Timestep on cylinder

vasepan · June 11, 2014, 17:12

I am running a few simulations and I found something odd. Since I am a new user probably what I might ask is silly.

I want to obtain a Reynolds number 60. Hence I set my freestream velocity to 1m/s and my diameter of the cylinder to 0.1m. Using icoFoam I have to define nu.Hence in order to get the Re I want I set it to 0.001667 which gives the required value. However this value doesnot correspond to a specific fluid. I then decide to choose air which has a viscosity of 0.000015328. Hence for the same cylinder i need a velocity of 0.01 approximately. All ok. The problem starts when I run the simulation. It takes 100 times more to produce vortex shedding. Which means set timestep to 0.2 and running simulation to 1000. What does this mean? That I need 1000secs to obtain same results whereas before I needed only 10? Doesnot sound physically correct or is it my idea?

Any help would be deeply appreciated.

vasepan · June 13, 2014, 12:13

I managed to figure out the above. It is correct since the non-dimensional time the phenomenon occurs is the same. However, I would like to ask you something. When I have a problem of Re=100 for a cylinder I noticed something. I run my simulations with timestep=0.002 and timestep=0.001 and monitor the fluctuating part of the velocity at some point downstream. The graphs are approximately the same. When I change my timestep to 0.0005 the solution at the specific point becomes "shifted" by a certain amount and notice fluctuations at a later point. Has this something to do with my fvschemes?

I use icoFoam and backward scheme for time and Gauss linear for the other parameters.

Any help would be deeply appreciated.

June 11, 2014, 17:12	Timestep on cylinder	#1
vasepan New Member Join Date: Jun 2014 Posts: 8 Rep Power: 0	I am running a few simulations and I found something odd. Since I am a new user probably what I might ask is silly. I want to obtain a Reynolds number 60. Hence I set my freestream velocity to 1m/s and my diameter of the cylinder to 0.1m. Using icoFoam I have to define nu.Hence in order to get the Re I want I set it to 0.001667 which gives the required value. However this value doesnot correspond to a specific fluid. I then decide to choose air which has a viscosity of 0.000015328. Hence for the same cylinder i need a velocity of 0.01 approximately. All ok. The problem starts when I run the simulation. It takes 100 times more to produce vortex shedding. Which means set timestep to 0.2 and running simulation to 1000. What does this mean? That I need 1000secs to obtain same results whereas before I needed only 10? Doesnot sound physically correct or is it my idea? Any help would be deeply appreciated.

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June 13, 2014, 12:13		#2
vasepan New Member Join Date: Jun 2014 Posts: 8 Rep Power: 0	I managed to figure out the above. It is correct since the non-dimensional time the phenomenon occurs is the same. However, I would like to ask you something. When I have a problem of Re=100 for a cylinder I noticed something. I run my simulations with timestep=0.002 and timestep=0.001 and monitor the fluctuating part of the velocity at some point downstream. The graphs are approximately the same. When I change my timestep to 0.0005 the solution at the specific point becomes "shifted" by a certain amount and notice fluctuations at a later point. Has this something to do with my fvschemes? I use icoFoam and backward scheme for time and Gauss linear for the other parameters. Any help would be deeply appreciated.