Can anybody explain pressure point constraint with a practical example in terms of CFD?
Navier Stokes and continuity equation solve pressure drop, not pressure itself. Therefore it requires a standard point from which pressure varies.
A pressure point constraint is to avoid infinity of solutions and setting the pressure P to zero. The stokes flow / creeping flow approximation can be used, since the typical dimensions of the given problem may give a very small Reynolds number.
Ex: chemical reactor creates a pressure that disturbs the flow,
Below is found the entire text in COMSOL's book.
What is meant is the definition of a constraint at a given point in the given domain, regarding how large the pressure is allowed to be at that point. The explanation is not more precise than that, as seen in the text below, taken from the guide.
From my own work on topology optimization I am aware that such constraints may cause the problem to behave less well.
It should be noted that the notion of pressure is quite similar to that of stress, so it may be easier to understand by thinking in terms of stress instead.
===COMSOL below===
Pressure Point Constraint
The Pressure Point Constraint feature adds a pressure constraint at a point. If it is not possible to specify the pressure level using a boundary condition, the pressure must be set in some other way, for example, by specifying a fixed pressure at a point.
PO I N T S E L E C T I O N From the Selection list, choose the points to use a pressure constraint.
P RE S S U R E C ON S T R A I N T Enter a point constraint for the Pressure p0 (SI unit: Pa).
C O N S T R A INT S E T T I N GS To display this section, click the Show button () and select Advanced Physics Options. To Apply reaction terms on all dependent variables, select All physics (symmetric). Or select Individual dependent variables to restrict the reaction terms as required.
Select the Use weak constraints check box to replace the standard constraints with a weak implementation.
Navier Stokes and continuity equation solve pressure drop, not pressure itself. Therefore it requires a standard point from which pressure varies.
In the case of incompressible flow pressure affects the flow field (see the momentum equation) not by level but with gradient (there are infinite number of solutions for the pressure field which satisfy governing equations). To define specific solution (to define pressure level) you should specify pressure at one point .
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