# Differing results on drag force for simulation and calculation

Question asked by Romano Capocci on Nov 20, 2014
Latest reply on Dec 3, 2014 by Romano Capocci

Hello,

I am carrying out a flow analysis on my model in which I wish to find out the drag on my vehicle when it is subjected to water at different flow rates. I am getting different results for my simulation and basic hand calculations.

I carried out a simulation on my model. The front surface area is 0.1064 m^2 and the flow conditions were as follows:

• water density 1035 kg/m^3
• velocity in the X direction - 3 knots (-1.54 m/s)
• laminar and turbulent flow

I set up a Global Goal of Force in the X direction. This was to check the drag on my model. The result I got was -6.8905 kgf. I tried it with different mesh settings and the results were similar. I then calculated the drag using the drag equation 1/2*(Density of water/gravity)*A*(V^2)*Cd. I used 0.9 for the Cd. When I carried out this equation I calculated the drag force to be -11.98 kgf.

To test the simulator I then tested a regular shape - a sphere. The sphere had a radius of 0.2 m giving it a front surface area of 0.125 m^2. I used the same flow conditions as before and the drag force in the X direction was 1.6 kgf at mesh setting 4 and 3.24 kgf at mesh setting 3. (mesh setting 8 gives similar results to mesh setting 4). I then calculated the drag using the equation above with a Cd 0.47. The result I got was 7.38 kgf.

These are large differences between simulation and calculation. It is baffling me because I also tried it with a cube with 0.4m*0.4m*0.4m dimensions and a Cd of 1.05 and the results of the simulations and calculations are similar.

Is there something simple I am missing? If anyone could help me with this problem I would be very grateful as I have spent weeks messing around with it.