What this step does

The patches are the groups of CAD faces assigned on the Geometry step. The list on the left groups them as inlets, outlets, outer walls, perforated walls and internal walls, followed by one entry per absorptive backing (Absorptive backing 1, 2, ...: the packing regions of the Geometry step, smallest region number first). Choosing a patch shows its card and highlights its faces in the model view; choosing a backing highlights the boundary faces of its region. Clicking a face in the model opens its card: a face of a backing's region opens the backing, unless it belongs to an inlet, outlet or perforated wall, which keep their own cards; a second click on a face of the entry already shown opens the face's other owner, so both can be reached from the model.

Inlets. The inlet mode (Inlet mode) imposes a uniform velocity normal to the inlet, a volume flow, a mass flow (Inlet density converts it for the incompressible solver) or a velocity vector. A flow rate is exact whatever the meshed area; a velocity delivers its flow through the meshed area. The inlet turbulence is estimated for fully developed pipe flow or set from an intensity and a mixing length (Inlet turbulence). A compressible case also needs the inlet temperature (Inlet temperature). The card shows the estimated inlet Mach number; saving asks for confirmation above 0.3 (incompressible) or 0.6 (compressible).

Outlets. A fixed static pressure relative to the ambient pressure of the Material step (Outlet gauge pressure); the absolute value is shown below it (Outlet absolute pressure).

Walls. All walls are no-slip with wall functions: Spalding's law for the turbulent viscosity, continuous from the viscous sublayer to the log layer (Spalding, 1961), with $k$ and $\omega$ wall functions. The low-Re treatment of the Mesh step switches to the low-Re forms. A roughness height (Wall roughness height, Roughness constant) switches the wall to the rough-wall function.

An internal wall (a zero-thickness baffle) becomes two wall patches in the mesh, <name> and <name>_slave; both sides receive the same settings and both are no-slip. With heat transfer, each outer wall has a thermal condition (Thermal condition of a wall): adiabatic, a fixed temperature, or heat loss through a shell and insulation to still or moving air, with radiation. Internal walls remain adiabatic.

Perforated walls. A perforated tube or plate is a zero-thickness porous jump on a cyclic baffle pair. The pressure drop across it follows a loss coefficient K, either typed or computed from the open-area ratio and the flow regime (Perforation type, Open-area ratio). For a perforated tube, K is updated from the solved flow (grazing correction). With acoustics on, the card also holds the wall's acoustic impedance model; a micro-perforated wall can take its non-linear (high-level) resistance from the hole velocity at the inlets' level (Non-linear resistance of a micro-perforate).

Absorptive backing. A packing is a fluid region ticked on the Geometry step's Domains tab; its card holds the material.

In the flow solution it is a Darcy-Forchheimer momentum sink on the cells whose centre lies inside the region (OpenFOAM user guide, 2026), $S = -(\mu d + \tfrac{1}{2}\rho |U| f)\,U$ with $d = R/\mu$ , set by Flow resistivity (or an estimate from Bulk density and Fibre type, see Resistivity input) and Inertial (Forchheimer) factor.
In the acoustic solution it is an equivalent fluid with a complex density and bulk modulus from Acoustic (poroacoustic) model of the packing: an empirical model of the flow resistivity alone (Delany-Bazley, Miki, Komatsu), the physical models JCA / JCAL with Porosity (JCA / JCAL), Tortuosity (JCA / JCAL), Viscous characteristic length (JCA / JCAL), Thermal characteristic length (JCA / JCAL) and Static thermal permeability (JCAL), or a user-defined set of constants (Variable X of the user constants, User constant C1 ... C8).

The flow resistivity is entered at room temperature; the flow and acoustic solutions scale it with the viscosity of the local gas. A packing is best bounded by perforated walls: an open interface between packing and gas follows the mesh resolution only. When the Kirby and Cummings impedance model is chosen, a perforated wall with packing on one side receives that model's porous-backing correction (Kirby and Cummings, 1998).

Acoustic ports. With acoustics on, every inlet and outlet card has an acoustic port: whether the inlet is excited, by an amplitude or a power and a phase (Excite this inlet), the outlet termination used for the insertion loss (Outlet: compute a radiation domain), and how the port absorbs the waves leaving the model: multimodal (default), plane-wave only, or a PML stub that also absorbs the evanescent modes (Anechoic termination). In an acoustics-only case the step shows only the ports, the perforated walls, the internal walls and the absorptive backing.

All settings are validated when the step is saved. The boundary conditions written to OpenFOAM's 0/ files are those documented for its boundary-condition types (OpenFOAM user guide, 2026).

Source

The boundary-condition types written to the solver are OpenFOAM's (OpenFOAM user guide, 2026); the wall law for the turbulent viscosity is Spalding's (Spalding, 1961).

Validation results of the program for the absorptive backing:

Straight two-body pipe: the packing's pressure drop $\Delta p = R\,U\,L$ is reproduced within 1.1 % ( $R = 500$ Pa·s/m², 10 m/s, 0.3 m).
Packed annulus in the 3-D acoustic solver: within 0.45 dB of a 1-D coupled-line solution with the same equivalent fluid, while absorbing up to 73 % of the incident power.

Valid range

Patch names

Inlets, outlets and perforated walls are numbered (inlet_1, outlet_1, perforate_1); unassigned outer faces are merged into the patch wall.

Saved mesh

Changing a patch's type after meshing (for example a perforated wall set back to open) makes the saved mesh unusable; Start run then asks to mesh again.

Packing cells

The packing takes the mesh cells whose centre lies inside its surface; a packing thinner than a few cells is represented coarsely.

JCA / JCAL

Checked in a full acoustic run (2026-11-11); every quality check passes.

Fields in this step 87

Field	Symbol	Unit	Default
Acoustic (poroacoustic) model of the packing			Miki
Acoustic impedance model of a perforated wall	$\zeta_p$		Elnady 2010 (new walls); old case files keep the global model they stored
Ambient temperature	$T_a$	°C	auto: 20 °C
Anechoic termination			Multimodal (port modes)
Bias (through-hole) Mach number	$M_b$		auto: from the CFD in a flow + acoustics run (cross-flow model: net through-flow / (σA), or Q / (σA)); 0 in an acoustics-only run
Bulk density	$\rho_b$	kg/m³	30 kg/m³ (shown when the bulk-density estimate is selected)
Darcy coefficient D	$D$	1/m²	0
Defined by (perforated plate)			open-area ratio (a new perforated wall); resistance coefficient K (a case file that stores no mode)
Discharge coefficient C_D (acoustics)	$C_D$		Auto: bilinear interpolation in the hole-diameter × wall-thickness table
Discharge coefficient C_D0	$C_{D0}$		Auto: interpolated in the hole-diameter × wall-thickness table
Emissivity	$\varepsilon$		auto: the wall-material library's value (0.8 for a user-defined material)
End-correction factor (thin plate)	$\epsilon$	× d	0.85 (Rayleigh)
Excitation input (amplitude or power)			amplitude
Excite this inlet			on for inlets
Fibre type			glass wool
Flow resistivity	$R$	Pa·s/m²	20 000 Pa·s/m²
Grazing correction			on
Grazing factor of a slit micro-perforate	$\beta$		0.08 (derived from Kabral et al. 2014 Fig. 6, not printed in the paper)
Grazing Mach number (acoustics)	$M_g$		auto: from the CFD (mapped field, else the perforate samples), else the entered flow through the tube, else 0
Grazing Mach number (flow)	$M$		0.1 (shown when nothing is stored)
Grazing velocity			CFD, tube core
Hole diameter	$d$	mm (stored in m)	none stored; the card pre-fills 3 mm
Hole diameter (acoustics)	$d$	mm	3.0 mm shown on a new wall (the case file stores none until the card is saved)
Hole edges of a micro-perforate	$\alpha$		As in Maa 1998 (α = 1/2)
Hole interaction (thin plate)	$f_{int}$		none
Incident amplitude	$\|A\|$	Pa	1 Pa
Incident power	$W_{in}$	W	1 W
Include bias flow	$M_b$		off
Inertial (Forchheimer) factor	$f$	1/m	0 (pure Darcy)
Inlet density	$\rho_{in}$	kg/m³	the density of the Material step
Inlet mode			Velocity (uniform_velocity)
Inlet temperature	$T_{in}$	K	the Material temperature (stored as 'follow the material')
Inlet turbulence			Fully developed
Inlet velocity / flow rate	$U,\ Q,\ \dot m$	m/s, m³/s or kg/s (kg/h shown on request)	10 m/s (velocity mode)
Inlet velocity vector, x component	$U_x$	m/s	the x component of the current inlet velocity along the inward normal
Inlet velocity vector, y component	$U_y$	m/s	the y component of the current inlet velocity along the inward normal
Inlet velocity vector, z component	$U_z$	m/s	the z component of the current inlet velocity along the inward normal
Insulation conductivity	$k_{ins}$	W/(m K)	0.06 W/(m K)
Insulation thickness	$t_{ins}$	mm (stored in m)	0 mm (no insulation)
Jet loss K0 (read-only)	$K_0$		computed: 1/(σ C_D0)²
K check interval		iterations	100
K relaxation	$\alpha_K$		1.0
Maximum K evaluations	$n_{max}$		5
Micro-perforate flow resistance	$\Delta p$	Pa	computed from the hole size, thickness and open-area ratio
Micro-perforation: hole diameter or slit width		mm	0.2 mm
Micro-perforation: holes or slits			circular holes
Mixing length	$\ell$	m	blank = 0.07 D_h of the inlet
Non-linear resistance of a micro-perforate	$\|\hat u_h\|$		Off (linear impedance)
Open-area ratio	$\sigma$		0.1
Open-area ratio for acoustics	$\sigma$		the wall's open-area ratio when it has one; otherwise as entered
Outer environment			Still air
Outer heat-transfer coefficient	$h$	W/(m² K)	10 W/(m² K) in the box (environment 'Manual h' only)
Outlet absolute pressure	$p_{abs}$	Pa	ambient pressure + gauge pressure
Outlet gauge pressure	$p_g$	Pa	0 Pa (discharge to the ambient)
Outlet: compute a radiation domain			off (the radiation impedance is chosen in Results)
Perforation type			perforated tube (a new perforated wall); perpendicular plate (a case file that does not store the key)
Phase of the incident wave	$\phi$	°	0°
Plate / wall thickness	$t$	mm (stored in m)	1 mm
Plate formula			thick plate (a new perforated wall); thin sharp-edged (a case file that stores none)
Porosity (JCA / JCAL)	$\phi$		0.95
Resistance coefficient K	$K$		6.89
Resistivity input			flow resistivity
Roughness constant	$C_s$		0.5
Shell conductivity	$k$	W/(m K)	20 W/(m K) in the box (material 'User defined' only)
Shell thickness	$t$	mm (stored in m)	1.5 mm
Single-hole resistance (Sullivan & Crocker)	$r$	rayl	2.4 rayl (kg m⁻² s⁻¹)
Slit width	$b$	mm	0.2 mm
Static thermal permeability (JCAL)	$k'_0$	m²	blank = φ Λ'² / 8 (the JCA value)
Thermal characteristic length (JCA / JCAL)	$\Lambda'$	µm (stored in m)	blank = 2 × viscous length
Thermal condition of a wall			Adiabatic
Thin plate: use C_D	$C_D$		off
Tortuosity (JCA / JCAL)	$\alpha_\infty$		1.0
Turbulence intensity	$I$		Medium, 0.05 (used only with 'Intensity and length')
User constant C1	$C_1$		0.122 (Miki's value)
User constant C2	$C_2$		0.618 (Miki's value)
User constant C3	$C_3$		0.18 (Miki's value)
User constant C4	$C_4$		0.618 (Miki's value)
User constant C5	$C_5$		0.079 (Miki's value)
User constant C6	$C_6$		0.632 (Miki's value)
User constant C7	$C_7$		0.12 (Miki's value)
User constant C8	$C_8$		0.632 (Miki's value)
Variable X of the user constants	$X$		ρ₀ f / R (Delany-Bazley, Miki form)
Vehicle speed	$V$	m/s	15 m/s in the box (environment 'Vehicle moving' only)
Viscous characteristic length (JCA / JCAL)	$\Lambda$	µm (stored in m)	blank = estimated from the flow resistivity
Wall material			Stainless steel 304 / 409 (k 20 W/(m K), emissivity 0.5)
Wall roughness height	$k_s$	m	0 m (smooth wall)
Wall temperature	$T_w$	°C	200 °C (shown when 'Fixed temperature' is chosen)

Boundary Conditions

What this step does

Source

Valid range

Fields in this step 87

References 3

What this step does

Source

Valid range

Fields in this step 87

Related 32

References 3