Home HomeSir James Lighthill, University College London
The recent three-volume history Twentieth Century Physics [1] includes my 118- page Chapter 10, Fluid Dynamics about a field where the 20th centurys first decade saw big breakthroughs in the analysis of nonlinear problems for which the physicists standard perturbation methods break down and which would later be named singular perturbation problems. Besides the 1904 elucidation of boundary-layer structure by Prandtl, these included the 1910 elucidation of shock-wave structure in complementary investigations by Rayleigh and by Taylor. Subsequent advances in shock wave dynamics brought crucial new discoveries on the structure and propagation of weak shocks, and also on shock-wave / boundary-layer interaction, both with major aeronautical implications; along with still harder investigations into the structure and propagation of strong shockwaves such as appear in explosions and implosions and also around spacecraft re-entering the earths atmosphere. It may, perhaps, be worth noting that questions which remain relatively simple for weak waves, like the nature of reflexion and diffraction by a solid body, raise formidable and intriguing difficulties for strong shock waves. In this introductory paper looking back over a century of shock wave dynamics, I highlight (i) key analytical approaches for both weak and strong shock waves, (ii) beautiful and effective optical methods for use in wind tunnels and shock tubes and (iii) powerful techniques for accurate shock capturing in computational fluid dynamics.
sv970129.pdf (Scanned)
TOPMalcolm J. Crocker, Auburn University (U.S.A.)
In recent years there have been rapid advances in digital computers, the miniaturization of electronic circuits and in the development of new materials. In the acoustics and vibration fields, these advances have led to a continual increase in computational power and speed of analysis, improved acoustics and vibration transducers and instrumentation and better measurement techniques. In many cases the developments have been synergistic; new experimental knowledge has led to improved theoretical models and approaches and vice versa. Improved computers have allowed the development of a host of computer programs and increasing numbers have become available as commercial acoustics and vibration software. Of particular importance has been the development of numerical calculation schemes such as the finite element method (FEM) and the boundary element method (BEM) which have led to much improved predictive capabilities in many fields. Fields that have shown advances in recent years include: Aeroacoustics, Underwater Sound, Ultrasonics, Mechanical Vibrations, Noise Control, Architectural Acoustics, Physiological Acoustics, Speech Communication, Musical Acoustics, Bioacoustics, Acoustical Measurements and Instrumentation and Transducer Design. As examples, a few of these advances will be concisely summarized as follows: increased knowledge and use of Sonochemistry, new Computational Aeroacoustics techniques, improved Underwater Transducers and Sonar System design, use of Ultrasonics in Medical Diagnostics and Medical Imaging and for Therapeutic Effects and the use of Ultrasonics in Nondestructive Testing, Active Noise and Vibration Control including the use of PVDF and PZT as piezoelectric structural sensors and actuators, improved Architecural Acoustics Design, increased Cochlear Mechanics knowledge and the use of Cochlear implants, improved techniques for Speech Coding and Recognition of Speech, improved Sound Intensity Measurements and their uses for Sound Power Determination and Noise Source Identification and Ranking, and better Digital Audio and Hearing Aid Transducer Design.
sv970025.pdf (Scanned) TOPHanno Heller, DLR, Institute of Design Aerodynamics (Germany)
Significant advances in understanding helicopter noise have been made through several recent major European dedicated rotor aeroacoustics research projects. The availability of the German Dutch Wind Tunnel (DNW) allowed research on relatively large main rotor models (up to 40% full-scale). The simultaneous application of blade-mounted absolute pressure sensors to probe into the aeroacoustic source mechanisms, of advanced acoustic-data acquisition systems for radiated noise, and of sophisticated flow-visualisation/-measurement techniques (Laser Light Sheet, Laser Doppler Velocimetry) to study the interrelationship of rotor aerodynamics and blade dynamic response, has provided insight into the aeroacoustics of rotors. Towards controlling rotor noise, projects dealt with higher harmonic or individual blade control to affect blade vortex interaction. Much data served as benchmark information to advance theoretical rotor noise prediction for moderate tip speed conditions based on linearised potential flow methods or Euler methods combined with the acoustic analogy approach", and for high tip speed conditions based on Computational Fluid Dynamics and Kirchhoff approaches. This paper discusses several major European helicopter aeroacoustics research projects (e.g. the EU-initiated HELINOISE and HELISHAPE projects, and the joint European-US HART project) to indicate recent relevant progress and how the findings provide guidance towards the ultimate objective - the quiet helicopter.
sv970081.pdf (Scanned) TOPChris R. Fuller, Virginia Polytechnic Institute & State University (U.S.A.)
Recent research and applications in the field of Active Structural Acoustic Control (ASAC), a technique for reducing low frequency sound radiation from structures, are discussed. The paper overviews some new advances in the ASAC component and system areas of actuators, sensors, controllers, analysis and optimization. Recent commercial applications of ASAC as well as some promising new uses are briefly outlined.
sv970054.pdf (Scanned) TOPDaniel J. Inman, Virginia Polytechnic Institute & State University (U.S.A.)
This work examines the use of piezoceramic devices, as well as viscoelastic elements in various configurations as damping treatments to suppress unwanted vibrations in structural elements such as satellite components, bladed disk assemblies and circuit boards. The combining of the use of piezoelectric materials with viscoelastic damping treatments is a form of hybrid damping referred to here as smart damping. Here we examine various configurations for combining viscoelastic and piezoceramic damping treatments in view of the mechanics of layered media and in view of controllability considerations. The various designs and configurations studied are based on trying to take advantage of the best properties of passive damping treatments and of active damping provided by smart structures actuation. These materials allow the construction of unobtrusive sensors and actuators fully integrated into a structural system along with any viscoelastic material. One advantage of using piezoceramic elements combined with viscoelastic elements is the possibility of using the active component to compensate for the temperature and frequency dependence of a viscoelastic element rendering a treatment that is insensitive to these effects. Previous work in the area is presented which is followed by a discussion of modeling issues, design considerations and a summary. Several examples illustrate the usefulness of this approach for vibration suppression in structural components.
sv970151.pdf (Scanned) TOPR.B. Randall, University of New South Wales (Australia)
For many years it has been recognised that so-called envelope analysis is a very powerful tool in the detection and particularly diagnosis of rolling element bearings. A recent quantitative study of the effects of masking signals has shown the benefits of performing the envelope analysis digitally, because of the better quality of filtering and the improved flexibility. This paper demonstrates the considerable enhancements which can be achieved by analysing the squared envelope signal, provided that the ratio of bearing to background signal can be made greater than unity, and discusses methods of achieving this by optimum bandpass filtering and self adaptive noise cancellation to remove masking by gear signals. Techniques are also discussed for dealing with very short signals, and varying signal paths suchas for planet bearings in epicyclic gearboxes. Developments are also proceeding in the field of gear diagnostics, made possible by the availability of new digital signal processing packages and for example direct downloading of vibration signals from a DAT recorder. An example is the increase in the application of time-frequency analysis techniques such as wavelet analysis and variants of the Wigner-Ville distribution. The paper discusses a number of techniques and compares the use of direct transmission error measurement with externally measured vibration signals.
sv970199.pdf (Scanned) TOPSamir N.Y. Gerges, Federal University of Santa Catarina (Brazil)
This paper addresses the important practical issues of hearing protectors attenuation that are used in industry to protect workers from high levels of noise. Comments on the difficulties in the measurement attenuation of hearing protectors are discussed. A new work on the numerical modelling of the outer ear-canal is presented that considers the eardrum acoustic impedance and examines finite element (FEM) and infinite FEM for the quantification of the noise attenuation of the protector. In addition, the numerical model considers the geometry of the outer ear, outer ear-canal and the eardrum acoustic characteristics. The model can serve as a quick and low cost tool for the optimisation of a protector design and the investigation of the effect of different parameters such as protector insertion, effect of leakage, materials, and others on the protector noise attenuation.
sv970060.pdf (Scanned) TOP