CHARACTERIZATION OF WAKE AND SUPPRESSION OF FLUID FORCES ACTING ON A ROW OF SQUARE CYLINDERS

Abstract

Flow over bluff bodies of circular and rectangular cross sections are studied for fundamental and engineering interests. The vortex shedding mechanism is same for these geometries except the separation point. For circular geometry, the separation point is dependent on the Reynolds number (Re) whereas, for rectangular/square geometry, the separation point is fixed at the sharp corners of the cylinder and almost independent of Reynolds number. The square cylinder is observed to be more bluff body than the circular cylinder. From engineering point of view, flow around structures that typically have rectangular or near rectangular cross sections are more similar to flow around square cylinders like bridge piers, offshore structures, buildings, chimneys, power lines, cooling system for nuclear power plants, electronic circuit cooling. The flow field is typically marked by the periodic, alternate formation of vortices from opposite sides of the cylinder. A regular pattern of vortices, formed behind a single or a group of bluff bodies in its wake, is known as the Karman vortex street. In many of these engineering applications, the strong periodic shedding of Karman vortices is responsible for problems with flow-induced vibration, noise, production of highly turbulent and three-dimensional fluid motion in its wake and causes considerable drag forces in the structures. The flow over a group of two to five square cylinders for in-line and side by side arrangements are investigated in uniform flow at three subcritical Reynolds numbers of 295, 485 and 775. When a group of two or more identical cylinders at different spacings are in in-line or side by side configurations, it exhibits different flow phenomena involving complex interactions between the shear layers, the vortices, and flow structures. The detailed flow field over square cylinders is investigated by using advanced techniques such as Particle image velocimetry (PIV), Hotwire anemometry (HWA), and flow visualization techniques. The spatial flow field information is captured using PIV whereas HWA gives the temporal information about the flow field and both together help us to investigate the flow physics in the wake. The emphasis is on acquiring an improved physical understanding of the fluid behavior and the dynamics of the vortical structures in the flow by varying the number of cylinders in a group of in-line or side by side cylinder arrangements. Also the effect of equal spacing between the cylinders and the effect of oscillations are of interest. For the group of in-line cylinders, the upstream cylinder is oscillated in transverse direction at a fixed amplitude of 0.1D (D = side of square cylinder) at iv different harmonics of vortex shedding frequency of an isolated stationary square cylinder. Similarly in case of side by side arrangement, the middle cylinder is oscillated in the transverse direction. For the in-line configuration depending upon the spacing between the cylinders, flow exhibits various patterns. The spacing ratios are varied from 1.5 to 5.0. The shear layers are separated from sharp edges of the upstream cylinder and reattached to the downstream cylinder. This reattachment largely depends on cylinder spacing and the number of cylinders. At smaller spacing ratio (1.5), the shear layers extended till the last cylinder before shedding occurs. This behavior is like an extended body flow. At spacing ratio of 3.0, steady vortex is formed behind the upstream cylinder. With further increase in spacing between the cylinders, synchronized or non-synchronized vortex shedding are formed by the cylinders. The effect of forcing frequencies on flow interference, wake oscillation frequencies, aerodynamic forces and turbulence statistics has been studied. Flow fields are investigated in terms of the time-averaged drag coefficient, stream traces, vorticity contours, and turbulence statistics. The instantaneous flow fields are captured using flow visualization, vorticity contours, and power spectra. The group of square cylinders in side by side configuration is also investigated using same techniques as discussed above. The objective is to acquire a better understanding of the various flow regimes and corresponding vortex interactions. The effects of proximity interference at spacing ratio of 1.5 to 5.0, for two to five cylinders are explored. Along with these, the effects of oscillations of middle cylinder on wake characteristics for three and five cylinders are examined. Different flow regimes are observed at various spacing ratios. The flow field is dominated by strong flow through the gaps between the cylinders. The physical aspects of each regime, such as interference between the shear layers, gap flow deflection, multiple flow modes, recirculation and vortex interaction are investigated with the time-average and instantaneous flow field.