On 16 April 2015 at 04:51, Emil Constantinescu <[email protected]> wrote: > > On 3/24/15 5:31 AM, Lisandro Dalcin wrote: >> >> Emil, is there any chance you can try my code with your problem? I >> really need some feedback to push this to PETSc, otherwise > > > Hi Lisandro - we checked ts_alpha_adapt and we tested it on a small system > (mildly stiff van der Pol ODE). I enclosed a Figure generated by Debo that > compares the error at the final time against ATOL - there alpha is the > original one (with original adaptor).
Original adaptor? Do you mean TSAlphaSetAdapt(ts,TSAlphaAdaptDefault,NULL)? In such a case, no surprises, TSAlphaAdaptDefault() is quite naive, it does not estimate the LTE, it is actually based in the change of the solution. Can you send/point me your code to take a look? > For this problem your adaptor works > better [ error should be close to ATOL]. > So from my perspective, your > adaptor works well. However, we need to keep the old too so that we have a > one step adaptor available ... since alpha is a one step method. > Sorry, I'm not following you. Generalized alpha is not truly a one-step method, because it retains Xdot from the previous time step. In the initial step, other codes out there assume that Xdot_prev=0. My "alpha1" implementation has a trick for the initial time step (using the same ideas, i.e. a BD formula) to both estimate Xdot_prev and the error. However, this is at the cost of performing two nonlinear solves in the initial step (IOW, two internal timesteps with backward Euler and dt/2). Please note this trick should be valid for any second-order method, at least to estimate the initial LTE in the first step. BTW, Have you compared my "-ts_type tsalpha1 -ts_alpha_adapt" with PETSc's "-ts_type cn -ts_theta adapt"? -- Lisandro Dalcin ============ Research Scientist Computer, Electrical and Mathematical Sciences & Engineering (CEMSE) Numerical Porous Media Center (NumPor) King Abdullah University of Science and Technology (KAUST) http://numpor.kaust.edu.sa/ 4700 King Abdullah University of Science and Technology al-Khawarizmi Bldg (Bldg 1), Office # 4332 Thuwal 23955-6900, Kingdom of Saudi Arabia http://www.kaust.edu.sa Office Phone: +966 12 808-0459
