Convergence Problems

Prateep Chatterjee

everybody:

this has never happened to me before. I'm encountering convergence difficulties in the unsteady simulation of laminar reacting flow. as always, I had generated a semi-converged steady solution (segregated solver, 2nd order schemes, DO radiation, single step Ch4-Air chemistry). instead of the residuals falling even more (which used to be the case earlier for another reacting flow simulation through a simpler geometry), they go up by an order of magnitude, especially the continuity one (which is the only one which refuses to come down).

I've reduced the time step to 10^(-8) s, but it still doesn't work. normally, for the kind of laminar combustion I've been working with, a time step of 10^(-5) s was a sufficiently low step size.

is grid adaptation a possible remedy ? based on what criterion should I adapt the grid ? I've checked gradients, and the only gradients which were high were the enthalpy ones [O(10^3)].

I'm using the second order scheme for pressure and PISO for p-v coupling.

the residuals are in the range of:

continuity 1.7377e+00

x-velocity 1.5850e-05

y-velocity 1.7548e-04

energy 5.3650e-08

do-intensi 8.7784e-08

ch4 6.8593e-08

o2 6.8611e-08

co2 7.2894e-08

h2o 7.4620e-08