CFD Online Discussion Forums

CFD Online Discussion Forums (
-   Phoenics (
-   -   about conservation of mass (

timur February 13, 2006 05:38

about conservation of mass
Cartesian coordinat system. Inlet absolute velocity VEL=20 m/s enter in furnace at angel, say, 45 degrees. Then Uvel=20*sin(PI/4), Vvel=20*cos(PI/4). In result file can see that mass from inlet M=ro*Uvel*Area_of_Inlet, and Vvel does not take into account. I want to see mass flow M=ro*VEL*Area_of_inlet. How can I solve the problem? Is only BFC helps me?

fevzi February 13, 2006 09:57

Re: about conservation of mass
Please correct me if I am wrong but by setting the Vvel as you did. Vvel will be tangent to inlet so may be there is no net in flow from that component. İs that right. best regards.

timur February 14, 2006 04:02

Re: about conservation of mass
Thank u fevzi for your reply. Yes, u are right about Vvel. Think only BFC can help me.

Mike February 15, 2006 05:13

Re: about conservation of mass
What is meant by "and Vvel does not take into account"? Does it mean that the NETT SOURCE printout in the RESULT file indicates zero momentum inflow rate for the velocity component parallel to the inlet plane Vvel, rather than m*Vvel, where m is the mass inflow rate?

I don't see why BFC is needed because if the VALues in the COVAL statements for the two Cartesian velocity components are 28.28 m/s and of the correct sign, then the momentum inflow rate should be correct. In the RESULT file, what COVAL statements appear for this inlet PATCH in the Group 13 printout? Also, in the VR Editor, is this inlet set up through an INLET or USER-DEFINED object?

timur February 15, 2006 07:04

Re: about conservation of mass
Thanks Mike for your reply. "Vvel does not take into account" meant that in the result file momentum does not indicates zero momentum inflow rate, but mass inflow (in result file) solve only for NORMAL velocity. Then if ABSolute velosity vector = 20 m/s, Vvel=20*sin(45 DEG)=14.14, Uvel=20*cos(45 DEG)=14.14. Net source for momentum is correct, but net mass flow in the inlet not correct, because of mass_flow_rate=ro*normal_vel*net_area. If I put (from patch and coval for P1) correct mass flow rate= ro*ABS_VELOCITY*net_area and UVel=Vvel=14.14 m/s I cann't get an convergence (because of momentum and mass do not conform).

Mike February 15, 2006 07:38

Re: about conservation of mass
Thank you. It is becoming a little clearer what you are doing.

If I have understood correctly, in your case the flow enters at angle of 45deg to the inlet plane. If so, then the PATCH and COVAL statements should be:


The velocity used in the P1 patch should be Q.A where Q is the velocity vector, and A is the inlet vector area. This means that the velocity component used to define the mass inflow rate is (Q.n), where n is the unit vector normal to the inlet plane.

Therefore, in the NETT source printout of the RESULT file we should see:

mass inflow rate=rho1*14.14*inlet_surface_area

U1 & V1 momentum inflow rate = mass inflow rate*14.14

timur February 15, 2006 09:25

Re: about conservation of mass
Yes, Mike you are right. But point is that if u change inlet flow angel (without changing ABS_velosity) mass flow rate changed too. If u want to fix mass flow rate - u must changing ABS_velosity and as result momentum source (((. Can u tell me can I change inlet flow angel wihtout changing momentum and mass flow rates in cartesian system?

Mike February 15, 2006 09:29

Re: about conservation of mass
I don't understand what you mean. It doesn't make much sense to me. I think you will have to tell me exactly how you are implementing the inflow conditions.

All times are GMT -4. The time now is 15:52.