Lift fluctuations in a steady simulation?

k.vimalakanthan · October 16, 2011, 15:00

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,

truffaldino · October 16, 2011, 15:22

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.

k.vimalakanthan · October 16, 2011, 15:29

Quote:

Originally Posted by truffaldino

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.

Many thanks truffaldino.

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?

truffaldino · October 17, 2011, 07:12

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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October 16, 2011, 15:22		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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.

October 17, 2011, 07:12		#4
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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.