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.

Looking at the sphere: the Reynolds number for a sphere of 0.2m diameter in a uniform water flow of 1.54 m/s is on the order of 10^5, well within the turbulent region. You can ignore laminar flow here, the turbulence model will take care of the boundary layer. Let's do a simple analysis using the drag equation:

Fd = .5 * (density) * (speed)^2 * (drag coefficient) * (projected area)

Fd = .5 * (1035 kg/m^3) * (1.54 m/s)^2 * .47 * (pi * [0.1m]^2) = 18.1 N

The mass required to produce 18.1 N under Earth gravity = 18.1 N / 9.81 m/s^2 = 1.85 kg, pretty darn close to what Flow Simulation spat back at you.

p.s. why are you using this strange "kg-f" unit? I never heard of that before reading your post. Sounds like something they use in the imperial unit system to deal with the confusion between pound-force and pound-mass.