Reference

Menter, F.R., Hüppe, A., Flad D., Garbaruk A.V., Matyushenko A:A:, Stabnikov A:S: Large eddy simulations for the Ahmed car at 25° slant angle at different Reynolds numbers. Flow, Turbulence and Combustion, 112, 321-343, (2024).