Which procedure to solve Ueqn without momentum predictor.
This is icoFoam's code,:
Hi, in the PISO method, U is predicted and then corrected in the pressure-velocity coupling loop, so that, if you don't predict the velocity, the velocity used to start the coupling loop is a derivative of the velocity in the last time step as is done in interFoam and other solvers.
In the code you've cited the matrix for the momentum equation is assembled so that you have the A and H methods available for the use in the PISO loop, the line:
HbyA = rAU*UEqn.H();
calculates a first approximation for U using the H operator applied to the velocity of the last-timestep or which was predicted when you solved the UEqn (if you did it) and the A operator. If you need it, please check the definition of these operators in chapter 2.7 of my Ph.D. thesis
and Jasak's Ph.D. and other's thesis.
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