Hi

It may say that you do the simulation with your own code, or with Fluent or CFX?

Seyed Mohammad Mousavi Thank you for your response. I apologize for not providing complete information earlier. I am using my own code for the simulation and it is an incompressible flow solver. The physical properties in my code jump across the interface without any diffusion.

I am not sure if Fluent or CFX can perform this kind of simulation. If they can, I would appreciate it if anyone could provide detailed information on the numerical method used to stabilize the simulation.

Dear Min Lu;

You may draw the geometry of the solution field and the boundary conditions and what you desired from this analysis for me, even if it cannot be solved directly with Fluent or CFX, it may be possible to create the desired conditions in these software by user defined functions.

Maintaining momentum and mass conservation in a two-phase flow simulation with inlet and outlet boundary conditions is crucial for accurate results. Here are some general guidelines to help achieve this:

1. Inlet Boundary Conditions:

· Specify the appropriate flow rates or velocities for each phase at the inlet.

· Ensure that the sum of the inlet mass flow rates for each phase is equal to the total mass flow rate.

· Consider the phase distribution and relative velocities when defining the inlet conditions.

· If necessary, use additional boundary conditions like slip velocity or phase fraction to accurately represent the two-phase flow behaviour at the inlet.

2. Outlet Boundary Conditions:

· Specify the appropriate pressure, backflow prevention, or mass flow rates for each phase at the outlet.

· Consider the phase distribution and relative velocities when defining the outlet conditions.

· Ensure that the sum of the outlet mass flow rates for each phase is equal to the total mass flow rate.

· If necessary, use additional boundary conditions like pressure drop models or backflow prevention mechanisms to maintain proper flow behaviour at the outlet.

3. Numerical Schemes and Convergence:

· Use numerical schemes that are consistent with the physics of two-phase flow, such as upwind differencing or hybrid schemes.

· Ensure that the simulation is properly converged by monitoring residuals, mass imbalances, and other relevant quantities.

· Adjust the convergence criteria and computational domain as needed to achieve accurate results.

· Verify the simulation results against experimental or analytical data to validate the accuracy of the model.

4. Interphase Momentum Transfer:

· Incorporate appropriate models to capture interphase momentum transfer, such as drag models for dispersed phases or interfacial momentum exchange models for separated phases.

· Ensure that the chosen models adequately represent the physics of the two-phase flow and validate them against experimental or numerical data when possible.

· Consider the effect of interfacial forces, such as surface tension, in maintaining momentum balance between the phases.

5. Grid Resolution:

· Ensure that the computational grid is appropriately resolved to capture the flow features and interfacial behaviour accurately.

· Pay attention to regions of high velocity gradients or significant phase interaction.

· Use grid refinement studies to assess the sensitivity of the results to grid resolution and adjust accordingly.

6. Sensible Assumptions:

· Make sensible assumptions about the behaviour of the two-phase flow, such as the choice of closure models or simplifications based on the flow regime.

· Validate and calibrate the assumptions against experimental or numerical data whenever possible

Min Lu I had a similar problem with gas-liquid bubbly flows. Is it the case with you? If yes I may tell you what would be possible to try

Jamel Chahed Yes, quite similar, I would appreciate it if you could provide some details, thank you in advance

Min Lu I used twenty years ago a two fluid model to simulate gaz-liquid vertical almost parallel bubbly flows. All was done so hat the model cheked with single phase works propely. When the gas phase is introduced at the inlet at the same velocity as the liquid, a loss of mass conservation was recorded. The problem came from the fact that the bubbles (low density) are highly accelarated at he start of the domain before reaching its final slip velocity. The problem was resolved in two ways: 1. Refining suitably the grid at the inlet. 2. Putting the bubble at the inlet with their final slip velocity. The gas velicity is thus equal to the liquid velocity plus the relative velocity to be obtained at its limit obtained far from the inlet.

Hello Min Lu, as a first glance I would say check if you have a 'match mass flow' condition at the outlet i.e check that you are not accumulating mass over time even before you attempt running a two-phase simulation. Set up a simple inflow/outflow problem of your choice and monitor the mass. The same amount of mass into the domain should be removed at the outflow.

Once that is done, I would check the discretization of the volume fraction equation. It has to be consistent with your NS solver. To check for consistency start off by initializing a bubble having the same fluid properties as the background fluid/phase and advect it across the domain using periodic boundary conditions such that it ends up in its initial position after one period. This should eliminate dependence of flux discretization in the volume fraction equation on the solution. If no problems show up increase the fluid property ratios gradually until you reach your bottleneck.

Hope this helps

Ali Fakhreddine Hi, thank you for your kindly reply. Yes, I understand. Actually, my code has been verified using benchmark two-phase flow cases. It has no problem with periodic boundary condition.

I kown how to enforce the mass conservation, as you mentioned, I should keep the void fraction of water a constant value during the simulation.

In order to ensure mass conservation in the scenario where a single two-dimensional jet evolves into multiple droplets, it is necessary to enforce the condition that the outlet flux, represented by the product of the cross-sectional area of the droplets (S2) and their velocity (u2), is equal to the inlet flux, represented by the product of the initial cross-sectional area of the jet (S1) and its velocity (u1). Since S2 is typically greater than S1, it follows that u2 must be smaller than u1 to maintain mass conservation. However, this approach alone does not guarantee momentum conservation.

I'm not sure if I've explained it clearly.