# NaN at low order **URL:** \ **Category:** Errors\ **Created:** [19 August 2026 19:47 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370 "2026-08-19T19:47:22Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Amirshau](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Amirshau](https://pyfr.discourse.group/u/Amirshau)\ **Post date:** [19 August 2026 19:47 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/1 "2026-08-19T19:47:22Z")
Hello, I’m trying to simulate a simple building shape starting to simulate with P =1 for residuals and instabilies, but at this low order I’m geting the NaN error over and over agin. Am I missimg something? Best The ini file: backend precision = single rank-allocator = linear backend-cuda device-id = local-rank mpi-type = cuda-aware constants ac-zeta=40 ;ac-zeta = 0.4 gamma = 1.4 Vin = 0 Win = 0 Pc = 101325 Pr = 0.72 nu = 0.000015 solver system = ac-navier-stokes order = 1 anti-alias = flux, surf-flux solver-time-integrator formulation = dual scheme = bdf2 pseudo-scheme = rk45 controller = none tstart = 0.0 tend = 5 dt = 0.5e-6 ;dt = 0.0016 atol = 0.00001 rtol = 0.00001 pseudo-dt = 0.025e-6 ;pseudo-dt = 4e-5 pseudo-niters-max = 20 pseudo-niters-min = 6 pseudo-resid-tol = 1e-4 pseudo-resid-tol-p = 1e-4 pseudo-resid-norm = l2 pseudo-controller = local-pi safety-fact = 0.8 min-fact = 0.98 max-fact = 1.01 pseudo-dt-max-mult = 2.5 solver-interfaces riemann-solver = rusanov ldg-beta = 0.5 ldg-tau = 0.1 solver-interfaces-line flux-pts = gauss-legendre quad-deg = 3 quad-pts = gauss-legendre solver-elements-tri soln-pts = williams-shunn quad-deg = 3 quad-pts = williams-shunn solver-elements-quad soln-pts = gauss-legendre quad-deg = 3 quad-pts = gauss-legendre solver-elements-hex soln-pts = gauss-legendre quad-deg = 3 quad-pts = gauss-legendre solver-elements-pri soln-pts = williams-shunn~gauss-legendre quad-deg = 3 quad-pts = williams-shunn~gauss-legendre solver-interfaces-tri flux-pts = williams-shunn quad-deg = 3 quad-pts = williams-shunn solver-interfaces-quad flux-pts = gauss-legendre quad-deg = 3 quad-pts = gauss-legendre soln-plugin-pseudostats flushsteps = 10 file = pseudostats.csv header = true soln-plugin-nancheck nsteps = 1 soln-plugin-writer tstart = 0.0 dt-out = 0.005 basedir = /home/amir.shaulov/ #basedir = . basename = Urban2-{t:.4f} soln-ics u =10 v = 0.0 w = 0.0 p = Pc soln-bcs-walls type = no-slp-wall u = 0 v = 0 w = 0 soln-bcs-inlet type = ac-char-riem-inv ac-zeta = 40 rho=1.185 p = Pc u = (0.49/0.41) \* log((z+0.0032)/0.0032) v = Vin w = Win soln-bcs-outlet type = ac-char-riem-inv ac-zeta=40 rho=1.185 p = Pc u = (0.49/0.41) \* log((z+0.0032)/0.0032) v = Vin w = Win soln-bcs-slip type = slp-wall ac-zeta=40 soln-bcs-farfield type = ac-char-riem-inv ac-zeta=40 rho=1.185 p = Pc u =10 v = Vin w = Win ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [22 August 2026 11:48 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/2 "2026-08-22T11:48:24Z")
Do you have a rendering of the flow just before it NaNs? Regards, Freddie. ---
**Author:** ![Amirshau](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Amirshau](https://pyfr.discourse.group/u/Amirshau)\ **Post date:** [23 August 2026 17:49 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/3 "2026-08-23T17:49:08Z")
This is the solution visualization (for second order solution) just before the error - the inlet boundary layer is almost interacts with the structure. ![image](https://global.discourse-cdn.com/free1/uploads/pyfr/original/1X/d275fa828859e766a1c3badb060658cb085fdfab.png) ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [26 August 2026 11:03 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/4 "2026-08-26T11:03:30Z")
This just looks like your grid is extremely under resolved. How did you decide on your element count for the problem? Regards, Freddie. ---
**Author:** ![Amirshau](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Amirshau](https://pyfr.discourse.group/u/Amirshau)\ **Post date:** [29 August 2026 05:46 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/5 "2026-08-29T05:46:04Z")
The elements near the wall are quite fine (y+~1) and the total cells number is about 350K. Basically this is a try to see if the results are reasonable. ---
**Author:** ![Amirshau](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Amirshau](https://pyfr.discourse.group/u/Amirshau)\ **Post date:** [1 September 2026 19:18 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/6 "2026-09-01T19:18:56Z")
Generally speaking, what is the recommended aspect ratio for the mesh? I’m sure that streched cells caused the simulation elapsing. ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [6 September 2026 13:45 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/7 "2026-09-06T13:45:31Z")
Generally, you want to keep the aspect ratio below 20:1, with 10:1 being preferred. This is substantially less than what you might encounter will RANS. Regards, Freddie. ---
**Author:** ![Amirshau](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Amirshau](https://pyfr.discourse.group/u/Amirshau)\ **Post date:** [6 September 2026 17:40 UTC](https://pyfr.discourse.group/t/nan-at-low-order/1370/8 "2026-09-06T17:40:52Z")
Thanks Freddie, that what I was aiming for. Are the low aspect ratio regions shold be kept in regions far a way from viscous stresses (i.e. walls regions)? best, Amir