Week 3 - External flow simulation over an Ahmed body.

Aim:

   CFD simulation of the External flow over an Ahmed body.

 

Objective: 

            The objective of this project is to determine the aerodynamic forces on the Ahmed body such as drag and lift coefficient and wake flow around the body to perform the different  grid independence test.

 

The expected results:

1. Velocity and pressure contours. 

2. The drag coefficient plot for a refined case. ( For a 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 independence test and provide the values of drag and lift with each case. 

 

 Introduction:

 The Ahmed body is simplified car body used in automotive field to study the impact of the external flow pattern on the drag. The external aerodynamics of the car defines many major factors of an automobile domain like stability, comfort and fuel consumption at high speeds. The flow around the vehicle is characterized by high turbulent and three dimensional flow separations as well as there is a growing need for more insight into the physical features of these dynamical flows. The Ahmed Body is a simplified car, used in automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of round front part, a moveable z slant plane in the rear of the body to study the detachment phenomena at different angles, and a rectangular box which link the front part and the rear slant plane. The principal objective to study such a simplified car body is to tackle the flow processes involved in drag production. Through perceiving the mechanisms involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximize vehicle performance.

 

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-epsilon` model and for the later stage `k-omega sst model is used.

 

Importance of Ahmed body:

 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.

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 its 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 :

The flow behind an object separates from the surface and creates a highly turbulent region behind the object, called the wake.

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 as long as 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:

In aerodynamics, flow separation results in reduced lift and increased pressure drag, caused by the pressure differential between the front and rear surfaces of the object. It causes buffeting of aircraft structures and control surfaces.

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.

 

Solving & Modelling approach:

 

 

 

Pre-processing and solver setting:

 

Case-1:  External flow simulation over Ahmed body by creating a single closure and

using default mesh value of ANSYS-FLUENT:

1. Ahmed body model:

 

 

2.Creating an enclosure across Ahmed body:

 

 

 

3. Creating a section plane  on the mid of the origin

create the plane to select the mid of the origin point to get three xyz plane

 4.click on section option to select symmetry plane of the Ahmed body

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. 

 

6.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:

 

 

Use F8 to hide the face

and create name of inner body - car wall

 

7.Performing meshing with default mesh size in ANSYS-FLUENT:

cut the geometry , and see the body aera

 

 

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.

8.It can be fixed by going back to Space claim, one of the copies should be suppressed for physics.

 

9.Setting up of physics:

10. Aera Calculation of Ahmed Body:

 

 

11. Set Velocity of Inlet:

Velocity= 25m/s

 

12. Viscous Model:

Choose K-Epsilon 

 

 

 

Hybrid initialization is selected and the solution is initialized by hitting 

 

Creating a cut plane to see the velocity change along Ahmed's body = cut-plane-z

 

14.Creating contour plot along cut plane z:

 

 

 

 

 

 

 

 

Vector :

0.4117949

Lift-coefficient  = 0.26947335

 

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

Turbulence model:  

 

$\mathrm{First\; refinement:}$

$\mathrm{Geometry:}$

 

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 also to make the mesh uniform.

 

 

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 $m$ 

Element size for the first enclosure:  or $m$

Maximum size for the first enclosure:  or $m$

Element size for the second enclosure:  

Element size of pillars of Ahmed body: $mm$                                                     

Number of elements :  187781

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

 

 

Results:

 Residual plot:

 

After 485 iteration the solution is convergent

Drag-coefficient plot:

 

 

Lift coefficient plot:

 

Velocity contour:

 

Velocity contour wake region:

 

 

Velocity contour - vector- wake region:

 

Pressure contour:

 

Pressure contour wake region:

 

 

Pressure contour-vector- wake region:

 

Drag coefficient	0.39994229
Lift coefficient	0.3214019

 

 

Case 2(b) : Second refinement

Turbulence model: 

$5mm$

 

Inflation layer:

First layer thickness: $mm$

Number of layers: 

Last layer  thickness:  

Number of elements: 263870

 

 

Results:

 Residual plot:

 

After 538 iteration the solution is convergent

Drag-coefficient plot:

 

Lift coefficient plot:

 

Velocity contour:

 Velocity contour wake region:

 Velocity contour - vector- wake region:

 

Pressure contour:

 

 

Pressure contour wake region:

 

 

Pressure contour-vector- wake region:

 

Drag coefficient	0.39532329
Lift coefficient	0.31734895

 

Case 2(c) : Second refinement

Turbulence model:  

 

$5mm$

 

Inflation layer:

First layer thickness: 0.15mm

Number of layers: 10

Last layer  thickness:  0.7739

Number of elements: 376776

 

Meshing:

 

Results:

Residual plot:

 

 

Drag-coefficient plot:

Lift coefficient plot:

Velocity contour:

Velocity contour wake region:

Velocity contour - vector- wake region:

Pressure contour:

Pressure contour wake region:

 

Pressure contour-vector- wake region:

 

Drag coefficient	0.26842712
Lift coefficient	0.37969136

 

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	$k-\epsilon$	$1$	0.26947335	0.4117949
First refinement	$k-\omega$$SST$	187781	0.3214019	0.39994229
Second refinement	$k-\omega$	263870	0.31734895	0.39532329
Third refinement	$k-\omega$	376776	0.37969136	0.26842712

 

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 $k-\omega$ 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.