Theory

Flow grazing past a hole sheds a shear layer over it, which adds resistance and blows away part of the oscillating mass at the hole ends.

Elnady adds $0.3\,M_g$ to the hole resistance (Elnady et al., 2010).
Kirby & Cummings use the friction velocity $u_*$ of the grazing flow, estimated from the grazing speed and the tube (or plate) diameter, equation (2) (Kirby and Cummings, 1998).
Lee & Ih use $M_g$ directly in their regression (Lee and Ih, 2003).
The thin-plate baseline, Sullivan & Crocker and the cross-flow model ignore it.

On auto the grazing speed $U_g$ is taken, in this order:

the tangential velocity of the mapped CFD field, averaged over the wall by area (acoustic medium mapped from the CFD, Acoustic medium);
the tube-core speed sampled by the perforate loss updates of the flow run (logs/perforate_k.json);
the total entered inflow divided by the cross-section of the perforated TUBE, $Q/(\pi r^2)$ , when the flow is on and the Mach estimate is enabled (a flat plate gets none);
0 otherwise (acoustics only: enter the Mach number).

The source used is written in the acoustic log and the report.

Equations

M_g = \frac{U_g}{c}, \qquad U_g = \frac{\sum_e |\mathbf{U}_e - (\mathbf{U}_e\cdot\mathbf{n}_e)\mathbf{n}_e|\,A_e}{\sum_e A_e}\ \text{(mapped field)}, \qquad U_g = \frac{Q}{\pi r^2}\ \text{(estimate)}

(1)

u_* = U_g\sqrt{\lambda/8}, \qquad \lambda = 0.306\,Re^{-1/4}, \qquad Re = \frac{U_g D}{\nu}

(2)

where	meaning	unit
$M_g$	Grazing Mach number	-
$U_g$	Grazing speed	m/s
$c$	Speed of sound of the local gas	m/s
$\mathbf{U}_e, \mathbf{n}_e, A_e$	Mean velocity, unit normal and area of wall triangle e	m/s, -, m²
$Q$	Total inflow	m³/s
$r$	Radius of the perforated tube (cylinder fit)	m
$u_*$	Friction velocity	m/s
$D$	Tube diameter, or equivalent diameter of a plate	m

Default: auto: from the CFD (mapped field, else the perforate samples), else the entered flow through the tube, else 0Computed by the program
Rule: Auto. A typed number overrides every source.
Source: Friction velocity: Lee & Ih eqs. 26-29 (Lee and Ih, 2003). The order of the sources is the program's rule.

Values

0 to 0.6.

Separate from the flow

The flow card's grazing Mach number (Grazing Mach number (flow)) is stored separately and does not change this value.

Model ranges

Lee & Ih: M ≤ 0.2; Kirby & Cummings: u* 0.48-2.19 m/s. Elnady et al. quote their grazing term from Bauer (1977) without a validity range.