Experimental Aerodynamics

Stefano Discetti, Andrea Ianiro, Stefano Discetti, Andrea Ianiro

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781498704045
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería mecánica

Experimental Aerodynamics provides an up to date study of this key area of aeronautical engineering. The field has undergone significant evolution with the development of 3D techniques, data processing methods, and the conjugation of simultaneous measurements of multiple quantities. Written for undergraduate and graduate students in Aerospace Engineering, the text features chapters by leading experts, with a consistent structure, level, and pedagogical approach. Fundamentals of measurements and recent research developments are introduced, supported by numerous examples, illustrations, and problems. The text will also be of interest to those studying mechanical systems, such as wind turbines.

SECTION III

Velocity measurements

CHAPTER NINE

Thermal anemometry

Ramis Örlü and Ricardo Vinuesa

Contents

9.1Introduction

Background

Reference literature and content

9.2Basic principles

Heat transfer characteristics

Modes of operation

9.3Probe design, manufacturing, and repair

Commercial versus in-house repaired/built probes

Hot-wire materials and geometrical constraints

Wire treatment: Etching versus plating

The prongs-wire connection: Soldering versus welding

Preaging, aging, and drift

9.4Calibration and its relations

Precautions and presettings

Single-wire probes

Multiwire probes

Temperature calibration

Calibrations for low velocities

9.5Measurements

9.6Limitations and corrections

Wall/probe interference and wall-position determination

Temporal and spatial resolution

Corrections for temperature fluctuations and drift

Acknowledgments

Problems

References

9.1Introduction

Background

The main objective of experimental fluid mechanics is the measurement of local flow velocities, and in this respect, hot-wire anemometry (HWA) is without doubt the most versatile and widely used laboratory method. The term “hot-wire anemometer” implies the usage of a hot, that is, heated, wire to measure wind speeds. Although the term has a historical justification since the early usage was restricted to measurements in air only, the so-called hot-film anemometers have been used in various liquids as well. Nonetheless, due to the emergence and advancements in laser Doppler velocimetry/anemometry (LDV/LDA) and particle image velocimetry (PIV) to be discussed in Chapter 10, HWA has again become more focused on measurements in gases, leaving the area of measurements in liquids primarily to optical measurement techniques. The measurement principle of the hot wire (and of thermal anemometers in general) is based on the fact that the local fluid velocity is measured by sensing the changes in forced convection from a small, electrically heated, sensor exposed to the flow of interest, which makes it an indirect measurement technique. Its small size and good frequency response as well as applicability to a wide velocity range with high accuracy and resolution makes it especially suitable for rapidly changing flow velocities such as in transient and turbulent flows.

HWA was also the first technique that enabled the study of turbulent fluctuations quantitatively. In fact, it was the only measurement technique capable of measuring high frequency and amplitude velocity fluctuations, prior to the development of LDV in the 1970s.* Although “modern” optical techniques such as LDV and PIV are often claimed to outrival the “very classical” technique of HWA [5], the latter remains the technique of choice when it comes to validation of numerical simulations or scaling laws, for example, in the field of turbulence, where the range of both spatial and temporal scales challenges these more “modern” techniques.^† Considering peer-reviewed publications, the occurrence of the aforementioned three common measurement techniques is depicted in Figure 9.1, which indeed confirms that optical measurement techniques are more prominent, but it also underlines that HWA preserves its importance in the research community since almost half a century. Nonetheless, faced with these facts,^‡ one cannot deny that LDV, and in particular PI...