CFD Online Discussion Forums

CFD Online Discussion Forums (
-   FLUENT (
-   -   mass flow inlet boundary condition (

vivian April 10, 2006 23:30

mass flow inlet boundary condition
Hi, everbody, applying mass flow inlet boundary condition, mass flux, static gauge pressure, total temperature and inlet flow direction must be specified. Then the total pressure adjusts to accommodate mass flow inputs. How does the total pressure be calculated to accommodate the mass flow? Which equations are used? More details are appreciated. Thank you!

Rags April 11, 2006 02:04

Re: mass flow inlet boundary condition
Hi vivian As we know P total= P static+0.5*rho*v^2 where v can be obtained from mass flow rate. by using mass flow rate= rho*Area*v Moreover, please be informed that static pressure is specified at the mass flow inlet only when flow is supersonic. Otherwise it is only for specifying intial guess if u r initialising the solution from mass flow inlet. hope this answers ur question

vivian April 11, 2006 04:32

Re: mass flow inlet boundary condition
Hi, Rags Thanks for your useful information! Does the Rho in the equation P total= P static+0.5*rho*v^2 need extrapolate? In my problem, the variety Rho is not constant. And another question, the gas dynamics equation Ptotal=Pstatic*(1+(k-1)*Ma*Ma/2.) can be used for this problem? (k is the adiabatic index 1.4, Ma is Mach number, Ma*Ma=u*u/(Rc*T*k)) Thanks a lot!

rags April 11, 2006 08:13

Re: mass flow inlet boundary condition
Hi Vivian The 2 equations P total= P static+0.5*rho*v^2 Ptotal=Pstatic*(1+(k-1)*Ma*Ma/2.) are one and same. U will come to know of it if u see the derivation of second equation. If Rho is not constant, Ptotal will also vary accordingly.In my opinion, you are trying to get the value of Ptotal at the inlet by hand calculation. If it is so, u can get the rho value at the temperature and pressure specified at the inlet.(I assume that u are using Ideal gas law for rho variation) Let me know if you have any probs..

vivian April 11, 2006 23:04

Re: mass flow inlet boundary condition
Hi, Rags I have understood how to set the mass flow inlet boundary condition. Thanks for your help! I have another problem.

I am writing a finite volume code for unsteady 2D laminar flow. It uses for numerical simulations of resonant oscillations in a tube which has one inflow and three walls (no outflow). It is like fluid entering a closed end box. The tube is filled with air under normal circumstances initially (T0=291.789, p0=101325pa).

I don't know how to set the boundary conditions. In my problem, I think the inlet velocity and inlet pressure can not be given simultaneously. Which boundary condition can be used, inlet velocity or inlet pressure?

The boundary conditions I use now is prescribed velocity u=u0*sin(wt) at the inlet. And the pressure at the boundary is extrapolated.

But I find if I use this inlet velocity boundary condition, I don't know how to get the absolute pressure. My code follows the pressure correction schemes for transient predictions and for compressible flow. The density need to be calculated through the equation of state Rho=P/(Rc*T).

So I enquire for the mass flow boundary condition, hoping for the absolute pressure can be calculated through the inlet velocity. Or the pressure boundary condition should be used for this problem? Can you give me some advice? Thank you!

Rajeev April 21, 2006 06:23

Re: mass flow inlet boundary condition *NM*

All times are GMT -4. The time now is 21:39.