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April 23, 2012, 23:59 
Lift & Drag Meshing Techniques

#1 
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Tyler
Join Date: Apr 2012
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Hey,
Having a few problems trying to model a wind tunnel experiment we were required to do. In the experiment we put a small cube in a wind tunnel to see what the lift and drag forces were on it. We have results of around 0.3N of force in drag and no lift (a face was square to the wind). Only problem is when we model this in Ansys CFX we can't get anywhere near that result. We are modelling it as if it were in the tunnel so the distance between the object and the walls/outlet aren't ideal. Inlet speed is 20m/s, Outlet average static pressure is 0Pa (relative to atmospheric). In nearly of the settings we tried, we haven't been able to get the drag force down below about 0.7N. If it was around about 0.4, I would be happy and would actually potentially be able to get some kind of meaningful result out of it but being over 100% out is a bit too much. We did only eyeball the angle so I'm not looking to model it exactly, but something sort of close would be nice. As such are there any techniques with meshing that someone would recommend for best capturing the lift and drag of an object? Inflation? Sphere of Influence? Any tips or recommendations would be great. Thanks Tyler 

April 24, 2012, 04:36 

#2 
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Sijal Ahmed Memon (turboenginner@gmail.com)
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Location: Islamabad Pakistan
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First of all, this problem is more related to solver than meshing.
First I discuss the meshing part: For drag prediction you need to ensure three things: 1. Y+ should be less than 1. 2. Boundary layer should be resolved properly with at least 3040 nodes 3. Wake region meshing requirements. 

April 24, 2012, 07:33 

#4 
Senior Member

Hi,
i have a few points to highlight.. 1) drag and lift convergence depends upon your the set of equations you have selected.. for eg..only euler equations with or without energy equations, or higher order equations involving viscosity, turbulence, reynolds stresses, etc..These affect your final value as you move closer to reality. No matter what, the numerical simulations have always resulted in a slightly higher value compared to experiments. This is because it is very very difficult to simulate real life conditions with approximations. 2) Apart from what far has told some other approximation for the solver are a)using ideal gas instead of constant density b)check your mach number..you might also want to use compressible flow instead of incompressible.. c) Dont use laminar flow models. d) try fluent as well.. 

April 25, 2012, 04:14 

#5  
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Tyler
Join Date: Apr 2012
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Quote:
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Also, when I have inflation turned on I can't get the momentum and mass equations to converge. Specifically the RMS V Moment and the RMS W Moment, which both sit around 3e03. The Mass and the RMS U moment both eventually go to below 1e04. Is this likely to be causing odd results and if so any ideas on how to get around it? Thanks Tyler 

April 27, 2012, 14:43 

#6 
Senior Member

1.Mach numberYeah the speed is very less..I just wanted you to make sure that the mach number on the surface of the airfoil does not cross 0.6.
2.ConvergenceFrom a first glance, i think you have suppressed some kind of variable (approximation) through the selection of model which might be getting generated during actual simulation.. (Huge temperature variations might also cause this problem. Try switching off the energy equation to confirm this hypothesis)..It must be affecting the final results but i am not expert enough to guess the weight age of effect.. All the best!!! 

May 2, 2012, 23:53 

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