June 2015 - Nektar-users - imperial.ac.uk

Stability of channel flow with Nektar++
by Stanisław Gepner 05 Nov '15

05 Nov '15

Dear Sirs, First of all thank you for developing this great software and making it available. I am planing to investigate stability of an incompressible channel flow using Nektar. I have studied the manual and the source (the version from the git repo) and repeated some of the examples from the IncNavierStokesSolver folders Tests and Examples. The channel flow stability test (Tests/ChanStability.xml) uses a base flow (.bse) and an initial guess (.rst) and converges to an expected result. Now, as practice I tried to play around with the settings, first by changing the Driver to Arpack (this works as well). Since I might not know the initial approximation I tried to get rid of the initial guess, and replaceed it with some random initial vector (I have put zeros, non zeros, the awgn()). This time the problem did not coverage at all (I increased the numbers of iterations, tried to modify the number and size of the time steps, change the dimension of the Krylov space). Is this an expected behaviour? And since this is a test, the problem is set up to be small and quick and would only converge with the proper initial vector? Should I expect a similar problem for a larger 3D case? My second question concerns the base flow. I tried to recover the base flow using the stationary solver (both classical and adaptive) but I failed. Could you point me at an example of how to set this up? Also I noticed (or I think I did) that the Example/ChanStability-Base.xml does not produce a correct flow field. Am I right? I would appreciate if you would comment on my questions. Best Regards, Stan Gepner

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Time averaged output file
by Kamil ÖZDEN 19 Jul '15

19 Jul '15

Dear All, I want to ask that if is it possible to obtain time averaged output file (.fld or .chk) in Nektar++ to get time averaged contour plots? Best Regards, Kamil

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How to do post-process with BiGlobal Floquet analysis
by Hongfu Zhang 30 Jun '15

30 Jun '15

Hi All, I want to computethe eigenmode of a period field using BiGlobal Floquet analysis. The attachments are the related files. (Cyl.xml ---- the file simulates base flow; Cyl-sta.xml ----- the file calculate eigenvalue and eigenmode of the base flow; Cyl-sta.fld----the calculated (by Cyl-sta.xml) results ). I have used Cyl.xml, Cyl-sta.fld or Cyl-sta.xml, Cyl-sta.fld to generate .vtu files with FiedConvert, but all failed when I see the field with Paraview. Could anyone tell me how to do to see the results in Paraview. Thanks very much in advance! Best Regards Cyl.xml <https://drive.google.com/file/d/0B86D0j5lPOCDU3dNeWVLOF8xWUE/edit?usp=drive…> Cyl-sta.fld <https://drive.google.com/file/d/0B86D0j5lPOCDNFRCTWpKX3hFLWc/edit?usp=drive…> Cyl-sta.xml <https://drive.google.com/file/d/0B86D0j5lPOCDcUxHUENjcEEyYm8/edit?usp=drive…>

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explicit discretization for diffusion term in case of 3DHomoneneous1D case
by Zhicheng Wang 18 Jun '15

18 Jun '15

Hi All, I'd like to use explicit scheme to discretize the diffusion term in the case of 3DHomogeneous1D in continuous Galerkin projection framework. For example, I have an array u, which stores the value of first component of velocity vector in Fourier spoace. and I want to calculate the inner product (grad w, grad u), where w is the base function, grad is gradient operator. Could following code get the correct result? m_fields[0]->PhysDeriv(0,u ,u_derv[0]); // gradient in x direction m_fields[0]->PhysDeriv(1,u, u_derv[1]); // gradient in y direction for(unsigned int i=0; i<num_planes; ++i) { double beta = 2*M_PI*m_trans->GetK(i)/m_homoLen; beta *= beta; m_fields[0]->GetPlane(i)->IProductWRTDerivBase(0, &u _derv[0]+i*np, &wkCoef1[0] +i*nc); // m_fields[0]->GetPlane(i)->IProductWRTDerivBase(1, &u _derv[1]+i*np, &wkCoef1[0]+i*nc); m_fields[0]->GetPlane(i)->IProductWRTBase(tmp1 = &u[0]+i*np, &wkCoef2[0]+i*nc,MultiRegions::eGlobal); Vmath::Smul(nc, beta, &wkCoef2[0] + i*nc, 1, &wkCoef2[0]+ i*nc, 1); } Vmath::Vadd(NumTotCoeffs,wkCoef0,1,wkCoef1,1,diff,1); Vmath::Vadd(NumTotCoeffs,wkCoef0,1,wkCoef2,1,diff,1); // array diff stores the final diffusion value

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Some problems with BiGlobal Floquet analysis
by 张洪福 15 Jun '15

15 Jun '15

Hi All, I encounter some problems with Nektar++. I want to calculate the eigenmode of a period field using BiGlobal Floquet analysis. 1, I have gotten a series base flow file (xx_1.bse, xx_2.bse ......). The Residual error of base period flow (long enough) , L 2 error (variable u), is about 10, and others are o(10^-1). *Is the result **convergence*? (i also got the lift force of the bluff body, and it shows period smoothly ) 2, The total structural quadrangular element of the 2D mesh is about 14000, order of P is 8, the Re number is 4000. The external flow is similar to the example in User Guide 8.6.12 (version 4.0.1), but i simulate a plan-like bluff body instead of cylinder. when i run the "mpirun -np 4 IncNavierStokesSolver XX.xml " , the XX_0.chk folder is generate, but it stay all day long in that state (no error, no respones). Simultaneously, I run "IncNavierStokesSolver XX.xml" ,i.e., using single processor, it works well (although slowly). Anyone can tell me why and what should I do? 3, How can I get the corresponding eigenmode of eigenvalue in .evl file? 4, I am a newbie, would anyone gives me some advises in my simulation? I will appreciate! PS, the part of .xml content (base flow): </SOLVERINFO> <PARAMETERS> TimeStep = 0.0001 NumSteps = 100000 IO_CheckSteps = 10000 IO_InfoSteps = 100 Re = 4000 Kinvis = 0.775/4000 </PARAMETERS>

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Coupled incompressible solver with periodic conditions and regular mesh
by Stanisław Gepner 09 Jun '15

09 Jun '15

Hi, I think I found a problem when importing Gmsh meshes into Nektar. This happens if I try to use the coupled solver and the periodic BC. Works with VelocityCorrection, or if BC are standard D / N. What I do: 1. Using the geometry in channel.geo create a mesh in Gmsh. This is a rectangular channel. I use Transfinite and Recombine to get a structured quad mesh. 2. I use MeshConvert with paralign to generate the channel.xml 3. If run with the attached cond2D.xml Nektar fails in AssemblyMap.cpp line 284, with "These values should never be the same" The problem is persistent with other recombined/transfinite meshes. If any of you is aware of how I could try to avoid the problem, I would be obliged. Regards, Stan Gepner

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