Hydrostatic Torque

[simar187]

I am using Cosmos Floworks to model and measure the hydrostatic torque that the flow of air would have on the disk of a butterfly damper.

The mass flow rate of air at the inlet is 15.3125 lbs/s, the pressure is approximately 14.477 psi. Since the system has a pressure drop of 0.725 psi, I am setting the pressure at the outlet to equal 13.752 psi. The CFD measures the vertical (x-axis; down is +) and horizontal (z-axis; back is +) forces on the back face of the disk (the flow is coming from the shaft side), as well as the torques in the y-axis. Using the pressures identified above, with the disc open to an angle of 80 degrees, the CFD returns the following data:

SG X - Component of Normal Force 1 [lbf] 343.5396509
SG Z - Component of Normal Force 1 [lbf] 61.96892146
SG X - Component of Force 1 [lbf] 343.545461
SG Z - Component of Force 1 [lbf] 61.92268904
SG Y - Component of Torque 1 [lbf*in] 911.5025292
SG Av Static Pressure 1 [lbf/in^2] 13.76498049
SG Av Static Pressure 2 [lbf/in^2] 13.752
Pressure Drop [lbf/in^2] 0.012980487

However, if I change the outlet pressure to atmosphere, and the approximate temperature at my mass flow rate to 1atm+0.725psi, the CFD generates the following data:

SG X - Component of Normal Force 1 [lbf] -3.283694062
SG Z - Component of Normal Force 1 [lbf] -0.86486757
SG X - Component of Force 1 [lbf] -3.278246323
SG Z - Component of Force 1 [lbf] -0.908334988
SG Y - Component of Torque 1 [lbf*in] -31.90967134
SG Av Static Pressure 1 [lbf/in^2] 14.70810313
SG Av Static Pressure 2 [lbf/in^2] 14.6959473
Pressure Drop [lbf/in^2] 0.012155828

Its apparent that the forces and torque are changing significantly, despite the 0.752 psi pressure differential.

Any insights on how to explain this phenomena would be greatly appreciated.

Thanks, Simarjit Chouhan

 

[JimCasey]

>>>and the approximate temperature at my mass flow rate to 1atm+0.725psi<<<
If you are really getting the program to accept a temperature input in pressure units as you state, it's probably as confused as I am.

Suggest you check your inputs carefully and be sure that your units are consistent. Pressure in psiA (not psi, that can be inconsistent) . I suspect you loaded in PSIA in most places and you dropped in a PSIG or an ATMOSPHERE somewhere, without telling the program. It appears the flow direction reversed.

 

[simar187]

Sorry about the confusion. The word "temperature" should actually be "pressure" in the following line:
"and the approximate PRESSURE (NOT temperature) at my mass flow rate to 1atm+0.725psi"

Furthermore, all pressure values that I input into CFD software and those mentioned in the initial thread are absolute pressure values in PSIA. Also 1 atm = 14.695 PSIA and the temperature at the inlet and outlet is 113 degrees Fahrenheit.

 

[bcd]

First place I would do is review of your inputs again and then compare convergence of your solutions. Not sure what CFD program you are using, but maybe the second solution is not fully converged.

 

[simar187]

I have reviewed the inputs and all of the parameter solutions have converged. I think the problem may have something to do with where the torque and force parameters are being measured. I will be starting a new thread to address this problem. Thanks for your help.