# Oscillating residuals in steady flow problem

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February 22, 2016, 06:08
Oscillating residuals in steady flow problem
#1
New Member

mohit
Join Date: Jun 2015
Location: Trivandrum
Posts: 13
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Hello,
I am recently trying to run a transonic flow simulation in cfd++ and have come across this phenomenon of oscillating residuals, even when the problem is specified as steady flow. I'm not able to interpret that and also not able to extract the data as it changes with changing residual. For your reference, htte snapshots of two of the cases have been attached herewith. Does anyone has faced such a problem or do anyone knows how to deal with such a case? Any kind of help would be appreciable.

Mohit
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 February 23, 2016, 12:38 #2 Senior Member   Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17 Have you considered the possibility that your flow isn't steady state? If you analyze an unsteady flow assuming it's steady your convergence can often behave as you have shown. If you are using an implicit solver, an iterative approach is used to advance the solution from a starting state (initialization) to a final (converged) state. This is true if the solution is one step in a transient problem or a final steady-state result. You can think of the iterations as a pseudo temporal progression that will still allow non-steady state behavior to develop.

February 23, 2016, 14:30
#3
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mohit
Join Date: Jun 2015
Location: Trivandrum
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Quote:
 Originally Posted by MBdonCFD Have you considered the possibility that your flow isn't steady state? If you analyze an unsteady flow assuming it's steady your convergence can often behave as you have shown. If you are using an implicit solver, an iterative approach is used to advance the solution from a starting state (initialization) to a final (converged) state. This is true if the solution is one step in a transient problem or a final steady-state result. You can think of the iterations as a pseudo temporal progression that will still allow non-steady state behavior to develop.
Hello Matt Sir,
First of all, thanks for your quick reply. As you mentioned flow may be an unsteady state since I'm simulating flow over payload fairing in transonic regime of M=0.9. But I would also like to mention that I have specified the simulation to be steady state in cfd++ i.e. the flow is supposed to be in steady state.
I would be grateful to you if you could please elaborate the meaning of solution being one step in a transient problem and the term pseudo temporal regression. What would you recommend that could be done in such a case?
Can you describe the significance of Courant-Freidrich-Lewy (CFL) number in a steady flow simulation?