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Parameter · 05 Boundary Conditions

Inlet velocity / flow rate

velocity_inlet.value Unit: m/s, m³/s or kg/s (kg/h shown on request) Step 05 · Boundary Conditions

The number the inlet mode imposes: a velocity in m/s, a volume flow in m³/s, or a mass flow in kg/s (the box can show kg/h; the case file always stores kg/s). Below it the card shows the estimated inlet Mach number.

Last updated

Theory

How the value becomes a boundary condition is described in Inlet mode. All modes are reduced to one equivalent uniform velocity UeqU_{eq} over the inlet patch, equation (1), which the program uses wherever a single inlet velocity is needed: the turbulence estimate (Inlet turbulence), the first-layer plan of the mesh, the transient end time (flow-through time), and the Mach number.

The Mach number shown on the card is computed live from what is typed, with the patch area from the Geometry step and the speed of sound of the case's gas at the inlet temperature (the Material temperature, or Inlet temperature for a compressible case), equation (2). The same formula decides the server's confirmation on saving. It is an estimate: the real velocity profile has a peak above the bulk value, and jets inside the muffler can be much faster than the inlet.

Equations

Ueq=U  (velocity),Ueq=QA,Ueq=m˙ρinAU_{eq} = U \ \ (\text{velocity}), \qquad U_{eq} = \frac{Q}{A}, \qquad U_{eq} = \frac{\dot m}{\rho_{in}A}
(1)
wheremeaningunit
UeqU_{eq}Equivalent uniform inlet velocitym/s
QQVolume flow ratem³/s
m˙\dot mMass flow ratekg/s
ρin\rho_{in}Inlet density (Inlet density; compressible: the Material density)kg/m³
AAInlet patch aream²
Ma=Ueqc(T),c=γ(T) R T,γ=cpcp−RMa = \frac{U_{eq}}{c(T)}, \qquad c = \sqrt{\gamma(T)\,R\,T}, \qquad \gamma = \frac{c_p}{c_p - R}
(2)
wheremeaningunit
MaMaEstimated inlet Mach number-
ccSpeed of sound of the gasm/s
γ\gammaRatio of specific heats at TT (from the gas model's cp(T)c_p(T))-
RRSpecific gas constant of the gasJ/(kg K)
TTInlet temperatureK

Default and rule

Default
10 m/s (velocity mode)
Rule

10 m/s in velocity mode. Switching the mass-flow unit converts the displayed number, so the stored kg/s value is kept.

Source

The incompressible limit Ma 0.3 is the usual rule of thumb. The compressible limit 0.6 is a conservative margin chosen from validation runs on case V1 (a 2.0 m multi-chamber muffler with a perforated pipe and three absorptive packings): the compressible solver ran at inlet Mach 0.15 and failed at 0.74; intermediate Mach numbers were not tested. Boundary conditions: (OpenFOAM user guide, 2026).

Valid range > 0

Values

Positive. A velocity of 0 gives no flow.

Incompressible above Mach 0.3

Saving asks for confirmation: density changes are no longer small, and the incompressible pressure drop becomes inaccurate. Choose a compressible analysis on the Model step.

Compressible above Mach 0.6

Saving asks for confirmation: on a real muffler the steady compressible solver clipped the pressure at its limits and aborted at inlet Mach 0.74.

References 1

  1. OpenCFD Ltd. (2026). OpenFOAM user guide. openfoam.com. Web page