Dear All,
I've several questions about outflow boundary conditions for boundary
regions and conditions specified below for a pipe flow problem:
*/<BOUNDARYREGIONS>/**/
/**/ <B ID="0"> C[2] </B> <!-- Inlet -->/**/
/**/ <B ID="1"> C[3] </B> <!-- Outlet -->/**/
/**/ <B ID="2"> C[1] </B> <!-- Wall -->/**/
/**/ </BOUNDARYREGIONS>/**/
/**/
/**/ <BOUNDARYCONDITIONS>/**/
/**/ <REGION REF="0">/**/
/**/ <D VAR="u" VALUE="0" />/**/
/**/ <D VAR="v" VALUE="0" />/**/
/**/ <D VAR="w" VALUE="1" />/**/
/**/ <N VAR="p" USERDEFINEDTYPE="H" VALUE="0" />/**/
/**/ </REGION>/**/
/**/ <REGION REF="1">/**/
/**/ <N VAR="u" VALUE="0" />/**/
/**/ <N VAR="v" VALUE="0" />/**/
/**/ <N VAR="w" VALUE="0" />/**/
/**/ <D VAR="p" VALUE="0" />/**/
/**/ </REGION>/**/
/**/ <REGION REF="2">/**/
/**/ <D VAR="u" VALUE="0" />/**/
/**/ <D VAR="v" VALUE="0" />/**/
/**/ <D VAR="w" VALUE="0" />/**/
/**/ <N VAR="p" USERDEFINEDTYPE="H" VALUE="0" />/**/
/**/ </REGION>/**/
/**/ </BOUNDARYCONDITIONS>/*
1) What is the meaning of */<N VAR="p" USERDEFINEDTYPE="H" VALUE="0" />
/*and why do we specify such a BC at the wall of the pipe? What happens
if I don't specify any pressure BC at the wall of the pipe?*/
/*2) Is there any other option to specify pressure BC at the outlet
other than*//**/*/<D VAR="p" VALUE="0" />/*/* ?*/*//*/*For example is it
possible to apply the pressure BC at the outlet given below which is
specified in a paper (Direct numerical simulation of stenotic flows.
Part 1. Steady flow, SONU S. VARGHESE, STEVEN H. FRANKEL AND PAUL F.
FISCHER) ?
/*In turbulent flows, it is possible to have vortices strong enough to
yield a (locally) negative flux at the outflow boundary. Since no flow
characteristics are specified on these boundaries, a negative flux
condition typically leads to instabilities with catastrophic results.
One way to ensure that the characteristics at the exit are always
pointing outwards is to force the exit flow through a nozzle,
effectively adding a mean axial component to the velocity field. In
contrast, schemes based on viscous *//*buffer zones require knowledge of
the anticipated space and time scales to ensure that vortical structures
are adequately damped as they pass through the buffer zone.*//*
*//*This nozzle effect can be imposed numerically without having to
change the mesh geometry by imparting a positive divergence to the flow
field near the exit (in the spirit of a supersonic nozzle). In the
current study, this is done by identifying the layer of elements
adjacent to the outflow and imposing a divergence function D(x) that is
zero at the upstream end of the layer and ramps to a fixed positive
value at the exit. Specifically, we set D(x)=C[1−(x⊥/L⊥)2], where x⊥ is
the distance normal to the boundary and L⊥ is the maximum thickness of
the last layer of elements. A net gain in mean velocity is obtained over
the extent of the layer by integrating the expression for D from x⊥/L⊥=1
to 0. The constant C is chosen such that the gain is equal to the mean
velocity prior to the correction.*//*
*/*/
/*Regards,
Kamil