The influence of water turbulence on surface deformations and the gas transfer rate across an air-water interface.

We present experimental results of a study on oxygen transfer rates in a water channel facility with varying turbulence inflow conditions set by an active grid. We compare the change in gas transfer rate with different turbulence characteristics of the flow set by four different water channel and grid configurations. It was found that the change in gas transfer rate correlates best with the turbulence intensity in the vertical direction.

Experimental study of the mutual interactions between waves and tailored turbulence.

When surface waves interact with ambient turbulence, the two affect each other mutually. Turbulent eddies get redirected, intensified and periodically stretched and compressed, while the waves suffer directional scattering. We study these mutual interactions experimentally in the water channel laboratory at the Norwegian University of Science and Technology (NTNU) Trondheim.

Bilateral shear layer between two parallel Couette flows.

We consider an unusual shear layer occuring between two parallel Couette flows. Contrary to the classical free shear layer, the width of the shear zone does not vary in the streamwise direction but rather exhibits a lateral variation. Based on some simplifying assumptions, an analytic solution is derived for this shear layer.

Temperature-independent Casimir-Polder forces despite large thermal photon numbers.

We demonstrate that Casimir-Polder potentials can be entirely independent of temperature even when allowing for the relevant thermal photon numbers to become large. This statement holds for potentials that are due to low-energy transitions of a molecule placed near a plane metal surface whose plasma frequency is much larger than any atomic resonance frequencies. For a molecule in an energy eigenstate, the temperature independence is a consequence of strong cancellations between nonresonant potential components and those due to evanescent waves.

Model for density waves in gravity-driven granular flow in narrow pipes.

A gravity-driven flow of grains through a narrow pipe in vacuum is studied by means of a one-dimensional model with two coefficients of restitution. Numerical simulations show clearly how density waves form when a strikingly simple criterion is fulfilled: that dissipation due to collisions between the grains and the walls of the pipe is greater per collision than that which stems from collisions between particles. Counterintuitively, the highest flow rate is observed when the number of grains per density wave grows large.

Electrodynamic Casimir effect in a medium-filled wedge. II.

We consider the Casimir energy in a geometry of an infinite magnetodielectric wedge closed by a circularly cylindrical, perfectly reflecting arc embedded in another magnetodielectric medium, under the condition that the speed of light be the same in both media. An expression for the Casimir energy corresponding to the arc is obtained and it is found that in the limit where the reflectivity of the wedge boundaries tends to unity the finite part of the Casimir energy of a perfectly conducting wedge-shaped sheet closed by a circular cylinder is regained. The energy of the latter geometry possesses divergences due to the presence of sharp corners.

Electrodynamic Casimir effect in a medium-filled wedge.

We re-examine the electrodynamic Casimir effect in a wedge defined by two perfect conductors making dihedral angle alpha=pi/p. This system is analogous to the system defined by a cosmic string. We consider the wedge region as filled with an azimuthally symmetric material, with permittivity and permeability epsilon1, micro1 for distance from the axis ra.

Nernst's heat theorem for Casimir-Lifshitz free energy.

Consideration of the Lifshitz expression for the Casimir free energy on the real frequency axis rather than the imaginary Matsubara frequencies as is customary sheds light on the ongoing debate regarding the thermodynamical consistency of this theory in combination with common permittivity models. It is argued that when permittivity is temperature independent over a temperature interval including zero temperature, a cavity made of causal material with continuous dispersion properties separated by vacuum cannot violate Nernst's theorem (the third law of thermodynamics). The purported violation of this theorem pertains to divergencies in the double limit in which frequency and temperature vanish simultaneously.

Temperature correction to Casimir-Lifshitz free energy at low temperatures: semiconductors.

The Casimir force and free energy at low temperatures have been the subject of focus for some time. We calculate the temperature correction to the Casimir-Lifshitz free energy between two parallel plates made of dielectric material possessing a constant conductivity at low temperatures, described through a Drude-type dielectric function. For the transverse magnetic (TM) mode such a calculation is made.

Analytical and numerical verification of the Nernst theorem for metals.

In view of the current discussion on the subject, an effort is made to show very accurately both analytically and numerically how the Drude dispersion model gives consistent results for the Casimir free energy at low temperatures. Specifically, for the free energy near T=0 we find the leading term proportional to T2 and the next-to-leading term proportional to T(5/2). These terms give rise to zero Casimir entropy as T-->0 and are thus in accordance with Nernst's theorem.