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Basic Concepts
Published in Noor A. Ahmed, Coanda Effect, 2019
Based on the works of Gebhart et al. [28], near the origin of a shear flow, a vortex sheet separates the fluid containing momentum from the stationary ambient fluid. This vortex sheet travels downstream. It soon becomes unstable and rolls up forming a series of discrete vortices.
A Tsuji burner in a counterflow
Published in Combustion Theory and Modelling, 2022
Brandon Li, Antonio L. Sánchez, Forman A. Williams
The nature of the flow arising for is further explored in Figure 5, which represents results corresponding to . For this case, the streamline pattern near the cylinder surface is markedly different from the potential-flow prediction, also shown in the figure. While the separating streamline of the potential flow remains close to the cylinder surface, separation of the vorticity layer results in the formation of a well-defined cavity bounded by a vortex sheet. The fluid inside the cavity is nearly stagnant, with velocities remaining everywhere comparable to the injection velocity . Since the pressure in the cavity is almost uniform, as corresponds to nearly stagnant flow, the velocity along the bounding surface must be constant, its value being , as revealed in Figure 4, and the associated streamwise pressure gradient correspondingly is vanishingly small, as can be seen on the right-hand-side plot of Figure 5. Separation of the vorticity layer from the cylinder surface is seen to occur just above , at a place (point C in the plots) where the pressure gradient, approaching zero, is still favourable, indicating that the type of inviscid separation observed here is not related to the familiar viscous-boundary-layer separation [10].
Experimental and CFD analyses of pollutant dispersion around an isolated cylindrical building
Published in Waves in Random and Complex Media, 2023
Amani Amamou, Hammouda Mahjoub, Khaled Al-Farhany, Nejla Mahjoub Said, Hervé Bournot
The effect of the wind velocity on turbulent structures was illustrated in Figure 7 for three velocity ratios between the chimney ejection and the wind source (R = v0/u∞). The tested values of the velocity ratio were R = 1.6, R = 1 and R = 0.67 for Reℓ = 9.4 × 104 (u∞ = 5 m/s), Reℓ = 1.5 × 105 (8 m/s) and Reℓ = 2.25 × 105 (12 m/s), respectively. To show the global evolution, we present three images that correspond to different times in each velocity ratio (R). Turbulent structures are clearly observed at the exit of the chimney for the different velocity ratios which are called Kelvin–Helmholtz instability. This phenomenon consists of shear layer instabilities of surfaces separating the two fluids moving with different velocities leading to a ‘vortex sheet’ formation on the border between the two fluid flows. From Figure 7, these turbulent structures are found to rotate in one direction or in the other. In fact, the direction of rotation depends on the ratio R between the ejection velocity and the wind velocity. For R = 0.67, it is shown that the existence of clockwise rotating vortices imposed by the transverse flow whose velocity is higher than that of the injection. For R = 1 when the velocities of the two flows are equal, bi-directional vortices are observed giving rise to instabilities formation which in turn lead to other vortices having also two directions of rotation. This results in regular ‘mushroom’ shaped vortices. For the case of R = 1.6, the interface instability generates vortices with a counterclockwise rotational direction, as shown in Figure 7.