Could someone give me a explanation of how a quasi-steady solution differs from the unsteady solution? Thanks!

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.

I would say that a non-steady-state flow viewed in a given reference frame is considered quasi-steady if the time-average (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 quasi-steady flows are periodic flows, though I think some authors would include in the quasi-steady category turbulent flows in which the turbulent quantity time-averaged statistics are independent of time.

I think what davoche describes is in thermodynamics referred to as quasi-static states of a thermodynamic system.

I don't understand how you could obtain a time dependant solution from an time average operation ?

Or maybe you think about phase average operation ?

Dear Himanshu,

"..........is considered quasi-steady if the time-average (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

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 quasi-steady 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 quasi-steady.

Yes, I was referring also to phase-averaging 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 quasi-steady. 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.