Drag Force

PPPO · December 20, 2012, 03:17

Hi
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, 07:25

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

PPPO · December 20, 2012, 09:05

Please give me an example.
Thanks

truffaldino · December 20, 2012, 13:30

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.

December 20, 2012, 03:17	Drag Force	#1
PPPO New Member Ali Join Date: Nov 2012 Posts: 21 Rep Power: 13	Hi 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?

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December 20, 2012, 07:25		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	The simplest way is to find difference between total x-momentum flow before and past obstacle

December 20, 2012, 09:05		#3
PPPO New Member Ali Join Date: Nov 2012 Posts: 21 Rep Power: 13	Please give me an example. Thanks

December 20, 2012, 13:30		#4
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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 SVnVx, 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.