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Mass Flow Rate or Mass Flow Distribution

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Old   March 6, 2020, 09:07
Smile Mass Flow Rate or Mass Flow Distribution
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Hello all,

I know it seems to be basic question or it might be a big mistake. Please excuse me and help me.

My model as a number of inlets and one outlet. One type of fluid flows in one inlet and the rest of them have other fluid. So, I have a total of two fluids. I have two velocity inlets and one pressure outlet with default settings of physics, solution method, and controls.

I have been trying for velocity profile and mass flow distribution. But when I hybrid initialize, plot and calculate.

I get a mass flow rate graph as follows in the attached image. I cannot see any values on Y-axis?

Please let me know, what all changes could I make or what is the problem.

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Old   March 8, 2020, 04:43
Default Values on abcissa
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The values are there but too small for the format chosen. You need to change the format to either scientific notation or increase the number of significant digits.
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Old   March 8, 2020, 11:35
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Hello Vinerm,

I am afraid, I could not get the point.

The model, I'm working on, has very small dimensions around in mm. The meshing size is also around 0.0001 mm. Did you mean this data?

Or can you please brief me on the solution or suggest the settings or steps necessary?

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Old   March 8, 2020, 12:09
Default Data on y-axis
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What I meant is that the field reported on the y-axis have rather small values, may be of the order of 0.000001. Now, if you display only four significant digits, number will appear as zero. However, if you use scientific notation for the plot, these will appear as 1.23e-6.
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Old   March 8, 2020, 12:41
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Hey Vinerm,

Yeah, thank you for your detailed reply.

Yeah, I got some results. It seems weird because all the values in Y abscissa are the same i.e., the value which I got, while I computed in mass flow rate.

Could you please give it a look and remark it?

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Old   March 8, 2020, 12:46
Default Setup
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No comments could be made about it until the case details are known. What you should check is the difference between mass flow rates at the inlets and the outlet. If the difference is very small as compared to flow rates at the inlets and outlet, say, less than 1%, then it is alright. Else, the case is not converged. Usually, a multiphase flow will not converge in 200 iterations, until it is a simple pipe flow. Secondly, mass flow error for multiphase flow is usually high, i.e., it is difficult to get mass conservation in a multiphase flow.
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Old   March 9, 2020, 05:07
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Okay. I am posting some information. Hope it could help me
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Old   March 9, 2020, 05:12
Default Mass Conservation
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The problem is with mass conservation. Are the fluids immiscible, like air and water? And are you running the simulation in double precision? For such small numbers, double precision would improve the accuracy. Secondly, the residuals appear to have stalled. Try changing the numerical scheme or URFs. Prefer Coupled with Pseduo-transient or run the simulation as transient for better convergence. You may also try changing the gradient limiter to cell-to-cell instead of default. This will help with stalled residuals.
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Old   March 9, 2020, 05:22
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Yeah, the fluids are immiscible. I am running in Double precision. Also, I have used Coupled with Pseduo-transient as my model is steady.

"You may also try changing the gradient limiter to cell-to-cell instead of the default. This will help with stalled residuals." I did not understand this, how can I do this?
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Old   March 9, 2020, 07:06
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This is available under Solution Controls > Advanced.

Since you have two immiscible fluids, I suppose you have gravity enabled, provided density ratio is high enough for gravity to have a significant effect. Furthermore, which multiphase model are you using?
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Old   March 9, 2020, 09:29
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I will change the spatial discretization to cell-to-cell and try to simulate.

Yeah, it is a high-density model. At present, I am just testing it with a single fluid. I will be using the VOF model in further cases.

To make the simulation more practical, I have applied gravity. But, I have no idea it will affect the model's density ratio.

Should I disable gravity or what should I consider?
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Old   March 9, 2020, 11:02
Default Gradient Limiter and Gravity
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For high density ratio system, gravity is a must. Gravity does not affect the density ratio, rather it affects the fluid flow if the density ratio is high. Gravity should always be enabled in your system and in the correct direction. Additionally, operating pressure reference location and operating density should be accurately specified.
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Old   March 9, 2020, 11:11
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Yeah, everything is fine then. I will just update all settings and try again.

Thanks, Mr. Vinerm

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