Set-up

The circular cylinder used in the current simulations has a diameter of 10 cm. Air flows into the domain with a velocity of 20 m/s and a turbulence intensity of 0.3%. Using these settings, the experiments showed a drag coefficient of 0.55 and a lift coefficient of -0.03.

The domain around the body is described in the image below. The size of the domain is created based on the diameter of the pipe. The depth of the domain is 1 diameter wide, modelling the pipe in quasi-2D.

Figure 1: domain

Mesh

Four meshes are generated to perform a mesh refinement study. In the image below the coarse mesh is shown. In the mesh, two refinement boxes are generated to account for the eddies coming from the pipe. Every mesh reduces the base size by a factor of 2, starting with a base size of 10cm for the coarse mesh and ending with a base size of 1.25cm for the finest mesh.

Figure 2: coarse mesh

Physics

For this simulation one phase, namely air, is used. The turbulence realistic k-ε  two layer model is used, with a turbulence intensity of 0.3% and a reference velocity of 20m/s.

Mesh refinement study

A mesh and time refinement study has been performed to find the closest approximation in drag coefficient compared to the test. The results of this study can be found in figure 3, The experimental drag coefficient is shown by the 0.55 box in the legend. From this figure it can be concluded that a maximum base size of 2.5cm is needed with a maximum timestep of 1ms. This leads to an error in drag coefficient of 2.5%. This error can be lowered to a minimum value of 1% by using the finest mesh with a timestep of 0.2ms or lower. However, an error of 2.5% is deemed acceptable for further study.

figure 3: mesh and time refinement study

Bibliography

[1]
J.-C. Bera, M. Michard, M. Sunyach and G. Comte-Bellot, “Changing lift and drag by jet oscillation: experiments on a circular cylinder with turbulent separation,” European Journal of Mechanics – B/Fluids, vol. 19, pp. 575-595, 2000.