- Pressure-Correction Methods
Pressure-Correction Methods refer to a class of methods used in
computational fluid dynamicsfor solving the Navier-Stokes equationsnormally for incompressible flows.
The equations solved in this approach arise from the implicit time integration of the incompressible
Due to the non-linearity of the convective term in the
momentum equationthat is written above, this problem is solved with a nested-loop approach. While so called "global"or "inner iterations" represent the real time-steps and are used to update the variables and , based on a linearized system, and boundary conditions; there is also an "outer loop" for updating the coefficients of the linearized system.
The outer iterations comprise two steps:
- solve the momentum equation for a "provisional" velocity based on the velocity and pressure of the previous outer loop.
- plug the new newly obtained velocity into the continuity equation to obtain a correction.
is computed by first calculating a residual value , resulting from spurious "mass flux", then using this "mass imbalance" to get a new pressure value. The pressure value that is attempted to compute, is such that when plugged into momentum equations a divergence-free velocity field results. The mass imbalance is often also used for control of the outer loop.
The name of this class of methods stems from the fact that the correction of the velocity field is comptued through the pressure-field.
The discretization of this is typically done with either the
finite element methodor the finite volume method. With the latter, one might also encounter the dual mesh, i.e. the computation grid obtained from connecting the centers of the cells that the initial subdivison into finite elements of the computation domain yielded.
Implicit Split-Update Procedures
Another approach which is typlically used in FEM is the following.
The aim of the correction step is to ensure "conservation of mass". In continuous form for compressible substances mass, conservation of mass is expressed by where is the square of the "speed of sound". For low
Mach numbers and incompressible media is assumed to be infinite, which is the reason for the above continuity equation to reduce to The way of obtaining a velocity field satisfying the above, is to compute a pressure which when substituted into the momentum equation leads to the desired correction of a preliminary computed intermediate velocity.
Applying the divergence operator to the copmpressible momentum equation yields
then provides the governing equation for pressure computation.
The idea of pressure-correction also exists in the case of variable density and high Mach numbers, although in this case there is a real physical meaning behind the coupling of
dynamic pressureand velocity as arising from the "continuity equation"
is with compressibility, still an additional variable that can be eliminated with algebraic operations, but its variablilty is not a pure artifice as in the compressible case, and the methods for its computation differ significantly from those with constant.
* M. Thomadakis, M. Leschinzer: A PRESSURE-CORRECTION METHOD FOR THE SOLUTION OF INCOMPRESSIBLE VISCOUS FLOWS ON UNSTRUCTURED GRIDS, Int. Journal for Numerical Meth. in Fluids, Vol. 22, 1996
* A. Meister, J. Struckmeier: Hyperbolic Partial Differential Equations, 1st Edition, Vieweg, 2002
* [http://ta.twi.tudelft.nl/isnas/isnas_mathmanual/mathmanual.html ISNaS - incompressible flow solver]
* [http://www.peconet.com/products/RootsBulletins/rm-135.pdf Application of Temperature and/or Pressure Correction Factors in Gas Measurement]
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