Transition to Turbulence in the Separated Shear Layers of Yawed Circular Cylinders
NAVAL UNDERSEA WARFARE CENTER DIV NEWPORT RI
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Spatial and temporal resolution of transition to turbulence inside the free-shear layers of two yawed circular cylinders is the subject of the present investigation. These physics were resolved using the large-eddy simulation LES methodology. An O-type grid was implemented such that the spatial scales of the LES computation fully resolved the energy range physics of the shear layers at Reynolds number ReD 8000 based on the cylinder diameter. The two test cases modeled the cylinder span skewed at angles 45 and 60 from the horizontal axis. Observations revealed the same transition process as the normal cross-flow state. Soon after separation, Tollmien-Schlichting disturbances were predicted that evolved into Kelvin-Helmholtz K-H eddies before absorption by the large-scale Karman-type vortices. These eddies defaulted to a spanwise wavy pattern similar to a normal cross-flow due to their threedimensional instability. No mixed modes were found between the K-H Bloor and Strouhal frequencies. The effect of yaw angle shortened the transition process. As a result, peak turbulence levels inside the wake formation zone approach the downstream cylinder periphery. In addition, the dimensionless frequencies of the K-H eddies lie above the normal cross-flow relationship as formulated by Bloor 1964. Disparity between the yawed and normal cross-flow states was further emphasized by the shear-layer transition characteristics. Although each property displayed the expected exponential growth during transition to turbulence, their dimensionless form was miss-aligned with those of the normal cross-flow case. Based on the present evidence, additional simulations andor experimental measurements are necessary to form conclusive arguments regarding the expected behavior of the transition characteristics within the free-shear layers of yawed circular cylinders.
- Numerical Mathematics
- Test Facilities, Equipment and Methods
- Fluid Mechanics