[Tensian]

Hi everyone,

I am triying to make the simulations of a complicated channel using simpleFoam, adding temperature equation as stated in somewhere in this forum and in turbulence regime.

The geometry consists in two inlets, an outlet and a wall.

I only have info about pressure values on inlets (0 Pa) and outlet (negative, for example -50 Pa), no velocity values are known and I want to simulate a turbulent flow, so some new files (from my previous laminar case) have to be added to "0" file.

I have been reading about how to compute the values for k, epsilon, nut and Nutilda, and I don´t know how to set up these values because for example for computing turbulence intensity I need to know the value of velocity ( for computing Reynolds) and it´s not known, so I cannot compute k.

Anybody knows a good setup of these files for my case?

Any suggestion will be really appreciated (beginner with turbulence)

[kornickel]

Why don't you calculate the inlet- and outlet velocities via Bernoulli's law and continuity equation? Since you know the inlet/outlet geometry and the pressure at inlet/outlet. With these velocities you should be able to calculate a good approximation of k and epsilon.

Edit: And about the folder structure: Take a look at the simpleFoam tutorial cases e.g. HERE

[Tensian]

Hi Frank,
Thanks for the fast reply. Let me see if I am understanding it well.

Supose we have a pressure at inlet of 0 Pa and a pressure at outlet of -40 Pa. The fluid is air with density rho=1.225 kg/m^3

So,

P_TOT=P_STATIC+P_DYNAMIC P_DYNAMIC=0.5*RHO*V^2

V=sqrt(2 (P_TOTAL-P_STATIC)/RHO)

Then in this case I obtain for inlet:

V=sqrt(2(101325-0)/1.225)

With these calculations I obtain a velocity V=406 m/s, and I guess I am doing something wrong...because I expect velocitites very much lower.

So my questions are:

1-Is this the V value that I should take to compute all turbulence quantities? 2-What am I doing wrong?
3. For a pipe (or similar) what is characteristic lenght: hydraulic dyameter or length of the pipe?

Sorry for that stupid questions, I have never worked with a pressure driven flow.

[kornickel]

Hi Tensian,

what I meant was use Bernoulli's principle AND the continuity equation. So with Bernoulli you are going to have:

P_in/rho + (v_in^2)/2 = P_out/rho + (v_out^2)/2

(I assume we can ignore the floor level difference)
So since you have incompressible case use the cont.-equ. next:

v_in * A_in = v_out * A_out

So you have two unknown variables (v_in and v_out) and two equations. Ta-da!

Depending on your inlet/outlet area you should get reasonable values.


I don't exactly know your case but I also assume you just can use the inlet velocity to calculate inlet k- and epsilon. Use a pressureVelocityInlet (in) and inletOutlet (out) for U field and totalPressure (inlet, calculate it with calculated v_in) and fixedValue (out) for the p field.

edit: characteristic length of a pipe is always hydraulic diameter. Look HERE.

[kornickel]

Almost forgot about your second inlet. You can just combine the two inlet areas to one big dummy inlet surface and calculate the inlet velocity this way. If the geometry is not that complicated both inlet velocities are the same due to same static pressure. Don't forget to add pressure loss in your tube if necessary.
So much for theory!

[Tensian]

Thank you so much Frank. I will make the calculations for my problem and I´ll let you know if I get a converged solution with the turbulence model!!

Cheers,

James.

[Many]

And what happens if the two inlet have different areas and perimeters? Are this computations still valid? I am a little confused

[Many]

I wonder if this inlet velocity can be used for computing Reynolds number. I am working with a similar geometry and I am not sure how to compute characteristic velocity and lenght (hydraulic dyameter of one inlet or both two?)

Thanks,

Many