Compressibility, Turbulence and High Speed Flow introduces the reader to the field of compressible turbulence and compressible turbulent flows across a broad speed range, through a unique complimentary treatment of both the theoretical foundations and the measurement and analysis tools currently used.

The book provides the reader with the necessary background and current trends in the theoretical and experimental aspects of compressible turbulent flows and compressible turbulence. Detailed derivations of the pertinent equations describing the motion of such turbulent flows is provided and an extensive discussion of the various approaches used in predicting both free shear and wall bounded flows is presented. Experimental measurement techniques common to the compressible flow regime are introduced with particular emphasis on the unique challenges presented by high speed flows. Both experimental and numerical simulation work is supplied throughout to provide the reader with an overall perspective of current trends.

An introduction to current techniques in compressible turbulent flow analysis
An approach that enables engineers to identify and solve complex compressible flow challenges
Prediction methodologies, including the Reynolds-averaged Navier Stokes (RANS) method, scale filtered methods and direct numerical simulation (DNS)
Current strategies focusing on compressible flow control

Turbulence in Porous Media introduces the reader to the characterisation of turbulent flow, heat and mass transfer in permeable media, including analytical data and a review of available experimental data. Such transport processes occurring a relatively high velocity in permeable media are present in a number of engineering and natural flows. This new edition features a completely updated text including two new chapters exploring Turbulent Combustion and Moving Porous Media. De Lemos has expertly brought together a text that compiles, details, compares and evaluates available methodologies for modelling and simulating flow, providing an essential tour for engineering students working within the field as well as those working in chemistry, physics, applied mathematics, and geological and environmental sciences.
Brings together groundbreaking and complex research on turbulence in porous mediaExtends the original model to situations including reactive systemsNow discusses movement of the porous matrix"

A Membrane BioReactor (MBR) is the combination of a membrane process (e.g. microfiltration/ ultrafiltration) with a suspended growth bioreactor. When used with domestic wastewater, MBR processes can produce effluent of high enough quality to be discharged to waterways, or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants.

The MBR Book covers all important aspects of Membrane BioReactors in water and waste water treatment, from the fundamentals of the processes via design principles to MBR technologies. Industrial case studies help interpret actual results and give pointers for best practice. Useful appendices provide data on commercial membranes and international membrane organizations.

The MBR book enables readers to:

Understand the fundamental processes involved in membrane and biotreatment technologiesCompare and contrast design options and work through sample calculationsReview commercial MBR systems in terms of specific applicationsLearn from case studies involving domestic and industrial effluent treatment and recyclingAnalyze process design, operation, performance and maintenance to draw conclusions appropriate to their requirements

New to the second edition:

45% more content than the first edition.

Over 120 contributors from the academic research and municipal/industrial practitioner communities.

Review of MBR status in ten countries.

Expanded section on anaerobic MBRs, micropollutant fate and hybrid systems.

Simplified design methodology, with biokinetics for dynamic modelling and cost benefit analysis.

Expanded operation and maintenance section, informed by expert panel of practitioners offering more than 40 years combined experience.

Over 40 MBR membrane products described, with most of the technical specifications provided.

Over 50 case studies provided, including key design, performance, and operation and maintenance data in almost all cases.
Membrane Bioreactors are a major growth area in the water and waste water treatment industriesInternationally-known author, one of the leading senior experts in MBR researchPrinciples and practice, backed by industrial case studies"

The complete reference for engineers and designers working on pump design and development, or using centrifugal pumps in the field. This authoritative guide has been developed with access to the technical expertise of the leading centrifugal pump developer, Sulzer. In addition to providing the most comprehensive centrifugal pump theory and design reference, with detailed material on cavitation, erosion, selection of materials, rotor vibration behaviour and forces acting on pumps, the Handbook also covers key pumping applications topics and operational issues including operating performance in various types of circuitry, drives and acceptance testing.
* Enables readers to understand, specify and utilise centrifugal pumps more effectively
* A complete centrifugal pump engineering guide. The Handbook covers theory, design and operation, with an emphasis on providing a high levels of rigorous data, design detail, and industry-leading quality and efficiency solutions for high capital outlay pump plant users.
* Draws on the world-beating experience of Sulzer, the leading centrifugal pump developer, making this a particularly valuable resource
* Third edition, updated to cover the latest design and technology developments, applications, test and reliability procedures, cavitation, erosion, selection of materials, rotor vibration behaviour, operating performance in various types of circuitry, new drive technology and acceptance testing.

The materials mechanics of the controlled separation of a body into two or more parts - cutting -- using a blade or tool or other mechanical implement is a ubiquitous process in most engineering disciplines, be it mechanical, civil, medical, food or process based, and a surprising number of engineers are involved in cutting and the design of cutting tools. There are a wide range of books available on the cutting of metals, but no other text is devoted to the cutting of materials generally, the mechanics of which (toughness, fracture, deformation, plasticity, tearing, grating, chewing, etc) have wide ranging implications for engineers, medics, manufacturers, and process engineers, making this text of particular interest to a wide range of engineers and specialists.
* The only book to explain and unify the process and techniques of cutting in metals AND non-metals. The emphasis on biomaterials, plastics and non-metals will be of considerable interest to many, while the transfer of knowledge from non-metals fields offers important benefits to metalcutters.
* Comprehensive, written with this well-known author's lightness of touch, the book will attract the attention of many readers in this under-served subject
* The clarity of the text is further enhanced by detailed examples and case studies, from the grating of cheese on an industrial scale to the design of scalpels, supporting online materials data and FEM modelling material.

Design Engineering Manual offers a practical guide to the key principles of design engineering. It features a compilation of extracts from several books within the range of Design Engineering books in the Elsevier collection. The book is organized into 11 sections. Beginning with a review of the processes of product development and design, the book goes on to describe systematic ways of choosing materials and processes. It details the properties of modern metallic alloys including commercial steels, cast irons, superalloys, titanium alloys, structural intermetallic compounds, and aluminum alloys. The book explains the human/system interface; procedures to assess the risks associated with job and task characteristics; and environmental factors that may be encountered at work and affect behavior. Product liability and safety rules are discussed. The final section on design techniques introduces the design process from an inventors perspective to a more formal model called total design. It also deals with the behavior of plastics that influence the application of practical and complex engineering equations and analysis in the design of products.

Fire and combustion presents a significant engineering challenge to mechanical, civil and dedicated fire engineers, as well as specialists in the process and chemical, safety, buildings and structural fields. We are reminded of the tragic outcomes of 'untenable' fire disasters such as at King's Cross underground station or Switzerland's St Gotthard tunnel. In these and many other cases, computational fluid dynamics (CFD) is at the forefront of active research into unravelling the probable causes of fires and helping to design structures and systems to ensure that they are less likely in the future.
Computational fluid dynamics (CFD) is routinely used as an analysis tool in fire and combustion engineering as it possesses the ability to handle the complex geometries and characteristics of combustion and fire. This book shows engineering students and professionals how to understand and use this powerful tool in the study of combustion processes, and in the engineering of safer or more fire resistant (or conversely, more fire-efficient) structures.
No other book is dedicated to computer-based fire dynamics tools and systems. It is supported by a rigorous pedagogy, including worked examples to illustrate the capabilities of different models, an introduction to the essential aspects of fire physics, examination and self-test exercises, fully worked solutions and a suite of accompanying software for use in industry standard modeling systems.
-Computational Fluid Dynamics (CFD) is widely used in engineering analysis; this is the only book dedicated to CFD modeling analysis in fire and combustion engineering
-Strong pedagogic features mean this book can be used as a text forgraduate level mechanical, civil, structural and fire engineering courses, while its coverage of the latest techniques and industry standard software make it an important reference for researchers and professional engineers in the mechanical and structural sectors, and by fire engineers, safety consultants and regulators
-Strong author team (CUHK is a recognized centre of excellence in fire eng) deliver an expert package for students and professionals, showing both theory and applications. Accompanied by CFD modeling code and ready to use simulations to run in industry-standard ANSYS-CFX and Fluent software.

The self-assembled nanostructured materials described in this book offer a number of advantages over conventional material technologies in a wide range of sectors. World leaders in the field of self-organisation of nanostructures review the current status of research and development in the field, and give an account of the formation, properties, and self-organisation of semiconductor nanostructures. Chapters on structural, electronic and optical properties, and devices based on self-organised nanostructures are also included.
Future research work on self-assembled nanostructures will connect diverse areas of material science, physics, chemistry, electronics and optoelectronics. This book will provide an excellent starting point for workers entering the field and a useful reference to the nanostructured materials research community. It will be useful to any scientist who is involved in nanotechnology and those wishing to gain a view of what is possible with modern fabrication technology.
Mohamed Henini is a Professor of Applied Physics at the University of Nottingham. He has authored and co-authored over 750 papers in international journals and conference proceedings and is the founder of two international conferences. He is the Editor-in-Chief of "Microelectronics Journal" and has edited three previous Elsevier books.
Key Features:
- Contributors are world leaders in the field
- Brings together all the factors which are essential in self-organisation of quantum nanostructures
- Reviews the current status of research and development in self-organised nanostructured materials
- Provides a ready source of information on a wide range of topics
- Useful to any scientist who is involved in nanotechnology
- Excellent starting point for workers entering the field
- Serves as an excellent reference manual

Cryogenics is the study of low temperature interactions - temperatures well below those existing in the natural universe.
The book covers a large spectrum of experimental cases, including basic vacuum techniques, indispensable in cryogenics. Guidance in solving experimental problems and numerous numerical examples are given, as are examples of the applications of cryogenics in such areas as underground detectors and space applications. Updated tables of low-temperature data on materials are also presented, and the book is supplemented with a rich bibliography.
Key Features include:
- Experiments described in technical detail
- Description of newest cryogenic apparatus
- Applications in multidisciplinary areas
- Data on cryogenic properties of new materials
- Current reference review
Researchers (graduate and above) in the fields of physics, engineering and chemistry with an interest in the technology and applications of low-temperature measurements, will find this book invaluable.
Key Features include:

- Experiments described in technical detail
- Description of newest cryogenic apparatus
- Applications in multidisciplinary areas
- Data on cryogenic properties of new materials
- Current reference review

The primary objective of this work is to give the reader an understanding of stress wave behaviour while taking into account the dynamic constitutive equations of elastic-plastic solids. The author has combined a 'materials characteristics' approach with a 'singularity surface' approach in this work, which readers will find to be a novel and unique route to solving their problems.
* Helps engineers understand the effects and behavior of stress waves in various materials
* Aids in the process of engineering design, testing, and evaluation

Dr Theodore Nicholas ran the High Cycle Fatigue Program for the US Air Force between 1995 and 2003 at Wright-Patterson Air Force Base, and is one of the world s leading authorities on the subject, having authored over 250 papers in leading archival journals and books.
Bringing his plethora of expertise to this book, Dr Nicholas discusses the subject of high cycle fatigue (HCF) from an engineering viewpoint in response to a series of HCF failures in the USAF and the concurrent realization that HCF failures in general were taking place universally in both civilian and military engines.
Topic covered include:
- Constant life diagrams
- Fatigue limits under combined LCF and HCF
- Notch fatigue under HCF conditions
- Foreign object damage (FOD)
* Brings years of the Author's US Air Force experience in high cycle fatigue together in one text
* Discusses HCF in the context of recent international military and civilian engine failures

Plasticity is concerned with understanding the behavior of metals and alloys when loaded beyond the elastic limit, whether as a result of being shaped or as they are employed for load bearing structures.
Basic Engineering Plasticity delivers a comprehensive and accessible introduction to the theories of plasticity. It draws upon numerical techniques and theoretical developments to support detailed examples of the application of plasticity theory. This blend of topics and supporting textbook features ensure that this introduction to the science of plasticity will be valuable for a wide range of mechanical and manufacturing engineering students and professionals.
-Brings together the elements of the mechanics of plasticity most pertinent to engineers, at both the micro- and macro-levels
-Covers the theory and application of topics such as Limit Analysis, Slip Line Field theory, Crystal Plasticity, Sheet and Bulk Metal Forming, as well as the use of Finite Element Analysis
-Clear and well-organized with extensive worked engineering application examples, end of chapter exercises and a separate worked solutions manual

Constitutive modelling is the mathematical description of how materials respond to various loadings. This is the most intensely researched field within solid mechanics because of its complexity and the importance of accurate constitutive models for practical engineering problems.
Topics covered include:
Elasticity - Plasticity theory - Creep theory - The nonlinear finite element method - Solution of nonlinear equilibrium equations - Integration of elastoplastic constitutive equations - The thermodynamic framework for constitutive modelling Thermoplasticity - Uniqueness and discontinuous bifurcations
More comprehensive in scope than competitive titles, with detailed discussion of thermodynamics and numerical methods.
Offers appropriate strategies for numerical solution, illustrated by discussion of specific models.
Demonstrates each topic in a complete and self-contained framework, with extensive referencing.

Electroplating: Basic Principles, Processes and Practice offers an understanding of the theoretical background to electroplating, which is essential if the practical results are to be as required. This book is different in that it explains HOW the electrodeposition processes work, covering such topics as the elctrodeposition of composites, multilayers, whisker formation and giant magnetoresistive effects. The section on R & D approaches will be especially useful for organisations in the field. This is the first English language version of a well-known German language book from a prestigious author of international repute. Electroplating is an invaluable resource for manufacturers of coatings, electrochemists, metal finishers and their customers and academics in surface engineering.
.Offers an understanding of the theoretical background to electroplating
.Explains how the electrodeposition processes work
.Prestigious author of international repute

Advanced Control Engineering provides a complete course in control engineering for undergraduates of all technical disciplines. Starting with a basic overview of elementary control theory this text quickly moves on to a rigorous examination of more advanced and cutting edge date aspects such as robust and intelligent control, including neural networks and genetic algorithms.
With examples from aeronautical, marine and many other types of engineering, Roland Burns draws on his extensive teaching and practical experience presents the subject in an easily understood and applied manner. Control Engineering is a core subject in most technical areas.
Problems in each chapter, numerous illustrations and free Matlab files on the accompanying website are brought together to provide a valuable resource for the engineering student and lecturer alike.

Complete Course in Control Engineering
Real life case studies
Numerous problems

This book is a companion text to Active Control of Sound by P.A. Nelson and S.J. Elliott, also published by Academic Press.
It summarizes the principles underlying active vibration control and its practical applications by combining material from vibrations, mechanics, signal processing, acoustics, and control theory. The emphasis of the book is on the active control of waves in structures, the active isolation of vibrations, the use of distributed strain actuators and sensors, and the active control of structurally radiated sound. The feedforward control of deterministic disturbances, the active control of structural waves and the active isolation of vibrations are covered in detail, as well as the more conventional work on modal feedback. The principles of the transducers used as actuateors and sensors for such control strategies are also given an in-depth description.
The reader will find particularly interesting the two chapters on the active control of sound radiation from structures: active structural acoustic control. The reason for controlling high frequency vibration is often to prevent sound radiation, and the principles and practical application of such techniques are presented here for both plates and cylinders. The volume is written in textbook style and is aimed at students, practicing engineers, and researchers.
* Combines material from vibrations, signal processing, mechanics, and controls
* Summarizes new research in the field

"Analysis of Turbulent Flows" is written by one of the most prolific authors in the field of CFD. Professor of Aerodynamics at SUPAERO and Director of DMAE at ONERA, Professor Tuncer Cebeci calls on both his academic and industrial experience when presenting this work. Each chapter has been specifically constructed to provide a comprehensive overview of turbulent flow and its measurement. "Analysis of Turbulent Flows" serves as an advanced textbook for PhD candidates working in the field of CFD and is essential reading for researchers, practitioners in industry and MSc and MEng students.

The field of CFD is strongly represented by the following corporate organizations: Boeing, Airbus, Thales, United Technologies and General Electric. Government bodies and academic institutions also have a strong interest in this exciting field.
An overview of the development and application of computational fluid dynamics (CFD), with real applications to industryContains a unique section on short-cut methods - simple approaches to practical engineering problems

Generally, welding produces welding deformation and residual stress in the products, which influences the quality and performance of the products. Although many engineers and researchers have made great effort how to control these incidents, they have still remained unresolved. "Welding Deformation and Residual Stress Prevention" provides a unique computational approach to the prediction of the effects of deformation and residual stress on materials. The goal is to provide engineers and designers with the ability to create their own computational system for predicting and possibly avoiding the problem altogether.
The basic theories including "theory of elastic-plastic analysis" and "inherent strain theory," and analysis procedures are described using a simple three-bar model.

Online simulation software to perform basic analysis on welding mechanics

Examples of strategic methods and procedures are illustrated to have solved various welding-related problems encountered in the process of construction.

Appendices present data bases for welding residual stresses, temperature dependent material properties, etc. "

Computational elastohydrodynamics, a part of tribology, has existed happily enough for about fifty years without the use of accurate models for the rheology of the liquids used as lubricants. For low molecular weight liquids, such as low viscosity mineral oils, it has been possible to calculate, with precision, the film thickness in a concentrated contact provided that the pressure and temperature are relatively low, even when the pressure variation of viscosity is not accurately modelled in detail. Other successes have been more qualitative in nature, using effective properties which come from the fitting of parameters used in calculations to experimental measurements of the contact behaviour, friction or film thickness.
High Pressure Rheology for Quantitative Elastohydrodynamics is intended to provide a sufficiently accurate framework for the rheology of liquids at elevated pressure that it may be possible for computational elastohydrodynamics to discover the relationships between the behaviour of a lubricated concentrated contact and the measurable properties of the liquid lubricant. The required high-pressure measurement techniques are revealed in detail and data are presented for chemically well-defined liquids that may be used as quantitative reference materials.
* Presents the property relations required for a quantitative calculation of the tribological behaviour of lubricated concentrated contacts.
* Details of high-pressure experimental techniques.
* Complete description of the pressure and temperature dependence of viscosity for high pressures.
* Some little-known limitations on EHL modelling.

Small-scale gas turbines, known as Microturbines, represent an exciting new development in gas turbine technology. They can run in size from small, human-scale machines down to micro-sized mini-machines that can barely be seen by the naked eye. They also run a great diversity of fuel types, from various types of commercial gases to waste-generated gases. This new book by industry expert Claire Soares will fully describe the various types of microturbines, their applications, and their particular requirements for installation, maintenance and repair. It will explain how a microturbine the size of a refrigerator can power an entire school, hospital or small factory, which is particularly useful for onsite, remote installations. The book will also show how microturbines can be paired with one or more fuel cells to form a hybrid energy source, or can be teamed with any source of distributed power, such as a mall hydro-turbine or a wind turbine. Moreover, the reader will learn how microturbines can run on a variety of fuels that are far cruder than those required by most standard gas turbines; they can be made to run, for instance, using gas from a landfill or biomass source. The reader will find detailed information on costs, specifications, and maintenance and repair guidelines. Ample references and resources will provide the reader with tools for finding manufacturers and product specifications for their own particular needs.

.Covers major categories of microturbines, including factors common to their design, installation, operation, optimization, maintenance, and repair
.Invaluable guidance on market factors and economics affecting microturbines and their applications, particularly for distributed power generation
.Provides current case studies showing microturbines used in hybrid systems with fuel cells and other types of power generation systems"

For a long time, conventional reliability analyses have been oriented towards selecting the more reliable system and preoccupied with maximising the reliability of engineering systems. On the basis of counterexamples however, we demonstrate that selecting the more reliable system does not necessarily mean selecting the system with the smaller losses from failures! As a result, reliability analyses should necessarily be risk-based, linked with the losses from failures.
Accordingly, a theoretical framework and models are presented which form the foundations of the reliability analysis and reliability allocation linked with the losses from failures.
An underlying theme in the book is the basic principle for a risk-based design: the larger the cost of failure associated with a component, the larger its minimum necessary reliability level. Even identical components should be designed to different reliability levels if their failures are associated with different losses.
According to a classical definition, the risk of failure is a product of the probability of failure and the cost given failure. This risk measure however cannot describe the risk of losses exceeding a maximum acceptable limit. Traditionally the losses from failures have been 'accounted for' by the average production availability (the ratio of the actual production capacity and the maximum production capacity). As demonstrated in the book by using a simple counterexample, two systems with the same production availability can be characterised by very different losses from failures.
As an alternative, a new aggregated risk measure based on the cumulative distribution of the potential losses has been introducedand the theoretical framework for risk analysis based on the concept potential losses has also been developed. This new risk measure incorporates the uncertainty associated with the exposure to losses and the uncertainty in the consequences given the exposure. For repairable systems with complex topology, the distribution of the potential losses can be revealed by simulating the behaviour of systems during their life-cycle. For this purpose, fast discrete event-driven simulators are presented capable of tracking the potential losses for systems with complex topology, composed of a large number of components. The simulators are based on new, very efficient algorithms for system reliability analysis of systems comprising thousands of components.
An important theme in the book are the generic principles and techniques for reducing technical risk. These have been classified into three major categories: preventive (reducing the likelihood of failure), protective (reducing the consequences from failure) and dual (reducing both, the likelihood and the consequences from failure). Many of these principles (for example: avoiding clustering of events, deliberately introducing weak links, reducing sensitivity, introducing changes with opposite sign, etc.) are discussed in the reliability literature for the first time.
Significant space has been allocated to component reliability. In the last chapter of the book, several applications are discussed of a powerful equation which constitutes the core of a new theory of locally initiated component failure by flaws whose number is a random variable.
This book has been written with the intention to fill two big gaps in the reliability and riskliterature: the risk-based reliability analysis as a powerful alternative to the traditional reliability analysis and the generic principles for reducing technical risk. I hope that the principles, models and algorithms presented in the book will help to fill these gaps and make the book useful to reliability and risk-analysts, researchers, consultants, students and practising engineers.
- Offers a shift in the existing paradigm for conducting reliability analyses.
- Covers risk-based reliability analysis and generic principles for reducing risk.
- Provides a new measure of risk based on the distribution of the potential losses from failure as well as the basic principles for risk-based design.
- Incorporates fast algorithms for system reliability analysis and discrete-event simulators.
- Includes the probability of failure of a structure with complex shape expressed with a simple equation.

This practical reference provides in-depth information required to understand and properly estimate compressor capabilities and to select the proper designs. Engineers and students will gain a thorough understanding of compression principles, equipment, applications, selection, sizing, installation, and maintenance. The many examples clearly illustrate key aspects to help readers understand the "real world" of compressor technology.
Compressors: Selection and Sizing, third edition is completely updated with new API standards. Additions requested by readers include a new section on diaphragm compressors in the reciprocating compressors chapter, and a new section on rotor dynamics stability in the chapter on diaphragm compressors.
The latest technology is presented in the areas of efficiency, 3-D geometry, electronics, CAD, and the use of plant computers. The critical chapter on negotiating the purchase of a compressor now reflects current industry practices for preparing detailed specifications, bid evaluations, engineering reviews, and installation. A key chapter compares the reliability of various types of compressors.
*Everything you need to select the right compressor for your specific application.
*Practical information on compression principles, equipment, applications, selection, sizing, installation, and maintenance.
*New sections on diaphragm compressors and an introduction to rotor dynamics stability.

This book is a practical account of pumping, piping and seals starting with basics and providing detailed but accessible information on all aspects of the pumping process and what can go wrong with it. Written by an acknowledged expert with years of teaching experience in the practical understanding of pumps and systems.
Aids understanding of pumps to minimize failures and time-out.
A practical handbook covering the basics of the pumping process.
Written by an acknowledged expert.

Valves are the components in a fluid flow or pressure system that regulate either the flow or the pressure of the fluid. They are used extensively in the process industries, especially petrochemical. Though there are only four basic types of valves, there is an enormous number of different kinds of valves within each category, each one used for a specific purpose. No other book on the market analyzes the use, construction, and selection of valves in such a comprehensive manner.
-Covers new environmentally-conscious equipment and practices, the most important hot-button issue in the petrochemical industry today
-Details new generations of valves for offshore projects, the oil industry's fastest-growing segment
-Includes numerous new products that have never before been written about in the mainstream literature

The topic of Introduction to Random Vibrations is the behavior of structural and mechanical systems when they are subjected to unpredictable, or random, vibrations. These vibrations may arise from natural phenomena such as earthquakes or wind, or from human-controlled causes such as the stresses placed on aircraft at takeoff and landing. Study and mastery of this topic enables engineers to design and maintain structures capable of withstanding random vibrations, thereby protecting human life.
Introduction to Random Vibrations will lead readers in a user-friendly fashion to a thorough understanding of vibrations of linear and nonlinear systems that undergo stochastic-random-excitation.
- Provides over 150 worked out example problems and, along with over 225 exercises, illustrates concepts with true-to-life engineering design problems
- Offers intuitive explanations of concepts within a context of mathematical rigor and relatively advanced analysis techniques.
- Essential for self-study by practicing engineers, and for instruction in the classroom.