Learning Forum

[Peppinfante]

Hello everybody, hope you are all doing well.

I am having troubles with solving the following hypersonic flow problem in Fluent:

I have a 45 degrees, Mach 21.4, inlet pressure 9.79 [Pa] flow coming from the left side and hitting a blunt body (the mesh is the one in figure with a zoom near the stagnation point) before leaving the field through the outlet on the right side. The body is the nose of a re-entry space vehicle. I am studying both the 2d and the 3d problem but I get into the same trouble regarding ricombination of N2. Let's start from the 2d case. The used settings are the following:

-2d, planar, steady, density based

-Models: Energy ON with 2-temp model activated, laminar flow, radiation on the wall, Species (with reactions) using the air-5species Park model

-Materials: Mixture of air with 5 species (automatically inserted by Fluent)

-Cell zone condition: 0 operating pressure

-Inlet BC: Pressure Far Field with p=9.79 Pa, M=21.4, 45°angle, 226K for the T, 0.233 mass fraction for the O2 (the others are 0 anche N2 is automatically calculated)

-Outlet BC: Pressure outlet with 9.79 Pa, 45° angle and same mass fractions (I also tried different combinations)

-Wall BC: Adiabatic with 0 heat flux and radiation towards the flow with epsilon=0.8, zero diffusive flux for the species

-Method: Implicit AUSM, Green Gauss Node Based, I did both First Order and Second Order

-Initialization from inlet with also a FMG initialization

My only (but big) problem is the fact that the Nitrogen starts recombinating in the boundary layer and so I obtain again 0.72 mass fraction for N2 on the wall. This is totally unexpected and makes me have high Temperatures on the wall (about 1800K) and as a consequence a really high radiation heat flux (about 450 kW/m^2, the radiative equilibrium is satisfied). I have experimental data (that unfortunately I cannot share) in which there is no recombination in the boundary layer so there is no creation of energy (due to the recomb. process) and so the temperature are far below (about 1400K) with a radiative heat flux of about 200 kW/m^2. I am also adding pictures regarding the species so that the situation is more clear. Thank you for your time and your patience, every suggestion is well accepted. If you need any other picture let me know.

[RK]

Hello,
Here are a few recommendations :
I would suggest you first run the model without species until the flowfield is established in the domain (turn on high speed numerics using the TUI command)
I would also suggest you do a grid adaptation
How many inflation layers do you have? You could add 20-25 layers.
You probably know about it, but we have a hypersonic tutorial : Chapter 7: Modeling Hypersonic Flow (ansys.com) , however, this is without species. I hope this helps!

[Peppinfante]

Hi, thanks for the reply.
Everything started from that particular tutorial and in fact I wanted to add some complications using the S2S radiation model and enabling the reactions. Before starting a new run I just wanted to ask you 2 things:
Where do I have to adapt the grid, on the shock (where I already did), on the wall or everywhere?
Do you want me to increase the first layer height in the inflation layer? Now I have 1e-5
Thank you

[RK]

Adapting the grid along the shock is good.
First layer height.. this should be good too.

[Peppinfante]

Hi, I did what you suggested. I refined the mesh along the shock and performed a simulation without radiation and without species enabled. After I reached a second order convergence, I enabled the reactions, fixing a certain T on the wall. Everything was good (even if the second order solution couldn't converge). Sticking with a first order converged solution I enabled the S2S model for the radiation, changing the the wall BC to 0 heat flux (adiabatic). Unfortunately the results obtained are the same as before with N2 fully recombinating on the wall. I cannot understand which is the problem. Maybe I am setting the radiation model in a wrong way or maybe I am using the wrong radiation model? I just want to study a wall radiation of a wall that doesn't conduct heat inside but just radiate outside.
Again, the radiative equilibrium is obtained but a temperature too high (about 1800 K).
Thanks for your help, I am posting some pictures of:
Field with no radiation and no reactions
Field with just reactions and a fixed T (n2)
Field with radiation and reactions considering 0 heat flux (n2)

[RK]

That's good, at least you have narrowed down on the issue. So, I am not sure if the radiation modeling is right, my expertise to radiation is limited. There could also be a chance you have not set it right! I would suggest you read the radiation models in Fluent theory guide to check if S2S is indeed the right approach. S2S stands for Surface to Surface and in this case, it is a blunt body exposed to free stream. I don't see a way for radiation to reflect off from one surface to another. Again, this is an educated guess, you might want to check out the theory guide.
I am sure you will find some journal / conference papers on the radiation model.