Basics questions about CFD computation

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10 years 11 months ago #4900 by guillaume23
Basics questions about CFD computation was created by guillaume23
Hello,

I have some basics questions about CFD modeling and simulation.

- does a CFD mesh need to be quadratic or just linear?

- When a simulation is finish, I don't know exactly whitch parameters I have to check.
I check the mass flow inlet and outlet, the normal resisual. Are there more? What is the derive and Rhs norm?

- How can I know that the number of iteration is enougth? What happen when it is too big?

Thank you for your advise.

Guillaume

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10 years 11 months ago #4901 by Claus
Replied by Claus on topic Re:Basics questions about CFD computation
Code_Saturne uses linear mesh - if a quadratic mesh is used, CS just converts it to linear.

The 'derive' *can* be used as an INDICATOR of convergence, but they aren't technically residuals of the PDE's - check the CS forum, there are a few posts on the subjects.

The best way currently, is to use monitoring points and watch them settle as you reach a steady state/convergence.

Regards,

Claus

Code_Aster release : STA11.4 on OpenSUSE 12.3 64 bits - EDF/Intel version

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10 years 11 months ago #4944 by guillaume23
Replied by guillaume23 on topic Re:Basics questions about CFD computation
Hello,
Thank you.
I think I have good results for laminar flow.
I try to make a simulation with turbulent flow but I don-t know how to calculate reynolds number and hydraulic diameter on the geometry.
I send a picture. It is an axisymetric problem with x axis rotation.
The geometry is like a nozzle with an annular inlet.

img243.imageshack.us/f/screenshot3ww.png/

If somebody have an idea to calculate this two parameters?


Bye

guillaume

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10 years 11 months ago #4946 by CAVT
The Reynolds number is an adimensional value computed as:

Re = (Rho * V * L) / mu

Rho is the density of the fluid, usually freestream (if you're using incompressible flow, it's the same everywhere, this is the most usual case). V is your reference velocity magnitude, usually it is the freestream velocity or the inlet velocity, but that is arbitrary and can be other velocity like local. L is your reference length, this is also an arbitrary choice, in the case of airfoils you take the airfoil chord, in your case you may want to take the hydraulic diameter. Mu is the fluid's dynamic viscosity, which can be found tabulated or you can calculate it with the Sutherland formula if it's a gas. The choice of the arbitrary parameters will depend on what's usually found in books for the case you're doing. For the nozzle I think with the inlet velocity, hydraulic diameter and inlet density you'll do perfect. Code_Saturne calculates Re automatically from the introduced data.

The hydraulic diameter is the diameter that would have an imaginary inlet with the same area as your inlet's cross section. Actually it's the wet area, but I suppose in your case the whole inlet area is wet by the fluid (a non fully wet area example could be a pipe with water until the midlevel only). For any cross section that you have, the formula is the same:

Dh = 4 * A / P

A is your inlet cross sectional area and P its perimeter. You say you have an anular inlet, is it anular or circular? An anulus is the area between two concentric circles, so the perimeter is the summatory of each circle's perimeter. If it's circular, then Dh = D (pretty obvious, :P ).
In Code_Saturne the Hydraulic diameter is used to make an initial guess of the turbulence parameters, so despite you must enter the value, it's not a vital parameter as it is used in the first step only. Actually you can define your own turbulence parameters in the user subroutines overriding the Dh definition, but that's another thing.

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