1.3.2 ViscosityViscosity can be thought of as the internal stickiness of a fluid. When a fluidflows, it deforms as one layer of fluid flows over another, and the rate ofdeformation is governed by internal shearing stresses that are set up withinthe fluid. The relationship between the shear stress and the rate of deform-ation for many fluids is governed by the simple linear relationship dudy1:2Where represents shear stress, represents the viscosity, u is the velocity inthe fluid, and y is a spatial coordinate. The quantity du=dy is a velocity gradientand can be interpreted as the rate at which strain or extension develops in thefluid. Fluids that are described by equation 1.2 are known as Newtonian fluids.Other types of behavior are possible, and fluids that deviate from equation 1.2are called non-Newtonian. Such fluids are discussed in detail in Chapter 5.The viscosity of a fluid is quite sensitive to the temperature, and liquidsshow a strong decrease in viscosity as temperature increases.1.3.3 Vapor pressureAll liquids show a greater or lesser tendency to vaporize and, if allowed tocome to equilibrium with its surroundings, a liquid will establish an equilib-rium across the liquid±vapor interface. The pressure exerted by the moleculesof the fluid in the vapor phase is specific to each liquid, and the equilibriumpressure is called the vapor pressure of the liquid. The vapor pressure is afunction of the temperature.If the vapor pressure of a liquid exceeds the prevailing total pressure, theliquid vaporizes rapidly by boiling. This phenomenon is commonly encoun-tered with water, which has a vapor pressure of 101.3 kPa at 100C. This is the

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6.9 Practice problems 6.10 Symbols Index //SYS21///INTEGRAS/B&H/IPF/FINAL_13-09-02/0750648856-CH000-PRELIMS.3D ± 7 ± [1±10/10] 23.9.20023:25PMPrefaceThis book deals with the transportation and handling of incompressiblefluids. This topic is important to most process engineers, because large quan-tities of material are transported in the process engineering industries. Theemphasis of this book is on suspensions of particulate solids although thebasic principles of simple Newtonian fluid flow form the basis of the devel-opment of models for the transportation of such material. Both settlingslurries and dense suspensions are considered. The latter invariably exhibitnon-Newtonian behavior. Transportation of slurries and other non-Newtonianfluids is generally treated inadequately or perfunctorily in most of the textsdealing with fluid transportation. This is a disservice to modern students inchemical, metallurgical, civil, and mining engineering, where problems relat-ing to the flow of slurries and other non-Newtonian fluids are commonlyencountered. Although the topics of non-Newtonian fluid flow and slurrytransportation are comprehensively covered in specialized texts, this bookattempts to consolidate these topics into a consistent treatment that followsnaturally from the conventional treatment of the transportation of incompres-sible Newtonian fluids in pipelines. In order to keep the book to a reasonablelength, solid±liquid systems that are of interest in the mineral processingindustries are emphasized at the expense of the many other fluid types thatare encountered in the process industries in general. This reflects the particu-lar interests of the author. However, the student should have no difficulty inadapting the methods that are described here to other application areas. Thelevel is kept to that of undergraduate courses in the various process engineer-ing disciplines, and this book could form the basis of a one-semester course

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2∆x1Figure 2.11 Schematic representation of the flow energyFlow of fluids in piping systems 23//SYS21///INTEGRAS/B&H/IPF/FINAL_13-09-02/0750648856-CH02.3D ± 24 ± [9±54/46] 23.9.2002 4:35PM2.4.4 The overall energy balanceWhen a fluid is moved from one location to another, such as when it ispumped through a piping system, there is usually a redistribution of energy.For example when a fluid flows downward in the gravitational field of theearth, potential energy can be converted to kinetic energy and the fluid cangain energy from a pump. Electrical energy consumed by a pump motor canbe converted to kinetic or potential energy of the fluid.As the fluid moves the changes in each type of energy must be balancedagainst the net loss or gain of energy by the fluid. An overall energy balancethat accounts for all types of energy that influences the behavior of fluids thatare of interest in typical engineering situations is given byÁu ÁPv gÁz 12Á"V2 q ÀW 2:37In this equation the symbol Á indicates the change in the specified energycomponent as the fluid changes its location. All energies are expressed perunit mass of fluid.The internal energy is determined largely by the temperature of the fluid

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Hindawi Publishing CorporationBoundary Value ProblemsVolume 2010, Article ID 257568, 20 pagesdoi:10.1155/2010/257568Research ArticleVariable Viscosity on MagnetohydrodynamicFluid Flow and Heat Transfer over an UnsteadyStretching Surface with Hall EffectS. Shateyi1and S. S. Motsa21School of Mathematical and Natural Sciences, University of Venda, Private Bag X5050,Thohoyandou 0950, South Africa2Department of Mathematics, University of Swaziland, Private Bag 4, Kwaluseni M201, SwazilandCorrespondence should be addressed to S. Shateyi, stanford.shateyi@univen.ac.zaReceived 16 July 2010; Accepted 16 August 2010Academic Editor: Vicentiu D. RadulescuCopyright q 2010 S. Shateyi and S. S. Motsa. This is an open access article distributed underthe Creative Commons Attribution License, which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly cited.The problem of magnetohydrodynamic ﬂow and heat transfer of a viscous, incompressible, andelectrically conducting ﬂuid past a semi-inﬁnite unsteady stretching sheet is analyzed numerically.The problem was studied under the eﬀects of Hall currents, variable viscosity, and variablethermal diﬀusivity. Using a similarity transformation, the governing fundamental equations areapproximated by a system of nonlinear ordinary diﬀerential equations. The resultant system ofordinary diﬀerential equations is then solved numerically by the successive linearization methodtogether with the Chebyshev pseudospectral method. Details ofthe velocity and temperature ﬁeldsas well as the local skin friction and the local Nusselt number for various values of the parameters

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presented in the API code 27 was shown to be a special case of this differential equation. The second equation was derived by combining the Euler equation with the a modification of the Darcy-Weisbach equation that is known to be valid for the lost head during laminar and non laminar flow in pipes and the equation of continuity for a real gas. Solutions were provided to the differential equations of this work by the Runge- Kutta algorithm. The accuracy of the first differential equation (derived by the combination of the Darcy law, the equation of continuity for a real gas and the Euler equation) was tested by data from the book of (Amyx et al., 1960). The book computed the permeability of a certain porous core as 72.5 millidracy while the solution to the first equation computed it as 72.56 millidarcy. The only modification made to the Darcy- Weisbach formula (for the lost head in a pipe) so that it could be applied to a porous medium was the replacement of the diameter of the pipe with the product of the pipe diameter and the porosity of the medium. Thus the solution to the second differential equation could be used for both pipe and porous medium. The solution to the second differential equation was tested by using it to calculate the dimensionless friction factor for a pipe (f) with data taken from the book of (Giles et al., 2009). The book had f = 0.0205, while the solution to the second differential equation obtained it as 0.02046. Further, the dimensionless friction factor for a certain core (fp ) calculated by the solution to the second differential equation plotted very well in a graph of fp versus the Reynolds number for porous media that was previously generated by (Ohirhian, 2008) through experimentation. Development of Equations The steps used in the development of the general differential equation for the steady flow of gas pipes can be used to develop a general differential equation for the flow of gas in porous media. The only difference between the cylindrical homogenous porous medium lies in the lost head term.

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flow is usually taken as the physical boundary of the part through which the flow is occurring.The control volume concept is used in fluid dynamics applications, utilizing the continuity,momentum, and energy principles mentioned at the beginning of this chapter. Once the controlvolume and its boundary are established, the various forms of energy crossing the boundary withthe fluid can be dealt with in equation form to solve the fluid problem. Since fluid flowproblems usually treat a fluid crossing the boundaries of a control volume, the control volumeapproach is referred to as an "open" system analysis, which is similar to the concepts studied inthermodynamics. There are special cases in the nuclear field where fluid does not cross thecontrol boundary. Such cases are studied utilizing the "closed" system approach.Regardless of the nature of the flow, all flow situations are found to be subject to the establishedbasic laws of nature that engineers have expressed in equation form. Conservation of mass andconservation of energy are always satisfied in fluid problems, along with Newton’s laws ofmotion. In addition, each problem will have physical constraints, referred to mathematically asboundary conditions, that must be satisfied before a solution to the problem will be consistentwith the physical results.HT-03 Page 8 Rev. 0Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.comFluid Flow CONTINUITY EQUATIONVolumetric Flow RateThe volumetric flow rate ( ) of a system is a measure of the volume of fluid passing a point in˙Vthe system per unit time. The volumetric flow rate can be calculated as the product of the cross-sectional area (A) for flow and the average flow velocity (v).(3-1)˙V AvIf area is measured in square feet and velocity in feet per second, Equation 3-1 results involumetric flow rate measured in cubic feet per second. Other common units for volumetric flowrate include gallons per minute, cubic centimeters per second, liters per minute, and gallons per

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Chapter 4: Fluid KinematicsChapter 4: Fluid KinematicsESOE 505221 Fluid Mechanics 2OverviewFluid Kinematics deals with the motion of fluids without necessarily considering the forces and moments which create the motion.Chapter 4: Fluid KinematicsESOE 505221 Fluid Mechanics 3Lagrangian DescriptionTwo ways to describe motion are Lagrangian and Eulerian descriptionLagrangian description of fluid flow tracks the position and velocity of individual particles. (eg. Brilliard ball on a pooltable.)Motion is described based upon Newton's laws. Difficult to use for practical flow analysis.Fluids are composed of billions of molecules.Interaction between molecules hard to describe/model. However, useful for specialized applicationsSprays, particles, bubble dynamics, rarefied gases.Coupled Eulerian-Lagrangian methods.Named after Italian mathematician Joseph Louis Lagrange (1736-1813).Chapter 4: Fluid KinematicsESOE 505221 Fluid Mechanics 4Lagrangian Description( )( )( )

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heat, hotter to cooler, the temperatures of the two fluids approach each other. Note thatthe hottest cold-fluid temperature is always less than the coldest hot-fluid temperature.ME-02 Rev. 0Page 4Heat Exchangers DOE-HDBK-1018/1-93 TYPES OF HEAT EXCHANGERSFigure 3 Parallel Flow Heat ExchangerCounter flow, as illustrated in Figure 4, exists when the two fluids flow in oppositedirections. Each of the fluids enters the heat exchanger at opposite ends. Because thecooler fluid exits the counter flow heat exchanger at the end where the hot fluid entersthe heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid.Counter flow heat exchangers are the most efficient of the three types. In contrast to theparallel flow heat exchanger, the counter flow heat exchanger can have the hottest cold-fluid temperature greater than the coldest hot-fluid temperatue.Figure 4 Counter Flow Heat ExchangeRev. 0 ME-02Page 5TYPES OF HEAT EXCHANGERS DOE-HDBK-1018/1-93 Heat ExchangersCross flow, as illustrated in Figure 5, exists when one fluid flows perpendicular to thesecond fluid; that is, one fluid flows through tubes and the second fluid passes around thetubes at 90° angle. Cross flow heat exchangers are usually found in applications whereone of the fluids changes state (2-phase flow). An example is a steam system'scondenser, in which the steam exiting the turbine enters the condenser shell side, and thecool water flowing in the tubes absorbs the heat from the steam, condensing it into water.Large volumes of vapor may be condensed using this type of heat exchanger flow.Figure 5 Cross Flow Heat ExchangerComparison of the Types of Heat ExchangersEach of the three types of heat exchangers has advantages and disadvantages. But of the three,the counter flow heat exchanger design is the most efficient when comparing heat transfer rateper unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that theaverage T (difference in temperature) between the two fluids over the length of the heat

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is an inevitable subject to be considered. It is involved in manipulating andanalysing fluids in structures on micrometer scale.[13] At this dimension, manyforces become dominant other than those experienced in macro devices.Additionally, when the device is used in biological or medical applications,biofluids will be involved, which makes the flow field even more complicated.1Therefore, all the factors mentioned above have to be taken into account when amicrofluidic device is designed.Specifically, a microfluidic device must contain the fluid to be studied andprovide couplings to let the fluid flow into and out of this container. Henceseparating and mixing various fluids are essential functions of microfluidicdevices. However, because of the high ratio of surface to volume in thesedevices, bubbles can easily form. Thus careful optimization shall be done toeliminate air bubbles. All the above topics will be further explored in this thesis.1.2Thesis ObjectiveA series of experiments are carried out to investigate the flow of complexfluids in microfluidic devices. Ultrasound is used as the external force to excitethe flow field so as to seek for its potential use in micro-separating and micromixing. Besides, in order to reduce bubbles in microfluidic devices so as tooptimize the micro-flow, the mechanism of air bubble formation in micro-filteris also explored through the experiments.In the study of micro-flows in the ultrasound field, the frequency ofultrasound actuator is tuned to form ultrasonic standing wave between the 2side walls of a rectangular microchannel. The standing wave is used as theexternal force to act on the microflows and the phenomena are recorded with

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vol. 178, no. 3-4, pp. 257–262, 1999.8 J H. He, “A coupling method of a homotopy technique and a perturbation technique for non-linearproblems,” International Journal of Non-Linear Mechanics, vol. 35, no. 1, pp. 37–43, 2000.9 J H. He, “Homotopy perturbation method: a new nonlinear analytical technique,” AppliedMathematics and Computation, vol. 135, no. 1, pp. 73–79, 2003.10 N. A. Khan, A. Ara, and A. Mahmood, “Approximate solution of time-fractional chemical engineeringequations: a comparative study,” International Journal of Chemical Reactor Engineering,vol.8,articleA19, 2010.11 N. A. Khan, N U. Khan, M. Ayaz, and A. Mahmood, “Analytical methods for solving the time-fractional Swift-Hohenberg S-H equation,” Computers and Mathematics with Applications. In press.12 N. A. Khan, A. Ara, S. A. Ali, and M. Jamil, “Orthognal ﬂow impinging on a wall with suction orblowing,” International Journal of Chemical Reactor Engineering. In press.13 A. Yıldırım, “Solution of BVPs for fourth-order integro-diﬀerential equations by using homotopyperturbation method,” Computers & Mathematics with Applications, vol. 56, no. 12, pp. 3175–3180, 2008.14 H. Koc¸ak, T.¨Ozis¸, and A. Yıldırım, “Homotopy perturbation method for the nonlinear dispersiveKm,n,1 equations with fractional time derivatives,” International Journal of Numerical Methods forHeat & Fluid Flow, vol. 20, no. 2, pp. 174–185, 2010.15 Y. Khan, N. Faraz, A. Yildirim, and Q. Wu, “A series solution of the long porous slider,” Tr ibologyTransac t i o n s , vol. 54, no. 2, pp. 187–191, 2011.16 N. A. Khan, A. Ara, S. A. Ali, and A. Mahmood, “Analytical study of Navier-Stokes equation withfractional orders using He’s homotopy perturbation and variational iteration methods,” InternationalJournal of Nonlinear Sciences and Numerical Simulation, vol. 10, no. 9, pp. 1127–1134, 2009.17 H. Aminikhah and J. Biazar, “A new HPM for ordinary diﬀerential equations,” Numerical Methods forPartial Diﬀerential Equations, vol. 26, no. 2, pp. 480–489, 2010.18 H. Aminikhah and M. Hemmatnezhad, “An eﬃcient method for quadratic Riccati diﬀerentialequation,” Communications in Nonlinear Science and Numerical Simulation, vol. 15, no. 4, pp. 835–839,2010.

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an answer to the first question." Due to often unnoticeably perturbations, a particular flow starting from given initial and boundary conditions can often progress reaching quite different flow patterns. It is a fact that most fluid flows are turbulent, and at the same time fluids occur, and in many cases represent the dominant physics, on all macroscopic scales throughout the known universe, from the interior of biological cells, to circulatory and respiratory systems of living creatures, to countless technological devices (all sizes of planes, wind farms, a wide range of structures, buildings, buildings arrays, etc) and household appliances of modern society, to geophysical and astrophysical phenomena including planetary interiors, oceans and atmospheres. And, despite the widespread occurrence of fluid flow, and the ubiquity of turbulence, the “problem of turbulence" remains to this day a challenge to physicists, engineers and fluid dynamics researchers in general. No one knows how to obtain stochastic solutions to the well-posed set of partial differential equations that govern turbulent flows. Averaging those non linear equations to obtain statistical quantities always leads to more unknowns than equations, and ad-hoc modeling is then necessary to close the problem. So, except for a rare few limiting cases, first-principle analytical solutions to the turbulence conundrum are not possible. The problem of turbulence has been studied by many of the greatest physicists and engineers of the 19th, 20th and early 21th Centuries, and yet we do not understand in complete detail how or why turbulence occurs, nor can we predict turbulent behavior with any degree of reliability, even in very simple (from an engineering perspective) flow situations. Thus, the study of turbulence is motivated both by its inherent intellectual challenge and by the practical utility of a thorough understanding of its nature. Wind Tunnels and Experimental Fluid Dynamics Research 198 Our particular concern is related with the low atmospheric turbulent boundary layer, that is, the part of the surface layer between ground level and a 400m height (this last value depends, more or less, upon the criteria of researchers). Inside this range of height most of

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8.16 PLANT AND FACILITIES ENGINEERINGFIGURE 4 Equivalent length of pipe ﬁttings and valves. (Crane Company.)Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2006 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.PIPING AND FLUID FLOW8.17FIGURE 5 Pressure loss in steam pipes based on the Fritzche formula. (Power.)Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2006 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.PIPING AND FLUID FLOW8.18 PLANT AND FACILITIES ENGINEERINGdetermine any one of the four variables listed above when the other three are known.In solving a problem on the chart in Fig. 6, use the steam-quantity lines to intersectpipe sizes and the steam-pressure lines to intersect steam velocities. Here are twotypical applications of this chart.Example. What size schedule 40 pipe is needed to deliver 8000 lb/h (3600kg/h) of 120-lb/in2(gage) (827.3-kPa) steam at a velocity of 5000 ft/min (1524m/min)?Solution. Enter Fig. 6 at the upper left at a velocity of 5000 ft /min (1524 m/min), and project along this velocity line until the 120-lb/in2(gage) (827.3-kPa)pressure line is intersected. From this intersection, project horizontally until the8000-lb/h (3600-kg/h) vertical line is intersected. Read the nearest pipe size as 4in (101.6 mm) on the nearest pipe-diameter curve.Example. What is the steam velocity in a 6-in (152.4-mm) pipe delivering

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rate.EO 1.6 CALCULATE either the mass flow rate or thevolumetric flow rate for a fluid system.EO 1.7 STATE the principle of conservation of mass.EO 1.8 CALCULATE the fluid velocity or flow rate in aspecified fluid system using the continuity equation.IntroductionFluid flow is an important part of most industrial processes; especially those involving thetransfer of heat. Frequently, when it is desired to remove heat from the point at which it isgenerated, some type of fluid is involved in the heat transfer process. Examples of this are thecooling water circulated through a gasoline or diesel engine, the air flow past the windings ofa motor, and the flow of water through the core of a nuclear reactor. Fluid flow systems are alsocommonly used to provide lubrication.Fluid flow in the nuclear field can be complex and is not always subject to rigorous mathematicalanalysis. Unlike solids, the particles of fluids move through piping and components at differentvelocities and are often subjected to different accelerations.Rev. 0 Page 1 HT-03Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.comCONTINUITY EQUATION Fluid FlowEven though a detailed analysis of fluid flow can be extremely difficult, the basic conceptsinvolved in fluid flow problems are fairly straightforward. These basic concepts can be appliedin solving fluid flow problems through the use of simplifying assumptions and average values,where appropriate. Even though this type of analysis would not be sufficient in the engineeringdesign of systems, it is very useful in understanding the operation of systems and predicting theapproximate response of fluid systems to changes in operating parameters.The basic principles of fluid flow include three concepts or principles; the first two of which thestudent has been exposed to in previous manuals. The first is the principle of momentum(leading to equations of fluid forces) which was covered in the manual on Classical Physics. Thesecond is the conservation of energy (leading to the First Law of Thermodynamics) which wasstudied in thermodynamics. The third is the conservation of mass (leading to the continuity

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Gesellschaft fur Technische Biologie und Bionik, Universitat des Saarlandes;1999:281-293.18. McIntosh A, Forman M: The efficiency of the explosive discharge of thebombardier beetle with possible biomimetic application. In Design andNature II–Comparing Design in Nature with Science and Engineering. Editedby: Collins MW, Brebbia CA. Southampton 2004:227-236.19. Amon CH: Advances in computational modeling of nano-scale heattransfer. Proceedings of 12th International Heat Transfer Conference Grenoble,France; 2002, 41-53.20. Chen S, Doolen GD: Lattice Boltzmann method for fluid flows. Annu RevFluid Mech 1998, 30:329-364.21. Maruyama S: Molecular dynamics methods in microscale heat transfer. InHeat Transfer and Fluid Flow in Microchannels. Edited by: Celata GP. NewYork: Begell House Inc; 2002.22. Bird GA: Molecular gas Dynamics and Direct Simulation of Gas Flows NewYork: Oxford Univ. Press; 1994.23. Wagner G, Flekkoy E, Fedder J, Jossang T: Coupling molecular dynamicsand continuum dynamics. Comput Phys Commun 2002, 147:670-673.24. Prizjev NV, Darhuber AA, Troian SM: Slip behavior in liquid films onsurfaces of patterned wettability: Comparison between continuum andmolecular dynamics simulations. Phys Rev E 2005, 71:041608.25. Adkins D, Yan YY: CFD simulation of fish-like body moving in viscousliquid. J Bionic Eng 2006,3:147-153.26.Zhu Q, Wolfgang MJ, Yue DKP, Triantafyllou MS: Three-dimensional flowstructures and vorticity control in fish-like swimming. J Fluid Mech 2002,468:l-28.27. Rohr JJ, Hendricks EW, Quigley L, Fish FE, Gilpatrick JW, Scardina-Ludwig J:Observations of dolphin swimming speed and Strouhal number. Space

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ϕ - film thickness in the "absolutely rigid" bearing. To determine it the expression (6) is used. Thus, the determination of pressures in the lubricating film and HMCh of EY TU is the related objective of the hydrodynamic lubrication theory and the theory of elasticity. Modeling of EY TU, compared with "absolutely rigid" bearings is supplemented by an elastic subproblem the purpose of which is to determine the strain state of the friction surface of a crank crosshead under the influence of complex loads. The method of solving the elastic subproblem is chosen according to the accepted approximating model of an EY bearing. In today's solutions for EY TU the compliance and stiffness matrix of the bearing is usually constructed using the FE method. The other side of modeling the elastic subsystem is adequate description of the entire complex of loads, causing the elastic deformation of the bearing housing and the conditions of fixing of FE model. One must consider not only the hydrodynamic pressure, but also the volume forces of rod inertia. The known methods of solving the elastohydrodynamic lubrication problem can be classified as follows: direct methods or methods of successive approximations, in which the solutions of the hydrodynamic and elastic subtasks are performed separately, with the subsequent jointing of the results in the direct iterative process; and system, oriented for the joint solution of equations of fluid flow and elastic deformation. In solving the problem of elastohydrodynamic lubrication of a bearing with the help of a direct iterative method, the hydrodynamic and elastic subproblems at each step of time discretization are solved sequentially in an iterative cycle. The main disadvantage of direct methods for the calculation of EHD is their slow convergence and the associated time-consumption. These difficulties are partially overcome by carefully selected prediction scheme and a number of techniques that accelerate the convergence of the iterative process in the form of restrictions on movement, load and move calculation. Methodology of Calculation of Dynamics and Hydromechanical Characteristics of Heavy-Loaded Tribounits, Lubricated with Structurally-Non-Uniform and Non-Newtonian Fluids

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The study reported herein analyzes the effect of periodic suction on the three dimensional flow of a viscous incompressible fluid past an infinite vertical porous plate embedded in a por[r]

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small (high-frequency) eddies of the laboratory flow are higher. The wind tunnel counterparts of full-scale high-frequency fluctuations are therefore partly suppressed by those stresses. This can affect significantly the extent to which laboratory and full-scale suctions are similar, especially in flow separation regions where the suctions are strong. Indeed, measurements have shown that, in zones of strong suctions, absolute values of pressure coefficients are far lower in the wind tunnel than at full scale (Fig. 4). 2.5 Wind speeds as functions of averaging times The relation between wind speeds averaged over different time intervals (e.g., the ratio between wind speeds averaged over 3 s and wind speeds averaged over 10 min) varies as a function of the time intervals owing to the presence of turbulence in the wind flow. Wind Tunnels and Experimental Fluid Dynamics Research 286 Fig. 4. Pressure coefficients measured at building corner, eave level, Texas Tech University Experimental Building (Long et al., 2006). The flow characteristics discussed in Sections 2.1 through 2.5 depend to a significant extent upon whether the storm to which the structure is subjected is of the large-scale extratropical (synoptic) type, or a hurricane, a thunderstorm, a chinook wind, and so forth. In current commercial practice the type of flow being simulated is the atmospheric boundary layer typical of synoptic storms (straight line winds), and it is assumed that simulations in this type of flow are adequate even if the structure is subjected to other types of storm. In wind engineering practice it is important to remember that the parameters of any given model of the wind flow are characterized by uncertainties in the sense that they can vary from storm to storm. Such variability should be accounted for in any uncertainty analysis of the wind effect estimates. 3. Aerodynamics and similitude Although computational fluid dynamics has made tremendous progress in the last decade thanks to high speed computing, its use has not reached routine level in the structural design of buildings, and its predictions need to be verified by experiments. Wind tunnel

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