scholarly journals Head‐on collision of a large vortex ring with a free surface

1992 ◽  
Vol 4 (7) ◽  
pp. 1457-1466 ◽  
Author(s):  
Museok Song ◽  
Luis P. Bernal ◽  
Grétar Tryggvason
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Large Vortex

The mechanism of vortex connection at a free surface

1999 ◽  
Vol 384 ◽  
pp. 207-241 ◽  
Author(s):  
CHIONG ZHANG ◽  
LIAN SHEN ◽  
DICK K. P. YUE
Keyword(s):  
Free Surface ◽  
Boundary Conditions ◽  
Vortex Ring ◽  
Stokes Equations ◽  
Incidence Angle ◽  
Surface Boundary ◽  
Viscous Layer ◽  
Vertical Vorticity ◽  
Vorticity Component ◽  
Stress Boundary Conditions

Vortex connections at the surface are fundamental and prominent features in free-surface vortical flows. To understand the detailed mechanism of such connection, we consider, as a canonical problem, the laminar vortex connections at a free surface when an oblique vortex ring impinges upon that surface. We perform numerical simulations of the Navier–Stokes equations with viscous free-surface boundary conditions. It is found that the key to understanding the mechanism of vortex connection at a free surface is the surface layers: a viscous layer resulting from the dynamic zero-stress boundary conditions at the free surface, and a thicker blockage layer which is due to the kinematic boundary condition at the surface. In the blockage layer, the vertical vorticity component increases due to vortex stretching and vortex turning (from the transverse vorticity component). The vertical vorticity is then transported to the free surface through viscous diffusion and vortex stretching in the viscous layer leading to increased surface-normal vorticity. These mechanisms take place at the aft-shoulder regions of the vortex ring. Connection at the free surface is different from that at a free-slip wall owing to the generation of surface secondary vorticity. We study the components of this surface vorticity in detail and find that the presence of a free surface accelerates the connection process. We investigate the connection time scale and its dependence on initial incidence angle, Froude and Reynolds numbers. It is found that a criterion based on the streamline topology provides a precise definition for connection time, and may be preferred over existing definitions, e.g. those based on free-surface elevation or net circulation.

Axisymmetric interaction between a vortex ring and a free surface

1994 ◽  
Vol 6 (1) ◽  
pp. 224-238 ◽  
Author(s):  
Peder A. Tyvand ◽  
Touvia Miloh
Keyword(s):  
Free Surface ◽  
Vortex Ring

scholarly journals Vortex ring dynamics at a free surface

1989 ◽  
Vol 1 (3) ◽  
pp. 449-451 ◽  
Author(s):  
L. P. Bernal ◽  
J. T. Kwon
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Ring Dynamics

scholarly journals The effect of a parallel free surface upon a submerged shallow synthetic jet

2020 ◽  
Author(s):  
Abhay Kumar ◽  
Ashish Karn
Keyword(s):  
Reynolds Number ◽  
Free Surface ◽  
Vortex Ring ◽  
Water Surface ◽  
Primary Objective ◽  
Synthetic Jet ◽  
Vortex Rings ◽  
Beam Laser ◽  
Jet Trajectory ◽  
Water Depths

The interaction of a submerged shallow synthetic jet with a parallel free surface has gathered substantial interest, owing to its relevance to the operation of marine vehicles viz. ships that move close to the water surface. However, despite exhaustive research on the perturbation on a free surface, very few studies have experimentally investigated the effect of unconfined water surface height on the evolution and propagation of a submerged synthetic jet. This study experimentally investigates a synthetic jet submerged in a quiescent flow at shallow depths ejecting parallel to the free surface, through qualitative analysis and quantitative measurements. The qualitative study includes the visualization of the flow using Plane Laser Induced Fluorescence (PLIF) technique, whereas the velocity measurements are carried out by a five-beam Laser Doppler Velocimetry (LDV) probe. The primary objective of these analysis and measurements is to gain a physical insight into the characteristics of vortex ring in a synthetic jet ejected from a fixed orifice at different water depths and at varying Reynolds number. Our studies indicate that the behavior of the vortex rings drastically changes as the depth of the jet crosses a certain threshold. Although no significant change in the path of synthetic jet is observed beyond a threshold depth in our experiments, the jet trajectory shows an interesting dependence on the Reynolds number based on circulation for shallow water depths. It has been found that in the shallow depths, the vortex ring drifts upwards and interacts with the free surface at lower Reynolds number, whereas for larger Reynolds number, the vortex ring rebounds near the free surface and moves downward. Based on our observations, it can be concluded that the phenomenon of upward/downward flection of vortex rings depends both upon its circulation and water depth.

Interaction of an obliquely rising vortex ring with a free surface in a viscous fluid

Meccanica ◽  
1996 ◽  
Vol 31 (6) ◽  
pp. 623-655 ◽  
Author(s):  
Samuel Ohring ◽  
Hans J. Lugt
Keyword(s):  
Free Surface ◽  
Viscous Fluid ◽  
Vortex Ring

scholarly journals Experimental studies of vortex disconnection and connection at a free surface

1996 ◽  
Vol 321 ◽  
pp. 59-86 ◽  
Author(s):  
Morteza Gharib ◽  
Alexander Weigand
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Experimental Studies ◽  
Local Strain ◽  
Surface Region ◽  
Vorticity Field ◽  
Core Diameter ◽  
Near Surface ◽  
Spatial And Temporal Scales ◽  
Rate Analysis

An experimental study is presented that examines the interaction of a vortex ring with a free surface. The main objective of this study is to identify the physical mechanisms that are responsible for the self-disconnection of vortex filaments in the near-surface region and the subsequent connection of disconnected vortex elements to the free surface. The understanding of those mechanisms is essential for the identification and estimation of the appropriate spatial and temporal scales of the disconnection and connection process. In this regard, the velocity and vorticity fields of an obliquely approaching laminar vortex ring with a Reynolds number of 1150 were mapped by using Digital Particle Image Velocimetry (DPIV). The evolution of the near-surface vorticity field indicates that the connection process starts in the side regions of the approaching vortex ring where surface-normal vorticity already exists in the bulk. A local strain rate analysis was conducted to support this conclusion. Disconnection in the near-surface tip region of the vortex ring occurs because of the removal of surfaceparallel vorticity by the viscous flux of vorticity through the surface. Temporal and spatial mapping of the vorticity field at the surface and in the perpendicular plane of symmetry shows that the viscous flux is balanced by a local deceleration of the flow at the surface. It is found that the observed timescales of the disconnection and connection process scale with the near-surface vorticity gradient rather than with the core diameter of the vortex ring.

The instability of a vortex ring impinging on a free surface

2009 ◽  
Vol 642 ◽  
pp. 79-94 ◽  
Author(s):  
P. J. ARCHER ◽  
T. G. THOMAS ◽  
G. N. COLEMAN
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Radial Direction ◽  
Slenderness Ratio ◽  
Long Wavelength ◽  
Short Waves ◽  
Radial Structure ◽  
Limiting Case ◽  
Crow Instability ◽  
Impermeable Boundary

Direct numerical simulation is used to study the development of a single laminar vortex ring as it impinges on a free surface directly from below. We consider the limiting case in which the Froude number approaches zero and the surface can be modelled with a stress-free rigid and impermeable boundary. We find that as the ring expands in the radial direction close to the surface, the natural Tsai–Widnall–Moore–Saffman (TWMS) instability is superseded by the development of the Crow instability. The Crow instability is able to further amplify the residual perturbations left by the TWMS instability despite being of differing radial structure and alignment. This occurs through realignment of the instability structure and shedding of a portion of its outer vorticity profile. As a result, the dominant wavenumber of the Crow instability reflects that of the TWMS instability, and is dependent upon the initial slenderness ratio of the ring. At higher Reynolds number a short-wavelength instability develops on the long-wavelength Crow instability. The wavelength of the short waves is found to vary around the ring dependent on the local displacement of the long waves.

scholarly journals Turbulent Vortex Ring/Free Surface Interaction

1995 ◽  
Vol 117 (3) ◽  
pp. 374-381 ◽  
Author(s):  
A. Weigand ◽  
M. Gharib
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Surface Interaction ◽  
Vortex Rings ◽  
Small Scale ◽  
Turbulent Vortex ◽  
Turbulent Transition ◽  
Vortex Loops ◽  
Scale Structures ◽  
Small Scale Structures

The interaction of turbulent vortex rings that approach a clean water surface under various angles is experimentally investigated. The temporal evolution of the vortex rings with an initial Reynolds number of Re0 = 7500 is characterized by the laminar/turbulent transition and asymptotic relaminarization of the flow. Using the shadowgraph technique, two major flow cases were identified as a result of the vortex-ring/free-surface interaction: a trifurcation case that results from the interaction during the transition stage, and a bifurcation case that evolves during the fully-developed turbulent stage. In contrast to the laminar interaction, the turbulent bifurcation pattern is characterized by the reconnection and mutual interaction of many small-scale structures. Simultaneous digital particle image velocimetry (DPIV) and shadowgraph measurements reveal that the evolution of the small-scale structures at the free surface is strongly dominated by the bifurcation pattern, which in turn is a consequence of the persisting laminar sublayer in the core regions of the reconnected turbulent vortex loops.

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