Visualising Compressible Flow
 

Something I was looking into while back but still not really found an answer, so thought I would try you lot.

I had been reading a beginners fluid mechanics book, which briefly introduced compressible flow with examples of nozzles, shock waves, Rayleigh and Fanno flow. It runs through the use of equations for basic calculations, but it doesn’t go very far with descriptions of what is physically happening with the fluid. Something I can’t seem to find with online searches either.

I think I’ve got my head round the movements and property changes to the fluid in sub/supersonic nozzles with isentropic flow. But when it looks at the effects of heat addition/removal and the effects of friction on sub/supersonic flows in a duct, I don’t seem to be able to picture/visualise what is happening that gives the strange changes in the fluid properties.

-- Below was the first stab at what is happening in Rayleigh flow, maybe someone will be able to see where I’m going wrong.--

Ideal gas with constant mass flow rate, no frictional effects, through constant cross-section duct with heat addition or removal.

Subsonic - heat added

I can see a dense gas expanding with the heat addition, the expansion pushing out giving this increase in velocity downstream, and with the relatively slow flow, acting somewhat against the upstream flow. So gives a drop in density and pressure across the heating point.

Subsonic – cooling

The flow energy is reduced with cooling and it slows (would there be a contraction?), this slower wall of gas has the upstream flow pushing on it increasing the pressure and density.

Supersonic – heat added

Having a guess here. The increase in energy is causing the molecules to become more excited and that increases the pressure, this wall of pressure is acting against the flow and slows it, but as the flow is moving very fast it doesn’t affect the upstream. It just puts a break on it as it passes the heating point. The flow backs-up downstream increasing the density and pressure further.

Supersonic - cooling

A guess. The reduction in the energy the molecules have after cooling, gives the reduction in pressure. This low pressure void that’s created allows the upstream to push through into the lower pressure downstream increasing the velocity. The molecules are moving off at a higher velocity giving the reduction in density.

-- After a reply on another forum, another though on what might be happening came up. --

Should I be thinking more towards the idea of the heating or cooling at the duct wall as creating a sort of artaficial diffuser or nozzle?

The property changes (Velocity, Density, Pressure, Temp) for supersonic heating match that of a supersonic diffuser (reduction in cross section in flow direction). It got me thinking maybe its just a reduction in the cross section of the main flow that gives the property changes.

Supersonic cooling matches a supersonic nozzle.
Subsonic heating matches a subsonic nozzle.
Subsonic cooling matches a subsonic diffuser.

Hopefully that makes sense.

Cheers

s34n