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Stefan Luther

Publications and source records attributed to Stefan Luther.

3 recordsLinked to original sources

Viscosity destabilizes sonoluminescing bubbles.

In single-bubble sonoluminescence (SBSL) microbubbles are trapped in a standing sound wave, typically in water or water-glycerol mixtures. However, in viscous liquids such as glycol, methylformamide, or sulphuric acid it is not possible to trap the bubble in a stable position. This is very peculiar as larger viscosity normally stabilizes the dynamics. Suslick and co-workers call this new mysterious state of SBSL "moving-SBSL." We identify the history force (a force nonlocal in time) as the origin of this destabilization and show that the instability is parametric. A force balance model quantitatively accounts for the observed quasiperiodic bubble trajectories.

Journal Article↗

Drag reduction in bubbly Taylor-Couette turbulence.

In Taylor-Couette flow the total energy dissipation rate and therefore the drag can be determined by measuring the torque on the system. We do so for Reynolds numbers between Re=7 x 10(4) and Re=10(6) after having injected (i) small bubbles (R=1 mm) up to a volume concentration of alpha=5% and (ii) buoyant particles (rhop/rhol=0.14) of comparable volume concentration. In case (i) we observe a crossover from little drag reduction at smaller Re to strong drag reduction up to 20% at Re=10(6). In case (ii) we observe at most little drag reduction throughout. Several theoretical models for bubbly drag reduction are discussed in view of our findings.

Journal Article↗

[Subjective quality of life of children and adolescents with psychiatric disorders: development and validation of a new assessment scale].

Within paediatric disciplines and in particular in child and adolescent psychiatry very little work is available regarding the quality of life (QoL) of patients. After using literature search and interviews for identifying the relevant QoL domains, a set of self report questionnaires (for the parents: proxy rating) was developed (C-version: children 10 to 14 years, A-version: adolescents 14 to 18 years, P-version: parental proxy rating about child/adolescent). It was aimed at an almost identical wording within each questionnaire version in regard to the different QoL domains. The questionnaires should not primarily address symptomatology (like e.g. the Child Behaviour Checklist by Achenbach and Edelbrock 1983). After a pilot phase ensuring adequacy and appropriateness of the forms, the questionnaires were field tested in a consecutive series of nearly 700 patients which were referred to the Cologne university department of child and adolescent psychiatry for diagnosis and treatment between 1996 and 2000. The intended structure of the questionnaire system with symptom and functioning scales concerning the domains: "physical performance, fatigue, anxiety, depression, peer group, school, family, beliefs, general QoL" and some further single questions was confirmed by psychometric testing (validity and reliability). Interscale correlations were in general moderate, exhibiting the expected pattern. Analyses of variances using the method of "known groups comparisons" showed e.g. age, sex, and diagnosis related effects hinting towards clinically meaningful differences. The questionnaire forms were well accepted and understood by patients and parents and the wording of the addressed topics was reported to be adequate. Further work will include the psychopathological findings and will focus on specific subgroups of patients (e.g. different diagnoses) as well as on the longitudinal observation of special patient groups, e.g. patients with eating disorders. It can be concluded that the new QoL instruments were successfully employed in a consecutive series of patients and showed very sufficient psychometric properties.

Adolescent↗