Thermally driven acoustic oscillations, part IV: Tubes with variable cross-section

The problem of thermally driven acoustic oscillations is treated for tubes with variable cross-section, with particular emphasis on the possible reduction of the necessary temperature ratio for excitation. Tubes with optimal conditions in the vicinity of the temperature jump, and with big cross-sections in parts with constant temperature are found to give the best performance in this respect. Included in the family of devices which were treated is the classical Sondhauss-tube. Experiments which give a striking confirmation of the theory are reported.     

Green's function of radial inhomogeneous spheres excited by internal sources

Green's function in the interior of penetrable bodies with inhomogeneous compressibility by sources placed inside them is evaluated through a Schwinger-Lippmann volume integral equation. In the case of a radial inhomogeneous sphere, the radial part of the unknown Green's function can be expanded in a double Dini's series, which allows analytical evaluation of the involved cumbersome integrals. The simple case treated here can be extended to more difficult situations involving inhomogeneous density as well as to the corresponding electromagnetic or elastic problem. Finally, numerical results are given for various inhomogeneous compressibility distributions.     

Thermally driven acoustic oscillations,part IV: Tubes with variable cross-section

Summary The problem of thermally driven acoustic oscillations is treated for tubes with variable cross-section, with particular emphasis on the possible reduction of the necessary temperature ratio for excitation. Tubes with optimal conditions in the vicinity of the temperature jump, and with big cross-sections in parts with constant temperature are found to give the best performance in this respect. Included in the family of devices which were treated is the classical Sondhauss-tube. Experiments which give a striking confirmation of the theory are reported.

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Zusammenfassung Das Problem der thermisch getriebenen akustischen Schwingungen wird für Rohre mit veränderlichem Querschnitt behandelt, mit besonderer Berücksichtigung der Frage der Reduktion des für die Anfachung nötigen Temperaturverhältnisses. Rohre, die optimale Radien in der Umgebung des Temperatursprunges haben, und möglichst grosse Querschnitte in Teilen mit konstanter Temperatur aufweisen, erweisen sich am besten in dieser Hinsicht. In der Familier der untersuchten Anordnungen its das klassische Sondhauss-Rohr miteinbezogen. Experimente gaben überzeugende Bestätigung für die verwendeten Theorien.

     

Thermally driven acoustic oscillations,part V: Gas-liquid oscillations

Summary Thermally driven gas oscillations are considered which drive a liquid column at the cold open end. The frequency of the oscillations is lowered by the inertia of the liquid and the Stokes boundary layer is thick. Optimal conditions for thermal driving in the gas occur at correspondingly big tube radii. Experiments conducted inN ₂ confirm the theoretically calculated stability limits. With hot air driving a water column, a low critical temperature ratio for the oscillations of about 1.5 was observed.

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Zusammenfassung Thermisch getriebene akustische Schwingungen werden untersucht die eine Flüssigkeitssäule am kalten offenen Ende antreiben. Die Frequenz der Schwingungen wird durch die Trägheit der Flüssigkeit vermindert und die Stokes'sche Grenzschicht ist dick. Die optimalen Bedingungen für thermisch getriebene Schwingungen im Gas werden bei entsprechend höheren Rohr-Radien auftreten. Experimente mitN ₂ haben die theoretisch berechneten Stabilitätsgrenzen bestätigt. Bei einer Anordnung, in der heisse Luft eine Wassersäule angetrieben hat, wurde ein tiefes kritisches Temperaturverhältnis von etwa 1.5 beobachtet.

     

Critical point estimation of the Lennard-Jones pure fluid and binary mixtures

The apparent critical point of the pure fluid and binary mixtures interacting with the Lennard-Jones potential has been calculated using Monte Carlo histogram reweighting techniques combined with either a fourth order cumulant calculation (Binder parameter) or a mixed-field study. By extrapolating these finite system size results through a finite size scaling analysis we estimate the infinite system size critical point. Excellent agreement is found between all methodologies as well as previous works, both for the pure fluid and the binary mixture studied. The combination of the proposed cumulant method with the use of finite size scaling is found to present advantages with respect to the mixed-field analysis since no matching to the Ising universal distribution is required while maintaining the same statistical efficiency. In addition, the accurate estimation of the finite critical point becomes straightforward while the scaling of density and composition is also possible and allows for the estimation of the line of critical points for a Lennard-Jones mixture.     

Best constants for higher-order Rellich inequalities in <Emphasis Type="Italic">L</Emphasis><Superscript><Emphasis Type="Italic">p</Emphasis></Superscript>(Ω)

We obtain a series improvement to higher-order L ^p -Rellich inequalities on a Riemannian manifold M. The improvement is shown to be sharp as each new term of the series is added.      

Trace estimates and invariance of the essential spectrum

We provide sufficient conditions under which the difference of the resolvents of two higher-order operators acting in belongs to trace classes . We provide explicit estimates on the norm of the resolvent difference in terms of L^p norms of the difference of the coefficients. Such inequalities are useful in estimating the effect of localized perturbations of the coefficients. Received: 5 September 2005; revised: 15 December 2005