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In this paper, the wake modes behind a circular cylinder under streamwisely forcing oscillating motion are studied at Reynolds number Re = 360–460 which are observed by laser-induced fluorescence flow visualization technique. The forcing frequency f e ranges from 0 to 6.85 f s , where f s is the vortex shedding frequency behind a stationary cylinder, and the forcing amplitude A / d = 0.2 , 0.5 , 1.0 , where d is the cylinder diameter. Both time-invariant and linearly ramping f e are investigated. Following our previous modal notation, the following conclusions can be drawn: firstly, three rarely reported modes in numerical studies, C-I, C-II and S-III, are now confirmed in experiments (though with differentiable appearance in their far wake behaviour) at higher A and/or f e ranges and the envelope lines of S modes and C modes, yielded from a vortex circulation model, are shown to be dependent on the peak relative velocity of the free stream to the cylinder surface. So is the occurrence of the S-II mode. Secondly, near the demarcation of A modes and S-I mode, wake mode undergoes constant transition in a stochastic manner at fixed Re, A and f e . Thirdly, a typical hysteretic effect can be observed when the oscillation frequency of the cylinder ramps up and down in a linear way, and the extent of delay is dependent on the ramping rate k. Finally, mode switching during frequency ramping obeys a unidirectional order. During k < 0 (ramp-down), when S-I (Type-II) mode switches to A-IV mode, or A-IV to A-III, the flow structure downstream is affected by the upstream and the entire wake flow eventually switched, which is classified as slow switches. In contrast, during k > 0 (ramp-up), a clear and abrupt switch can be observed in the wake when A-IV or A-III switch to S-I (Type-II) modes, which are jump switches. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical investigations on the flow characteristics past a circular cylinder with a jet injection at the rear stagnation point are performed. Transient, incompressible, laminar, and isothermal flow governing equations are solved with finite volume method. Numerical simulations have been carried out at a Reynolds number of 150 with different injection ratios (IR) ranging from 0.5 to 7. Time evolutions of the coefficients of drag and lift, and streamline patterns are plotted. Three different flow pattern ranges are observed, namely wake dominant (IR=0-1.5), transition (IR=1.5-2.5) and jet dominant (IR > 2.5), and they are characterized by the combined effects of vortex shedding, undulation and jet dominant phenomena appearing in the flow downstream. It is also found that Cd decreases slightly with the injection ratio up to 1.5, after that it monotonically increases with the injection ratio. The similar incremental trend is observed in the Strouhal number variation with IR up to 1.5; then, it increases almost linearly till IR=4. When IR is greater than 4, there is a sudden drop in the Strouhal number value equal to zero and it remains constant after that for all the IR values considered in this study. The power spectral density of Cl indicates that the dominant frequency is present for the lower IR up to 4 and that no dominant frequency appears in the higher injection ratio range of 5-7 due to a complete suppression of the vortex shedding behind the cylinder by a dominant jet mechanism. [ABSTRACT FROM AUTHOR]

The influence of the velocity ratio (VR) between the jet and main flow on the wake structure and aerodynamic loss of the trailing edge jet is studied using particle image velocimetry and numerical simulations. Three different velocity ratios, namely, VR = 0.5, 1.0, 1.5, are chosen for this comparative study. The Reynolds number (Reh) based on the slot height (h) and the mainstream velocity (U0) are 3380. Results show that the influence of jet on wake structure is significant such that the wake region shrinks and the turbulent kinetic energy is enhanced as the velocity ratio increases. The distribution area of strong vorticity is enlarged with increasing velocity ratio. By using proper orthogonal decomposition and fast Fourier transfer analysis, the variation of velocity ratio demonstrates significant impact on vortex shedding and turbulent kinetic energy. The aerodynamic loss coefficient is nearly constant between VR = 0.5 and 1.0, but increases by 3.25 % as the velocity ratio increases from 1.0 to 1.5. [ABSTRACT FROM AUTHOR]

Visualization experiments for flow around a cylinder with parallel slit placed inside a circular pipe are carried out with water as the working medium. Two different color dyes are employed to visualize the complex vortex formation mechanism behind the bluff bodies. The objective of this study is to explore the potential of cylinder with parallel slit as an improved vortex generator for various practical applications. Wake parameters, such as Strouhal Number, wake width, and vortex formation length, are calculated from the recorded images. Three different slit widths with non-dimensional separation ratio of 0.1, 0.2, and 0.4 are chosen to study the effect on the vortex formation mechanism and the corresponding wake parameters. The Reynolds number range covered in this study is Re = 200-2300. Symmetric vortex formation from the outer surface is observed for lower slit widths, whereas for high slit widths, both in-phase and anti-phase shedding are observed. A separation bubble from each of the inner surface is formed which detaches from the bluff body to form a vortex at higher Reynolds numbers. The separation bubble is found to be both bi-stable and symmetric for lower slit widths. The interaction of the separation bubble with the outer vortices is seen to affect the strength of the primary/outer vortex. The slit width is seen to be an important parameter in determining the strength of the vortices. At higher slit width, the configuration behaves like a dual bluff body and strong crisscross vortex shedding is observed. Such novel kind of vortex formation mechanism can be utilized to improve the performance of numerous practical applications, such as flowmeters, heat exchangers, etc. Graphical abstract: [ABSTRACT FROM AUTHOR]

The appearance of a ground surface can play an important role in the flow structures for the flows past a flat plate. We conduct two-dimensional numerical simulations on viscous flows past a flat plate inclined at an angle of attack of $$20^\circ $$ with ground effects using a finite-volume method. Results show that the effects on the separated flow from the ground are highly dependent on the gap ( G) between the plate and the ground. As the gap decreases, the strength of vortices generated from the trailing edge is restrained, which is consistent with experimental observations. Further decrease in the gap even eliminates the vortex shedding and yields a steady flow. It is also found that the flow between the gap can either be accelerated at large gap ratios ( $${G/L >1}$$ , G is the gap, L is the plate length), or be decelerated at small gap ratios ( $${G/L <1}$$ ). Furthermore, the numerical results show that the wake flow behind the plate can significantly change the distribution of surface shear stress on the ground. Specifically, the mean shear stress on the ground in the downstream region at a gap ratio $$G/L = 2.0$$ is one order of magnitude larger than that at a small gap ratio $$G/L = 0.3$$ , and the length of the downstream region where the shear stress can be effectively changed is much larger than the plate length, which provides a guideline to manipulate the ground wall surface shear stress using an inclined plate in the vicinity of the wall. [ABSTRACT FROM AUTHOR]