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shirley February 5, 2004 11:58

nozzle
 
Ií»v been working on a simple problem, a converging-diverging nozzle. I am a fairly new user and was quite confused at several things. The dimensions of my nozzle are as following: Inlet pressure 200Pa, Outlet pressure 0.5Pa, Inlet diameter: 12mm, Throat diameter: 2mm, Outlet diameter: 12mm, First of all, I doní»t know which Solver is best for my case. I tried segregated/implicit and the Residuals was set to 10^-6. Then I set the pressure inlet and pressure outlet. It looks like the Residual for continuity would never reach 10^-6. So I tried Pressure Far-Field boundary, and set Mach number as 0.6(default). I doní»t know what this Mach number mean? Is it the Mach number at the outlet? How can I get the correct value for it? Of course the solutions were converged, but I really doní»t know whether the result is reasonable or not? The second thing is that I use the default Laminarin the viscous model . Is that mean the flow at the inlet and outlet is Laminar? I hope someone can give me some hints and suggestions!

Thank you very much, Best regards, Shirley

Nandu February 6, 2004 06:37

Re: nozzle
 
Hi,

for your specific case according to isentropic relaitons for air (gamma = 1.4) the required relations to pressure are:

area ratio = 6 Mach number = 3.36 Pe/Po = 0.0158

hence for an inlet pressure of 200 kPa (i am assuming this is a stagnation pressue at the inlet) the outlet pressure would be 3.6 kPa. The flow through the outlet would be supersonic and hance it will make no difference what pressure u set at the outlet, as the pressure for a supersonic outlet will be extrapolated from the interior of the domain.

I would suggest you try the following, set up your problem again using the coupled solver with the explicit option. solve the problem as an inviscid analysis at first. set the inlet as a pressure inlet with the staganation conditions of the flow (pressure and temperature) or as a far field inlet with the static pressure, temperature and Mach number for the flow where mach number = sqrt(1.4*static temperature* 287.3). place a pressure outlet boundary at the outlet with the pressure as would be got from isentropic relations, but for this case as the flow is supersonic as mentioned earlier it would not make any difference.

NOTE: set the operating pressure = 0, and the other pressure values relative to this.

hope it works cheers nandu


shirley February 6, 2004 12:33

Re: nozzle
 
Thans very much, Nandu,

I'll try your idea right now, but by the way what do you mean Mach number = sqrt(1.4*static temperature* 287.3). Isn't it the speed of sound (= sqrt(1.4*static temperature* 287.3)? How can I get the Mach number at the inlet? Do you have any idea? Thanks, Shirley

Nandu February 9, 2004 01:58

Re: nozzle
 
yes, my mistake with that one, the formula is for the speed of sound......

M = V/[sqrt(1.4*static temperature* 287.3)] where V = the velocity at the inlet

this is easist when u use the far field inlet.....

if you choose to use the pressure basd inlet then u have to enter the total pressure, static pressure and total temperature at the inlet...the formulae can be picked off any decent gas dynamics (or fluid dynamics, compressible flow) text book

cheers nandu

shan February 17, 2004 01:43

Re: nozzle
 
why you choose laminar as viscous model. In your case,the flow in nozzle must be turblence flow.

shirley February 23, 2004 12:08

Re: nozzle
 
The Re at the throat is less than 1000, that why I use the laminar model. Is that Ok?


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