Danielle S. Tan

Application of potential theory to steady flow past two cylinders in tandem arrangement

The wake flow patterns associated with flow past a cylinder and a cylinder-pair in tandem configuration are revisited, compared, and evaluated with respect to the streamline patterns generated based on potential flow theory and superposition of various potential flow elements. The wakes, which are vortex shedding in the lee of the cylinder(s), are reproduced by placing pairs of equal but opposite circulation elements in the potential flow field. The strength of the circulation elements determines the size of the vortices produced. The streamline patterns of flow past a pair of unequal cylinders in tandem configuration provide an indirect means to establish the threshold condition for the wake transition from that of a single bluff body to alternating reattachment behavior. This threshold condition is found to be a function of the diameter ratio, (diameters and , ), spacing ratio, (centre-to-centre distance, , to cylinder diameter, ), and equivalent incident flow speed, A unique functional relationship (, , ) of this threshold condition is established.

Wake flow behaviour behind a smaller cylinder oscillating in the wake of an upstream stationary cylinder

The flow patterns around a cylinder oscillating freely in the wake of a larger cylinder upstream were investigated using the particle image velocimetry technique. The upstream cylinder was fixed at both ends while the downstream smaller cylinder was held by springs such that it was free to oscillate in the transverse direction. The flow patterns, amplitudes of oscillation and vortex shedding frequencies were compared with those of a single cylinder. In the presence of the upstream cylinder, the three parameters characterizing the oscillation response of the smaller cylinder—amplitude of oscillation, vortex shedding frequency and Reynolds stresses—were greatly reduced. While their magnitude increased with gap ratio, these three parameters were still smaller than the corresponding magnitudes for a single oscillating cylinder. The peak values of turbulence statistics such as Reynolds shear stress and normal stress behind the oscillating downstream cylinder were similarly reduced, and increased with gap ratios.

A model for the onset of oscillations near the stopping angle in an inclined granular flow

Abstract.We propose an explanation for the onset of oscillations seen in numerical simulations of dense, inclined flows of inelastic, frictional spheres. It is based on a phase transition between disordered and ordered collisional states that may be interrupted by the formation of force chains. Low-frequency oscillations between ordered and disordered states take place over weakly bumpy bases; higher-frequency oscillations over strongly bumpy bases involve the formation of particle chains that extend to the base and interrupt the phase change. The predicted frequency and amplitude of the oscillations induced by the unstable part of the equation of state are similar to those seen in the simulations and they depend upon the contact stiffness in the same way. Such oscillations could be the source of sound produced by flowing sand.Graphical abstract

Acoustic signals generated in inclined granular flows

Spontaneous avalanching in specific deserts produces a low-frequency sound known as “booming.” This creates a puzzle, because avalanches down the face of a dune result in collisions between sand grains that occur at much higher frequencies. Reproducing this phenomenon in the laboratory permits a better understanding of the underlying mechanisms for the generation of such lower frequency acoustic emissions, which may also be relevant to other dry granular flows. Here we report measurements of low-frequency acoustical signals, produced by dried “sounding” sand (sand capable of booming in the desert) flowing down an inclined chute. The amplitude of the signal diminishes over time but reappears upon drying of the sand. We show that the presence of this sound in the experiments may provide supporting evidence for a previously published “waveguide” explanation for booming. Also, we propose a model based on kinetic theory for a sheared inclined flow in which the flowing layer exhibits “breathing” modes superimposed on steady shearing. The predicted oscillation frequency is of a similar order of magnitude as the measurements, indicating that small perturbations can sustain oscillations of a low frequency. However, the frequency is underestimated, which indicates that the stiffness has been underestimated. Also, the model predicts a discrete spectrum of frequencies, instead of the broadband spectrum measured experimentally.

Reaeration Model for a Still-Water Body

AbstractAn investigation on the influence of temperature (21.6–39.6°C) on the reaeration rate was performed. The recipient still water body was homogeneous and unstratified. The findings of the experimental study showed that the reaeration rate increases with temperature within the range of the temperature investigated. A mathematical model for estimating the reaeration rate is also presented. A new temperature correction factor for the conversion of the kinetic rate coefficient at different temperatures has been developed and was found to better reflect the reaction rates associated with the reaeration process.

Experimental Study on the Flow Around a Moving Cylinder

An experimental investigation of flow behavior behind a moving circular cylinder was carried out in an open channel. The cylinder was mounted on a platform and extended to submerge in the water. The platform was rested on two ball-bearing-filled tracks which were designed to reduce the friction between the platform and the tracks. The cylinder was maintained perpendicular to the platform. The platform was free to move in a horizontal plane. The flow fields behind the suspended cylinder were captured using the technique of Particle Image Velocimetry (PIV) and the trajectory of the cylinder was captured using a camera operated in video mode (60 frames per second). Instantaneous flow patterns and orbital responses of the moving cylinder were analyzed and the findings were presented herein.Copyright