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Detlef Lohse

Publications and source records attributed to Detlef Lohse.

8 recordsLinked to original sources
PubMed2003

Harmonic enhancement of single-bubble sonoluminescence.

It is known from experiment that the light emission from a sonoluminescing bubble can be increased by using more than one driving frequency. In this paper, a systematic method to determine the optimal conditions of pressure amplitude and relative phase for this effect is described. As a specific application, a two-frequency system--26.5 kHz and 53 kHz--is considered. It is found that the maximum temperatures achievable can be appreciably increased with respect to single-frequency drive, still maintaining spherical stability, provided the dissolved inert gas concentration is kept extremely low in order to maintain diffusive stability.

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PubMed2003

Response maxima in modulated turbulence.

Isotropic and homogeneous turbulence driven by an energy input modulated in time is studied within a variable range mean-field theory. The response of the system, observed in the second-order moment of the large-scale velocity difference D(L,t)=<<(u(x+L)-u(x))(2)>> proportional, variant Re(2)(t), is calculated for varying modulation frequencies omega and weak modulation amplitudes. For low frequencies the system follows the modulation of the driving with almost constant amplitude, whereas for higher driving frequencies the amplitude of the response decreases on average proportional, variant 1/omega. In addition, at certain frequencies the amplitude of the response either almost vanishes or is strongly enhanced. These frequencies are connected with the frequency scale of the energy cascade and multiples thereof.

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PubMed2003

Ultimate state of thermal convection.

The ultimate regime of thermal convection, the so-called Kraichnan regime [R. H. Kraichnan, Phys. Fluids 5, 1374 (1962)]], hitherto has been elusive. Here numerical evidence for that regime is presented by performing simulations of the bulk of turbulence only, eliminating the thermal and kinetic boundary layers and replacing them with periodic boundary conditions.

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PubMed2002

Competitive clustering in a bidisperse granular gas.

A bidisperse granular gas in a compartmentalized system is experimentally found to cluster competitively: Depending on the shaking strength, the clustering can be directed either towards the compartment initially containing mainly small particles or to the one containing mainly large particles. The experimental observations are quantitatively explained within a flux model.

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PubMed2002

Prandtl and Rayleigh number dependence of the Reynolds number in turbulent thermal convection.

The Prandtl and Rayleigh number dependences of the Reynolds number in turbulent thermal convection following from the unifying theory by Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000); Phys. Rev. Lett. 86, 3316 (2001)] are presented and compared with various recent experimental findings. This dependence Re(Ra,Pr) is more complicated than a simple global power law. For Pr=5.5 and 10(8)<Ra<10(10) the effective or local power law exponent of Re as a function of Ra is definitely less than 0.50, namely, Re approximately Ra(0.45), in agreement with Qiu and Tong's experimental findings [Phys. Rev. E 64, 036304 (2001)]. We also calculated the kinetic boundary layer width. Both in magnitude and in Ra scaling it is consistent with the data.

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PubMed2002

Sudden collapse of a granular cluster.

Single clusters in a vibro-fluidized granular gas in N connected compartments become unstable at strong shaking. They are experimentally shown to collapse very abruptly. The observed cluster lifetime (as a function of the driving intensity) is analytically calculated within a flux model, making use of the self-similarity of the process. After collapse, the cluster diffuses out into the uniform distribution in a self-similar way, with an anomalous diffusion exponent 1/3.

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PubMed2002

Suppressing dissociation in sonoluminescing bubbles: the effect of excluded volume.

Recent theoretical work in single-bubble sonoluminescence has suggested that water vapor in the collapsing bubble leads to energy-consuming chemical reactions, restricting the peak temperatures to values for which hardly any light emission could occur. Analyzing the reaction thermodynamics within the dense, collapsed bubble, we demonstrate that the excluded volume of the nonideal gas results in pronounced suppression of the particle-producing endothermic reactions. Thus, sufficiently high temperatures for considerable bremsstrahlung emission can be achieved.

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