Capturing Shaock Diamonds
 

Hello, I am using a combustion problem and using K-Epsilon modelling. It is a Supersonic flow from a Nozle. I am supposed to capture atleast 5 to 6 shock diamonds before the flow diffuses. But I get only one shock! I have tried,, both Boundary and Gradint adaption. No use. Can any one suggest me a way to cature more shock diamonds and suggets a better model of turbulence, if any?

Srivatsan

Re: Capturing Shaock Diamonds
 

Hi Srivatsan,

I am also looking at a supesonic flow through a nozzle. My flow is underexpanding, but all I can see is one mach cell, barely, and then the flow diffuses. I have tried a lot of different models as well. How many cells did you use?

I used 188,000 cells in my whole domain, and they are structural elements in the critical area, and triangular in the not so critical area.

Did you stop the iteration process when the mass flux was close to zero? If you look at the pathlines, do they span all the way down to the edge of your domain?

i do not know what criterion to use for convergence. talk to you soon.

Re: Capturing Shaock Diamonds
 

What kind of nozzle are you modelling the flow in. What model do you use to capture the combustion.?

If the shock is due to the combustion in the domain then for capturing the shock you need to use a good model for cumbustion. The standard model used in fluent should do the job fine as long as you set the parameters right . As far as the model for turbulence K-E should work fine.Also do not set any parameters which you do not have much idea about. It might have a significant effect on the solution of the problem.

Re: Capturing Shaock Diamonds
 

Hi I am using a Conv-Div Nozzle. It is combustion of Propylene and Oxygen and so I use "species transport" model. The shock is expected outside the nozzle. The exit Mach number is and shd be 2 to 2.3. I dont touch the parameters of the K-E model and leave it default. The problem is I could not still find, more that 1 shock...But in relity there are up to 7 shocks!

Srivatsan

Re: Capturing Shaock Diamonds
 

Try to check the mach number at the exit.Also the best way i would suggest for modelling the shock at the exit is a simple oblique shock model. In supersonic flow shock is mostly an acoustic effect and i would suggest you keep the flow laminar and also 2-d outside the nozzle and try to capture the shock .

Re: Capturing Shaock Diamonds
 

Hi thanks for your suggestion.

I have tried a laminar appraoch. But the in practical it is turbulent. So I am adviced to do something else by my supervisor! My Aim now is to extend the supersonic zone. That is right now, the flow extends onbly up to 75 mm from the exit. I want to etend it up to atleast 120mm from the exit. Pls suggets me another alternative.

Also my flow dissipates quickly. Let me know a way to stop it..

Srivatsan

Re: Capturing Shaock Diamonds
 

Hi Srivatsan,

I am pretty sure that you wont be able to capture 7 shock cells using Fluent's current solver. If you are the please let me know, would be immense help. What i would suggest is to try and look at the centreline static pressure and try to infer shock positions from that. If you are using the second order solver in fluent, then the shock cells tend to get diffused rather quickly. the turbulence model will also adds numerical dissipation into the solution. try solving it as an inviscid solution, or use a laminar model, these will reduce the numerical diffusion introduced by the turbulence model, and ould tell you how many shock cells you will capture in the solution. almost a limitng case exercise.

The ideal way it to use higher order solvers, asn the shock cells after the 1st few are fairly weak, and the gradients across them probably wont get resolved with a second order solution. might want to try the new 3rd order MUSCL scheme in fluent 6.2

cheers Nandu

Hi there. Even I have been trying to simulate the diamond shock formation at the exit of a supersonic C-D nozzle. Can you briefly explain the setup procedure to capture these diamond shocks in the first place? and is it possible to capture these diamonds using just air as the jet with no combustion? Please reply quickly if possible. Thank you.