Laser Doppler Anemometer System Research Articles

EFFECT OF STABLE DENSITY STRATIFICATION SECONDARY FLOW IN COMPOUND CHANNEL

Flow measurements were undertaken in a compound channel to investigate the effect of stratification on secondary flow in two-layer density-stratified flow. Secondary flow and salinity were measured using a laser induced fluorescence technique together with a laser Doppler anemometer system. Flow visualisation was also carried out using a digital video camera to demonstrate the effect of stratification on flow. The measured data shows that the size of typical twin vortices in the compound channel reduces as the degree of stratification increases. Two generation mechanisms of secondary flow were recognized on the floodplain, namely non-isotropic turbulence driven secondary flow and density driven secondary flow. The production of turbulent kinetic energy due to buoyancy was found to be not significant in most region but is significant in the water surface region

AN EXPERIMENTAL STUDY OF DISPERSED LIQUID/LIQUID TWO-PHASE UPFLOW IN A PIPE

This paper presents experimental data for dispersed liquid/liquid upflows. Water was the continuous phase and mineral oil was the dispersed droplet phase. For this flow regime reduced gravity bubbly flow phenomena was simulated because the mineral oil and water had almost the same density. The mean velocity and turbulence fields, the size distributions of the oil droplets, the volume fraction, and interfacial area density distribution were measured using fiber optic Laser Doppler Anemometer (LDA) and phase Doppler Anemometer (PDA) systems. Significantly, the results presented in this paper are similar to those for bubbly air/water flows in microgravity conditions (Kamp el at., 1995).

Characterization of vortices using pulsed-wave Doppler ultrasound.

The detection and characterization of vortices from a Kaman vortex generator by means of a 20 MHz pulsed-wave Doppler ultrasound system were assessed. Measurements were made at different steady flowrates in a 10 mm internal diameter polyurethane tube, 14 mm distal to a circular cylinder of diameter 2 mm, placed across the tube inlet. The results were compared with those obtained with a two-component laser Doppler anemometer system. There was generally good agreement between the two techniques in the measurement of convective flow velocity, frequency of vortex shedding and the circulation velocity of the vortices. It is concluded that pulsed-wave Doppler ultrasound is a suitable technique for investigating vortical flow structures.

AN EXPERIMENTAL STUDY OF PHASE DISTRIBUTION AND TURBULENCE STRUCTURE FOR SOLID/LIQUID FLOW IN A HORIZONTAL PIPE

Local measurements of the velocity and phase distribution were made for solid/liquid two-phase flows in a horizontal pipe using a multidimensional laser Doppler anemometer (LDA) system. The pipe used had a 30.6 mm inner diameter and was made of a special optically clear material, Fluorinated Ethylene Propylene (FEP), which has the same index of refraction as water. The test section and laser probes were submerged in water so that no optical corrections were necessary. Simultaneous measurements of the liquid and particle phases were obtained and discriminated based on differences in the residence time that each particle had when crossing the LDA's measurement volume. Positive buoyant and negative buoyant solid spherical particles, about 2 mm in diameter, were used in this study. A complete data set of the phasic mean velocities, turbulence and volume fractions were taken for each particle type. The results presented herein are complete and self-consistent, and can be used to assess the predictive capabilities of multidimensional two-fluid computational fluid dynamic (CFD) models.

Experimental and numerical results of optical preamplification in a laser Doppler anemometer receiving head

The first application of a fiber amplifier in a laser Doppler anemometer (LDA) system is presented. Using an Nd-doped fiber amplifier as optical preamplifier in a conventional 1064-nm LDA, a small signal gain of 30 and 35 dB signal-to-noise (SNR) enhancement of the LDA radiation are achieved. The amplifier model, numerical, and experimental results are described.

Laser Doppler anemometer measurement of the velocity pulsations of large particles

A procedure has been developed for estimating the velocity pulsations of polydisperse particle velocities. The mean velocities of glass particles as well as their root-mean-square deviations were measured in trials for various sensitivities of the laser Doppler anemometer system used.

Loading Rate Effect on Woven Glass Laminated Plates by Penetration Force

The effect of the loading rate on 2.9 mm thick E-glass/epoxy woven composite laminates subjected to quasi-static contact and impact forces applied in the transverse direction was investigated. The quasi-static contact test was performed using a 12.7 mm hemispherically-tipped indentor. On the other hand, impact was conducted using a gas gun with projectiles of the same frontal shape weighing 8.7 g, 17.7 g and 35.5 g. The test speeds were from several m/s to the ballistic limits of the laminates. The impact response was measured using the self-developed laser Doppler anemometer (LDA) system and was subsequently converted into the force and displacement histories. Significant loading rate effects were observed when comparing the laminates loaded quasi-statically to those struck by projectiles with a four-fold difference in mass. These effects included differences in the impact force history, a drastic increase of the force peak and the absorbed energy, and a completely different delamination damage pattern. Thus, the loading rate effect for composite laminates reinforced by glass fibers cannot be overlooked. On the other hand, fiber breakage throughout the plies on the distal side of laminates was found to absorb a significant amount of impact energy. The absorbed energy versus the initial projectile energy for the dynamically loaded specimens, when plotted in a non-dimensionalized manner, was found to have a bilinear relationship regardless of the projectile mass employed. This bilinear line was also closely bounded from below by the quasi-static results.

Flow development downstream of a standard tube bundle and three different porous plate flow conditioners

The development of the mean velocity and turbulence intensity downstream of a standard tube bundle and three different slotted plates has been measured in a four inch pipe using a 1-D laser Doppler anemometer system. The results show how far the standard tube bundle must be placed upstream of the orifice plate to obtain fully developed mean velocity and turbulence. Numerical analyses of how shaping the velocity profile exiting a flow conditioner effects the development of both the turbulence and mean velocity have been performed. These analyses indicate that a power law profile (1/7th) with a 9% turbulence intensity produces a fully developed flow in the shortest pipe length, 20D. A parabolic profile requires a slightly longer development length. A uniform flow required the longest length of pipe. The effect of the turbulence intensity produced by the flow conditioner was also investigated. Three different slotted plates were manufactured to produce a 1/7th power law, parabolic, and uniform velocity profile downstream of the plate. The development of the mean velocity and turbulence have been measured. These data were compared to the predictions with very favorable results. The simulations accurately represented the flow field and turbulence intensity distributions.

Measurements of a longitudinal vortex generated by a rectangular jet in a turbulent boundary layer

Measurements were made on the mean flowfield created through the interaction between a two-dimensional flat plate turbulent boundary layer on a flat plate and an inclined jet. The jet was generated by a nozzle of rectangular exit and was pitched and skewed to the oncoming flow. A total of three pitch angles and two jet velocity ratios were tested. The measurements were performed with a three-component laser Doppler anemometer system in a low speed wind tunnel. The dominant feature of the flow physics is a single longitudinal vortex produced as a result of jet/boundary layer interaction, with additional induced secondary features in the near-wall area in the spanwise direction. A salient feature of the flow is its maximum vorticity position which is located underneath the center of the vortex. Within the measured flow parameter range, the vortex development differs also from that of a round jet, differences in the velocity distribution are observed. The study provides contributions to flow physics.

Velocity field in a twin‐screw extruder

Velocity measurements inside the screw channels of a co‐rotating, self‐wiping, twin‐screw extruder were carried out. A laser Doppler anemometer (LDA) system was used to measure tangential and axial velocity components in the translational region of one of the screws of a twin‐screw extruder. The velocity measurements were carried out using heavy corn syrup at different screw speeds and for two forwards conveying elements differing in their pitch. It was found that velocity profiles in the translational region were substantially affected by the pitch of the screw element, however, screw speed (30–90 r.p.m.) did not have a significant effect on the shape of the velocity profiles. The range of velocity values in a 14 mm pitch screw element was wider than that in a 28 mm pitch screw element. Experimentally measured velocity profiles were compared with predictions of a one‐dimensional model, which showed that the model underpredicted the velocity values, for the 14 mm pitch element.

The mean flow field generated by a pitched blade turbine: Changes in the circulation pattern due to impeller geometry

AbstractThe mean flow field in a tank stirred with a pitched blade turbine was measured using a two‐component Laser Doppler Anemometer system (LDA). The effects of impeller clearances and impeller geometries (number of blades, blade angle and blade size) on the mean flow field have been studied. The primary pumping number, induced pumping number of the primary circulation loop and the induced pumping number of the secondary circulation loop, which often has been ignored, are reported. The flow patterns and circulation loops are more complex than those traditional ones, which vary with the geometries of the PTD and the clearances.

A comparison of laser Doppler and single-fibre reflection probes for the measurement of the velocity of solids in a gas-solid circulating fluidized bed

In this study, an attempt has been made to compare an intrusive solids velocity measurement technique with the laser Doppler anemometer (LDA) which is a typical non-intrusive solids velocity measurement technique. Single-fibre reflection (SFR) probe and LDA measurements were directly compared under identical operating conditions in the upper dilute zone of a circulating fluidized bed (CFB). This experimental unit was a cold model CFB (inner diameter of the riser, 0.4 m; height, 15.6 m). The measurements were taken at a height of 6 m above the distributor. Quartz sand with a mean diameter of 166 µm was fluidized at a superficial gas velocity of 4 m s −1. The cross-sectional average solids volume concentration was 1.2 vol.%. The highest instantaneous local solids concentration was 5 vol.% close to the riser wall. LDA measurements were taken with a miniaturized measuring head (diameter, 14 mm) operating in the backscatter mode and inserted in a probe casing (diameter, 22 mm). The focal point of the laser beams was located at a distance of 19 mm from the probe casing window. This allows the LDA measuring head to be moved through the fluidized bed and valid LDA measurements to be acquired inside relatively dense suspensions. The fibre-optical probe measurements were taken with a two-channel SFR probe (fibre: diameter, 600 µm; probe: diameter, 8 mm; centre-to-centre distance between the two sensing surfaces, 2.3 mm). The measurements at a superficial gas velocity of 4 m s −1 showed mean solids velocities between 10 m s −1 upwards and 3ms −1 downwards. The LDA system faces problems in the regions of higher solids concentration located close to the riser wall where data acquisition rates decrease considerably. The good agreement between the local average solids velocities acquired with the two systems proves that intrusive probe measurements cause negligible distortion of the solids velocity Pattern in a CFB environment.

Phase-Averaged Wall Shear Stress, Wall Pressure, and Near-Wall Velocity Field Measurements in a Whirling Annular Seal

The flow field inside a 50 percent eccentric whirling annular seal operating at a Reynolds number of 24,000 and a Taylor number of 6600 has been measured using a three-dimensional laser-Doppler anemometer system. Flush mount pressure and wall shear stress probes have been used to measure the stresses (normal and shear) along the length of the stator. The rotor was mounted eccentrically on the shaft so that the rotor orbit was circular and rotated at the same speed as the shaft (a whirl ratio of 1.0). This paper presents mean pressure, mean wall shear stress magnitude, and mean wall shear stress direction distributions along the length of the seal. Phase-averaged wall pressure and wall shear stress are presented along with phase-averaged mean velocity and turbulence kinetic energy distributions located 0.16c from the stator wall, where c is the seal clearance. The relationships between the velocity, turbulence, wall pressure, and wall shear stress are very complex and do not follow simple bulk flow predictions.

Electrostatic effect on the flow behavior of a dilute gas/cohesive particle flow system

AbstractCohesive (Group C) particles have been widely used in various industries. To handle and process such fine particles, a clear understanding of the flow behavior and interparticle force, is needed. To achieve that objective, a Laser Doppler Anemometer system was used to measure particle velocity, fluctuating velocity, and size and extent of agglomeration or cluster formation of particles in a dilute gas/fine oil shale particle flow system with particle density of 2,082 kg/m3, average particle volumetric concentration of 1.5%, and average particle mass flux of about 100 kg/m2·s in a controlled‐moisture environment. The flow behavior of the particles was also studied for a mixture of 99% shale particles and 1% antistatic agent (Larostat powder, a quaternary ammonium compound) to examine the role of electrostatic force in gas/cohesive particle flow behavior. The addition of Larostat powder significantly reduced the electrostatic force and, in turn, made Group C particles behave similar to Group A or in some cases to Group B particles. In addition, our experimental data showed that the Maxwellian distribution function is a reasonable assumption to describe the velocity probability density function of the shale particles with or without antistatic agents.

A maximum likelihood based autocorrelation processor for a photon-resolved laser Doppler anemometer

Maximum likelihood estimation (MLE) techniques were used in the design of an autocorrelation processor for estimating the period (frequency) of a photon-resolved laser Doppler anemometer (LDA) signal. Implementation of these techniques required the derivation and numerical computation of the covariance matrix for the autocorrelation function of the photodetector signal. The processor is insensitive to the particle number density of the system and will work with photon-resolved and non-photon-resolved signals. The processor provides an accurate estimate of the error in each period estimate. Examples are given for the use of a Fisher matrix, derived as part of the MLE procedure, to predict the optimal processor performance of a LDA system as a function of the processor parameters and the system parameters. For periods of time over which the flow is steady the processor can be configured such that improved accuracy for each subsequent estimate can be achieved by utilizing the results from previous estimates. Computer simulations were used to test the processor design.

Comparison measurements for selection of suitable photodetectors for use in Nd: YAG LDA systems

Laser-diode-pumped Nd:YAG lasers (λ=1064nm) are compact, single-transverse mode laser sources with a higher output power than diode lasers (λ ∼ 800 nm). They are suitable light sources for miniaturized laser Doppler anemometer systems. On the other hand, the sensitivity of conventional detectors and the scattering efficiency particularly of particles smaller than 1 μm are reduced, when these light sources are used. The paper describes comparison measurements of commercial photodetectors based on Si, Ge and InGaAs in a test LDA with an Nd:YAG laser. The achievable signal-to-noise ratios (SNR) of the Doppler bursts versus the laser power in the measuring volume are compared. This shows that Silicon Avalanche photo diodes (APD) for the near-infrared using a special technology and InGaAs-APDs with low-noise pre-amplifiers can be used advantageously. Similar comparison measurements with an LDA system at λ=830 nm show that an Nd:YAG laser based LDA is more sensitive and leads to higher SNRs because of its higher output power, even though the Mie scattering efficiency and the detector sensitivity are reduced.

A study on the pulsatile flow characteristics of Newtonian and non-Newtonian fluids in the bifurcated tubes

Experimental and numerical studies for three-dimensional pulsatile flows are conducted to investigate the flow characteristics in the bifurcated tubes. Velocity measurements in experimental study were made by both Pulsed Doppler Ultrasound(PDU) machine and Laser Doppler Anemometer(LDA) system. Glycerin is used for experimental study. Experimental results are used to verify the results of the numerical simulation. Flow characteristics of Newtonian fluid and blood in the bifurcated tubes under the steady and pulsatlie flows are numerically investigated. Finite volume method is employed for three-dimensional numerical simulations. Blood is considered as a non-Newtonian fluid and the constitutive equation of blood is used for the numerical analysis. Numerical analyses are focused on the flow patterns for various branch angles ranging from 30.deg. to 90.deg. and diameter ratios such as 1.0, 0.8, and 0.6. Pulsatile flow characteristics of blood are compared with those of Newtonian fluid. Parameter effects on axial velocity, pressure and wall shear stress distribution along the bifurcated tubes are discussed in terms of the branch angle, diameter ratio, and Reynolds number.

Apparent wall slip velocity coefficients in concentrated suspensions of noncolloidal particles

The apparent wall slip velocity coefficient of concentrated monodisperse suspensions of 90 μm PMMA particles in a viscous Newtonian fluid was determined experimentally in a narrow gap Couette device by measuring the particle velocities across the gap using a laser doppler anemometer system and then extrapolating the results to the two walls. The slip coefficients thereby obtained were found to be insensitive to the magnitude of the applied shear rate and to equal, approximately, λ/8 for particle concentrations φ in the range 0.45≤φ≤0.52, where λ refers to the relative effective viscosity of the suspension with respect to that of the pure fluid.

Analysis and quantification of a solid-state laser Doppler anemometer

A single component, visible wavelength, heterodyne laser Doppler anemometer was constructed based on solid-state laser diode technology for use in fluid dynamic research. With the recent advancements in solid-state laser diodes and solid-state avalanche photodiodes, it is possible to construct miniature laser Doppler anemometer systems solely employing solid-state technology. Such systems have the advantages of small size, light weight, low-power consumption, low cost, and ruggedness. The subject of the current research was to quantitatively evaluate a miniaturized laser Doppler anemometer system constructed with visible laser diodes and an avalanche photodiode detector. Evaluation measurements in a turbulent, stagnating jet are presented and compared to both hot-wire and five-hole probe data. The miniature solid-state laser Doppler anemometer system was found to agree within 2% for mean velocities and agree within 5-20% for root mean square turbulent velocities.

Woven glass/epoxy laminates subject to projectile impact

Impact results of woven E-glass/epoxy composite laminates are presented in this paper. A gas gun was employed to launch 12.7 mm diameter hemispherically-tipped projectiles 35.5 g in weight. The developed laser Doppler anemometer (LDA) system was successfully employed to record the projectile velocity history, whose range was from several m/s to twice the ballistic limit of the target, and was converted into the force history by using both the optimization and the polynomial curve-fitting methods. It was found that, when the projectile initial velocity was lower than the ballistic limit, the delamination area was approximately proportional to the initial energy of the projectile, and the relationship between the absorbed energy and initial energy of the projectile followed a straight line of unit slope when it was expressed non-dimensionally. Also, the incremental rate of the peak impact force became gradually smaller and finally reached a constant value at the ballistic limit, and the impact force history became progessively asymmetric. Beyond the ballistic limit, the absorbed energy remained approximately constant regardless of the increase of the initial striking velocity of the projectile. The peak impact force, on the other hand, was surprisingly found to increase again.