Divergence of Continuity Residual

[jfe293]

Hi,

I was simulating a large stormwater pond model having around 35 million cells. The stormwater pond has two inlets and one outlet. 

The inflromation of my model setup is given below: 

Time: Steady

BCs:
Inlet 1: Mass Flow Inlet (3680 kg/s)
Inlet 2: Mass flow Inlet (1360 Kg/s)
Outlet: Outflow

Pressure-Velocity Coupling Scheme: SIMPLE

Discretization for Momentum and Turblence Equations: First Order Upwind

URFs

Pressure: 0.2

Momentum: 0.5

All other controls: by default

Initialization Methods: Standard Initialization computed from all zones

The issue is that my continuity residual is not still converging even after 2250 iterations. Instead, it's drifting upward very slowly (Image attached).

I would appreciate any guidance from this forum on how to address this issue.

Thanks.

 

[Rob]

How does the flow field look? Is the model fixed lid single phase or VOF? 

[jfe293]

Hi Rob,

The model is a fixed lid single phase set up. At the free surface zero shear BC condition is used.

I am attaching here the residual plots and velocity contour after 6700 iteration:

Please note the following:

Oulet Bc conditios:

Outflow: iteration 0 to iteration 4272

Pressure outlet: iteration 4272 to iteration 4700

Outflow: iteration 4700 to iteration 5200

Pressure outlet: iteration 5200 to iteration 6706

 

[Rob]

OK, fixed lid and no gravity? Do NOT use the outflow condition, it's old and really should have been removed in Fluent 6. For the above velocity in & pressure outlet should be fine. Mass flow inlets are ideally for compressible flow - as you know the fluid mass flow, density and inlet area do the maths your self to get an inlet velocity. 

Looking at those residuals and the flow speed I suspect you've got some transient effects in the flow. Add some monitor points into areas of interest and plot the above contour every 10 iterations for 100-200 iterations then look at the images. What I suspect you'll see is the higher speed flow wandering around in the domain: that's normal where flow momentum is low.