Laser Doppler Velocimetry Measurements Research Articles

LDV measurements of particle velocity distribution in an annular stripper

Abstract Particle descent velocities in an annular stripper were measured by a laser Doppler velocimetry (LDV) system. In the radial direction, particle descent velocity was relatively constant in the mid-region of the stripper and increased towards the walls on both sides, exhibiting an anti-U-shaped distribution. Particle descent velocity in the radial mid-region increased with the increase of superficial gas velocity, and the maximum in the outer wall region increased significantly with the increase of solid mass flux. Superficial stripping gas velocity had stronger effect on particle velocity distributions near the stripper gas distributor, and such effect weakened with the increase of the distance from the distributor. Local particle velocity and its radial profiles could be adjusted by changing the superficial stripping gas velocity. Empirical formulas were established to describe the relationships between the local particle velocity and cross-sectional averaged velocity based on the effects of operating conditions and measuring positions. The result showed that the predicted data was in good agreement with the experimental value.

Experimental investigation and numerical simulation of flow in the draft tube elbow of a Francis turbine over its entire operating range

Hydraulic power plants are increasingly required to extend their operating range, enabling the seamless large scale integration of renewable energy sources into the electrical grid. In Francis turbines, the complex cavitation patterns appearing in the draft tube cone at off-design operation cause a decline in turbine performance and cyclic pressure pulsations that may lead to dangerous hydro-acoustic instabilities. The accurate prediction of the velocity distribution and the pressure pulsations by numerical simulation in the design phase is important for manufacturing stable and more efficient hydraulic machines over a wide operating range. This work seeks to improve the accuracy of numerical flow models for predicting the dynamic turbine characteristics through a systematic comparison of the velocity fields simulated with CFD and measured with Laser Doppler Velocimetry (LDV) in the draft tube cone. The numerical simulation and experimental measurements are performed in flow conditions for six values of discharge factor, four part load comprised 40%, 60%, 80%, 90% of the discharge value at the Best Efficiency Point (BEP), at the BEP, and one full load of 110%. The steady Reynolds-Averaged Navier-Stokes (RANS) simulation are performed for numerical analysis by the commercial flow solver ANSYS CFX. The LDV measurements are performed on a reduced scale model with a specific speed of nQE=0.20, installed the test rig of EPFL Laboratory for Hydraulic Machines.

Experimental and numerical investigation of flows inside draft tube of a pump-turbine

To determine the flow patterns and validate the flow simulation inside a pump-turbine, Laser Doppler Velocimetry (LDV) measurements and 3D flow simulation in the draft tube of a high-head pump-turbine have been performed at normal operating conditions in turbine and pump modes. Velocity distributions were measured for operating conditions corresponding to the rated head H=600m and the optimum head H=750m at turbine mode for various loads ranging from 40% to 120% of the rated power. The 3D flows through the full pump-turbine model have also been investigated with numerical simulations at all the measured operating points. Both measured and simulated velocity profiles are presented and analyzed to show the influence of operating parameters on the velocity and vortex characteristics in draft tube. Comparison of the measurement and simulation results shows rather good agreement in the vortex-free zone but clear discrepancies at part loads far from BEP. The experimental data such as the velocity profiles, vortex ropes visualization and frequency characteristics of the instantaneous tangential velocity at typical operating points are used to validate the precision of the model and method in the numerical simulation. The presented methodology and techniques in this paper have been demonstrated very helpful in R&D of pump-turbines at two Pumped storage Power plants in China

Experimental investigation of flow structure and energy separation of Ranque–Hilsch vortex tube with LDV measurement

In this study, transparent vortex tubes of diameter 30 mm were designed for the main tube visualisation and a 2D laser Doppler velocimetry (LDV) measurement was adopted to investigate the mean flow field on the meridian plane. The distributions of the axial and radial velocities, and particularly the reverse flow boundary, were obtained for understanding the characteristics of large-scale vortex structures. Various operation conditions with different cold mass fractions (0.1–0.9), tube lengths (360 mm, 600 mm, and 900 mm), and inlet pressures (0.1 bar and 0.2 bar) were studied to reveal how the reverse flow boundary affects the energy separation performance. The LDV results show that the mean axial velocity distribution present good axial symmetry, and increasing the cold mass fraction results in an expansion of the reverse flow boundary as reported for previous numerical calculations. A deterioration in the energy separation performance due to an excessively short or long tube was revealed, and the proposed tube optimization criteria were proven.

Axisymmetric rotating flow with free surface in a cylindrical tank

The flow induced by a disk rotating at the bottom of a cylindrical tank is characterised using numerical techniques – computation of steady solutions or time-averaged two-dimensional and three-dimensional direct simulations – as well as laser-Doppler velocimetry measurements. Axisymmetric steady solutions reveal the structure of the toroidal flow located at the periphery of the central solid body rotation region. When viewed in a meridional plane, this flow cell is found to be bordered by four layers, two at the solid boundaries, one at the free surface and one located at the edge of the central region, which possesses a sinuous shape. The cell intensity and geometry are determined for several fluid-layer aspect ratios; the flow is shown to depend very weakly on Froude number (associated with surface deformation) or on Reynolds number if sufficiently large. The paper then focuses on the high Reynolds number regime for which the flow has become unsteady and three-dimensional while the surface is still almost flat. Direct numerical simulations show that the averaged flow shares many similarities with the above steady axisymmetric solutions. Experimental measurements corroborate most of the numerical results and also allow for the spatio-temporal characterisation of the fluctuations, in particular the azimuthal structure and frequency spectrum. Mean azimuthal velocity profiles obtained in this transitional regime are eventually compared to existing theoretical models.

Statistical Inference Method for Liner Impedance Eduction with a Shear Grazing Flow

This paper presents a statistical inference method for impedance eduction in a flow-duct facility. The acoustic impedance is recast into a random variable, and Bayes’s theorem is used to obtain the posterior probability density function of both its real and imaginary parts, thus expressing the knowledge/uncertainty one has on the impedance value, given a certain experimental data uncertainty. An evolutionary Markov chain Monte Carlo technique is selected to explore the probability space, and a surrogate model based on the method of snapshots is employed to speed up the calculations. The linearized Euler equations are solved using a two-dimensional discontinuous Galerkin scheme, accounting for the presence of a grazing flow. The inference process is first validated on published NASA Grazing Incidence Tube results, in which acoustic-pressure measurements on the wall opposite the liner are used as inputs. Then, the same procedure is applied to educe the impedance of a conventional single degree-of-freedom liner in the ONERA–The French Aerospace Lab B2A acoustic bench, in which a laser Doppler velocimetry (LDV) technique is used to measure the two components of the acoustic-velocity fields above the liner. The primary conclusion of the study is that the Bayesian inference method allows for consistent impedance eductions, as compared to a classical deterministic eduction approach, for both microphone and LDV measurements. Furthermore, it yields the credibility intervals of the identified impedance, which represent the uncertainty on the identified impedance values, given an uncertain measurement. The identified parameters are less correlated using an LDV-based inference than a microphone-based inference, which might be due to the more limited number of data.

Effects of a Diverging Cup on Swirl Number, Flow Pattern, and Topology of Premixed Flames

Impact of the diverging cup angle of a swirling injector on the flow pattern and stabilization of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experiments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments.

Experimental investigation of flows inside draft tube of a high-head pump-turbine

Abstract Due to fast and frequently switch between pumping and generating modes several times daily and continuously increase to operate at off-design conditions, understanding the detail of flows is a prerequisite to the design and safety operation of a pump-turbine. Because complex flow phenomena occur inside the draft tube at different operating conditions, its flow patterns are highly demanding to be accurately investigated with reliable approach. Laser Doppler Velocimetry measurements and vortex ropes visualization inside the draft tube of a high-head pump-turbine with the specific speed nq = 24 have been performed at various operating conditions both in turbine and pump modes. Measurements and observations were taken for the speed factor nED = 0.21 and 0.19 at loads ranging from 40% to 120% of the rated power at turbine mode, and the possible operating range at pump mode. The distribution of measured axial and tangential velocities, the size of backflow zones and the rotation of vortex rope are all in line with the hydraulic design and model test results. The LDV measurement has been applied to accurately analyze the swirl flow pattern and quantify swirling intensity, the region of backflow and vortex rope, as well as to precisely determine the location of the rope-free zone combining with vortex ropes visualization. Measured instantaneous tangential velocities have been applied to accurately distinguish the processional frequency of a rotating vortex rope inside the draft tube from other fluctuations at part loads. Influence of the splitter blades can be also observed in both time and frequency domain of measured instantaneous velocities. This study can provide rather valuable information for analysis and validation in different operating regimes. The presented methodology and techniques have been demonstrated very helpful to successfully developed high-head pump-turbines for two pumped storage power plants in China.

A space–time integral minimisation method for the reconstruction of velocity fields from measured scalar fields

Scalar image velocimetry (SIV) is the technique to extract velocity vectors from scalar field measurements. The usual technique involves minimising a cost functional, that penalises the deviation from the scalar conservation equation. This approach requires the measured scalar field to be sufficiently resolved and relatively noise free, such that space and time derivatives of the measured scalar field can be accurately evaluated. We quantify these requirements for a synthetic two-dimensional (2-D) turbulent flow field by evaluating the velocity reconstruction accuracy as a function of the temporal and spatial resolution and the noise level. We propose an improved SIV scheme, that reconstructs not only the velocity field but also the scalar field, which does not require approximating the space and time derivatives of the measured scalar field. Improved velocity reconstruction is demonstrated for the 2-D synthetic field. We furthermore apply the scheme to interferograms of the thickness field of a falling soap film, where 2-D turbulence is generated by an array of cylindrical obstacles. The statistics of the reconstructed velocity field are within 10 % of laser Doppler velocimetry measurements.

High speed photography technique for measuring impact strength of porous concrete

In this study, an impact strength measurement method based on high speed photography, that has been developed for testing porous concrete, was introduced. In the experiments, a drop weight impact test set-up instrumented with a high speed camera was used. The impact strength analyses were conducted using impedance mismatch method, where the wave reverberations were investigated in detail. The measurement configuration carries the advantage of being a fast, non-contact and accurate experimental method where only the known material properties and particle velocity data of the drop weight are required. The target cementitious material itself is not actually involved in the measurements or the subsequent analyses. The method also facilitates the observation of the crack patterns throughout the experiment. The measurement method was verified to be accurate and consistent by testing different types of porous concretes and by comparing the results with those from other dynamic monitoring techniques, such as laser Doppler velocimetry and direct stress gauge measurements.

Experimental investigation of flows in a high-head pump-turbine draft tube at turbine mode

The accurate prediction of flow patterns is essential during the optimization design of a pump-turbine. To determine the flow patterns and validate the flow simulation inside a pump-turbine, Laser Doppler Velocimetry (LDV) measurements and 3D flow simulation in the draft tube cone of a high-head pump-turbine model have been performed at normal operating conditions in turbine and pump modes. Velocity distributions were measured for operating conditions covering the speed factor nED=0.21 and 0.19 at turbine mode for loads ranging from 40% to 100% the rated power as well as best efficiency point. Measurements of unsteady flows inside the draft tube cone were conducted at a high precision hydraulic model test rig with LDV system from TSI Co. The 3D flows through the pump-turbine model were numerically simulated with ANSYS/CFX® software at all the measured operating points. Both measured and simulated velocity profiles are presented and analyzed to show the influence of operating parameters on the velocity and swirl characteristics in draft tube inlet. Comparison of the measurement and simulation shows rather good agreement at the best efficiency point and full load but clear discrepancies at part load. The experiment data such as the velocity profiles, pressure signals, and amplitude and frequency characteristics of the instantaneous tangential velocity at various loads offers the possibility to assess the precision in performance prediction in the pump-turbine using the numerical simulation methods. The measured instantaneous tangential velocity at part loads reveals a periodic peak, which is asynchronous to that of runner’s rotating speed and can be used to determine the characteristic frequency of rotating vortex rope at part loads. Axial and tangential velocity fields obtained from LDV measurements at normal operating points can be used to validate the numerical simulation in optimization design.

Streamwise variations of turbulence statistics up to maximum drag reduction state in turbulent boundary layer flow due to surfactant injection

We investigate streamwise variations of turbulence statistics in the wide range of drag reduction (DR) up to the maximum drag reduction (MDR; DR ≥ 60%) state for the turbulent boundary layer flow due to surfactant injection. One-component laser-Doppler velocimetry (LDV) measurements show that the DR is drastically varied from the low drag reduction (LDR) to the high drag reduction (HDR) regions and is saturated in the MDR region, and such variation is sensitive to the free-stream velocity and dependent on the process of diffusion of injected surfactant solution. Both two-component LDV measurements and particle image velocimetry (PIV) measurements clarify that the mean velocity in wall units agrees with the so-called Virk’s ultimate profile in the MDR region, where both wall-normal turbulence intensity and the Reynolds shear stress with outer scaling are considerably suppressed compared to those in the LDR and HDR regions, while the maximum of streamwise turbulence intensity is comparable with that of water. Such behavior is independent of flow types such as internal and external flows and kinds of additives such as polymer and surfactant. The principal axis angle in joint probability density function of streamwise and wall-normal velocity fluctuations near the wall is the most promising index as the amount of DR based on the LDR, HDR, and MDR. The PIV measurements also show that the sheet-like structures in the HDR and MDR regions expand to around the maximum location of streamwise turbulence intensity, at which spanwise length scales are evaluated.

Hydraulic performance evaluation of a micro-turbine with counter rotating runners by experimental investigation and numerical simulation

Abstract The development of energy recovery systems on existing water utility networks is a new challenge for increasing the sustainability of electricity generation mix. The concept of the new micro-turbine with counter rotating runners braces this goal by allowing the recovery of the energy which is spoiled in pressure reducing valves in many water supply networks. The hydraulic performances of the micro-turbine are evaluated through an analysis on the angular momentum balance applied to the investigated cross-sections of the machine. For this analysis, an experimental campaign involving 2D Laser Doppler Velocimetry (LDV) is performed to measure the velocity field of the flow at the inlet and outlet sections of the machine. Since the optical access to perform LDV measurements is blocked close to the hub of the runners, a further analysis on the mass balance and numerical simulations are performed to complete the velocity profiles and to evaluate the machine performance. The combination of numerical and experimental methods grants a full study on the operating condition of the micro-turbine and strengthens the results achieved. The results back up the best operating condition of the machine for different discharges and support further improvements of the runner blades shape.

Characterization of the effects of a dielectric barrier discharge plasma actuator on a coaxial jet in a Bunsen burner

The following work details the experimental and numerical characterization of jet flow control in a coaxial Bunsen burner obtained by an alternate current driven dielectric barrier discharge plasma actuator (AC-DBDPA). Fuel and oxidizer flows could be inverted between the inner circular tube and outer annular tube. Use of the AC-DBDPA was intended to improve lean combustion performance. The high voltage (HV) electrode of the AC-DBDPA was powered with sinusoidal voltage excitation at 20 kHz frequency and peak-to-peak voltage of about 12 kV. Two burner fueling configurations were tested, characterized by different flow rates in the inner and annular tubes. Values of actuator power dissipation, which were deduced through electrical characterization, ranged between 25 and 28 W.A preliminary analysis of the air-methane flame behavior in presence and absence of plasma actuation was performed. Images of the flame and chemiluminescence emissions showed that the flame luminosity changed and OH radical production increased in presence of the discharge.The thermal and fluid dynamic effects of the plasma actuation on the flow were investigated by means of a non-reacting flow characterization (absence of combustion), where air was used as both fuel and oxidizer. The flow pattern and turbulence in both presence and absence of actuation were studied by laser Doppler velocimetry (LDV) measurements. Flow temperature was acquired using thermocouples. The non-reacting flow experimental results showed that, for a fixed inlet volumetric flow rate, the plasma actuation increased jet velocity and air temperature. Interesting results were obtained from a spectral analysis using the time resolved velocity signals: plasma actuation affected jet mixing and impacted energy distribution between the different flow scales.Complementary numerical simulations identified increasing temperature as primarily responsible for increasing flow velocity. Moreover, numerical predictions permitted better identification of flow regimes under different test conditions.

Ability of URANS approach in prediction of unsteady turbulent flows in an unbaffled stirred tank

Three-dimensional, unsteady Navier-Stokes equations are numerically solved to investigate the turbulent flows in a stirred vessel. The computational domain consists of an unbaffled, cylindrical vessel with a pitched-blade turbine impeller. An Eulerian-Eulerian multiphase flow model is applied to determine the shape of the free-surface vortex core. This numerical method is validated by comparing its results with laser Doppler velocimetry measurements in terms of velocity distribution and turbulence kinetic energy profiles at different positions. In the present study, URANS approach with a hybrid zonal turbulence model, k−ω SST and SST-SAS, is used to predict the unsteady pressure and velocity fluctuations within the vessel. Pressure and inward-outward radial velocity waves are generated by the impeller rotation and are captured in the time/space domain close to the impeller trailing edge. The pressure and velocity spectra are computed to characterize the blade passing frequency as the main source of unsteadiness in the turbulent flow within the vessel. The results indicate that the current URANS approach with a proper turbulence model and well-resolved grids can be used as a predictive tool for the flow field and large turbulence scales in the stirred tanks.

Detailed Analysis of the Velocity Fields from 60 kW Swirl-Stabilized Coal Flames in CO2/O2- and N2/O2-Atmospheres by Means of Laser Doppler Velocimetry and Particle Image Velocimetry

ABSTRACTThis work presents experimental results from flow field measurements of swirl stabilized pulverized-coal flames in oxy-fuel conditions and in conventional atmospheres to characterize the influence of the mixture upon flame aerodynamics. Experiments were carried out on flames inside a cylindrical combustion chamber equipped with a pulverized-coal swirl burner. Flow fields for different flame configurations were measured by means of laser Doppler velocimetry (LDV) and are complemented by particle image velocimetry (PIV). The experimental setups and results are presented, compared, and discussed. LDV measurements provide radial profiles for mean and RMS axial and tangential velocity components at different cross sections inside the combustion chamber. PIV measurements provide planar two-dimensional flow field information about mean values for the axial and the radial velocity components of the flow, as well as qualitative information about solid particle distribution inside the combustion chamber. The performance of LDV and PIV as optical measurement techniques is evaluated for the present case. Coal particles with a wide size distribution are used as flow tracers in the experiments. The influences of particle size upon obtained results, the measured flow fields, and qualitative particle distributions are discussed in detail.

Feature Correlation Velocimetry for Measuring Instantaneous Liquid Sheet Velocity

We describe a novel nonintrusive velocimetry technique for measuring the instantaneous velocity field on a liquid sheet. Short wavelength corrugations are naturally formed on the surface of a liquid sheet when the sheet interacts with ambient air. This method, called feature correlation velocimetry (FCV), relies on cross-correlation of such short wavelength corrugations visualized on the liquid sheet surface when captured using a high-speed camera. An experimental setup was created for producing a liquid sheet of known thickness and velocity. After imaging the liquid sheet with a high-speed camera, cross-correlation was employed at various spatial locations on the liquid sheet. To examine the fidelity of the method, laser Doppler velocimetry (LDV) measurements were obtained for a range of flow rates at the same spatial locations and were compared with the FCV values. The FCV values were found to be consistently within 7% of the LDV readings with the FCV measurements being consistently less than those from the LDV. In order to examine the cause of the bias error, a theoretical model of the liquid sheet has been developed. Based on the model predictions, the bias error was observed to scale as U3/2, where U is the local instantaneous liquid sheet velocity. After correcting for this bias error, a good match was observed between the FCV and the LDV readings. As an application of the FCV method, the near-nozzle region of an annular sheet exiting a spray injector has been characterized.

Experimental study of double-cavity flow

The flow through two facing, identical cavities (double-cavity) is characterized experimentally, as the inflow velocity and the distance between the cavities is varied. Standard 2D2C particle image velocimetry measurements in the spanwise mid-plane provide information on the instantaneous and mean velocity flow fields. Laser Doppler velocimetry measurements at several points in the double-cavity domain reveal the global character of the streamwise fluctuating velocity spectra. The flow is characterized based on time series, recorded in the shear layer of one of the cavities, for a wide range of inflow velocities and intercavity distances. In a detailed spectral study, we show how the shear layer spectra get affected when the two cavities are brought closer together. Based on the experimental data, a temporal local linear stability analysis was carried out, which was able to explain why the frequency peaks for close intercavity distances broaden and move to higher Strouhal numbers.

Review of Experimental Modelling in Vascular Access for Hemodialysis.

This paper reviews applications of experimental modelling in vascular access for hemodialysis. Different techniques that are used in in-vitro experiments are bulk pressure and flow rate measurements, Laser Doppler Velocimetry and Vector Doppler Ultrasound point velocity measurements, and whole-field measurements such as Particle Image Velocimetry, Ultrasound Imaging Velocimetry, Colour Doppler Ultrasound, and Planar Laser Induced Fluorescence. Of these methods, the ultrasound techniques can also be used in-vivo, to provide realistic boundary conditions to in-vitro experiments or numerical simulations. In the reviewed work, experimental modelling is mainly used to support computational models, but also in some cases as a tool on its own. It is concluded that, to further advance the utility of computational modelling in vascular access research, a rigorous verification and validation procedure should be adopted. Experimental modelling can play an important role in both in-vitro validation, and the quantification of the accuracy, uncertainty, and reproducibility of in-vivo measurement methods.

Study the Flow behind a Semi-Circular Step Cylinder (Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD))

Laser Doppler Velocimetry (LDV) measurements, flow visualizations and unsteady Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) simulations have been carried out to study the turbulent wake that is formed behind a semi-circular step cylinder at a constant flow rate. The semi-circular cylinder has two diameters, a so-called step cylinder. The results from the LDV measurements indicate that wake length and vortex shedding frequency varies with the cylinder diameter. This implies that a step cylinder can be used to attract fish of different size. By visualizations of the formation of a recirculation region and the well-known von Kármán vortex street behind the cylinder are disclosed. The simulation results predict the wake length and shedding frequency well for the flow behind the large cylinder but fail to capture the dynamics of the flow near the step in diameter to some extent and the flow behind the small cylinder to a larger extent when compared with measurements.