Instrumentation, Measurements, and Experiments in Fluids, Second Edition

Ethirajan Rathakrishnan

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781351848060
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Energy

Instrumentation, Measurements, and Experiments in Fluids, Second Edition is primarily focused on essentials required for experimentation in fluids, explaining basic principles, and addressing the tools and methods needed for advanced experimentation. It also provides insight into the vital topics and issues associated with the devices and instruments used for fluid mechanics and gas dynamics experiments. The second edition adds exercise problems with answers, along with PIV systems of flow visualization, water flow channel for flow visualization, and pictures with Schlieren and shadowgraph—from which possible quantitative information can be extracted. Ancillary materials include detailed solutions manual and lecture slides for the instructors.

Chapter 1

Needs and Objectives of Experimental Study

1.1 Introduction

As we know, the theory of potential flow is based on simplifying assumptions such as the fluid is barotropic, inviscid, and so on. Therefore, the potential flow theory cannot account for the profile drag acting on an object present in the flow field and for the boundary layer effects. Also, we know that the mathematical theory of boundary layer motion is highly complex. A considerable amount of theoretical work has already been done on inviscid compressible fluid flow at subsonic and supersonic speeds. But the theory of boundary layer for compressible flow has to develop a lot. Therefore, the theory available for fluid flow analysis is incomplete and needs to be supplemented by experiments. From the design point of view, experiments have two principal objectives:

1. They make it possible to determine the influence of various features of design, and modifications to them, in a safe, quick, direct, and relatively less expensive manner.

2. They provide information of a fundamental nature, usually in conjunction with theoretical work. By this means, the theory is confirmed or extended, thereby laying the foundation for future design improvements of a fundamental character.

The aim of this book is to discuss the fundamental aspects of the experimentation in fluids. In other words, our aim here is to acquaint ourselves with the need for experimental study and to gain insight concerning various applications of the available techniques for experimental study of fluid flows.

1.2 Some Fluid Mechanics Measurements

A large variety of measurement techniques are used in the field of experimental fluid mechanics. To have an idea about the different types of measurements, let us see the examples given below:

1.2.1 Wind Tunnel Studies

Wind tunnels are used for numerous investigations ranging from fundamental research to industrial aerodynamics. Many wind tunnel studies aim at the determination of forces on scaled models of aircraft, aircraft components, automobiles, buildings, and so on. Forces such as lift and drag acting on the models being tested are known to obey the following law of similitude

$\boxed{F=\frac{1}{2}\rho V^{2}S{C}_N}$

(1.1)

where S is the surface area or cross-sectional area of the model, depending on the application. The force coefficient C_N is known to be a function of several nondimensional parameters. The prime ones, among such dimensionless parameters used, in aerodynamics are

$\begin{array}{l}\text{Reynolds number}=\frac{\text{Inertia force}}{\text{Viscous force}}=\frac{\rho Vl}{\mu }\\ \text{Mach number}=\frac{\text{Inertia force}}{\text{Elastic force}}=\frac{V}{a}\end{array}$

where ρ and V are the density and velocity of flow, respectively, µ is the dynamic viscosity coefficient, l is the characteristic length, and a is the speed of sound.

To correlate data, velocity is measured with pitot–static tube or hot–wire anemometer or laser Doppler anemometer, and the temperature and pressure are obtained with appropriate instrumentation. The forces and the moments on a model are usually determined with specially designed balance or surface pressure measurements. The density is usually calculated from the measured pressure and temperature.

1.2.2 Analogue Methods

By analogue methods, problems may be solved by setting up another physical system, such as an electric field, for which the governing equations are of the same form as those for the problem to be solved, with corresponding boundary conditions. The solution of the original problem may be obtained experimentally from measurements on the analogous system. Some of the well-known analogue methods for solving fluid flow problems are the Hele–Shaw analogy, the electrolytic tank, and the surface waves in a ripple tank.

1.2.3 Flow Visualization

Apart from the conventional methods of experimentally investigating flow patterns by means of pressure and velocity surveys, fluid flows lend themselves to numerous visualization techniques. Some of the popularly employed flow visualization methods for fluid flow analysis are flow visualization with smoke, tuft, chemical coating, interferometer, schlieren, and shadowgraph.

1.3 Measurement Systems

Basically, the main components of a measuring system may be classified into the following three categories.

1. The sensing element.

2. The signal converter.

3. The display.

1.3.1 Sensing Element

A sensing element is also called a transd...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• About the Book
• About the Author
• 1 Needs and Objectives of Experimental Study
• 2 Fundamentals of Fluid Mechanics
• 3 Wind Tunnels
• 4 Flow Visualization
• 5 Hot-Wire Anemometry
• 6 Analogue Methods
• 7 Pressure Measurement Techniques
• 8 Velocity Measurements
• 9 Temperature Measurement
• 10 Measurement of Wall Shear Stress
• 11 Mass and Volume Flow Measurements
• 12 Special Flows
• 13 Data Acquisition and Processing
• 14 Uncertainty Analysis
• References
• Index