Week 3 - External flow simulation over an Ahmed body.
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:
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 which according to its symmetry it will be possible to use the split command in order to obtain half the geometry being able to refine the mesh even more, and still obtaining a desired symmetry results convergence.
Meshing geometry set up:
Baseline mesh
- Open the meshing console.
- Click on mesh option on the left side of work tree diagram.
- Click on generate mesh.
- A general mesh pattern will be created.
- Create section plane on one half of the geometry.
Refined meshing
- Open meshing console in ansys fluent.
- Create named section of the entire geometry
- inlet - front face of Ahmed body outer encloser.
- outlet - Rear face of Ahmed body outer encloser.
- Ahmed body - Wall
- Rest faces - Symmetry.
Outer encloser:
Element size:0.1m
Inner encloser :
Element size : 0.05m
Inflation layer :
Method - Total layer thickness - 0.0036
Growth rate - 1.2
No . Of layers - 6
Face sizing :
Element size - 0.005m
SOLUTION SETUP:
Boundary Conditions:
Inlet - 25 m/s -velocity inlet
Outlet - Pressure outlet-0
Ahmed body - wall
Rest faces - Symmetry
Initialization - Hybrid initialization
Solver:
pressure based steady state solver
RESULTS:
Residuals plot:
Drag plot:
Lift plot:
Velocity contour:
PRESSURE CONTOUR:
CASE 2
MESH
Outer encloser:
Element size:0.1m
Inner encloser :
Element size : 0.04m
Inflation layer :
Method - Total layer thickness - 0.0036
Growth rate - 1.2
No . Of layers - 6
DRAG PLOT:
LIFT PLOT:
Velocity contour:
PRESSURE CONTOUR:
CASE 3
MESH
Outer encloser:
Element size:0.1m
Inner encloser :
Element size : 0.03m
Inflation layer :
Method - Total layer thickness - 0.0036
Growth rate - 1.2
No . Of layers - 6
DRAG PLOT:
LIFT PLOT:
VELOCITY CONTOUR:
PRESSURE CONTOUR:
GRID INDEPENDECE TEST:
| GRID NUMBER | ELEMENT SIZE(m) | Cd | Cl |
| 1. | 0.05 | 0.342 | 0.183 |
| 2. | 0.04 | 0.326 | 0.217 |
| 3. | 0.03 | 0.307 | 0.218 |
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:
- By having simulated several steady state simulations, performing a mesh dependency with three cases on a Ahmed geometry, obtaining for each one of them their own significant convergence according to the velocity, drag and lift coefficients to a quasi-steady results, the final (average) drag and lift coefficients were calculated for all of them.
- When We compare the CFD results to experimental results, all cases are close to the experimental results.