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October 16, 2011, 15:00 |
Lift fluctuations in a steady simulation?
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#1 |
Member
Kisorthman Vimalakanthan
Join Date: Apr 2011
Posts: 49
Rep Power: 15 |
Hi guys,
I'm modelling an airfoil problem at high angle of attack (14 deg) using Ansys CFX (quasi 2D). Currently I'm facing fluctuations in the coefficient of lift value with iterations (please see attached pic). I have tired the simulation with higher physical time scale (even up to 50s), still it does not seems to converge with platoa convergence. After addressing this issue to some of my friends, they suggest that the simulation is exhibiting a unsteady behaviour in a steady simulation, which i'm not able to comprehend. Is this even possible? If this is the case shouldn't my increased physical time scale even out the unsteadiness of the flow? Any help is greatly appreciated,
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Kisorthman Vimalakanthan Dept. of Power and Propulsion Cranfield University Email: k.vimalakanthan@gmail.com |
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October 16, 2011, 15:22 |
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#2 |
Senior Member
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The steady solution at such aoa is not stable, at this aoa you will have a separation bubble which sheds vortices, just as in case of von-Karman votices around cylinder. Steady solution does not make sence here. You should run transient and then average forces over vortex shedding time period, if you need a mean force.
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October 16, 2011, 15:29 |
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#3 | |
Member
Kisorthman Vimalakanthan
Join Date: Apr 2011
Posts: 49
Rep Power: 15 |
Quote:
Do you have any idea why changing the local and physical time scales does not effect these 'bouncy' results? also is it reasonable to average the forces over this periodic steady simulation? Could you please also suggest any good reference for explaining these 'time marching' CFD techniques?
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Kisorthman Vimalakanthan Dept. of Power and Propulsion Cranfield University Email: k.vimalakanthan@gmail.com |
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October 17, 2011, 07:12 |
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#4 |
Senior Member
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I think averaging oscillations of steady solver does not make much sence. You should swithch to unsteady. In this situation I usually use implicit scheme with time stepping with courant number somewhere between .1 and 1. Courant number is the ratio of flow travel distance during a timestep to grid spacing C=v*dt/dl, where v-flow velocity, dt-time step, dl-grid step.
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Tags |
airfoil, fluctuations, lift, unsteadyness |
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