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PPPO December 20, 2012 04:17

Drag Force
drag force coefficients in a viscous fluid flow past obstacles depend on velocity differentials, fluid viscosity , and pressure p:
I have velocity distribution around obstacle.

How can i calculate drag force?

truffaldino December 20, 2012 08:25

The simplest way is to find difference between total x-momentum flow before and past obstacle

PPPO December 20, 2012 10:05

Please give me an example.

truffaldino December 20, 2012 14:30

Take a surface enclosing your object which is far away from the object

If you are doing to calculate the drag force numerically, split the surface into small faces and sum x-momentum flows over all faces (I suppose that the flow far upstreams goes in the x-direction). If the face area is S, then the x-momentum flow through this face is S*Vn*Vx, where Vn is the velocity component normal to the face.

The sum over all faces equals, according to the 2nd and 3rd Newton laws, to the Drag force.

If you choose your surface as a rectangular box whoose two biggest sides are perpendicular to the x-direction, then Vn=Vx on the upstream plane of the box and Vn=-Vx on the downstream side of the box. If other sides of the box are placed far away from the object (or they are frictionless walls with slip condition on them) you can put Vn=0 on these far sides and the drag force will be difference between the total flows through the upstream side and downstream side.

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