Week 3 - External flow simulation over an Ahmed body.

Objective:-

• The Aerodynamic behaviour of a Ahmed body is numerically investigated in the present work by using the Commerical Software ANSYS FLUENT for the simulation using a Turbulent model. In this case we are considering the only half Ahmed body for the best practice where you can save on the number of cells and get the results faster as well.
• Also, the Grid dependency test is performed to validate the results against the experimental data and compute the coefficient of drag and lift maintaining the y+ in the appropriate range.

Theory:-

The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated, it is used to design new models of the car. So, it becomes a benchmark for aerodynamic simulation tools. The aim of the benchmark validation was to evaluate the ANSYS, computational fluid dynamics (CFD) software, and compare its results with the measurement data. The particular goal of this benchmark is to compare the Drag Coefficient, Velocity Profiles, Velocity field, and the time-averaged Wake structure of the ANSYS simulation results with the real experimental measurement in the wind tunnel.

Modelling Approach:-

The Flow for this case we are using is Turbulent flow which is based on the Reynolds number means the Re >2000 for this flow and turbulent model is k-epsilon. The K-epsilon model is used for the High reynolds number, Steady, Equilibrium flows without a large adverse pressure gradient explicity modelling the effect of the innermost portion of the turbluent layer is not neccesary.In this challenge we are using Ahmed bodyThe Ahmed body represents a simplified, ground vehicle geometry of a bluff body type. Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent Flow, k-epsilon interface.

Here, is the given geometry of the Ahmed body that we are consider for validating the simulation.

2D geometry of Ahmed body

The simple geometrical shape has a length of 1.044m, height of 0.288m and width of 0.389m. The shape of cylinderical pillars that is attached to the bottom of the body has a dimension of 0.5 m and the slant height of the rear surface ϕ=30°.The Ahmed body with the slant angle of 25° belongs to the fastback vehicles, which exhibit very strong inward rotating longitudinal vortices at the C-pillars.

These vortices extend far into wake and generate a field of downwind in the enclosed region. At stant angle of 30 degrees, i.e, directly before transition to the full squareback flow, this pair of vortices is devolped most strongly, and the flow separates at the lower edge of the stant. This produces strong under-pressures at that position, So that the aerodyanmic drag reaches its maximum in this state here, is the 3D model of the Ahmed body.

The flow for this model we are consider is Turbluent which is based on the Reynold's number determined by the body length and the inlet velocity. The simulation solves for the turbulent kinetic energy and dissipation on addition to the velocity and pressure fields.

Problem Statement:-

• Describe Ahmed's body and its importance.
• Explain the reason for the negative pressure in the wake region.
• Explain the significance of the point of separation.

Expected Results:

• Velocity and pressure contours.
• The drag coefficient plot for a refined case.
• The vector plot clearly showing the wake region.

Solution Approach:-

Step1 Creating a Geometry

In this case we are going to load a geomtery in ANSYS FLUENT space claim from a stp file format.

• The Ahmed body is enclosed by a virtural wind tunnel created by an enclosure option in prepare toolbox. In this case first we split the body in half part for fastest convergence as mentioned in the above. Then create a virtual fluid domain by an enclosure option and uncheck the symmtery for the editing the required dimensions.
• We have created a 2 enclosure boxes the inner one is for add local refinement region right in front of Ahmed body and also in the wake region.
• The outer one is for fluid domain where we need to know the flow features around a body So, we need to create a mesh on the body not inside the body.
• Select a plane perpendicular to the Ahmed body and check the interference in the cross sectional view and then resolved the interference problem by the interfernce command line.

Resolved the Interference problem for the interruption carried out by the two enclosure boxes.

• After the solution of the interference problem do not forget to share the fluid properties of fluid flow in the 2 enclosure's by the share topology command.

Here, is the 3D model in the Space claim

• 

Step2 Meshing

• After opening the Mesh set up First we creates name by selection command like inlet, outlet, symmetry and car walls

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• After that we create first the baseline mesh of element size is 100mm and generate the mesh For the hexahedral we have to go insert sizing and select the multizone for the different different mesh patterns. It will generate hexahedral baseline automatically bcoz hexahedral meshes are more accurate that tetrahedral meshes.

• For the local refinement region mesh size we are using it 50mm for beacuse the as the mesh is more finer the more accurate the result It will captures the flow behaviour at every point. Right click on the mesh go to insert option and then select the sizing option. Select the inner box then give the element size 50mm and generate mesh.

• After the refinement of the inner part the mesh around the cylinderical pillars of the Ahmed body.
• To refine the Ahmed body part. Hide all the parts expect the car walls of the Ahmed body then select the box tool to select all the faces of car wall. Then give element size 10mm and click on generate mesh After that the cylinerical pillars are still not refine as much

• To refine the cylinderical pillars of Ahmed body, click on sizing select the single selection tool and then select the only face of the pillars of the cylinderical portion give element size 5mm and generate mesh.

Here, we introduce the Important term Y+

The y+ value is a non-dimensional distance (based on local cell fluid velocity) from the wall to the first mesh node, as you can see in the image below. To use a wall function approach for a particular turbulence model with confidence, we need to ensure that our y+ values are within a certain range.

• Now to add the body fitted mesh on the surface of the Ahmed body for this we use the inflation layer command Then go to inflation. Select geomerty from named selection choose car wall. Type of inflation layer is Total thickness. Give the no. of layers and total thickness based on first layer thickness. click on Generate mesh

Here, we are Calculate the total thickness on the basis of first cell height we got from the assumed Y+ value bascially Y+ value tells where our mesh lies So, the total thickness of the inflation layer is

`Sn = a1(1-r^n)/(1-r)`

a1 = first term of first layer of thickness

r = growth rate

n = no. of layers

`Sn = 5((1-(1.2)^6)/(1-1.2))`

Sn = 34.75mm

Step3 Start Fluent Setup

• Select Density-Based solver because It solves for high speed incompressible flows whose Mach no. is greater than 0.3. It solves Energy, momentum, continuty and species transport equations simultaneously.

• Viscous model is selected as K- Epsilon model. Wall treatment standerd.

• Check the Fluid properties as default it is saved the air fluid properties as we want so leave it as it is.

• Then set up the boundary conditions and the refrence values the inlet velocity is 25 m/s and the projected area of the Ahmed body is 0.065741 m2 and the length of the entire domain is 1.044m.

• Now, to plots the velocity and pressure contours in the fluid domain or over the Ahmed body first we have to create a plan in the xy direction at the z=0 then create reports of velocity and pressure contours

• Then, create reports and calculate the coefficient of drag and lift go in definations in the solutions then force reports. report of cd and cl

• After the complete data set up first we do the initialzation In this case we are using the hybrid initilazation because It solves for potential flow probelms and also provides good initial conditions. Method we are using in this case is SIMPLE implicit method.

• After the initilazation the problem go in Run and calculation type the number of iterations as you want to run number of times your simulation and then hit calculate.

Step4 Results

Iteration convergence plot - Residual plot

Coeffcient of drag

Coeffcient of lift

Velocity Contour

Pressure contour

Vectors

Vectors in 3D show wake region

Graph of velocity profile at different locations of y

Grid Independcy test:

In this study we have set about 4 cases, by refining the mesh and varriation in the Y+ values to check the effect on Coefficient of drag and lift values.

Y+ value	Ist cell thickness (in mm)	Total thickness (in mm)	Baseline Mesh size (in mm)	No of layers	Coefficient of Drag	Coefficient of Lift
240	3.5	34.75	50	6	0.34715	0.2533
240	3.5	34.75	25	6	0.3030	0.2409
342	5	49.64	50	6	0.34914	0.23258
410	6	59.75	50	6	0.34922	0.2864

Case1: y+ = 240 mesh size = 50mm

Iteration convergence plot - Residual

Coeffcient of drag

Coeffcient of Lift

Velocity contour

Pressure contour

Vectors

Vectors in 3D show wake region

Graph of velocity profile at different locations of y

Case2: y+ = 342 and mesh size = 50mm

Iteration convergence plot - Residual

Coeffcient of drag

Coeffcient of Lift

Velocity contour

Pressure contour

Vectors

Vectors show wake region

Graph of velocity profile at different locations of y

Case4 : y+ = 410 and mesh size = 50mm

Iteration convergence plot - Residual

Coefficient of Drag

Coefficient of Lift

Velocity contour

Pressure contour

Vectors

Vectors in 3D show wake region

Graph of velocity profile at different locations of y

Case4 : Y+ = 240 and mesh size = 25mm

Iteration convergence plot - Residual

Coeffcient of drag

Coeffcient of Lift

Velocity contour

Pressure contour

Vectors

Vectors 3D show wake region

Graph of velocity profile at different locations of y

Conclusion:-

We can clearly conclude that In the wake region the velocity is high as compared the velocity in the backside i.e, tail region and the Velocity vectors is also very high in the wake region It is because the obstruction offered by an Ahmed body in the wake region is more as compared in the tail region. In the Tail region of the Ahmed body the circulating flow is more that results in low velocity.The pressure in the front is very high but in the wake region we got is Adverse pressure or called negative Pressure and here, we got the term Negative pressure

Negative Pressure Gradient: When an object moves in a fluid the fluid sticks on the surface of the body and formed a boundary layer over it. Now, for certain bodies and type of motion , there is a point on the body where the fluid detaches from its surface, thus breaking the boundary layer, this point at which fluid detaches from the surface is called flow of separation. The boundary layer maintains till the velocity gradient is greater than zero and when the velocity gradient goes decrease or negative The flow of separation takes place.

Flow seperation

In this case flow seperation takes place at the curved surface interface of the Ahmed body. Separation occurs due to an adverse pressure gradient and also negative velocity gradient encountered as the flow expands and its resulting in the recirculating flow.

• The coefficient of drag and the lift is increases when the of y+ value is increases and also when the mesh is finer the coefficient of drag and lift is also less that is because the larger initial y+ value, smaller overall mesh count and faster run times. Expect flow separation and the accurate prediction of the separation point will have an impact result, such as the drag or lift forces experienced by the ellipse then would be advised to resolve the boundary layer all the way to the wall with a finer mesh.
• The residuals continously dropping as we refining the mesh with the Y+ value means the error is significantly reduced.
• The flow separation and apprearence of vortices gets more prominent as Y+ is increased by refining the mesh wake region is captured properly.
• From the pressure contours we can clearly see that the pressure at the frontal area is more and also the velocity is more at the frontal area.