Flow over Ahmed Body using Ansys FLUENT

Abstract:

The aerodynamic behaviour of Ahmed body is numerically investigated in the present work. Commercial FLUENT software is used for CFD simulation using a turbulent model. Grid dependency test is performed to validate the results against the experimental data. Computation of drag and lift fairly agrees with the numerical simulation.

Theory:

The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

Modeling Airflow Over an Ahmed Body

In the Airflow Over an Ahmed Body verification model, our Ahmed body has a 25-degree slant and is placed in the following domain, measuring 8.352-by-2.088-by-2.088 meters, to compute the flow field.

The front of the body is placed at a distance of 2 car lengths (2L) from the flow inlet. To reduce the computational cost, a symmetry plane is introduced to model half of the model.

The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy in addition to the velocity field. For this simulation, we need a larger mesh size than what is usually common to resolve the turbulent flow. More specifically, we use a finer mesh downstream of the model to capture the wake zone.

The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields.

The total drag coefficient of the Ahmed body is the key measurement for this simulation. It is made up of measurements for the pressure coefficients in the front, slant, and base of the body as well as the body’s skin friction. In the results of our simulation, the total drag is very well predicted but the individual measurements deviate from the experimental results in varying amounts.

A few different factors cause these deviations in data. For the front and roof of the body, wall functions used in the simulation aren’t effective at predicting the flow transitions observed in the experiments.

In terms of deviations in the slant coefficient data, we see the flow along the slant shown by streamlines in the figure below. The thickness of these streamlines is determined by turbulent kinetic energy.

In the experimental data, these lines show that the flow along the slant is attached almost everywhere and that there are two small recirculation zones behind the base. In the simulation results, this effect is captured but the extent of the recirculation zones is over-predicted.

The pressure drag for the slant coefficient is very sensitive to the exact shape and location of the recirculation regions, which accounts for the deviation in measurements.

Although the data has quantitative differences, it is qualitatively equal to experimental results because the total drag coefficients are so close. There may be deviating details in the smaller data but the simulation still captures the major features of the flow over an Ahmed body. This simulation is more than adequate for calculating the overall drag coefficient.

Problem statement:

1. Describe Ahmed's body and its importance.

2. Explain the reason for the negative pressure in the wake region.

3. Explain the significance of the point of separation.

Expected Results:

1. Velocity and pressure contours.

2. The drag coefficient plot for a refined case. ( For velocity of 25m/sec, the drag coefficient should be around 0.33).

3. The vector plot clearly showing the wake region.

4. Perform the grid independency test and provide the values of drag and lift with each case.

Geometrical dimension of Ahmed body:

Different views of ahmed body with dimension is shown below

Problem solving and modelling approach

meshing methodology

Geometry case set up:

The model is loaded in ansys space claim from stp file format.

The ahmed body is enclosed by a virtual wind tunnel created by enclosure option in prepare physics toolbox. Uncheck the symmetric option.

Create an inner box using the enclosure option.

Select a plane perpendicular to the ahmed body to check the cross-section view.

Checking for interference. Separate the outer enclosure from the inner box and also the ahmed body from the inner box.

After interference checking, click on split body command to split the ahmed body and the enclosure in one half.

split1-outer enclosure

split2-inner enclosure with ahmed body

split3-supressing Ahmed body

Meshing geometry set up:

Baseline mesh

• Open the meshing console.
• Click on mesh option on the left side of work tree diagram.
• Click on generate mesh.
• A general mesh pattern will be created.
• Create section plane on one half of the geometry.

Refined meshing

• Open meshing console in ansys fluent.
• Create named section of the entire geometry
• inlet
• 
• outlet
• 
• symmetry
• 
• car wall
• 
• Click on the mesh option on the left-hand side of work tree diagram.
• A general mesh will be created.
• 
• To refine mesh, click on automatic methods. Select the outer wind tunnel box. Go to methods and select multi zone option. Click on generate mesh.
• 
• To give required size, click on mesh and give the required mesh size.
• 
• To refine the inner box, click on sizing option by right click on mesh. Select the inner box. Give the required mesh size. Click on generate mesh.
• 
• To refine the Ahmed body part. Hide all the faces other than the car wall.
• Use box select tool to select the faces of car wall.
• 
• Give the required size of mesh. Generate mesh.
• 
• Now to refine the cylindrical pillars of Ahmed body, click on sizing, select face selection option. Use single select tool. Select the cylindrical portion.
• 
• Give the sizing. Click on generate mesh.
• Now go to mesh. Click on inflation. Select geometry from named section. Choose car wall. Give the no of layers and total thickness based on y+. generate mesh.
• 

Setting up pre-processing physics:

• Select density-based solver
• 
• Viscous model is selected as k-epsilon model.
• 
• Setting up boundary conditions.
• Set up inlet condition as 25m/s.
• 
• Set up the reference values.
• 
• Create report definition of drag and lift plot.
• Create a plane using point and normal method.
• 
• Set up initialization by choosing hybrid initialization method.
• 
• Create pressure and velocity contour plot across the cut plane.
• Create animation plot of velocity and pressure contour.
• Set the no of iteration to run the fluent solver.

Computing first cell thickness from y+ factor:

The y+ factor is selected in between 30 to 300 for better capturing the effect of drag and lift at the boundary regions of ahmed body.

Certain computation regarding the cell thickness and Reynolds no from the given data

Length of body=1.044m

Area of Ahmed body=0.05601sqm

Inlet velocity=25m/s

Density of air=1.225kg/m3

Viscosity=1.7894e-5

`Re=(UρL)/(μ)`

`C_f=0.026/(Re^(1/7) )`

`τ_wall=1/2 ρU^2 C_f`

`U_friction=√(τ_wall)/(ρ)`

`∆s=(y^+ μ)/(U_friction ρ)`

Post processing physics for baseline mesh:

Mesh symmetry case

Iteration convergence

Drag plot

Lift plot

Post processing velocity vector plot

Post processing pressure vector plot

Grid independency test:

In this study we have set about 4 cases, by refining the mesh at different sizes and also at different y+ values. The table below shows the different cases considered for grid independence test.

Outer wind tunnel	Inner box enclosure	Ahmed body	Nodes	Elements
120mm	50mm	5mm	32546	109022
100mm	45mm	5mm	44857	140606
80mm	40mm	5mm	63842	188238
60mm	35mm	5mm	97716	267184

The table below shows different y+ set up taken:

Y+	1st cell thickness	Total thickness	No of layers	Cd	Cl	Drag Error(%)
70	1.028mm	3.069mm	7	0.3928	0.198167	15.98
100	1.46mm	7.533mm	10	0.38088	0.23094	13.36
150	2.196mm	19.579mm	13	0.376958	0.27345	12.45
200	2.929mm	37.605mm	15	0.37049	0.25378	10.92

Post processing physics for grid independence test refined mesh:

Case1:

Outer wind tunnel: 120mm

Inner enclosure box: 50mm

Ahmed body: 5mm

Y+=70

Mesh symmetry case

Iteration convergence

Drag plot

Lift plot

computed drag and lift values

Post processing velocity vector plot

Post processing pressure vector plot

Case2:

Outer wind tunnel: 100mm

Inner enclosure box: 45mm

Ahmed body: 5mm

Y+=100

Mesh symmetry case

Iteration convergence

Drag plot

Lift plot

computed drag and lift values

Post processing velocity vector plot

Post processing pressure vector plot

Case3:

Outer wind tunnel: 80mm

Inner enclosure box: 40mm

Ahmed body: 5mm

Y+=150

Mesh symmetry case

Iteration convergence

Drag plot

Lift plot

computed drag and lift values

Post processing velocity vector plot

Post processing pressure vector plot

Case4:

Outer wind tunnel: 60mm

Inner enclosure box: 35mm

Ahmed body: 5mm

Y+=200

Mesh symmetry case

Iteration convergence

Drag plot

Lift plot

computed drag and lift values

Post processing velocity vector plot

Post processing pressure vector plot

Negative pressure gradient

When fluid velocity increases, the pressure decrease. The wake region is formed due to adverse pressure in the rear end causes the boundary layer to separate out from the body. The wall shear stress and velocity becomes zero and wake formation starts. These wakes are nothing but small vortices that create negative pressure. These create circulation of velocity and induces drag forces.

Point of seperation

Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow.When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag.

Conclusion

1. From running a baseline simulation, the post CFD results gave inaccurate results at the region of mesh overlapping.
2. The grid independency test shows best results close to the experimental data. With refining the mesh size with y+ factor, the error is significantly reduced.
3. The y+ is chosen in between a stable range to give a correct prediction of velocity and pressure contour plots.
4. The flow seperation and appearence of vortices gets more prominent as y+ is increased. As seen in baseline meshing no wake formation is captured properly, on refining the mesh, the flow is captured properly.