Regularization modeling for large-eddy simulation of homogeneous isotropic decaying turbulence

Inviscid regularization modeling of turbulent flow is investigated. Homogeneous, isotropic, decaying turbulence is simulated at a range of filter widths. A coarse-graining of turbulent flow arises from the direct regularization of the convective nonlinearity in the Navier–Stokes equations. The regularization is translated into its corresponding sub-filter model to close the equations for large-eddy simulation (LES). The accuracy with which primary turbulent flow features are captured by this modeling is investigated for the Leray regularization, the Navier–Stokes-α formulation (NS-α), the simplified Bardina model and a modified Leray approach. On a PDE level, each regularization principle is known to possess a unique, strong solution with known regularity properties. When used as turbulence closure for numerical simulations, significant differences between these models are observed. Through a comparison with direct numerical simulation (DNS) results, a detailed assessment of these regularization principles is made. The regularization models retain much of the small-scale variability in the solution. The smaller resolved scales are dominated by the specific sub-filter model adopted. We find that the Leray model is in general closest to the filtered DNS results, the modified Leray model is found least accurate and the simplified Bardina and NS-α models are in between, as far as accuracy is concerned. This rough ordering is based on the energy decay, the Taylor Reynolds number and the velocity skewness, and on detailed characteristics of the energy dynamics, including spectra of the energy, the energy transfer and the transfer power. At filter widths up to about 10% of the computational domain-size, the Leray and NS-α predictions were found to correlate well with the filtered DNS data. Each of the regularization models underestimates the energy decay rate and overestimates the tail of the energy spectrum. The correspondence with unfiltered DNS spectra was observed often to be closer than with filtered DNS for several of the regularization models

Numerical experiments in modulated turbulence

This thesis is devoted to turbulent mixing. Various flow problems arising in science and technology are intimately connected to properties of turbulent transport. Improvements and control of mixing efficiency rely directly on the understanding of turbulence phenomena. From the mathematical point of view, turbulence is governed by the Navier-Stokes equations. The convective nonlinearity in these equations induces an interaction between a wide range of scales in a flow. In additions, the dissipation term acts mainly on the smallest flow scales, characterized by the fluid viscosity. In the absence of external forces theses two terms fully dictate the dynamics of decaying turbulence. In numerical studies of turbulence dynamics one usually incorporates an external forcing to supply energy to the largest flow scales. Correspondingly, we observe an average energy cascading process from the largest to the smallest scales that is well depicted by the so-called Kolmogorov K41 theory

Kinematic and dynamic simulation of the functioning of torsionally flexible metal coupling

The article presents the process of visualization and the accuracy of performance of the prototype of bidirectional torsionally flexible metal coupling using Autodesk® Inventor® Professional 2009. Selected figures from the simulations are presented and discussed on the basis of a virtual model of the coupling

Nonlinear dynamics of a two-stage gear system with consideration given to the torsional stiffness of the countershaft

In the article presented and discussed are research results obtained during computer simulation conduced on dynamic model of a two-stage gear system. The investigation attempted to identify the impact of torsional stiffness of the countershaft on nonlinear gear stage behavior. Analysis of the results showed significant influence of the investigated constructional parameter on dynamic state of analyzed gear. First of all, lack of nonlinear gear behavior for adopted range of change in mesh torsional stiffness was observed

Modeling flow, heat and mass transfer in a porous biomass plug - when used in an electrically heated tobacco system

Heating porous biomass samples is utilized in many industries for drying or extracting Volatile Organic Compounds (VOC) from the biomass. The heating may trigger physical and chemical processes within the material, such as release of VOC, thermal degradation and evaporation. Most of the processes triggered by the increased temperature are occurring simultaneously and are strongly interdependent. For most practical applications, it is important to have control of the complex processes occurring during heating to generate stable and controllable release of VOC. This is the case for products delivering the released VOC to consumers by inhalation, as is the case of an Electrically Heated Tobacco System (EHTS). Please download the full abstract below

Response maxima in time-modulated turbulence: Direct Numerical Simulations

The response of turbulent flow to time-modulated forcing is studied by direct numerical simulations of the Navier-Stokes equations. The large-scale forcing is modulated via periodic energy input variations at frequency $\omega$. The response is maximal for frequencies in the range of the inverse of the large eddy turnover time, confirming the mean-field predictions of von der Heydt, Grossmann and Lohse (Phys. Rev. E 67, 046308 (2003)). In accordance with the theory the response maximum shows only a small dependence on the Reynolds number and is also quite insensitive to the particular flow-quantity that is monitored, e.g., kinetic energy, dissipation-rate, or Taylor-Reynolds number. At sufficiently high frequencies the amplitude of the kinetic energy response decreases as $1/\omega$. For frequencies beyond the range of maximal response, a significant change in phase-shift relative to the time-modulated forcing is observed.Comment: submitted to Europhysics Letters (EPL), 8 pages, 8 Postscript figures, uses epl.cl

Observations of Rough-Toothed Dolphins (Steno bredanensis) off the Coast of Utila, Honduras

Local dive shop operators and fishermen report that rough-toothed dolphins (Steno bredanensis) are frequently encountered off the coast of Utila, Honduras, (16°05\u2746.5 N 86°55\u2747.8 W). Our observations suggest that at least some of these animals may constitute a resident population, although the extent of the resident group’s home range has not been determined. Twenty-eight rough-toothed dolphins were identified using photoidentification techniques, 15 of which were re-sighted on two or more occasions. The 12 animals that were re-sighted four or more times were typically seen together, suggesting that they constitute a stable social group. At least one of these dolphins is an adult male, and his continued presence in this social group may reflect a social structure for rough-toothed dolphins that differs from that described for other dolphin species. Social interactions often involved tactile behaviours such as pectoral fin rubbing and side rubbing. The observed dolphins sometimes expressed interest in the research vessel and other boats by approaching, and on separate occasions examined a hydrophone and slow moving propeller visually and echoically. Overall, our behavioural observations suggest: (1) synchronous behaviours and ‘tight’ groupings are common while rough-toothed dolphins are travelling; (2) tactile contact is an important aspect of social interactions for rough-toothed dolphins; (3) cooperative behaviour occurs during play; and (4) rough-toothed dolphins are curious

Out of the Mouth of Babes: Lessons from Research on Human Infants

Marine mammal behavior and cognition researchers often face a number of challenges, including the research subjects’ lack of interest and verbal abilities, as well as choosing a paradigm with appropriate stimuli for the subjects’ perceptual and cognitive abilities. Researchers who work with human infants often encounter similar challenges when studying infant cognition and have developed strategies to overcome these challenges, including using stimuli that capture the infants’ attention, determining what tasks are age-appropriate, and using conditioned responses to test discrimination abilities. This paper encourages marine mammal researchers to learn from the research paradigms and techniques used in human infant research and alter them appropriately for the intended study subjects. The conditioned head-turn response, Violation-of-Expectation paradigm, and the help/hinder paradigm have all been used in infant cognition research and show great promise for furthering the current understanding of marine mammal behavior and cognition. In addition, studying a subject’s spontaneous behavior can provide valuable insight in areas such as problem solving skills, creativity, and individual differences. Care must be taken to adapt the paradigms and use stimuli to fit each species’ perceptual abilities. For example, avoiding a task that requires color discrimination for species that do not possess color vision or using stimuli that fall within a particular species’ hearing range are necessary steps in designing an ecologically valid and informative study. Adapting paradigms previously used with human infants can help expand the current understanding of marine mammal communication, cognitive abilities, and social behavior

Capturing aerosol droplet nucleation and condensation bursts using PISO and TVD schemes

A mathematical model for single-species aerosol production and transport is formulated, and solved using an adapted PISO algorithm. The model is applied to a laminar flow diffusion chamber, using a finite volume method on a collocated grid. In tran- sient simulations, a sharp scalar front (e.g., vapor mass fraction), is shown to introduce unphysical oscillation in the solution, when applying a second order linear interpolation in the convective terms. At increased grid resolution, these oscillations are strongly at- tenuated. When applying a TVD scheme (here the MUSCL scheme), a time-accurate monotonicity-preserving solution is obtained. The numerical dissipation introduced by the MUSCL scheme implies increased spatial resolution to restore high accuracy levels. We develop a one-dimensional grid refinement algorithm, which relates the grid density in one direction to the magnitude of the scalar gradient. In combination with the MUSCL scheme, this gives accurate results, with a significant reduction in computational effort, in comparison with a uniform fine grid

How Selective is Social Learning in Dolphins?

Social learning is an important aspect of dolphin social life and dolphin behavioral development. In addition to vocal social learning, dolphins discover behaviors for foraging, play, and social interactions by observing other members of their social group. But dolphins neither indiscriminately observe nor mindlessly mimic other dolphins. To the contrary, dolphin calves are quite selective in their choices of who to observe and/or imitate. Calves are most likely to learn foraging behaviors from their mothers, but they are more likely to watch and reproduce the play behaviors of other calves than the play behaviors of adult dolphins (including their mothers). But not all calves are equally likely to be good models. Instead, calves are more likely to observe and mimic the behaviors of other calves that are producing either novel behaviors or more complex forms of behaviors that the observing calf already knows. As a result, there is a general tendency for calves to watch and learn from calves that are older than they are. But differences in age are only part of the story. In fact, dolphin personality may be more important than dolphin age in determining the efficacy of a model