Barry: here is an attempt to explain how variable switching works in pflotran (we may need to iterate on this if it's not clear). I will consider as an example the 2-phase, 3-component water-supercritical CO2 system.
Any grid cell (we use finite volume) can exist in one of three possible states: single-phase pure liquid water (l); single-phase pure supercritical CO2 (g); or two-phase coexistence of water and supercritical CO2 (2ph). Each of these states is described by three independent (nonunique) state variables. One possibility is: l: (p_l, T, X_CO2^l) g: (p_g, T, X_CO2^g) 2ph: (p_g, T, s_g) where p_l,g = liquid or gas pressure, T = temperature, and s_g = volume fraction of pore space occupied by supercritical CO2. Where variable switching comes into play is when a grid cell undergoes a phase change with the following possibilities: l -> 2ph g -> 2ph 2ph -> l 2ph -> g Note that we do not include directly l <-> g. As a consequence different regions of space have different primary variables the boundaries of which are changing with time along with the state variables. Line search is not feasible with the current implementation in petsc. After a completed time step or Newton iteration, the stability of the solution in each grid cell must be checked for a change of phase. For example, if the cell is in a 2 phase state, but the solution gives s_g > 1 or s_g < 1, then a phase change occurs from 2ph -> g or 2ph -> l, respectively, and a change of variables is necessary: (p_g, T, s_g) -> (p_l, T, X_CO2^l) or (p_g, T, s_g) -> (p_g, T, X_CO2^g) …Peter > On Jun 12, 2015, at 18:14, Barry Smith <[email protected]> wrote: > > > John, > > We can provide any Fortran stubs you need but before we do could you please > outline to us (in algebraic notation; not in reference to PETSc code) how you > plan to handle your "variable switching". That is, what is the problem being > solved, what are the constraints, when do you need to switch variables etc? > > I would say everyone who has tried to do variable switching in PETSc before > has failed because we (the PETSc developers) did not help them enough and > understand what is needed in the Newton solver. I suspect we need to provide > a bit more functionality in PETSc to make variable switching a success and > I'd like to understand what that is and add it to the PETSc algorithms. Also > if you know of any references that actually explain variable switching in > detail, that would be great, all I've found are vague hand-wavy discussions > that leave too many important details unexplained. > > Thanks > > Barry > >> On Jun 12, 2015, at 5:54 PM, John O'Sullivan <[email protected]> >> wrote: >> >> Hi, >> >> We're working on changing variables during an SNES solve so that we can >> handle phase changes. >> >> I'm looking into using SNESSetUpdate but there doesn't seem to be a fortran >> interface. I get an error: >> >> Undefined symbols for architecture x86_64: >> "_snessetupdate_", referenced from: >> _MAIN__ in phaseChange.o >> >> I've looked in the interface files and can't find it either. I tried writing >> one myself but things get a bit too complicated to guess... >> >> Can an interface be added please? >> >> Is this the best to do variable switching? If not where would be the right >> place and are there Fortran interfaces? We will be changing from Pressure, >> Temperature to Pressure, Saturation etc. But for now I'm just working on a >> simple example code. >> >> Cheers >> John >> -- >> Dr John O'Sullivan >> Lecturer >> Department of Engineering Science >> University of Auckland, New Zealand >> email: >> jp.osullivan at auckland.ac.nz >> >> tel: +64 (0)9 923 85353 >> >
