Basics of Timescale

FluidDynamics · March 20, 2012, 06:12

Dear All,

Please can anyone tell me why we use Timescale/ Timestep in CFD simulations?

Regards

lava12005 · March 20, 2012, 08:50

? what do you mean?
Are you asking why there is temporal discretization in CFD?

FluidDynamics · March 20, 2012, 11:10

For example in CFX we use Physical / Automatic Timescale or in FLuent we used Timestep.

I just want to know that what is the purpose of using timescale.

delaneyluke · March 22, 2012, 07:03

Firstly, a timescale is used in CFX as it is a fully implicit solver AND in Fluent (for the explicit solver) it uses relaxation factors.
The idea is to under relax your discretized equations as your solution proceeds to a steady state converged state. By under-relaxing your variables you are allowing only a part of the old value to be used to calculate the new value and so on so that your solution doesn't go out of hand and diverge.
I hope this makes sense to you.

In CFX the timescale refers to this under relaxation and is in no way related to the transient timestep.

Regards
Luke

FluidDynamics · March 22, 2012, 07:05

Luke,

Thanks a lot.

March 20, 2012, 06:12	Basics of Timescale	#1
FluidDynamics New Member Neal Join Date: Nov 2010 Posts: 9 Rep Power: 0	Dear All, Please can anyone tell me why we use Timescale/ Timestep in CFD simulations? Regards

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March 20, 2012, 08:50		#2
lava12005 Senior Member KHB Join Date: Aug 2010 Location: Singapore Posts: 118 Rep Power: 15	? what do you mean? Are you asking why there is temporal discretization in CFD?

March 20, 2012, 11:10		#3
FluidDynamics New Member Neal Join Date: Nov 2010 Posts: 9 Rep Power: 0	For example in CFX we use Physical / Automatic Timescale or in FLuent we used Timestep. I just want to know that what is the purpose of using timescale.

March 22, 2012, 07:03		#4
delaneyluke Senior Member Join Date: Mar 2009 Location: Indiana, US Posts: 186 Rep Power: 17	Firstly, a timescale is used in CFX as it is a fully implicit solver AND in Fluent (for the explicit solver) it uses relaxation factors. The idea is to under relax your discretized equations as your solution proceeds to a steady state converged state. By under-relaxing your variables you are allowing only a part of the old value to be used to calculate the new value and so on so that your solution doesn't go out of hand and diverge. I hope this makes sense to you. In CFX the timescale refers to this under relaxation and is in no way related to the transient timestep. Regards Luke

March 22, 2012, 07:05		#5
FluidDynamics New Member Neal Join Date: Nov 2010 Posts: 9 Rep Power: 0	Luke, Thanks a lot.