Algorithm Options

Conservative vs. Non-conservative

The following must be preceded by “ns.”

	Description	Type
do_mom_diff	If 0, solve velocity equation in convective form, else use conservation form	Int
do_cons_trac	If 0, solve for a passively advected tracer, else advect conservatively	Int
do_cons_trac2	If 0, solve for a passively advected 2nd tracer, else advect conservatively	Int

Note that Temperature is only non-conservative. For more details, see Fluid Equations.

Advection

IAMR computes the advective terms with an unsplit Godunov scheme (piecewise linear or piecewise parabolic reconstruction) or with the Bell-Dawson-Shubin (BDS) scheme. The following must be preceded by “ns.”

	Description	Type	Default
advection_scheme	Godunov_PLM, Godunov_PPM or BDS. Godunov_PPM and BDS are not available with embedded boundaries.	String	Godunov_PLM
do_scalminmax	After the advective update, clamp each scalar other than density to the min/max of its old-time values in the surrounding 3x3 (3x3x3 in 3D) cells.	Int	0
do_denminmax	After the advective update, clamp density to the min/max of its old-time values in the surrounding 3x3 (3x3x3 in 3D) cells.	Int	0

Note that the old ns.use_godunov key and the MOL scheme have been removed; setting either aborts the run.

Note that ns.do_scalminmax = 1 (ns.do_denminmax = 1 for density) keeps the advective update from creating new extrema, which the unsplit Godunov schemes can otherwise produce in a strongly deforming flow (a tracer initialized in [0,1] can leave that range by a few percent). Conservatively advected scalars are clamped as \(s = (\rho s)/\rho\). With embedded boundaries, covered cells are neither clamped nor included in the min/max. The clamps are not conservative, and they act on the advective update only: the diffusive update and, on multilevel runs, the sync correction are applied afterwards and are not clamped.

For problems without embedded boundaries, there is an additional option for the Godunov method. The following must be preceded by “godunov.”

	Description	Type	Default
use_forces_in_trans	Use external forcing terms in constructing transverse derivatives	bool	false

Diffusion

The following must be preceded by “ns.”

	Description	Type	Default
be_cn_theta	Diffusion solve fully implicit (1.0) or semi-implicit (<1 && >0.5)	Real	0.5

Note the default value of ns.be_cn_theta = 0.5 corresponds to the Crank-Nicolson method.

Large Eddy Simulation

IAMR can add a subgrid-scale eddy viscosity to the viscous terms. The following must be preceded by “ns.”

	Description	Type	Default
do_LES	Add a subgrid-scale eddy viscosity to the molecular viscosity	Int	0
LES_model	Which model to use: Smagorinsky or Sigma. Any other value aborts. Sigma is 3D only.	String	Smagorinsky
smago_Cs_cst	Model constant, used only when LES_model = Smagorinsky	Real	0.18
sigma_Cs_cst	Model constant, used only when LES_model = Sigma	Real	1.5
getLESVerbose	Print the model and constant in use from the LES routine	Int	0

Note that each model reads its own constant, so changing ns.smago_Cs_cst has no effect when ns.LES_model = Sigma, and vice versa. Both models compute a kinematic eddy viscosity \(\nu_t\); since IAMR’s viscosity is dynamic, \(\mu_t = \rho \nu_t\) (with \(\rho\) averaged to faces) is what is added to it. The Sigma model is described in Nicoud et al., Using singular values to build a subgrid-scale model for large eddy simulations, Phys. Fluids 23, 085106 (2011).

Embedded Boundaries

These apply to builds with USE_EB=TRUE; see Constructing Embedded Boundaries for how the geometry itself is constructed. The following must be preceded by “ns.”

	Description	Type	Default
redistribution_type	How the advective update of a small cut cell is redistributed to its neighbours: StateRedist, FluxRedist or NoRedist. Any other value aborts.	String	StateRedist
refine_cutcells	Tag every cut cell for refinement, so that the embedded boundary never crosses a coarse/fine boundary. Setting 0 allows a partially refined EB, which is still under development and issues a warning.	Int	1

Note that ns.advection_scheme = Godunov_PPM and BDS are not available with embedded boundaries.