如何设置阻力

phoenics

Q1. Question: - How do I set resistance terms, such as the Ergun Equation?
Answer:
Introduction
The ERGUN equation represents the pressure drop associated with flow through a packed bed, or more generally, a porous medium.
In PHOENICS, pressure drops are applied as sinks in the momentum equation. The units of the momentum equation are those of force, so the pressure drop must also be turned into a force.
The ERGUN equation can be written as:
dp/dx = a*U1 + b*U12
where a=µ/K and b is an empirical constant. Here, µ is the dynamic viscosity of the fluid flowing through the porous media and K is an empirical constant called the permeability.
The pressure drop can be turned  into a force, as follows:
Fx = Area * dp
= Area * dp/dx * dx
Now Area = dy * dz;
so dx * Area = Volume
Hence:
Fx = Volume * (a*U1 + b*U12)
Implementation
The velocities used in the equation are superficial velocities, so
This can be introduced as two patches:
PATCH(ERGUN1, VOLUME, ........
COVAL(ERGUN1, U1, a, 0.0)
PATCH(ERGUN2, VOLUME, ........
COVAL(ERGUN2, U1, -b, 0.0)
Note that the -ve COefficient in patch ERGUN2 is activating a quadratic source - as stated in POLIS.
In the VR-Editor, the above can be achieved by giving an object the type User-Defined, and setting two patches with the Attributes as described above.
Q2. Question: - How do I set a Stagnation Pressure boundary condition?
Answer:
For incompressible flows, the stagnation pressure is:
Pt = P + 0.5 . r. vel**2
This can be manipulated to give the mass flux:
r . vel = Ö ( 2 . r. (Pt - P) )
For P1 (and also P2), a negative coefficient flags a mass source of the form:
Sm = T . Ö ( abs ( Cm . (Vm - Pp)))
where Cm and Vm are the coefficient and value for mass (P1).
The desired stagnation source can thus be set by putting:
coefficient = - 2 * r
value = Pt
type = appropriate area
These settings must be supplemented by a COVAL for the velocity component normal to the boundary, with coefficient set to ONLYMS, and value set to SAME.
PIL example:
PATCH (PSTAG, LOW, 1,N,1,NY, 1,1, 1,LSTEP)
COVAL (PSTAG, P1, -2*RHO1, PSTAG)
COVAL (PSTAG, W1, ONLYMS, SAME)
Note that for compressible flows, it is still possible to set stagnation conditions, though not in such an easy fashion.
In the VR-Editor, the above can be achieved by giving an object the type User-Defined, and setting the Attributes as described above

易拉罐

怎么设置对流体的完全阻碍效果？

xhfan

你能否说的再明白一点？