[pramodnarayan]

Hi all,

I am currently simulating the water level change in a 2-D vessel using the Volume of Fluid Approach. I want to include the Residence Time Distribution calculations also in this simulation. This is the approach that I took to do this.

Geometry Details:
Width =30.48m
Height = 69.4944m
Jet Velocity =0.236 m/s

A common inlet-outlet is provided at the bottom of this vessel. The diameter of this inlet/outlet port is 1.2192 m. A jet of water with a velocity of 0.236 m/s comes from the bottom of the vessel and because of this, the level in the tank increases. The tank is initially filled with 65% of water. The other 35% of the tank is filled with air. There is an outlet vent provided at the top of vessel to avoid pressure buildup in the vessel.

Notes:
Turbulence Model - Realizable K-episolon model with Standard Wall Functions.
Turbulence Parameters at the Velocity inlet and Outlet vent : Turbulence Intensity = 3.3263% and Hydraulic Diameter =1.2192m.
VOF Scheme Used - Implicit
Pressure - Velocity Coupling Scheme - PISO, Pressure - Body Force Weighted , Volume Fraction - Modified HRIC, Turbulent Kinetic Energy and Turbulent Dissipation Rate - Second Order, Transient Formulation- Bounded Second Order Implicit.

I have no problems simulating just the level change using the VOF. When i try to include the Species Transport, my continuity equation blows up. I start at a very small time step of 1e-25 and slowly reduce it by an order of magnitude of 10. But When I reduce it till a particular value, the solution blows up and shows reverse flow at the inlet. I have defined a tracer with the same properties of water. I have patched the tracer with a concentration of 1 for the initial volume of water. My objective is too have water coming in the charging cycle. So the level goes up till 95% of the tank. Then there is a discharge cycle where there is a volume change of 20-25%. During the discharge cycle, I will create a surface monitor at the common inlet/outlet port and track the concentration of tracer as the water is pulled out of the tank. With this data, I can calculate the age of the water

I am not able to understand what the problem is with my setup. I request your guidance and suggestions on this. Any help is appreciated.

[pramodnarayan]

First of all, I want to thank the entire CFD Online community for guiding us, novices in the big ocean of CFD. I have learnt a lot by using this forum.I have some issues that I need help with. I request some guidance here.

1) I have a Negatively buoyant problem where in a I have a jet of water coming in to a tank which is filled with water. Since the inlet temperature is less than the temperature of the tank, negative buoyancy condition is created. I had defined a tracer with the same properties as that of water and patched the initial volume of water in the tank with a concentration of 1. I have assumed an uniform distribution of water and tracer in the tank as an initial condition. Then I have a jet of water coming in. Because of the negative buoyancy, the momentum of the jet quickly should quickly dissipate. With the species transport equation, the jet behaves like a isothermal jet.

Is buoyancy ignored in the species transport equation? I am not able to understand what is going on. I looked into the forum and could not find any satisfactory reply. Could someone please help my about this?

Since the buoyancy effects were being ignored with the species transport, I decided to use a UDS to solve the scalar equation. What is the meaning of Specified Value and Specified Flux in the User Defined Scalar boundary condition?