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 K.S.Ravichandran August 28, 2000 08:15

stagnation pressure in transient flow simulation

Hi

While simulating transient flow in blow-down wind tunnel geometry, I noticed wall static pressures nearly double the upstream stagnation pressure. I know that the energy equation in terms of total enthalpy shows that the latter is proportional to the time derivative of the pressure and hence particle enthalpy can exceed its upstream stagnation value during motion. Is there a similar equation for stagnation pressure? what is a simple intuitive physical explanation of pressure being higher than stagnation pressure?

KSR

 John C. Chien August 31, 2000 00:26

Re: stagnation pressure in transient flow simulati

(1). Stagnation pressure is a derived local variable. It should be higher than the local static pressure, unless you are using the wrong equation. (2). If you trace the local fluid particle back to the upstream tank, then the original total pressure of the fluid particle in the tank should also be higher than the local total pressure due to total pressure loss from the tank to the current location. (3). So, you can trace the fluid particle anywhere in the tunnel back to its original state in the tank. (4). Now if the fluid particle in the tank keep their total pressure constant, then the static pressure everywhere should always be lower than the total pressure upstream. (5). But if the total pressure in the tank changes as a function of time, then the fluid particles released will have different total pressure of its own. In this case, it is possible to have downstream static pressure higher than upstream total pressure. (6). This is assuming that there is no numerical errors in your formulation and calculations.

 K.S.Ravichandran September 5, 2000 00:34

Re: stagnation pressure in transient flow simulati

John!

I must thank you for your response. However I must say that your arguments are not convincing. Specifically, I have observed that in impulsively started flows past blunt bodies in external flows, the stagnation pressure at early times shows a value that is higher than that at infinity. The initial condition here is that uniform free stream at all grid points (meaning stagnation pressure at every grid point is equal to that at infinity) and zero normal velocity on the body contour. Since dp/dt is positive at the nose at early times, the total enthalpy increases during the early transient. If the Mach number is low enough, the same should hold for stagnation pressure as well. Of course your arguments are correct for steady flow, but that is not what is under discussion. regards

Ravichandran

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