Frequently Asked Question

An inlet condition supplies the information entering the computational domain. Depending on the problem, this may be velocity, mass flow rate, total pressure and temperature, or a profile measured or calculated upstream.

A physically useful inlet specification includes the variables required by the governing equations and any additional transported quantities, such as temperature, species fractions, or turbulence statistics. The profile should be compatible with the expected upstream flow; a uniform profile imposed immediately before a developing region is an assumption, not a measurement.

Checklist: confirm direction, units, reference frame, area, density model, turbulence intensity or fluctuation scale, and thermal/species composition. If a specified mass flow is ˙m, the area-averaged normal velocity for uniform density is approximately U_n = ˙m/(ρA).

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.