## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Fluent Reports: How to calculate the mass of fluid in a tank?

• student_18
Subscriber

Hi,

I need to simulate a tank filling with an electrolyte (incompressible). Initially the tank is filled with air (ideal gas). Through a pressure difference at a pressure inlet, the electrolyte flows in the tank compressing the air, there is no outlet.

After a certain flow time I want to calculate the mass of electrolyte in the tank.

I have used two different approaches:

•  Report Definitions>Flux Report> Mass Flow Rate on Inlet Boundary (attached figure). To approximate the area under the time depended data points I used trapezoidal rule to get the mass.

•  Results>Reports>Volume Integrals>Mass (Integration of phase volume fraction)

Comparing the calculated mass, I have 10% deviation between the two methods.

Which approach calculating mass would you recommend as more accurate?

Or might the difference be a sign of mistakes in the simulation?

Best regards

• klu
Ansys Employee

Hi, I think the second method should be used. The mass of a particular phase is computed by summing the product of phase density, cell volume, and phase volume fraction in Volume Integral (https://www.sharcnet.ca/Software/Ansys/16.2.3/en-us/help/flu_th/flu_th_sec_compute_volint.html#flu_th_sec_report_volint_mass). It should return the computed mass by Fluent.

For the first method, please check:

1. Integration errors when using the trapezoidal rule.

2. if the report evaluates the mass flow rate of electrolyte but not the mixture at the inlet. If the mixture was selected, the reports returns mass flow rate of mixture.

Thanks.

• DrAmine
Ansys Employee

Go for the second method to identify the mass of the phase you are interested to get.