Resistance coefficient K

The loss coefficient of a perpendicular perforated plate that is defined by its resistance coefficient. The pressure falls across the plate by $\tfrac12\rho K U_n^2$ , with $U_n$ the velocity normal to the plate face (not the velocity in the holes).

Last updated 2026-11-03

Theory

The plate is a zero-thickness porous jump between two cyclic patches (OpenFOAM porousBafflePressure (OpenFOAM user guide, 2026)). OpenFOAM writes the jump per unit length; the program converts $K$ to the inertial coefficient $I = K/t$ , with the plate thickness $t$ as the jump's length (Plate / wall thickness), so that only the product $I\,t = K$ matters, equation (1). The value is used unchanged for the whole run.

$K$ refers to the face (approach) velocity. A coefficient quoted on the hole velocity $U_h = U_n/\sigma$ must be converted with equation (2).

Equations

\Delta p = \tfrac12\rho\,I\,\lvert U_n\rvert U_n\,t = \tfrac12\rho\,K\,\lvert U_n\rvert U_n, \qquad I = \frac{K}{t}

(1)

K = \frac{K_h}{\sigma^2} \quad \text{(from a coefficient } K_h \text{ on the hole velocity)}

(2)

where	meaning	unit
$\Delta p$	Pressure jump across the plate	Pa
$\rho$	Gas density	kg/m³
$U_n$	Velocity normal to the plate face	m/s
$I$	Inertial coefficient written to the boundary condition	1/m
$t$	Plate thickness	m
$\sigma$	Open-area ratio	-
$K_h$	Loss coefficient on the hole velocity	-

Default and rule

Default: 6.89
Rule: 6.89, the value used for a case file that stores none. A new perforated wall is set up from the open-area ratio instead (Defined by (perforated plate)), so this value normally appears only when the mode is switched.
Source: The jump law and the meaning of $I$ and of the length: OpenFOAM's porousBafflePressure (OpenFOAM user guide, 2026). The default 6.89 is a placeholder with no published basis; enter a value for the actual plate. Validation of the jump (perforated plate on a straight two-body pipe): 465 Pa in the CFD against 428 Pa predicted from the same $K$ (+8.7 %); the difference comes from a slightly narrowed plate rim where the baffle's boundary layers sit.

Valid range K ≥ 0

Values

$K \ge 0$ ; 0 makes the plate fully open (no loss).

Plate only

Shown only for a perpendicular plate defined by its resistance coefficient. A perforated tube computes its own $K$ (Jet loss K0 (read-only)).

References 1

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

Theory

Equations

Default and rule

Valid range K ≥ 0

Related 5

References 1