/BEM/FLOW (Obsolete)

Block Format Keyword

/BEM/FLOW - Incompressible Fluid Flow by Boundary Elements Method

Description

Describes the incompressible fluid flow by boundary elements method.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
/BEM/FLOW/flow_ID/unit_ID
flow_title
surf_IDex	Nio	Iinside	Ifsp	Fscalesp		Ascalesp
grn_IDaux	Itest	Tole
Rho		Ivinf
surf_IDio	fct_IDvel	fct_IDpr		Fscalenv		Fscalepres		Ascalet
Iform	Ipri	Dtflow
Ifvinf	Fscalevel		Ascalevel
Dirx		Diry		Dirz

Field	Contents	SI Unit Example
flow_ID	Incompressible flow block identifier (Integer, maximum 10 digits)
unit_ID	Optional unit identifier (Integer, maximum 10 digits)
flow_title	Incompressible flow block title (Character, maximum 100 characters)
surf_IDex	Flow external surface identifier (Integer)
Nio	Number of inflow-outflow surfaces (Integer)
Iinside	Inside or outside flow flag Default = 1 (Integer) = 1: Flow is computed inside the surface defined by surf_IDex. The surface element normals must be oriented outwards. = 2: Flow is computed outside the surface defined by surf_IDex. The surface element normals must be oriented inwards.
Ifsp	Stagnation pressure curve number (Integer)
Fscalesp	Stagnation pressure scale factor Default = 1.0 (Real)
Ascalespc	Abcissa scale factor for stagnation pressure curve Default = 1.0 (Real)
grn_IDaux	Auxiliary nodes group identifier (see Comment 2) (Integer)
Itest	Test auxiliary nodes flag (see Comment 2) (Integer > 0)
Tole	A dimensional tolerance (see Comment 2) Default = 1.e-5 (Real)
Rho	Fluid density (Real)
Ivinf	Additional velocity field flag (see Comment 3) (Integer > 0)
surf_IDio	Inflow-Outflow surface identifier (see Comment 4) (Integer)
fct_IDnv	Normal velocity curve (see Comment 4) (Integer)
fct_IDpres	Imposed pressure curve (see Comment 5) (Integer)
Fscalenv	Normal velocity scale factor Default = 1.0 (Real)
Fscalepres	Imposed pressure scale factor Default = 1.0 (Real)
Ascalet	Abscissa scale factor for normal velocity curve and imposed pressure curve Default = 1.0 (Real)
Iform	Formulation flag (see Comment 6) (Integer > 1) = 1: fluid flow is computed using BEM with a collocation approach to solve the integral equation. = 2: fluid flow is computed using BEM with a galerkin approach to solve the integral equation.
Ipri	Output level (Integer > 1)
Dtflow	Time step for BEM matrices assembly (see Comment 7) Default = 0 (Real)
Ifvinf	Velocity curve (Integer)
Fscalevel	Velocity scale factor Default = 1.0 (Real)
Ascalevel	Abscissa scale factor for velocity curve Default = 1.0 (Real)
Dirx	X component of the additional field direction vector (Real)
Diry	Y component of the additional field direction vector (Real)
Dirz	Z component of the additional field direction vector (Real)

1.	The surf_IDex must define a closed surface.

2.	Using BEM, the flow potential, velocity and pressure are computed for nodes belonging to the surface defined by surf_IDex.

For visual and post-treatment concerns, the flow characteristics can be computed for a set of nodes inside the flow belonging to grn_IDaux.

If Itest = 1, whether the auxiliary nodes are actually located inside (if Iinside =1) or outside (if Iinside =2), the surface defined by surf_IDex at each time step is tested. Wrong nodes are then canceled for the current time step.

Tolerance Tole is used to perform the point-inside-closed-surface test.

3.	Flag Ivinf is only effective for flow computation in an unbounded domain outside the surface defined by surf_IDex (Iinside =2).

If Ivinf = 1, an inflow condition is defined by an additional homogeneous flow defined in free space. The computed flow will be identical to the additional flow at an infinite distance from the surface defined by surf_IDex.

4.	If Iinside = 0: there must be at least one surface where the normal velocity is imposed and one, and only one surface where the normal velocity is left free. The velocity at the free surface will be computed thanks to flux equilibrium on the global surface defined by surf_IDex.

If Iinside = 2 and Ivinf = 0: same as above.

If Iinside = 2 and Ivinf = 1: the number of surfaces is free and the normal velocity must be imposed on all of them.

5.	In order to reduce pressure from the velocity field, one and only one pressure must be imposed for the entire flow computation: it can be whether the global stagnation pressure or the pressure at one of the inflow-outflow surfaces.

6.	The collocation approach is faster but may not be robust enough to handle very complex geometries.

The galerkin approach works in every situation but is significantly slower.

7.	BEM matrices depend only on the geometry of the surface.

If Dtflow = 0 (default), they are assembled at every cycle of the simulation (the time step being classically given by the stability condition of finite elements).

If Dtflow ≠ 0:,max(Dtflow, Dt) is used to update to BEM matrices; where Dt is the finite element time step.