- Nektar-users - imperial.ac.uk

Which file in the director depicting how to apply boundary conditions for the diffusion equations.
by 丁老师 16 Oct '16

16 Oct '16

Dear Nektar group: I Want to know the method of applying first, and second BC for the diffusion equations. Which file in the source directory deicting about this. Regards Your Sincerely Ding

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Default operator evaluation in Nektar++ and error "NaN found during time integration"
by Selbach, Fabian 10 Oct '16

10 Oct '16

Dear Nektar++ -Community, please can you tell me the default operator evaluation in Nektar++? In the papers "From h to p efficiently" there are mentioned three kind of types: 1. global matrix approach 2. local matrix approach 3. sum-factorization I want to choose the best hp/ configuration for my 3D simulation and this -of course- depends on the operator evaluation strategy. I think actually Nektar++ uses the global matrix approach by default, but I'm not sure by 100%. My second question depends on the error message: "Fatal : Level 0 assertion violation NaN found during time integration." While using this configuration: ======================================================================= EquationType: EulerCFE Session Name: 3Dmeshcube_h10 Spatial Dim.: 3 Max SEM Exp. Order: 3 Num. Processes: 24 Expansion Dim.: 3 Riemann Solver: HLLC Advection Type: Projection Type: Discontinuous Galerkin Diffusion Type: Advection: explicit AdvectionType: WeakDG Diffusion: explicit Time Step: 0.01 No. of Steps: 10000 Checkpoints (steps): 1000 Integration Type: RungeKutta2_ImprovedEuler Problem Type: General ======================================================================= Initial Conditions: - Field rho: rhoInf + (0.01 * exp(-0.077*(x^2 + y^2 + z^2))) - Field rhou: rhoInf * uInf - Field rhov: rhoInf * vInf - Field rhow: rhoInf * wInf - Field E: (pInf / (Gamma - 1)) + ((rhoInf/2) * uInf^2) + ((0.01/(Gamma-1))*exp(-0.077*(x^2 + y^2 + z^2))) +( (0.01/2) * uInf^2 * exp(-0.077*(x^2 + y^2 + z^2))) I already changed my time-step and the refinement of my mesh and i think this error message is because of oscillation of my solution. So maybe my mesh is not fine enough or i must increase p. But i want to use a fix h to p constellation to test the efficiency. Can anybody help me with this problem? I already tried to use the spectral hp dealiasing, the smooth advection and artificial viscosity, but it doesn't worked or helped... Thanks for any answer. Best regards Fabian

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2d axisymmetric model and LES requeriments
by Manuel F. Mejía De Alba 06 Oct '16

06 Oct '16

Dear Nektar team, I trying to model the experiment developed by HIshida 1987. (Hishida, K., Maeda, M., 1987. Turbulent characteristics of gas–solids two-phase confined jet: effect of particle density. Jpn. J. Multiphase Flow 1, 56–69.) The problem is the Re number is near to 3E4, then the model is computationally expensive. Here my questions: 1. Is it possible to developed an axisymmetric model in Nektar++ ? I found this file http://prism.ac.uk/wp-content/uploads/2014/07/gabriele2014.pdf, on this said that G. Rocco is working or worked on it. May I implement the model based on his work ? 2. For this 3D model, the mesh requirements are about 3E10, If I use a LES model with SVVCutoffRatio=0.5N and SVVDiffCoeff=0.5, how can I evaluate the new mesh requirements Thanks in advance for your comments, --- Manuel Felipe Mejía De Alba.

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Error with Hybrid Meshes
by Jian Yu 02 Oct '16

02 Oct '16

Hi everyone, I ran a 2D case with hybrid meshes(triangular and quadrilateral elements), and encountered the following error: Fatal : Level 0 assertion violation eid must be between 0 and 2 Could anyone help me figure out the problem? The xml file is attached. Thank you in advance! Best wishes Jian

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Question about a Steady Case
by mohammadali alidoost 21 Sep '16

21 Sep '16

Dear Nektar++ Developers, We intend to do a blood flow simulation for a *steady* case by using the “velocity correction scheme”. In order to achieve the *steady* state, we have questions as follows: 1) What quantity(s) should be monitored? 2) Where should this quantity be tracked? 3) What criteria with which tolerance should be considered? Moreover, if we have a high order mesh produced by Nektar++ consisting of different types of elements with Pth order polynomials, what will be the equivalent number of linear elements of that mesh in 2D and 3D? We would be so pleased if you could help us about above-mentioned questions. Kind Regards, Mohammadali Alidoost

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Laminar supersonic compression ramp
by Grisham, James Ray, IV 13 Sep '16

13 Sep '16

Hello, I'm trying to use the compressible flow solver in Nektar++ for a laminar supersonic compression ramp case. I'm using version 4.3.4. After running into issues and digging around in the code a bit, I figured out that the only Riemann solver which includes the viscous fluxes is HLLC. After switching the UpwindType to HLLC, the code runs, but it quits after the second time step. Here is the output of the code: ======================================================================= Initial Conditions: - Field rho: rhoInf - Field rhou: rhoInf * uInf - Field rhov: 0 - Field E: pInf / (Gamma - 1) + 0.5 * rhoInf * uInf * uInf - Field eps: 0 Writing: "ramp_0_0.chk" Steps: 1 Time: 1e-09 CPU Time: 0.922648s Writing: "ramp_0_1.chk" Steps: 2 Time: 2e-09 CPU Time: 0.866426s Fatal : Level 0 assertion violation NaN found during time integration. I tried different time integration schemes to see if that helped, to no avail. Any insights or pointers on what is causing the NaNs? A coarse mesh file and session file are attached. Thanks for your time, James

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Nektar++ v4.3.4 Released
by Chris Cantwell 09 Sep '16

09 Sep '16

Dear all, We are pleased to announce the release of Nektar++ v4.3.4. This release includes a number of further bug fixes and minor enhancements to the 4.3 release: **Library:** - Fix performance issue with `v_ExtractDataToCoeffs` for post-processing of large simulations (!672) - Added additional assertions to ensure homogeneous simulations have an even number of planes per process (!666) - Fix compilation with NEKTAR_USE_MESHGEN option - Fix IterativeFull solver in parallel (!685) **IncNavierStokesSolver:** - Fix 2nd order time-integration for VCSMapping (!687) As always, if you encounter any problems please feel free to contact us on the mailing list nektar-users(a)imperial.ac.uk. Cheers, Chris -- Chris Cantwell Imperial College London South Kensington Campus London SW7 2AZ Email: c.cantwell(a)imperial.ac.uk www.imperial.ac.uk/people/c.cantwell

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ctest fails for the 13th test
by 宋科 06 Sep '16

06 Sep '16

Dear Nektar group: I installed a serial version of Nektar++_4.3.3 . When I ran ctest, the 13th test showed the following messages, and then it looked like no response or to be paused. Start 13: StdRegions_StdProject2D_Quad_Orth_P6_Q7 13: Test command: /home/sk1/nektar/build/tests/Tester "/home/sk1/nektar/library/Demos/StdRegions/Tests/StdProject2D_Quad_Orth_P6_Q7.tst" 13: Test timeout computed to be: 9.99988e+06 When I reduced the order by replacing "4 1 1 6 6 7 7" with "4 1 1 4 4 7 7" in the file "StdProject2D_Quad_Orth_P6_Q7.tst", the 13th test passed. After trying many times ,I found order1>4 would lead to this trouble. Moreover ,a tutorial case "basics-advection-diffusion" encountered similar problem. In the file "ADR_conditions.xml", NUMMODES>4 for COMPOSITE="C[2]" would lead to problems. I got the messages, and it looked like no response. ADRSolver ADR_mesh_aligned.xml ADR_conditions.xml ======================================================================= EquationType: UnsteadyAdvection Session Name: ADR_mesh_aligned Spatial Dim.: 2 Max SEM Exp. Order: 5 Expansion Dim.: 2 Riemann Solver: Upwind Advection Type: Projection Type: Discontinuous Galerkin Advection: explicit AdvectionType: WeakDG Diffusion: explicit Time Step: 0.001 No. of Steps: 1000 Checkpoints (steps): 100 Integration Type: ClassicalRungeKutta4 ======================================================================= Initial Conditions: I noticed that quadrilateral mesh was used in above two problems . The 13th test used quadrilateral mesh, while the "basics-advection-diffusion" case used multi-region mesh (region 2 was quadrilateral, region 1 and 3 was triangle ), and the NUMMODES>4 for quadrilateral region would lead to problems. Why? How to fix it? Thanks. SONG Ke

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How to compile in debug mode?
by Saviz Mowlavi 02 Sep '16

02 Sep '16

Hi all, I am a graduate student new to Nektar++ and I have a small question about compiling the source code. Specifically, I am trying to compile the DiffusionSolver in debug mode (i.e. like what the ‘-g’ flag produces when running gcc). Do I need to modify the corresponding Makefile, and which part should I change? Thanks for the help! Saviz Mowlavi

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extracting 1D line data in homogeneous direction
by Asim Onder 29 Aug '16

29 Aug '16

Dear all, I'm doing 3D1H DNS simulations, and I would like to do some statistical analysis (power-spectra, PDF, etc.) on certain lines in the homogeneous direction. Is there any practical way to extract line data in z-direction? I guess I can seed a family of points with probes filter in the direction of a line. However, I have around 2000 modes in the homogeneous direction, which would make this method probably expensive for increasing number of lines. Thanks, Asim ________________________________ Important: This email is confidential and may be privileged. If you are not the intended recipient, please delete it and notify us immediately; you should not copy or use it for any purpose, nor disclose its contents to any other person. Thank you.

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