arXiv subjects

Edmund W. Tedford

Publications and source records attributed to Edmund W. Tedford.

6 recordsLinked to original sources

arXiv2022

Salt-fingering in seasonally ice-covered lakes

When ice forms on lakes, dissolved salts are rejected, which can lead to under-ice salt-finger formation. We performed a series of laboratory experiments to visualize these fingers. While we detected salt-fingers in our camera recordings, the signal of these fingers is nearly absent in the temperature record. We quantify the velocity of the salt plumes and measure the bottom salinity increase from these fingers. Further, we estimate that the salinity is often distributed evenly with depth. Comparing the salt fluxes in our experiments with a typical salt flux in lakes, we suggest that conditions are favorable for salt-fingering in most seasonally ice-covered lakes.

physics.geo-ph↗

arXiv2020

The Cooling Box Problem: Convection with a quadratic equation of state

We investigate the convective cooling of a fluid with a quadratic equation of state by performing three-dimensional direct numerical simulations of a flow with a fixed top-boundary temperature, which is lower than the initial fluid temperature. We consider fluid temperatures near the density maximum, where the nonlinearity is expected to be important. When the equation of state is nonlinear, the resultant vertical transport of heat is fundamentally different and significantly lower than the predictions derived for a linear equation of state. Further, three dimensionless groups parameterize the convective system: the Rayleigh number ($\hbox{Ra}_0$), the Prandtl number (Pr), and the dimensionless bottom water temperature $(T_B)$. We further define an effective Rayleigh number ($\hbox{Ra}_{eff} = Ra_0 \ T_B^2$), which is equivalent to the traditional Rayleigh number used with a linear equation of state. We present a predictive model for the vertical heat flux, the top boundary-layer thickness, and the turbulent kinetic energy of the system. We show that this model agrees well with the direct numerical simulations. This model could be used to understand how quickly freshwater lakes cool in high latitude environments.

physics.flu-dyn↗

arXiv2018

Sensitivity of vortex pairing and mixing to initial perturbations in stratified shear flows

The effects of different initial perturbations on the evolution of stratified shear flows that are subject to Kelvin-Helmholtz instability and vortex pairing have been investigated through Direct Numerical Simulation (DNS). The effects of purely random perturbations of the background flow are sensitive to the phase of the subharmonic component of the perturbation that has a wavelength double that of the Kelvin-Helmholtz instability. If the phase relationship between the Kelvin-Helmholtz mode and its subharmonic mode is optimal, or close to it, vortex pairing occurs. Vortex paring is delayed when there is a phase difference, and this delay increases with increasing phase difference. In three dimensional simulations vortex pairing is suppressed if the phase difference is sufficiently large, reducing the amount of mixing and mixing efficiency. For a given phase difference close enough to the optimal phase, the response of the flow to eigenvalues perturbations is very similar to the response to random perturbations. In addition to traditional diagnostics, we show quantitatively that a non-modal Fourier component in a random perturbation quickly evolves to be modal and describe the process of vortex pairing using Lagrangian trajectories.

physics.flu-dyn↗

arXiv2016

Observation of superradiance in a vortex flow

Wave scattering phenomena are ubiquitous to almost all Sciences, from Biology to Physics. When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. Since the scatterer absorbs part of the incident energy, the reflected wave carries less energy than the incident one. However, if the obstacle is rotating, this process can be reversed and waves can be amplified, extracting energy from the scatterer. Even though this phenomenon, known as superradiance, has been thoroughly analysed in several theoretical scenarios (from eletromagnetic radiation scattering on a rotating cylinder to gravitational waves incident upon a rotating black hole), it has never been observed. Here we describe in detail the first laboratory detection of superradiance. We observed that plane waves propagating on the surface of water are amplified after being scattered by a draining vortex. The maximum amplification measured in the experiment was 20%, obtained for 3.70 Hz waves, in a 6.25 cm deep fluid. Our results are consistent with superradiant scattering caused by rapid rotation. In particular, a draining fluid can transfer part of its rotational energy to incident low-frequency waves. Our experimental findings will shed new light on Black Hole Physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole. We believe, especially in view of the recent observations of gravitational waves, that our results will motivate further research (both theoretical and experimental) on the observation of superradiance of gravitational waves.

gr-qc↗

arXiv2013

Classical aspects of Hawking radiation verified in analogue gravity experiment

There is an analogy between the propagation of fields on a curved spacetime and shallow water waves in an open channel flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long (shallow water) waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated Hawking emission by a white hole (the time inverse of a black hole). The measurements of amplitudes of the converted waves demonstrate that they appear in pairs and are classically correlated; the spectra of the conversion process is described by a Boltzmann-distribution; and the Boltzmann-distribution is determined by the determined by the change in flow across the white hole horizon.

gr-qc↗

arXiv2010

Measurement of stimulated Hawking emission in an analogue system

There is a mathematical analogy between the propagation of fields in a general relativistic space-time and long (shallow water) surface waves on moving water. Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. Similar arguments predict the same effect near wave horizons in fluid flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process.

gr-qc↗