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Lesson 16 : Mass flow rate = 0
Posted by Narendar on December 26, 2023 at 8:04 amI have followed the video with the exact steps, but while running the simulation both the inlet and outlet mfr are showing a value of 0. Can someone help me out here?
T R Abhishek replied 2 months, 3 weeks ago 3 Members · 12 Replies 
12 Replies

Hi Narendar,
We can help you better if you could provide more details regarding the case setup, boundary conditions, solution convergence etc that you have used. Please give us some idea of the mesh and residuals as well with screenshots.
Best regards
Rahul
Flowthermolab support
Mesh : followed as exactly shown in the video
Valve height : 5mm
Model : komega SST
Medium : Water
Inlet ( Pressure ) : GTP = 400000 Pa , IGP = 399900 Pa , Turbulence : TI = 5% Hydraulic dia = 0.051m
Outlet ( Pressure ) : GP = 300000 Pa
Reference values : computed from inlet
Methods : Coupled (all second order)
Report definitions : mfr inlet, mfr outlet, force on the disc
Hybrid initialization
PFA the scaled residuals and the reports generated.


Hi Narendar,
The solution does not seem to be converged as it is only 40 iterations. Please use lower residuals of 1e5 or 1e6 and run for more iterations.
Also, check the velocity contours/vectors on the midplane to check if flow is happening as intended.
Also, please take a look at the operating pressure and pressure boundary conditions values and verify it with the lesson.
Please take a look at these and let us know.
Best regards
Rahul
Flowthermolab support
Hey! Thanks for replying.
I didnt run the simulation further as i was not getting any value for MFR.
I’ll run it till convergence and let you know

Try it out. Since the force value is nonzero flow seems to be happening in the domain. But the mass flow rate also does not seem to be zero since it is fluctuating. The value may be lower than the precision of the graph. Running for more iterations and checking the contour plots should give you a better idea.


Hi Narendar
The sign of the force value depends on the direction which we specify in the Force report definition. Please take a look if the value is correct.
Is the mass flow rate higher now? Please ensure that the problem follows mass conservation. Check the difference between inlet and outlet mass flow rates.
Best regards
Rahul
Could you share a screenshot of which surface you are taking for the mass flow rate and the corresponding value shown in the dialog box?
Also, can you check the mass flow rate manually? Check the mass flow averaged velocity at the outlet and use the equation for mass flow rate calculation. Please share the velocity value at outlet as well.
Could you also check the working fluid density and the outlet diameter and let me know that as well.
Best regards
Rahul 
While generating report / graph for mass flow rate, if you select “inlet” as well as “outlet”, the plot will show the difference between the two. You might get values of the order e5 etc. (nearly zero. I faced the same), which is logically correct. The mass that comes in goes out.
If you select “inlet” only, you will get a positive value. Selecting “outlet” alone will give negetive value, but numerically almost same as inlet mdot.





Also, the boundary layer seems to be a bit coarse. Use the last ratio option for boundary layer and use a constant 1st cell thickness through. It seems the 1st cell height and growth rate for layers is not constant in the domain in your case.
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