Technology & Engineering
Viscous Liquid
A viscous liquid is a substance that has a thick, sticky consistency and resists flowing easily. It exhibits a high resistance to deformation and tends to flow slowly. Viscous liquids are commonly found in various industrial and engineering applications, such as lubricants, hydraulic fluids, and polymers, where their flow properties are important for their intended use.
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9 Key excerpts on "Viscous Liquid"
- George E. Totten, Victor J. De Negri(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
The importance of viscosity is far more complex than described here, but these simple descriptions are a starting point for a discussion of viscosity and its relevance to hydraulic fluids. 4.1.1 A BSOLUTE V ISCOSITY AND N EWTON ’ S L AW Viscosity is a fluid’s resistance to flow. The viscosity of a fluid, either liquid or gas, describes the opposition to a change in shape or to movement. A mathematical description was first developed by Sir Isaac Newton as a special case of his second law of motion: Shear stress (Coefficient of viscosity) (Shear rate), = × (4.1) where the shear stress is the force per area and the shear rate is the velocity gradient. This leads to a description of viscosity as the force that must be overcome to cause a given fluid motion. Newton’s viscosity law applies when laminar, or non-turbulent, flow occurs. Laminar flow can be imagined as numerous, discrete layers (lamina) of the fluid moving in the same direction, but with the velocity of each layer varying, depending on the distance from a system boundary. The concept of velocity as a function of distance is known as the “velocity gradient.” Fluid flow through a tube caused by an applied pressure, known as “Poiseuelle flow,” is repre-sented in Figure 4.1. The arrows represent layers moving in streamlines between stationary, parallel walls. The layer closest to a wall is assumed to adhere to the wall and, in turn, will cause a frictional drag on the next closest layer, which is moving because of the applied force. Successive layers expe-rience gradually reduced frictional drag and thus flow at successively higher velocities. The central layer encounters the least friction from adjacent layers and, consequently, has the highest velocity. This change in velocity—from zero at the boundaries to the highest at the center—describes the term velocity gradient.
eBook - PDFMeeting the Pump Users Needs
The Proceedings of the 12th International Pump Technical Conference
- Sam Stuart(Author)
- 2013(Publication Date)
- Elsevier(Publisher)
PUMPING VISCOUS FLUIDS WITH PROGRESSING CAVITY PUMPS Alan G. Wild Director of Engineering Robbins & Myers, Inc. Since the rheological properties of viscous fluids often change in response to system changes, viscous fluids often present unique pumping opportunities. The viscosity of a fluid is defined as the ratio of the shearing stress between adjacent layers of fluid to the rate of change of velocity perpendicular to the direction of motion. More simply stated, viscosity is that property of a fluid which causes the fluid to resist flow. The higher the viscosity of a fluid the more it will resist flowing. Although it is obvious that extremely viscous fluids, such as heavy greases, can only be pumped efficiently with positive displacement pumps, many low viscosity fluids are also best handled by positive displacement pumps, and in particular, progressing cavity pumps. Progressing cavity pumps are known for their low shear rates and many low viscosity fluids, such as blood, are best handled with progressing cavity pumps due to the shear sensitivity of these fluids. Isaac Newton, who is credited with the original definition of viscosity, assumed that all fluids have, at a given temperature, a viscosity that is independent of the shear rate. Fluids that behave according to Newton's assumption are identified as Newtonian fluids. Water and light lubricating oils are typical examples of Newtonian fluids. Designing pumps and systems for moving viscous fluids would not be difficult if all viscous fluids were Newtonian. Unfortunately, relatively few fluids fall into this category. Far more fluids are non-Newtonian and their viscosity will change as a function of shear rate and some are also affected by the length of time they are subjected to shearing. To make the problem more interesting the viscosity of some fluids is directly proportional to the shear rate while the viscosity of other fluids is inversely proportional to the shear rate.
eBook - PDF- Ferdinand Rodriguez, Claude Cohen, Christopher K. Ober, Lynden Archer(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
Chapter 8 extends this coverage to elastic properties of polymers, which can be as important as viscous flow properties in fabrication processes. Chapters 9 and 10 describe mechan-ical properties and failure characteristics of polymers. 7.2 VISCOSITY 7.2.1 D EFINITIONS AND M ICROSCOPIC O RIGINS We have used the term viscosity up to now without a strict definition. The magnitude of a fluid’s viscosity provides a measure of its resistance to flow. Viscosity is also related to the energy dissipated by a fluid in motion under the action of an applied force. In polymers, energy dissipation arises fundamentally from friction between molecules as they slide by each other in flow. Both the geometry of the polymer repeat unit and the strength of intermolecular forces between these units therefore determine the magnitude of the viscosity. Polymer solutions are generally less vis-cous than their melts because solvent present in the former mediate and in many cases lubricate sliding contacts between molecules. Polymer liquids made up of higher molecular-weight molecules are also usually more viscous because of the larger number of repeat units per polymer chain, which increases the overall fric-tion. Thermodynamic state variables such as pressure and temperature can also affect viscosity. Typically the viscosity of polymer melts decreases with increasing tem-perature and increases with pressure. The flow resistance of a polymer during a fab-rication process is therefore a function of the unique chemical characteristics of the material and the processing conditions such as temperature and applied force. The device depicted in Figure 7.1. is termed a planar Couette shear cell. It pro-vides a simple means for measuring the viscosity of a polymer and also allows for an operational definition of viscosity. In this device, a liquid is sandwiched between two parallel planes separated by a distance d in the y direction.
eBook - PDFFood Texture and Viscosity
Concept and Measurement
- G.F. Stewart, B.S. Schweigert, John Hawthorn(Authors)
- 2012(Publication Date)
- Academic Press(Publisher)
CHAPTER 5 Viscosity and Consistency Introduction The previous two chapters dealt with texture of solid foods. This chapter deals with fluid foods. Unfortunately, the distinction between solid and fluid is not sharp and clear; consequently there is some overlap between the discussion in this chapter and the previous chapter. The tendency of a fluid to flow easily or with difficulty has been a subject of great practical and intellectual importance to mankind for centuries. The famous English physicist Sir Isaac Newton (1642-1727) was one of the earliest re-searchers to study the flow of fluids. In his Principia, the section entitled 4 O n the Circular Motion of L i q u i d s , he stated the hypothesis that the resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another. This principle, that the flow of fluid is directly proportional to the force that is applied, is used to describe the class of liquids known as Newtonian fluids. Water is the best-known Newtonian fluid. Other scientists have studied more complex liquids; for example, Schlubler in a 1828 paper on T h e Fatty Oils of G e r m a n y included within the physical constants a fluidity ratio using an instrument that is similar to some of the simple instruments that are currently used. Poiseuille (1797-1869) performed an elegant study of the flow of fluids in capillary tubes and may be considered as one of the founders of modern viscometry. Sir George Gabriel Stokes (1819— 1903), who was president of the Royal Society from 1885 to 1892, studied the flow of liquids through an orifice and can be considered the founder of the efflux type of viscometer. 199 200 5. Viscosity and Consistency Some important definitions in viscometry are set out in the following: Laminar flow is streamline flow in a fluid.
- Malcolm C. Bourne(Author)
- 2014(Publication Date)
- Academic Press(Publisher)
CHAPTER 5 Viscosity and Consistency Introduction The previous two chapters dealt with texture of solid foods. This chapter deals with fluid foods. Unfortunately, the distinction between solid and fluid is not sharp and clear; consequently there is some overlap between the discussion in this chapter and the previous chapter. The tendency of a fluid to flow easily or with difficulty has been a subject of great practical and intellectual importance to mankind for centuries. The famous English physicist Sir Isaac Newton (1642-1727) was one of the earliest re-searchers to study the flow of fluids. In his Principia, the section entitled 4 O n the Circular Motion of Liquids, he stated the hypothesis that the resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another. This principle, that the flow of fluid is directly proportional to the force that is applied, is used to describe the class of liquids known as Newtonian fluids. Water is the best-known Newtonian fluid. Other scientists have studied more complex liquids; for example, Schlubler in a 1828 paper on The Fatty Oils of Germany included within the physical constants a fluidity ratio using an instrument that is similar to some of the simple instruments that are currently used. Poiseuille (1797-1869) performed an elegant study of the flow of fluids in capillary tubes and may be considered as one of the founders of modern viscometry. Sir George Gabriel Stokes (1819— 1903), who was president of the Royal Society from 1885 to 1892, studied the flow of liquids through an orifice and can be considered the founder of the efflux type of viscometer. 199 200 5. Viscosity and Consistency Some important definitions in viscometry are set out in the following: Laminar flow is streamline flow in a fluid.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter 7 Important Concepts of Viscosity 1. Newtonian fluid A Newtonian fluid (named after Isaac Newton) is a fluid whose stress versus strain rate curve is linear and passes through the origin. The constant of proportionality is known as the viscosity. Definition A simple equation to describe Newtonian fluid behaviour is where τ is the shear stress exerted by the fluid (drag) [Pa] μ is the fluid viscosity - a constant of proportionality [Pa·s] is the velocity gradient perpendicular to the direction of shear, or equivalently the strain rate [s −1 ] In common terms, this means the fluid continues to flow, regardless of the forces acting on it. For example, water is Newtonian, because it continues to exemplify fluid properties no matter how fast it is stirred or mixed. Other examples may be aqueous solutions, emulsions. Contrast this with a non-Newtonian fluid, in which stirring can either leave a hole behind (that gradually fills up over time - this behavior is seen in materials such as pudding and oobleck, or, to a less rigorous extent, sand), or climb the stirring rod (the Weissenberg effect) because of shear thinning, the drop in viscosity causing it to flow more (this is seen in non-drip paints, which brush on easily but become more viscous when on walls). ________________________ WORLD TECHNOLOGIES ________________________ For a Newtonian fluid, the viscosity, by definition, depends only on temperature and pressure (and also the chemical composition of the fluid if the fluid is not a pure substance), not on the forces acting upon it.
eBook - PDF- A. Dorinson, K.C. Ludema(Authors)
- 1985(Publication Date)
- North Holland(Publisher)
The deeper, fundamental difference between liquids and gases l i e s i n their internal structure, a subject of considerable complexity. The physicochenical differentiation of the liquid state from the gaseous and the solid states requires elaborate and formal treatment. But characterization of the liquid state in a fashion useful for lubrica- tion problems can be made much simpler than is required by exact theory. It will suffice f o r o u r purposes to begin with the treatment of liquid viscosity in descriptive terms. Then those constitutive and structural aspects of liquids and the liquid state which influence viscosity will be discussed. Similar treatment will be applied to the density and compres- sibility of liquids. 60 VISCOSITY AND VISCOMETRY 4.2. NEWTONIAN AND NON-NEWTONIAN VISCOSITY In deriving the equations for the flow of a simple viscous fluid the theoretical physicist uses a definition of viscosity based on a mathe- matical statement rather than a physical model. Let us define viscosity from a physical point of view. Consider two planes in the body of a fluid a distance dy apart, as shown in Fig. 4-1. If we apply tangential stress T~~ along one of these planes and observe a rate of shear i , then we define the differential viscosity q v a s Equation 4-1 can in turn be used to define the unit of viscosity. In SI units, when the force F x is one newton, the area A one square meter and the velocity gradient one meter per second per meter, then the unit of viscosity is one newton-second per square meter, or alternately one pascal-second. The c.g.s. unit of viscosity, the poise, is one dyne- second per square centimeter. One pascal-second is therefore equal to 10 poise. The fundamental dimensions of viscosity are ML-lT-’. Figure 4-1. Simple laminar shear between close parallel planes in the body of a fluid. What we have done in Eqn 4-1 is to define model applicable to any material which flows.
eBook - PDF- Gwidon Stachowiak, Andrew W Batchelor(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
It can also be affected by the velocities of the operating surfaces (shear rates). The knowledge of the viscosity characteristics of a lubricant is therefore important in the design and in the prediction of the behavior of a lubricated mechanical system. In this chapter a simplified concept of viscosity, sufficient for most engineering applications, is considered. Refinements to this model incorporating, for example, transfer of momentum between the adjacent layers of lubricant and transient visco-elastic effects, can be found in more specialized literature. Dynamic Viscosity Consider two flat surfaces separated by a film of fluid of thickness ‘ h ’ as shown in Figure 2.1. The force required to move the upper surface is proportional to the wetted area ‘ A ’ and the velocity gradient ‘ u/h ’, as the individual fluid layer in a thicker film will be subjected to less shear than in a thin film, i.e.: F α A × u/h (2.1) h u = 0 F area Infinitely thin fluid ‘layers’ A u F IGURE 2.1 Schematic representation of the fluid separating two surfaces. This relationship is maintained for most fluids. Different fluids will exhibit a different proportionality constant ‘ η ’, called the ‘ dynamic viscosity ’. The relationship (2.1) can be written as: F = η × A × u/h (2.2) Rearranging gives: η = (F/A) / (u/h) or η = τ / (u/h) (2.3) P HYSICAL P ROPERTIES OF L UBRICANTS 13 where: η is the dynamic viscosity [Pas]; τ is the shear stress acting on the fluid [Pa]; u/h is the shear rate, i.e., velocity gradient normal to the shear stress [s -1 ]. Before the introduction of the SI system the most commonly used dynamic viscosity unit was the Poise. Incidentally this name originated not from an engineer but from a French medical doctor, Poiseuille, who studied the flow of blood. For practical applications the Poise [P] was far too large, thus a smaller unit, the centipoise [cP], was more commonly used. The SI unit for dynamic viscosity is Pascal-second [Pas].
eBook - ePubMcDonald’s Blood Flow in Arteries
Theoretical, Experimental and Clinical Principles
- Wilmer W. Nichols, Michael O'Rourke, Elazer R. Edelman, Charalambos Vlachopoulos, Wilmer W. Nichols, Michael O'Rourke, Elazer R. Edelman, Charalambos Vlachopoulos(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
3 Nature of Flow of a LiquidWilmer NicholsDOI: 10.1201/9781351253765-3It is convenient in mechanics to classify matter into solids and fluids. A solid substance has a definite size and shape and can be either perfectly elastic or plastic (see Chapter 4 ). A fluid is a substance that can flow and includes both liquids and gases. Gases are easily compressed, whereas liquids are almost incompressible. This text deals almost exclusively with liquids and the physical laws that govern their flow.In considering the vascular system with its contained blood, we are naturally concerned with the physical laws governing the flow of liquids through solid tubes. The simplest example is that of a straight, uniform, rigid tube with a steady (or constant) rate of nonturbulent liquid flow through it. To maintain such a steady flow, there must be a constant head of pressure applied to the liquid because of its viscosity or “internal friction”. Steady flow in a rigid cylindrical tube is described by the well-known Poiseuille equation, which states that the pressure drop (or gradient) along the tube is directly proportional to the length of the tube, the rate of flow through the tube and the viscosity of the liquid and is inversely proportional to the fourth power of the internal radius. If dye is injected into the tube, the liquid in the axis of the tube moves much faster than that near the wall, and the front of the dye assumes a paraboloid shape. This occurs because the particles of liquid are flowing in a series of laminae parallel to the sides of the tube, the liquid in contact with the wall is stationary, and each successive lamina is slipping against the viscous friction of the lamina outside it. When flow occurs in such parallel laminae, it is called “laminar” (Kaufmann, 1963). If two tubes are joined to form a trunk and dye is injected into only one of them, there is no mixing transversely across the tube, and the two streams remain distinct and are seen to flow side by side in the main trunk; this phenomenon is called “streamlining” (McDonald, 1974; Nichols and O’Rourke, 1990, 1998). Hence laminar flow is often called “streamline flow”. Alternatively, it may be called “Poiseuille-type flow” because it obeys the Poiseuille equation (Caro et al., 1978; Milnor, 1989; Fung, 1997; Wood, 1999; Nakayama and Bouncher, 1999; Wang et al., 2015; Zhang et al
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