John W. M. Bush — Page 2

arXiv2014

The hydraulic bump: The surface signature of a plunging jet

When a falling jet of fluid strikes a horizontal fluid layer, a hydraulic jump arises downstream of the point of impact provided a critical flow rate is exceeded. We here examine a phenomenon that arises below this jump threshold, a circular deflection of relatively small amplitude on the free surface, that we call the hydraulic bump. The form of the circular bump can be simply understood in terms of the underlying vortex structure and its height simply deduced with Bernoulli arguments. As the incoming flux increases, a breaking of axial symmetry leads to polygonal hydraulic bumps. The relation between this polygonal instability and that arising in the hydraulic jump is discussed. The coexistence of hydraulic jumps and bumps can give rise to striking nested structures with polygonal jumps bound within polygonal bumps. The absence of a pronounced surface signature on the hydraulic bump indicates the dominant influence of the subsurface vorticity on its instability.

physics.flu-dyn↗

arXiv2012

The Tibetan singing bowl: acoustics and fluid dynamics

We present the results of an experimental investigation of the acoustics and fluid dynamics of Tibetan singing bowls. Their acoustic behavior is rationalized in terms of the related dynamics of standing bells and wine glasses. Striking or rubbing a fluid-filled bowl excites wall vibrations, and concomitant waves at the fluid surface. Acoustic excitation of the bowl's natural vibrational modes allows for a controlled study in which the evolution of the surface waves with increasing forcing amplitude is detailed. Particular attention is given to rationalizing the observed criteria for the onset of edge-induced Faraday waves and droplet generation via surface fracture. Our study indicates that drops may be levitated on the fluid surface, induced to bounce on or skip across the vibrating fluid surface.

physics.flu-dyn↗

arXiv2011

Tibetan Singing Bowls

We present the results of an experimental investigation of the acoustics and fluid dynamics of Tibetan singing bowls. Their acoustic behavior is rationalized in terms of the related dynamics of standing bells and wine glasses. Striking or rubbing a fluid-filled bowl excites wall vibrations, and concomitant waves at the fluid surface. Acoustic excitation of the bowl's natural vibrational modes allows for a controlled study in which the evolution of the surface waves with increasing forcing amplitude is detailed. Particular attention is given to rationalizing the observed criteria for the onset of edge-induced Faraday waves and droplet generation via surface fracture. Our study indicates that drops may be levitated on the fluid surface, induced to bounce on or skip across the vibrating fluid surface.

physics.flu-dyn↗

arXiv2010

The Tibetan Singing Bowl

The Tibetan singing bowl is a type of standing bell. Originating from Himalayan fire cults as early as the 5th century BC, they have since been used in religious ceremonies, for shamanic journeying, exorcism, meditation and shakra adjustment. A singing bowl is played by striking or rubbing its rim with a wooden or leather-wrapped mallet. The sides and rim of the bowl then vibrate to produce a rich sound. When the bowl is filled with water, this excitation can cause crispation of the water surface that can be followed by more complicated surface wave patterns and ultimately the creation of droplets. We here demonstrate the means by which the Tibetan singing bowl can levitate droplets. This is a sample arXiv article illustrating the use of fluid dynamics videos.

nlin.PS↗

arXiv2008

The Clapping Book

A steady horizontal air stream flows across a book clamped at its downstream end. Pages lift off to form a growing bent stack whose shape is determined by the torques associated with aerodynamic forces, weight and elastic resistance to bending. As more pages lift off to join the bent stack, the increasing importance of bending rigidity to dynamic pressure eventually causes the book to clap shut. The process restarts, and self-sustained oscillations emerge. [Fluid dynamics video]

physics.flu-dyn↗

arXiv2008

Crawling beneath the free surface: Water snail locomotion

Land snails move via adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit waves of shear stress through a thin layer of mucus onto a solid substrate. Since a free surface cannot support shear stress, adhesive locomotion is not a viable propulsion mechanism for water snails that travel inverted beneath the free surface. Nevertheless, the motion of the freshwater snail, Sorbeoconcha physidae, is reminiscent of that of its terrestrial counterparts, being generated by the undulation of the snail foot that is separated from the free surface by a thin layer of mucus. Here, a lubrication model is used to describe the mucus flow in the limit of small amplitude interfacial deformations. By assuming the shape of the snail foot to be a traveling sine wave and the mucus to be Newtonian, an evolution equation for the interface shape is obtained and the resulting propulsive force on the snail is calculated. This propulsive force is found to be non-zero for moderate values of Capillary number but vanishes in the limits of high and low Capillary numbers. Physically, this force arises because the snail's foot deforms the free surface, thereby generating curvature pressures and lubrication flows inside the mucus layer that couple to the topography of the foot.

physics.bio-ph↗