flow around a cylinder with different step using icoFoam

Aaron_L · August 24, 2016, 03:38

Hi, foamers
I run a flow around a circular cylinder using icoFoam, and I get everything OK,lift and drag coefficient, St number are in agreement with literature, but the lift coefficient different with different time step. can you give me some suggestions?

Artur · September 1, 2016, 08:24

HI,

It seems the difference is only in phase shift between the two signals, correct? If so then I wouldn't worry about it too much as the period up to when the flow becomes fully developed is not very physical anyway (you probably star the simulation with U=Uinf everywhere, including right next to the cylinder?).

All the best,

A

Aaron_L · September 2, 2016, 05:45

Hi,Artur
Thanks for your reply, my consideration is that flow over a cylinder should be time-independent, if I reduce time-step, the simulation phenomena should be the same, why I using icoFoam and PISO algorithm, the lift coefficient phase change? Is the PISO algorithm problem?or I get something wrong?

Best wishes,
Aaron

Artur · September 2, 2016, 05:52

What I'm trying to say is that your initial condition may not be physically correct and if that's the case the initial transients are not going to be either. If you'd like to simulate development of the instabilities you should initialise your simulation with zero velocity and then ramp it up to the speed you want (analogous to putting the geometry into a wind tunnel and turning it on). Then you should get similar results with both time steps. For problems like this one it's customary to just discard the initial period and only consider the data once the flow is fully developed, however.

All the best,

A

Aaron_L · September 2, 2016, 06:06

hi, Artur
many thanks for your detailed explanation, I will try it

best wishes,
Aaron

lorirosanp · November 16, 2023, 22:58

Quote:

Originally Posted by Aaron_L

Hi, foamers
I run a flow around a circular cylinder using icoFoam, and I get everything OK,lift and drag coefficient, St number are in agreement with literature, but the lift coefficient different with different time step. can you give me some suggestions?

Hey Aaron, do you have the case setup for the cylinder? I am encountering some issues. Also did you manage to solve your problem?

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September 1, 2016, 08:24		#2
Artur Senior Member Artur Join Date: May 2013 Location: Southampton, UK Posts: 372 Rep Power: 20	HI, It seems the difference is only in phase shift between the two signals, correct? If so then I wouldn't worry about it too much as the period up to when the flow becomes fully developed is not very physical anyway (you probably star the simulation with U=Uinf everywhere, including right next to the cylinder?). All the best, A

September 2, 2016, 05:45		#3
Aaron_L New Member Aaron Join Date: Apr 2016 Posts: 24 Rep Power: 10	Hi,Artur Thanks for your reply, my consideration is that flow over a cylinder should be time-independent, if I reduce time-step, the simulation phenomena should be the same, why I using icoFoam and PISO algorithm, the lift coefficient phase change? Is the PISO algorithm problem?or I get something wrong? Best wishes, Aaron

September 2, 2016, 05:52		#4
Artur Senior Member Artur Join Date: May 2013 Location: Southampton, UK Posts: 372 Rep Power: 20	What I'm trying to say is that your initial condition may not be physically correct and if that's the case the initial transients are not going to be either. If you'd like to simulate development of the instabilities you should initialise your simulation with zero velocity and then ramp it up to the speed you want (analogous to putting the geometry into a wind tunnel and turning it on). Then you should get similar results with both time steps. For problems like this one it's customary to just discard the initial period and only consider the data once the flow is fully developed, however. All the best, A

September 2, 2016, 06:06		#5
Aaron_L New Member Aaron Join Date: Apr 2016 Posts: 24 Rep Power: 10	hi, Artur many thanks for your detailed explanation, I will try it best wishes, Aaron