QuasiSteady vs. Unsteady
Could someone give me a explanation of how a quasisteady solution differs from the unsteady solution? Thanks!

Re: QuasiSteady vs. Unsteady
From a physical point of view, a quasi steady solution refers to phenomena for which each temporal states of the evolution of your system can be found independantly from a steady state (by applying the same conditions). A contrarion, unsteady phenomena cannot.

Re: QuasiSteady vs. Unsteady
I would say that a nonsteadystate flow viewed in a given reference frame is considered quasisteady if the timeaverage (over a suitable time scale or period) of the flow quantities at each reference spatial location in the flow is independent of time. This would seem to imply that quasisteady flows are periodic flows, though I think some authors would include in the quasisteady category turbulent flows in which the turbulent quantity timeaveraged statistics are independent of time.
I think what davoche describes is in thermodynamics referred to as quasistatic states of a thermodynamic system. 
Re: QuasiSteady vs. Unsteady
I don't understand how you could obtain a time dependant solution from an time average operation ?

Re: QuasiSteady vs. Unsteady
Or maybe you think about phase average operation ?

Re: QuasiSteady vs. Unsteady
Dear Himanshu,
"..........is considered quasisteady if the timeaverage (over a suitable time scale or period) of the flow quantities at each reference spatial location in the flow is independent of time". I thought the definition looked more appropriate for stationary flows. A flow is said to be quasi steady if temporal variations at a spatial location are much smaller (they would be zero if the flow was steady) ompared to spatial variations for any quantity. Regards, Ganesh 
Re: QuasiSteady vs. Unsteady
That could be so, Ganesh. I was thinking after my post that turbulent flows whose statistics (mean flow and averaged turbulence) were steady would be classified as stationary. Some authors likely do use quasisteady to mean that the time variations are much smaller than the spatial variations. I was writing from vague memory, but I still believe that some authors refer to periodic flows as quasisteady.

Re: QuasiSteady vs. Unsteady
Yes, I was referring also to phaseaveraging for periodic flows. Because the flow pattern repeats itself periodically, when viewed over one or multiple periods, the flow appears steady, and hence is referred to as quasisteady. I seem to remember reading about periodic flows being classified as such, though my memory could be deceiving me, and I am too lazy to flip through my textbooks at this time.

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