Week 3 - External flow simulation over an Ahmed body.
OBJECTIVE
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.
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).
AHMED BODY USED FOR ANALYSIS
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.
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.
GEOMETRY USED:
Ahmed Body Geometry
PROCEDURE
1. Import the given geometry to spaceClaim
2.Now after importing the model into SpaceClaim we create two enclosures and then check for any interference. Also using the split body command we cut half the ahmed body and then with the help of symmetry we model it and run through which we can get a faster output
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 boundaries for the same.Here shows the image below.
Checking For Interference
After Using Split body Command
3.Now after the model is set up now we move onto mechanical module for meshing
4. 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 enclosure chosen as a method of the Multizone and this will create the Hexa mesh
Mesh Model
Multi-Zone Mesh Selection
Hexa Mesh Deployment
5.Next for inner enclosure we choose body sizing method to make more refinement,the element size we use for this is 0.04m.
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.
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 capture along the body perfectly.We take the number of inflation layers as 6,
INFLATION LAYER CALCULATION ACCORDING TO Y+ VALUES
For getting the total thickness we are taking the Y+ = 5. 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
6.After meshing now we have to give named selection y selecting the face and clicking the shortcut key N
INLET
OUTLET
SYMMETRY
CAR-WALL
6. We now proceed with the setup for which we move to the fluent module
SOLVER SETUP:
We are using K-omega model as viscous property as it accounts for the wall viscous effects 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 modification.
SETUP
Solver type : Pressure based
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
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.
RESULTS:
CASE 1:50mm-104 iteration
Drag Coefficient Plot for 50mm
CASE II:40mm 63 iterations
CASE II 40mm Drag coefficient plot
CASE III:45mm 58 iterations
Of all the three results as case, II converged with least number of iterations and with a D.C of 0.33 this case is considered ideal
CASE I:
VELOCITY CONTOURS:
CASE II:
PRESSURE CONTOUR
VELOCITY CONTOUR
VELOCITY VECTOR PLOTS
CASE III 45mm:
PRESSURE CONTOUR
VELOCITY CONTOUR
VELOCITY VECTOR PLOTS
CONCLUSION
1.As air from the inlet hits the front face of the body has maximum pressure and flow gets separated to two regions above and below the body. As Ahmed body is cubical in shape so separated air travels along the body merely come in contact with rear part. This phenomenon creates a void at rear leads to negative pressure
2. Refining mesh leads to clearer visibility of wake region