Laser Doppler Anemometer Research Articles

Analysis of the limits of applicability of a photomultiplier as part of a laser Doppler anemometer

Analysis of the limits of applicability of a photomultiplier as part of a laser Doppler anemometer

Analysis of the limits of applicability of photomultiplier as part of a laser Doppler anemometer

This paper considers the use of laser Doppler anemometry methods for non-contact measurement of the velocity of a moving medium in various phase states. It notes the specifi city of conditions of application of laser Doppler anemometers, in that in a number of cases there are restrictions on the use of photomultipliers. In order to find an alternative to the classic photomultiplier of a laser Doppler anemometer, three types of photomultipliers were investigated: classic electric vacuum; silicon avalanche multipixel; with microchannel plate. Physical experiments were conducted to measure the velocity of aero- and hydrodynamic fl ows by laser Doppler anemometer with photomultipliers of three types. The velocities of a rotating glass disk, aerodynamic fl ow in a channel of rectangular cross-section, and aerosol flow, as well as hydrodynamic flooded jet, were measured. The obtained experimental data were analyzed according to the criteria for assessing the quality (efficiency) of operation and finding the limits of applicability of the specified photomultipliers. Based on the physical properties of the studied photomultipliers, their efficiency indicators in various experiments are explained. The boundaries of the effective use of these photomultipliers in a wide class of physical experiments are determined. The results obtained are important for researchers involved in the study of aero- and hydrodynamic flows, where accurate and reliable measurements of flow velocity are required. The choice of an optimal photomultiplier tube for the laser anemometer allows increasing the accuracy and ease of obtaining experimental data and reducing the cost of the final equipment.

Flow dynamics and heat transfer analysis of a sweeping air jet – An experimental approach

Technological and environmental constraints are driving the development of enhanced heat transfer alternatives. In this context, the scope of this study encompasses sweeping jets generated by fluidic oscillators, which offer considerable potential for enhancing heat transfer. Initially, an experimental setup was developed to measure the flow velocity using a Laser Doppler Anemometer (LDA) and the heat transfer onto a target surface using a flux sensor. Moreover, videos capturing the free jet under various flow regimes were recorded using a Nikon® D850 camera. Concerning the flow dynamics of the sweeping jet, an upward trend in the estimated deflection angle of the jet and its oscillation frequency was noted at higher air flow rates. Heat transfer measurements on a surface indicated that an increase in air flow rate leads to improved heat transfer effectiveness of the jet. Finally, a comparison between the sweeping jet and the industry standard jet for surface cooling was carried out. This comparison revealed that the sweeping jet is highly sensitive to the flow regime at a distance from the plate of H/D= 6. In laminar flow, the sweeping jet outperforms the impinging jet in heat transfer performance. However, as the jet transitions to turbulent flow, the impinging jet exhibits superior thermal characteristics for enhancing heat transfer.

Heat transfer across an air curtain for heat confinement in road tunnels: Influence of the heat source

Experiments were carried out to study the influence of the heat source using double-flow double-jet curtains (DS-TJ) to confine heat in fire safety problems. For this purpose, a small-scale road tunnel facility in 1:34 was built. The heat source used is an electric resistance. Simultaneous two-dimensional temperature and velocity measurements were made across the DS-TJ air curtain and in its immediate vicinity using a fine thermocouple and a 2D laser Doppler anemometer (LDA), respectively. Four cases were studied, by modifying the air curtain velocities at the nozzle exits and the heat source power. It is noted that hot-cold jets and heat source play a role in the curtain heat confinement by modifying the development of the two jets. Both the entrainment of the upward flow rising from the heat source as well as the Kelvin-Helmholtz instabilities contribute to this. On the other hand, the nozzle exit velocities determine the behavior of the Reynolds stress profiles as well as the production of the velocities fluctuations across the curtain, showing a non-correlating effect on the hot side, so that the profiles take values around zero, suggesting a heat source influence. In contrast in the cold side negative values indicate energy-draining behavior. The intensity of thermal turbulence reveals that the hot jet has higher values than the cold jet, constituting a thermal barrier to turbulent heat flow, and all activity is relegated to the zone of interaction between the two jets. In fact, the production derived from temperature fluctuations occurs mainly in the interaction of hot-cold jets, and negative values are due to large turbulent eddies, such as Kelvin-Helmholtz instabilities and Görtler-type structures. The Richardson number shows that buoyancy forces prevail on both sides of the curtain (Ri > 0.1), especially on the hot side, due to the heat source. Lower Ri values within the curtain indicate the presence of a thermal barrier, attenuating heat transfer through the curtain and favoring thermal sealing.

A numerical method to mimic an experimental wind gust generator: The immersed boundary gust generator

To generate horizontal wind gusts in a classical wind tunnel, Wood, Breuer, and Neumann [A novel approach for artificially generating horizontal wind gusts based on a movable plate: The paddle,” J. Wind Eng. Ind. Aerodyn. 230, 105170 (2022)] developed a new wind gust generator denoted the “paddle.” The working principle relies on the partial blocking of the outlet of the wind tunnel nozzle by a plate that vertically moves into the free-stream. Based on laser-Doppler anemometer measurements of the velocity at only a few locations, the basic functionality of the device was proven. The objective of the present contribution is to numerically mimic the gust generator and the flow field induced by the paddle in the test section. Contrary to the single-point measurements, the three-dimensional time-resolved simulation delivers the entire flow field and thus allows to investigate all details of the generated gust. To describe the paddle motion, the immersed boundary method with a continuous and direct forcing approach is implemented into a finite-volume flow solver for large-eddy simulations. A uniform and a non-uniform distribution of the Lagrangian markers are investigated where the latter ensures that an excessive increase in the computational resources required can be avoided. The predictions allow to characterize the resulting flow features induced by the paddle in great detail. Furthermore, a comparison of the numerical and experimental results is carried out based on the time histories of the streamwise and vertical velocity components at certain positions showing a close agreement. Finally, the forces acting on the fluid by the moving paddle are evaluated.

Constant stress layer characteristics in simulated stratified air flows: Implications for aeolian transport

Varying thermal atmospheric stability conditions and their effects on shearing flows has long been a subject of interest for researchers working in atmospheric science. The development of new instrument technologies now offers an opportunity to study flows with high spatial and temporal resolutions in wind tunnel atmospheric boundary layers. In the presented study, we use a laser Doppler anemometer within the Trent Environmental Wind Tunnel Laboratory to investigate the influence of thermal stratification on the constant stress layer. Analyses of the thermal stratification represented by the gradient Richardson number and the apparent von Kármán parameter, shear velocity, and the slope of the streamwise velocity profiles reveal strong linear relationships. An exponential relationship between thermal stability and the apparent roughness length is also revealed. Profiles of the streamwise and vertical velocity and turbulence intensity, as well as the dimensionless Reynolds stress, are influenced by the gradient Richardson number. These findings have implications for producing accurate models of sediment entrainment and transport by wind in non-neutral conditions.

Airborne four channel fiber coupled vector laser Doppler anemometer system.

Using an airborne vector laser Doppler anemometer (LDA) to measure the air flow outside of the boundary layer of an airplane is a promising optical technique. Measurement of the primary flight data like true airspeed, the angle of attack, and the angle of sideslip can be directly derived from the measured wind vector. We developed an experimental system with interchangeable telescopes to study the change in the LDA sensitivity and signal rate, depending on the focusing of the measurement beam. The system has a real-time-capable field programmable gate array data acquisition system, which also can record full data dumps for off-line analysis. This paper presents the first results of an airborne measurement campaign. The true airspeed is measured with an residual error of 1% compared to the five-hole flow sensor. The angle of sideslip and the angle of attack show a standard deviation of the residual error of 0.6∘ for the angle of sideslip and 0.2∘ for the angle of attack.

Effects of Shape and Size on Microplastic Atmospheric Settling Velocity.

Microplastics (MPs) have been found in all terrestrial, marine, and riparian environments, including remote regions. This implies that atmospheric transport is an important pathway when considering MP sources and global budgets. However, limited empirical data exist to aid in effective development and parameterization of MP atmospheric transport models. This study measured the atmospheric settling and horizontal drift velocities of various sizes and shapes of MPs in two specially designed settling columns using a laser Doppler anemometer. The settling velocities were generally lower than modeled values, while shape exerted the most significant influence on the rate of settling. Rather than conforming to well-established, power-law models, each class of MP exhibited a linear but different relationship between MP size and settling velocity, with markedly higher slopes for the spheres and cylinders as compared to the films and fibers. Shape also had a substantial influence on particle drift, with the fibers and films exhibiting the greatest horizontal motion, as suggestive of their changing orientation in response to particle interactions and fluid drag. As a consequence, microplastic particles identified within atmospheric deposition samples collected at a single point may derive from entirely different sources representing a wide range in transport distance.

Dual wavelength laser Doppler anemometer for simultaneous velocity and particulate size distribution measurements in submarine environments.

An in-situ laser Doppler current probe (LDCP) for the simultaneous measurements of the micro-scale subsurface current speed and the characterizations of micron particles is dedicated in this paper. The LDCP performs as an extension sensor for the state-of-the-art laser Doppler anemometry (LDA). The all-fiber LDCP utilized a compact dual wavelength (491 nm and 532 nm) diode pumped solid state laser as the light source to achieve the simultaneous measurements of the two components of the current speed. Besides its ability for the measurements of the current speed, the LDCP is also capable of obtaining the equivalent spherical size distribution of the suspended particles within small size range. The micro-scale measurement volume formed by two intersecting coherent laser beams makes it possible to accurately estimate the size distribution of the micron suspended particles with high temporal and spatial resolution. With its deployment during the field campaign at Yellow Sea, the LDCP has been experimentally demonstrated as an effective instrument to capture the micro-scale subsurface ocean current speed. The algorithm for retrieving the size distribution of the small suspended particles (2∼7.5µm) has been developed and validated. The combined LDCP system could be applied to the continuous long-term observations of plankton community structure, ocean water optical parameter over a wide range, and useful to elucidate the processes and interactions of the carbon cycles in the upper ocean.

Noise induced effects in the axisymmetric spherical Couette flow.

We study the axisymmetric, wide gap, spherical Couette flow in the presence of noise in numerical simulations and experiments. Such studies are important because most of the flows in nature are subjected to random fluctuations. Noise is introduced into the flow by adding fluctuations to the inner sphere rotation which are random in time with zero mean. Flows of a viscous incompressible fluid are induced either by rotation of the inner sphere only or by the co-rotation of the spheres. Mean flow generation was found to occur under the action of additive noise. A higher relative amplification of meridional kinetic energy compared to the azimuthal component was also observed under certain conditions. Calculated flow velocities were validated by laser Doppler anemometer measurements. A model is proposed to elucidate the rapid growth of meridional kinetic energy for flows induced by varying the co-rotation of the spheres. Our linear stability analysis for flows induced by the rotation of the inner sphere revealed a decrease in the critical Reynolds number, corresponding to the onset of the first instability. Also, in this case, a local minimum of the mean flow generation on approaching the critical Reynolds number was observed, which is consistent with the available theoretical predictions. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

Coupled fluid-structure simulations and experiments of DBD plasma actuator for damping of plate vibrations

In the present work, an experimental and numerical study has been performed to evaluate the suitability of the dielectric barrier discharge plasma actuators to reduce free vibrations of plates. Two configurations composed of two different plexiglass plates and identical actuators were manufactured. Experimental campaigns, in terms of laser doppler anemometer velocity measurements, flow visualization and proper orthogonal decomposition were carried out to characterize the plasma actuator and to validate the numerical models. The plasma-induced virtual body force for the numerical analysis has been calibrated based on the experimental body force and the induced flow field. Then numerical studies of the plasma effect on the flow around an oscillating plate was performed. An experimental modal analysis has been also performed to understand the vibration characteristics of the plate and to support the numerical study. A reduced-order model of the system, based on the harmonic oscillator, was developed to properly design an open loop controller (system natural frequency, amplitude and timing of the attenuation force). Eventually, the work demonstrates that the control strategy based on the plasma actuation is effective in reducing the plate vibrations. However, a deeper study and a more sophisticated control strategy would be required to significantly mitigate the flutter oscillations of airfoils in real applications.

The influence of noises with different spectra on viscous incompressible fluid flows

The results of an experimental study of the effects of noises on flows in a rotating spherical layer are presented. Noise is introduced into the flow in the form of random fluctuations of the inner sphere rotational rate relative to a time constant mean value. The flow velocity was measured with a laser Doppler anemometer. It was found that the noises of the equal amplitude, but with different spectra cause the generation of mean flows of different intensity. It is shown that the increase in the average velocities under the action of the noise is larger for the stable flows compared to unstable ones Keywords: noise, flows with rotation, spherical Couette flow.

Влияние шумов с различными спектрами на течения вязкой несжимаемой жидкости

The results of an experimental study of the effects of noises on flows in a rotating spherical layer are presented. Noise is introduced into the flow in the form of random fluctuations of the inner sphere rotational rate relative to a time constant mean value. The flow velocity was measured with a laser Doppler anemometer. It was found that the noises of the equal amplitude, but with different spectra cause the generation of mean flows of different intensity. It is shown that the increase in the average velocities under the action of the noise is larger for the stable flows compared to unstable ones.

Gas flow standards and their uncertainty

We review primary and working gas flow standards that are used to calibrate gas flow meters. For each type of primary standard, we describe the principles and practical considerations of its operation and describe one example implementation. We identify the practical limits of the example’s performance, and, in many instances, we provide an uncertainty budget for a typical flow. The reviewed standards span the flow range 2.2 × 10−7 g s−1 to 520 kg s−1. The references point to standards that operate at higher and at lower flows. The reviewed standards include volumetric flow standards (piston and bell provers, pressure–volume–temperature–time standards, rate of rise standards), static and dynamic gravimetric flow standards, and velocity × area standards such as critical flow venturis, laser doppler anemometer surveys, and multipath-ultrasonic meters. Finally, we describe working standard flow meters used in parallel to attain higher flows than economically practical via primary methods.

Uncertainty analysis of the air velocity standard based on LDA and wind tunnel

This paper deals with uncertainty analysis an air velocity standard consisting of a wind tunnel and a laser Doppler anemometer (LDA) measuring system. The analysis is performed for air velocities between 0.3 m/s and 40 m/s, and a calibration procedure with the readings from the LDA and the device under test (DUT) taken simultaneously. The calculation of the reference air velocity includes a systematic correction of the velocity measured by the LDA to the position of the DUT. The study provides a complete and detailed methodology for estimating all major uncertainty contributions attributed to the LDA standard, position correction including velocity field uniformity, stability and uncorrected blockage effect. The contribution of the blockage effect was estimated for three different anemometers. For the Pitot static tube, the contribution of position correction predominates; however, for the other two larger anemometers, the contribution of the uncorrected blockage effect predominates at higher velocities.

A novel laser Doppler anemometer (LDA) for high-accuracy turbulence measurements

High accuracy and dynamic range have been some of the most prominent challenges when it comes to fine-scale turbulence measurements. The current commercial LDA processors, which perform the signal processing of Doppler bursts directly using hardware components, are essentially black boxes and in particular are renown for suffering from practical limitations that reduce the measurement reliability and accuracy. A transparently functioning novel LDA, utilizing advanced technologies and up-to-date hardware and software has therefore been developed to enhance the measurement quality and the dynamic range. In addition, the self-developed software comes with a highly flexible functionality for the signal processing and data interpretation. The LDA setup and the combined forward/side scattering optical alignment (to minimize the effective measuring volume) are described first, followed by a description of the signal processing aspects. The round turbulent jet has been used as the test bed since it presents a wide range of degree of difficulty for the LDA processor (accuracy, dynamic range etc.) across the different radial distances and downstream development. The data are diagnosed for dynamic range in residence and interarrival times, and compared to a typical hardware driven processor. The radial profiles of measured mean streamwise velocity and variance agree well with previous studies of the round jet. The spatial turbulent kinetic energy spectra in the fully developed region perfectly match the expected (and in this region well established) -5/3 power law even for the largest measured distances from the centerline (where shear and turbulence intensity are significant).

Performance Evaluation of Force Transducer for the Observation of Sediment Entrainment in Rapidly Varied Flows

Abstract Direct measurement of forces within the rough bed layer have been limited by previous spatial-averaging shear force studies. A highly sensitive force transducer assembled with a target sphere was used to measure and record the instantaneous three-dimensional forces of sediment at incipient motion. In the current study, a laser Doppler anemometer, ultrasonic displacement meter, and a force transducer accompanied by video recordings were used to experimentally investigate the incipient motion of sediment. The developed experimental setup have the potential to resolve and improve fundamental classical hypotheses regarding the incipient sediment motion. Experiments conducted in a large recirculating flume verified that the force transducer detects instantaneous forces at incipient motion under varies hydrodynamic conditions. Depth time series, instantaneous horizontal, vertical, and lateral forces are presented for dam-break and tidal breaking bores. Evidence suggests that the uplift vertical force plays an important role in destabilizing and in the incipient motion of particles. A sudden decrease in horizontal force was observed in tidal breaking bore due to flow reversal; however, a rapid rise was observed due to initial impact of dam-break bore. Bore velocity seems to have a larger effect on dam-break force than bore height. Furthermore, lateral force has the least influence during tidal breaking bore, while sediment particles are subjected to additional lateral force during dam-break bore.

A novel approach for artificially generating horizontal wind gusts based on a movable plate: The Paddle

The paper is concerned with a novel approach to generate horizontal wind gusts in classical wind tunnels. Assuming an open test section of an Eiffel- or Göttingen-type wind tunnel, the new wind gust generator denoted “The Paddle” can be easily retrofitted to such experimental setups at low costs. The device is constructed based on commercially available components including a programmable software tool that allows to adjust all relevant kinematic parameters. Five predefined motion patterns of the paddle are investigated which differ according to the achieved blocking ratio of the wind tunnel nozzle as well as the velocity and acceleration of the downward and upward motion of the paddle. It is shown that the shape and intensity of the generated gusts can be fully controlled by these parameters. That guarantees a strong individual adjustability and customization of the induced gusts. Furthermore, a synchronization of the wind gust generator with measurement devices such as laser-Doppler anemometer or particle-image velocimetry is easy to implement. An extensive measurement campaign has shown that the generated horizontal gusts are highly reproducible for the presently investigated laminar boundary layer and vary in the core area of the gust measurements solely according to free-stream turbulence of the wind tunnel.

Similarity scaling of the axisymmetric turbulent jet

In this work, we find that a free axisymmetric jet in air displays self-similarity in the fully developed part of the jet (≳30 jet exit diameters from the jet exit). We report accurate measurements of first, second, and third order spatially averaged statistical functions of the axial velocity component performed with a laser Doppler anemometer, including the outer (high intensity, ≳20%) regions of the jet. The measurements are compared to predictions derived from a simple jet model, which is described in a separate publication, and we discuss the implications for the further study of self-similarity in a free jet. It appears that all statistical functions included in this study can be scaled with a single geometrical scaling factor—the downstream distance from a common virtual origin, z−z0.

Experimental Study on a New Combined Gas–Liquid Separator

Gas–liquid separation at natural gas wellheads has always been a key technical problem in the fields of natural gas transportation and storage. Developing a gas–liquid separation device that is both universal and highly efficient is the current challenge. A new type of combined gas–liquid separation device was designed in this study, and the efficiency of the separator was studied using a laser Doppler anemometer and phase Doppler particle analyzer at a flow rate of 10–60 Nm3/h. The results showed that the separation efficiency of the combined separator was above 95% at each experimental flow rate, verifying the strong applicability of the combined separator. Moreover, the separation efficiency was as high as 99% at the flow rates of 10 and 60 Nm3/h, thereby realizing efficient separation. This study is significant to the development of gas–liquid separation devices.