[Panboy]

Originally Posted by krishna13j

Thanks Panboy. I am trying to capture the boundary layer transition on a re-entry capsule fro which all effects such as the finite rate chemistry, shock standoff distance, etc, need to be accurately predicted. I am new to modelling chemistry in flows so am not sure as to what the third body efficiency means and am not using them for now.

The problem I am facing when I try to run the solver, seems to be a technical one. When I try to run after defining the reaction, the error I get is that the Arrhenius number does not have the correct dimensions. The problem is that the CFX-pre does not have an option to specify any dimensions for the Arrhenius number.

For now, I believe you'll be ok without third body efficiencies. You should be able to run a simulation and get it to converge without. Third bodies will usually have small effect on the reactions.

I'm not sure what CFX units are as I haven't used CFX for reactions. CFX should have a manual that should state the dimensions of the Arrhenius rates. One question, how do you know the Arrhenius rates aren't the correct dimension?

[rvp604]

Originally Posted by AdeeMij

I've tried to provide a thorough description of my problem without overloading it with detail. If anyone has specific questions I'd be glad to provide as much info as I can.

For the past several months I've been putting together a solver setup for Fluent (12.0.16) to handle external hypersonic flow. The application problems will be 2D airfoil sections, 3D wings, and possibly simplified 3D vehicles.

I've gotten reasonable results for most of the validation cases I've run, at least as far as flow features: shock types and angles look right, expansion fans, boundary layer formation, etc show up where they're supposed to. The general flowfield looks good. However, the magnitude of surface profiles are way off, such as temperature and pressure distributions being off by an order of magnitude or more.

The biggest suspect to me is the chemistry. I looked into including finite-rate reactions early on, but had a lot of trouble getting them to run properly, so I dropped them. The target simulations will be run at fairly low ambient pressures (<1,000 Pa), and toward the lower hypersonic range (M=5.0-10.0). I was hoping that the errors would be acceptable, but that doesn't seem to be the case.

So I've turned back to including the finite-rate chemistry. I can converge to a reasonable solution with species transport, everything except reactions, but the moment I turn on the reactions Fluent fails. I was hoping someone here would be able to point me in the right direction or find a flaw in my chemistry model.

The general solver and model settings I'm using are:

• Density-Based Solver, Steady-State
• Energy Equation On
• k-w SST Turbulence Model
• Discrete Ordinate Radiation Model
• Species Transport, Volumetric Reactions, Eddy-Dissipation Concept Model
• Aungier-Redlich-Kwong Real Gas Model for Density
• Pressure Inlet/Outlet BCs, Wall BCs
• Boundary Turbulence Specification: I = 0.05%, Turbulent Viscosity Ratio = 1.0
• Calculating in Absolute Pressure (0 Pa Operating Pressure)
• AUSM Gradient Method
• All equations second order
• Solution Steering - Hypersonic, No FMG Initialization

If I left anything out, it's probably left as default, but feel free to ask.

I've poured over the Fluent docs to make sure I'm not missing some incompatibility between these models, and worked through all the tutorials I can find that relate. I have had trouble finding a good tutorial for setting up a custom chemistry model, or a solid hypersonic tutorial, although I have seen them referenced in several places.

The chemistry model I'm using is a 5-species, 5-reaction model:

Species Transport, Volumetric Reactions, Eddy Dissipation Concept
Third Body Efficiencies (for dissociation reactions), Calculate Backward Rates

Species:
N2, O2, N, O, NO

Reactions and Arrhenius Parameters:
(A = Pre-Exponential Factor, beta = Temperature Exponent, Ea = Activation Energy)

Oxygen Dissociation:

• O2 + M' -> O + O + M'
• A = 2.70 e+19, beta = -1.00, Ea = 494,000 J/kgmol
• Rate Exponent: [O2] = 2.00

Nitrogen Dissociation:

• N2 + M'' -> N + N + M''
• A = 3.70 e+21, beta = -1.60, Ea = 941,000 J/kgmol
• Rate Exponent: [N2] = 2.00

NO Dissociation:

• NO + M''' -> N + O + M'''
• A = 2.90 e+15, beta = 0.00, Ea = 621,000 J/kgmol
• Rate Exponent: [NO] = 2.00

N2-O Exchange:

• N2 + O -> NO + N
• A = 1.82 e+14, beta = 0.00, Ea = 319,000 J/kgmol
• Rate Exponents: [N2] = 1.00, [O] = 1.00

NO-O Exchange:

• NO + O -> N + O2
• A = 3.80 e+9, beta = 0.00, Ea = 173,100 J/kgmol
• Rate Exponents: [NO] = 1.00, [O] = 1.00

Third Body Efficiencies:
M' vs:

• O2 = 1.00
• N2 = 0.10
• O = 2.80
• N = 0.10
• NO = 0.10

M'' vs:

• O2 = 0.10
• N2 = 1.00
• O = 0.10
• N = 2.80
• NO = 0.10

M''' vs:

• O2 = 0.05
• N2 = 0.05
• O = 1.00
• N = 1.00
• NO = 1.00

Boundary Species Distribution:

• NO - 0.0
• N - 0.0
• O - 0.0
• O2 - 0.209476
• N2 - Bulk Species

I can attach screenshots of exactly how I've set these up in Fluent if it will help. I pulled the parameters and model from a paper I can dig up if someone wants. I pulled the reaction orders from the NIST Kinetics Database.

My grids vary; some are structured, some are unstructured. All use near-wall placement of cells to capture boundary layer effects (Y+ < 10). Some examples of the cases I've been running are:

• Diamond airfoils with various wedge angles
• Cylinder cases
• Ramp in wind tunnel
• Step in wind tunnel
• etc...

I can provide more details on the specific validation cases I'm running if requested. For now I'm focusing on the ramp case, which can be found here: http://www.grc.nasa.gov/WWW/wind/val...p/hypramp.html

The solution residuals will converge to 1e-6 without much trouble. General flow field and, where applicable, drag monitors also indicate convergence. When reactions are turned on, I receive the following error:

dasac failure at temperature = ...

I've seen this error pop up many times on these forums, but it never seems to get a solid reply. Hopefully I've provided enough information here for someone to point me in the right direction. This site has been a fantastic resource for sorting out problems, and I appreciate any help you can provide.

Hey there! I am trying to simulate the hypersonic re-entry case and am relatively new to species modelling. I am trying to incorporate the flow over a cone-flare and employ the above stated 5 species 5 step chemistry. The flow without the chemistry is in good accordance with the expected values and the shock formations are accurate as well. It will be very helpful if you could share me screenshots of the setup of the species modelling. Also since monoatomic oxygen and nitrogen aren't present in the database, I created 2 new materials and I have doubts regarding the properties that i have keyed in. Am I wrong in my approach please do clarify. Thanks in advance

Regards
rvp604

[Panboy]

Originally Posted by rvp604

Hey there! I am trying to simulate the hypersonic re-entry case and am relatively new to species modelling. I am trying to incorporate the flow over a cone-flare and employ the above stated 5 species 5 step chemistry. The flow without the chemistry is in good accordance with the expected values and the shock formations are accurate as well. It will be very helpful if you could share me screenshots of the setup of the species modelling. Also since monoatomic oxygen and nitrogen aren't present in the database, I created 2 new materials and I have doubts regarding the properties that i have keyed in. Am I wrong in my approach please do clarify. Thanks in advance

Regards
rvp604

This thread has lots of detail that you should be able to step through the setup pretty well. What properties for monoatomic oxygen and nitrogen are you doubtful about?