Laser Doppler Anemometry Technique Research Articles

Laser Doppler Anemometry technique to study the flow field in the nozzle and in the water jet of a Pelton turbine

The constant availability of water from the mountain basins is crucial to feed the hydropower plants that currently represent the most important renewable energy source in many countries, like Italy. The long dry periods that are nowadays registered during the summer periods have imposed the managers of the hydropower plants to operate the machines at very low flow rates, close to the lowest allowed values. In this work we focus our attention on the fluid-dynamics of the water jet of a Pelton machine and of the corresponding nozzle, with particular attention to the low flow-rate ranges. The nozzle has been manufactured in PMMA to be able to study the internal fluid-dynamics using the Laser Doppler Anemometry (LDA) technique. The shape of the nozzle has been adapted to limit the laser beam distortion and an algorithm has been implemented to correct the position of the measurement volume and the evaluation of the velocity components. Moreover, a specific measurement layout has been proposed and implemented to be able to study the velocity of the water jet downstream the nozzle and to study the homogeneity of the flow velocity in the jet, which is a crucial characteristic to grant a proper operation of the Pelton runner. The specific set-up of the test bench and the LDA parameters are presented; the signal of the LDA receiver proved to be very consistent and reliable. Moreover, the results of the measurements have been used to validate CFD simulations and to extend the results of the analyses to other operating conditions of the Pelton nozzle

Spectral analysis of the transition to turbulence downstream a delta winglet pair vortex generator in an airflow channel

The purpose of this experimental study is to provide a detailed analysis of the transition to turbulence in an airflow channel perturbed by a delta winglet pair vortex generator. Measurements of instantaneous velocity are carried out by a laser Doppler anemometry technique. Experiments are conducted for Reynolds numbers (based on the hydraulic diameter) ranging from Re = 400 to Re=12 000 and at several axial position downstream of the vortex generator. The flow behavior is characterized in each regime by means of a frequency analysis. We identified the inlet conditions corresponding to laminar weakly unsteady, transitional, and fully turbulent flow regimes. Furthermore, we have constructed a correlation between the Strouhal number and the Reynolds number that characterizes the vortex instability mechanism. Finally, under turbulent flow conditions, the dissipation rate was estimated, which showed an increase followed by an exponential decrease with the distance downstream of the vortex generator.

Enhanced outer peaks in turbulent boundary layer using uniform blowing at moderate Reynolds number

Uniform blowing through a permeable surface acts as an active flow control method in wall bounded flows. Such control technique was investigated in a zero pressure gradient turbulent boundary layer over a flat plate. Measurement data were obtained with the help of Laser Doppler Anemometry technique. Besides the drag reduction characteristics of such flow control method, time averaged measurement of stream-wise and wall-normal velocity components was taken at Reynolds number based on momentum thickness () of 1100–3670. Due to the difference in surface condition with and without blowing, mean properties of the boundary condition at wall influence the flow properties when scaled with outer scaling properties. Enhanced turbulence is observed in Reynolds stresses using statistical analysis including the thickening of the boundary layer.

A novel ventilation method to prevent obstruction phenomenon within sewer networks

The phenomenon of blockage is a widely encountered issue in industrial applications with multiphase flows such as sewer networks. This paper proposes a solution to eliminate the blocking phenomenon in such confined horizontal channels. The study presents alternative longitudinal ventilation using inclined jets. Numerical simulations were performed to evaluate several parameters such as the position of the jets, their orientation and the velocity ratio between the main air inlet and the jets. The Volume of Fluid model (VOF) is used to follow the evolution of the interface between the air and water phases. Numerical results of the axial velocity were confronted to the experimental data obtained by the Laser Doppler Anemometry (LDA) technique and gave good agreement. The parametric study shows that the lateral injection has a direct effect on the dynamic of the air-water flow inside channels. With the optimal depicted position and orientation of the injectors, the obstruction is avoided for low air velocities and is moved away from the entrance for the higher ones. Results show also that side-by-side lateral injection all over the channel with an optimal ventilation ratio fully avoid the occurrence of slug flow and thus the obstruction of the channel.

Experimental assessment of the rotor outlet flow in a twin-entry radial turbine by means of Laser Doppler Anemometry

The current paper presents the validation of some hypotheses used for developing a one-dimensional twin-entry turbine model with experimental measurements. A Laser Doppler Anemometry (LDA) technique has been used for measuring the axial Mach number and for counting the number of particles downstream of the rotor outlet. These measurements have been done for different mass flow ratio (MFR) and reduced turbocharger speed conditions. The flow coming from each turbine entry does not fully mix with the other within the rotor since, downstream of the rotor, they can still be differentiated. Thus, the hypothesis of studying twin-entry turbines as two separated single-entry turbines in one-dimensional models is corroborated. Moreover, the rotor outlet area corresponding to each flow branch has linear trends with the MFR value. Therefore, the rotor outlet effective area used for one-dimensional models should vary linearly with the MFR value.

The effect of air velocity on slugs in a confined channel

The present work focuses on the study of slugs occurring in a two-phase flow of a confined rectangular channel: conditions of appearance and effect on the flow behavior. Three-dimensional numerical simulations have been carried out to examine the effect of superficial air velocity on flow behavior. The Volume Of Fluid model (VOF) is used to track the air-water interface. Validation of the numerical model is obtained by comparing the results of the simulated axial velocity with experimental data determined using the Laser Doppler Anemometry (LDA) technique. The numerical results revealed that for a fixed water level and superficial water velocity, higher superficial air velocities generate a slug flow that causes channel blockage. The position of these slugs and the timing of their occurrence were correlated in terms of air and water superficial Reynolds numbers.

Unsteady flow fluctuations in a centrifugal pump measured by laser Doppler anemometry and pressure pulsation

Turbulent flow generated by the intense rotor–stator interaction is detrimental to the safe running of the centrifugal pump. In order to gain more insight into unsteady velocity pulsation characteristics, this research applies the laser Doppler anemometry technique to capture velocity signals at various flow rates. Besides, pressure transducers are arranged on the model pump to sample transient pressure pulsation signals. The study paid particular attention to pulsation signals in the diffusion section of the volute. Results show that at low flow rates, a prominent hump phenomenon generated within the pump head curve, which indicates the development of rotating stall in the model pump. As noted from the spectra, the discrete blade passing frequency and impeller rotating frequency dominate the velocity and pressure spectra. Root mean square values of velocity signals increase rapidly at off-design flow rates, especially within the rotating stall region, and a similar trend is observed for pressure amplitude at the blade passing frequency. From the measuring point (P3) at the inlet of the discharge channel upstream of the volute tongue to the point (P10) at the volute outlet, pressure amplitude rises significantly. Meanwhile, the minimum point of pressure amplitude vs flow rate occurs around 0.4ΦN, and again, the findings show differences for comparisons between measuring points in the core flow region of the volute. This resulting phenomenon can be attributed to flow patterns in the diffusion section.

Experimental measurement for non-spherical bulk materials flow behaviour in rectangular enclosure

Dust control is a major concern in food industries since dust emission is generally unavoidable during bulk material handling. Obtaining a better understanding of the way particles are pulled away from a stream of particles and the generation of a dust cloud will assist in the design of more efficient dust control systems. In this paper, the particles- air interactions during free fall of bulk powder is reviewed. An experimental investigation was performed to elucidate the effect of conveying velocity and particle properties on velocity profiles. The Laser Doppler Anemometry (LDA) technique was employed to measure the particle velocity during free fall. Results revealed that the particle properties (particles with flaky shapes and lower bulk density) had a great influence on dust cloud generation as compared to the conveying velocity. The results presented here may facilitate the development of a quantitative tools for dust control system design that deal with a falling stream of bulk powder.

Measurements of local droplet velocities in horizontal gas-liquid pipe flow with low liquid loading

New data on streamwise droplet velocity profiles for low liquid loading pipe flows are reported. The fluids used in the experiments are SF6 (gas-phase) and Exxsol D60 (oil-phase). Experiments are conducted in a 10 cm pipe diameter high-pressure (∼780 kPa, absolute) flow loop to reproduce gas-condensate field conditions. Instantaneous streamwise velocity data, obtained using the non-intrusive Laser Doppler Anemometry (LDA) technique, are used to calculate mean and root-mean-squared (RMS) local velocities. Asymmetric droplet velocity profiles with respect to the pipe center-line are observed especially for the stratified low atomization flow conditions. However, as the flow momentum increases, the droplet velocity profiles seem to become more symmetric. Also, these data suggest that, irrespective of the conditions studied, the single-phase gas flow characteristics are preserved closer to the top pipe wall. The data from the LDA imply that the bottom pipe half region is highly influenced by the gas-liquid interfacial characteristics. This results in high streamwise turbulence intensity in the region influenced by the interfacial waves (interfacial turbulence). An isokinetic sampling instrument is also used to measure the local instantaneous dynamic pressure. The dynamic pressure and the locally extracted liquid (droplets) volume rate under isokinetic conditions are used to calculate local fluid velocity. Excellent agreement has been obtained when comparing this calculated local velocity from the isokinetic instrument to the LDA data.

Determining the parameters of vortex structures in a hydrodynamic vortex chamber

This study focused on the large-scale vortex structures. The working section was a tangential vortex chamber with a cylindrical cross-section. The flow was swirled by tangential supply of fluid into the chamber, using 12 direct-flow rectangular nozzles. The design swirl parameter was equal to 6.6 and was determined based on the vortex chamber geometry. The Reynolds number was maintained equal to 30000. Quantitative measurements were realized with the use of the optical technique of laser Doppler anemometry. The results of optical measurements allowed determining main parameters of the vortex core for two basic modes of the vortex flow, including the central direct-flow vortex and the spiral vortex.

Flow patterns and turbulence effects in large cylinder arrays

Studies on flow-induced vibrations in large tube bundles are usually focused solely on frequency analysis, without considering the flow patterns which are responsible for the fluid forces. Furthermore, investigations which involve variations in the spacing ratios do not separate transversal and longitudinal proximity effects. The purpose of this article is to separately analyze the influence of the transversal (T/D) and longitudinal (L/D) spacing ratios of a confined in-line cylinder array with five rows on the flow characteristics and to identify flow patterns. The laser Doppler anemometry technique was employed to acquire the mean velocity and its fluctuations in the transversal and longitudinal directions between the cylinder rows. Strouhal numbers and regimes reported in the literature were identified in the experiments. The same regime did not always persist along all cylinder rows for a given spacing ratio, as a result of the combined longitudinal and transversal proximity effects and also of the generation of turbulence by the array. For the smallest T/D ratio, a quasi-steady behavior associated with the biased flow pattern was noted in the experimental set-up and flip-flopping was observed in one case. Additionally, the flow characteristics in these arrays diverged from tube bundle classifications described in the literature. The behavior of the fluid forces and susceptibility to vibrations in the array were predicted based on the turbulence intensity of the incident flow of the cylinders. The results reinforced the need to extend flow pattern investigations to arrays with more cylinder rows and to consider both transversal and longitudinal proximity effects, when studying flow-induced vibrations.

Flame spray drying: Droplet and particle flow dynamics

ABSTRACTThe work presents the fundamental research of dispersed phase flow during flame spray drying. Particle dynamics analysis and laser Doppler anemometry technique were applied to determine particle size distribution and particle velocity distribution. Results of the study showed that stable combustion and longer length of the flame were observed for coarse spray (large droplets, low atomization pressure, small spraying angle, and low concentration of droplets in spray). Particle residence time in the combustion zone is the main factor affecting the increase in particle diameters due to puffing. Complex flow dynamics of dispersed phase observed at the outward regions of the flame in recirculation zones results in the formation of particle agglomerates.

Aerodynamic effect of the aircraft carrier island on flight deck flow with cross wind

Regarding the importance of helicopter operations over aircraft carriers and the increment of the workload of the pilot during take-off and landing manoeuvres, it has been performed a study of the flow downstream the island of an aircraft carrier in cross-wind conditions. The main objective of this work is to characterize the aerodynamic flow in that critical area providing insight for future investigations directed to reduce the workload and improve safe operational conditions. Then, a wind tunnel test campaign was carried out using particle image velocimetry and laser Doppler anemometry techniques. Finally, wind tunnel results have been compared with on-board measurements showing a good agreement between full-scale and model-scale wind tunnel tests.

Change in the direction of electric wind from a wire electrode tilted relative to a grounded plane

We report on experimental investigations of the structure of electric (ionic) wind in the wire-versus-plane electrode system with a constant interelectrode gap and variable wire tilt angle relative to the plane. Characteristic wind velocity distributions were determined using a laser Doppler anemometry technique. It is established that the position of the narrow central jet of the wind significantly changes depending on the tilt of the corona-forming electrode.

Experimental and numerical study of particle velocity distribution in the vertical pipe after a 90° elbow

A major problem of utilizing co-firing technique is controllably distributing fuel mixtures of pulverized coal and granular biomass in a common pipeline. This research related into particle velocity distribution in the vertical pipe after a right angle elbow was undertaken using the Laser Doppler Anemometry (LDA) technique and a coupled computational fluid dynamics (CFD)-discrete element method (DEM) simulation. According to the similarity criterion of Stokes Number, three types of glass beads were used to model the dilute gas-solid flow of pulverized coal or biomass pellets. In the vertical pipe after an elbow (R/D=1.3, R is the bend radius as 100mm and D is pipe diameter as 75mm), a horseshoe shape feature has been found on cross-sectional distributions of the axial particle velocity for all three types of glass beads on the first section which is 15mm away from the blend exit. At the further downstream sections, the horseshoe-shaped feature is gradually distorted until it completely disappears. The distance for total disintegration is about 300mm away from the first section for the first type of glass beads, 150mm away for the second type and 75mm away for the third one. As for particle number rate, its cross-sectional distribution on the first section shows that a rope is formed at the pipe outer wall where the maximum particle number rate occurs. On the whole, the rope formed by the first type of glass beads can be still observed at the section which is 450mm away from the first section. For the second type of glass beads, the rope will disintegrate from the section (300mm away) according to the cross-sectional distributions of the dimensional value of particle axial velocity divided by air conveying velocity. Overall, the roping characteristic is not obviously shown in the gas-solid flow of the third type of glass beads after the first section. All of these indicate that ropes formed from larger particles disperse more easily, for reasons perhaps related to their higher inertia. In addition, CFD-DEM analysis was employed to determine the particle characteristics to confirm this assumption. The numerical results indicated that the flow pattern has the highest axial velocity near the elbow to form the ‘horseshoe’ pattern. As the particles with lower Stokes Number were easier to follow the air flow, this is the reason why the horseshoe pattern of smaller particles was more obvious than larger particles.

On near‐wall jets in a disc‐like gas vortex unit

To clarify the three‐dimensional (3D) structure of near‐wall jets observed in disc‐like gas vortex units (GVUs), experimental and numerical studies are performed. The experimental results are obtained using stereoscopic particle image velocimetry (PIV), laser doppler anemometry, pressure probes and surface oil flow visualization techniques. The first three techniques have been used to investigate the bulk flow hydrodynamics of the vortex unit. Surface oil flow visualization is adopted to visualize streamlines near the end‐walls of the vortex unit. The surface streamlines help to determine the azimuthal and radial velocity components of the radial near‐wall jets. Simulations of the vortex unit using FLUENT® v.14a are simultaneously performed, computationally resolving the near‐wall jet regions in the axial direction. The simulation results together with the surface oil flow visualization establish the 3D structure of the near‐wall jets in GVUs for the first time in literature. It is also conjectured that the near‐wall jets develop due to the combined effect of bulk flow acceleration and swirl. The centrifugal force diminishes in the vicinity of the end‐walls. The radially inward pressure gradient in these regions, no longer balanced by the centrifugal force, pushes gas radially inward thus developing the near‐wall jets. © 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 63: 1740–1756, 2017

In vitro and in silico (IVIS) flow characterization in an idealized human airway geometry using laser Doppler anemometry and computational fluid dynamics techniques

The air flow in idealized human airway geometry was studied using computational and experimental methods. A computational fluid dynamics model developed to determine the air flow characteristics in airways was validated by comparison of the experimental velocity profiles obtained with laser Doppler anemometric measurements with numerical data. A good correlation was found between the values obtained with the two methods. Both the measurements and the calculations showed the flow to be laminar in the trachea region of the airway model, but it is affected by the airway geometry in subsequent airways.

Numerical Investigation of Flow Characteristics in the Obstructed Realistic Human Upper Airway.

The flow characteristics in the realistic human upper airway (HUA) with obstruction that resulted from pharyngeal collapse were numerically investigated. The 3D anatomically accurate HUA model was reconstructed from CT-scan images of a Chinese male patient (38 years, BMI 25.7). The computational fluid dynamics (CFD) with the large eddy simulation (LES) method was applied to simulate the airflow dynamics within the HUA model in both inspiration and expiration processes. The laser Doppler anemometry (LDA) technique was simultaneously adopted to measure the airflow fields in the HUA model for the purpose of testifying the reliability of LES approach. In the simulations, the representative respiration intensities of 16.8 L/min (slight breathing), 30 L/min (moderate breathing), and 60 L/min (severe breathing) were conducted under continuous inspiration and expiration conditions. The airflow velocity field and static pressure field were obtained and discussed in detail. The results indicated the airflow experiences unsteady transitional/turbulent flow in the HUA model under low Reynolds number. The airflow fields cause occurrence of forceful injection phenomenon due to the narrowing of pharynx caused by the respiratory illness in inspiration and expiration. There also exist strong flow separation and back flow inside obstructed HUA owing to the vigorous jet flow effect in the pharynx. The present results would provide theoretical guidance for the treatment of obstructive respiratory disease.

On the possibility of using of different turbulence models for modeling flow hydrodynamics and power consumption in mixing vessels with turbine impellers

The results of the mathematical modeling of a liquid flow in a mixing vessel with a turbine impeller are presented in the paper. The calculations were carried out using the numerical codes of Computational Fluid Dynamic (CFD) and Reynolds-Averaged Navier-Stokes equations (RANS) approach. As a closure method, three different two-equation turbulence models of the k-ɛ family were applied, i.e., the Standard k-ɛ, the renormalization-group (RNG) k-ɛ and the Realizable k-ɛ model. The results of CFD simulations were verified by measurements carried out using the Laser Doppler Anemometry (LDA) technique.

Investigation of turbulent flow field in a Kenics static mixer by Laser Doppler Anemometry

AbstractThe main purpose of the present paper was to apply the Laser Doppler Anemometry (LDA) technique to measure turbulent liquid flow in a Kenics static mixer. The LDA set-up was a one-channel backscatter system with argon-ion laser. Measurements in the static mixer were carried out for three values of the Reynolds number: 5000, 10000, and 18000. Water was used as the process liquid. Values of the axial and tangential components of the local, mean, and root mean square velocities were measured inside the static mixer. It was observed that the shape of the velocity profile depends strongly on the Reynolds number, Re, as well as on the axial, h, and radial, α, position of the measurement point. Strong dependence of the velocity fluctuations on the Reynolds number was found in the investigated range of Re and the measurement point position. Furthermore, one-dimensional energy spectra of the velocity fluctuations were also obtained by means of the Fast Fourier Transform. Fluctuation spectra of the axial and tangential velocities provided information about the energy density of velocity fluctuations in the observed range of Reynolds numbers. A study of the energy spectra led to the conclusion that the energy density increases with the increasing radial distance from the mixer walls at constant values of h, Re, and α. Minor variations in the mean value of the energy density, E, were observed together with variations of the measurement point angular position, α. In addition, it was observed that an increase of the Reynolds number causes significant increase of the power spectral density.