Developments in Laser Techniques and Applications to Fluid Mechanics

Low-Reynolds-number effects on the inner region of a zero pressure gradient turbulent boundary layer have been investigated using a two-component laser-Doppler velocimetry (LDV) system. The momentum thickness Reynolds number R _θ is in the range 400 to 1320. The wall shear stress is determined from the mean velocity gradient close to the wall, allowing scaling on wall variables to be examined unambiguously. The results indicate that, for the present R _θ range, this scaling is not appropriate.

The flow structure in a concentric annular geometry with a large aspect ratio and a radius ratio of 0.506 has been investigated for inner cylinder (centrebody) rotation for one Newtonian and two non-Newtonian liquids: glucose, Xanthan gum and a blend of Laponite and CMC. Tangential velocity distributions have been measured for circular Couette flow (i.e. sub-critical Taylor numbers) and found to be in excellent agreement with theory. Tangential and axial velocity components have been measured for high-Taylor numbers to reveal the internal structure of the Taylor vortices. The axial-velocity data have been used to construct streamline plots of the radial-axial flowfields. The shear-thinning nature of the two non-Newtonian fluids is shown to have a strong influence on both the Couette flow and also the Taylor cell structure. Axial drift of the Taylor cells for the two non-Newtonian liquids is attributed to differences in their rheological characteristics (viscoelasticity versus thixotropy).

LDV measurements of the three-dimensional velocity flowfield in the confined swirling flow generated by a rotating cone are presented. The reported measurements covered two particular regimes exhibiting bubble-type of vortex breakdown. The corresponding Reynolds numbers were Re = 2200 and 2570, with values of the gap ratio H/R = 2 and 3, respectively. The former regime displayed single breakdown, while the latter regime was characterized by double breakdown. The present measurements and flow visualization have corroborated the approximate axisymmetry of the flowfield associated to bubble-type of vortex breakdown already observed in different flow geometries.

A new type of instability during the laminar-turbulent transition of a viscous incompressible fluid flow in the gap between two concentric spheres, where only the inner sphere rotates (spherical Couette flow), was detected. In case of two relatively wide gap widths (β = 0.33 and β = 0.5) it was found that the well-known Taylor-instability does not exist. At the stability threshold, where the laminar basic flow loses its stability, the first instability manifests itself as a break of the spatial symmetry and non-axisymmetric secondary waves with spiral arms appear. They spread from the pole to the equator. With increasing the Reynolds number above the critical one, the number of secondary waves with spiral arms decreases. Flow visualization studies and simultaneously laser-Doppler-velocimeter measurements show that the transition of the secondary wave flow with spiral arms is periodic and quasi-periodic before small scale turbulent structures occur.

Swirling flow, in which there is large scale rotation of the mean flow, is one of the well recognised configurations of flow. The use of vortex type flows are extremely widespread in a range of different devices and applications (1). The list of industrial operations in which they are used includes such diverse applications as the separation of gases of different molecular weights, Ranque-Hilsch tubes, cyclone dust separators, spray dryers, gas scrubbers, flash dryers, cyclone evaporators, combustion devices, plasma flame stabilisation, in agitators and in the piping associated with fluid turbo-equipment (2)(3)(4)(5)(6)(7)(8)(9)(10). Also fluidic vortex valves, and gas-core nuclear rockets are among the numerous practical devices, the performance of which is dominated by a confined turbulent vortex created by tangential injection of all or part of the through-flow (11)

This contribution describes the development of a data collection system in which a stationary fibre optic probe is used to provide LDA measurements for flows through rotating passages. The application of the technique is shown in the investigation of turbulent flow through a U-bend that rotates in orthogonal mode. A stationary 2-channel fibre optic probe is employed to collect instantaneous flow measurements from the rotating test section. The flow Reynolds number is 100,000, and the U-bend curvature ratio is 0.65. Three cases have been examined: flow through a stationary U-bend, flow through a U-bend rotating positively (the trailing side coinciding with the bend outer side) at a rotation number (Ro = Ω D/U _m ) of +0.2, and rotation at a rotation number of −0.2. The resulting measurements produce a detailed mapping of the mean and fluctuating flow fields, which reveal the effects of strong curvature and rotation on the development on turbulent flow.

Measurements of the three Reynolds shear stresses in a fully developed flow in a mildly curved flume were executed using a 3-dimensional laser-Doppler velocimeter. An optical arrangement for turbulence measurements in shallow water flow in a flume was developed. For such flows equal eddy viscosity coefficients for the different Reynolds shear stresses are often assumed, which depend only on the main flow. However, this assumption appears not to be justified. It leads to an underestimation of the secondary flow intensity in a mildly curved flow.

Laser-induced fluorescence is used to quantify the thickness of liquid films. An optimisation of the fluorescent tracer to make it evaporate at the same ratios as gasoline is performed since the final application is the study of evaporating films, as those found in intake pipes of port-injected spark-ignited engines. The liquid studied is iso-octane and the tracer is a ketone. Because of distillation, the tracer concentration in the liquid film may not remain constant. The most suitable tracers are C₆ ketones.

The optical set-up uses a single optical fibre for both laser excitation and fluorescence collection. It is designed for local measurements through transparent walls. A precise liquid wedge is used for calibration. An application is shown on a transparent pipe, with stationary air flow and pulsed low-pressure injection. The technique has proven to be able to show the unstationary phenomena. The impact of the spray, the deposition of a large quantity of fuel and the displacement of liquid waves are quantified and followed with time.

Distributions of droplet size, axial velocity and liquid flux have been measured downstream of a model inlet valve with a phase-Doppler velocimeter. The valve had a diameter of 40mm, a 45° bevelled edge and was arranged at the exit of a straight tube of inside diameter 34mm with the lifts of 2, 4 and 6mm. Suction was applied by a variable speed electric fan to simulate the intake stroke of a cylinder of 400cc displacement at 1200 RPM, with an air flow rate for the 4mm valve lift of 0.99m³/min, corresponding to the bulk velocity of 38m/s in the valve gap. Gasoline was supplied either by a commercial injector located 70mm from the valve or by its introduction on the surfaces of the valve and port. The injection of fuel with an open valve led to Sauter mean diameters between 80 and 1001.m, lower than the 130µm appropriate to the free unconfined spray. With 2mm valve lift, the spray emerged between angles of 10° to 70° to the horizontal axis and with larger valve lifts it emerged with narrow angle close to that of the valve head surface immediately upstream of the valve bevel, that is 25°. The introduction of fuel as a liquid streams led to a Sauter mean diameter of 240µm with the maximum flux emerging from the edges of the valve and seat bevels, at angles of 50° and 40° to the horizontal axis, respectively.

Thermal spraying is an expanding technology with still growing markets. Especially the variety of plasma spraying techniques offers a number of possibilities for new and advanced applications. The spraying of sophisticated materials, the demand of generating coating structures or substrate/coating combinations with novel properties and the improvement of the spraying efficiency require a more detailed understanding of the physical and chemical reactions inside the plasma, in order to achieve an appropriate process control. The present paper demonstrates that phase-Doppler anemometry (PDA) is applicable as a powerful tool for process diagnostics in plasma spraying. Yielding simultaneous measurements of particle size and velocity at discrete locations, the PDA-technique generates new possibilities to characterize and further improve the spraying process.

Measurements of Reynolds shear stress and heat flux, by means of laser Doppler velocimetry and compensated fine wire thermocouples, have been made in the shear layer surrounding a swirl — induced recirculation bubble stabilising an unconfined non-premixed flame. The burner was operated at a Reynolds number of 27000, based on the air bulk velocity of 8.5 m/s and on a characteristic burner exit radius, R, of 50 mm, a swirl number of 1.07 and at a natural gas flow rate corresponding to an equivalence ratio of 0.85 and a potential heat release rate of 50 kW. The sign of the flux of the shear stress was found to be in accordance with that expected from a gradient diffusion hypothesis but the sign of the heat flux was in the counter-gradient direction for axial distances up to 0.5 R from the exit of the quart. Arguments based on the shape of the weighted joint probability distribution of radial velocity and temperature fluctuations, W(v", t") = v" t" P(v", t"), show that the counter-gradient contributions are due to large amplitude, comparatively rare departures of temperature from the local mean and also suggest that the corresponding radial flux of the Favre-averaged conserved scalar, ‹v"ξ"›, will also be in the counter-gradient sense. Even in those regions of the flow where (v" t") is in the gradient sense, the magnitude of W(v", t") in quadrants 2 and 4, which represent counter-gradient flux, remains strong in comparison with quadrants 1 and 3, which represent gradient diffusion. The quantity ‹v"ξ"› is used in the prediction of non-premixed flames and these results show that predictions of this kind of flow must be based on second moment, rather than on effective viscosity, turbulence model closures so as to capture the effects of \( \bar \xi ''\partial P/\partial{x_i} \) on heat fluxes.

Turbulent premixed flames stabilized in a bluff-body are experimentally investigated in order to discuss flame structure and morphology. Integral scales were measured within the reacting flow, based on the integral of the lateral velocity correlations obtained by using two laser-Doppler velocimetry systems. The quantitative results obtained are compared with flame shadowgraphs and complemented with simultaneous measurements of time-resolved velocity and temperature obtained by combining laser-Doppler velocimetry and laser-Rayleigh scattering. The results are used to quantify the relative magnitudes of the terms involving the balance equations of momentum, turbulent kinetic energy and heat flux. In addition, the interaction of pressure gradients and temperature fluctuations is shown to affect the heat release in the flame studied, revealing the existence of large zones characterized by non-gradient scalar fluxes.

A new method is described for the simultaneous detection of the two-dimensional (2D) temperature and the 2D OH concentration distributions in a high turbulent premixed flame inside a contained combustion chamber. The temperature information is obtained by Rayleigh scattering and the OH concentration from laser-induced predissociated fluorescence (LIFE). A newly designed detection optics for the separation of the synchronously induced Rayleigh and fluorescence signals allows the use of only one single laser source in combination with only one single 2D detector.

A droplet cloud of liquid fuel produced by an ultrasonic atomizer was ignited by a spark, and the flame ball propagating outward was observed in order to elucidate the mechanism of flame propagation and complicated group combustion behaviors of spray flames. For that purpose, the instantaneous images of droplet clusters, OH-radical chemiluminescence and C₂-band flame luminosity were taken simultaneously. Furthermore, the light emission signals in OH- and CH-bands, Mie-scattering signal from droplets, and the size and velocity of droplets were monitored simultaneously in time series. It was found that a nonluminous flame propagated ahead of a luminous flame, and that droplets disappeared in the luminous flame zone due to rapid evaporation, where a number of small-scaled droplet clusters were burning in diffusion combustion mode associated with solid-body emissions.

Velocity measurements based on the transport of turbulence induced refractive index structures are investigated. Particle scattering is assumed to be negligible. The scattering from collective structures is analyzed with a phase screen model. A system based on reference beam detection and combining Doppler and time-of-flight concepts may provide the best spatial resolution and utilize a broad band region of the high frequency turbulence for light scattering. In the optical region it is so that a long wavelength is preferable and reference beam detection is necessary. An potential hybrid system for plasma diagnostics is presented. It is shown that time resolved measurements may be possible.

GLMT (generalized Lorenz-Mie theory) and its applications to the design of phase-Doppler instruments are discussed. A particular emphasis is stressed on Gaussian beam errors (trajectory ambiguity effects) and their elimination. Very recent concepts, even if not yet fully developed and/or implemented, are discussed too.

In this paper we evaluate the effect of quantization of Doppler signals. We do this by calculating Cramér-Rao lower bounds for one bit quantization and by doing computer simulations. Both approaches show that there are serious problems with one bit quantization around certain critical frequencies. Contrasting, it is shown that 4 bit quantization performs nearly as good as no quantization. The conclusion is that 4 bits are adequate while fewer number of bits give unreliable results.

Conventional directional LDAs are based on the frequency shifting technique. In this paper a technique has been proposed for the realization of a direction sensitive HF-pulsed LDA on the basis of switch demultiplexing and quadrature signal processing. This technique allows one to take advantage of HF-pulsed LDAs and to use only one high-frequency pulsed laser diode and one detector for directional flow velocity measurements without frequency shifting. The first experimental verification of this technique has been made. Improvements to the experimental set-up and practical miniaturized realizations of a directional HF-pulsed LDA using light fibres and integrated optical devices are described.

That paper is devoted to the analysis of the importance of the finite size of the laser beam on the quality of bubble measurements by a Phase Doppler Anemometer (PDA) (trajectory ambiguity or Gaussian beam effect). Detection at 30 ° and 70° are studied for a standard PDA, a non-standard PDA and dual burst configurations. The possibility to sort bubbles and perfectly reflecting particles is discussed.

In the present paper theoretical and experimental studies of the radiation pressure and of the intensity distribution in the rainbow region are presented. It has been shown that both radiation pressure and rainbow position show an oscillatory behaviour in their dependence on droplet radius. This property has been used to detect changes in radius of optically levitated droplets. From the theoretical calculations it follows that the period of these oscillations is practically independent of size and refractive index for a wide range of both quantities. Furthermore, it could be shown theoretically and experimentally that the oscillations of radiation pressure and rainbow position are strongly correlated and have the same period.

A standard phase-Doppler system was modified to determine the feasibility of refractive index measurements utilizing new optical techniques. With the use of two laser beam systems of different wavelength and a single receiving unit different optical configurations can be explored by varying the scattering, elevation and crossing angles independently for each system. The index of refraction of a droplet can be extracted from the phase-shift ratio of two distinct pairs of apertures. Selecting the proper optical parameters yields a sensitive relationship between phase ratio and refractive index.

For measurements an LDA configuration with two transmitting optics was chosen because of its widespread use and standard components. The two probes were situated 40° relative to each other. This was done to maximize the relationship between the ratio of phase shifts and the refractive index. Tests were conducted on water-glucose mixtures with varying refractive index.

The anisotropy of laser light scattered by a droplet exhibits a strong dependence on droplet size and temperature. Around the geometrical rainbow angle this dependence is such that it can be used to determine these two parameters. This determination is done with the help of the Airy theory for the rainbow to avoid an extensive calibration of the angular scattered light intensity. A comparison between the Airy and the Mie theories shows that one has to be careful in applying the Airy theory for this purpose. The Airy theory has also been compared with the experimental first order “monochromatic rainbow” created by single falling droplets crossing a laser beam. There is a qualitative good agreement between experiment and theory after the measured rainbow pattern has been properly smoothed.

An instrument based on an imaging technique and laser Doppler velocimetry (LDV) was developed and evaluated for sizing particles of irregular shape. The signals from particles were detected by a 32 channel linear photodiode array focused on the measuring volume and the image was reconstructed from ‘sliced’ images of the particle provided by temporal outputs from each channel. The present instrument consists of a multi-channel transient recorder and independent, single-channel LDV processor, and provided size, velocity, shape and trajectory information of particles ranging from 30 to 250 µm with velocity up to 3 m/s; the particle size was obtained from the area of the shadow image and the trajectory was inferred for defocused images by cross-correlation between two images projected by two incident beams of the conventional LDV. For determination of particle size distribution, the biasing effect, caused by the size dependence of the sampling space, were eliminated by a correction scheme. The accuracy of the average size for spheres was better than 3.0% referred to that measured by a microscope. The size distribution of non-spherical particles was similar to microscopic measurement and the maximum difference in arithmetic average diameter was approximately 10%.

A holographic particle image velocimetry (HPIV) system has been developed to measure instantaneously all three components of velocity at order 10⁶ points in a three-dimensional volume. A high speed interrogation system and an automated vector validation procedure is used to make this measurement technique applicable to turbulence research. The procedure by which holographic particle images are analyzed to obtain the three-dimensional velocity measurements from a single hologram, and how these measurements are validated using automated statistical algorithms is described in this paper.

A 3-D Scanning Particle Image Velocimetry (3-D SPIV) technique has been developed to overcome the resolution problem associated with conventional 3-D Particle Tracking Velocimetry (3-D PTV) without requiring the complexity of 3-D holography. The technique employs a high repetition rate Copper Vapor Laser synchronized with a scanning mirror drum and sheet forming optics to scan the volume of interest. 25 or 50 individual mirrors are adjusted to form an array relatively thick laser sheets parallel to each another, hence effectively covering the entire volume of interest (typically 100×100×100 mm³). This device is capable of recording the time evolution of a flow field in 3-D with a temporal resolution of 200 to 400 Hz, which is more than sufficient for most water flows. The digitized 16mm movies are then processed by a combination of techniques: conventional 2-D cross-correlation and 2-D PTV within each plane. Due to the 2D nature of the recorded information, relatively high seeding densities can be used to yield 20,000 to 50,000 vectors per scan, but yet provide the temporal information associated with successive scans at a rapid rate. The physical implementation of this technique, the overview of the processing and some sample images are presented in this paper.

Holographic recording promises the ability to extend Particle Image Velocimetry (PIV) to a fully 3-D measuring method, where PIV is no longer limited to the instantaneous registration of 2-D velocity vectors within a single sheet. The recording of small particles in a 3-D volume introduces, however, additional noise in the interrogation step. This is investigated in more detail. First the holographic recording of a continuous deep volume is considered. Sampling the depth by spaced lightsheets allows a first improvement of the signal to noise ratio and provides a higher validation rate in the interrogation step. In addition, the light for illumination of the particles is used in a more economic way. If dense sampling is required the concept of simultaneous recording of all light sheets, but reconstruction of single sheets each can be used. This is possible by taking advantage of the limited coherence of the laser. A demonstration of this method is included. The validation characteristics of typical evaluation methods like autocorrelation and crosscorrelation techniques are estimated by Monte-Carlo-simulations. This gives the ability to find the method best suited for the fluid mechanical problem under investigation.

The Particle Image Velocimetry (PIV) is a technique which is utilized for measuring complete velocity vector fields in a few microseconds time. However, a lot of time is required during the evaluation of the PIV recordings. If the auto-correlation technique is used it is necessary to calculate a few thousand auto-correlation functions (ACFs) for each PIV recording. In this paper an evaluation system for photographical PIV recordings will be described, which allows a completely analog-optical determination of the ACFs of small sub areas of the recording. An optically addressable liquid crystal spatial light modulator was utilized to store the Young’s fringes pattern appearing in the frequency plane at the output of a first optical Fourier processor and to provide them as the input of a second optical Fourier processor. Our analog optical autocorrelator offers a high processing speed and simultaneously a high resolution in the autocorrelation plane. The realized set up is compact and very easy to handle.

The Particle Tracking Velocimetry (PTV) technique consists in recording, at different instances in time, positions of small tracers particles following a fluid flow and illuminated by a sheet, or pseudo sheet, of light. It aims to recognize each particle trajectory, constituted of n different spots and thus to determine each particle velocity vector. In the present paper, we devise a new method, taking into account a notion of global consistency between the trajectories to be extracted, in terms of visual perception and physical properties. It is based on a graph-theoretic formulation of the particle tracking problem and on the use of original neural networks, called pulsed neural networks.

We present an analysis of Filtered Rayleigh Scattering, a diagnostic technique for precision measurements of velocity, temperature, and density in unseeded air flows. Precision measurements require that the filter characteristics be accurately known and that the geometrical factors be properly modeled. Failure to account for these effects in low F number optical systems will lead to large errors in both the velocity and temperature measurements. The best signal-to-noise is achieved with a sharp cut-off blocking filter whose width exceeds the linewidth of the Rayleigh scattered light. The signal-to-noise can be further enhanced by operating in the ultraviolet region of the spectrum where large enhancements in the scattering cross section and better optical filters are available.