37,613 research outputs found

### Existence of N\'eel order in the S=1 bilinear-biquadratic Heisenberg model via random loops

We consider the general spin-1 SU(2) invariant Heisenberg model with a
two-body interaction. A random loop model is introduced and relations to
quantum spin systems is proved. Using this relation it is shown that for
dimensions 3 and above N\'eel order occurs for a large range of values of the
relative strength of the bilinear ($-J_1$) and biquadratic ($-J_2$) interaction
terms. The proof uses the method of reflection positivity and infrared bounds.
Links between spin correlations and loop correlations are proved.Comment: 24 pages, 5 figure

### A Discussion of the Application of the Prandtl-Glauert Method to Subsonic Compressible Flow over a Slender Body of Revolution

The Prandtl-Glauert method for subsonic potential flow of a compressible fluid has generally been believed to lead to an increase in the pressures over a slender body of revolution by a factor 1/([sqrt](1-M[sub]1^2)) (where M[sub]1 is Mach number in undisturbed flow) as compared with the pressures in incompressible flow. Recent German work on this problem has indicated, however, that the factor 1/([sqrt](1-M[sub]1^2)) is not applicable in this case. In the present discussion a more careful application of the
Prandtl-Glauert method to three-dimensional flow gives the following results:
The Prandtl-Glauert method does not lead to a universal velocity or pressure correction formula that is independent of the shape of the body. The factor 1/([sqrt](1-M[sub]1^2)) is applicable only to the case of two-dimensional flow.
The increase with Mach number of the pressures over a slender body of revolution is much less rapid than for a two-dimensional airfoil. An approximate formula from which the increase can be estimated is derived theoretically.
The increase with Mach number of the maximum axial interference velocity on a slender body of revolution in a closed wind tunnel is given approximately by the factor 1/((1-M[sub]1^2)^-3/2), rather than by the factor 1/([sqrt](1-M[sub]1^2)) previously obtained by Goldstein and Young and by Tsien and Lees

### The Stability of the Laminar Boundary Layer

The present papcr is a continuation of a theoretical investigation of the stability of the laminar boundary layer in a compressible fluid. An approximate estimate for the minimum critical Reynolds number Re[sub]cr[sub-sub]min, or stability limit, is obtained in terms of the distribution of the kinematic viscosity and the product of the mean density [rho][super][bar]* and mean vorticity [formula] across the
boundary layer. With the help of this estimate for Re[sub]cr[sub-sub]min it is shown that withdrawing heat from the fluid through the solid surface increases RRe[sub]cr[sub-sub]min and stabilizes the flow, as compared with the flow over an insulated surface at the same Mach number. Conduction of heat to the fluid through the solid surface has exactly the opposite effect. The value of Re[sub]cr[sub-sub]min for the insulated surface decreases as the Mach number increases for the case of a uniform free-stream velocity. These general conclusions are supplemented by detailed calculations of the curves of wave number (inverse wave length) against Reynolds number for the neutral disturbances for 10 representative cases of insulated and noninsulated surfaces.
So far as laminar stability is concerned, an important difference exists between the case of a subsonic and supersonic free-stream velocity outside the boundary layer. The neutral boundary-layer disturbances that are significant for laminar stability die out exponentially with distance from the solid surface; therefore, the phase velocity c* of these disturbances is subsonic relative to the free-stream velocity [symbol] or [symbol], [symbol] where is the local sonic velocity. When [symbol]<1, (where M[sub]0 is free-stream Mach number), it follows that [inequalities] and any laminar boundary-1ayer flow is ultimately unstable at sufficiently high Reynolds numbers because of the destabilizing action of viscosity near the solid surface, as
explained by Prandtl for the incompressible fluid. When M[sub]0 >1, however, [inequalities]. If the quantity [forumla] is large enough negatively, the rate at which energy passes from the disturbance to the mean flow, which is proportional to [formula], can always be large enough to counterbalance the rate at which energy passes from the mean flow to the disturbance because of the destabilizing action of viscosity near the solid surface. In that case only damped disturbances exist and the laminar boundary layer is completely stable at all Reynolds numbers. This condition occurs when the rate at which heat is withdrawn from the fluid through the solid surface reaches or exceeds a critical value that depends only on the Mach number and the properties of the gas. Calculations show that for M[sub]0 > 3 (approx.) the laminar boundary-layer flow for thermal equilibrium -- where the heat conduction through the solid surface balances the heat radiated from the surface -- is completely stable at all Reynolds numbers under free-flight
conditions if the free-stream velocity is uniform.
The results of the analysis of the stability of the laminar boundary layer must be applied with care to discussions of transition; however, withdrawing heat from the fluid through the solid surface, for example, not only increases Re[sub]cr[sub-sub]min but also decreases the initial rate of amplification of the self-excited disturbances, which is roughly proportional fo 1/[sqrt]Re[sub]cr[sub-sub]min. Thus, the effect of the thermal conditions at the solid sufice on the transition Reynolds number Re[sub]tt, is similar to the effect on Re[sub]cr[sub-sub]min. A comparison between this conclusion and experimental investigations of the effect of surface heating on transition at low speeds shows that the results of the present paper give the proper direction of this effect.
The extension of the results of the stability analysis to laminar boundary-layer gas flows with a pressure gradient in the direction of the free stream is discussed

### What are the factors which contribute to level one social work students failing to progress or achieving low grades?

This article was first published in the Wolverhampton Intellectual Repository and E-Theses (WIRE). There is no printed version.This study is a preliminary review of the possible reasons for low achievement among some level one social work undergraduates. These may be viewed as challenges to the individual, attempting to study in a particular social context, or as challenges to the institution in raising achievement and accommodating differing needs. Much of the literature is concerned with the experiences of students from particular social groups. In some studies, these concerns are integrated with the identification of individual strategies for success and/or institutional practices which foster or inhibit achievement

### Factors associated with changing efficacy of emamectin benzoate against infestations of Lepeophtheirus salmonison Scottish salmon farms

The availability and use of medicines to control infestations of sea lice on Atlantic salmon, Salmosalar L., farms in Scotland has changed considerably in the last decade (Lees, Gettinby & Revie 2008b). Whereas hydrogen peroxide and organophosphate compounds were used widely throughout the 1990s and in the early 2000s, only two therapeutants have remained in common use since 2005: topical cypermethrin (Excis; Novartis Animal Health, Camberley, UK) and an oral formulation of emamectin benzoate (SLICE; Schering Plough Animal Health, Uxbridge, UK)

### Self-protected electrodes limit field-emission current

One cathode includes array of square-shaped conductor columns. Columns are electrically interconnected by conducting plate on bottom. Each column provides field-emission current. Another cathode includes array of rodlike conductors. Layer is covered by control film made of conducting material. Film is isolated from each conductor

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