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-   -   Setup/monitor points of pressure and force coefficients (http://www.cfd-online.com/Forums/cfx/81225-setup-monitor-points-pressure-force-coefficients.html)

 siw October 20, 2010 10:12

Setup/monitor points of pressure and force coefficients

Hi,

I'm simulating the flow over a half aircraft model at http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaa-dpw/.

The flow is Mach No. = 0.85, Re = 5 x 10^6 (based on mean aerodynamic chord of about 7 m). Reference (stagnation) temperature = 100 °F. The target lift coefficient = 0.5. The wing (reference) area and mean aerodynamic chord are also given. All this data is given at the above website.

I've done some hand calcs to get the velocity and pressure for CFX:

Vx = 280.9 m/s, Vy = Vz = 0 m/s
p = 3384.6 Pa (quite low because of the given Re)

In CFX I've set the Reference Pressure = 0 Pa and the Relative Pressure = 3384.5 Pa based on the information in the User Guide (but the opposite setup is used for Tutorial 8 - Supersonic Flow over a Wing) which quotes:

"...a reference pressure of 0 Pa can be used without any problems when the dynamic pressure changes are significant compared to the absolute pressure level."

The dynamic pressure here is: q = 1/2 * gamma * p * M^2 = 1711.7 Pa.

The static temperture was obtained from the stagnation tempertaure and Mach number so: T = 271.7 K. I've based the timescale on this velocity and reference chord: t = 0.025 s.

The fluid domain is a hemisphere with a velocity inlet and pressure outlet. I've set the Avergare Static Pressure = 3384.6 Pa at this boundary.

I've set up some expressions for montior points and post-processing: coefficients of lift, drag and pressure as follows:

Drag = force_x()@AIRCRAFT
Drag Coef = Drag/(DynPress*RefArea)
DynPress = 0.5*1.4*areaAve(Absolute Pressure)@Inlet*(MachNo^2)
Lift = force_z()@AIRCRAFT
Lift Coef = Lift/(DynPress*RefArea)
MachNo = areaAve(Mach Number)@Inlet
PhysTime = RefLen/VelIn
PressCoef = (Pressure-aveArea(Absolute Pressure)@Inlet)/DynPress
RefArea = 383.7 [m^2]
RefLen = 7 [m]
RefPress = 0 [Pa]
RelPress = 3384.6 [Pa]
StatTemp = 271.7 [K]
VelIn = 280.6 [m s^-1]

I'm currently watching the solution progress and it's up to 25 iterations but the convergence is very slow and the momentum terms are not below 1.0e-03 and don't look like they'll drop much further. The monitor points of Drag Coef and Lift Coef are between 50 to 80.

My questions are:

1) How can I speed up convergence and get the residuals to get closer to the targets? Ok, I'm aware that I'm using the coarest mesh but it's still y+=1, ~9 million elements.

2) Are you in agreement/disagreement with the way I've setup the pressure i.e. relative vs. reference?

3) Are you agreement/disagreement with the way I've setup the expressions i.e. use of absolute pressure rather than total pressure?

Thanks

 ghorrocks October 20, 2010 18:22

FAQ for point 1:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria

 siw October 22, 2010 02:48

Thanks for the notice Glenn,

The simulation completed after 24hrs (the pain of not having a HPC facility but needing to run large meshes) and the residuals all held constant from 100 - 200 iterations at 1e^-2 to 1e^-3 for the momentum equations which was not the target. I guess the coarse mesh was too coarse but I'm also building medium and fine resolution meshes for comparisons so they may get to the target residuals. The CL and CD monitor points didn't start to hold constant until nearly 200 iterations so will need longer.

Can anyone confirm if my Expressions are correct. They are basic aerodynamics equations which I'm know are right but just concerned that I've called a wrong pressure or something, even though I've read the Guide.

Thanks.

 ghorrocks October 22, 2010 06:07

As mentioned in the FAQ, steady state convergence on finer meshes is often harder to achieve.

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