Computational Fluid Dynamics (CFD) Modeling of Turbulence Induced Drag in Vehicle Aerodynamics.
ABSTRACT
This research work focused on the use of an hybrid RANS-LES turbulence model in simulating and checking the drag performance characteristics of an aerodynamic vehicle.
The Spalart-Allmaras shear stress transport (SST)- Scale Adaptive Simulation (SAS) Turbulence model was selected for use, and with the help of OpenFOAM Finite Volume Computational Fluid Dynamic Solver the simulation algorithm was set and carried out in a total time step of 4000.
Convergence was achieved under the stipulated time step based on our set convergence criteria. The geometries were downloaded in their stereolitography (STL) format, then SnappyHex Mesh meshing tool was used to generate our mesh grid. The first simulation which took 674870secs to run gave a coefficient of drag value of 0.522497.
Then with the help of FreeCAD Open Source Software, the vehicle shape was carefully altered and on running the simulation on the new shape it produced a drag coefficient value of 0.415411 in 669116secs, which represents a coefficient of drag reduction of about 20.49%.
Thus OpenFOAM CFD Solver helped to carry out analysis on the turbulent flow over a motorbike and aided in reducing the drag coefficient of the motorbike.
Further optimal shape analysis studies if carried out can help an aerodynamic engineer produce more fuel economical vehicles for consumer use.
INTRODUCTION
To save energy and to protect the global environment, fuel consumption reduction is a primary concern of automotive development.
In vehicle body development, reduction of drag is essential for improving fuel consumption and driving performance, and if an aerodynamically refined body is also aesthetically attractive, it will contribute much to increase the vehicle‟s appeal to potential customers.
The main driver for lower aerodynamic drag is fuel economy (Kranson, 1985). As long as standards for fuel economy increase and fuel costs go up, aerodynamic drag will have to be improved.
However, as the passenger car must have enough capacity to accommodate passengers and baggage in addition to minimum necessary space for its engine and other components, it is extremely difficult to realize an aerodynamically ideal body shape.
The car is therefore obliged to have a body shape that is rather aerodynamically bluff, not an ideal streamline shape as seen on fish and birds. Such a body shape is inevitably accompanied by flow separation at the rear end.
Two elements that have major influence on the drag coefficient of a bluff object are the roundness of its front corners and the degree of taper at its rear end.
When aerodynamics is considered from a fuel economy standpoint, the primary focus is the coefficient of drag ” “. Essentially, this is how easily a vehicle moves through the air, though drag isn’t the only factor that is considered.
There’s more to aerodynamics than just drag, there’s down force and lift and there is yawing moment, which is basically when in a crosswind, how much the vehicle gets steered by the wind that is pushing on it, and then there’s noise (Kranson, 1985).
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