# Tutorial 10 – Flow around a cylinder

In this tutorial, the simulation around a two-dimensional circular cylinder at $Re_D=200$ and Ma=0.2$is considered. The goal of this tutorial is to introduce the usage of a new functionality of the posti_visualizerecordpoints tool and the new tool posti_dmd. ### Compiler options Make sure that FLEXI is compiled with the CMake options listed in the following table. Option Value Comment CMAKE_BUILD_TYPE Release FLEXI_2D ON FLEXI_EQYNSYSNAME navierstokes FLEXI_PARABOLIC ON FLEXI_MPI ON optional POSTI_BUILD_VISUALIZERECORDPOINTS ON POSTI_DMD ON Table: Cmake options for the cylinder simulation. To check whether they are set, change to your build folder and open the CMake GUI ccmake [flexi root directory]  If necessary, set the above options and then compile the code by issuing make  ### Mesh Generation with HOPR The mesh file used by FLEXI is created by HOPR ./hopr parameter_hopr.ini This creates the mesh file Cylinder_Re200_mesh.h5 in HDF5 format. If HOPR is not available, the mesh file is supplied in this tutorial. ### Flow Simulation with FLEXI The simulation setup is defined in parameter_flexi.ini. The initial condition is selected via the variable vector RefState=(/1.,1.0,0.,0.,17.857/) which represents the vector of primitive solution variables$(\rho, u, v, w, p)^T$. Material properties are given in the table below. Based on the ideal gas law, we get $$Ma=1/\sqrt{\kappa p/\rho}=0.2$$ Note that in this non-dimensional setup the mesh is scaled such that the reference length is unity, i.e.$D=1$. Then to arrive at$Re=\rho u D / \mu = 200$, the viscosity is set to $$\mu = \rho u D / Re = 1/Re = 0.005$$ Property Variable Value dynamic viscosity$\mu$mu0 0.005 ideal gas constant$R$R 17.857 Prandtl number Pr 0.72 isentropic coefficient$\kappa$kappa 1.4 Table: Material properties set in the parameter file ### Numerical settings The DG solution on the mesh is represented by piecewise polynomials and the polynomial degree in this tutorial is chosen as$N=4$. The main code settings are shown in the table below. Variable Description Value N Polynomial degree 4 MeshFile Mesh file to be used Cylinder_Re200_mesh.h5 tend end time of the simulation 300 Analyze_dt time interval for analysis 0.01 nWriteData dump solution every n’th Analyze_dt 500 CFLscale 0.9 DFLscale 0.9 Table: Numerical settings ### Boundary conditions The boundary conditions were already set in the mesh file by HOPR. Thus, the simulation runs without specifying the boundary conditions in the FLEXI parameter file. The freestream boundaries of the mesh are Dirichlet boundaries using the same state as the initialization, the wall is modeled as an adiabatic wall. The boundary conditions in$z$direction are not relevant for this 2D example, but would be realized as periodic boundaries for a 3D simulation. All boundary conditions used are listed below. Name Type State Alpha BC_cylinder 3 0 0 BC_farfield 2 0 0  ### Running the code We proceed by running the code in parallel. For example, using 4 processors, use the following command mpirun -np 4 flexi parameter_flexi.ini  The simulation runs for 300 convective time units to achieve periodic vortex shedding, thus the simulation can take up to one to two hours. ### Evaluation of Strouhal number The Strouhal number (which is a non-dimensional frequency,$Sr=\frac{f \cdot D}{u}$, describing the oscillatory motion of the flow) is estimated using the forces acting on the cylinder induced by the vortex shedding. The forces are calculated on the fly during runtime. The associated flags in the parameter file are CalcBodyForces=T WriteBodyForces=T  The first line activates the calculation of the forces at each Analyze_dt, the second line enforces output of the forces to a file. In the following figure the force in y-direction is plotted. By measuring the time from peak to peak over several periods the Strouhal number can be estimated to$0.1959$which is close to the expected value from literature. ### Evaluation of the separation angle The mean separation angle is evaluated using the record points-tool as introduced in the tutorial about the NACA 0012 airfoil. The simulation setup already contains the record points set and the record points are written during the simulation. The record points set contains a plane within the boundary layer of the upper cylinder side. This time we want to use the Plane_doBLProps functionality within the posti_visualizerecordpoints tool. With this tool we want to analyze the boundary layer properties such as the wall friction to estimate the separation point. The parameter needed are already set in the parameter_visualizeRecordpoints.ini file. You can run the tool using posti_visualizerecordpoints parameter_visualizeRecordpoints.ini Cylinder_Re200_RP_*  After executing the tool, you will get a file named Cylinder_RP_BLProps_upperSide_BLPla000001.vts which can be visualized with ParaView. The Data we want to visualize is one dimensional, so you won’t be able to see the data in the render view. To visualize it you need the apply the plot over time filter. ParaView should automatically apply the correct range to plot on. By plotting tau_w over the circumference you can estimate the separation angle to$113~deg$(the intersection with the$y=0$line). ### Dynamic Mode Decomposition In this part of the tutorial we want to introduce the posti tool posti_dmd. The dynamic mode decomposition is an algorithm divide a temporal series into a set of modes which are associated with a frequency and grow/decay rate. With this tool we are also capable to determine the Strouhal frequency. The dynamic mode decomposition (DMD) is implemented according to Schmid et al. (Dynamic mode decomposition and proper orthogonal decomposition of flow in a lid-driven cylindrical cavity, 2009). To use this tool, we need a higher temporal resolution of the written state files. Thus, we change the time tend to$310$and nWriteData to$1$. We restart the simulation from the latest state file: flexi parameter_flexi.ini Cylinder_Re200_State_0000300.000000000.h5  To execute the DMD on the density run the following command: posti_dmd parameter_dmd.ini Cylinder_Re200_State_00003*  Depending on the available memory you might have to decrease the number of input state files. After execution you will see two additional files Cylinder_Re200_DMD_0000300.000000000.h5 and Cylinder_Re200_DMD_Spec_0000300.000000000.dat. The first file contains the field representation of the different modes and the second file contains the ritz spectrum of the modes. To visualize the field run the following command: posti_visu parameter_postivisu.ini Cylinder_Re200_DMD_0000300.000000000.h5  The new file Cylinder_Re200_Solution_0000300.000000000.vtu now contains five modes to visualize. The figure below shows the steady, the global, the first and the second harmonic mode. The global mode is the mode of the considered Strouhal number. With the python script plot_RitzSpectrum.py the Ritz spectrum of the DMD can be plotted. The script is placed in the tools folder of FLEXI. To plot the spectrum execute: python plot_RitzSpectrum.py -d Cylinder_Re200_DMD_Spec_0000300.000000000.dat  The result is a Ritz spectrum as shown in the next figure. On the x-axis the frequency of the modes and on the y-axis the growth/decay factor is plotted, whereat modes with$\omega_r<0\$ are damped. The modes placed directly on the x-axis are the already discussed modes, from left to right the global, the first, the second harmonic mode and so on. The color and size of the plotted  modes represent the Euclidian norm of the mode which can be interpreted as an energy norm of the mode. 