Frequently Asked Question

Boundary conditions define how the flow interacts with the edges of your simulation domain — they're one of the most common sources of both convergence trouble and inaccurate results if set up incorrectly.

Common boundary condition types:

• Inlet conditions — Options usually include velocity inlet (specify flow speed/direction), mass flow inlet, or pressure inlet (specify total or static pressure). Choose based on what's physically known about your system — if you know the actual flow rate, use a velocity or mass flow inlet; if you know the upstream pressure but not flow rate, use a pressure inlet.
• Outlet conditions — Typically a pressure outlet (specify static pressure, often atmospheric/gauge zero) or, less commonly, an outflow condition. The outlet should be placed far enough downstream that flow has stabilized and isn't affected by nearby geometry features (recirculation crossing the outlet boundary can cause convergence issues).
• Wall conditions — No-slip (velocity = 0 at the wall, standard for viscous flows) is most common; some cases use slip walls for idealized/symmetry situations. Wall roughness and thermal boundary conditions (adiabatic, fixed temperature, fixed heat flux) also get specified here.
• Symmetry planes — Used to reduce computational domain size when the geometry and expected flow are symmetric; cuts mesh count roughly in half (or more, for multiple symmetry planes) without sacrificing accuracy, provided the assumption genuinely holds.

Common setup mistakes:

• Placing outlet boundaries too close to areas of flow disturbance (wakes, recirculation) — push them further downstream
• Using symmetry conditions on flows that aren't actually symmetric (e.g., a case with any yaw angle or asymmetric geometry feature)
• Mismatched units between CAD geometry and solver setup — always double-check unit consistency before running

A general principle: Boundary conditions should reflect physically real, known quantities wherever possible. When a boundary condition is a rough guess rather than a known value, treat the resulting simulation results with proportionally more caution.

Well-posed boundaries

Boundary conditions provide the information needed to make a mathematical problem well posed. They must match the physical location and the variables that are actually known. Specifying velocity, mass flow, pressure, temperature, turbulence, species, or phase fraction redundantly can over-constrain a problem or hide an unintended assumption. Artificial boundaries should be placed far enough from the feature of interest that their condition does not control the result.

ṁ is mass flow, A is the boundary area, n is the outward normal, and Δp is the pressure difference with a stated reference.

Worked example

For an inlet area of 0.01 m², uniform speed of 5 m/s, and density of 1.2 kg/m³, ṁ = 1.2×5×0.01 = 0.06 kg/s. Compare that value with integrated outlet flux. A mismatch requires investigation of normals, compressibility, sources, leakage, or boundary placement.

Check: vary domain extent or outlet treatment when recirculation or strong gradients reach the boundary.