Boundary Conditions: Setup

This chapter covers configuring specific boundaries — inlets, outlets, and walls — the transient nature of the LBM solver, and how to verify and troubleshoot your boundary setup.

Setting an Inlet

1. Add a boundary (+ on the Boundaries container) and pick your painted zone in the Associated zone dropdown
2. Open the Boundary type dropdown — it reads Select until you choose
3. Choose Velocity Inlet
4. Fill in all three fields:
   • Magnitude [m/s] — default 1.0
   • Direction — a 3-component vector, pre-filled from the painted face normal and drawn as an arrow in the viewer
   • Profile — parabolic for developed pipe flow (vessels), plug for a uniform inflow (airways)

Setting an Outlet

1. Add a boundary and associate its zone
2. Open the Boundary type dropdown
3. Choose Pressure Outlet
4. Set Gauge value [Pa] — relative to the 101 325 Pa reference

Time-Varying Boundaries

A boundary can be driven by a Python function instead of a constant. Tick Time variable on the boundary; LeaRes generates a stub like:

def property1_driver(time):
    return amplitude * math.sin(2 * math.pi * frequency * time)

The function is injected into the solver deck and evaluated at every timestep — this is how you drive a cardiac or respiratory waveform.

Note: the interface marks this feature “Not validated”. Treat results from time-varying boundaries as exploratory.

Walls

You do not paint or assign walls. Any surface cell you leave unpainted is treated as a no-slip wall by the solver. This keeps setup simple: define only the openings, and the rest of the geometry is wall by default.

Transient Nature of LBM

MOEBIUS is a Lattice Boltzmann solver and is intrinsically transient — every simulation evolves in time. A “steady” flow condition is reached by integrating until the solution stops changing, not by switching to a separate steady-state solver.

Practical implications:

• For a constant inlet velocity, the run continues until the flow reaches stationarity; the convergence autostop (covered in the next tutorial) monitors this automatically
• For time-varying inlet conditions, the simulation must run long enough to capture the relevant temporal behavior before you analyze the results

Verification

Before running, confirm:

• Each opening has a boundary with a painted zone and a type assigned from Select
• At least one inlet and one outlet are defined so fluid can enter and leave
• The unpainted surface correctly represents the walls

Troubleshooting

No Type Options Appear

Problem: The Select dropdown is empty for a boundary.

Solutions:

• Make sure the boundary was created with the + button and has a zone selected in Associated zone
• Finish the boundary you are currently editing — the interface refuses to add a new one until the current is complete

The type menu itself does not depend on the flow type: the same nine base types are returned for every project.

Flow Has Nowhere to Go

Problem: The simulation cannot start or behaves unphysically because there is no outlet.

Solutions:

• Add a Pressure Outlet boundary at the exit opening
• Ensure you have at least one inlet and one outlet

Velocity or Pressure Seems Off

Problem: Results look unrealistic.

Solutions:

• Recheck the inlet velocity and outlet pressure values on each boundary
• Confirm the inlet is painted at the entrance and the outlet at the exit (not swapped)

Next Steps

With boundaries configured, you’re ready to launch a simulation. Continue to Running a Simulation.