OpenFOAM Outlet Boundary Conditions: Pressure, Velocity and Backflow

Outlet boundary conditions are the most common source of divergence in OpenFOAM simulations that beginners overlook. Picking the wrong BC type, placing the outlet too close to a recirculation zone, or failing to handle backflow correctly can destabilize an otherwise well-configured case. This guide covers every outlet BC type and when to use each one.

By CFDpilot · Updated April 2026

1. The fundamental outlet BC pair

Most OpenFOAM outlet configurations use a pressure-velocity pair. The pressure outlet sets the reference pressure; the velocity outlet must be compatible with whatever flow exits naturally.

The standard incompressible outlet pair:

// 0/p
outlet
{
    type    fixedValue;
    value   uniform 0;   // gauge pressure = 0 Pa
}

// 0/U
outlet
{
    type    zeroGradient;
}

This assumes the flow exits cleanly with no recirculation at the outlet face. The pressure is fixed at zero gauge, and the velocity is extrapolated from the interior — whatever is leaving, leaves freely.

This pair works well when the outlet is placed far enough downstream. When there is recirculation or the flow is not fully developed at the outlet, this pair breaks down and the simulation diverges.

2. zeroGradient vs fixedValue

zeroGradient tells the solver: assume the gradient of this field in the direction normal to the boundary is zero. This means the value at the boundary face is extrapolated from the last interior cell. For velocity at outlets, this is physically correct for fully developed flow — the velocity profile is not changing as it leaves.

fixedValue imposes a specific value at the boundary, regardless of what the interior solution is doing. For pressure at outlets, this sets the reference pressure level for the whole simulation. For velocity, fixedValue at outlets is rarely appropriate — it forces a specific velocity profile that may conflict with the global mass balance.

A common mistake is using fixedValue for both velocity and pressure at an outlet. This overconstrains the problem — you have set both the pressure level and the velocity, leaving the mass balance nowhere to adjust, which causes divergence.

3. pressureInletOutletVelocity: the robust outlet velocity BC

pressureInletOutletVelocity is a smarter version of zeroGradient for velocity at outlets. It works as follows:

• When flow is exiting (positive flux outward): behaves like zeroGradient — velocity is extrapolated from the interior
• When flow is entering (backflow, negative flux): applies a fixedValue with the specified inlet velocity (typically zero or a specified direction)

// 0/U
outlet
{
    type            pressureInletOutletVelocity;
    value           uniform (0 0 0);
}

This BC prevents the simulation from blowing up when a small amount of backflow occurs at the outlet. The backflowing fluid enters with zero velocity rather than with whatever the extrapolated exterior value would be (which is numerically undefined). Use this instead of zeroGradient whenever there is any risk of backflow.

4. inletOutlet: for scalar fields with backflow

inletOutlet is the general-purpose backflow-handling BC for scalar fields (k, epsilon, omega, T, nut, etc.):

// 0/k at outlet
outlet
{
    type            inletOutlet;
    inletValue      uniform 0.01;   // value when flow enters
    value           uniform 0.01;
}

When flow exits, inletOutlet applies zeroGradient. When flow enters (backflow), it applies fixedValue with the inletValue. This prevents unphysical values of k, epsilon, or omega from entering the domain through the outlet.

The inletValue for turbulent quantities should be set to a physically reasonable value — the free-stream turbulence intensity value or a small positive number. Setting it to zero can cause division-by-zero in the turbulence model.

5. Fully corrected outlet for turbulent scalars

For a complete incompressible RANS outlet with proper backflow handling:

// p
outlet { type fixedValue; value uniform 0; }

// U
outlet { type pressureInletOutletVelocity; value uniform (0 0 0); }

// k
outlet { type inletOutlet; inletValue uniform 0.1; value uniform 0.1; }

// epsilon
outlet { type inletOutlet; inletValue uniform 0.01; value uniform 0.01; }

// nut
outlet { type calculated; value uniform 0; }

6. Compressible outlet BCs

Compressible solvers (rhoSimpleFoam, rhoPimpleFoam) require different outlet treatment because pressure and density are coupled.

Subsonic outlet (fixedValue pressure)

// p (absolute pressure)
outlet { type fixedValue; value uniform 101325; }

// U
outlet { type pressureInletOutletVelocity; value uniform (0 0 0); }

// T or h
outlet { type inletOutlet; inletValue uniform 300; value uniform 300; }

// rho
outlet { type zeroGradient; }

Non-reflective outlet (waveTransmissive)

For aeroacoustics or cases where pressure waves should leave the domain without reflection:

outlet
{
    type            waveTransmissive;
    field           p;
    psi             thermo:psi;
    gamma           1.4;
    fieldInf        101325;
    lInf            1.0;    // characteristic length
    value           uniform 101325;
}

7. Backflow instability: diagnosis and fixes

Backflow instability manifests as oscillating residuals, warnings like Negative flux at outlet in the log, or divergence starting from the outlet region. The root cause is almost always backflow — reversed flow entering the domain through the outlet patch.

Diagnosing backflow

In the log, look for:

// OpenFOAM prints this when backflow occurs
Reversing flux at X faces on outlet

In ParaView, visualize the velocity field at the outlet patch. Any blue (negative x-velocity for a streamwise outlet) cells indicate backflow.

Fix 1: Move the outlet downstream

This is the cleanest fix. Extend the domain 5–15 diameters downstream of the last flow disturbance (bluff body, diffuser, junction). The boundary condition assumptions (fully developed, no backflow) are only valid where the flow has reattached and is moving uniformly toward the outlet.

Fix 2: Use backflow-resistant BCs

Switch from zeroGradient to pressureInletOutletVelocity for velocity and use inletOutlet for all scalar fields. These BCs handle the occasional backflow without destabilizing the simulation.

Fix 3: Add a pressure outlet buffer

For complex geometries where outlet placement is constrained, add a short buffer region of simplified geometry (straight duct extension) before the outlet boundary. This forces the flow to become more uniform before hitting the boundary condition.

Fix 4: Use outletPhaseMeanVelocity for multiphase

In VOF simulations with interFoam, backflow instability is particularly severe because the phase fraction field is affected. Use outletPhaseMeanVelocity which adjusts the mean velocity to maintain phase balance while allowing the profile to vary.

8. Outlet for periodic/cyclic cases

For channel flows, pipe flows, or any geometry with translational periodicity, you can replace the inlet/outlet pair with a cyclic patch pair. This is the most accurate approach — there is no inlet or outlet at all, just periodic repetition:

// In constant/polyMesh/boundary
left
{
    type            cyclic;
    neighbourPatch  right;
}
right
{
    type            cyclic;
    neighbourPatch  left;
}

Add a body force in fvOptions to drive the flow at the desired mean velocity or pressure gradient. This eliminates all outlet BC issues by construction.

FAQ

What is the difference between zeroGradient and fixedValue at an outlet in OpenFOAM?

zeroGradient extrapolates the field value from the interior — appropriate for velocity at outlets where the flow exits freely. fixedValue imposes a specific value regardless of the interior solution — appropriate for pressure at outlets to set the reference pressure. Never use fixedValue for both U and p at the same outlet patch.

What boundary condition should I use for velocity at an OpenFOAM outlet?

Use pressureInletOutletVelocity at pressure-fixed outlets. It applies zeroGradient for outflow and a fixed zero value for any backflow. This is more stable than pure zeroGradient when there is any risk of flow reversal at the outlet.

How do I handle backflow at an outlet in OpenFOAM?

Use inletOutlet for velocity and all scalar fields (k, epsilon, T). inletOutlet switches between zeroGradient (outflow) and fixedValue with a physically reasonable backflow value. Also extend the domain to push the outlet further from recirculation zones.

Where should I place the outlet boundary in OpenFOAM?

At least 10–15 diameters downstream of any flow disturbance (bluff body, bend, junction). The flow should be as close to fully developed and unidirectional as possible at the outlet. Large recirculation zones at the outlet boundary cause backflow and divergence.

Why is my OpenFOAM simulation diverging at the outlet?

Most likely cause is backflow — reversed flow entering through the outlet patch. Fix by switching to pressureInletOutletVelocity for U and inletOutlet for scalar fields, extending the domain, or adding a straight duct extension before the outlet.

What pressure BC should I use at an OpenFOAM outlet?

For incompressible solvers, use fixedValue uniform 0 (gauge pressure). For compressible solvers, use fixedValue with the absolute pressure in Pascals. For buoyancy-driven flows with buoyantSimpleFoam, set fixedValue on p_rgh, not p.