I am trying to perform a CFD on a car model but after simulation I observed that the direction of velocity vectors went in opposite direction. I have applied velocity inlet condition at inlet. Why it is happening?? Also I am getting negative value of drag coefficient. Using K-epsilon model.
The pattern of velocity vectors demonstrates a wake (very low flow) and flow vortex region at the front of the car and this incorrect, the wake region should be at the rare of the back. Hence, you should check the boundary conditions, are you set the velocity at the inlet of the physical domain? Are you set the pressure at the end (outlet) of the domain?
Try a smaller size of time-step and refine your mesh. Also, have a look at your boundary conditions to make sure that you’re not over constraining!
What is the direction of your coordinate axis at the inlet cross-section? Is the positive direction of the coordinate axis pointing outward of the domain? In this case the specification of a positive inlet velocity value would potentially mean, that your velocity is not directed from the inlet cross-section towards the car mdoel, but pointing away from the car model.
Yes I have applied correct boundary conditions
Problem with co-ordinate system???
Thomas Frank
Sir, right now I don't have it because I have changed the conditions later on. But I can show you the axes system and orientation of my model. Click on the image to see the full view.
Seems like my suspicion was right. The x-axis is pointing to the outside of the domain at your inlet boundary condition (to the left of your posted image). If you specify a positive velocity magnitude, it probably will automatically lead to an inward pointing velocity vector with a negative x-velocity component (u_x). If you specify all three velocity vector components individually, than the u_x component needs to have a negative value, otherwise you deine at the inlet boundary condition a velocity block profile, which is actually sucking out the air from your domain, leading to the observation in your first posted image (velocity field in the opposite direction than expected).
Regards,
Dr. Th. Frank.
Thomas Frank Thank you sir my problem is solved but now I am getting negative value of Co-efficient of drag
Why this is happening??
Drag is essentially a normalized forced. The force has a direction with respect to the used coordinate system. You can either use a coordinate system which points with its x-coordinate vector from the inlet towards your car model or you can just simply change the sign of the drag force, from which you deduce the drag coefficient.
Regards,
Dr. Th. Frank.
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