Modelling the rapid spherical compression of isotropic turbulence

Rapid distortion analysis is used to modify the form of the closure model for the dissipation rate of the turbulent kinetic energy. The modification is such that the evolution of the dissipation rate during a rapid compression is predicted exactly; good agreement between the model prediction and direct simulation data is obtained. Previous closure proposals fail to properly predict the rapid compression case. The reason for the difference between the present and previous models is traced to the fact that previous workers neglected variations of kinematic viscosit

The Role of Cutaneous Receptors in the Menton Tap Silent Period

The neuromuscular mechanisms of the electromyographic silent period are poorly understood but clinically significant. Our purpose was to isolate the contribution of cutaneous afferents from the chin to the latency and the duration of the EMG silent period. Under the conditions of our experiment, the afferents of the mental nerve have no observable influence on the latency or the duration of the silent period educed by a menton tap.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66492/2/10.1177_00220345790580011001.pd

Surface-focused Seismic Holography of Sunspots: I. Observations

We present a comprehensive set of observations of the interaction of p-mode oscillations with sunspots using surface-focused seismic holography. Maps of travel-time shifts, relative to quiet-Sun travel times, are shown for incoming and outgoing p modes as well as their mean and difference. We compare results using phase-speed filters with results obtained with filters that isolate single p-mode ridges, and further divide the data into multiple temporal frequency bandpasses. The f mode is removed from the data. The variations of the resulting travel-time shifts with magnetic-field strength and with the filter parameters are explored. We find that spatial averages of these shifts within sunspot umbrae, penumbrae, and surrounding plage often show strong frequency variations at fixed phase speed. In addition, we find that positive values of the mean and difference travel-time shifts appear exclusively in waves observed with phase-speed filters that are dominated by power in the low-frequency wing of the p1 ridge. We assess the ratio of incoming to outgoing p-mode power using the ridge filters and compare surface-focused holography measurements with the results of earlier published p-mode scattering measurements using Fourier-Hankel decomposition.Comment: Solar Physics, accepte

Rapid distortion theory for compressible homogenous turbulence under isotropic mean strain

Isotropic compressible turbulence subjected to rapid isotropic compression is studied using inviscid rapid distortion theory (RDT) and direct numerical simulation. An exact solution to the rapid distortion problem is given. Comparisons are made between the simulation results and the RDT solution, as well as previously studied limiting cases of the RDT solution. The comparisons illustrate the range of applicability of the RDT solutions. Implications for the use of RDT results in modeling compressible turbulent flows are briefly discusse

Reduced Gravity Cryogenic Propellant Tank Re-Supply Simulation and Geyser Prediction

Computational modeling of microgravity geyser phenomena is considered in an effort to establish the maturity of one computational tool for its simulation. Verification and validation of the tool against data for a turbulent channel flow, data for the spread of a selfpreserving turbulent axisymmetric jet, and experiment data from geyser flows in a drop tower experiment is presented. A mesh convergence study demonstrates that the mesh used in previous studies was not quite sufficiently refined to produce mesh independent simulations. Critical review of the drop tower experiment and previous studies revealed that more accurate definition of the experiment parameters could be made. Surprisingly, simulations using the improved parameters and the better mesh were in poorer agreement with the experimentally measured geyser height than has been previously reported. With the insight gained from the critical review of the experiment, alterations to a previously proposed correlation for the prediction of geyser height are proposed. The new correlation outperforms its predecessors by producing an RMS error of only 0.09 when used to predict the geyser heights reported for the experiment

A high heating rate pyrolysis model for the Phenolic Impregnated Carbon Ablator (PICA) based on mass spectroscopy experiments

A novel model for the pyrolysis of the Phenolic Impregnated Carbon Ablator (PICA) at high heating rate is developed and calibrated based on high fidelity thermal decomposition experiments. The calibration relies on accurate quantification of pyrolysis gases obtained from mass spectroscopy analysis during thermal decomposition at fast heating rates simulating flight conditions. Model calibration is achieved by coupling the Porous material Analysis Toolbox based on OpenFOAM (PATO) with an optimization software (Dakota). A multi-objective genetic algorithm is used to fit the experimental data by optimizing the model parameters for an element and a species-based formulation. The new model captures both the material mass loss and the gaseous species produced during pyrolysis.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

One-equation near-wall turbulence modeling with the aid of direct simulation data

Die Laengenmassstaebe in den Beziehungen fuer die Wirbelviskositaet und die Dissipationsrate in Eingleichungs-Turbulenzmodellen wurden aus direkten Simulations-(DMS)-Daten fuer entwickelte Kanalstroemung und Grenzschichtstroemung fuer jeweils zwei Reynoldszahlen ermittelt. Als Basis fuer die weitere Auswertung werden die aus den DNS Rechnungen folgenden Verteilungen der wichtigsten Hauptstroemungs- und Turbulenzgroessen vorgestellt und diskutiert, auch im Hinblick auf den Reynoldszahleinfluss und die Unterschiede zwischen Kanal- und Grenzschichtstroemung. Ein alternatives Eingleichungsmodell wird untersucht, in welchem (anti #upsilon#'"2)"1"/"2 anstatt k"1"/"2 als Geschwindigkeitsmassstab verwendet wird. Die nun im Modell erscheinenden Laengenmasse folgen gut einer linearen Verteilung in Wandnaehe. Die resultierenden Laengenmassbeziehungen zusammen mit einer auf den DNS-Daten basierenden Beziehung zwischen anti #upsilon#'"2/k und y"* = k"1"/"2y/#upsilon# bilden ein neues Eingleichungsmodell fuer den Gebrauch in wandnahen Zonen. Das neue Modell wurde als Komponente fuer den wandnahen Bereich in einem Zweischichtenmodell getestet, und zwar durch Anwendung auf eine entwickelte Kanalstroemung, eine Grenzschichtstroemung und die Stroemung ueber eine zurueckspringende Stufe. (orig.)The length scales appearing in the relations for the eddy viscosity and dissipation rate in one-equation models were evaluated from direct numerical (DNS) simulation data for developed channel and boundary-layer flow at two Reynolds numbers each. To prepare the ground for the evaluation, the distribution of the most relevant mean-flow and turbulence quantities is presented and discussed, also with respect to Reynolds-number influence and to differences between channel and boundary-layer flow. An alternative model is examined in which (anti #upsilon#'"2)"1"/"2 is used as velocity scale instead of k"1"/"2. With this velocity scale, the length scales now appearing in the model follow closely a linear relationship near the wall. The resulting length-scale relations together with a DNS based relation between anti #upsilon#'"2/k and y"* = k"1"/"2y/#nu# form a new one-equation model for use in near-wall regions. The new model was tested as near wall component of a two-layer model by application to developed-channel, boundary-layer and backward-facing-step flows. (orig.)SIGLEAvailable from TIB Hannover: RO 1728(93) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman