/BEM/FLOW
Block Format Keyword Describes the incompressible fluid flow by boundary elements method.
Format
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/BEM/FLOW/flow_ID/unit_ID  
flow_title  
surf_ID_{ex}  Nio  Iinside  Ifsp  Fscale_{sp}  Ascale_{sp}  
grn_ID_{aux}  Itest  Tole  
Rho  Ivinf  
surf_ID_{io}  fct_ID_{vel}  fct_ID_{pr}  Fscale_{nv}  Fscale_{pres}  Ascale_{t}  
I_{form}  Ipri  Dtflow  
Ifvinf  Fscale_{vel}  Ascale_{vel}  
Dir_{x}  Dir_{y}  Dir_{z} 
Definitions
Field  Contents  SI Unit Example 

flow_ID  Incompressible flow block
identifier. (Integer, maximum 10 digits) 

unit_ID  Unit Identifier. (Integer, maximum 10 digits) 

flow_title  Incompressible flow block
title. (Character, maximum 100 characters) 

surf_ID_{ex}  Flow external surface
identifier. (Integer) 

Nio  Number of inflowoutflow
surfaces. (Integer) 

Iinside  Inside or outside flow flag.
(Integer) 

Ifsp  Stagnation pressure curve
number. (Integer) 

Fscale_{sp}  Stagnation pressure scale
factor. Default = 1.0 (Real) 
$\left[\text{Pa}\right]$ 
Ascale_{spc}  Abcissa scale factor for
stagnation pressure curve. Default = 1.0 (Real) 
$\left[\text{s}\right]$ 
grn_ID_{aux}  Auxiliary nodes group
identifier. 2 (Integer) 

Itest  Test auxiliary nodes flag.
2 (Integer > 0) 

Tole  A dimensional tolerance.
2 Default = 1.e5 (Real) 

Rho  Fluid
density. (Real) 
$\left[\frac{\text{kg}}{{\text{m}}^{3}}\right]$ 
Ivinf  Additional velocity field
flag. 3 (Integer > 0) 

surf_ID_{io}  InflowOutflow surface
identifier. 4 (Integer) 

fct_ID_{nv}  Normal velocity curve.
4 (Integer) 

fct_ID_{pres}  Imposed pressure curve.
5 (Integer) 

Fscale_{nv}  Normal velocity scale
factor. Default = 1.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
Fscale_{pres}  Imposed pressure scale
factor. Default = 1.0 (Real) 
$\left[\text{Pa}\right]$ 
Ascale_{t}  Abscissa scale factor for
normal velocity curve and imposed pressure curve. Default = 1.0 (Real) 
$\left[\text{s}\right]$ 
I_{form}  Formulation flag. 6
(Integer > 1) 

Ipri  Output level. (Integer > 1) 

Dtflow  Time step for BEM matrices
assembly. 7 Default = 0 (Real) 
$\left[\text{s}\right]$ 
Ifvinf  Velocity
curve. (Integer) 

Fscale_{vel}  Velocity scale
factor. Default = 1.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
Ascale_{vel}  Abscissa scale factor for
velocity curve. Default = 1.0 (Real) 
$\left[\text{s}\right]$ 
Dir_{x}  X component of the
additional field direction vector. (Real) 

Dir_{y}  Y component of the
additional field direction vector. (Real) 

Dir_{z}  Z component of the
additional field direction vector. (Real) 
Comments
 The surf_ID_{ex} must define a closed surface.
 Using BEM, the flow
potential, velocity and pressure are computed for nodes belonging to the surface
defined by surf_ID_{ex}.
For visual and posttreatment concerns, the flow characteristics can be computed for a set of nodes inside the flow belonging to grn_ID_{aux}.
If Itest = 1, whether the auxiliary nodes are actually located inside (if Iinside =1) or outside (if Iinside =2), the surface defined by surf_ID_{ex} at each time step is tested. Wrong nodes are then canceled for the current time step.
Tolerance Tole is used to perform the pointinsideclosedsurface test.
 Flag
Ivinf is only effective for flow computation in an
unbounded domain outside the surface defined by
surf_ID_{ex}
(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_ID_{ex}.
 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_ID_{ex}.
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.
 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 inflowoutflow surfaces.
 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.
 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.