Laminar counterflow parallel-plate heat exchangers: Exact and approximate solutions

Multilayered, counterflow, parallel-plate heat exchangers are analyzed numerically and theoretically. The analysis, carried out for constant property fluids, considers a hydrodynamically developed laminar flow and neglects longitudinal conduction both in the fluid and in the plates. The solution for the temperature field involves eigenfunction expansions that can be solved in terms of Whittaker functions using standard symbolic algebra packages, leading to analytical expressions that provide the eigenvalues numerically. It is seen that the approximate solution obtained by retaining the first two modes in the eigenfunction expansion provides an accurate representation for the temperature away from the entrance regions, specially for long heat exchangers, thereby enabling simplified expressions for the wall and bulk temperatures, local heat-transfer rate, overall heat-transfer coefficient, and outlet bulk temperatures. The agreement between the numerical and theoretical results suggests the possibility of using the analytical solutions presented herein as benchmark problems for computational heat-transfer codes.

FTCS finite difference scheme GPGPU parallel computing for the heat conduction equation = Programación en paralelo GPGPU del método en diferencias finitas FTCS para la ecuación del calor

En el presente artículo se muestran las ventajas de la programación en paralelo resolviendo numéricamente la ecuación del calor en dos dimensiones a través del método de diferencias finitas explícito centrado en el espacio FTCS. De las conclusiones de este trabajo se pone de manifiesto la importancia de la programación en paralelo para tratar problemas grandes, en los que se requiere un elevado número de cálculos, para los cuales la programación secuencial resulta impracticable por el elevado tiempo de ejecución. En la primera sección se describe brevemente los conceptos básicos de programación en paralelo. Seguidamente se resume el método de diferencias finitas explícito centrado en el espacio FTCS aplicado a la ecuación parabólica del calor. Seguidamente se describe el problema de condiciones de contorno y valores iniciales específico al que se va a aplicar el método de diferencias finitas FTCS, proporcionando pseudocódigos de una implementación secuencial y dos implementaciones en paralelo. Finalmente tras la discusión de los resultados se presentan algunas conclusiones. In this paper the advantages of parallel computing are shown by solving the heat conduction equation in two dimensions with the forward in time central in space (FTCS) finite difference method. Two different levels of parallelization are consider and compared with traditional serial procedures. We show in this work the importance of parallel computing when dealing with large problems that are impractical or impossible to solve them with a serial computing procedure. In the first section a summary of parallel computing approach is presented. Subsequently, the forward in time central in space (FTCS) finite difference method for the heat conduction equation is outline, describing how the heat flow equation is derived in two dimensions and the particularities of the finite difference numerical technique considered. Then, a specific initial boundary value problem is solved by the FTCS finite difference method and serial and parallel pseudo codes are provided. Finally after results are discussed some conclusions are presented.

Bilingual Polytechnic Dictionary of Metaphors: Spanish to English

This paper provides an overview of an ongoing research project work: “A Polytechnical Bilingual Dictionary of Metaphors: Spanish-English/English-Spanish” done by the UPM consolidated research group “DISCYT” (Estudios Cognitivos del Discurso Científico-Técnico). A detailed explanation of the method adopted to identify key metaphors collected from the different subject areas is included. Drawing from recognized empirical methods (Pragglejaz 2007, Cameron 2007, Steen 2007), the examples have been examined according to the main tenets of conceptual metaphor and conceptual integration theory (Deignan 2005, Gibbs 2008, Lakoff 1993, Lakoff & Johnson 1999, Steen 2007, Fauconnier & Turner 2008). This forthcoming dictionary comprises metaphors of over 10 scientific and technical areas such as Aeronautical engineering, Agronomy, Architecture, Biotechnology, Civil engineering, Geology and Mining, Mechanical engineering, Nanotechnology, Naval and Maritime engineering, Sports and Telecommunications. In this paper, we focus on the study of examples taken from civil engineering, materials engineering and naval engineering. Representative cases are analyzed from several points of view (multimodal metaphor, linguistic information strategies and translation into target language) highlighting cross linguistic variations between Spanish and English.

Parallel sets and morphological measurements of CT images of soil pore structure in a vineyard

Important physical and biological processes in soil-plant-microbial systems are dominated by the geometry of soil pore space, and a correct model of this geometry is critical for understanding them. We analyze the geometry of soil pore space with the X-ray computed tomography (CT) of intact soil columns. We present here some preliminary results of our investigation on Minkowski functionals of parallel sets to characterize soil structure. We also show how the evolution of Minkowski morphological measurements of parallel sets may help to characterize the influence of conventional tillage and permanent cover crop of resident vegetation on soil structure in a Spanish Mediterranean vineyard.

Neurite, a finite difference large scale parallel program for the simulation of the electrical signal propagation in neurites under mechanical loading

With the growing body of research on traumatic brain injury and spinal cord injury, computational neuroscience has recently focused its modeling efforts on neuronal functional deficits following mechanical loading. However, in most of these efforts, cell damage is generally only characterized by purely mechanistic criteria, function of quantities such as stress, strain or their corresponding rates. The modeling of functional deficits in neurites as a consequence of macroscopic mechanical insults has been rarely explored. In particular, a quantitative mechanically based model of electrophysiological impairment in neuronal cells has only very recently been proposed (Jerusalem et al., 2013). In this paper, we present the implementation details of Neurite: the finite difference parallel program used in this reference. Following the application of a macroscopic strain at a given strain rate produced by a mechanical insult, Neurite is able to simulate the resulting neuronal electrical signal propagation, and thus the corresponding functional deficits. The simulation of the coupled mechanical and electrophysiological behaviors requires computational expensive calculations that increase in complexity as the network of the simulated cells grows. The solvers implemented in Neurite-explicit and implicit-were therefore parallelized using graphics processing units in order to reduce the burden of the simulation costs of large scale scenarios. Cable Theory and Hodgkin-Huxley models were implemented to account for the electrophysiological passive and active regions of a neurite, respectively, whereas a coupled mechanical model accounting for the neurite mechanical behavior within its surrounding medium was adopted as a link between lectrophysiology and mechanics (Jerusalem et al., 2013). This paper provides the details of the parallel implementation of Neurite, along with three different application examples: a long myelinated axon, a segmented dendritic tree, and a damaged axon. The capabilities of the program to deal with large scale scenarios, segmented neuronal structures, and functional deficits under mechanical loading are specifically highlighted.

Morphological Functions with Parallel Sets for the Pore Space of X-ray CT Images of Soil Columns

During the last few decades, new imaging techniques like X-ray computed tomography have made available rich and detailed information of the spatial arrangement of soil constituents, usually referred to as soil structure. Mathematical morphology provides a plethora of mathematical techniques to analyze and parameterize the geometry of soil structure. They provide a guide to design the process from image analysis to the generation of synthetic models of soil structure in order to investigate key features of flow and transport phenomena in soil. In this work, we explore the ability of morphological functions built over Minkowski functionals with parallel sets of the pore space to characterize and quantify pore space geometry of columns of intact soil. These morphological functions seem to discriminate the effects on soil pore space geometry of contrasting management practices in a Mediterranean vineyard, and they provide the first step toward identifying the statistical significance of the observed differences.

El paralelo. Bosquejo de un método gráfico = The parallel. Sketch of a graphical method

El paralelo gráfico ha sido -y continúa siendo- un excepcional método para conocer, aprender, investigar y difundir la forma arquitectónica y urbana. Aquí intentamos esbozar los principios que rigen su elaboración y echar un leve vistazo a alguno de los jalones de su intensa historia, que merecería una atención más pausada.

Parallel Virtual Urban Workshops A “resasonable-cost” methodology for academic internationalization in problem-solving oriented postgraduate subjects

Descripción y análisis critic de una metodología de taller de posgrado a realizar entre dos universidades en idioma ingles y con el apoyo de las nuevas tecnologías

Multiphase Parallel Interleaved And Primary-Parallel Secondary-Series Forward Micro-Inverter Comparison.

Using multiphase technique is interesting in PV AC-module application due to light-load efficiency improvement by applying phase shedding, and the possibility of low-profile implementation. This paper presents a comparison, in terms of size and efficiency, of the parallel interleaved and the parallel-series connected multiphase configurations, as a function of the number of phases, for a forward micro-inverter operated in DCM. 8-phase prototypes of both multiphase configurations are built and compared between them and with the single phase forward micro-inverter, validating the presented analysis.

Comparative theoretical analysis between parallel and perpendicular geometries for 2D particle patterning in photovoltaic ferroelectric substrates

This paper describes the dielectrophoretic potential created by the evanescent electric field acting on a particle near a photovoltaic crystal surface depending on the crystal cut. This electric field is obtained from the steady state solution of the Kukhtarev equations for the photovoltaic effect, where the diffusion term has been disregarded. First, the space charge field generated by a small, square, light spot where d _ l (being d a side of the square and l the crystal thickness) is studied. The surface charge density generated in both geometries is calculated and compared as their relation determines the different properties of the dielectrophoretic potential for both cuts. The shape of the dielectrophoretic potential is obtained and compared for several distances to the sample. Afterwards other light patterns are studied by the superposition of square spots, and the resulting trapping profiles are analysed. Finally the surface charge densities and trapping profiles for different d/l relations are studied.

Advanced Control Strategies for a 6 DoF Hydraulic Parallel Robot Based on the Dynamic Model

Nowadays robots have made their way into real applications that were prohibitive and unthinkable thirty years ago. This is mainly due to the increase in power computations and the evolution in the theoretical field of robotics and control. Even though there is plenty of information in the current literature on this topics, it is not easy to find clear concepts of how to proceed in order to design and implement a controller for a robot. In general, the design of a controller requires of a complete understanding and knowledge of the system to be controlled. Therefore, for advanced control techniques the systems must be first identified. Once again this particular objective is cumbersome and is never straight forward requiring of great expertise and some criteria must be adopted. On the other hand, the particular problem of designing a controller is even more complex when dealing with Parallel Manipulators (PM), since their closed-loop structures give rise to a highly nonlinear system. Under this basis the current work is developed, which intends to resume and gather all the concepts and experiences involve for the control of an Hydraulic Parallel Manipulator. The main objective of this thesis is to provide a guide remarking all the steps involve in the designing of advanced control technique for PMs. The analysis of the PM under study is minced up to the core of the mechanism: the hydraulic actuators. The actuators are modeled and experimental identified. Additionally, some consideration regarding traditional PID controllers are presented and an adaptive controller is finally implemented. From a macro perspective the kinematic and dynamic model of the PM are presented. Based on the model of the system and extending the adaptive controller of the actuator, a control strategy for the PM is developed and its performance is analyzed with simulation.

El tiempo como sustancia de la forma : una aproximación al Museo de Arte Romano de Mérida desde los presupuestos del vitalismo

El modelo dominante durante la Era Moderna asume la presencia del hombre como sujeto dentro del gran engranaje mecánico del Cosmos. Asimismo, recoge una idea acerca del ser natural dentro de la tradición ontológica iniciada por el eleatismo presocrático que concibe a éste como lo inmutable y estático frente al cambio y al movimiento, los cuales se constituyen como meras apariencias. Durante el periodo anterior a la aparición de los grandes filósofos griegos se produce una transformación en donde, de la inicial cosmología vinculada a un tiempo primordial, se pasará a una visión del Universo como ente indestructible, atemporal, inmutable, perfecto, geométrico y espacial. Si en Demócrito se admite un universo sometido al azar y a la necesidad, en Platón el Universo sólo atiende a la necesidad. Este modelo ontológico se pone en entredicho cuando el hombre ya no es concebido como pieza de un sistema más amplio, sino como centro radical del pensamiento. La condición radical del hombre es entonces su propia vida, siendo éste el concepto troncal del denominado vitalismo cuyo más influyente representante en España es José Ortega y Gasset. El estatismo del ser –del hombre- pierde sentido; en palabras del propio Ortega, “no es un ser sino un estar siendo” lo que caracteriza a la vida humana. La razón cartesiana es ahora la razón vital y su objeto de estudio no es la naturaleza sino el propio devenir, es decir, el tiempo, la historia. Este planteamiento es fundamental para comprender el edificio que es objeto de este estudio, el Museo de Arte Romano de Mérida (1980-1985) de Rafael Moneo. Por ello el concepto de tiempo es utilizado como marco y estructura de la presente tesis, a sabiendas del notorio y muy significativo papel que este edificio desempeñó en la carrera de su autor y en el panorama nacional e internacional de la arquitectura y de la museología. Este proyecto nos permite acercarnos al pensamiento de su autor a través de un edificio que, aun habiendo sido ampliamente reconocido, no cuenta con un estudio suficientemente exhaustivo que recoja la amplitud y riqueza que encierra. Esta tesis no es un compendio de lo que ya se ha investigado sobre Mérida; es una aproximación global e interpretativa cuyo sentido sólo puede concebirse al vislumbrar la estructura completa de la misma en sintonía con el "lógos" vital, histórico y narrativo que el proyecto encierra. Se revisará la concepción histórica según la cual, la configuración espacial de la forma habría tenido primacía respecto a su configuración temporal, al remitir esta última a una condición espacializada y circunstancial. La componente vicaria de la circunstancia será elevada por Ortega a la categoría de esencial, visualizando así una paradoja cuya reformulación nos lleva a la concepción de un tiempo sustancial. El diccionario de la RAE, en su tercera acepción, define la sustancia como “aquello que permanece en algo que cambia”, lo cual nos remite al pensamiento antiguo. Se mostrará que lo que permanece no necesariamente implica una concepción estática y eleática de la forma, que la arquitectura esencial no es unívocamente la arquitectura atemporal del platonismo y que cabe concebir la "firmitas" desde la atención a la "durée" bergsoniana. Al asociar tradicionalmente la sustancia con el referido estatismo, se margina al tiempo y a la duración a lo no sustancial; por ello, se tratará de aproximar los términos de tiempo y sustancia para definir la forma. Ello conllevará al estudio de las notables patologías derivadas de la asunción de un tiempo cronológico en nuestra contemporaneidad frente a las cuales, las intuiciones contenidas en Mérida, se alinearán con la actual actitud revisionista en el ámbito del pensamiento filosófico y científico. En Mérida, la memoria recogerá los aspectos de la conciencia así como los aspectos vinculados a la experiencia íntima y colectiva como soporte para la consecución de un discurso. La dualidad entre intuición e inteligencia será recogida por Moneo con idea de trascender su incomunicabilidad, mediante una operación que consistirá en la reivindicación de una memoria irreductible, cuya morada estaría incardinada en el propio tiempo de la duración y de la vida y no en la espacialidad coextensiva del presente y de la acción funcional sobre la materia. Moneo asumirá el papel de la memoria como condición central de una forma que se encarnará al concebirse como un teatro. En la respuesta a la contradicción entre el hecho físico y el efecto psíquico de la experiencia humana residirá la pertinencia de un tiempo narrativo. Será entonces el lenguaje el encargado de aportar sentido a la obra mediante el recurso fundado en la dramatización de la experiencia, es decir, a través de una conexión entre la conciencia íntima y el carácter social y colectivo intrínseco en la arquitectura. ABSTRACT TIME AS A SUBSTANCE OF FORM. AN APPROACH TO THE ROMAN ART MUSEUM OF MÉRIDA FROM THE VIEWPOINT OF VITALISM. The dominant model during the Modern Era placed man as a subject inside the great mechanism of the cosmos. It is also based in an idea about natural being within the ontological tradition initiated by the pre-Socratic Eleatism that conceives it as something immutable and static in respect with change and movement, which are considered as mere appearances. Prior to the emergence of the great Greek philosophers occurred a transformation where concepts of cosmology linked to a primordial time, changed to a view of the universe as indestructible, timeless, unchanging, perfect, geometric and spatial. If Democritus accepted a universe subjected to randomness and necessity, Plato thought that the universe only worked by necessity. This ontological model is called into question when man is not conceived as a piece of a broader system, but as a radical center of thinking. The radical condition of man then is his own life. This is the core concept of so-called Vitalism, whose most influential representative in Spain was José Ortega y Gasset. The stillness of being – of man - loses its meaning; in the words of Ortega, “it is not being but being in progress” that characterizes human life. The Cartesian reason is now the vital reason and its subject of study is no longer nature but its own evolution, in other words, time and history. This approach is fundamental to understand the building which is the subject of this study, the Museum of Roman Art in Mérida (1980-1985) by Rafael Moneo. The concept of time is used as a framework and structure of this thesis, demonstrating the notorious and very significant role this building has implied in the career of its author and in the national and international panorama of architecture and museology. This project allow us to approach the thought of its author through a building that, even whilst widely recognized, does not yet have a sufficiently comprehensive study covering its breadth and richness. This thesis is not a compendium of what already has been researched on Merida; it is a global and interpretative approach whose meaning can only be conceived as a study of its complete structure in line with the vital, historical and narrative logos the project implies. We will review the historical idea where spatial configuration of the form would have had primacy with respect to temporary configuration, because the latter refers to a spatial and circumstantial condition. The vicarious nature of the circumstance will be elevated by Ortega to the category of essential, thus showing a paradox which reformulation leads us to the conception of a substantial time. The dictionary of the Spanish Royal Academy, in its third meaning, defines substance as "that which remains in something that changes". This is a reference to ancient thought. It will be shown that what remains does not necessarily imply a static and Eleatic conception of form. It will also be shown that the essential architecture is not uniquely the timeless architecture of Platonism and that it is possible to conceive the "firmitas" parallel to the "durée" of Henri Bergson. As a result of this traditional association between substance and stillness, it marginalizes the time and the duration to the non-substantial; for this reason, we will try to approach terms of time and substance to define the shape. This will involve studying significant pathologies resulting from an assumption of chronological time in our contemporary world against which, the insights contained in Merida, will be aligned with the current revisionist attitude in the fields of philosophical and scientific thought. In Merida, memory includes aspects of consciousness as well as aspects linked to the intimate and collective experience as a foundation for the achievement of discourse. The duality between intuition and intelligence is put forward by Moneo with the idea of transcending its lack of communication, by means of a resource consisting of the vindication of an irreducible memory, whose home would be embodied in the time of duration and life and not in the coextensive spatiality of the present and in the functional action on the matter. Moneo demonstrates the role of memory as a central condition of form as a theatre. In response to the contradiction between the physical fact and the psychological effect of human experience lies the relevance of narrative time. Language then assumes the responsibility of giving meaning to the work through the dramatization of experience, i.e., through a connection between the intimate consciousness and the intrinsic social and collective character of architecture.

Parallel Computer Vision Algorithms for Graphics Processing Units

La evolución de los teléfonos móviles inteligentes, dotados de cámaras digitales, está provocando una creciente demanda de aplicaciones cada vez más complejas que necesitan algoritmos de visión artificial en tiempo real; puesto que el tamaño de las señales de vídeo no hace sino aumentar y en cambio el rendimiento de los procesadores de un solo núcleo se ha estancado, los nuevos algoritmos que se diseñen para visión artificial han de ser paralelos para poder ejecutarse en múltiples procesadores y ser computacionalmente escalables. Una de las clases de procesadores más interesantes en la actualidad se encuentra en las tarjetas gráficas (GPU), que son dispositivos que ofrecen un alto grado de paralelismo, un excelente rendimiento numérico y una creciente versatilidad, lo que los hace interesantes para llevar a cabo computación científica. En esta tesis se exploran dos aplicaciones de visión artificial que revisten una gran complejidad computacional y no pueden ser ejecutadas en tiempo real empleando procesadores tradicionales. En cambio, como se demuestra en esta tesis, la paralelización de las distintas subtareas y su implementación sobre una GPU arrojan los resultados deseados de ejecución con tasas de refresco interactivas. Asimismo, se propone una técnica para la evaluación rápida de funciones de complejidad arbitraria especialmente indicada para su uso en una GPU. En primer lugar se estudia la aplicación de técnicas de síntesis de imágenes virtuales a partir de únicamente dos cámaras lejanas y no paralelas—en contraste con la configuración habitual en TV 3D de cámaras cercanas y paralelas—con información de color y profundidad. Empleando filtros de mediana modificados para la elaboración de un mapa de profundidad virtual y proyecciones inversas, se comprueba que estas técnicas son adecuadas para una libre elección del punto de vista. Además, se demuestra que la codificación de la información de profundidad con respecto a un sistema de referencia global es sumamente perjudicial y debería ser evitada. Por otro lado se propone un sistema de detección de objetos móviles basado en técnicas de estimación de densidad con funciones locales. Este tipo de técnicas es muy adecuada para el modelado de escenas complejas con fondos multimodales, pero ha recibido poco uso debido a su gran complejidad computacional. El sistema propuesto, implementado en tiempo real sobre una GPU, incluye propuestas para la estimación dinámica de los anchos de banda de las funciones locales, actualización selectiva del modelo de fondo, actualización de la posición de las muestras de referencia del modelo de primer plano empleando un filtro de partículas multirregión y selección automática de regiones de interés para reducir el coste computacional. Los resultados, evaluados sobre diversas bases de datos y comparados con otros algoritmos del estado del arte, demuestran la gran versatilidad y calidad de la propuesta. Finalmente se propone un método para la aproximación de funciones arbitrarias empleando funciones continuas lineales a tramos, especialmente indicada para su implementación en una GPU mediante el uso de las unidades de filtraje de texturas, normalmente no utilizadas para cómputo numérico. La propuesta incluye un riguroso análisis matemático del error cometido en la aproximación en función del número de muestras empleadas, así como un método para la obtención de una partición cuasióptima del dominio de la función para minimizar el error. ABSTRACT The evolution of smartphones, all equipped with digital cameras, is driving a growing demand for ever more complex applications that need to rely on real-time computer vision algorithms. However, video signals are only increasing in size, whereas the performance of single-core processors has somewhat stagnated in the past few years. Consequently, new computer vision algorithms will need to be parallel to run on multiple processors and be computationally scalable. One of the most promising classes of processors nowadays can be found in graphics processing units (GPU). These are devices offering a high parallelism degree, excellent numerical performance and increasing versatility, which makes them interesting to run scientific computations. In this thesis, we explore two computer vision applications with a high computational complexity that precludes them from running in real time on traditional uniprocessors. However, we show that by parallelizing subtasks and implementing them on a GPU, both applications attain their goals of running at interactive frame rates. In addition, we propose a technique for fast evaluation of arbitrarily complex functions, specially designed for GPU implementation. First, we explore the application of depth-image–based rendering techniques to the unusual configuration of two convergent, wide baseline cameras, in contrast to the usual configuration used in 3D TV, which are narrow baseline, parallel cameras. By using a backward mapping approach with a depth inpainting scheme based on median filters, we show that these techniques are adequate for free viewpoint video applications. In addition, we show that referring depth information to a global reference system is ill-advised and should be avoided. Then, we propose a background subtraction system based on kernel density estimation techniques. These techniques are very adequate for modelling complex scenes featuring multimodal backgrounds, but have not been so popular due to their huge computational and memory complexity. The proposed system, implemented in real time on a GPU, features novel proposals for dynamic kernel bandwidth estimation for the background model, selective update of the background model, update of the position of reference samples of the foreground model using a multi-region particle filter, and automatic selection of regions of interest to reduce computational cost. The results, evaluated on several databases and compared to other state-of-the-art algorithms, demonstrate the high quality and versatility of our proposal. Finally, we propose a general method for the approximation of arbitrarily complex functions using continuous piecewise linear functions, specially formulated for GPU implementation by leveraging their texture filtering units, normally unused for numerical computation. Our proposal features a rigorous mathematical analysis of the approximation error in function of the number of samples, as well as a method to obtain a suboptimal partition of the domain of the function to minimize approximation error.

Grid-Connected Forward Microinverter With Primary-Parallel Secondary-Series Transformer

This paper presents a primary-parallel secondaryseries multicore forward microinverter for photovoltaic ac-module application. The presented microinverter operates with a constant off-time boundary mode control, providing MPPT capability and unity power factor. The proposed multitransformer solution allows using low-profile unitary turns ratio transformers. Therefore, the transformers are better coupled and the overall performance of the microinverter is improved. Due to the multiphase solution, the number of devices increases but the current stress and losses per device are reduced contributing to an easier thermal management. Furthermore, the decoupling capacitor is split among the phases, contributing to a low-profile solution without electrolytic capacitors suitable to be mounted in the frame of a PV module. The proposed solution is compared to the classical parallel-interleaved approach, showing better efficiency in a wide power range and improving the weighted efficiency.

Parallel-up structure evidences the molecular directionality during biosynthesis of bacterial cellulose

The “parallel-up” packing in cellulose Iα and Iβ unit cells was experimentally demonstrated by a combination of direct-staining the reducing ends of cellulose chains and microdiffraction-tilting electron crystallographic analysis. Microdiffraction investigation of nascent bacterial cellulose microfibrils showed that the reducing end of the growing cellulose chains points away from the bacterium, and this provides direct evidence that polymerization by the cellulose synthase takes place at the nonreducing end of the growing cellulose chains. This mechanism is likely to be valid also for a number of processive glycosyltransferases such as chitin synthases, hyaluronan synthases, and proteins involved in the synthesis of nodulation factor backbones.