## Exam (elaborations)

# Exam (elaborations) TEST BANK FOR Fluid Mechanics Fundamentals and Applications 2nd Edition By Yunus A. Çengel, John M. Cimbala - (Instructor’s solution manual) McGraw-Hill (2006)

Solution We are to define internal, external, and open-channel flows. Analysis External flow is the flow of an unbounded fluid over a surface such as a plate, a wire, or a pipe. The flow in a pipe or duct is internal flow if the fluid is completely bounded by solid surfaces. The flow of liquids in a pipe is called open-channel flow if the pipe is partially filled with the liquid and there is a free surface, such as the flow of water in rivers and irrigation ditches. Discussion As we shall see in later chapters, there different approximations are used in the analysis of fluid flows based on their classification. 1-2C Solution We are to define incompressible and compressible flow, and discuss fluid compressibility. Analysis A fluid flow during which the density of the fluid remains nearly constant is called incompressible flow. A flow in which density varies significantly is called compressible flow. A fluid whose density is practically independent of pressure (such as a liquid) is commonly referred to as an “incompressible fluid,” although it is more proper to refer to incompressible flow. The flow of compressible fluid (such as air) does not necessarily need to be treated as compressible since the density of a compressible fluid may still remain nearly constant during flow – especially flow at low speeds. Discussion It turns out that the Mach number is the critical parameter to determine whether the flow of a gas can be approximated as an incompressible flow. If Ma is less than about 0.3, the incompressible approximation yields results that are in error by less than a couple percent. 1-3C Solution We are to define the no-slip condition and its cause. Analysis A fluid in direct contact with a solid surface sticks to the surface and there is no slip. This is known as the no-slip condition, and it is due to the viscosity of the fluid. Discussion There is no such thing as an inviscid fluid, since all fluids have viscosity. 1-4C Solution We are to define forced flow and discuss the difference between forced and natural flow. We are also to discuss whether wind-driven flows are forced or natural. Analysis In forced flow, the fluid is forced to flow over a surface or in a tube by external means such as a pump or a fan. In natural flow, any fluid motion is caused by natural means such as the buoyancy effect that manifests itself as the rise of the warmer fluid and the fall of the cooler fluid. The flow caused by winds is natural flow for the earth, but it is forced flow for bodies subjected to the winds since for the body it makes no difference whether the air motion is caused by a fan or by the winds. Discussion As seen here, the classification of forced vs. natural flow may depend on your frame of reference. . © 2006 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission. 1-2 Chapter 1 Introduction and Basic Concepts PROPRIETARY MATERIAL. © 2006 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission. 1-3 1-5C Solution We are to define a boundary layer, and discuss its cause. Analysis When a fluid stream encounters a solid surface that is at rest, the fluid velocity assumes a value of zero at that surface. The velocity then varies from zero at the surface to the freestream value sufficiently far from the surface. The region of flow in which the velocity gradients are significant and frictional effects are important is called the boundary layer. The development of a boundary layer is caused by the no-slip condition. Discussion As we shall see later, flow within a boundary layer is rotational (individual fluid particles rotate), while that outside the boundary layer is typically irrotational (individual fluid particles move, but do not rotate). 1-6C Solution We are to discuss the differences between classical and statistical approaches. Analysis The classical approach is a macroscopic approach, based on experiments or analysis of the gross behavior of a fluid, without knowledge of individual molecules, whereas the statistical approach is a microscopic approach based on the average behavior of large groups of individual molecules. Discussion The classical approach is easier and much more common in fluid flow analysis. 1-7C Solution We are to define a steady-flow process. Analysis A process is said to be steady if it involves no changes with time anywhere within the system or at the system boundaries. Discussion The opposite of steady flow is unsteady flow, which involves changes with time. 1-8C Solution We are to define stress, normal stress, shear stress, and pressure. Analysis Stress is defined as force per unit area, and is determined by dividing the force by the area upon which it acts. The normal component of a force acting on a surface per unit area is called the normal stress, and the tangential component of a force acting on a surface per unit area is called shear stress. In a fluid at rest, the normal stress is called pressure. Discussion Fluids in motion may have additional normal stresses, but when a fluid is at rest, the only normal stress is the pressure. 1-9C Solution We are to define system, surroundings, and boundary. Analysis Asystem is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary. Discussion Some authors like to define closed systems and open systems, while others use the notation “system” to mean a closed system and “control volume” to mean an open system. This has been a source of confusion for students for many years. [See the next question for further discussion about this.] Chapter 1 Introduction and Basic Concepts PROPRIETARY MATERIAL 1-10C Solution We are to discuss when a system is considered closed or open. Analysis Systems may be considered to be closed or open, depending on whether a fixed mass or a volume in space is chosen for study. A closed system (also known as a control mass or simply a system) consists of a fixed amount of mass, and no mass can cross its boundary. An open system, or a control volume, is a properly selected region in space. Discussion In thermodynamics, it is more common to use the terms open system and closed system, but in fluid mechanics, it is more common to use the terms system and control volume to mean the same things, respectively. Mass, Force, and Units 1-