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CFDstudent123 May 16, 2011 02:43

Drag forces
hii.can anybody tell me how to calculate pressure drag and skin friction drag seperately in fluent

Amir May 16, 2011 08:47

report->forces (set the drag direction ....)

CFDstudent123 May 16, 2011 09:34

thanks for reply..but from moniter forces ..we can find the total drag.but i need to calculate the pressure drag and the skin friction drag seperately

saclu May 16, 2011 09:44

if you want both don't use the drag monitor but
Report> forces, be careful to the director vector (give right values for X and Y). This will give you the pressure and viscous forces in the monitor

CFDstudent123 May 16, 2011 09:53

can you plz tell me that what will be the value of X any Y for pressure and Viscous Drag

shimmyya May 16, 2011 10:35

pressure should be same vector as flow direction

saclu May 16, 2011 10:43

What is your geometry and flow parameters?

CFDstudent123 May 17, 2011 00:55

i have a semi circular cylinder in a rectangular shaped domain . the left BC is velocity inlet , the right is velocity outlet and the top and bottom is symmetric.and its in 2D.
plz help me

saclu May 17, 2011 09:12

You can have a look to this link, it's an example for an airfoil

you will find how to proceed...

CFDstudent123 May 17, 2011 11:20

@saclu ..thanks for the reply..but here in the sample problem they have fount the lift coefficient and drag coefficient changing the force vector.but i need to find the coefficient of pressure drag and coefficient of skin friction drag separately....

saclu May 18, 2011 02:57

the example was just to show you how to find the right values for the drag direction vector (X et Y). Once you have these values go in Report>force, choose the the wall zone, completed the direction vector with the values you calculated then press print and you will have the pressure and viscuous force separetly on your monitor.

CFDstudent123 May 19, 2011 03:15

@saclu...hi your reply was very useful to me ..but now i need to find the time average value of coefficent of pressure drag....and i am solving for an unsteady state problem

saclu May 20, 2011 04:44


I wouldn't know how to do without using a UDF (User Defined Function) but it isn't a good idea if your not familiar with it. May be you could have an approximate value by solving a steady problem.

CFDstudent123 May 23, 2011 08:03

@saclu ..can u plz help me in the UDF part ...
plz tell me which function is used to get the pressure drag for writing the UDF

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