Week 3 - External flow simulation over an Ahmed body.
1. INTRODUCTION
The Ahmed body is a simplified car body used in the automotive field to study the impact of the flow pattern on 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 the automotive field to investigate flow analysis and find the wake flow around the body. Ahmed's body is made up of a 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 that links 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 reduce 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 Ahmed's body is shown in the figure below. The slant angle is adjustable and is the main variable model parameter 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 :
- The right face is located at a distance of 5m from the right side face of Ahmed's body along the X-axis.
- The left face is located at a distance of 2m from the left side face of Ahmed's body along the X-axis.
- The top face of the enclosure 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 enclosure is attached to the bottom face of Ahmed's body along the YZ plane.
- The front and back face of the enclosure 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 the K-E model as viscous property and taking the initial Velocity as 25m/s. Pressure-based solver and steady-state simulation are to be generated, We are going to observe the Velocity & Pressure contours to estimate the results and which can be useful in further future modification.
Velocity Contour Plane Pressure Contour plane
Drag Coefficient Lift Coefficient
Here from the contours, the results are not up to the required experimental values so we need to refine more to make the results get up to the required values from the velocity contour we can observe that there is a 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 force of Ahmed's Body reaches the maximum when the slant angle is 30 deg. For slant angles higher than this value, the adverse pressure gradient between the slant and the roof is so intense that the flow fully detaches over the slant. Below this critical slant angle (30 degrees), 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 separates at the upstream end of the slant can be coupled further to the downstream. The flow around Ahmed's body has several flow separations from the front to the rear of the vehicle. The flow recirculation caused by these flow detachments contributes to 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 ENCLOSURE :
- The right face is located at a distance of 1 m from the right side face of Ahmed's body along the X-axis.
- The left face is located at a distance of 0.5 m from the left side face of Ahmed's body along the X-axis.
- The 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 enclosure is attached to the bottom face of Ahmed's body along the YZ plane.
- The front and back face of the enclosure 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 enclosers so that the mesh gets overlapped which gets not acceptable so to avoid this we made share topology and get deleted the Interference region 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 boundaries 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 types of meshing processes for this model.
In this, we follow three stages,
1. Meshing will be done for the outer enclosure chosen 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 the inner enclosure we choose the 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 capturing the flow very finer way and the mesh will also be captured 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 Ahmed's body. we can see clearly that the because of the renting mesh the wake formation is captured properly at the rear end of the Ahmed body and reined it further to get 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 types of meshing processes for this model.
In this, we follow three stages,
1. Meshing will be done for the outer enclosure chosen 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 the inner enclosure we choose the 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 capturing the flow very finer way and the mesh will also be captured 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 Ahmed's body. we can see clearly that the because of the renting mesh the wake formation is captured properly at the rear end of Ahmed's body and reined it further to get more wake region.
PRESSURE CONTOUR:
| MULTI-ZONE | BODY SIZING | Total Thickness | DRAG COEFFICIENT | LIFT COEFFICIENT | ELEMENT SIZE |
| BASELINE | - | - | 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 causing the boundary layer to separate out from the body. The wall shear stress and velocity become zero and wake formation starts. These wakes are nothing but small vortices that create negative pressure. These create a circulation of velocity and induce drag forces.
Point of separation
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 in the region of mesh overlapping.
- The grid independency test shows the best results close to the experimental data. By refining the mesh size with the 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 separation and appearance of vortices get 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