5 Replies Latest reply on Aug 11, 2010 3:29 PM by Bill McEachern

# How to use FloWorks to Compute the force of drag?

I am currently trying to find the force acting on a component that will be attached to a airplane wing using fluid dynamics (Drag force= 0.5*density*Drag Coefficient*A*v^2) and checking my results with FloWorks. I started with a simple sphere which I know I can calculate correctly analytically and comparing the results to the results from FloWorks. The results are different and my question is what am I doing wrong.

I selected:

-External Flow

-Air as the fluid

-Velocity of 50 m/s

-Global goal (Force in the direction of flow)

-Run

The only thing I could think of is that selecting the global goal is only accounting for normal forces and not shear forces but I'm not sure how FloWorks works.....?

Any help would be great.

Thanks;

Nick

• ###### Re: How to use FloWorks to Compute the force of drag?

use a surface goal and pick the force component goals you want as surface goals. You need a relatively fine mesh to get sensible results. You cna check the verification manual for flow over a 2D cylinder if you want to see how it does - pretty well really up to R'numbers of about ~6 million.

• ###### Re: How to use FloWorks to Compute the force of drag?

I tried a global goal and a surface goal for a sphere. I selected the force and the normal force in the direction of flow and the results being very similar were both about half the amount of the results I calculated analytically. Is it because I need a higher mesh.

• ###### Re: How to use FloWorks to Compute the force of drag?

maybe. depends on the flow regime. What r'number did you do your hand calc at? the Cd varies with R'number - goes from like .5 to ~.2 if I recall at super critical.

• ###### Re: How to use FloWorks to Compute the force of drag?

The velocity I imput into the general settings was 50m/s so that gives a reynolds number of 6,780,898  (Re=[v*l*ρ]/μ : ρ=1.207kg/m^3 μ=0.0000178 kg/m*s)   The only term im not positive on the is l (i treated it as the depth the compoment so for a sphere it would be the dia).

• ###### Re: How to use FloWorks to Compute the force of drag?

The K-epsilon turbulence  model that Flow sim uses starts to diverge from experimental in the flow regime you are considering. Check the validation manual and pick a super cirical R'number that Flow sim does a good job of and figure out what it takes to get a good number and then you can see how you do in the 6 million range. You can always put on the auto-adaptive meshing and through it to result resolution to 8 and give it a rip. That should produce as good a number as it is likely to get with out much mucking about. Though it might take a bit to compute.