III. Essential theoretical data

Aerodynamics department

Laboratory work №1

Wind tunnels, experimental methods,

Measurement instrumentation

Authors:

professor - E.P.Udarzev,

associate professor - V.G. Zhila,

associate professor - O.M. Pereverzev

Reviewer:

professor Kasynov V.A.

Kiev - 2005

LABORATORY WORK №1

Wind tunnels, experimental methods,

Measurement instrumentation

I. INTRODUCTION

In this laboratory work we describe briefly some of the devices which are used for experimental work in aerodynamics in the laboratory. The most important and widely used tool for such work is the wind tunnel, by means of which we create an airflow in which may be placed and tested a small scale model of the wing, aircraft or other body in which we are interested. It is expected that from the measured behaviour of the model we may infer something of the behaviour of the full scale body. In the interpretation of wind tunnel results, it is always important to take account of scale effect, since the Reynolds number is usually much less than that associated with the full scale body. It is often the aim to provide Reynolds numbers which are as high as possible, in order to minimize scale effects, and this leads to the use of bigger and bigger tunnels, and perhaps to pressurization. In any case, in quoting experimental data, it is always essential to quote the Reynolds number at which the tests were carried out. Another problem is the interference between model and tunnel walls.

Most of our attention in this laboratory work will be devoted to a description of the design and use of wind tunnels, together with the associated instrumentation.

The basic requirement is for a region of uniform flow with low turbulence, in which a model may be placed and tested. This region is called the working section.

II. THE METHODICAL INSTRUCTIONS

Before beginning the performance of laboratory work, the student should BE ABLE:

1. Using knowledge of physical properties of a liquid and gas define the basic thermodynamic parameters of atmospheric air: temperature, pressure, density.

Temperature changes in the atmosphere. Effect of temperature and pressure on density.

2. Using the equation of state for a gas, the equation of continuity, the Bernoulli’s equation for both incompressible and compressible flow find and explain interrelation between the basic thermodynamic parameters of a flow of a liquid (or gas), that flows in the channel of different forms.

3. Define parameters of atmospheric air, using the table of a standard atmosphere.

The literature: [1, гл.1]; [2, гл.1]; [3. гл.1, § 1]

III. ESSENTIAL THEORETICAL DATA

The behaviour of an aerodynamic body immersed in a moving fluid is governed by the physical properties of the fluid itself. Aircraft operate within the mass of air that surrounds the surface of the earth and permeates the region above that surface. This mass of air is called the atmosphere, and a detailed knowledge of the physical properties of the air within the atmosphere is essential to any study of aircraft behaviour. The most important of these properties are:

(a) Temperature.

No attempt is made here to give a formal definition of temperature. It is assumed that the concept is familiar to all readers, who are referred for further discussion to the standard text-books on thermodynamics. It is measured in this book on the absolute scale in degrees Kelvin (°K = °C + 273).

(b) Pressure.

The term pressure is used here to relate to the force per unit area exerted by the air on an immersed body by virtue of its static presence, and not by virtue of any relative motion which may exist. Other related concepts (dynamic pressure, total pressure) 'will be introduced later, and will be formally defined. Where the term pressure alone is used, it is this static pressure which is implied. In the atmosphere it derives, of course, from the weight of the mass of air located above the point in question.

(c) Density.

Density is the mass of unit volume of the air.

(d) Viscosity.

Viscosity in fluids is related to friction in solids. It exists in the form of tangential stress distributed within the fluid wherever relative motion, and hence a velocity gradient, exists, in particular where the fluid moves over the surface of an immersed body. It is measured by the coefficient of viscosity, m, defined as the ratio of the viscous stress, t, to the velocity gradient.

However, the values of these properties vary, not only with height above the earth's surface, but also with locality and, indeed, from day to day. Fluctuations with time may be rapid and largely unpredictable; and the pattern of variation with altitude may at times be completely irregular.

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