As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster as well.
The geometry which is provided with the challenge needs a modification which you should be able to do at this stage of a course. You need to use the 'split body' command in SpaceClaim to perform the operation and then you can use the symmetry boundary condition in fluent to perform the simulation.
Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.
For this challenge, you will have to provide answers to the following questions:
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.
1.INTRODUCTION
The Ahmed body is simplified car body used in automotive field to study the impact of the flow pattern on the drag. The external aerodynamics of the car defines many major traits of an automobile 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 :
BODY MODEL
The “Ahmed” body, has the form of a highly simplified car, consisting of a blunt nose with rounded edges fixed onto a box-like middle section and a rear end that has an upper slanted surface (like a “hatch-back” car), the angle of which can be varied. The model is supported on circular-sectioned legs or stilts, rather than wheels. Despite neglecting a number of features of a real car (rotating wheels, rough underside, surface projections, etc.) the Ahmed body generates the essential features of flow around a car, namely: flow impingement and displacement around the nose, relatively uniform flow around the middle and flow separation and wake generation at the rear.
Besides the analyses of the velocity field, the Ahmed Body is also used for drag force and lift force analyses and rolling, pitching and yawing moments. The slant angle on the back allows studying the separation region and its influence on the velocity profile and forces.
The geometry of the Ahmed body is shown in the figure below. The slant angle is adjustable and is the main variable model parameters in the experimental investigations. The Ahmed body, of length (L) 1044 mm, height (H) 288 mm, and width (W) 389 mm, was placed at 50 mm from the ground (G).
GENERATING SIMULATION FOR BASE MESH
OUTER ENCLOSER :
Right face is located at a distance of 5m from the right side face of Ahmed body along the X axis.
Left face is located at a distance of 2m from the left side face of Ahmed body along the X axis.
Top face of the encloser is at a distance of 1 m from the top side face of the Ahmed body along the Y axis.
The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
The front and back face of the encloser is at a distance of 0.5 m along the Z axis.
MESH GENERATION :
BASE MESH SECTIONAL VIEW OF MESH
SOLVER SETUP:
We are using K-E model as viscous property and taking the initial Velocity as 25m/s. Pressure based solver and steady state simulation is to be generated ,We are going to observe the Velocity & Pressure contours to estimate the results and which can be useful in further future modefication.
Velocity Contour Plane Pressure Contour plane
Drag Coefficient Lift Coefficient
Here from the countours the results are not upto the required experimental values so we need to refine more to make the reults get upto the require values and from the velocity countour we can observe that there is low pressure developing due to the slant angle.
Negative pressure at Wake region and Point of Separation :-
The flow around the Ahmed reference body is complicated. Due to the slant in the rear of the vehicle,flow separation and counter rotating vortices are generated at the slant edges. The drag forces of Ahmed Body reaches at maximum when the slant angle is 30 deg. For slant angles higher than this value, the adverse pressure gradient in between the slant and the roof is so intense that the flow fully detaches over the slant. Below this critical slant angles (30 degree), the flow still separates but the pressure difference between the slant region and the side walls is still sturdy enough to generate substantial stream-wise vortices at the lateral slant edges. These prompt a downward motion over the slant, mainly in the downstream part. As a result, the flow separating at the upstream end of the slant can coupled further to the downstream. The flow around the Ahmed body has several flow separations from the front to rear of the vehicle. The flow recirculation caused by these flow detachment contributes the vehicle’s drag. The location point at which the flow separates determines the size of the separation zone, and accordingly the drag force, thus an exact simulation of the wake flow and of the separation process is essential for the accurate result of drag predictions.
INNER ENCLOUSER :
Right face is located at a distance of 1 m from the right side face of Ahmed body along the X axis.
Left face is located at a distance of 0.5 m from the left side face of Ahmed body along the X axis.
Top face of the encloser is at a distance of 0.5m from the top side face of the Ahmed body along the Y axis.
The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
The front and back face of the encloser is at a distance of 0.26 m along the Z axis.
INTERFERENCE REGION :-
Initially there was a huge interference between the Outer and Inner encloser so that the mesh gets overlapped which gets not acceptable so to avoid this we made share topology and get deleted the Interference region that by using the interference command after this we get a good interaction between the Inner and Outer enclosers so that we can get the mesh properly and also share the boundaroies for the uid ow.Here shows the image below.
MESHING :
In this meshing we can assign the named selection for Inlet,Outlet,Symmetry wall and Car Wall,We can use the different type of meshing process for this model.
In this we follow three stages,
1. Meshing will be done for the outer enclouser choosen as a method of the Multizone and this will create the Hexa mesh,The element size used for this is 0.08m.
2.Next for inner enclouser we choose body sizing method to make more refinement,the element size we use for this is 0.04m.
3.Next we select the face sizing method to select the legs of the body to capture them with better refinement,we take the element length as 0.005m.
4.Next to capture the flow over the body we make the inflation layer which helps us in caputuring the flow very finer way and the mesh will also be capture along the body perfectly.We take the number of inflation layers as 6, Total thickness value as 0.0036.
For getting the total thickness we are taking the Y+ = 100.
Some of the calculations needed to find these values,
Where ,
ρ , µ are fluid density and viscosity ; v is the free stream velocity ; L is characteristic length ; Uf is frictional velocity ; Tw is wall stress , Cf is skin friction coefficient
SETUP :
Solver type : Pressure based
Check the Energy equation
Turblent Model : K-Omega SST model
Fluid Material : Air
Boundary conditions
Inlet velocity : 25 m/s
Outlet pressure : 0 pa
Symmetry : For side walls
Wall : Bottom ,
Creating a surface plane to observe the contours.
RESULTS :
VELOCITY CONTOUR :
Velocity Contour with mesh and vectors
The above figure represents the velocity vector over the Ahmed body.we can see clearly that the because of the rening mesh the wake formation is captured properly at the rear end of the Ahmed body and reined it futher more to get the more wake region.
PRESSURE CONTOUR:
Plots :
Drag Coefficient Drag Force
After the calculations we get the value of the Drag Coeff value as 0.36 ,Drag Force 16.
Lift Coefficient Lift Force Scaling residuals
After the calculations we get the value of the Lift Coeff value as 0.225,Lift Force 10.
FINAL REFINEMENT :
MESHING:
In this meshing we can assign the named selection for Inlet,Outlet,Symmetry wall and Car Wall,We can use the different type of meshing process for this model.
In this we follow three stages,
1. Meshing will be done for the outer enclouser choosen as a method of the Multizone and this will create the Hexa mesh,The element size used for this is 0.08m.
2.Next for inner enclouser we choose body sizing method to make more refinement,the element size we use for this is 0.0326m.
3.Next we select the face sizing method to select the legs of the body to capture them with better refinement,we take the element length as 0.005m.
4.Next to capture the flow over the body we make the inflation layer which helps us in caputuring the flow very finer way and the mesh will also be capture along the body perfectly.We take the number of inflation layers as 6, Total thickness value as 0.0036.
For getting the total thickness we are taking the Y+ = 100.
Some of the calculations needed to find these values,
Where ,
ρ , µ are fluid density and viscosity ; v is the free stream velocity ; L is characteristic length ; Uf is frictional velocity ; Tw is wall stress , Cf is skin friction coefficient
SETUP :
Solver type : Pressure based
Check the Energy equation
Turblent Model : K-Omega SST model
Fluid Material : Air
Boundary conditions
Inlet velocity : 25 m/s
Outlet pressure : 0 pa
Symmetry : For side walls
Wall : Bottom ,
Creating a surface plane to observe the contours.
RESULTS :
VELOCITY CONTOUR :
Velocity contour with mesh and vector
The above figure represents the velocity vector over the Ahmed body.we can see clearly that the because of the rening mesh the wake formation is captured properly at the rear end of the Ahmed body and reined it futher more to get the more wake region.
PRESSURE CONTOUR:
MULTI ZONE BODY SIZING Total Thickness DRAG COEFFICIENT LIFT COEFFICIENT ELEMENT SIZE
BASE LINE - - 0.87 0.51 89141
0.08 0.04 0.00363 0.36 0.2225 319247
0.08 0.0326 0.00363 0.33 0.229 499875
VECTOR GENERATION AT WAKE REGION FOR VELOCITY CONTOUR :-
X= 1100 mm X = 1200 mm
X = 1400 mm X = 1600 mm
With XY plane and vector
DRAG COEFFICIENT & DRAG FORCE :-
Drag Coeff Drag Force
LIFT COEFFICIENT & LIFT FORCE:-
Lift Coeff Lift Force
STREAMLINE VIEW :
Negative pressure gradient
When fluid velocity increases, the pressure decrease. The wake region is formed due to adverse pressure in the rear end causes the boundary layer to separate out from the body. The wall shear stress and velocity becomes zero and wake formation starts. These wakes are nothing but small vortices that create negative pressure. These create circulation of velocity and induces drag forces.
Point of seperation
Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow.When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag.
Conclusion
From running a baseline simulation, the post CFD results gave inaccurate results at the region of mesh overlapping.
The grid independency test shows best results close to the experimental data. With refining the mesh size with y+ factor, the error is significantly reduced.
The y+ is chosen in between a stable range to give a correct prediction of velocity and pressure contour plots.
The flow seperation and appearence of vortices gets more prominent as y+ is increased. As seen in baseline meshing no wake formation is captured properly, on refining the mesh, the flow is captured properly.
