Double-pulsed Holography Research Articles

Wingtip Vortex Simulation by Using Nonequilibrium Eddy Viscosity Model

W INGTIP vortex is an important subject in fluid mechanics, as it is involved inmany practical engineering problems that can be found, for example, in [1–4]. Thus, it has long been the subject of numerous experimental and/or numerical investigations. Chigier and Corsiglia [5] pioneered the experimental study of thewingtip vortex. Theymeasured, by using hot-wire anemometry, thewingtip vortex of a square-tipped rectangular wing up to a 12c downstream distance. The maximum axial velocity at the vortex core at x0=c 0:25 was 1:4U1. They showed that themaximum tangential velocity increases with the angle of attack, while the vortex core radius remains unchanged. Green [6] and Green and Acosta [7] used double-pulsed holography to measure the instantaneous velocity distribution in trailing vortices of a round-tipped rectangular wing. At a 10 deg angle of attack, the axial velocity at the vortex core was 1:6U1 at x0=c 2:0. Chow et al. [8] used hot-wire and seven-hole probes to measure the flow over and immediately downstream of a roundtipped rectangular wing with a NACA0012 section at Rec 4:6 10 and a 10 deg angle of attack. The measurement data of this study will be used as reference data in this work. Numerical studies began to appear in the late 1980s. Srinivasan et al. [9] used a thin-layer Navier–Stokes solver with the Baldwin– Lomax turbulence model [10] to examine the influence of the tip-cap shape and tip planform on the wingtip vortex. Their results showed good agreement with the experimental data of Spivey and Moorehouse [11] for the surface pressures, except for the surface pressure suction peak induced by the vortex in the vicinity of the wingtip. The computed vorticity contours of the tip vortex, however, were rather poor in the sense that theywere highly distorted andmore diffusive with the downstream distance. Dacles-Mariani and Zilliac [12] studied, both numerically and experimentally, the wingtip vortex in the nearfield in conjunctionwithChowet al.’s experimental study [8]. The Baldwin–Barth turbulence model [13] was used with the modified production term suggested by Spalart through private communication [12] to reduce the eddy viscosity at the vortex core. The predicted velocity profile was in good agreement with the measured data, but the core static pressure was underpredicted up to 25%at the downstreamboundary. Craft et al. [14] recently performed a numerical simulation of the experiment of Chow et al. [8]. Three turbulence models, the eddy viscosity model, the nonlinear eddy viscosity model of Suga [15], and the two-component-limit (TCL) second-moment closure model [16], were adopted in their simulation. The results obtained using linear and nonlinear eddy viscosity models showed a far too rapid decay of the vortex core. Only the TCL model successfully reproduced the principal features of the vortex flowfield. The TCL model is a Reynolds stress transport model, which is much more complicated for application to practical flows than the two-equation models in spite of its distinguished merits. It is well known that conventional two-equation models perform rather poorly for flows that are not in near-equilibrium flow. To remedy this situation, Yoshizawa et al. [17] suggested a nonequilibrium eddy viscosity model, which works well for swirling (or strong vorticity) flows. The motivation of the present work is to investigate whether this nonequilibrium model performs well for wingtip vortex flow in comparison with the TCL model, which is a first attempt to the authors’ knowledge. It is found that the model performs almost as good as the TCL model; hence, it is strongly recommended for practical flow computations of wingtip vortex flow.

Fourier analysis of holographic data from a noisy disc brake and its implication for modelling

Double pulsed laser holography is used to obtain images with fringe lines giving detailed information about the mode of vibration of a braking system generating high-frequency noise or squeal. If the images obtained are digitized, it is shown to be possible, using image processing techniques, to develop algorithms that determine the displacement field of the exposed part of the disc surface. Using least-squares mathematical approximations it is then feasible to generate Fourier series type representations of the out-of-plane motion of the vibrating disc. Holograms of the disc rim can also be analysed to show how the vibrating motion varies throughout the thickness of the disc (in-plane motion). The results demonstrate the existence of travelling waves as well as significantly large amplitude and quite complex in-plane vibrations. Implications for the construction of mathematical models of disc brake noise are discussed.

Fast and full-field measurement of brake squeal using pulsed ESPI technique

In the field of structural testing, optical techniques have already been successfully applied for the determination of the dynamic structure response. Recently electronic speckle pattern interferometry (ESPI) systems using pulsed lasers are increasingly used to replace conventional double pulse holography interferometers. ESPI takes over not only the main benefits of holography toward conventional measuring techniques, such as full-field, noncontact measurement and sensitivity, but also uses modern camera and computer techniques for image capturing and processing. The capabilities of pulsed ESPI have already been further extended to 3-D measurements of the dynamic response and modal analysis of vibrations. An important area of application has already been demonstrated in the field of brake disks analysis, where pulsed ESPI systems provide complete maps of the vibration of any component of brakes during testing (brake disks, drums, calipers, and pads). Using the 3-D technique, completely automatic brake squeal inspection systems have been designed, which capture the squealing signal, automatically set and fire the laser, and provide a complete full-field deformation map of the component under test. We describe recent developments of the 3-D ESPI technique using pulsed lasers, and give examples of applications in the fields of dynamic structure response, with an emphasis in the field of brake squeal analysis. The application for the determination of the complex amplitude of vibration is demonstrated, which opens further potential into the field of operational deflection measurement and modal analysis. Conventional techniques using accelerometers and vibrometers have a very high temporal resolution and therefore are predestined to measure the frequency response of the vibrating object. ESPI overcomes this feature with very high spatial resolution, allowing a full-field measurement also for transient events.

An algorithm for evaluating overlapping bubble images recorded by double pulsed laser holography

The present paper describes a method for evaluating images of a bubbly flow in stirred aerated tanks which are typical when pulsed laser holography is applied as the measuring technique. Features of the brightness histograms of reconstructed bubble images are discussed. A procedure is presented to evaluate the bubble images taken from a reconstructed hologram in order to determine the center of gravity of the bubble image. Double pulsed holograms were taken to measure bubble velocities and diameters simultaneously. In this case, overlapping bubble images are sometimes observed in the reconstruction. This significantly impedes the evaluation of the characteristics of the bubbles. Thus, an algorithm is presented in this work to distinguish between single and overlapping bubble images and to separate the overlapping bubble image in a two-dimensional image for a bubbly flow at low void fraction recorded from double pulse holograms. This algorithm was confirmed to be effective if the bubble images are extracted from the entire image.

Double pulsed holography used to investigate noisy brakes

The vibrational characteristics of a noisy passenger car disc brake have been studied using the double pulsed holographic technique which has been developed to allow three orthogonal visual images of a vibrating brake system to be recorded simultaneously. These images show the disc to be vibrating in a bending mode whereas the pad is seen to be excited in a variety of modes such as bending, torsion, and often a combination of both. The development of the technique includes alternative ways of triggering the laser and typical results from the application of these differing methods are also included along with mechanical signals which confirm the visual interpretations. Final results, using a laser trigger delay technique, show that the disc mode waveform rotates about the disc at a rate equivalent to the frequency of vibration divided by the diametral mode order. Early work on a passenger car drum brake is also introduced, this complementing commercial ‘noise fix’ solutions and a proposed theoretical model.

TV-Holography and holographic interferometry: Industrial applications

A detailed vibration analysis is vital for industry. However, in particular in the automotive industry, one is interested in the acoustical field or in the determination of strain and stress. The acoustical field can easily be measured by microphones or intensity microprobes; however, it is also important to understand by which vibration modes the noise is emanating. TV-Holography, also called electronic speckle interferometry (ESPI), and double or multipulse (three or four) holography are two famous techniques of holography (Marwitz H., Praxis der Holographie, Expert Verlag, 1990). In speckle interferometry two images of the speckle field at different times are recorded directly with a CCD camera and then subtracted from each other by an image processing system. The resulting interference fringes are available in real time. In double pulse holography a ruby laser is triggered such that the interesting object vibration is recorded by two consecutive laser pulses. The interference fringes can be evaluated at reconstruction time by the phase shift technique, if two reference beams are used. The multi-pulse method is very useful for the investigation of operational vibration modes or of transient processes like shock waves. The calculation of the acoustical field and frequency spectrum requires the knowledge of the contour for the generation of boundary elements. The contour data can be taken either from CAD or FEM data or by measurement with optical 3D measurement systems. The acoustical field is calculated by solving the Helmholtz integral. For experimental strain and stress analysis the 3D shape and the 3D displacement are also necessary. For acoustical prediction the component of the displacement, which is perpendicular to the surface, is determined. On the other hand, for strain/stress calculation the tangential components to the surface are used.

Simultaneous acquisition of π/2 phase-stepped interferograms with a photorefractive Bi_12GeO_20 crystal: application to real-time double-pulse holography

We present a novel method for real-time analysis of vibrations in double-pulse laser holography. Two pi/2 phase-stepped interferograms are obtained simultaneously. An experimental demonstration gives a phase measurement with an accuracy of 4 degrees .

Holographic studies of the bubble generated by a seismic airgun

Using a Nd:YAG laser in single and double pulse mode, the dynamic behavior of bubbles generated by a small airgun, placed in a water tank, has been investigated. Single pulse holograms of the bubble have been obtained with the purpose of investigating the shape and smoothness of the surface. Interferometric holography, or double pulse holography, of the bubble has been performed in order to detect the pressure wave from the gun at the early stages of the bubble expansion. The optical phase delay of the laser beams traversing the test section is caused by a local change in the refractive index of the water in the tank due to the acoustic disturbance. A theory for relating the change of the refractive index to bubble parameters is presented. Modeling of interferometric holograms has been performed with the aim of qualitatively comparing them with the recorded holograms. From the holograms it is observed that the surface of the bubble is relatively smooth during the first expansion. During the contraction, and the following oscillations, the bubble surface becomes highly nonsmooth and chaotic due to the developed turbulence in the surrounding water.

Drop properties after secondary breakup

Drop properties during and after secondary breakup in the bag, multimode and shear breakup regimes were observed for shock-wave-initiated disturbances in air at normal temperature and pressure. Test liquids included water, n- heptane , ethyl alcohol and glycerol mixtures to yield Weber numbers of 15–600, Ohnesorge numbers of 0.0025–0.039, liquid/gas density ratios of 579–985 and Reynolds numbers of 1060–15080. Measurements included pulsed shadowgraphy and double-pulsed holography to find drop sizes and velocities after breakup. Drop size distributions after breakup satisfied Simmons' universal root normal distribution in all three breakup regimes, after removing the core (or drop-forming) drop from the drop population for shear breakup. The size and velocity of the core drop after shear breakup was correlated separately based on the observation that the end of drop stripping corresponded to a constant Eötvös number. The relative velocities of the drop liquid were significantly reduced during secondary breakup, due both to the large drag coefficients caused by drop deformation and the reduced relaxation times of smaller drops. These effects were correlated successfully based on a simplified phenomenological theory.

Continuous- and dispersed-phase structure of dense nonevaporating pressure-atomized sprays

The structure and breakup properties of the dense-spray region of nonevaporating pressure-atomized sprays were studied. The multiphase mixing layer near the injector exit was emphasized, considering large-scale (9.5mm injector diameter) water jets injected vertically downward in still room air. Phase-discriminating laser velocimetry and double-pulse holography were used to measure phase velocities and drop-size properties for both nonturbulent and turbulent jet exit conditions. Present test conditions involved two types of primary breakup: 1) aerodynamic breakup for nonturbulent jets where properties could be correlated using earlier aerodynamic breakup theories; and 2) turbulent breakup for turbulent jets where drop properties could be related to liquid turbulence properties. Both mechanisms yielded Weber numbers exceeding secondary drop breakup limits near the liquid surface. Significant effects of separated flow were observed for present test conditions; however, scaling analysis suggests reduced effects of separated flow at higher injector velocities and ambient pressures—largely due to finer atomization.

Implementation of double-pulsed holography in evaluation of whole-body vibration

This study used a unique holographic technique to evaluate the effects of vibration on soft tissues and bones. It was possible to record forced whole-body vibration in humans by holograph interferometry using a double-pulsed ruby laser. The study investigated the manner in which the muscles of the back and vertebral column are affected by vibrations applied to the human buttocks in the sitting position. The subject was exposed to vibration at two frequencies: 40 and 60 Hz (vertical Z axis). Transmission of the vibrations along the subject's back was recorded by means of double-pulsed holography and electromyography. Evaluation of the vibration pattern showed that the vibrations are transmitted along the back all the way up to the neck and head. The pattern of vibration in the muscles of the back and vertebral column showed that the greatest effect was exerted on the lumbar region of the back and the area of transition between the thoracic and cervical regions.

Unsteady flow in trailing vortices

The instantaneous velocity distribution in trailing vortices generated by lifting hydrofoils has been measured in the Low Turbulence Water Tunnel at the California Institute of Technology. Two different rectangular planform hydrofoils with small aspect ratios were tested. Double-pulsed holography of injected microbubbles, which act much as Lagrangian flow tracers, was used to determine instantaneous axial and tangential velocities. Measurements were made at various free-stream velocities, angles of attack, and downstream distances. The vortex core mean axial velocity is consistently greater than the free-stream velocity near the hydrofoil trailing edge, and decreases with downstream distance. The mean axial velocity is strongly Reynolds-number dependent.Axial flow in the trailing vortex is highly unsteady for all the flow conditions studied; peak-to-peak fluctuations on the centreline as large as the free-stream velocity have been observed. The amplitude of these fluctuations falls rapidly with increasing distance from the centreline. For an angle of attack of 10° the fluctuations consist of both ‘fast’ and ‘slow’ components, whereas for α = 5° only ‘fast’ fluctuations have been observed. Peak decelerations of the centreline fluid occur with amplitude comparable to the maximum centripetal acceleration around the centreline. Certain unusual structures of the vortex core - regions in which the flow direction quickly diverges from the free-stream direction, and then equally quickly recovers - have been labelled ‘vortex kinks.’

Structure of the near-injector region of nonevaporating pressure-atomized sprays

The dense-spray region of pressure-atom ized nonevaporating sprays was studied, emphasizing the properties of the multiphase mixing layer that surrounds the liquid core during atomization breakup. The dispersed-phase properties of a large-scale (9.5-mm injector diameter) water jet injected vertically downward in still air were measured using single- and double-pulse holography for both fully developed and slug flow jet exit conditions. The inner portion of the mixing layer contained large irregularly shaped liquid elements and drops, and the proportion of spherical drops increased and drop sizes decreased with increasing radial distance. For present test conditions, the liquid core and the large liquid elements cause mean liquid volume fractions to be high near the axis; however, the gas-containing region was relatively dilute at each instant. Additionally, the velocities of large drops were generally much larger than small drops and predictions based on the locally homogeneous flow approximation, providing direct evidence of significant separated-flow effects in the flow. Finally, the degree of flow development at the jet exit had a substantial effect on the structure of the mixing layer, with increased turbulence levels increasing the number and size of large irregular liquid elements through distortion of the surface of the liquid core—enhancing rates of removal of liquid from the core.

Three-dimensional flow visualization of shock wave using double-pulsed holographic interferometry. 1st report, Three-dimensional measurement of shock wave configuration generated from the flying u-shaped pin in a supersonic flow and its date processing.

It is extremely important to obtain basic knowledge of shock waves formed in turbo machines rotating at supersonic speed. The main purpose of this research is to develop an advanced 3-dimensional visualization technique using double-pulsed holographic interferometry with diffused laser light. As the initial step of this research, this technique was applied to observation of the oblique shock waves generated at a "U" -shaped pin, which was utilized in order to get better understanding of the 3-dimensionality of an object flying counter to the supersonic flow in this 1st report. The oblique shock waves generated at each edge could be visualized by the interference fringes and clearly separated in the reconstructed real image. Also by employing the reconstructed process coupled with a spatial filter, it could be verified that this diffused double-pulsed holography is effectively applicable to the 3-dimensional measuring technique of the shock wave. Furthermore, the algorithm for real-time data processing was developed.

Local Measurement of Particle Concentration, Size and Velocity in the Sheet of a Flat Spray Nozzle with Double Pulse Holography

Article Local Measurement of Particle Concentration, Size and Velocity in the Sheet of a Flat Spray Nozzle with Double Pulse Holography was published on March 1, 1989 in the journal International Journal of Turbo & Jet-Engines (volume 6, issue 1).

Investigation of the State and Transport of Particulate Phases in a Turbulent Tube Flow

A comprehensive description of gas-particle flows requires the knowledge of the distribution states of the particulate phase. To determine these distribution states in the first instance, optical methods are suitable, in particular double pulse holography. It yields a maximum amount of information, because as an imaging method it allows the determination of position, size and velocity of each individual particle of a collective simultaneously. The consequence is that the spatial distribution of the particles can be determined, as well as the frequency distributions of all the relevant quantities, such as diameter, velocity, momentum and energy as well as their local dependence and correlation. In the present case double pulse holography was employed to study different gas-particle flows at the outlet of a vertical tube (length 1 m, diameter 50 mm, mean gas velocity 3 m/s, Re = 10000). The dispersed phases consisted of water droplets with a broad size spectrum from 3 to 100 m̈m, and of glass spheres ranging from 20 to 35 μm. The most important results which are discussed in this paper are: 1 the local dependence of concentration, mean size and mean particle velocity, 2 the frequency distributions of particle size and velocity, 3 the distributions of energy and momentum as function of particle size, 4 the correlation of particle velocity with particle size. The results reflect the different behaviour of the two particulate phases used.

Holographic investigation of the impact response of human heads.

The impact response of intact human cadaver heads was studied by means of double-pulsed holography using a ruby laser, and the transient deformation process at the surface of the skull was recorded. Frequency and acceleration measurements following impact were obtained. On frontal impact, the shock wave reached the opposite pole after 0.2 to 0.3 msec, and maximum amplitudes were found in the temporal regions. On impact to the vault, the deformation wave reached the skull base after 0.15 to 0.175 msec. Centers of maximum amplitude were recorded in the parietal and temporal regions. Deviations in the interference lines occurred at some sutures, revealing discontinuities. High-frequency components of the shock wave were demonstrated. The authors believe that fracture mechanisms may be elucidated by further studies of impact responses.

Measurement of whole-body vibration by double-pulsed holography

A technique for registering vibration and deformation patterns has been adapted for the measurement of whole-body vibration in humans. Double-exposure holographic interferometry produces three dimensional pictures of the body, allowing exact measurement of the subject's movement between the two pulses. In this study an interval of 600 μs between the two pulses was used, producing measurements with a resolution of less than 0.3 × 10 −6 m. The subject standing in a fixed posture, was exposed to the laser beams first without vibration and then with vibration. The picture without vibration is needed as certain movements due to life functions of the body such as heartbeat, blood circulation etc are involved therein. This basic pattern should be considered when analysing the pictures with vibration. Different types of vibration in various postures were studied. Tests were also conducted when a reflective coating was applied to the skin. The results show that the method is applicable for measuring whole-body vibration and suggests further tests with more modern laser equipment which is now available. Such equipment can produce pulses with a high repetition rate and of much better quality than those obtained in this study. Once coordinated to the heartbeat and to the working frequency of the vibrating object, a reliable analysis of whole-body vibration can be maintained.

Three-dimensional shock structure in a transonic flutter cascade

Rapid double-pulse holography was employed to obtain detailed, two-dimensional images of the shock forming during simulated flutter in a transonic flowfield. The experiment comprised a linear cascade of airfoils externally oscillated in torsion and viewed tangentially at the shock surface. Three biconvex airfoils were subjected to harmonic pitching motion about the midchord axis at a frequency of 0.53 while immersed in a Mach 0.81 flow. Failure to produce observable shocks led to use of choked flow with a Mach number near one, of which 50 holograms were taken. The images revealed a narrow shock surface with a spanwise variation in the shock properties. The method is concluded to be useful for examining transonic flowfield shocks in the presence of airfoil flutter.

Ejection of material from shocked surfaces

Velocity interferometry and double-pulse holography have been used to study material ejected from surfaces which are impulsively loaded with plane shock waves. Experiments performed on aluminum shocked to 25 GPa (250 kbar) provide the average mass and velocity distribution as well as the spatial distribution of material ejected from the surface. A total mass of about 3 μg/cm2 was ejected when the shock arrived at the surface in the present experiments, and a substantial part of this material resulted from jetting at small pits on the surface.