An entropic understanding of flow maldistribution in thermally isolated parallel channels

Flow across heated parallel channel systems exists in many applications. The performance of such systems experiencing multiphase flow could suffer from the deleterious effects of flow non-uniformity or maldistribution. Modeling the behavior of such systems is challenging due to the inherent non-linearity associated with the multiphase flow and the difficulty in determining the actual flow among several possible flow distributions. This study addresses the challenge by analyzing the entropy production in such systems. Using experiments on two thermally isolated, nominally identical, and externally heated parallel channels, we quantify irreversibility in the resulting multiphase flow by evaluating the entropy generation rate. Our experiments reveal that certain flow conditions result in severe maldistribution (flow ratio > 10) in the channels, associated with a sharp rise in entropy production. Such an increase is not predicted for uniform flow distribution across parallel channels, making maldistributed flow a thermodynamically favored state over equally distributed flow. We extend this understanding to non-identical parallel channels as well. With entropy analysis providing additional insight besides the fundamental equations governing mass, momentum, and energy conservation, this approach is valuable in predicting and controlling flow distribution in parallel channel systems

Application of entropy analysis in the prediction of flow distribution in parallel channels

Multiphase flow in parallel channels is often an efficient approach to manage heat and energy distribution in engineering systems. However, two-phase flow with heating in parallel channels is prone to maldistribution, resulting in sub-optimal performance and in some cases, permanent damage. This challenge requires accurate flow modeling in parallel channels to mitigate or design against the adverse effect of two-phase flow maldistribution. The nonlinear nature of multiphase flow results in a multiplicity of predicted solutions for the same condition, thereby creating significant challenges in modeling flow distribution. Therefore, this study focuses on solving this challenge by applying entropy generation analysis and the conservation of mass, momentum balance, and energy balance to predict two-phase flow distribution in a two-parallel-channel assembly with a numerical model. Both model predictions and experimental data show that equally distributed flow becomes severely maldistributed with a decrease in flow rate, resulting in significant change (>30%) in the entropy generation rate. We show that the entropy analysis can be applied in distinguishing between stable and unstable flow distribution, like the linear stability analysis used in previous studies. We also surpass the limit of applying linear stability analysis by using entropy analysis to identify the most feasible end state in a maldistribution process

Models and Analysis of Vocal Emissions for Biomedical Applications

The International Workshop on Models and Analysis of Vocal Emissions for Biomedical Applications (MAVEBA) came into being in 1999 from the particularly felt need of sharing know-how, objectives and results between areas that until then seemed quite distinct such as bioengineering, medicine and singing. MAVEBA deals with all aspects concerning the study of the human voice with applications ranging from the neonate to the adult and elderly. Over the years the initial issues have grown and spread also in other aspects of research such as occupational voice disorders, neurology, rehabilitation, image and video analysis. MAVEBA takes place every two years always in Firenze, Italy. This edition celebrates twenty years of uninterrupted and succesfully research in the field of voice analysis

Towards a solution of the closure problem for convective atmospheric boundary-layer turbulence

We consider the closure problem for turbulence in the dry convective atmospheric boundary layer (CBL). Transport in the CBL is carried by small scale eddies near the surface and large plumes in the well mixed middle part up to the inversion that separates the CBL from the stably stratified air above. An analytically tractable model based on a multivariate Delta-PDF approach is developed. It is an extension of the model of Gryanik and Hartmann [1] (GH02) that additionally includes a term for background turbulence. Thus an exact solution is derived and all higher order moments (HOMs) are explained by second order moments, correlation coefficients and the skewness. The solution provides a proof of the extended universality hypothesis of GH02 which is the refinement of the Millionshchikov hypothesis (quasi- normality of FOM). This refined hypothesis states that CBL turbulence can be considered as result of a linear interpolation between the Gaussian and the very skewed turbulence regimes. Although the extended universality hypothesis was confirmed by results of field measurements, LES and DNS simulations (see e.g. [2-4]), several questions remained unexplained. These are now answered by the new model including the reasons of the universality of the functional form of the HOMs, the significant scatter of the values of the coefficients and the source of the magic of the linear interpolation. Finally, the closures 61 predicted by the model are tested against measurements and LES data. Some of the other issues of CBL turbulence, e.g. familiar kurtosis-skewness relationships and relation of area coverage parameters of plumes (so called filling factors) with HOM will be discussed also

Nonlinear Dynamics

This volume covers a diverse collection of topics dealing with some of the fundamental concepts and applications embodied in the study of nonlinear dynamics. Each of the 15 chapters contained in this compendium generally fit into one of five topical areas: physics applications, nonlinear oscillators, electrical and mechanical systems, biological and behavioral applications or random processes. The authors of these chapters have contributed a stimulating cross section of new results, which provide a fertile spectrum of ideas that will inspire both seasoned researches and students

Numerical Investigation of Liquid Film Dynamics and Atomisation in Jet Engine Fuel Injectors

Today’s aerospace industry continues to exploit liquid hydrocarbon fossil fuels. Motivated by operational considerations, continued availability and cost, this is likely to be the case for many years, despite the obvious environmental concerns. The interplay of liquid atomisation, spray vaporisation and the combustion process are intricately linked. However, the physical process of fuel injection and its atomisation into tiny droplets prior to combustion remains poorly understood. Because atomisation governs the size of the fuel droplets, and therefore their subsequent evaporation rate, adjusting the injection sequence is of paramount importance and will have far-reaching repercussions on many aspects of the combustion process, for example pollutant formation. In the context of jet engines, kerosene is usually injected in its liquid form via an airblast-type fuel injector. A coflowing high-speed airstream destabilises the liquid fuel, which is thus sprayed into fine droplets into the combustion chamber. The prediction of this phenomenon for various operating conditions relevant to the aeronautical industry requires a deeper understanding of the mechanisms involved in the interaction of the two fluids. A key element in predicting the complex behaviour of spray formation and evolution in jet engines is accurate modelling of fuel atomisation. Atomisation represents one of the key challenges that remains to be undertaken to make predictive computational simulations possible. However, the inherent multi-physics and multi-scale nature of this process limits numerical investigations. Thanks to the steady progress in computer power and Computational Fluid Dynamics (CFD) methods, computational modelling of injection systems emerges as a promising tool that can drive the design of future devices. This research project sets out to investigate the atomisation process in detail, in particular in providing physical insight into the fundamental physics of the phenomenon, in conjunction with an analysis on wetting behaviours and liquid droplet tracking. High-fidelity numerical simulations are performed using a novel in-house state-of-the-art multiphase flow modelling capability, RCLSFoam. The performance of the numerical scheme is demonstrated on typical two-dimensional and three-dimensional benchmark test cases relevant to both multiphase flow modelling and atomisation, and validated against other computational methods. An informed and systematic qualitative assessment of the topological variations of the phase interface during primary atomisation of a liquid film is made through dynamical analysis, while investigating an extensive domain of operating conditions at ambient and aero-engine injection conditions relevant to industry. This analysis demonstrated the influence of shear-driven instabilities on the atomisation process. The shear stress and difference in inertia between liquid and gas are observed to play a significant role in the atomisation process. In addition, the key physical mechanisms and their competing effects have been mapped out in order to predict the evolution of the process according to the operating conditions of the injection system. The proposed cartography gathers four different atomisation mechanisms. In particular, for sufficiently high liquid injection speeds, three-dimensional wave modes were observed to co-exist (the “3-D wave mode” regime). For very low liquid flow rates, accumulated liquid at the atomising edge undergoes deformation by which droplets are generated (the “accumulation” regime). For an increasing gas injection speed and a fixed liquid velocity, the effects of surface tension were observed to result in the generation of streamwise ligaments only, which tend to pair up (the “ligament-merging” regime). Finally, “vortex action” is another observed mechanism by which the liquid film is fragmented. Overall, this research project culminated in (i) the study of dynamic wetting behaviours, with the implementation and validation against experimental data of the Kistler dynamic contact model; and (ii) the demonstration of an algorithm for droplet capture and subsequent post-processing analysis of the droplet characteristics.Rolls Royce plc. and EPSR

Symbolization-based analysis of engineering time series

Data symbolization, derived from the study of symbolic dynamics, involves discretization of measurement data to aid in observing and characterizing temporal patterns. In this study, symbolization-based methods are developed for analysis of time series from experimental engineering systems to test hypotheses concerning stationarity, temporal reversibility, and synchronization. Stationarity is examined in the context of process control and dynamical state matching; temporal reversibility, in the context of model discrimination and selection of control schemes (linear versus nonlinear); and synchronization, in the context of modes of interactions between system components. Statistical significance is estimated using the method of surrogate data with Monte Carlo probabilities

Effects of errorless learning on the acquisition of velopharyngeal movement control

Session 1pSC - Speech Communication: Cross-Linguistic Studies of Speech Sound Learning of the Languages of Hong Kong (Poster Session)The implicit motor learning literature suggests a benefit for learning if errors are minimized during practice. This study investigated whether the same principle holds for learning velopharyngeal movement control. Normal speaking participants learned to produce hypernasal speech in either an errorless learning condition (in which the possibility for errors was limited) or an errorful learning condition (in which the possibility for errors was not limited). Nasality level of the participants’ speech was measured by nasometer and reflected by nasalance scores (in %). Errorless learners practiced producing hypernasal speech with a threshold nasalance score of 10% at the beginning, which gradually increased to a threshold of 50% at the end. The same set of threshold targets were presented to errorful learners but in a reversed order. Errors were defined by the proportion of speech with a nasalance score below the threshold. The results showed that, relative to errorful learners, errorless learners displayed fewer errors (50.7% vs. 17.7%) and a higher mean nasalance score (31.3% vs. 46.7%) during the acquisition phase. Furthermore, errorless learners outperformed errorful learners in both retention and novel transfer tests. Acknowledgment: Supported by The University of Hong Kong Strategic Research Theme for Sciences of Learning © 2012 Acoustical Society of Americapublished_or_final_versio

Proceedings of the First International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Kruger Park, 8-10 April 2002.This lecture is a principle-based review of a growing body of fundamental work stimulated by multiple opportunities to optimize geometric form (shape, structure, configuration, rhythm, topology, architecture, geography) in systems for heat and fluid flow. Currents flow against resistances, and by generating entropy (irreversibility) they force the system global performance to levels lower than the theoretical limit. The system design is destined to remain imperfect because of constraints (finite sizes, costs, times). Improvements can be achieved by properly balancing the resistances, i.e., by spreading the imperfections through the system. Optimal spreading means to endow the system with geometric form. The system construction springs out of the constrained maximization of global performance. This 'constructal' design principle is reviewed by highlighting applications from heat transfer engineering. Several examples illustrate the optimized internal structure of convection cooled packages of electronics. The origin of optimal geometric features lies in the global effort to use every volume element to the maximum, i.e., to pack the element not only with the most heat generating components, but also with the most flow, in such a way that every fluid packet is effectively engaged in cooling. In flows that connect a point to a volume or an area, the resulting structure is a tree with high conductivity branches and low-conductivity interstices.tm201

14th Conference on Dynamical Systems Theory and Applications DSTA 2017 ABSTRACTS

From Preface: This is the fourteen time when the conference “Dynamical Systems – Theory and Applications” gathers a numerous group of outstanding scientists and engineers, who deal with widely understood problems of theoretical and applied dynamics. Organization of the conference would not have been possible without a great effort of the staff of the Department of Automation, Biomechanics and Mechatronics. The patronage over the conference has been taken by the Committee of Mechanics of the Polish Academy of Sciences and the Ministry of Science and Higher Education. It is a great pleasure that our invitation has been accepted by so many people, including good colleagues and friends as well as a large group of researchers and scientists, who decided to participate in the conference for the first time. With proud and satisfaction we welcome nearly 250 persons from 38 countries all over the world. They decided to share the results of their research and many years experiences in the discipline of dynamical systems by submitting many very interesting papers. This booklet contains a collection of 375 abstracts, which have gained the acceptance of referees and have been qualified for publication in the conference proceedings [...]