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-   -   Supersonic Jet Impingement (http://www.cfd-online.com/Forums/fluent/35775-supersonic-jet-impingement.html)

manish February 18, 2005 03:33

Supersonic Jet Impingement
 
Hello All,

I am trying to simulate a Mach 4.5 exit jet impingement of air on a flat plate. I am using a laval nozzle to obtain this. I have an inlet stag pressure of 180 psi and an ambient pressure of .1 psi in the chamber. I am using ambient temperature at the inlet, and I approximated the ambient temperature in the chamber. Are these the temperatures I should put into the bc? Should I use stag T? I do not know how to calculate that, because I do not know the mach number at the inlet? Did i set up my pressure bc wrong as well? I AM SO CONFUSED ON HOW TO DO THIS

If you can provide me with the methodology for modeling this problem, I would very much appreciate it. I am a new user in FLUENT.

I may have set my bc wrong. PLEASE HELP- Thank you.

-manish

N Menon February 20, 2005 07:32

Re: Supersonic Jet Impingement
 
if you are using a laval nozzle, then you should know the conditions at the throat of the nozzle. you can set the inlet conditions as the sonic velocity, and the pressure based on the stagnation pressure and a simple isentropic compression to the sonic condition at the throat.

cheers Nandu

manish February 20, 2005 09:34

Re: Supersonic Jet Impingement
 
Hi Nandu,

Unfortunately, I do not know the exact location of where the throat is(it is the minimum area, but it is hard to find that location). I was wondering if I could make the inlet conditions near the chamber. I know the stag pressure and mass flow rate. It is a 2d simulation, and I was wondering if I could make the exit of the nozzle as the pressure outlet, and make the surroundings of my chamber for now as a pressure outlet, and the plate as a wall. I have the centerline as symmetry condition. I am using a coupled implicit solver. I was wondering if this would be appropriate bc for good convergence. Should I also incremently bring the pressure up to the final pressure to assist convergence? Convergence is a very big problem for me. Any more advice that you could provide would be very beneficial. Do you think I can show you my mesh and bc? I hope to get this resolved soon. THANK YOU SO MUCH FOR YOUR HELP!

-manish

N Menon February 20, 2005 11:23

Re: Supersonic Jet Impingement
 
well i have got it working fine on fluent, as i am looking at supersonic jet impingement from square and elliptical nozzles.

u can send ur mesh to nmenon@mech.wits.ac.za

in any jet flow case you have to be very careful how you are analysing convergance. there are always slight instabilities in the jet boundary which is a very strong shear layer. monitering convergence in the traditional sense of looking at the residuals of the equations might not help. the residuals might just flatten out and not reach the low convergence value that u set. might be easier to compare match fluxes entering and leaving the domain, as a measure of convergence. i have found this to be pretty useful, and it matches well with expermental results in literature with regards to the shock structures.

implicit coupled solver is fine, but star with a low courant number. explicit coupled is more stable byut might take a lil longer to solve.

with regards to bc's use a wall for the impingement wall, the other boundaries of the chamber can be pressure outlets with ambient conditions as the bc. the inlet can be a far field condition or a pressure inlet, if u know twhat the stagnation values or the actual inlet Mach number is. hope this helps

nandu

manish February 21, 2005 19:28

Re: Supersonic Jet Impingement
 
Dear Nandu Menon:

I have sent you a private email. Thank you for your help. I just had certain questions on how to set up this problem. I am running it rite now. I am incremently increasing my inlet pressure(stag) to the desired pressure. Is this a good way of going? I left the outlet pressure and operating conditions the same for each run. I am running a first order-S-A turbulence model, coupled implicit solver. I have set it as an ideal gas. I am not able to get the influx and outward flux to match up(a large difference -300). i let it run for 1000 iterations. hope to hear from you soon. thanks again. manish

manish February 21, 2005 20:26

Re: Supersonic Jet Impingement
 
Anyone who ran supersonic impingement model see slight oscillations on their residual continuity plot as the simulation was running? Did they also see it a bit on their force plots? the best convergence I get is between 2-3 e-1. i cannot get it below that.

this is a first order run. i am waiting to reach my desired pressure. i am increasing the increment by .2. do you think this is a good idea?

thanks. manish

manish February 21, 2005 21:14

Re: Supersonic Jet Impingement
 
Is it necessary to increment the inlet conditions for a supersonic jet implingement? can i go directly to my case, and still maintain convergence. hope to hear from you soon. manish


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