The communication of vibration, motion or physical resistance to a user is called:

Elementary Acoustics

Sound consists of mechanical vibrations that propagate through a medium. Sound induces movements or displacements of the particles in the medium. Imagine a small sphere that expands to create a denser area. This compression will propagate as particles are displaced in the direction of propagation. If the sphere then contracts, it can create an area of rarefaction, or lower density, and this also can propagate outward. These compressions or rarefactions can be expressed in terms of particle displacement or as a pressure differential.

Now imagine a sound source that creates a series of compressions and rarefactions that propagate through the medium. A source with a purely sinusoidal pattern of compression and rarefaction would produce energy at only one frequency. The frequency of this sound is measured in cycles per second. A sound that takes t seconds to make a full cycle has a frequency f = t− 1. Older references may refer to frequency in cycles per second, but the modern unit of frequency is the Hertz (Hz) and a frequency of 1000 Hz is expressed as one kiloHertz (1 kHz). If a sound took 1 s for a full cycle, it would have a frequency of 1 Hz. The wavelength of a tonal sound is the distance from one measurement of the maximum pressure to the next maximum. The speed of sound is approximately 1500 m s− 1 in water, roughly five times the value in air, 340 m s− 1. The speed of a sound c is related in a simple way to the frequency f and the wavelength λ by c = λf. An under-water sound with f = 1 Hz would have λ = 1500 m; for f = 1500 Hz, λ = 1 m. Not all sounds have energy limited to one frequency. Sounds that have energy in a range of frequencies, say in the frequency range between 2000 and 3000 Hz (2 and 3 kHz), would be described as having a bandwidth of 1 kHz.

One can imagine a sound wave as a growing sphere propagating outward from a compression or rarefaction generated by a point source. The initial movement of the source will have transmitted a certain amount of energy to the medium. If none of this energy is lost as the sound propagates, then it will be evenly diluted over the growing sphere. The acoustic intensity is defined as the amount of energy flowing through an area over a unit of time. As the sphere increases in radius from 1 to r, the surface area increases to 4πr2. The intensity of a sound thus declines as the inverse of the square of the range from the source (r− 2). A sound in the middle of the ocean can be thought of as spreading in this way until it encounters a boundary such as the surface or seafloor that might cause reflection, or an inhomogeneity in the medium that might cause refraction. One fascinating acoustic feature of the deep ocean is that sound rays propagating upward may refract downward as they encounter warmer water near the surface, and downward-propagating rays will refract upward as they encounter denser water at depth. When one is far from a sound source compared to the ocean depth, the sound energy may be concentrated by refraction in the deep ocean sound channel. This sound can be thought of as spreading in a plane, to a first approximation. In this case, sound intensity would decline as the inverse of the first power of the range, or r− 1. This involves much lower loss than the inverse square spreading loss in an unbounded medium.

Sound spreading is a “dilution” factor and is not a true loss of sound energy. Absorption, on the other hand, is conversion of acoustic energy to heat. The attenuation of sound due to absorption is a constant per unit distance, but this constant is dependent upon signal frequency. While absorption yields trivial effects at frequencies below 100 Hz, it can significantly limit the range of higher frequencies, particularly above 40 kHz or so. A 100 Hz sound can travel over a whole ocean basin with little absorption loss, while a 100 kHz sound would lose half its energy just traveling about 100 m.

II.E Ultrasonic Inspection

Ultrasonic methods use a beam of mechanical vibrations of wavelength in the range 0.5–10 mm (i.e., frequencies in the range 500 kHz to 10 MHz). Being mechanical vibrations, the particles of the specimen are displaced temporarily as the wave passes through, but the displacement is very small and the specimen is not damaged. Several types of wave motion are possible, the most important for NDT being:

Compressional waves. The particle displacement is parallel to the direction of wave propagation. Such waves are possible in both solids and liquids.

Shear waves. The particle displacement is at right angels to the direction of propagation, and such waves can only occur in solids.

Surface waves (also called Rayleigh waves). The particles vibrate in an ellipse perpendicular to the direction of wave propagation.

Lamb waves. These have a complex mode of propagation and occur chiefly in thin-sheet material.

Most ultrasonic flaw detection uses compressional or shear waves and uses short pulses of waves rather than a continuous wave.

Ultrasonic flaw-detection equipment uses discs of piezoelectric material to produce and detect ultrasonic waves. If a disc of such material, of appropriate thickness, is given an electrical pulse it vibrates, and if laid on the specimen surface with a suitable couplant, this pulse of vibrations is transmitted into the specimen as a beam of ultrasonic energy, initially of the same beamwidth as the piezoelectric disc. Similarly, a piezoelectric disc can absorb ultrasonic energy from the specimen and convert this into an electrical pulse, which can be amplified and displayed.

Four physical properties of ultrasound are of importance:

The velocity and wavelength depend on the elastic properties of the material. For example, a 5-MHz beam in steel has a velocity of 5900 m/s for compressional waves and 3230 m/s for shear waves.

At any interface where there is a change in elastic properties, some ultrasonic energy will be transmitted and some reflected.

If an ultrasonic beam is incident on an interface at any angle except 90°, mode conversion occurs and the transmitted and reflected beams may be compressional, or shear, or both, depending on the angle of incidence (Snell's law applies).

The ultrasonic beam is not uniform in intensity, either over its cross section or along its axis.

There are broadly two main types of ultrasonic probe (Figs. 4 and 5). Figure 4 shows a compressional-wave probe designed to transmit a beam at right angles to the specimen surface. Figure 5 shows a shear-wave probe in which the initial compressional wave from the piezoelectric crystal is converted into a shear wave at the interface of the Perspex (Lucite) shoe and the specimen surface. The angle of the shoe is chosen so that a transmitted compressional wave is not transmitted into the specimen.

The most common method of data presentation is the A-scan technique as shown in Figure 4, also commonly known as “pulse echo.” The flaw signal is not an image of the flaw as would be the case in radiography-on-film, but the position of the flaw signal along the trace is an accurate measure of the depth of the flaw below the surface; the height of the signal is related to the flaw size, and the shape of the signal envelope can sometimes be related to the nature of the flaw. Interpretation is therefore complicated and requires skill and experience.

In the past, this A-scan technique with a hand-held probe acting as both transmitter and receiver, or a pair of probes, was the most widely used technique, particularly for flaw detection in welds. Mechanically scanning multiprobe systems are now more common and digitization of the received signals, followed by computer analysis and flaw-image reconstruction, has developed rapidly.

Many probe types and configurations are used, the most important are:

Focused probes, which produce a more intense beam.

Electromagnetic-acoustic (EMA) probes, which can be used without contact couplant and will operate through paint or scale.

Immersion and water-jet probes for underwater scanning.

Double-crystal probes with “transmit” and “receive” built into one head.

Alternative techniques for special applications are:

Through-transmission techniques, often used for bond testing.

Time-of-flight diffraction (TOFD) measurements used to accurately locate the tips of an internal crack by using the diffracted ultrasonic energy and the transit-time between two probes.

Acoustic holography to produce three-dimensional information.

Synthetic aperture focusing (SAFT) to extract more information from the ultrasonic signals and build up a more detailed flaw image.

Small battery-powered ultrasonic thickness gauges with a digital read-out.

Lamb waves for plate, bar, and wire testing.

Velocity and attenuation measurements to investigate material properties.

Ultra-high-frequencies (400 MHz and higher) for high resolution on very thin specimens—sometimes called “ultrasonic microscopy.”

For some applications, air-transmission probes can be used, which do not require liquid couplant to the specimen surface.

The advantages of ultrasonic testing are:

The basic electronic equipment needed is not expensive and is basically the same whatever the thickness or material of the specimen, although different probes may be necessary.

There is no hazard to the operator.

Thicknesses up to several meters of fine-grain metal can be examined.

Crack sensitivity can be very good, given good operator skills.

The disadvantages are:

The complications of interpretation, from A-scan indication of a flaw to flaw identification. On complex-shaped specimens where there can be multiple-mode conversions, the received signal can be very complex.

On nonisotropic materials with large grain size, such as some austenitic steels and copper alloys, the ultrasonic beam is scattered and bent, and such materials can be very difficult or impossible to inspect satisfactorily.

10.1 INTRODUCTION

In recent years, the effect of mechanical vibration on the stability threshold of thermal systems has been the subject of numerous studies. In thermo-vibrational convection, the energy of mechanical vibration in the presence of a temperature or a concentration gradient can be used to control the onset of convective motion. This type of convective motion, in which the buoyancy force may be thought of as time dependent, has attracted the attention of many researchers. Theoretical studies concerning linear and weakly nonlinear stability analysis of the Rayleigh–Bénard convection subjected to a sinusoidal acceleration modulation have been conducted by several researchers, e.g. Gershuni et al. (1970), Gresho and Sani (1970), Biringen and Peltier (1990), and Clever et al. (1993). Relative to the classical problem of the Horton–Rogers–Lapwood which has been documented in many books, see, for example, Ingham and Pop (1998, 2002) and Nield and Bejan (1999), only a few works have been devoted to the onset of convection under the action of harmonic vibration. Among existing thermo-vibrational studies in porous media saturated by a pure fluid we mention the works of Zen'kovskaya (1992) and Zen'kovskaya and Rogovenko (1999) in an infinite layer heated from below or above, Khallouf et al. (1996) and Sovran et al. (2000) in a rectangular cavity heated differentially, Bardan and Mojtabi (2000) in a rectangular cavity heated from below. Also, Jounet and Bardan (2001) consider the thermohaline problem in a rectangular cavity. Finally Sovran et al. (2002) consider the effect of vibration on the onset of Soret driven convection in a rectangular cavity. In addition, Rees and Pop (2000, 2001, 2003) have recently reported the effect of g-jitter on some boundary-layer problems.

It should be emphasized that vibration-induced natural convection may exist even under weightlessness. This phenomenon is in contradiction with the common belief that natural convection cannot exist in space. Further research showed that a spacecraft in orbit is subject to many disturbing influences, see Nelson (1994). These influences result in the production of residual accelerations, which are commonly referred to as ‘g-jitter’. It is important to note that these accelerations occurring on microgravity platforms may induce disturbances in space experiments that deal with liquids in the presence of density gradients. The construction of the international space station has increased interest in the influence of g-jitter on convective phenomena. The term g-jitter is used to describe a residual acceleration of 10−3 g fluctuating with a frequency that varies from 10−3 to hundreds of hertz. This acceleration is the result of crew activity as well as machinery on board the space station. One of the aims of the space station is to perform experiments under zero gravity conditions, i.e. without natural convection. It is well known that the g-jitter can produce drastic disturbances in space experiments as, for instance, in solidification processes during which mushy zones modelled as porous media may appear.

With reduced gravity, other forces, which are normally masked on earth, may play a dominant role in buoyancy-driven convection. For a better understanding of the g-jitter effects, it was suggested that harmonic oscillations might be used to model this phenomenon, see Alexander (1994).

The objective of this chapter is to study the effect of this vibrational mechanism on double-diffusive convection in a porous medium with or without Soret effect. We first present the so-called time averaged formulation applied to the double-convective oscillation in a porous medium in the framework of a Darcy–Boussinesq approximation. This formulation, restricted to the limiting case of high-frequency and small-amplitude vibration, can be effectively applied to investigate thermo-soluto-vibrational convection. We will later show, by using the scale analysis method, what is meant by high frequency and small amplitude.

Under the Soret effect, the temperature gradient can produce mass flux in a multi-component system, see for example De Groot and Mazur (1984). The influence of vibration on the thermo-solutal convective motion with Soret effect has been studied in an infinite horizontal fluid layer, see Gershuni et al. (1997, 1999). The governing equations were described by a time-averaged formulation, which can be adopted under the condition of high-frequency and small-amplitude vibration. Their results showed that vibrations could drastically change the stable zones in the stability diagram. Generally, vertical vibrations (parallel to the temperature gradient) increase the stability threshold of the conductive mode. Smorodin et al. (2002) studied the same problem under low frequency vibration. They concluded that the synchronous mode has a stabilizing effect on the onset of convection.

In this chapter, we describe the problem of vibrational double-diffusive convection and write down their basic system of equations in the framework of the standard Darcy–Boussinesq approximation. The system for the mean field is obtained by applying the averaging technique. In the first part of the chapter, the thermo-convective motion in an infinite horizontal layer and confined cavity saturated by a binary mixture is studied. A linear stability analysis is carried out. The influence of the direction of vibration on the stability threshold is investigated. Stationary and Hopf bifurcations are investigated and the corresponding convective structures under the combined effects of vibration and gravitational accelerations are examined. In the second part, a numerical simulation has been carried out which allow us to corroborate the results obtained from the linear stability analysis.

The Benefits of Multiple Bellows in Industrial Plants

Bellows can vibrate both from internal fluid flow, and externally imposed mechanical vibrations. At high flow, velocities and flow induced resonance produces bellows fatigue. Multiple bellows are less susceptible to vibration failures because of the damping effect of the interplay friction. The benefits of multiple rubber expansion joints are given below:

increase in flexibility and reducing deflection forces

ability to cope with higher pressures and lower thrust forces

lower spring rates and higher elasticity

minimal installation length due to high elasticity

fail safe design

high corrosion resistance can be attained economically.

15.3.1.2 The Eardrum: Interface Between Outer and Middle Ear

Airborne sound waves reach only as far as the eardrum. Here they are converted into mechanical vibrations in the solid materials of the middle ear. Sounds (air pressure waves) first set up sympathetic vibrations in the taunt membrane of the eardrum, just as they do in the diaphragm of some types of microphones. The eardrum passes these vibrations on to the middle ear structure.

Although we recognize noise pollution as a major environmental problem, it is difficult to quantify the effects it has on human health. Exposure to excessive noise has been shown to cause hearing problems, stress, poor concentration, productivity losses in the workplace, communication difficulties, fatigue from lack of sleep, and a loss of psychological well-being.

3.8 Instabilities produced by the control system

Steam generation systems are subjected to flow instabilities due to parametric fluctuations and inlet conditions, which may result in mechanical vibrations of components and system control problems.

Oscillations of flow rate and system pressure can cause mechanical vibrations; problems of system control in extreme circumstances disturb the heat transfer characteristics so that the heat transfer surface may burn out.

Of the various types of oscillations, those generated from control systems response are the most common. Controllers, such as the master recirculation flow controller, are typically more stable at the high end of their control band than at the low end. To account for this problem, interlocks and procedures prevent automatic master flow control below some value (typically < 45%). Other control systems that effect BWR oscillations are the pressure control system and the feedwater control system. Even with the constant modulation of the turbine control valves to regulate reactor pressure and feedwater pump steam supply valves or feedwater regulating valves to control feedwater flow, periodic oscillations can be observed in reactor power during steady state operation. The amplitude of these observed oscillations has ranged from a few percent to 15%. Oscillations that occur from control system responses are not normally divergent and do not challenge fuel safety limits. In general, these are due to the malfunction of reactor hardware.

Resuspension

Deposited particles on room surfaces, especially in the size range 0.7–10 μm, can become resuspended in the indoor air, particularly through activities in the indoor environment. Factors such as mechanical vibration, surface structure and chemistry, surface loading, and aerodynamic and electrostatic forces can achieve a stronger effect than that of gravitation and hence influence the resuspension of particles. In different field studies, it was shown that activities in the indoor environment (e.g., running, children playing) resulted in a significant increase in PM contents, whereby essentially coarse particles were stirred up. Moreover, it was shown that the resuspension in rooms with wall-to-wall carpet was significantly higher than that in rooms with smooth flooring. Another source of indoor PM could be mattresses and clothing. In a fully controlled chamber, PM in the diameter range of 0.5–10 μm were deposited on new fabrics, which were moved in a standardized manner to observe whether clothing might act as a transport vector for inhaled airborne particles. On average, 0.3%–3% of deposited particles were subsequently released with fabric motion, confirming that clothing can act as a vehicle for transportation of airborne particles via increased resuspension with vigor of movement and dust loading.

7.3.1 Noise sources in atomic force microscopy

The limitations of the metrological capabilities of an AFM due to thermal noise are well documented [11]. However, not only thermal but all noise sources need to be systematically investigated and their particular contributions to the total amount of the noise quantified for metrological purposes [12]. Note that most of the discussions on noise in AFM are also of relevance to other forms of SPM.

Noise source can be either external, including:

variations of temperature and air humidity;

air motion (for example, air-conditioning, air circulation, draughts, exhaust heat);

mechanical vibrations (for example, due to structural vibrations, pumps – see section 3.9);

acoustic (for example, impact sound, ambient noise – see section 3.9.6).

or internal noise (intrinsic noise), including:

high-voltage amplifiers;

control loops;

detection systems;

digitization.

It is also well known that adjustments made by the user (for example, the control loop parameters, scan field size and speed) also have a substantial influence on the measurement [13].

To reduce the total noise, the subcomponents of noise must be investigated. The total amount of the z axis noise can be determined by static or dynamic measurements [14] as described in the following section.

7.3.1.1 Static noise determination

To determine the static noise of an SPM, the probe is placed in contact with the sample, the distance is actively controlled, but the xy scan is disabled, i.e. the scan size is zero. The z axis signal is recorded and analysed (for example, RMS determination or calculation of the fast Fourier transform to identify dominant frequencies which then serve to identify causes of noise). An example of a noise signal for an AFM is shown in Figure 7.3; the RMS noise is 13 pm in this case (represented as an Rq parameter – see section 8.2.7.2).

FIGURE 7.3. Noise results from an AFM. The upper image shows an example of a static noise investigation on a bare silicon wafer. The noise-equivalent roughness is Rq = 0.013 nm. For comparison, the lower image shows the wafer surface: scan size 1 μm by 1 μm, Rq = 0.081 nm.

SONIC DRILLING

Sonic drilling is a relatively new technique for obtaining vadose zone soil samples for pore liquid analyses (www.sonic-drill.com). This technique is essentially a dual-cased drilling system that uses high-frequency mechanical vibrations to allow continuous core sampling and advancement into the profile. The drill head uses offset counterrotating weights to generate sinusoidal wave energy, which operates at frequencies close to the natural frequency of the steel drill column (up to 150 cycles per second) (Figure 7.9). The counterrotating balance weights are designed to direct 100% of the vibration at 0 degrees and 180 degrees. This action causes the column to vibrate elastically along its entire length. Resonance occurs when the vibrations coincide with the natural resonant frequency of the steel drill casing. This allows the rig to transfer timed vibrational energy to the top of the drill string. The high-energy vibrations are transmitted down to the face of the drill bit, producing the cutting action needed for penetration. Rotation and application of a downward force causes the drill string to advance through the profile. During drilling, the walls of the steel pipe expand and contract, causing the fluidization of soil particles along the drill string, which enhances drilling speed.

A dual-string assembly allows the use of an outer casing to hold the borehole open and an inner core barrel for collecting samples. Diameters of the outer casing range up to 12 in (30.5 cm). Diameters of the sampling core barrel range from 3 in (7.5 cm) to 10 in (25 cm). When the borehole is drilled, the core barrel is advanced ahead of the outer casing in 1 to 30 ft (95 m) increments, depending on physical conditions. After the core barrel is removed from the borehole, a plastic sheath is slipped over the barrel. The sample is extruded into the sheath. Subsamples are taken and stored in appropriate containers. Lined split-spoon samplers or Shelby tubes can also be used for sampling. In addition to the drilling of vertical holes, sonic rigs can angle-drill holes up to 75 degrees from horizontal. This is advantageous when sampling beneath existing waste disposal facilities such as landfills and impoundments.

The principal advantages of sonic drilling are that water is not required, perched water can be identified, and there is a relatively safe operating environment. Other advantages include the ability to collect continuous cores; drilling rates (up to 10 times faster than hollow-stem rigs [no drilling fluids needed]; fewer drill cuttings to dispose of (up to 80% less than other rigs); the ability to drill through bedrock, cobbles, and boulders; and the ability to drill to greater depths than hollow-stem augers (sonic drills can provide 3-in cores to a depth of 500 ft [153m]). Disadvantages include a higher cost than that for hollow-stem augers, equipment breakdowns, and potential volatilization of VOCs due to the heat that is generated.

3.2.5 Hydraulic Core Extraction

Conventionally, cores were extracted by knocking on the inner barrel after it was uninstalled and suspended. For large-caliber drill rigs, this not only involves high labor intensity, long extraction duration, and dangerous operation, but also subjects the plastic or fragile core samples to deformation and the brittle core samples to damage due to blockage inside the tube, mechanical vibration, or free fall of the cores, causing man-made destruction of the primary information of the strata, and adding to the difficulty of core description, scanning, cutting, and analysis.

With the hydraulic core extraction device we produced, cores are integrally and evenly pushed out of the inner barrel under the pressure of the mud pump, simply by coupling a mud pipe on the upper joint of the rig, and sealing the annular gap at the lower end of the inner and outer tubes. This device eliminates all aspects of the conventional extraction approach, meaning higher operational safety and significantly lower labor intensity and shorter supporting work time. More importantly, this method creatively ensures damage-free extraction of cores. The successful application of this technical output has provided high-quality physical data of the strata. The principle and field operation of the device are shown in Fig. 3.10.