Learning Forum

[Usurper Shark]

I was trying to calculate the drag coefficient for a long thin rectangularr cylinder. The value of drag coffecient changes with the size of the enclosure I choose. I found values of -1.5, 1.14, 0.7. I do not understand the problem properly. Previously with other meshes the problem used to be solved when I used correct values for projected area. But for thsi mesh nothin is working. Can someone help me with a guidline. Thanks in anvance. 

[Rob]

The mesh quality wants some looking at too. Has the solution converged? 

As a general point, if the enclosure is too small you'll see an excessive flow acceleration around the object and pressure increase on the outer domain walls. That then adds too much force to the object which then causes the drag to not be as expected. 

[Usurper Shark]

What would be an excepable value for drag coefficient for a long rectangular cylinder. Should it be close to 1.5 ? I also got a negative drag coefficient once. What can be the reason for that ?

[Usurper Shark]

Yes my solution has converged. Mesh quality is not good enough. But the values of drag coefficient is so random that I thought it should be for some other reason.

[Rob]

I don't know what value you should expect, most of the correlations assume a cylinder of infinite length so you'll need to factor in end effects too. 

As the coefficient is a correlation it'll depend on what the area should be, and that may come down to what the correlation uses. Monitor the forces to see if they change (much) instead.