Week 3 - External flow simulation over an Ahmed body.

AIM: External flow simulation over an Ahmed body.

We want to simulate the flow at outer faces of the Ahmed body. That is where the mesh will created and we donot want mesh inside the Ahmed body.

INTRODUCTION THEORY:

AHMED BODY: Ahmed body is a 3D model that is reprasentative of standard Passenger car. The reason why this model is developed is to help in validating CFD codes. This is research based model which is used for experimental analysis.

PROCEDURE: Create an enclosure.

ENCLOSURE: is an single volume as spaceclaim is taken asingle volume bu substracting the volume of Ahmed body from the volume of enclosure.the single volume will generate automatically because we have use enclosure option.

Prepare > Enclosure > click on Ahmed body > uncheck the symmetry option > give dimensions > click on tick option.

* disabling of symmetry option because we want more control over the geometry because of that we will enter all the dimensions of the enclosure.

*

Preprocessing and the solver settings

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

Turbulence model:

k epsilon + standard + enhamced wall treatment

Ahmed body model

Creating an enclosure across Ahmed body

Creating a plane section on the origin

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 SpaceClaim 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.

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 outlet)

Symmetry boundary condition

Creating named selection for car walls

To create the named selection for car walls we need to hide the faces of the enclosure by hitting function key f8.

Performing meshing with default mesh size in ANSYS-FLUENT

Element size: 0.4136m

Number of elements: 83781

The geometry is cut into two halves by using a section plane

Thus the mesh is created inside Ahmed's body which is not good for simulation because the flow outside Ahmed's body is taken into account. It can be fixed by going back to Spaceclaim, one of the copies should be suppressed for physics.

The required geometry is then loaded into the ANSYS-FLUENT window.

Simulation procedure

Setting up of physics

Reference values

The frontal area of Ahmed body

Material properties

Viscous model

> Select boundary conditions.

> inlet

> outlet

> Create the difinitions for the various Quantities.

1) drag coefficient

Definitions > new > force report > drag > select name as Drag > select car wall > print to console >ok

2) lift coefficient

Definitions > new > force report > lift > select name as lift > select car wall > print to console > ok.

3) Skin friction coefficients

Definitions > new > area weighted average > select wall fluxes as field variable > select skin friction coefficient in the sub domain of field variable > car wall > print to console > ok.

> Disable the convergence limit for all the residuals plot

Hybrid initialization is selected and the solution is initialized.

Creating a cut plane to see the velocity change along Ahmed's body.

Creating contour plot along cut plane z

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

• Since the Ahmed body is symmetric about the X-Y plane (shown above), the body is to be split into two symmetric half and the 'symmetry' boundary condition is to be applied. This helps to save a lot of computational cells that can be used to further refine the wake region.
• The given problem is external aerodynamic simulation. A wind tunnel is created over the Ahmed body using the 'Enclosure' option in Spaceclaim. This outer enclosure contains a smaller, inner enclosure (for local refinement) which in turn contains the surface of the Ahmed body. While creating the enclosures, the 'Interference' option was used to remove any geometric interference and common regions created by the double enclosures. The 'topology' option is set to 'Share' to ensure data is shared between different enclosures. The dimensions of the enclosure are shown:

Outer Enclosure and Inner Enclosure:

• The outer enclosure is of bigger dimensions to make the inflow and outflow boundary conditions not be influenced by the presence of the Ahmed body. But it is impractical to make the whole domain thoroughly refined because it would result in a waste of computational resources. Hence a smaller inner enclosure is created for the purpose of local refinement. It is of immense importance to refine the mesh much more, closer to the geometry, and is shown below.

> Create enclosures.

Prepare > enclosure > click on ahmed body > disable the symmetry option > enter dimensions as shown in below images > click on green colour tick mark.

similarly create small enclosures.

> After creating the two enclosure generate the planes.

Design > Plane tool > Click on origin > getting three sets of planes > select xy plane .

> split the body in two halves by using the split body command.

Design > split body > select entire surface using box selection > select cutter > select xy plane > remove all other surfaces using the command REGIONS TO REMOVE.

* select cutter

* select command REGIONS TO REMOVE as shown in the image below.

> Perform interference and enable share topology.

• Shown below is the final geometry that is to be used for meshing. Note that the 'split body' command has already been used on the geometry.
  
  

MESHING:

1) Multizone

2) body sizing

3) Face sizing

4) Inflation

• Next, the mesh module is opened. Since the geometry of the outer enclosure is made of straight lines, the 'hex' mesh is used. The inner enclosure holds the surface of the Ahmed body and hence is 'triangle dominant'. The 'Multizone' mesh method is applied to the outer enclosure to connect between different mesh types. The base mesh size is set to 0.1 meters.
• The inner enclosure is given 'mesh body sizing' and is set to a size of 0.05 meters. Mesh 'face sizing' with a size of 0.005 meters is given on the cylindrical stands of the Ahmed body to prevent geometry distortion at the stands. To capture the near-wall physics accurately, a boundary layer mesh (Body fitted Mesh) (or) Mesh inflation is provided to the geometry. To come up with the right mesh size, the following calculations are done. Shown below is the mesh obtained.

> Perform the meshing with element size of 100mm for whole body.

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

> Perform Mulizone meshing for the outer enclosure with element size of 100mm.

Mesh > Right click > method > select outer body by using body selection > select apply on geometry selection > method > multizones > element size as 100mm > go to sizing under mesh > select maximum size as 100mm for getting the evenly sized mesh > generate mesh.

> Below images shows the multizone for the outer body.

2) BODY SIZING:It is defined as increasing the number of elements by decreasing the element size. The element size of the second closure is 50mm.

Use the diffrent mesh size for smaller box by hiding the faces of outer box and then select the smaller box using box selection and perform meshing with element size of 50mm.

Right click on mesh > Insert > sizing > select internal enclosure by hiding the outer encloure > change element size to 50mm > update mesh.

3) FACE SIZING:

The four cylinder supports those are supporting the ahmed body are not captured well meaning the shape of the cylinder from circular it become polygon to eleminate this use mesh element size of 5mm for four supports of the Ahmed body.

Right click on mesh > Insert > sizing > select face mode > Select two faces those are circular > give element size as 5mm > update.

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

Mesh > inflation > use automatic inflation as all faces choosen name selection > name selection as car wall > First layer height as 0.4mm > maximum layers as 6.

SETUP:

>

SOLVER	STEADY STATE, PRESSURE BASED
VISCOUS MODEL	K-EPSILON STANDARD ENHANCED WALL TREATMENT
MATERIAL	AIR
BOUNDARY CONDITIONS(INLET)	VELOCITY INLET AS 25m/s
Boundary conditions(outlet)	pressure outlet as 0 gauge pressure.
boundary conditions (car wall)	no slip condition

How to calculate y+ value.

Reports > surface integrals > Area weighted average > field variable as turbulence > sub domain as wall y plus > car wall > compute > y+ value as 9.524.

The y+ value depends upon the mesh element size > if mesh element size decreases > then the Y+ value also decreases.

> RESIDUAL PLOTS:

1. The number of iterations was set to 500.
2. But it is observed that the residuals have fallen below the specified tolerances and the solution attains convergence in ~84 iterations.

> DRAG:

> LIFT:

> SKIN FRICTION COEFFICIENT

1) The formation of the low-pressure wake region dictates how high is the drag coefficient. The steady-state drag coefficient is found to be 0.3499

2) The formation of vortices in the wake results in the lift. The steady-state lift coefficient is found to be 0.1924

> Wall shear stress and the point of separation:

) Wall shear stress and the Point of separation:

1. Boundary layer separation occurs when there is an adverse pressure gradient and zero normal velocity gradient at the wall (i.e zero wall shear stress) simultaneously. The possibility of this occurring in the Ahmed body is at the slant surface.
2. But, from literature, it is known that the flow does not separate at slant angles less than 30.
3. The LHS plot is the wall shear stress plot along the entire length of the Ahmed body. The plot on the RHS is the zoomed-in range of the wall shear stress close to zero ranging from 0.0 Pa to 0.2 Pa.
4. The result obtained is in accordance with the literature. The minimum wall shear stress obtained along the surface of the entire surface is always greater than 0.1.
5. However, the flow separation occurs right at the end of the slant surface at the rear of the geometry. This flow separation results in a recirculation region of low pressure and determines the coefficient of drag of the body.

POST PROCESSING:

velocity contour at cut plane z.

Pressure contour at cut plane z:

Pressure at car wall:

Velocity vector at cut palne z:

1. The contours are generated on an X-Y plane at Z = 0.
2. Just behind the slanted surface of the Ahmed body, a recirculation region is formed. This will be demonstrated more clearly by the vector plots in the coming section. The recirculation region has a lower pressure core and is represented by the light green patches in the contour. The scale defined in the contour represents the gauge pressure and hence shows negative pressure values at the recirculation region.
3. The pressure is maximum at the leading boundary of the Ahmed body because the flow is stagnated here. Also, at points where there exists curvature such as the filleted region at the leading boundary and the sharp beginning of the slant, there exist local low-pressure regions due to a sudden change in the geometry.
4. The velocity contour shows the flow over the slanted region. The flow does separate till the end of the slanted surface, after which separation occurs. A low-velocity region is formed at the recirculation region. This low-pressure region extends far into the wake of the car along with the formation of counter-rotating vortices that is shown in the next section.
5. At the different curvatures on the surface of the Ahmed body, where local low pressure exists, the regions have relatively higher velocities.

> YZ planes at diffrent locations:

> velocity at yz plane at a distance of 1.044m from the origin.

> Velocity vector at yz plane at a distance of 1.044m from the origin.

> velocity vector at distance 1.1m from the origin

> velocity vector at a distance of 1.2 m from the origin

> velocity vector at distance of 1.5 m from the origin

> velocity vector at a distance of 2 m from the origin

> velocity vector at distance of 2.3 m from the origin

> Velocity vector at diffrent location in yz plane.

1. These vector plots are generated at specific Y-Z plane probes as shown above at distances 1.044, 1.1m, 1.2m, 1.3m, 1.5m, 2m, 2.3m respectively from the origin. The Origin is located at the front end of the Ahmed body.
2. It is observed that with increasing distance in the wake, the vortices become fully developed. The distance up to which the vortices influence the wake is slightly above 2 meters.
3. It is to be noted that the images shown above represent only one vortex as the geometry was split and symmetry BC was applied. Hence, it can be assumed that a similar vortex is formed on the right-hand side of the displayed vortex.
4. finally, the wake dies out and the flow is reattached. At a distance of 2.5 meters, it is observed that the flow is the usual freestream flow.

> Velocity profiles at diffrent line probes.

> create a line between two points having coordinates of point 1 as (1.044,-0.05,0) in m and point 2 as (1.044,0.288,0) in m.

> create a line between two points having coordinates of point 1 as (1.1,-0.05,0) in m and point 2 as (1.1,0.288,0) in m

> create a line between two points having coordinates of point 1 as (1.2,-0.05,0) in m and point 2 as (1.2,0.288,0) in m

> create a line between two points having coordinates of point 1 as (1.5,-0.05,0) in m and point 2 as (1.5,0.288,0) in m

> create a line between two points having coordinates of point 1 as (2,-0.05,0) in m and point 2 as (2,0.288,0) in m

> create a line between two points having coordinates of point 1 as (2.5,-0.05,0) in m and point 2 as (2.5,0.288,0) in m.

> Velocity profiles at these diifrent lines.

velocity graph at line which is at distance of 1.044 m from the origin.

velocity graph at line which is at a distance of 1.1 m from the origin.

> Velocity graph at line which is at distance of 1.2m from the origin.

> Velocity graph at line which is at a distance of 1.5 m from the origin.

> Velocity graph at a line which is at a distance of 2m from the origin.

> Velocity graph at line which is at distance of 2.3m from the origin.

1. Various line probes are drawn at distances of 1.044.1.1 m, 1.2 m, 1.5 m, 2 m, 2.3 m respectively from the origin. The Origin is located at the front end of the Ahmed body.
2. The velocity profile is drawn at each of these lines probes wrt the distance in the Y direction.
3. For the first three probes, the velocity reduces from the freestream speed to a minimum at the recirculation region and climbs steadily up to a value after which the influence of the wake is not present. From here, the gradient in the velocity is very small and it attains the free stream speed. For the probe at X=0, the flow reduces from freestream to 0 at the stagnation point and then follows the same trend as before.
4. The rest of the probes that are present in the influence of the wake, have velocities lesser than the freestream speed and gradually rise to the freestream speed. No points of minimum speed are observed at these probes.

GRID INDEPENDENCE TEST:

The case is set up using meshes of increasing local refinement in an attempt to get mesh-independent results. The following are the results obtained for the combination of different mesh sizes. The maximum number of cells that can be used in the academic version of fluent is 512000 cells.

CASE	OUTER ENCLOSURE (MULTIZONE) Mesh in m	INNER ENCLOSURE (BODY SIZING) Mesh in m	FACE SIZING	NUMBER OF CELLS	DRAG	LIFT	SKIN FRICTION COEFFICIENT
CASE 1	0.150m	0.1m	0.005m	51600	0.3400	0.1458	0.003112
CASE 2	0.1m	0.05m	0.005m	98457	0.3499	0.1924	0.00353
CASE3	0.250m	0.05m	0.005m	35337	0.379	0.184	0.002712

CASE 1:

> DRAG

> LIFT

> SKIN FRICTION COEFFICIENT

CASE 2 :

> CASE 3:

> SCALED RESIDUALS:

> DRAG

LIFT:

> SKIN FRICTION COEFFICIENT:

CONCLUSIONS:

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.

The flow over the Ahmed body is conducted and the wake region is visualized using Vector and Contour plots and analyzed. The coefficient of drag and coefficient of lift is estimated. The point of separation and its influence is talked upon. The grid Independence test is conducted.

• It can be inferred that the grid independence test is performed to know the smaller cell size for convergence.
• As the setup is refined drag coefficient, lift coefficient decreases.
• As the setup is refined skin friction 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.