Week 3 - External flow simulation over an Ahmed body.
Introduction: This study is to understand external aerodynamics over an experimental body (Ahmed body). To assess drag and lift and make sense from the contour plots.
Ahmed Body is an experimental struture which mimics an automobile. Many experiments have been conducted with this body and the observation and data extracted from these experiments have been documented and using this one can conduct simulations to validate the setup and can conduct simulation on similair structure (automobile).
As the flow passes over the body it creates a boundary layer, as no-slip condition exist in the flow there is zero velocity to the first layer of air. As the rest of the flow takes place the fluid (air) loses its momentem due to the viscous forces of the bottom layers. Due to this loss of momentum the fluid losses its energy and the oncoming fluid piles-up at this location, this causes increase in pressure, also known as adverse pressure gradient. This causes the flow to turn back (flowing against the general flow). Due to its slow velocity and high momentum of the upstreme air the reversed flow is not able to flow ahead hence, it detaches from the surface, this point on the surface is the Point of separation leading to creation of wake at the rear of the Point of separation.
As this low pressure wake is created, the fluid that is ahead of this wake flows back but due to the oncoming air there occurs a recirculation. This is bad a it gives rise to the pressure drag which is bad for aerodynamic performance of the vehicle.
The following result is from a study conducted with velocity of air as 25m/s with a grid independence test,
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Plots:
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Contours:
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Conclusion: As the grid independency is conducted in this case it seems that the grid is not yet independent as there dosen't seem to be anu congruence in the obtained Coefficient of Drag. Although, we could get to the point where refinement would not change the Cd but as limited by the licencing we cannot know for sure.