62,382 research outputs found

### Small-amplitude perturbations of shape for a nearly spherical bubble in an inviscid straining flow (steady shapes and oscillatory motion)

The method of domain perturbations is used to study the problem of a nearly spherical bubble in an inviscid, axisymmetric straining flow. Steady-state shapes and axisymmetric oscillatory motions are considered. The steady-state solutions suggest the existence of a limit point at a critical Weber number, beyond which no solution exists on the steady-state solution branch which includes the spherical equilibrium state in the absence of flow (e.g. the critical value of 1.73 is estimated from the third-order solution). In addition, the first-order steady-state shape exhibits a maximum radius at θ = 1/6π which clearly indicates the barrel-like shape that was found earlier via numerical finite-deformation theories for higher Weber numbers. The oscillatory motion of a nearly spherical bubble is considered in two different ways. First, a small perturbation to a spherical base state is studied with the ad hoc assumption that the steady-state shape is spherical for the complete Weber-number range of interest. This analysis shows that the frequency of oscillation decreases as Weber number increases, and that a spherical bubble shape is unstable if Weber number is larger than 4.62. Secondly, the correct steady-state shape up to O(W) is included to obtain a rigorous asymptotic formula for the frequency change at small Weber number. This asymptotic analysis also shows that the frequency decreases as Weber number increases; for example, in the case of the principal mode (n = 2), ω^2 = ω_0^0(1−0.31W), where ω_0 is the oscillation frequency of a bubble in a quiescent fluid

### Bubble dynamics in time-periodic straining flows

The dynamics and breakup of a bubble in an axisymmetric, time-periodic straining flow has been investigated via analysis of an approximate dynamic model and also by time-dependent numerical solutions of the full fluid mechanics problem. The analyses reveal that in the neighbourhood of a stable steady solution, an O(ϵ1/3) time-dependent change of bubble shape can be obtained from an O(ε) resonant forcing. Furthermore, the probability of bubble breakup at subcritical Weber numbers can be maximized by choosing an optimal forcing frequency for a fixed forcing amplitude

### Transport property analysis method for thermoelectric materials: material quality factor and the effective mass model

Thermoelectric semiconducting materials are often evaluated by their
figure-of-merit, zT. However, by using zT as the metric for showing
improvements, it is not immediately clear whether the improvement is from an
enhancement of the inherent material property or from optimization of the
carrier concentration. Here, we review the quality factor approach which allows
one to separate these two contributions even without Hall measurements. We
introduce practical methods that can be used without numerical integration. We
discuss the underlying effective mass model behind this method and show how it
can be further advanced to study complex band structures using the Seebeck
effective mass. We thereby dispel the common misconception that the usefulness
of effective band models is limited to single parabolic band materials.Comment: 5 pages, 3 figure

### Magnetoresistivity Modulated Response in Bichromatic Microwave Irradiated Two Dimensional Electron Systems

We analyze the effect of bichromatic microwave irradiation on the
magnetoresistivity of a two dimensional electron system. We follow the model of
microwave driven Larmor orbits in a regime where two different microwave lights
with different frequencies are illuminating the sample ($w_{1}$ and $w_{2}$).
Our calculated results demonstrate that now the electronic orbit centers are
driven by the superposition of two harmonic oscillatory movements with the
frequencies of the microwave sources. As a result the magnetoresisitivity
response presents modulated pulses in the amplitude with a frequency of
$\frac{w_{1}-w_{2}}{2}$, whereas the main response oscillates with
$\frac{w_{1}+w_{2}}{2}$.Comment: 4 pages, 3 figures Accepted in Applied Physics Letter

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