Week 3 - External flow simulation over an Ahmed body.

Aim: To Study External flow simulation over an Ahmed body.

Objective:

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.

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.

Theory :

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

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.

Outer Closer:

• 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.

Inner Closer

• 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.