A Dye-Tracer Technique for Experimentally Obtaining Impingement Characteristics of Arbitrary Bodies and a Method for Determining Droplet Size Distribution

A dye-tracer technique has been developed whereby the quantity of dyed water collected on a blotter-wrapped body exposed to an air stream containing a dyed-water spray cloud can be colorimetrically determined in order to obtain local collection efficiencies, total collection efficiency, and rearward extent of impingement on the body. In addition, a method has been developed whereby the impingement characteristics obtained experimentally for a body can be related to theoretical impingement data for the same body in order to determine the droplet size distribution of the impinging cloud. Several cylinders, a ribbon, and an aspirating device to measure cloud liquid-water content were used in the studies presented herein for the purpose of evaluating the dye-tracer technique. Although the experimental techniques used in the dye-tracer technique require careful control, the methods presented herein should be applicable for any wind tunnel provided the humidity of the air stream can be maintained near saturation

Modelling of the evolution of a droplet cloud in a turbulent flow

The effects of droplet inertia and turbulent mixing on the droplet number density distribution in a turbulent flow field are studied. A formulation of the turbulent convective diffusion equation for the droplet number density, based on the modified Fully Lagrangian Approach, is proposed. The Fully Lagrangian Approach for the dispersed phase is extended to account for the Hessian of transformation from Eulerian to Lagrangian variables. Droplets with moderate inertia are assumed to be transported and dispersed by large scale structures of a filtered field in the Large Eddy Simulation (LES) framework. Turbulent fluctuations, not visible in the filtered solution for the droplet velocity field, induce an additional diffusion mass flux and hence additional dispersion of the droplets. The Lagrangian formulation of the transport equation for the droplet number density and the modified Fully Lagrangian Approach (FLA) make it possible to resolve the flow regions with intersecting droplet trajectories in the filtered flow field. Thus, we can cope successfully with the problems of multivalued filtered droplet velocity regions and caustic formation. The spatial derivatives for the droplet number density are calculated by projecting the FLA solution on the Eulerian mesh, resulting in a hybrid Lagrangian–Eulerian approach to the problem. The main approximations for the method are supported by the calculation of droplet mixing in an unsteady one-dimensional flow field formed by large-scale oscillations with an imposed small-scale modulation. The results of the calculations for droplet mixing in decaying homogeneous and isotropic turbulence are validated by the results of Direct Numerical Simulations (DNS) for several values of the Stokes number

Turbulence-induced cloud voids: observation and interpretation

The phenomenon of “cloud voids”, i.e., elongated volumes inside a cloud that are devoid of droplets, was observed with laser sheet photography in clouds at a mountain-top station. Two experimental cases, similar in turbulence conditions yet with diverse droplet size distributions and cloud void prevalence, are reported. A theoretical explanation is proposed based on the study of heavy inertial sedimenting particles inside a Burgers vortex. A general conclusion regarding void appearance is drawn from theoretical analysis. Numerical simulations of polydisperse droplet motion with realistic vortex parameters and Mie scattering visual effects accounted for can explain the presence of voids with sizes similar to that of the observed ones. Clustering and segregation effects in a vortex tube are discussed for reasonable cloud conditions

Pattern Anomaly Detection based on Sequence-to-Sequence Regularity Learning

Anomaly detection in traffic surveillance videos is a challenging task due to the ambiguity of anomaly definition and the complexity of scenes. In this paper, we propose to detect anomalous trajectories for vehicle behavior analysis via learning regularities in data. First, we train a sequence-to-sequence model under the autoencoder architecture and propose a new reconstruction error function for model optimization and anomaly evaluation. As such, the model is forced to learn the regular trajectory patterns in an unsupervised manner. Then, at the inference stage, we use the learned model to encode the test trajectory sample into a compact representation and generate a new trajectory sequence in the learned regular pattern. An anomaly score is computed based on the deviation of the generated trajectory from the test sample. Finally, we can find out the anomalous trajectories with an adaptive threshold. We evaluate the proposed method on two real-world traffic datasets and the experiments show favorable results against state-of-the-art algorithms. This paper\u27s research on sequence-to-sequence regularity learning can provide theoretical and practical support for pattern anomaly detection

Revealing Dynamic Mechanisms of Cell Fate Decisions From Single-Cell Transcriptomic Data.

Cell fate decisions play a pivotal role in development, but technologies for dissecting them are limited. We developed a multifunction new method, Topographer, to construct a "quantitative" Waddington's landscape of single-cell transcriptomic data. This method is able to identify complex cell-state transition trajectories and to estimate complex cell-type dynamics characterized by fate and transition probabilities. It also infers both marker gene networks and their dynamic changes as well as dynamic characteristics of transcriptional bursting along the cell-state transition trajectories. Applying this method to single-cell RNA-seq data on the differentiation of primary human myoblasts, we not only identified three known cell types, but also estimated both their fate probabilities and transition probabilities among them. We found that the percent of genes expressed in a bursty manner is significantly higher at (or near) the branch point (~97%) than before or after branch (below 80%), and that both gene-gene and cell-cell correlation degrees are apparently lower near the branch point than away from the branching. Topographer allows revealing of cell fate mechanisms in a coherent way at three scales: cell lineage (macroscopic), gene network (mesoscopic), and gene expression (microscopic)

Mixing and Demixing Processes in Multiphase Flows With Application to Propulsion Systems

A workshop on transport processes in multiphase flow was held at the Marshall Space Flight Center on February 25 and 26, 1988. The program, abstracts and text of the presentations at this workshop are presented. The objective of the workshop was to enhance our understanding of mass, momentum, and energy transport processes in laminar and turbulent multiphase shear flows in combustion and propulsion environments

Microfluidics: Fluid physics at the nanoliter scale

Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Péclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world

Numerical Analysis of a Single Droplet Combustion: Jet-A1, N-Dodecane, N-Hexadecane

There has been an increase in concerns about the planet earth and its resources throughout the past decades. The dependency on fossil fuels created a critical dilemma since transportation is currently fueled by traditional, not sustainable power sources. The originated climate crisis on fossil fuels demands action from mankind, specifically concerning the research on alternative ways of fueling the current methods of transportation. The implementation of Biofuels in transportation encourages future scientists and engineers as a realistic option among other different paths constructed to develop sustainable fuels. The study of the injection, impinging, evaporation, and combustion allows the improvement of the burning characteristics assuming a specific fuel within a combustion chamber. These investigations of the combustion and evaporation procedures improve the burning droplet performance and thus reduce the emitted emissions under the same circumstances. This study intends to numerically simulate the single droplet evaporation and combustion of fuel droplets in a drop tube furnace (DTF) that has the capacity of varying the ambient temperature. The numerical approach simplifies the physical phenomena by employing an Eulerian-Lagrangian approach, considering a discrete and a continuous phase, which is further accomplished while running in a CFD software. The continuous phase is computed recurring to a turbulence modeling, while the dispersed phase is separately computed using the discrete phase model. The computation of the combustion phenomenon is deeply related to the evaporation of the discrete phase employing the non-premixed combustion provided by the operating software. There is a 2D planar simplification of the 3D axisymmetric experimental cylinder followed by the respective discretization of the mathematical equations and pressure-velocity coupling. This work numerically simulates the burning phenomenon of n-dodecane, jet fuel, and nhexadecane single droplets. The obtained results of the droplet size reduction relating to time display agreement with the d 2 law and respective experimentally obtained data. The acquired outcomes also allow the establishment of correlations between the combustion characteristics and the droplet physics properties, such as velocity, displaying a reduction of the droplet velocity alongside the shrink of the droplet diameter. This information is visible for different temperature environments and fuels, suggesting a physical association. Besides this interpretation, the imposed droplet initial velocity variations (1.0 m/s until 1.3 m/s) do not affect the combustion characteristics outcomes. This study demonstrates a precise relation between the ambient temperature of the drop tube furnace (DTF) and the improvement of the combustion process and burning properties. Additionally, the chemical composition of the fuels influences the combustion characteristics and their performances. Overall, the numerical performed simulation can be improved and thus approximate the implemented simulation to the occurring physical event, allowing the development of the additional knowledge in this thematic.As preocupações relativamente ao planeta Terra e os seus recursos naturais têm vindo a aumentar nas últimas décadas. A dependência de combustíveis fósseis provocou um problema delicado, visto que o transporte atualmente é essencialmente abastecido por fontes de energia tradicionais e não sustentáveis. A crise climática originada devido à utilização de combustíveis fósseis exige ações da humanidade, especificamente no que se refere à pesquisa de formas inovadoras de abastecer os atuais meios de transporte. A implementação de biocombustíveis nos transportes atuais, desperta o interesse dos cientistas e engenheiros como uma opção viável entre os distintos possíveis caminhos para se desenvolver opções de combustíveis sustentáveis. O estudo de fenómenos como a injeção, a colisão de gotas, a evaporação e a combustão permite a melhoria das características de queima de um determinado combustível dentro da câmara de combustão, porém o estudo da combustão e da evaporação revela ser a forma mais eficiente para reduzir as emissões e melhorarando significativamente o desempenho da queima. Este estudo pretende simular numericamente a evaporação e combustão de gotículas de combustível dentro de um forno tubular com capacidade de variar a temperatura ambiente. A abordagem numérica simplifica os fenómenos físicos, empregando uma abordagem EulerLagrange realizada com auxílio de um software de Dinâmica de Fluidos Computacional (DFC). A fase contínua é calculada recorrendo a um modelo de turbulência, enquanto a fase dispersa é calculada separadamente utilizando um modelo de fase discreta. O cálculo do fenómeno de combustão está profundamente relacionado à evaporação da fase discreta empregando posteriormente o modelo de combustão fornecida pelo software utilizado. Adicionalmente, existe uma simplificação 2D do domínio e a respetiva discretização das equações matemáticas em conformidade com o acoplamento pressão-velocidade do modelo numérico. Neste trabalho o fenómeno de queima de gotas é simulado para gotas isoladas de n-dodecano, jet fuel e n-hexadecano. Os resultados obtidos relativamente à evolução temporal do diâmetro da gota mostram concordância com a lei d 2 e respetivos dados obtidos experimentalmente. Os resultados adquiridos também permitem uma correlação das características da combustão e a dinâmica das gotas, apresentando uma redução entre velocidade das gotas e a respetiva redução do diâmetro das gotas. Esta relação ocorre para diferentes temperaturas do meio continuo e utilizando diferentes combustíveis, sugerindo uma associação física. Além dessa interpretação, a variação da velocidade inicial da gota não afeta os resultados das características de combustão. Durante o estudo é demonstrada uma relação entre a temperatura ambiente do forno tubular e a indução do processo de combustão bem como as propriedades de queima da gota. Além disso, as composições químicas dos combustíveis utilizados aparenta influenciar as características de combustão da gota e o seu desempenho global na câmara de combustão. De uma forma geral, a simulação numérica poderá ser optimizada em trabalhos futuros e aproximar a simulação ao fenómeno fisico, permitindo assim o desenvolvimento de conhecimentos nesta temática

Advances in Modeling of Fluid Dynamics

This book contains twelve chapters detailing significant advances and applications in fluid dynamics modeling with focus on biomedical, bioengineering, chemical, civil and environmental engineering, aeronautics, astronautics, and automotive. We hope this book can be a useful resource to scientists and engineers who are interested in fundamentals and applications of fluid dynamics