# When can we decide a flow is unsteady or periodic? and How?

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 April 26, 2017, 09:08 When can we decide a flow is unsteady or periodic? and How? #1 Member   vidyadhar Join Date: Jul 2016 Posts: 68 Rep Power: 3 Sponsored Links Hi All, I am running fluid(water) flow through a square channel at different Re. For small values of Re, I can observe that the streamlines are not changing with time (after certain duration of simulation) and I conclude that the flow has reached steady state. [Is it correct?] But, for higher values of Re, the streamlines are varying with time even after certain duration of simulation run. For how long should I run the simulation, at higher Re, to verify if the flow reaches steady state or if it remains unsteady? Or is there a way to decide that the flow will be in unsteady state or periodic (after certain Re). The mesh contains nearly 5 million cells. Regards, vidyadhar

 April 27, 2017, 01:07 #3 Member   vidyadhar Join Date: Jul 2016 Posts: 68 Rep Power: 3 Thank you Pilz for the reply. I am using a transient solver. If the flow becomes unsteady at higher Re, you are asking me to monitor the frequency of a flow variable. But, in my case, the pressure at outlet is equal to ambient and fixed. Can I try to plot the frequency of the flow velocity. I do not know how to plot the forces at the wall. Thank you.

 April 27, 2017, 04:02 #4 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 263 Rep Power: 4 > I do not know how to plot the forces at the wall. Put in system/controlDict Code: ```functions { forces { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( nameOfMyWall ); pName p; UName U; rhoName rhoInf; log true; rhoInf 1; liftDir (0 1 0); dragDir (1 0 0); CofR (-0.5 0 0); pitchAxis (0 1 0); magUInf 1; lRef 1; Aref 1; } }``` You have to adapt the name(s) of your wall(s) and the direction of the main stream which results in dragDir and liftDir. Beside this, the functions needs to now the free stream velocity magUInf and some other values. __________________ Uwe Pilz -- Sie ahnen nicht, wieviel Poesie in der Berechnung einer Logarithmentafel enthalten ist (Carl Friedrich Gauß)

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