Dear All, I have been conducting a direct numerical simulation of turbulent pipe flow using nek5000 at a moderately high Reynolds number (at present Re_tau=550). For this, I have to use a very fine grid in order to capture the flow physics accurately (for a length of 25R, I am using a total of 450 million grid points when assuming a polynomial order 7). However, I have been having problems with the stability of the code when using TORDER = 3, i.e. parameter 027. The way I conducted my simulation was as follows: - I started with polynomial order 3, TORDER set to 3, and run for 4 flow through times, starting from random noise. At this point I had the projection parameters, i.e. P094 and P095 set to non-zero values. - Afterwards, I increased the polynomial order to 5, with TORDER kept at 3, and ran for another 2 flow through times. Here, I set the projection parameters P094 and P095 to zero. - Now, starting from the last field of the previous run, I changed to my target polynomial order 7, and keeping TORDER = 3. However, the codes explodes shortly after restarting. The error is: "failed in HMHOLTZ." I also tried a considerably lower time-step, but the problem persisted. Meanwhile, when I change TORDER to 2, the code runs fine and does not explode. The "physical" results are absolutely fine, and the statistics I get are in perfect agreement to our expectations. I did not get the same problem while running a turbulent pipe flow at a lower Reynolds number (Re_tau=180) and with less grid points. Probably also of importance, I did test with various ways of restarting, i.e. storing in fld or f files, using 4 or 8 byte accuracy, and storing multiple consecutive fields for the multi-step time scheme. None of these changes made significant impact. Also, I am using overintegration and filtering (0.01 in parameter P103); increasing the filtering (by a factor of 5) did also not help. I am wondering if any faced a similar issue before, and if would have an insight on how to tackle it. Could my problem maybe be related to the restart-issue that Adam brought up recently? Best regards George ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.