November 2016 - Nektar-users

Nonphysical terminated simulation - CompressibleFlowSolver
by Selbach, Fabian 16 Dec '16

16 Dec '16

Dear users, I hope you can help one more time. I got nonphysical oscillations during my airfoil-simulation. At that time my simulations terminate with the message: "NaN found during time integration". For more information of the error and parameters see pictures below. I tried to solve this by a higher polynomial degree up to p=7 (this generates 1.5 Mio grid-points here for a 2D simulation), but the simulation terminates still. The time step itself is computed by a fixed CFL=0.9 condition. Maybe this is the wrong way, because this condition is not sufficient? So please can somebody give me a hint about that? Do I have to use another kind of grid, because the resolution of the boundary at the peak of the airfoil is not high enough or the grid lines have no be orthogonal to the surface? I am already using the spherigons-tool and tried to get a smooth boundary of my airfoil. I am using the CompressibleFlowSolver on a parallel run up to 96 CPUs by solving the Euler-Equations (EulerCFE) and the Navier-Stokes-Equations (NavierStokesCFE). The simulation terminates for both. By the way what is the difference between EulerCFE and EulerADCFE (there are no information about it anywhere)? I tried to run the example NACA0012 of the paper "Nektar++: An open-source spectral/hp element framework". The conditions use the EulerADCFE, but by running this example the simulation does not terminate, but also does not change (values of every variable in the whole domain are constant over the time - no changes?). Thanks for any help! Best regards Fabian Visualization of the grid at a polynomial degree of p=2 just before terminating: [cid:71608e01-1455-4c20-abbb-9508cb48f805] With auto-scale: [cid:bba140ce-246a-4689-a539-79ff287727b0] Same solution with scaling by user: [cid:a6bda960-d10e-4f91-a833-2e5537f8758c] Please take into account the scaling above. Error and conditions for example p=2: [cid:0ebd995c-1bae-4c43-9d08-91fe685022bf] [cid:300abb94-0f24-4dde-8cda-f67f326ea4f8] [cid:a43f5fef-3537-4385-a2d9-2f49afbcf006] Visualization of the grid at a polynomial degree of p=7 just before terminating: [cid:4d0f11c2-718a-4f85-860f-e5c90684699e] (a few time steps after that the simulation terminates and the values go against inf...)

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merging partitions
by Asım Önder 29 Nov '16

29 Nov '16

Dear all, Due to memory limitations, often I have to partition large data files using something like FieldConvert --part-only=100 config.xml fields/field_1.chk , and postprocess them piece by piece: FieldConvert -v -m meanstats config_xml/P000000"$p".xml fields/field_1.chk meanFields/meanField_1_p"$p".fld In the case of meanmode module for example, the output is a collection of many small files associated with partitions, which can be analyzed piece by piece. I was wondering if there is a way to merge the outputs back into one piece again like one piece fld file. Any feedback is appreciated. Thanks, Asim

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Wall bounded incompressible flows: how to set pressure origin?
by F Mellibovsky 28 Nov '16

28 Nov '16

Dear all, I was trying to run a simple lid-driven cavity flow benchmark and I get rather strange results. The drag force on the lid is strongly mesh dependent, there are some numerical issues at the top right corner, the shear stress on the lid has a non-zero wall normal component and the pressure takes awfully large values across the domain. Before bothering you with setup or mesh files I was wandering if not setting a pressure origin somewhere may be causing some kind of trouble. Is it possible to set the pressure to a given value at a point of choice (e.g. p=0 on bottom left corner)? Cheers, Fer Mellibovsky

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Question on mesh partitions for post processing.
by ashok jallepalli 28 Nov '16

28 Nov '16

Hi Everyone, Does any know a smart way to partition to mesh(Nektar XML) file to contain all element in a bounding box? Example: If i have 10x10 element structure mesh on domain of [0,1]x[0,1]. I need to partition the mesh such that, the new partition file has all elements in the region (a,b) to (c,d). Assuming a,b,c,d are in [0,1]. - If there is no command line option for the above scenario, How can I accomplish it using the API? I have tried using the part-only and part-only-overlapping. I am unable to accomplish what I want. Thanks in advance, Ashok.

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Output turbulent kinetic energy, dissipation and divergence velocity
by Selbach, Fabian 20 Nov '16

20 Nov '16

Dear users, I have two problems/questions: First I would like to compute the time averaged field of the 'turbulent kinetic energy' and 'dissipation' at the end of my compressible flow solver simulation. How can I generate them? Is there already any filter? In the user-guide I realized the option "to compute time-averaged fields for each variable defined in the session file" at page 40. By default my variables in the compressible flow solver are: rho, rho*u, rho*v, rho*w and E. So it's only possibly to generate the time-averaged values of them or do I have any other option? And my second question: How can I compute the divergence u at an instantaneous time step? Best regards Fabian

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hwo to change the name of variables in the output *.vtu file
by ztdepyahoo＠163.com 15 Nov '16

15 Nov '16

Dear professor: In the vtu file, the u and v velocity must have name MomentumX and MomentumY, How to resolve this. Regards. ztdepyahoo(a)163.com

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bug in aero-forces filter
by Asım Önder 11 Nov '16

11 Nov '16

Dear all, I think there is a major bug in aero-forces filter for 3D1H case. It does not take into account the length in homogenous direction, LZ, while evaluating integrals. So LZ seems to be always 1. So basically, all the calculated forces are wrong for 3D1H problems. My LZ is close to unity so I almost didn't notice it. I was wondering if it is just calculating force on one slice or do some hidden normalization in third direction. Best. Asim

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