8/12/2019 MSc CFDAutoAssignment2014

1/3

CFD Short Course Assignment: External flow over an

Ahmed body

4th March 2014

Introduction

The Ahmed body has been used extensively as a benchmark for CFD solvers since the experimentaldata was published by Ahmed et al in 1984 [1]. The geometry of the Ahmed body is shown in Figure1. The flow over the Ahmed body features a sharp change in the drag, occuring at ~300 slant angle(Figure 2) due to the change in the flow structure.

The case has been adopted as a standard benchmark by the European Research Communityon Flow, Turbulence and Combustion (ERCOFTAC). The modelling challenges are associated withthe complex structure of the flow over the slant (Figure 3), with subcritical angles of the slant beingtypically more difficult to model.

In this assignment you are asked to conduct CFD modelling of a flow around Ahmed body usinga CFD solver and a RANS model of your choice at a sub-critical slant angle of 25o. The accuracy ofyour results should be evaluated through a comparison with the experimental data of of Ahmed etal [1] and Lienhart and Becker [2, 3, 4]. The key parameters for comparison are:

The structure of the flow, with a particular emphasis on the flow over the slant and corner vor-tices.

The total drag coefficient

Profiles of U and W velocity at selected locations over the slant x=-203; -183; -163; -143; -123;-103; -83; -63; -43; -23; -3 (the origin of the coordinate system is at ground level in the symmetryplane at the end of the model).

Please pay particular attention to the following points:

Choice of the size of the computational domain.

Choice of the inlet boundary conditions.

Meshing requirements and their correspondence to the choice of the turbulence model.

Sensitivity of the results to the computational mesh.

Figure 1: Geometry of the Ahmed body

1

8/12/2019 MSc CFDAutoAssignment2014

2/3

Figure 2: Ahmed body drag transition

2

8/12/2019 MSc CFDAutoAssignment2014

3/3