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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe v...
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe vortices around a circular cylinder with a cavity (slot) placed upstream. Through the cavity, no mass flow addition (blowing) or reduction (suction) is applied. With the upstream cavity, adverse pressure gradient is weakened upstream of the cavity whereas it is strengthened downstream of the cavity. Furthermore, a single vortex system is found to form immediately upstream of the cylinder instead of a typical two vortex (primary and secondary vortices) system observed in the absence of the upstream cavity. The strength of the primary vortex is weakened due to the fluid stream engulfed in to the upstream cavity, resulting in diffusion of the mainstream. Consequently, the circulation of the primary vortex is weakened.
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摘要 :
Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe v...
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe vortices around a circular cylinder with a cavity (slot) placed upstream. Through the cavity, no mass flow addition (blowing) or reduction (suction) is applied. With the upstream cavity, adverse pressure gradient is weakened upstream of the cavity whereas it is strengthened downstream of the cavity. Furthermore, a single vortex system is found to form immediately upstream of the cylinder instead of a typical two vortex (primary and secondary vortices) system observed in the absence of the upstream cavity. The strength of the primary vortex is weakened due to the fluid stream engulfed in to the upstream cavity, resulting in diffusion of the mainstream. Consequently, the circulation of the primary vortex is weakened.
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Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vert...
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Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vertical plates a Particle Image Velocimetry (PIV) and dye visualization techniques were used. Instantaneous and time-averaged velocity vector fields, corresponding streamline topology, vorticity values were analyzed. Boundary layer separation, developing vortices, secondary or counter-rotating vortices, merging of developing and counter-rotating vortices which results in a primary horseshoe vortex system and corner vortex were demonstrated qualitatively and quantitatively.
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In this paper, we sought to investigate unsteady cavitating turbulent flows around a twisted three-dimensional NACA 16012 hydrofoil using the large eddy simulation (LES) and volume of fluid (VOF) methods. The one-equation eddy vis...
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In this paper, we sought to investigate unsteady cavitating turbulent flows around a twisted three-dimensional NACA 16012 hydrofoil using the large eddy simulation (LES) and volume of fluid (VOF) methods. The one-equation eddy viscosity model (OEEVM) was used to calculate the sub-grid scale (SGS) stress tensor. The compressive velocity VOF technique and the Kunz cavitation model were also employed. Numerical simulation was performed using the interPhaseChangeFoam solver within the OpenFOAM framework. Cavitation simulation was performed at three cavitation numbers within the cloud cavitation regime; that is to say, cavitation numbers: 0.95, 1.15, and 1.35. Afterwards, the details of flow predictions including the pressure, velocity, streamlines, volume fraction of water phase, and turbulent kinetic energy were reported. Cavity behaviours, including growth, shedding, and collapse of the cavity, were considered in detail. Three-dimensional cavity structures such as primary and secondary shedding and the shedding of the U-shaped horseshoe vortex were reported as well. The present work illustrated the side-entrant jets and the radially diverging re-entrant jet corresponding to the three-dimensional effect of the twisted wing. The cavity pattern and the shedding cycle frequency agreed well with the available experimental observations.
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The development of Goertler vortices with pre-set wavelength of 15 mm has been visualized in the boundary-layer on a concave surface of 2.0 m radius of curvature at a free-stream velocity of 3.0 m/s. The wavelength of vortices was...
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The development of Goertler vortices with pre-set wavelength of 15 mm has been visualized in the boundary-layer on a concave surface of 2.0 m radius of curvature at a free-stream velocity of 3.0 m/s. The wavelength of vortices was pre-set by vertical wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several stream wise locations show the growth and breakdown of the vortices. Three different regions can be identified based on different growth rate of the vortices. The occurrence of a secondary instability mode is indicated by the formation of a small horseshoe eddies generated between the two neighboring vortices traveling streamwise, to form mushroom-like structures as a consequence of the non-linear growth of the Gortler vortices.
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The interaction of a short cylindrical vortex elongated along the transverse coordinate with a shear flow near a solid surface is considered. A cylindrical vortex with its ends lowered to the bottom of the channel is shown to turn...
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The interaction of a short cylindrical vortex elongated along the transverse coordinate with a shear flow near a solid surface is considered. A cylindrical vortex with its ends lowered to the bottom of the channel is shown to turn into a vortex ring. The obtained qualitative model is verified by experiment.
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This study is motivated by applications to near-wall shear flow (ⅰ) as a longitudinal wall shaping starts, (ⅱ) around a surface obstacle, or (ⅲ) through a pipe bend. All are shown to be governed, at relatively high flow rates, ...
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This study is motivated by applications to near-wall shear flow (ⅰ) as a longitudinal wall shaping starts, (ⅱ) around a surface obstacle, or (ⅲ) through a pipe bend. All are shown to be governed, at relatively high flow rates, by essentially the same theoretical problem. This concerns three-dimensional nonlinear longitudinal vortex-like motion under a prescribed displacement which continues to increase with distance downstream. Symmetry-plane solutions are obtained mainly through forward marching computation followed by analysis of the far-downstream response. The behaviour far downstream is found to involve either a strengthening attachment or an increasing three-dimensional separation (lift-off) with no backflow.
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This study examines several finite length NACA 0012 airfoils to explore how the angle of attack (α), the sweep angle (Λ) and the Reynolds number (Re) affect the junction vortex and horseshoe vortex. Upstream floor roughness and ...
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This study examines several finite length NACA 0012 airfoils to explore how the angle of attack (α), the sweep angle (Λ) and the Reynolds number (Re) affect the junction vortex and horseshoe vortex. Upstream floor roughness and turbulence intensity (T.I.) influence the wing-junction flow was also studied. The junction-flow structures at low Reynolds numbers were visualized using the smoke-wire technique. The smoke-streak flow patterns were classified into two characteristic modes - horseshoe vortex and non-horseshoe vortex. The horseshoe-vortex patterns were further categorized as the junction-vortex mode and non-junction-vortex mode. The velocity vectors were measured using the particle-image velocimetry (PIV), and the data was utilized to calculate the junction vorticity (Ω). Experimental results indicate that the straight wing has the maximum junction vorticity. The Ω decreases with increasing α and Λ and with decreasing Re. The Ω decreases with increasing T.I. The upstream T.I. generated by the mesh fences was more significant than that produced by sandpapers.
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A numerical simulation of a square jet ejected transversely into a laminar boundary-layer flow was performed at a jet-to-main-flow velocity ratio of 9.78 and jet Reynolds number of 6330. The jet consisted of a single pulse with a ...
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A numerical simulation of a square jet ejected transversely into a laminar boundary-layer flow was performed at a jet-to-main-flow velocity ratio of 9.78 and jet Reynolds number of 6330. The jet consisted of a single pulse with a duration equal to the time required for the jet fluid to travel 173 jet widths. A strongly-favourable streamwise pressure gradient was applied to the boundary layer and produced a freestream acceleration that is above the typical threshold required for relaminarization. The results of the simulation illustrate the effect of the favourable streamwise pressure gradient on the flowfield created by the transverse jet. Notably, the horseshoe vortex system created upwind of the jet remains steady in time and does not induce noticeable fluctuations in the jet flow. The upwind and downwind shear layers of the jet roll-up through a Kelvin–Helmholtz-like instability into discrete shear-layer vortices. Jet vorticity in the upwind and downwind shear layers accumulates near the corners of the jet and produces two sets of vortex pairs, the former of which couple with the shear-layer vortices to produce large, counter-rotating vortices in the freestream, while the latter are unstable and periodically produce hairpin vortices in the main-flow boundary layer and elongated vortices in the freestream behind the jet. The departure of the jet flowfield from the vortical structures typically observed in transverse jets illustrates the substantive effect of the favourable streamwise pressure gradient on the flowfield created by the jet.
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Accurate prediction of secondary flows and associated heat transfer phenomena in a cascade of turbine blades or vanes remains to be a challenging task, despite the great effort made in this area for several past decades. In a revi...
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Accurate prediction of secondary flows and associated heat transfer phenomena in a cascade of turbine blades or vanes remains to be a challenging task, despite the great effort made in this area for several past decades. In a review of secondary flow literature, covering up to 2000, Langston [1] has stated also achievements and shortcomings of secondary flow CFD predictions at that moment. The present contribution objective is to reveal the progress achieved for the last decade. The main focus is on prediction of the horseshoe vortex system and endwall heat transfer.
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