Week 3 - External flow simulation over an Ahmed body.

AHMED BODY
AIM: - Steady state external flow simulation over an Ahmed body to study it’s Aerodynamic properties using Ansys fluent.
OBJECTIVE: - Calculate coefficient of drag and lift. To perform grid independency test on Ahmed body.
INTRODUCTION: - The flow is largely turbulent around the vehicle under normal operating condition. In vehicles running at high speed the aerodynamic drag is exerted by the air flow over the body. The point at which the flow separates from the body causes turbulence in wake region of the body and decides the size of the force. It directly affects the vehicle performance in terms of fuel consumption, noise generation and road stability. Current design of vehicle includes the styling and shape of the car as an important aspect. In general, aerodynamic drag of passenger car is 0.3~0.5 for calculating Road-Load Power of the car. Coefficient of drag is the overall value of drag from body, skin friction and resistance to the air flow through the grill and radiator.
The passenger cars in general case are bluff body where large part of the surface is covered in flow and experiences the pressure drag. In the wake region the car body the large-scale flow separation and large vortices are formed. The major objective of the vehicle of aerodynamic design is to avoid the flow separation or control separation. The first objective is not completely possible at majority of operating conditions.
Ahmed Body: - Aerodynamics analysis is used in the design of car to reduce drag coefficient. Ahmed body is the standard representation of passenger car which has typical flow field around it. The general flow around the automobile body was captured and characterised by the experimental work of S. R. Ahmed. The Ahmed Body is used to validate the flow both in wind tunnel and in CFD environment. It serves as a reference model which describes the typical flow around the body. Further the car model is refined by validating against the reference model. It is useful tool to design the details of the feature of the car on the exterior. The flow separates at the rear of the body and in wake region separation zones are formed. Simulation helps us to decide the more accurate the drag coefficient of the Ahmed Body.
The Amed Body is having front rectangular part with round edged at all four edges followed by box. The box connects two parts. The rear part has slanting at top edge of the box running downward slope to the rear.
Length = 1.044m
Width = 0.389m
Height = 0.338m.
The 4 cylinders are attached to bottom face. The cylinder size is 0.03m. The cylinder height is 0.015m. At the rear the slant angle is 20°.