Different calc for lift between: force coefficient report and 6-DOF body force report

hxax · January 15, 2020, 10:16

This is for a hydrofoil vessel. Lift coeff force report say 80 tons, 6-DOF body force report 90. About 10 ton difference. What is this?

fluid23 · January 15, 2020, 10:35

A lift coefficient report will not have units of tons. You will have to provide more info about your report setups if you want any useful feedback.

hxax · January 15, 2020, 20:21

well, the body mass lifted in DFBI is about ten tons higher than the lift capability predicted by the standard force coefficient reports.

Is the pressure somehow accounted for is some additional manner?

ton= N*E3/(9,81).

fluid23 · January 16, 2020, 11:08

Since a force coefficient report doesn't directly indicate lift capability on its own, you have to make sure that your nondimensionalization parameters are appropriate. The coefficient report properties include reference values for pressure, density, velocity and area. If these aren't defined correctly then the force report and coefficient report will not agree. Likewise, if you are taking a coefficient and redimensionalizing it from a coefficient into a scalar value with units (i.e. N or lbf) then you need to make sure that the reference values used correspond to the coefficient assumptions. For that matter, also make sure that both reports include all the same surfaces/regions... it seems trivial but its easy to overlook the small stuff.

Now your second question is much more specific and probably a little easier to answer. I am not very familiar with 6DOF, but a quick read of the software documentation reveals a difference in the calculation of a force report and a 6DOF force report. Namely, it is the inclusion of user forces and gravity in 6DOF. A standard force report will only include pressure and shear forces.

That being said, it does sound like something is off. One wouldn't expect the inclusion of gravity to increase the net lift force.

I would suggest double checking your DFBI and report setups. Its impossible to troubleshoot without more info.

January 15, 2020, 10:16	Different calc for lift between: force coefficient report and 6-DOF body force report	#1
hxax New Member Norge Join Date: Nov 2017 Location: Norway Posts: 21 Rep Power: 8	This is for a hydrofoil vessel. Lift coeff force report say 80 tons, 6-DOF body force report 90. About 10 ton difference. What is this?

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January 15, 2020, 10:35		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	A lift coefficient report will not have units of tons. You will have to provide more info about your report setups if you want any useful feedback.

January 15, 2020, 20:21		#3
hxax New Member Norge Join Date: Nov 2017 Location: Norway Posts: 21 Rep Power: 8	well, the body mass lifted in DFBI is about ten tons higher than the lift capability predicted by the standard force coefficient reports. Is the pressure somehow accounted for is some additional manner? ton= N*E3/(9,81).

January 16, 2020, 11:08		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	Since a force coefficient report doesn't directly indicate lift capability on its own, you have to make sure that your nondimensionalization parameters are appropriate. The coefficient report properties include reference values for pressure, density, velocity and area. If these aren't defined correctly then the force report and coefficient report will not agree. Likewise, if you are taking a coefficient and redimensionalizing it from a coefficient into a scalar value with units (i.e. N or lbf) then you need to make sure that the reference values used correspond to the coefficient assumptions. For that matter, also make sure that both reports include all the same surfaces/regions... it seems trivial but its easy to overlook the small stuff. Now your second question is much more specific and probably a little easier to answer. I am not very familiar with 6DOF, but a quick read of the software documentation reveals a difference in the calculation of a force report and a 6DOF force report. Namely, it is the inclusion of user forces and gravity in 6DOF. A standard force report will only include pressure and shear forces. That being said, it does sound like something is off. One wouldn't expect the inclusion of gravity to increase the net lift force. I would suggest double checking your DFBI and report setups. Its impossible to troubleshoot without more info.