Frequently Asked Question

An outlet condition should allow information to leave the domain without artificially reflecting disturbances. A pressure condition is often appropriate when downstream pressure is known, while a prescribed flow condition is appropriate when the outgoing flow rate is known.

Place the outlet far enough downstream that the flow is reasonably developed. If strong recirculation reaches the boundary, the assumed one-way outlet behavior may be invalid. Backflow, if physically possible, must have sensible temperature, composition, and turbulence values.

Diagnostic: move the outlet farther downstream and compare the quantity of interest. A substantial change indicates boundary influence and insufficient domain extent.

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.