Drag coefficient and added mass

amine3002 · August 3, 2011, 04:18

Hi, I want to know how i can determinate the drag coefficient and the added mass of a vehicle moving in the water

cfdnewbie · August 3, 2011, 04:34

I dont know what you mean by added mass (may sth like fake mass due to negative lift??), but you determine the drag coefficient by integrating the pressure and viscous forces along the hull - the force opposed to the direction of motion is the drag.

truffaldino · August 9, 2011, 16:00

To accelerate vechile in water you need greater force than in the vacuum even if the steady flow drag is zero, so effect of the water can be seen as a faked mass added to vechile.

The roughest method to suggest to find this additional mass might be to integrate kinetic energy of water (i.e. get total water kinetic energy) multiplied by two and divide it by vechile velocity squared.

However I am not sure this method is completely correct as it assumes quasistacionarity of fluid motion as vechile accelerates. It will not work for sure in 2D for non-zero circulation flow, but perhaps it might be a realistic approximation in 3D.

August 3, 2011, 04:18	Drag coefficient and added mass	#1
amine3002 New Member Amine Fakhfekh Join Date: Jun 2011 Posts: 13 Rep Power: 14	Hi, I want to know how i can determinate the drag coefficient and the added mass of a vehicle moving in the water

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August 3, 2011, 04:34		#2
cfdnewbie Senior Member cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20	I dont know what you mean by added mass (may sth like fake mass due to negative lift??), but you determine the drag coefficient by integrating the pressure and viscous forces along the hull - the force opposed to the direction of motion is the drag.

August 9, 2011, 16:00		#3
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	To accelerate vechile in water you need greater force than in the vacuum even if the steady flow drag is zero, so effect of the water can be seen as a faked mass added to vechile. The roughest method to suggest to find this additional mass might be to integrate kinetic energy of water (i.e. get total water kinetic energy) multiplied by two and divide it by vechile velocity squared. However I am not sure this method is completely correct as it assumes quasistacionarity of fluid motion as vechile accelerates. It will not work for sure in 2D for non-zero circulation flow, but perhaps it might be a realistic approximation in 3D.