The control of horizontal square cylinder wake using thermal buoyancy has been experimentally investigated at low Reynolds numbers. The cylinder with an aspect ratio of 60 is mounted in a vertical test cell. The cylinder is electrically heated such that the buoyancy aids to the inertia of the mean flow. The operating parameters i.e. Reynolds number (87–118) and Richardson number (0.065–0.171) are varied to examine the flow behaviour over a range of experimental conditions. Laser schlieren-interferometry has been used for visualization and analysis of flow structures. The complete vortex shedding sequence has been recorded using a highspeed camera. The suppression of vortex shedding by heat input has been demonstrated by schlieren image visualization, time traces of light intensity, corresponding power spectra and Strouhal number. The study provides new experimental information on processes and mechanisms involved in the heat-induced changes of the vortex structures under the influence of buoyancy. The formation length of the vortex structures increases with increase in Richardson number i.e. heating level. The sequence of instantaneous schlieren images show that shape of vortex structures becomes slender at a sufficiently high Richardson number and the vortices from opposite shear layers rub with each other without increasing the circulation level and the two shear layers combine to form a single plume. The plume becomes steady at critical value of heat input leading to suppression of vortex shedding. The corresponding spectra evolve from having a clear peak at the vortex shedding frequency to broadband spectra when vortex shedding is suppressed.
An experimental and computational study is performed of the wake flow behind a single yawed cylinder and a pair of parallel yawed cylinders placed in tandem. The experiments are performed for a yawed cylinder and a pair of yawed cylinders towed in a tank. Laser-induced fluorescence is used for flow visualization and particle-image velocimetry is used for quantitative velocity and vorticity measurement. Computations are performed using a second-order accurate block-structured finite-volume method with periodic boundary conditions along the cylinder axis. Results are applied to assess the applicability of a quasi-two-dimensional approximation, which assumes that the flow field is the same for any slice of the flow over the cylinder cross section. For a single cylinder, it is found that the cylinder wake vortices approach a quasi-two-dimensional state away from the cylinder upstream end for all cases examined (in which the cylinder yaw angle covers the range 0⩽ϕ⩽60°). Within the upstream region, the vortex orientation is found to be influenced by the tank side-wall boundary condition relative to the cylinder. For the case of two parallel yawed cylinders, vortices shed from the upstream cylinder are found to remain nearly quasi-two-dimensional as they are advected back and reach within about a cylinder diameter from the face of the downstream cylinder. As the vortices advect closer to the cylinder, the vortex cores become highly deformed and wrap around the downstream cylinder face. Three-dimensional perturbations of the upstream vortices are amplified as the vortices impact upon the downstream cylinder, such that during the final stages of vortex impact the quasi-two-dimensional nature of the flow breaks down and the vorticity field for the impacting vortices acquire significant three-dimensional perturbations. Quasi-two-dimensional and fully three-dimensional computational results are compared to assess the accuracy of the quasi-two-dimensional approximation in prediction of drag and lift coefficients of the cylinders.
The Formation of Vortex Structures in a Screen Cylinder Wake