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The nonlinear steady states of perturbation equations describing the instability of wavy shear flows to counter-rotating vortical structures aligned with the flow in shallow water layers is considered. The structures are described by the Craik–Leibovich equations and are known as Langmuir circulation; they arise through an instability that requires the presence of shearand differential driftof the same sign provided a threshold Rayleigh number is exceeded. Of specific interest here is the aspect ratio of the Langmuir circulation and how that ratio is affected by nonlinearities when the layer is shallow, as in coastal waters and estuaries. For context it is known from observation that the aspect ratio (width of two cells to depth) is two to three in deep water, whereas in shallow waters it can range up to ten. Accordingly, while the ratio for deep water is well predicted by linear theory, the ratio for shallow water is not, which explains why nonlinearities are of interest. Present always, but of key importance in effecting a preferred spacing with nonzero wavenumberlin shallow water Langmuir circulation, is an extra stress induced by the perturbed motion at the free surface. This stress is reflected in the boundary conditions. Moreover, since it is most influential in the limit, it is instructive to expose that influence through a small-lasymptotic approximation. It is found that nonlinearities ensure supercritical stability and that the critical wavenumber at onset to instability is significantly less than its linear counterpart. This results from a subcritical bifurcation due to symmetry breaking of the governing equations. The precise level of reduction is affected by the distributions ofand, but herein it ranges from 50 to 70%. This means that nonlinearities act to effect aspect ratios up to twice those given by linear theory and which are in good agreement with observation in shallow coastal waters. Finally, the extra stress at the free surface is found to play no rôle in the ratio of linear to nonlinear spacing but, as in the linear case, acts to ensure the spanwise wavenumber at onset to instability is nonzero. [ABSTRACT FROM PUBLISHER]

The rôle of boundary conditions applied to perturbation equations describing the instability of wavy shear flows to counter-rotating vortical structures aligned with the flow in shallow water layers is considered. In the ocean context the structures are described by the Craik-Leibovich equations and are known as Langmuir circulation (LC); they arise via an instability mechanism known as CL2, which requires the presence of shearand differential driftof the same sense and that a threshold Rayleigh number be exceeded. While Neumann or stress-free boundary conditions applied at the free surface in deep water act to yield a physically plausible spanwise wavenumberlat onset, that is not the case in shallow layers, where non-zerolat onset is ensured only when mixed boundary conditions are applied. This paper questions why that is so and goes on to determine how onset is affected by distributions ofand. In order to proceed analytically, results are computed by a small-lasymptotic approximation for which an algorithm is generated enabling computation to, essentially, any order. It is found that the extra stress induced by the perturbed motion, which is reflected in mixed but not Neumann boundary conditions, acts to suppress the growth rate of the instability over all wavenumbers, thereby restricting instability to non-zerol; further details are seen to depend on the distributions ofand. Specifically, while the critical wavenumber is not overly sensitive to precise distributions ofand, the critical Rayleigh number is sensitive. Minor variations inandcan also lead to multiple rather than single layers of LC and even contrive to ensure non-zerolat onset with Neumann boundary conditions. Finally, although crafted as an asymptotic approximation for, the results concur closely with numerical calculations at much larger l. [ABSTRACT FROM AUTHOR]

This paper is concerned with the evaluation of two Lagrangian measures which arise in oscillatory or fluctuating shear flows when the fluctuating field is rotational and the spectrum of wave numbers which comprise it is continuous. The measures are the drift and pseudomomentum. Phillips |J. Fluid Mech. 430, 209 (2001)] has shown that the measures are, in such instances, succinctly expressed in terms of Lagrangian integrals of Eulerian space-time correlations. But they are difficult to interpret, and the present work begins by expressing them in a more insightful form. This is achieved by assuming the space-time correlations are separable as magnitude, determined by one-point velocity correlations, and spatial diminution. The measures then parse into terms comprised of the mean Eulerian velocity, one-point velocity correlations, and a family of integrals of spatial diminution, which in turn define a series of Lagrangian time and velocity scales. The pseudomomentum is seen to be strictly negative and related to the turbulence kinetic energy, while the drift is mixed and strongly influenced by the Reynolds stress. Both are calculated for turbulent channel flow for a range of Reynolds numbers and appear, as the Reynolds number increases, to approach a terminal form. At all Reynolds numbers studied, the pseudomomentum has a sole peak located in wall units in the low teens, while at the highest Reynolds number studied, Reτ = 5200, the drift is negative in the vicinity of that peak, positive elsewhere, and largest near the rigid boundary. In contrast, the time and velocity scales grow almost logarithmically over much of the layer. Finally, the drift and pseudomomentum are discussed in the context of coherent wall layer structures with which they are intricately linked. [ABSTRACT FROM AUTHOR]

The Lagrangian drift in an O(ϵ) monochromatic wave field on a shear flow, whose characteristic velocity is O(ϵ) smaller than the phase velocity of the waves, is considered. It is found that although shear has only a minor influence on drift in deep-water waves, its influence becomes increasingly important as the depth decreases, to the point that it plays a significant role in shallow-water waves. Details of the shear flow likewise affect the drift. Because of this, two temporal cases common in coastal waters are studied, viz. stress-induced shear, as would arise were the boundary layer wind-driven, and a current-driven shear, as would arise from coastal currents. In the former, the magnitude of the drift (maximum minus minimum) in shallow-water waves is increased significantly above its counterpart, viz. the Stokes drift, in like waves in otherwise quiescent surroundings. In the latter, on the other hand, the magnitude decreases. However, while the drift at the free surface is always oriented in the direction of wave propagation in stress-driven shear, this is not always the case in current-driven shear, especially in long waves as the boundary layer grows to fill the layer. This latter finding is of particular interest vis-à-vis Langmuir circulations, which arise through an instability that requires differential drift and shear of the same sign. This means that while Langmuir circulations form near the surface and grow downwards (top down), perhaps to fill the layer, in stress-driven shear, their counterparts in current-driven flows grow from the sea floor upwards (bottom up) but can never fill the layer. [ABSTRACT FROM AUTHOR]

The evolution of an unbounded inviscid free surface subjected to a velocity potential of Gaussian form and also to the influence of inertial, interfacial and gravitational forces is considered. This construct was motivated by the occurrence of lung haemorrhage resulting from ultrasonic imaging and pursues the notion that bursts of ultrasound act to expel droplets that puncture the soft air-filled sacs in the lung plural surface, allowing them to fill with blood. The tissue adjacent to the sacs is modelled as a liquid and the air–tissue interface in the sacs as a free surface. The evolution of the free surface is described by a boundary-integral formulation and, since the free surface evolves slowly relative to the bursts of ultrasound, they are realized as an impulse at the free surface, represented by the velocity potential. As the free surface evolves, it is seen to form axisymmetric surface jets, waves or droplets, depending upon the levels of gravity and surface tension. Moreover the droplets may be spherical and ejected away from the surface or an inverted tear shape and fall back to the surface. These conclusions are expressed in a phase diagram of inverse Froude number Fr−1 versus inverse Weber number We−1. Specifically, while axisymmetric surface jets form in the absence of surface tension and gravity, gravity acts to bound their height, rendering them waves, although instability overrides the calculation prior to its reaching that bound. Surface tension acts to suppress the instability (provided that We−1 > 0.045) and to form drops; if sufficiently strong it can also damp the evolving wave, causing it to collapse. The pinchoff which effects spherical drops is of power-law type with exponent 2/3, and the universal constant that relates the necking radius to the time from pinchoff, thereby realizing a finite-time singularity, has the value ${\mathfrak{K}} \,{=}\, 0.45 \pm 0.025$. Finally, drops can occur once the mechanical index, a dimensional measure used in ultrasonography, exceeds 0.5. [ABSTRACT FROM AUTHOR]

The ground-state band in 110Mo has been observed for the first time. The band, comprising six levels, has been populated in the spontaneous fission of 248Cm and studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array. The ratio Eexe (4+)=Eexe(2+) suggests that the deformation of 110Mo is smaller than that in 108Mo but may stabilize at higher neutron number, where an oblate shape is expected. The new data suggests that the deformation of Sr and Zr isotopes decreases above neutron number N = 64. [ABSTRACT FROM AUTHOR]

High response rates from physicians are key to obtaining valid and generalizable data regarding their sexually transmitted disease (STD) diagnosis, treatment, and control practices. A factorial (3 x 2) study was designed using varying cash incentives ($0, $15, $25) and delivery modes (Federal Express, U.S. mail). Surveys, with three follow-up mailings, were sent to a national probability sample of 311 physicians in OB-GYN, family practice, internal and emergency medicine, and pediatrics specialties. Overall, 156 physicians returned completed surveys (56% overall response rate). Significant effects for incentive level (F = 28.2, df = 2, p < .01) and delivery mode (F = 4.1, df = 1, p < .05) existed. Highest response was among physicians in the $25-FedEx condition (81%). High response rates from busy practicing physicians can be achieved if surveys are relevant to clinical practice, sponsored by a reputable organization (the Centers for Disease Control and Prevention), include a monetary incentive, and are delivered by courier. [ABSTRACT FROM AUTHOR]

An analysis is undertaken of a nominally two-dimensional turbulent boundary layer in zero pressure gradient. In accord with Yajnik [J. Fluid Mech. 42, 411 (1970)], the well known law-of-the-wall similarity relationship for the mean velocity, Reynolds stress, and turbulence fields in the wall region is shown to be only the leading term in an expansion, the higher-order terms of which are dependent upon streamwise Reynolds number. The asymptotic behavior of the streamwise velocity, Reynolds stress, and turbulence intensities is deduced to leading order for two parts of the wall region, one immediately next to the wall and the other far enough from it for the effects of viscosity to be unimportant. A continuous function is then deduced for the velocity that precisely satisfies and links the asymptotics in each region; the corresponding distributions of Reynolds stress and turbulence intensities follow. Agreement with experiment at high Reynolds number is excellent. No ad hoc turbulence assumptions are made. [ABSTRACT FROM AUTHOR]

Matched asymptotic expansions are used as a framework from which to derive differential equations that describe the mean velocity and turbulence fields in the outer region of a zero-pressure-gradient turbulent boundary layer. Attention is focused upon solutions to these equations in the very outer region or superlayer, where the boundary layer merges with the outer flow. It is found that both the velocity and turbulence fields approach their free-stream values exponentially fast as Townsend [The Structure of Turbulent Shear Flow (Cambridge U.P., Cambridge, 1976)] had foreseen, but not necessarily in the detailed manner he conjectured. These details are used to help construct approximate solutions for the mean velocity and turbulence fields in the outer region that display the correct asymptotic form both in the logarithmic region and the superlayer. The resulting solution for mean velocity is closely in accord with Coles’ law of the wake and accurately reproduces data over the complete Reynolds number range for which the boundary layer is turbulent; likewise the profiles for the turbulence intensities. It is further shown that the turbulence intensities conform to a law analogous to the law of the wake. [ABSTRACT FROM AUTHOR]

An air-core superconducting solenoid, with a diameter of 0.2 m and a length of 0.4 m, has been configured for use as a heavy-ion reaction-product spectrometer (E/A≤5 MeV/u) near θ=0 °. The spectrometer has a large solid angle (10–35 msr) and properties suitable for time-of-flight measurements with flight paths >= 2 m. The performance of the spectrometer was established using α-particle sources and nuclear-reaction products from heavy-ion collisions. The characteristics of air-core magnets are compared to those of steel-yoke magnets. The simplicity and ease of operation of the air-core magnet, without significant problems from the (axial) fringe fields, suggests that larger air-core magnets with dΩ≥20 msr and capable of focusing ions with E/A≥30 MeV/u are feasible. Other applications of solenoids and combinations of solenoids with radial electric-field lenses (ELCO lenses) are also discussed, including designs which focus more than one charge state simultaneously. [ABSTRACT FROM AUTHOR]

This paper is concerned with the generic form of space-time correlations of instantaneous velocity fluctuations in turbulent shear flows. The study has as its basis the Kovasznay-Corrsin conjecture modified to account for inhomogeneities. Details of the modifications were dictated by comparison with direct numerical simulations of turbulent channel flow. For example, analysis of the simulations shows that the space-time correlations at optimal delay depict, when appropriately normalized, a form independent of turbulence component. It is also evident that the half width of the correlation grows, in the outer region of the layer, like V[sup n+1], while the Lagrangian time scale grows like V[sup n], where 2

Presents a letter to the editor of the 'Journal of the American Medical Association,' about hospitals' adoption of smoke-free policies.

The EUROGAM-2 array has been used to study γ rays emitted following the spontaneous fission of a 248Cm source. Analysis of Doppler-broadened lineshapes corresponding to transitions from excited rotational states in neutron-rich Sr, Zr, Mo, Ru, and Pd fission fragments has enabled measurements of state lifetimes in these nuclei at nuclear spins of I ∼ 10. Extracted transition quadrupole moments are compared with various recent theoretical approaches, as well as with measurements at lower spin, and an interpretation of the data is made within the context of prolate-oblate shape competition in this region. [ABSTRACT FROM AUTHOR]