Ministry of education and science of Ukraine

National Aviation University

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:

1. 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).

2. 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.

3. Density.

Density is the mass of unit volume of the air.

4. 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, , defined as the ratio of the viscous stress, , 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.

The International Standard Atmosphere (ISA)

To obviate this difficulty, an entirely fictitious set of values has been postulated to represent some kind of average conditions. The values have been established by international agreement, and this set of figures giving variation with altitude of physical properties of air is known as the International Standard Atmosphere. It does not, of course, represent actual conditions anywhere at any given time, but it is useful for reference purposes. Thus it is possible to quote the performance of an aircraft at a certain altitude; and this is under­stood to mean the performance under the physical conditions associated with that altitude in the Standard Atmosphere.

The ISA gives the variation with altitude of temperature, pressure, density, speed of sound, coefficients of absolute and kinematic of viscosity.

Sea level values are denoted by the suffix 0. It is also useful to refer to relative pressure, relative temperature and relative density, by which is implied in each case the local value divided by the sea level value.

The standard sea level temperature of air, T₀, is 288*2 °K. With increases in altitude, the temperature falls initially, at a steady rate, which is called the lapse rate and denoted by A; but once a certain height is reached, it remains constant with further increases in altitude. The range of altitudes within which the tem­perature falls steadily is called the troposphere. Within this region the lapse rate in the standard atmosphere is approximately 6,50 °K per kilometer.

The point at which the temperature ceases to fall is called the tropopause. It occurs at a height very slightly over 11 km, at which point the temperature is 216,7 °K. The region above this altitude, within which the temperature remains constant at his figure, is called the stratosphere.

The standard sea level air pressure is 1,013*10⁵ N/m², and on the altitude of 11 km – 2,227*10⁴ N/m².

The standard sea level density of air is 1,225 kg/m³.

Static, Total and Dynamic Pressure

(3.13)

It follows from Bernoulli's Equation that, in a given field of flow, any increase in velocity is accompanied by a reduction in pressure, and vice versa. The pressure is lower at points in the flow where the speed is higher. The greatest pressure is achieved when the speed is zero, i.e., at stagnation points, and the pressure at such points is called the stagnation pressure, usually denoted by p₀. The pressure and velocity at any given point in the flow are then related by the equation

, (1)

This shows that the stagnation pressure is a constant, i.e., that the pressure is the same at all stagnation points in a given flow, and further that the concept of stagnation pressure has significance even in a flow where there are no actual stagnation points. For this reason, it is often referred to not as stagnation pressure but as the total pressure associated with the flow.

Consider now the quantity p₀ — p, i.e., the difference between the total pressure of the flow and the local value of the pressure. This quantity has the dimensions of pressure and is a pressure associated with the motion of the fluid. It is therefore called the dynamic pressure. In the case of incompressible flow considered here, the dynamic pressure is equal to .

To distinguish it from the dynamic and total pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure.

The following observations, arising from the above definitions, should be noted:

(a) Total pressure = static pressure + dynamic pressure.

(b) Total pressure is constant throughout the field of flow.

(c) The dynamic pressure and static pressure generally vary through the field.