Week 3 - External flow simulation over an Ahmed body.

 

Q1. Describe Ahmed's body and its importance.

Q2. Explain the reason for the negative pressure in the wake region. 

Q3. 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.

 

Ahmed body: The Ahmed body was first created by S.R. Ahmed. It is an important benchmark for aerodynamic simulation tools. It will be clearly shown in the sketch below that it has a length of 1.044 meters, the height of 0.288 meters and the width of 0.389 meters. It also has a 50-millimeter cylindrical legs attached to the bottom of the body and the rear surface which has a slant that falls of at 35 degrees. 

The flow for the model is turbulent which is based on velocity and body length. For the default mesh size in ANSYS-FLUENT we will use k−ε model and for the later stage k−ω SST model is used. 

Importance of Ahmed body 

The drag force is an important factor considering fuel consumption. As the drag force increases the fuel consumption also increases which in turn becomes expensive for the car owners. As the burning of fossil fuels becomes an issue, the manufacturers are pressing for fuel-efficient cars. One of the main factors considering the car is fuel-efficient or not is the car's geometry. 

Complex shaped cars are very challenging to model and it's difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools.  

Negative pressure in the wake region  

When the air moving over the vehicle is separated at the rear end, it leaves a large low-pressure turbulent region behind the vehicle which is known as the wake. This contributes to the formation of pressure drag which affects the vehicle performance. 

When the vehicle is moving at a certain velocity the fluid imparts viscous force to the vehicle if it is in the viscous sublayer of the boundary. When the fluid reaches the rear end of the vehicle it gets detached from the surface of the body causing negative pressure in the wake region. The movement of the fluid around the vehicles depends on the geometry of the car and the Reynolds number. 

 

 

Significance of point of separation 

The point of separation will occur when there is a discontinuity in the surface or a region of the negative pressure gradient (a sharp end or rear portion of the car). 

When the flow is attached to the body of the vehicle the viscous force will increase which will increase the pressure and a decrease in velocity before the rear end of the vehicle. As soon as the air encounters the sharp turn causing the velocity to decrease resulting in the negative pressure region which is in the wake region. The recirculation region happens beyond the point of separation. 

 
 

If the rate of change of pressure is large the mixing process will not occur and the boundary layer flow stops. 

The above figure shows that at the point of turning the flow gets separated from the boundary layer and there is a region of recirculation after which the flow attaches itself to the boundary layer. 

In order to reduce the separation of fluid from the boundary layer, it is advisable to have a smooth edge to reduce the effect of drag on the vehicle. 

  

Modeling approach 

Baseline simulation approach using k−ε model. 

• Ahmed's body is loaded into the Space claim. 
• A single enclosure is drawn across Ahmed's body. 
• Now the geometry is loaded for meshing. 
• A baseline simulation is set up with the default element size of Ansys (413.6 millimeters). 
• A named selection is created for the enclosure viz; inlet, outlet, symmetry. 
• To create the named selection for the car the enclosure faces are hidden, and box select is selected from the meshing toolbar and (car wall) name is given to it. 
• Ansys Fluent window is opened, and the geometry is loaded. 
• Steady-state, the density-based solver is selected. 
• Reference values are updated as per the prescribed conditions. 
• In the inlet boundary condition, inlet velocity is 25 msec and 300 K temperature are set. 
• Drag-coefficient plot and lift-coefficient plot is created by setting the reference as (car walls). 
• Hybrid initialization is selected. 
• The solution is initialized by hitting t=0. 
• The cut-plane-z is created along Ahmed's body. 
• The velocity contour, pressure contour is created along the cut-plane-z. 
• The animation video is created for the velocity contour.
   

Simulation with a finer mesh using k−ω SST. 

• The first simulation ran successfully. 
• The geometry is loaded into the Space Claim. 
• The region of local refinement is created along Ahmed's body. 
• As the body is perfectly symmetric, the simulation is executed by considering the only half body. This is the best practice where we can save on the number of cells and get the results faster as well.  
• The geometry is loaded for meshing. The multizone method is selected for the outer enclosure to provide hexahedral mesh which provides accurate results in simulation.  
• The element size for the first enclosure will vary as (100mm, 90mm, 80mm), the element size for the second enclosure will vary as (50mm, 45mm, 40mm), the element size of the leg of Ahmed's body will be 5mm, the first layer thickness, inflation layer, and the maximum thickness will be varied accordingly to imitate the experimental results. 
• The rest procedure is the same as above, the necessary contour plot and graphs have been uploaded respectively. 

Preprocessing and the solver settings 

 Case-1:  External flow simulation over Ahmed body by creating a single closure and using default mesh value of ANSYS-FLUENT 

Turbulence model: k−ε  

Ahmed body model 

 

Creating an enclosure across Ahmed body 

 

The three planes are normal to the x,y,z axes respectively. 

As the body is perfectly symmetric, we can simulate by considering only half the body. This is the best practice where we can save on the number of cells and get the results faster as well.  

To get half of Ahmed's body, the section mode is selected in Space Claim and the front view plain is selected to yield the desired result accordingly. 

 

Once the geometry part is complete, it is loaded in the meshing window to perform the baseline meshing. 

mesh is created as base line mesh in Default 416 mm as mesh element size

Simulating with a default mesh size of Ansys-Fluent 

Creating named selection to set up physics and boundary conditions. 

Inlet boundary condition (velocity inlet)

outlet boundary condition (pressure inlet) 

Results 

Residual plot 

 

Drag-coefficient plot 

 

Lift-coefficient plot 

 

Velocity contour 

 

Pressure contour 

 

The vector plot showing the wake region 

Drag-coefficient	0.394
Lift-coefficient	0.225

It can be inferred from the following figure that the results of the base simulation are distorted and do not capture the turbulent flow behind the body perfectly. This simulation captures the wake length, high velocity, low-velocity areas which can be used to further refine the mesh value around the Ahmed body and add a local refinement around. 

  

Case-2 (a) External flow simulation over Ahmed body with finer mesh in a double enclosure. 

Turbulence model: k−ω SST. 

First refinement 

A region of local refinement is added to the Ahmed body. 

Share Topology: It is set to share. It is used for sharing the information between the two enclosures and to make the mesh uniform. 

 

The geometry is loaded into the meshing window. 

Meshing 

The meshing involves the following steps 

Method 

Outside the box, the hexahedral mesh is used. The mesh is of the highest quality and gives higher accuracy. 

Multizone method:  It is used to create hexahedral meshes wherever possible and for other regions where different mesh types are used it will connect it properly. 

 

 

Sizing: It is defined as increasing the number of elements by decreasing the element size. The element size of the second closure is 0.05m  

Element size for the first enclosure: 100mm (about 3.94 in) or 0.1m 

Maximum size for the first enclosure: 100100mmmm or 0.1m 

Element size for the second enclosure:  50mm (about 1.97 in) 

Element size of pillars of Ahmed body: 5mm (about 0.2 in)                                                      

Number of elements: 1,92,628 

Inflation layer: It is the process of adding layers to the boundary to cover the boundary layer thickness fully. 

The first cell height predicted value is 5mm (about 0.2 in) 

Inflation layer: 5 

Last layer thickness: 15mm (about 0.59 in) 

Y+ value: 345 

 

Results 

Residual plot 

 

Drag-coefficient plot 

 

Lift-coefficient plot 

 

Velocity contour 

Pressure contour 

 

The vector plot showing the wake region 

 

Drag-coefficient	0.376
Lift-coefficient	0.290

 
 

Case 2(b): Second refinement 

Turbulence model: k−ω SST. 

The multizone was used for the second area. The number of element size for the outer enclosure is taken as 90mm (about 3.54 in) whereas the element size for the inner closure is taken as 45mm (about 1.77 in). Face sizing was implemented for the pillars of Ahmed's body (5mm). 

Inflation layer 

First layer thickness: 3mm (about 0.12 in) 

Number of layers: 10 

Last layer thickness:  15.4793m 

Number of elements: 253152 

The value of y+ is around 207. 

 

 

Results 

Residual plot 

 

Drag-coefficient plot 

 

Lift-coefficient plot 

 

Velocity contour 

 

Pressure contour 

 

The vector plot showing the wake region 

 

Drag-coefficient	0.362
Lift-coefficient	0.288

 

 

Case 2(c): Third refinement 

Turbulence model: k−ω SST. 

Element size in the first enclosure: 80mm (about 3.15 in) 

Element size in the second enclosure: 40mm (about 1.57 in) 

Element size of the bottom leg of Ahmed body: 5mm (about 0.2 in) 

Number of inflation layer: 10 

First layer thickness: 0.15mm (about 0.01 in) 

Last layer thickness: 0.77mm (about 0.03 in) 

Number of elements: 348448 

Y+: 10 

 

 
 

Results 

Residual plot 

 

Drag-coefficient plot 

 

Lift-coefficient plot 

 

Velocity contour 

 

Pressure contour 

 

The vector plot showing the wake region 

 

Drag-coefficient	0.343
Lift-coefficient	0.246

 

 

Demonstration of grid independence test 

Comparison of all cases 

Steady-state, Turbulent flow, Density-based, Fluid Material-Air
Grid Independence Test	Turbulence-model	Number of elements	Lift-Coefficient	Drag-Coefficient
Baseline mesh	k−ε	83781	0.225	0.394
First refinement	K−ω SST	192628	0.290	0.376
Second refinement	K−ω SST	253152	0.288	0.362
Third refinement	K−ω SST	348448	0.246	0.343

  

It can be inferred that the imitation of the experimental result is obtained during the second refinement. Thus, the grid independence test saves a lot of computational time and convergence can be achieved by using smaller cell sizes for calculation. 

Conclusion 

• It can be inferred that the grid independence test is performed to know the smaller cell size for convergence. 
• Apart from mesh refinement, convergence depends on the Y+ value, inflation layer, it is advisable to use k−ω SST as the turbulence value because it calculates the values well near the wall. 
• As the setup is refined drag coefficient, lift coefficient decreases. 
• Y+ plays an important role in determining the value of drag. 
• The negative pressure in the wake region is primarily because there is no flow in that region. This can be further attributed to various factors such as separation and recirculation which is evident from the vector plot near the wake region. 
• The point of separation occurs at the rear end of the vehicle, the separation occurs beyond this point where it can be observed that the flow recirculates and attaches it to the boundary layer.