## Fluids

#### Divergence detected in AMG solver. VOF.

• Anakin
Subscriber

Hi guys. I'm simulating air-water flow through a T pipe with 26mm inner diameter. The pipe was created in a 2D domain, there are two inlets (one for air and the other for water) and one outlet. The two inlets have velocity inlet conditions and the outlet has pressure outlet conditions. The pipe is filled with water and then water and air start to flow from the inlets with their respective inlet velocities. The time-step was calculated based on the CFL condition and the total flow time is 6 seconds. The simulation is presenting the following problems after the bubbles reach the outlet (which occurred approximately in 2,3 seconds):

# Divergence detected in AMG for pressure correction: protective actions enabled!

# Divergence detected in AMG for pressure correction, temporarily solve with BCGSTAB!

Divergence detected in AMG solver: pressure correction# Divergence detected in AMG for pressure correction: protective actions enabled!

# Divergence detected in AMG for pressure correction, temporarily solve with BCGSTAB!

Divergence detected in AMG solver: pressure correction# Divergence detected in AMG for k: protective actions enabled!

# Divergence detected in AMG for k, temporarily solve with BCGSTAB!

Divergence detected in AMG solver: k# Divergence detected in AMG for omega: protective actions enabled!

# Divergence detected in AMG for omega, temporarily solve with BCGSTAB!

Divergence detected in AMG solver: omega

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: k

Divergence detected in AMG solver: omega

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: k

Divergence detected in AMG solver: omega

reversed flow in 63 faces on pressure-outlet 9.

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: pressure correction

Divergence detected in AMG solver: k

Divergence detected in AMG solver: omega

Error at host: floating point exception

Error at Node 0: floating point exception

Error at Node 1: floating point exception

Error at Node 2: floating point exception

Error at Node 3: floating point exception

Error: floating point exception

Error Object: #f

I checked the mesh quality and it looks good (min orthogonal quality = 0.73; max skewness = 0.53; max aspect ratio = 21.16). I tried to solve the problem by reducing the time step in half and decreasing the relaxation factors, but it didn't work. I don't know how to fix this problem, so I really need some help and I'm open to answering any questions. I'm going to list every piece of information I find relevant about the simulation.

General:

Parallel

Solver type: Pressure-Based

Time: Transient

Planar 2D space

Gravity on

Models:

Multiphase VOF with explicit formulation and Surface Tension Force Modeling (CSF)

Default Courant number: 0,25

Implicit Body Force

Turbulence Model: SST k-omega

Cell Zone Conditions:

Specified Operating Density: 1.225 kg/m³ (air density)

Boundary conditions:

air inlet velocity = 0.936 m/s

water inlet velocity = 0.37

m/s

outlet Gauge Pressure (pascal) = 0

pipe wall: stationary, no slip

Solution Methods:

PISO Scheme

PRESTO! Pressure

volume fraction: Geo-Reconstruct

Momentum, Turbulent Kinetic Energy and Specific Dissipation Rate = Second order upwind

Transient Formulation = First order implicit

Solution Controls: Default

Residuals = 10e-5

Standard initialization with default initial values

Ansys Employee
I assume you have patched the volume before start of iterations.
Please make sure that you reduce your timestep as 6 sec looks too large.
Please search through forum and you will find many posts on VOF.
Regards Keyur