Difference between revisions of "UFR 335 Test Case"
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Revision as of 15:01, 20 August 2019
Cylinderwall junction flow
Test Case Experiments
The experimental data were acquired by conducting planar monoscopic 2D2C PIV in the vertical symmetry plane upstream of the cylinder. The PIV snapshots were evaluated by the standard interrogation window based crosscorrelation of . Doing so, we achieved instantaneous velocity fields of the streamwise () and the wallnormal () velocity component. From these data the timeaveraged turbulent statistics were calculated in the postprocessing. We used a CCDcamera with a square sensor. The size of a pixel was , therefore the spatial resolution of the images was , of the PIV data however, it was . The temporal resolution was , which is approximately twice as the macro time scale . The light sheet was approximately 2mm thick provided by a Nd:YAG laser. The fnumber and the focal length of the lens was and , respectively.
At the measurement section, the flume had transparent walls. Therefore, the laser light, which entered the flow from above could pass with a minimum amount of surface reflections. However, an acrylic glass plate had to be mounted at the waterair interface to suppress the bow waves of the cylinder and let the light sheet enter the water body perpendicularly. The influence of this device at the water surface was tested and considered to be of minor importance for the cylinderwall junction.
Hollow glass spheres were used as seeding and had a diameter of . The corresponding Stokes number was , and therefore, the particles were considered to follow the flow precisely.
The hydraulic boundary condition of a turbulent boundary layer developed naturally due to the long entry length and by the use of vortex generators as recommended by (Counihan 1969). The total number of timesteps was , the timedelay between two image frames of a timestep was . Therefore, the total sampling time was or . During the experiment seeding and other particles accumulated along the bottom plate, which undermined the image quality by increasing the surface reflection. Therefore, the data acquisition was stopped after images to allow surface cleaning and to empty the limited capacity of the laboratory PC's RAM. The sampling time of such a batch was or .
The data acquisition time and number of valid vectors was validated by the convergence of statistical moments. In the centre of the HV the number valid samples had its minimum. Therefore, the timeseries at the centre of the HV was analysed as a reference for the entire flow field. The standard error of the mean was times the standard deviation, the corresponding error in the fourth central moment is .
The experimental parameters are listed in Table 1:
Description  Value  Unit 

Cylinder diameter  []  
Flow depth  []  
Channel width  []  
Flow rate  []  
Depthaveraged velocity of approach flow  []  
Kinematic viscosity  []  
Reynolds number  [] 
CFD Codes and Methods
As the numerical details of our large eddy simulation (LES) can be found in (Schanderl & Manhart2016), we provide a brief summary here. The setup was intended to be identical to the experimental infrastructure. To model the bottom and side walls, we set the boundary conditions to noslip, whereas the free surface was modelled by a slip boundary condition. Therefore, the Froude number in the LES was infinitesimal, and no surface waves occurred.
We used our inhouse finite volume code MGLET with a staggered nonequidistant Cartesian grid. The RungeKutta timeintegration was of third order, the spatial approximation of second order and the maximum of the CFL number was in the range of 0.55 to 0.82. To model the cylindrical body, a second order immersed boundary method was applied. The subgrid scales were modelled using the WallAdapting Local EddyViscosity (WALE) model, and the portion of the modelled dissipation is about 30% of the total dissipation rate.
By conducting a precursor simulation a fully developed turbulent boundary layer was generated. The streamwise boundary conditions were periodic, and the precursor domain had a length of 30D to prevent the flow from superstructures. The wall resolution of the precursor grid was 7.5 wall units; thus, no wall model was applied. When the statistics of the precursor simulation converged, the fully developed the turbulent boundary layer was fed into the main simulation domain as inflow condition. Around the cylinder, the grid was refined in three steps, each with a factor of two. Schanderl & Manhart (2016) showed while performing a grid study that three refinement levels were enough to achieve 0.95 wall units at the cylinder and based on the oncoming wallshear stress. When using the local shear stress, the spatial resolution slightly decreased to 1.6 wall units. Furthermore, the sensitivity of the HV system regarding the inflow conditions was also investigated by Schanderl & Manhart (2016).
File:UFR335 gridLVL.png (Schanderl 2018)
Grid  Level of refinement  Cells per diameter
horizontal / vertical 
Grid spacing

Number of grid cells 

Precursor  0  
Base  0  
Grid 1  1  
Grid 2  2  
Grid 3  3 
Contributed by: Ulrich Jenssen, Wolfgang Schanderl, Michael Manhart — Technical University Munich
© copyright ERCOFTAC 2019