arXiv subjects

Clifford Chafin

Publications and source records attributed to Clifford Chafin.

25 records · Page 2Linked to original sources

arXiv2014

A Set of Statistical Variables for Hydrodynamic Flow

Through a discussion of some typical unsteady hydrodynamic flows, we argue that the time averaged hydrodynamic functions at each point give a rather sparse filling of the local jet space. This situation then suggests a set of time dependent probability functions that are shown to give evolution uniquely defined by the Navier-Stokes equations through a set of "differential distribution equations." The closure relations are therefore unique and have no ad hoc characteristics. Annealing methods are proposed as a way to arrive at the stable stationary solutions corresponding to time averaged fluid flow with constant driving forces and fixed boundary conditions. Some applications of this method to quantum statistical mechanics and kinetic theory to higher orders are suggested.

physics.flu-dyn↗

arXiv2014

The Superrotation of Venus: Where's the Torque?

The superrotation of the atmosphere of Venus requires a large torque on the up- per atmosphere. Mechanisms for providing a net balancing of this through waves or ionospheric motions to other parts of the atmosphere have been proposed but all have difficulties. Here we demonstrate that the albedo gradient from the day to night side of the cloud layer allows a gradient of light pressure that is sufficient to provide an external torque to drive this flow.

astro-ph.EP↗

arXiv2014

Persistent Thermal Inhomogeneities in a Gas-Cluster Mixture

Surface tension of small grains and droplets makes them stable only at a much lower temperature than in bulk. This makes spontaneous nucleation unfavorable in many cases. Kinetic approaches are delicate in that one can easily generate models that do not agree with thermodynamics in the large N limit. Here it is shown that thermodynamics itself dictates a kind of temperature suppression inside each small cluster in any gas-cluster mixture. This gives a different perspective on the "translation-rotation" paradox in that this gives a time averaged steady state thermal inhomogeneity rather than just temporal fluctuations in the energy. This not only reduces the barrier to nucleation but also suggests a change in the thermal radiation spectrum from such a mixture that is not just a result of the inhibited radiation spectrum from Mie radiators. Either verification or refutation of this effect will be shown have important consequences for thermodynamics. An understanding of this effect will be essential to kinetic approaches to nucleation theory.

cond-mat.stat-mech↗

arXiv2014

The Transient Neutral Flux in Plasma: An Explanation of Heating for the Solar Corona?

In this short note, we discuss a mechanism for the transport of energy, momentum and dipole moment via transient neutral carriers in plasma. This gives a way to rapidly convert bulk hydrodynamic flow energy into thermal energy over a distance of several mean free paths. In the transition region of the solar corona we estimate various processes and their potential to introduce the high energies needed to to reach the 2 x10^6K observed there. It implies that kinetic methods may be essential for modeling the corona and that there are more gentle but still robust means than reconnection to relax magnetic fields in plasmas.

astro-ph.SR↗

arXiv2013

Scale breaking and fluid dynamics in a dilute two-dimensional Fermi gas

We study two observables related to the anomalous breaking of scale invariance in a dilute two dimensional Fermi gas, the frequency shift and damping rate of the monopole mode in a harmonic confinement potential. For this purpose we compute the speed of sound and the bulk viscosity of the two dimensional gas in the high temperature limit. We show that the anomaly in the speed of sound scales as $(2P-ρc_s^2)/P\sim z/[\log(T/E_B)]^2$, and that the bulk viscosity $ζ$ scales as $ζ/η\sim z^2/[\log(T/E_B)]^6$. Here, $P$ is the pressure, $c_s^2$ is the speed of sound, $η$ is the shear viscosity, $z$ is the fugacity, and $E_B$ is the two-body binding energy. We show that our results are consistent with the experimental results of Vogt et al. [Phys. Rev. Lett. 108, 070404 (2012)]. Vogt et al. reported a frequency shift $δω/ω$ of the order of a few percent, and a damping rate smaller than the background rate $Γ/ω_0\sim 5%$.

cond-mat.quant-gas↗

arXiv2013

Hydrodynamic fluctuations and the minimum shear viscosity of the dilute Fermi gas at unitarity

We study hydrodynamic fluctuations in a non-relativistic fluid. We show that in three dimensions fluctuations lead to a minimum in the shear viscosity to entropy density ratio $η/s$ as a function of the temperature. The minimum provides a bound on $η/s$ which is independent of the conjectured bound in string theory, $η/s \geq \hbar/(4πk_B)$, where $s$ is the entropy density. For the dilute Fermi gas at unitarity we find $η/s\gsim 0.2\hbar$. This bound is not universal -- it depends on thermodynamic properties of the unitary Fermi gas, and on empirical information about the range of validity of hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic mode with frequency $ω$ diverges as $1/\sqrtω$, and that the shear viscosity in two dimensions diverges as $\log(1/ ω)$.

cond-mat.quant-gas↗

arXiv2010

Scaling Flows and Dissipation in the Dilute Fermi Gas at Unitarity

We describe recent attempts to extract the shear viscosity of the dilute Fermi gas at unitarity from experiments involving scaling flows. A scaling flow is a solution of the hydrodynamic equations that preserves the shape of the density distribution. The scaling flows that have been explored in the laboratory are the transverse expansion from a deformed trap ("elliptic flow"), the expansion from a rotating trap, and collective oscillations. We discuss advantages and disadvantages of the different experiments, and point to improvements of the theoretical analysis that are needed in order to achieve definitive results. A conservative bound based on the current data is that the minimum of the shear viscosity to entropy density ration is that eta/s is less or equal to 0.5 hbar/k_B.

cond-mat.quant-gas↗