# UDf for calculating Cd in unsteady (transient) analysis

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 September 9, 2013, 05:39 UDf for calculating Cd in unsteady (transient) analysis #1 Member     Join Date: Jul 2011 Posts: 73 Blog Entries: 1 Rep Power: 10 Anyone had experience with making UDF for calculating drag coeff (Cd) in unsteady (transient) analysis? For example, a cube is rotating (sliding mesh, unsteady problem) and I want to plot (and write) Cd of the cube after every time step. __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 9, 2013, 06:16 #2 Senior Member   duri Join Date: May 2010 Posts: 130 Rep Power: 7 Why do you need UDF? Force monitor will work.

 September 9, 2013, 06:25 #3 Member     Join Date: Jul 2011 Posts: 73 Blog Entries: 1 Rep Power: 10 Duri, thanks for reply. Can you be more precise? What do you mean by "Force monitor will work"? Do you mean I can somehow setup this in Monitor options? Drag force = 0,5*density*Cd*A*v2 Keep in mind that A is changing during time, as well as Cd - and consequently drag force. Drag force is calculated in each step via integral(p*ds), so i guess if one knows change of A in each time step, then Cd can be calculated. But I am not sure how at this moment. __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 10, 2013, 04:29 #4 Senior Member   duri Join Date: May 2010 Posts: 130 Rep Power: 7 A is the reference area it is not necessarily the normal blockage area at that instant. (for eg., reference area of airfoil is chord length and it is same for all angle of attacks) It could be surface area or any reference area. If you are changing reference area to calculate cd at each instant, then whole purpose of drag coefficient is lost. In such case better use drag force itself.

 September 10, 2013, 04:44 #5 Member     Join Date: Jul 2011 Posts: 73 Blog Entries: 1 Rep Power: 10 For my purpose (irregular rotating object), A (maximal exposed area to flow) is changing as the body moves through the air (and rotate in the same time). For example, my body moves with velocity in excess of 4 Mach, and randomly tumbles. Velocity is lost exponentially, as well as kinetic energy. So, I want to determine fluctuations of Cd (from Cdmin to Cdmax) as body moves without stability through air. With that results I could use software for determination of maximal and minimal range of that body. __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 10, 2013, 05:09 #6 Senior Member   duri Join Date: May 2010 Posts: 130 Rep Power: 7 Reference area is used for engineering convenience and does not represent physics. Force are really critical for design and not the coefficient. It doesn't matter whether an object rotates or oscillates or what ever it does. Reference area is based on some geometry characteristics. (eg., In aircraft aerodynamics wing planform area is taken as reference area at all mach numbers and for all kind of trajectories, it doesn't matter it takes a cobra maneuver or cruise)

 September 10, 2013, 05:16 #7 Member     Join Date: Jul 2011 Posts: 73 Blog Entries: 1 Rep Power: 10 so, in short, what do you suggest, if you can explain it step by step? __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 10, 2013, 05:31 #8 Senior Member   duri Join Date: May 2010 Posts: 130 Rep Power: 7 Just calculated cd using some relevent geometric area and flow condition. Mention the reference conditions when you are supplying the data or publishing the data (not in numbers). Eg., like free stream condtion and base area. Then your drag coefficient would be very generic. Represent drag coefficient with respect to rotational angles or something relevant. When you represent interms of instantaneous area. It would be very difficult for the users to interpret, interpolate and retrive any information from your data.

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