The effect of holes on drag reduction and the near-wake of a circular cylinder in cross-flow

The main aim of this research is to assess the effect of double rows of holes on time-averaged drag reduction and the near-wake flow structure of a circular cylinder in cross-flow. Experiments were carried out for various combinations of six hole diameters, d = 0.075D, 0.1D, 0.125D, 0.15D, 0.2D and 0.25D, six hole-to-hole distances, l = 2d, 3d, 4d, 5d, 6d and 7d, nineteen angles of incidence (α), from 0° to 90° in steps of 5°, and twenty nine Reynolds numbers (Re), from 4,000 to 32,000 in steps of 1,000 where d and D (40 mm) are the diameters of hole and cylinder, respectively. It was found that the double rows of hole have great importance in terms of drag reduction. The is minimum at α = 0° regardless of test models. The drag reduction mechanism is directly relevant to that the jet filled the near-wake region with fluid and stretched the separated shear layers from the cylinder to form further downstream. So that, time-averaged drag coefficient, , can be reduced by 37% when compared to single isolated cylinder (d/D = 0.25, l/d = 2). This reduction is generally independent of Re. However, it depends severely on d, l and α. There are no direct specific relations between the above-mentioned parameters and . At small α values, from 0° to 15°, the reduction in was increased as hole diameter increased regardless of l. The generally increased proportional to the angle of incidence up to 30° regardless of test models. However, the of the test models are still lower than that of the single isolated cylinder. At relatively higher α values, from 55° – 60° to 65° – 75°, the values increased regardless of d. The level of increase is directly related to the l. As l reduced, the level of increase in increased, that is, the test models with lower hole number are generally more effective in terms of drag reduction in this α range. This unexpected increase in was interpreted as a change in flow structure inside the test models. The is remarkably reduced once again at α = 85° – 90° due to the transformation of two-dimensional vortices into weakened three-dimensional vortices by aid of holes which behave like a passive vortex generator. The whole field velocity measurements via PIV also supported this idea. The decreased fluid entrainment into the test models also contributed this reduction in . In the thesis, the near-wake flow topology, wake characteristics, velocity distributions, vortex shedding frequencies, vorticity and turbulent kinetic energy contours obtained via PIV measurements were presented in order to explain the advantages and drawbacks in aerodynamic/hydrodynamic aspect of double rows of holes as a passive flow control method.