Challenge

Week 3 - External flow simulation over an Ahmed body.

IInosri Jana18 Nov 2022

External flow simulation over an Ahmed body

Aim:To simulate external flow over an Ahmed body.

Introduction and Theory:(Ans for Q1)

Ahmed's body and its importance:The Ahmed body is a geometric shape first proposed by Ahmed and Ramm in 1984. The shape provides a model to study geometric effects on the wakes of ground vehicles (like cars).The airflow around the Ahmed body captures the essential flow features around an automobile .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.

As the burning of fossil fuels becomes a more pressing issue, manufacturers are introducing more fuel efficient cars to the market. One main contributor to fuel burn is the car’s aerodynamic drag. Complexly 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. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

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 simple geometry of an Ahmed body

Solving & Modelling approach:

Base line modelling:

Geomtry creation:Importing geomtry in spaceclaim.

Meshing:

set up:

Solution:

Velocity contour plot:

Pressure contour plot:

Refined Modelling:

Geometry setting:In present case, the body is symmetry along z axis. Thus the xy plane is used to split body in spaceclaim using Split body command. An enclosure is created around the body as shown in the figure.

Mesh setting:

1. In meshing the outer enclosure is meshed with the multizone method to adjust with inner enclosure. Interference between inner enclosure and outer enclosure is removed and topology between two is made to share.

2. Body sizing method is used for the inner enclosure to refine the mesh.

3. Facing size in given to the bottom stands. Mesh around the face is refined to keep its cross section circular in shape.

4. Inflation is given for car wall. Total thickness method is given .

If first layer thikness=0.0007164 m,no of layers=5

using online y+ calculator:

It's important that your mesh near the wall is properly sized to ensure accurate simulation of the flowfield. This calculator computes the height of the first mesh cell off the wall required to achieve a desired Y+ using flat-plate boundary layer theory.

Input

Reset to sea level conditions

U_∞:freestream velocity (m/s)

ρ:freestream density (kg/m3)

μ:dynamic viscosity (kg/m s)

L:1.044reference length (m)

y+:50 desired y+

k-epsilon :y+(30-300)

k-omega:y+(1-5)

Compute Wall Spacing

Δs:0.000716483234694654 wall spacing (m):first layer thickness

Re_x:1786772.1023806867 Reynolds number

The drag coefficient `C_d` is defined as,

`C_d=(2*F_d)/(rho*u^2.A)`

where:

`F_d`= the drag force, which is by definition the force component in the direction of the flow velocity;
^`rho`=the mass density of the fluid;
`u`= the flow speed of the object relative to the fluid;
`A`=the reference area

5. Faces are named according to their function. inlet,outlet,wall(symmetry:all side walls) and car-wall.

Setup:

General:

Viscous Model:

Set the reference values from inlet :(change the area to projected area)

Projected area is to be calculated from result-reports

Velocity of fluid at inlet=25m/s(given)

Fluid: Air

Turbulence model= k-epsilon

No. of iterations=300

Scaled residuals:

Drag-coefficient plot(refined):

Drag coefficient,Cd=0.3005143

Lift-coefficient plot(refined):

Lift coefficient,Cl=0.24824794

Cd=

0.3005143

Velocity contour plot:

Pressure contour plot:

The vector plot showing the wake region:

Grid independency test:

Case 1:

Body sizing:Element size:0.04m

Drag-coefficient plot(case 1):

Lift-coefficient plot(case 1):

Case 1:

Body sizing:Element size:0.06m

Drag-coefficient plot(case 2):

Lift-coefficient plot(case 2):

cases	drag coefficient(Cd)	lift coefficient(Cl)
1.(Element size 0 .05m)(body sizing)	0.3005143	0.24824794
2.(Element size 0 .04m)(body sizing)	0.30897883	0.16502426
3..(Element size 0 .06m)(body sizing)	0.33728987	0.21885791

The reason for the negative pressure in the wake region (Ans for Q2):

The flow behind an object separates from the surface and creates a highly turbulent region behind the object, called the wake. The pressure inside the wake region remains low as the flow separates and a net pressure force (pressure drag) is produced. In other words, due to the production of pressure drag being produced from flow separation, the pressure in the wake region is low. This is what is causing the vacuum.

Although the wake particles push the vehicle forward but this effect is minimized on the lead vehicle due to the drag it is producing, however it is beneficial for any cars following closely behind (this concept is called 'drafting'). With drafting, the lead car creates a envelope of less of air behind it, reducing the friction drag for the cars behind it. As the lead car moves it creates a low pressure region, that partial vacuum, behind it which essentially sucks in the car behind it and reduces pressure drag.

The significance of the point of separation (Ans for Q3):

The point of separation of flow in the car body determines the size of the wake region. If the separation of the flow is occurring early then it would result in large wake region and higher drag force. Increased drag force would affect the fuel efficiency of the vehicle. To achieve a low drag coefficient, optimization in aerodynamic design is done to shift the point of flow separation. In the case of Ahmed body, by adjusting the slant angle we can control the flow separation.

Suppose there is a stationary average car, in which air all around it at atmospheric pressure. Now when the car just starts to move forward, the air would flow from the front to the back of the car. There would be streamlined flow over the car. Now, when the flow reaches a separation point, i.e., where it can no longer maintain the streamlined nature, the flow leaves the surface.

The point where the flow leaves the surface is called the separation point. In scientific terms, it is the point where there is no more influence of the wall i.e., there is no zero wall shear stress. This flow with it would carry the air particles behind the car when the car was at rest. So, the number of molecules behind the car has now considerably gone down, which implies the pressure has reduced. This is the negative pressure region that is developed behind the car.

Conclusions:

A base line mesh is created before refining the mesh to start the simulation.Grid independency tests are performed by changing body mesh element size .The mesh can be refined more but limited at our end.Simulation has successesfully performed.

References:

1.Flow and Turbulence Structures in the Wake of a Simplified Car Model

2.Experiments and numerical simulations on the aerodynamics of the Ahmed body

3.https://www.cfd-online.com/Wiki/Ahmed_body