# Studying The Airflow over a car using an Ahmed Model

Objective: Significance of Ahmed model and determine the Coefficient of drag and Coefficient of lift over the model. Introduction: The Ahmed body was described originally by S. R. Ahmed in 1984 [1]. Three main features were seen in the wake: 1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of model 2. The B recirculation region that is formed due to the separation at the base of the model. 3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges. As the burning of fossil fuels becomes a more pressing issue, manufacturers are introducing more fuel-efficient cars to the market. One main contributor to fuel burn is the car’s aerodynamic drag. Complexly shaped, cars are very challenging to model and it’s difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model while maintaining car-like geometry features. Why Study the Drag Coefficient in Cars? The drag coefficient quantifies the resistance of an object in a fluid environment. It is not an absolute constant for a body’s shape because it varies with the speed and direction of flow, object shape and size, and the density and viscosity of the fluid. The lower the drag coefficient of an object, the less aerodynamic or hydrodynamic drag occurs. In terms of a car, the lower the drag coefficient, the more efficient the car is. As well as affecting the top speed of a vehicle, the drag coefficient also affects the handling. Cars with a low drag coefficient are sought after, but decreasing the drag drastically can reduce the downforce and lead to a loss in road traction and a higher chance of car accidents. Drag Calculation using ANSYS. For any computational process we required following steps are: 1. Create a CAD model of ahmed model 2. Meshing 3. Pre-processing 4. Setting up the physics 5. Post-processing Creating CAD model: Here cad is created using Creo 5.0 and imported into ANSYS Design modeler Meshing: Solver preference used is fluent solver with patch conforming algorithms.meshing is done of 503141 elements and 105793 nodes. Inflation is provided in the outer surface of Ahmed model make meshing more accurate. Pre-processing: It is a steady-state pressure base based solver with absolute velocity formulation. Here gravity is neglected. Viscous K-epsilon realizable with the scalable wall function model. Initial condition is selected with free flow velocity is 35m/s with air density is 1.225𝑘𝑔/𝑚3, Viscosity of air is 1.7894e-05 Solution method used is SIMPLE pressure velocity coupling scheme with second-order pressure, momentum, and turbulent K.E and dissipation rate along with least square cell-based gradient. Result: The system initiated the calculation using hybrid initialization for 500 iterations. Conclusion: The value of Cd and Cl is 0.47046 and 0.38961 respectively. Coefficient of drag and coefficient of lift of Ahmed model Observation: 1. The pressure is negative in certain regions mostly near the wall. 2. The pressure is high at the front of the model. 3. Velocity becomes almost zero at the backside of the model as shown in velocity contour representation. 4. Two eddy vortex can also be observed at the back trail of model. 5. The coefficient of drag(Cd) is greater than the coefficient of lift(Cl) of Ahmed model.

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