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Umarahmad12 April 14, 2013 07:50

Internal Flow - Incompresible Fluid with Changing Temperature
Hello, firstly thankyou for taking the time to read this!
I am fairly new to CFX and indeed ANSYS, I am trying to model a flow involving an Ejector Nozzle, it is derivative work based on some validation studies undertaken by NASA in the early 1990's. The Ejector Nozzle is comprised of a converging diverging nozzle with a smaller primary nozzle at the inlet as can be seen by the accompanying picture. The original work that I am basing this simulation on states that the flows involved are all less than a mach number of 0.3 at the inlet, obviously indicating that the flows are incompressible. However, the original work also states that the two inlets are not just at differing mach numbers, but also differing temperatures and pressures, after leaving the primary nozzle exit the two fluids enter a mixing chamber where the pressure and temperature of the mixed fluid changes. My question is essentially, is it possible to simulate such a flow?... that is incompressible and has changing temperatures and pressure. Or is it safe to assume that the mach numbers will increase to a level of 0.3 and above when passing through the diverging section of the nozzle?

Any help that could be offered to me on this simulation would be greatly appreciated!!

The original work can be found at this web address:

Thanks in advance guys! :)

ghorrocks April 14, 2013 08:29

Temperature differences can be modelled as incompressible flow. So can pressure differences. But if a density difference is important then you must use a compressible flow model.

I would simply run both compressible and incompressible and see for yourself what the difference is. If you cannot run the whole simulation then just run enough so you can see what will happen.

Umarahmad12 April 14, 2013 08:47

Thankyou very much! It really is much appreciated.

I have found an orginal study that the NASA simulation was based on that states the two air streams are compressible and therefore i will model as such.

I have one more question if you have the time to answer?
What inaccuracies are found when simulating an incompressible flow with a compressible flow model, if any?

Again, many thanks.

ghorrocks April 14, 2013 18:29

If you model resistance along a duct or over an object at M=0.3 the difference between incompressible and compressible is usually a few percent. At lower Ma it is smaller, at higher it is bigger.

But some effects require compressible flow at low mach number. These include water hammer and other transient pressure waves; or things where the gas is compressed or expanded. Both of these are transient effects so the decision between incompressible and compressible is more complex for a transient flow compared to a steady state flow.

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