Computational Setup

A total of six simulations are performed. For three Reynolds numbers, 3900, 10 000 and 50 000, simulations are performed for both fully turbulent boundary layers and with the transition model.

The simulations are performed on an O-grid, with a computational domain extending 25D in the radial direction and π×D in the transverse direction. The full computational domain is shown in Figure 31.

Figure 31 Scheme of the computational domain

The boundary conditions used for the numerical simulations can be defined as follows. Velocity will be assigned as fixed value in the inlet. On the cylinder boundary no slip condition for the velocity will be applied and in the outlet velocity is zero gradient type. The pressure boundary condition in inlet is of a type zero gradient on the cylinder. This is happening because there is no flow through the wall so no pressure gradient should exist normal to the wall. On the other boundaries, the boundary condition is fixed value pressure.

51

The time step is set to maintain a Courant number of approximately 0.5. The total simulation time is U/D=300 and statistics are taken from the last U/D=100 times.

Since the results of CFD simulations are typically stored at the cell centers, it is important to interpolate the results from cell centers to the face centers, to obtain the fluxes for the surface integrals in the transport equation. The discretization scheme used for the convective term is the Linear Upwind Stabilized Transport (LUST) for the momentum equation and the limited linear scheme for the remaining equations. The time discretization scheme used is the second-order backward scheme.

7.3 Grid sensitivity study

To evaluate the required grid size, a grid sensitivity was performed for the Reynolds number 3900. The number of grid cells in the radial, tangential and transverse directions are denoted 𝑁_𝑟, 𝑁_𝜃 and𝑁_𝑧, respectively. Here, 𝑁_𝜃 = 𝑁_𝑟 and𝑁_𝑧= ^𝑁𝑟

2. Four different grid resolutions were considered, 𝑁_𝑟 = 60, 80, 100 and 120.

The total expansion ratio has been calculated corresponding to y⁺ ~ 1. It is important that the mesh near the wall is properly sized to ensure accurate simulation of flow field. Thus we use online calculator to compute the height of the first mesh cell off the wall required to achieve y⁺

~ 1 using flat-plate boundary layer theory. For a Reynolds number of 3900, this gives a first grid cell size of 4 mm. The grid that has been created for this case has been illustrated in the Figure 32 and Figure 33.

52

Figure 32 Mesh for the whole domain

Figure 33 Mesh as close-up

53

Figure 34 shows the drag coefficient for the four grids. For the last two grids, the coefficient converges to a value of 1 and so the results appear to converge for𝑁_𝑟 = 100.

The instantaneous flow field at the final time step is shown in Figure 35, for a resolution of𝑁_𝑟 = 100. The plot shows an isocontour of the Q-criterion with value 0.25, colored by the velocity magnitude. We see the shear layer separating from the cylinder surface and breaking up into vortices.

Figure 34 Drag coefficient for 4 grids 0

0.2 0.4 0.6 0.8 1 1.2

0 20 40 60 80 100 120 140

CD

Number of grid points in radial direction

54

Figure 35 Q-criterion=0.25 colored by velocity magnitude

The results are also compared in terms of pressure coefficient along the surface, and velocity in the wake. Figure 36 illustrate the pressure coefficient with respect to angle of stagnation. There is a good agreement between the simulation results and the experiments.

55

Figure 37 compares the statistics for the near wake of the cylinder with the experiments results at three different downstream locations in the very near wake (X/D = 1.06, 1.54, 2.02) for Nr= 100. Again, there is good agreement between the experiment and the simulation.

With this resolution, the mesh expansion ratio is 1.06. For the remaining simulations, the first cell size is calculated to keep y⁺ = 1, and the same expansion ratio is used. This means that the number of grid cells increases with the Reynolds number.

Figure 36 CP coefficient with respect of Angle from separation point

56

Figure 37 Mean velocity at different locations in the wake of a circular cylinder at Re=3900

57

7.4 Results

The drag coefficients for three different Reynolds numbers with turbulent model and transitional model are shown in Figure 38 next to the experimental results of Wieselsberger [28].

Figure 38 Drag coefficient results for two type of models in three different Re numbers

Comparing the computed drag coefficient as a function of Reynolds number with defined models and experimental results, the transitional model appear to be in better agreement with experimental results than the fully turbulent model. In particular, the fully turbulent results appear not to capture the flattening of the drag coefficients before the drag crisis occurs. This is expected, since for fully turbulent boundary layer, the flow separates later, which for the current geometry, means a smaller wake and lower drag values.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

50 500 5000 50000 500000

Drag Coefficient

Reynolds Number

Series1

Turbulent Model Transition Model

58

The biggest difference is observed for Reynolds number 50 000. Figure 39 shows a comparison between the mean velocities for the two models. The fully turbulent case clearly has a later separation compared to the transition case. This leads to a smaller wake and hence also lower drag values.

Figure 39 Comparison of mean velocity field for Re=50 000 with fully turbulent boundary layer (top) and transition model (bottom).

59

For the lower Reynolds numbers, the difference between the two models are smaller. The reason for this can be explained by looking at the intermittency values. Figure 40 compares intermittency values for Reynolds numbers 3900 and 50 000. For Re=3900, the intermittency value is low, meaning the boundary layer is laminar. For Re=50 000, however, there is a transition from laminar to turbulent flow at around the top/bottom of the cylinder.

Figure 40 Intermittency values along the cylinder

Following figures compare the velocity magnitude in the boundary layer at a 45 degree angle from the front of the cylinder. We see that for the lower Reynolds number the difference between the transitional and fully turbulent model is not as pronounced as for the higher Reynolds number. For the higher Reynolds number the higher turbulence in the fully turbulent model leads to separation at a later angle.

0 0.2 0.4 0.6 0.8 1 1.2

0 50 100 150 200

Intermittency

Angle along cylinder surface

Re=3900 Re=50 000

60 Figure 41 Comparison between transitional and fully turbulent model at Re=3900

Figure 42 Comparison between transitional and fully turbulent model at Re=50000

61