September 2019 - Nektar-users

Element with max CFL
by Vishal Saini 02 Apr '20

02 Apr '20

Hi all, Quick question: Has anyone tried to check the location of a particular element in the mesh? A log file reports the maximum (?) CFL and the corresponding element as: "CFL: 0.0337915 (in elmt 94488)"? Is there a quick way to locate "elmt 94488"? Cheers, Vishal

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Manipulate 3DH1 chk/fld file in Fourier space
by F Mellibovsky 16 Mar '20

16 Mar '20

Dear all, I have run a 3DH1 simulation and I need to set all Fourier coefficients to zero except those corresponding to a fundamental wavelength and its harmonics (e.g. keep only modes 0, +-5, +-10, etc and set all the rest to zero). I have the intuition that some combination of FieldConvert options can accomplish exactly this, but I am unsure of what is the best (or some) way to proceed. My guess is that the "homplane:wavespace:planeid=?" can be used to extract the individual Fourier modes, but I am unsure as to how the Fourier modes are sorted and which ones must be extracted to recover a chosen fundamental wavelength and its harmonics (both real and imaginary parts, or modulus and argument, or however the Fourier modes are stored). The only one I'm pretty sure of is mode 0 or mean mode. Once I get the individual fld files for each one of the modes, I was intending to use the "addfld:fromfld=mode1.fld:scale=1 mode1.xml mode2.fld newfile.fld" sequentially to build a solution from the chosen modes. The issue here is that the fields extracted with "homplane" seem to be 2D and would therefore need to be recast in 3DH1 form before proceeding to "addfld". How can this be done? Thanks in advance for your help Fer Mellibovksy

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Product between base flow derivative and the aerodynamic forces
by Daiane Iglesia Dolci 13 Oct '19

13 Oct '19

Dear all, I need to solve a mathematical operation that is shown in the figure attached below. I am simulating an adjoint field and I wanna compute the product between a derivative of the base flow velocity and the aerodynamic force of the adjoint field, at the wall boundary. I know that in the FilterAeroforces.cpp, the forces are computed in each element, then an integral is applied. So I would like to know how I can take the derivative of the base flow in these elements by reading the baseflow.fld. If someone can help me, I thank you. Daiane I. Dolci.

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implementation details of the adjoint linearised NSE
by Simon Schmidt 26 Sep '19

26 Sep '19

Hi everyone, This might be a trivial question but I have some trouble understanding the implementation of the linearised adjoint (incompressible) navier stokes equations in nektar++. From Barkley, Blackburn and Sherwin (2008) it seems to me that the only differences between the direct and adjoint equations, are switched signs for the time derivative (du/dt -> -du/dt) and one of the advective terms ((U nabla) u -> -(U nabla) u). From my understanding the necessary modifications to the existing direct solver are then to modify this advection term and to choose a negative time step to "reverse" the time integration. The modification to the advection term are taken into account in AdjointAdvection.cpp. However, I cannot find any reference in the code where the negative time derivative is considered. In the parameter files for the adjoint cylinder example the time step is as well positive. Am I just missing the respective line of code or are my assumptions wrong? I would really appreciate any insight to help me understand the procedure :) Cheers Simon

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Convergence issues with compressible flow solver
by Thibaut Appel 20 Sep '19

20 Sep '19

Dear Nektar++ developers/users, We're encountering some difficulties with the compressible flow solver of Nektar++. We're trying to replicate a laminar flat plate case from SU2 (https://su2code.github.io/tutorials/Laminar_Flat_Plate/) where the free-stream Mach is 0.2 and with a rather high unit Reynolds number of 4.2 million. The structured mesh was generated with Gmsh, we impose a Blasius profile at the inlet and a similarity solution as the initial condition. This results in a transient wavepacket advected downstream with the iterations. To get the simulation running, we had to resort to using a very small time step (1e-8) but it still blows up after a certain number of iterations. You can find attached a screenshot of the last flow field (u,v) saved as well as our XML parameter file. We read carefully the user guide and went through the different threads of the mailing list, with some questions: - is it hopeless to try and solve such a high Reynolds number, regardless of the mesh resolution? Is it likely we're suffering from a under-resolution issue? - are there any other parameters implemented recently in the master branch that we could change? The compressible flow solver section of the user guide is not really verbose and for example, we found out about the PressureOutflowNonReflective b.c. Our XML file was set up with some inspiration from the 3D hump of De Grazia. - are there any sponge layers implemented for the compressible flow solver? - we noticed there is a CompressibleBL.cpp source file in the Utilities folder, could it be useful to compute straight away a similarity solution? What was the original purpose of that source file? We'd appreciate if you could notify us of anything suspicious in our XML file. Thank you for your support, Thibaut Appel PhD Student Virtual Product Engineering, Airbus CR&T Department of Mathematics, Imperial College London

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Sponge-layer
by 蒋树杰 18 Sep '19

18 Sep '19

Hi all, First, I want to say thanks to everyone for helping me so much! Now I want to use a time-depentdent sponge-layer. Is there anyone using this function now? The simeluation will diverge very soon. In the simualtion, It is an empty domain and gust moves with flow. The boundary condition is the same with the reference fields. I use the Compressible Flow Solver with Euler euqations. My reference like this: <FUNCTION NAME="referencefields"> <E VAR="rho" VALUE="rhoInf" /> <E VAR="rhou" VALUE="rhoInf*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" /> <E VAR="rhov" VALUE="rhoInf*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" /> <E VAR="E" VALUE="pInf/(Gamma-1)+0.5*rhoInf*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)+0.5*rhoInf*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" /> </FUNCTION> Of course with the <REFFLOWTIME> tag: <FORCING> <FORCE TYPE="Absorption"> <COEFF> SpongeCoefficient </COEFF> <REFFLOWTIME> referencefields </REFFLOWTIME> </FORCE> </FORCING> The reference field is the function of x, y and t (time-dependent). Is there any thing wrong with my reference fields? I test the reference fields without x,y and t. It works well. It looks like: <FUNCTION NAME="referencefields"> <E VAR="rho" VALUE="rhoInf" /> <E VAR="rhou" VALUE="rhoInf*uInf" /> <E VAR="rhov" VALUE="rhoInf*vInf" /> <E VAR="E" VALUE="pInf/(Gamma-1)+0.5*rhoInf*uInf^2+0.5*rhoInf*vInf^2" /> </FUNCTION> So I think maybe my sponge-layer coffecient is fine. If anyone has some idea please let me. I can send my case file and mesh file to you. Besides, I am using the Compressible Flow Solver to do some areoacoustic research. I found there is some reflection on the boundary when I use far-field boundary conditon. At first I think the far-field boundary condition is a non-reflection boundary conditions when relate with solving the Euler euqaitons.(in the paper: A guide to the implementation of boundary conditons in compact high-order methods for compressible aerodynamics). I do not know if there is any misunderstanding in that paper. So I want to use the sponge-layer to reduce the reflection. Kind regards, Shujie University of Southampton

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Coordinate transformations
by 蒋树杰 15 Sep '19

15 Sep '19

Hi All, I am using the Coordinate transformations for generating the wavy wing.  <FUNCTION NAME="Mapping"> <E VAR="x" VALUE="x - wavyAmplitude/2 * cos(2*PI*z/wavyLambda)" /> </FUNCTION> <MAPPING TYPE="XofZ"> <COORDS>Mapping</COORDS> </MAPPING> When the computation is finished, I use FieldConvert -m mapping wavyWing-aoa12.xml wavyWing-aoa12.fld wavyWing-aoa12.plt to get the result. But in tecplot, I can not see the deformed geometry. The first picture is the slices on y=0. If someone is familiar with this, please no hesitate to contact me. Many thanks for your guys! Shujie University of Southampton

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3DH1D interppoints module
by Guillermo 15 Sep '19

15 Sep '19

Dear all, I have performed a quasi 3D simulation, being the z-axis the homogeneous direction, with an homogeneous length equal to 0.1 (LZ=0.1) and up to 3 Fourier modes (HomModesZ = 4). In that sense, my planes are located at: 0, 0.025, 0.05 and 0.075. I want to interpolate the solution to a series of points, thus I am using the interppoints module. These points are located along the z direction and between the Fourier planes, e.g., z=0.033, 0.066 etc. The output file from the FieldConvert -m interppoints command is quite strange, since each variable is constant along the z direction, and of course, it is not the actual case. I have also tested it for another set of points. In this second test I have changed the z-coordinates and fixed it to the Fourier planes. However, the result is the same as before, constant measure for each variable and with exactly the same value as for the first set of points. So, is the interppoints module implemented for the 3DH1D simulations? I would like to mention that on the user guide, the commands shown for the usage of the interppoints module, except for the first example, they are missing the output "file2.dat". Thank you. Regards, Guillermo

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Error - tutorial NACA
by Felipe Silva Maffei 12 Sep '19

12 Sep '19

Hi, I am master student in universidade de São Paulo. As part of a discipline, I need to desenvolve a project using nektar++. I am following the tutorial of NACA, but when I try to generate the .xml file by tapping $NEK/NekMesh, I get the following message: No such module: input: mcf. What would I be doing wrong? Best regards, Felipe Silva Maffei <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campai…> Livre de vírus. www.avast.com <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campai…>. <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>

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Unable to use Compressible flow solver for transonic/supersonic flow problems
by Gaurav Kumar 01 Sep '19

01 Sep '19

Hi! I am trying to simulate a hypersonic flow over wedge configuration. I have successfully simulated the flow problem at a subsonic Mach number (M=0.3). But increasing Mach number to a supersonic range, Let's say M=3, the solution diverges even with a decrease in time step size. I think my problem could be with the use of artificial viscosity or aliasing errors. Can somebody share the setup files for a supersonic case, maybe flow past NACA0012 airfoil test case from the user manual? And, any other suggestion or documentation relating to such a problem will be very helpful to me. Thank you Regards Gaurav Kumar Ph.D Student, Department of Aerospace Engineering, Indian Institute of Technology, Kanpur

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