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Home Forums ANSYS Fluent Beginners to Advanced level Lesson 16 : Mass flow rate = 0

• # Lesson 16 : Mass flow rate = 0

Posted by on December 26, 2023 at 8:04 am

I 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?

3 Members · 12 Replies
• 12 Replies
• ### Rahul

Member
December 26, 2023 at 10:04 am

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

• ### Narendar

Member
December 29, 2023 at 7:45 am

Mesh : followed as exactly shown in the video

Valve height : 5mm

Model : k-omega 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.

• ### Rahul

Member
December 29, 2023 at 8:38 am

Hi Narendar,
The solution does not seem to be converged as it is only 40 iterations. Please use lower residuals of 1e-5 or 1e-6 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

• ### Narendar

Member
January 2, 2024 at 7:49 am

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

• ### Rahul

Member
January 2, 2024 at 8:33 am

Try it out. Since the force value is non-zero 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.

• ### Narendar

Member
January 4, 2024 at 6:21 am

The solution converged at 110 iteration for 1e-06 as the convergence criteria. The flow is happening which can be seen by the pressure and velocity graphs. I have attached them here.

What I’m unable to understand is that the force on the disc is in negative.

• ### Rahul

Member
January 4, 2024 at 6:28 am

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

• ### Narendar

Member
January 4, 2024 at 6:33 am

The mass flow rate is still showing as 0 only.

• ### Rahul

Member
January 4, 2024 at 6:57 am

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

• ### T R Abhishek

Member
January 22, 2024 at 5:08 am

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 e-5 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.

• ### Rahul

Member
December 29, 2023 at 9:06 am

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.

• ### Narendar

Member
January 2, 2024 at 7:49 am

I followed the video step by step as shown