July 2018 - Nektar-users - imperial.ac.uk

Reading Boundary conditions from a customized file
by wasim sarwar 30 Jul '18

30 Jul '18

Hi all, Is it possible in nektar++ to read the time-dependent boundary conditions from a file. Actually, I am simulating Vortex induced Vibration (VIV) of cylinder with blowing and suction from the surface. The motion of the cylinder is defined by the flag "userdefinedtype"=movingwall which accounts for the structural dynamics under the action of fluid forces. In order to implement blowing and suction, I need to update the value of velocity field at the cylinder surface at each time step. I cannot use the flag "userdefinedtype" twice in the same boundary condition tag to introduce this time-dependence. The way i could think of to implement this idea is to the read this time-dependent boundary condition from the file. I would also need to overwrite the content of the boundary file at each time step so that i donot need to write all file corresponding to the time steps. So, my question is, is there a way to generate the time-dependent boundary condition through a given function or by any other means, and then read the same time-dependent boundary condition from a given file for the next time step. cheers

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Incompressible Navier-Stokes solver result problem
by Csaba Biró 15 Jul '18

15 Jul '18

Dear all, I am trying to solve an edge tone configuration related problem. I am using the Incompressible Navier.Stokes Solver to calculate the effects of blowing a jet flow, that impinges a wedge-shaped object. My problems are, that there seems to be an insufficient amount of outflow at the outlet ("u" velocity doesn't change), as if the airflow got stuck and that the errors shown after the calculation is complete are very high. Changing the time step or the number of modes did not decrease these errors considerably. Following the link you can see the used xml file (inc.xml). I attached a picture about the resulting "u" velocity and the geo+mesh. https://drive.google.com/open?id=1PYMXzPJo26wcIEns4OA71oI1lwWMCoYH I am only simulating half of the the problem. (hence the symmetry axis). I think there should be a much greater change in the "u" velocity overall. I double checked my boundary conditions. Please give me your opinion. Best regards, Csaba Biró

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Problem in vtk fields
by Asım Önder 15 Jul '18

15 Jul '18

Dear all, I am postprocessing turbulent channel flow data from a 3D1H simulation, in which I used a coordinate mapping to impose a waviness on the bottom wall and specified NUMMODES="9" TYPE="GLL_LAGRANGE_SEM" in inhomogeneous directions. To analyze the data, I calculate the gradient (FieldConvert -m mapping -m gradient) of instantaneous fields and write the output in vtu format. After the conversion to physical coordinates in Paraview, I extract a horizontal plane from the resulting vtk fields where I see some spurious peaks on element borders in the streamwise derivative fields (u_x, v_x, w_x). I attached a figure showing an example of this, where on the left the problematic "u_x" is shown and on the right "u_x" derived from the continuity equation is shown, i.e., "-v_y-w_z". It can be seen that the fields are the same except some artifacts on element borders in direct "u_x". Is this maybe a bug? Thanks for any feedback. Regards, Asim

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Calling Fortran modules in compressible flow solver
by Aidyn Aitzhan 15 Jul '18

15 Jul '18

Hi all, I am planning to call an external Fortran module in the compressible flow solver of Nektar++ 4.4.1. Could you please guide me about how I should modify the make (CMakeLists.txt) files? Basically, I have Fortran subroutines that allow me to compute temperature for non-constant heat capacities, so I want to use them. Thanks in advance, Aidyn

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Vortex induced Vibration
by wasim sarwar 08 Jul '18

08 Jul '18

Hi all, I am interested to simulate flow past cylinder with vortex induced vibration. I went through a tutorial where finite-strip theory was used to model VIV of flexible cable. In the xml file of the tutorial they have already prescribed the motion of the cable via analytic expressions. Is it possible to compute the displacements and velocities of the cable arising from fluid forces, i.e. lift and drag rather than giving an explicit expression beforehand? Regards

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