Laser Doppler Velocimetry System Research Articles

Pelton turbine jet flow investigations using laser Doppler velocimetry

Hydropower is a sustainable and clean renewable energy source. The Pelton turbine is an impulse type of hydraulic turbine and operates under high head and low discharge. It comprises an injector, which converts high-pressure water into kinetic energy as a high-speed free surface water jet that impinges on the runner generating power coupled with an electric generator. The water is highly accelerated in the nozzle, which produces turbulent kinetic energy in the jet flow. Before impinging on the runner, the surrounding air is entrained into the jet, which causes the jet dispersion. The effective conversion of water energy into mechanical energy and the quality of a jet depend significantly on the uniformity of the jet. However, the jet encounters instabilities and non-uniformity caused by secondary flow and turbulence. Jet surface and velocity distribution do not remain symmetric. Investigation of the jet's velocity profile and turbulence intensities is important to understand the flow behavior and improvements in the design of injector. In this study, the experimental investigation of the jet has been performed using the laser Doppler velocimetry (LDV) system. The turbulent intensity and instantaneous velocity of the jet are analyzed at different locations along the jet flow. The velocity wake zone occurs around the center of the jet, which diminishes in the axial direction of the jet, with non-dimensional jet velocities values 0.91, 0.92, and 0.94 at x/Do of 0.5, 1, and 2 for ∼98% nozzle opening. The secondary flow velocity magnitude is larger for 90°–60° injector jet flow than for 90°–50°. The jet velocity profile obtained from the LDV has been compared with computational fluid dynamics analysis.

Laser Velocimetry for the In Situ Sensing of Deep-Sea Hydrothermal Flow Velocity.

Laser Doppler velocimetry (LDV) based on a differential laser Doppler system has been widely used in fluid mechanics to measure particle velocity. However, the two outgoing lights must intersect strictly at the measurement position. In cross-interface applications, due to interface effects, two beams of light become easily disjointed. To address the issue, we present a laser velocimeter in a coaxial arrangement consisting of the following components: a single-frequency laser (wavelength λ = 532 nm) and a Twyman-Green interferometer. In contrast to previous LDV systems, a laser velocimeter based on the Twyman-Green interferometer has the advantage of realizing cross-interface measurement. At the same time, the sensitive direction of the instrument can be changed according to the direction of the measured speed. We have developed a 4000 m level laser hydrothermal flow velocity measurement prototype suitable for deep-sea in situ measurement. The system underwent a withstand voltage test at the Qingdao Deep Sea Base, and the signal obtained was normal under a high pressure of 40 MPa. The velocity contrast measurement was carried out at the China Institute of Water Resources and Hydropower Research. The maximum relative error of the measurement was 8.82% when compared with the acoustic Doppler velocimeter at the low-speed range of 0.1-1 m/s. The maximum relative error of the measurement was 1.98% when compared with the nozzle standard velocity system at the high-speed range of 1-7 m/s. Finally, the prototype system was successfully evaluated in the shallow sea in Lingshui, Hainan, with it demonstrating great potential for the in situ measurement of fluid velocity at marine hydrothermal vents.

The non-uniform inflow structure of reactor coolant pump based on Laser Doppler Velocimetry

The hydrodynamic fields generated by the steam generator (SG) head at the reactor coolant pump (RCP) inlet are experimentally and numerically investigated in this paper. The velocity profiles and turbulence intensity rate were obtained with a Laser Doppler Velocimetry (LDV) system. The velocity profiles under non-uniform inflow and uniform inflow conditions were obtained and compared. The numerical results on measurement sections were qualitatively and quantitatively validated against LDV data. The results indicated that the numerical method have a good agreement with the experiments. It can be known from the study that hydraulic performance of pump deteriorates with its inflow. In addition, the velocity profile of non-uniform inflow can be divided into three regions. These regions are constant high velocity region, large velocity gradient region and back flow region respectively. Furtherly, from the turbulence intensity rate analysis, it can be concluded that the non-uniform inflow is highly turbulent, which would bring significant performance changes to the pump.

Passive flow control of vortex-induced vibrations of a low mass ratio circular cylinder oscillating in two degrees-of-freedom

Offshore wind turbine towers are often placed vertically before installation at ports. At certain wind speeds, these towers are subject to vortex-induced vibrations and can lose up to a third of their fatigue live within a few days. So far, helical strakes have primarily been used in the upper part of the towers. These have so far proven to be not effective enough to suppress vortex-induced vibrations. The aim of this study was to develop a passive flow control method for offshore wind turbine towers and to experimentally investigate its efficiency.Experiments were performed in a circulating water tank to investigate using fin plates to suppress vortex-induced vibration of fully submerged flexible cylinders in flows at Reynolds numbers, referred to the cylinder diameter, between 1.4 × 104 and 8.0 × 104. The corresponding non-dimensional reduced velocities (normalized via the free stream velocity, the excitation frequency, and the diameter of the cylinder) were between 1.1 and 6.1. The cylinders’ of aspect ratio 18.8 had a mass ratio of 0.535. This mass ratio, referred to the mass of an oscillating cylinder normalized by the mass of displaced fluid, was below the critical mass ratio of 0.545. Each cylinder consisted of a polyvinyl chloride pipe mounted as a cantilever beam on a six degrees-of-freedom piezoelectric transducer. The top end of the cylinders was free to oscillate in the streamwise and lateral directions. An accelerometer was placed near their tops to measure accelerations and, via twofold integration, to retrieve top-end cylinder motions. A Laser Doppler Velocimetry system was employed to measure free stream velocities at the inlet of the water channel and wake velocities behind the cylinder. The wake velocities were used to calculate shedding vortex frequencies. Dynamic forces and displacements were also measured. Results demonstrated that fin plates attached to the cylinders mitigated their vibrations considerably. Optimized fin plate arrangements reduced amplitudes of streamwise and lateral vibration by 81 and 75%, respectively, and amplitudes of lateral forces acting on the cylinders by between 40 and 75%. Low uncertainty estimates of measured values substantiated the reliability of the results.

Improvement of Rudder Cavitation Performance Using Rudder Inflow Measurements in Large Cavitation Tunnel

One of the important things in a ship’s rudder design is the incidence angle of the rudder inflow to improve rudder cavitation performance. In the present study, the rudder inflow is measured by a 3-D LDV (laser Doppler velocimetry) system in a large cavitation tunnel, where the ship model tests are performed at a rather high Reynolds number of 107~108. Through the evaluation of the transverse velocity component using water injection experiments in the test section, the reliability of the 3-D LDV measurements is confirmed. The three-dimensional velocity components of rudder inflow between the rudder and propeller are successfully measured and are compared with numerical simulation results to see good agreement. The incidence angle distribution of the rudder inflow is obtained from the 3-D velocity components and shows a large angle at the range of 0.5 < Z/Span < 0.9, where the cavitation mainly occurred. The asymmetry-type rudder proposed based on rudder inflow measurements showed outstanding improvement of cavitation performance, compared with the flat-type rudder.

Experimentally investigated vortex-induced vibration of a high aspect ratio and small mass ratio circular cylinder oscillating in low reduced velocity flows

Experimental investigations were performed for a circular cantilevered cylinder located in a circulating water channel. The cylinder of mass ratio 1.78 and aspect ratio of 17.8 was vertically fixed at one end, and its other end was free to oscillate in the stream-wise and lateral directions. The reduced velocities of the flow past the cylinder, ranging from 0.65 to 5.18, corresponded to Reynolds numbers between 6.38 ∙ 10 3 and 5.1 ∙ 10 4 . At the inlet of the water channel, a Laser doppler velocimetry system measured stream-wise and span-wise flow velocity profiles and calculated the corresponding turbulence intensities. For differing reduced velocities, cylinder accelerations, displacements, and forces acting on the cylinder in the stream-wise and lateral directions were measured. Vortex shedding frequencies were compared with cylinder response frequencies in the stream-wise and lateral directions. Strouhal numbers were obtained at different water channel depths and widths. Coefficients of forces acting in the cylinder and trajectories of the cylinder's motions and accelerations were presented and analyzed. The main objectives were to investigate the effect of relevant parameters for a cylinder oscillating in 2-DOF, such as measuring the universal wake Strouhal number. The universal Strouhal number calculations were based on the vortex shedding frequency and then compared with published data. Directly measured shedding vortex frequencies at different locations around the cylinder and at different reduced velocities were compared with the frequencies of a stationary cylinder to obtain further insight of shedding vortex frequencies. Directly measured were cylinder accelerations, cylinder oscillations, and inertial forces acting on the cylinder and their trajectories. The maximum oscillation recorded in the lateral direction of the cylinder recorded was 0.8D, where D is the cylinder diameter, at which the lock-in phenomenon was observed, and figure eight–shaped (Lissajous) trajectory patterns were captured. The universal wake Strouhal number for different reduced velocities decreased almost linearly from 0.201 at U r = 0.6 to 0.141 at U r = 5.18. At different depths and widths, the Strouhal number was almost constant for each reduced velocity considered. Another objective was to directly measure the forces coefficients. The stream-wise force coefficients were close to the coefficients for the stationary cylinder, and, at higher Reynolds numbers, they increased. Furthermore, the uncertainty methods used here were explained in details and low uncertainties of measured values substantiated the reliability of our results. The experiments provided reliable benchmark data suitable for numerical validation of vortex-induced vibration for a circular cylinder oscillating in two degrees-of-freedom. 1 Experimental investigation of Vortex induced vibration in water circulation channel. 2 Velocity profiles and Turbulence intensities of the circulation water channel using Laser Doppler velocimetry. 3 Direct measurements of forces and dynamic response in lateral and transverse directions at different reduced velocities. 4 Shedding vortex Strouhal number measurements for oscillating cylinder in two degree of freedom. 5 Fluid-structure interaction measurements uncertainty analysis.

Numerical and experimental investigation of the water flow through PWR spacer grids

Spacer grids are one of main components of a Pressurized Water Reactor (PWR) fuel assembly. They are able to improve heat transfer from rod bundles to the water flow by increasing turbulence and mixture of this flow. On the other hand the pressure drop increases because spacer grids. Experimental and Computational Fluid Dynamics (CFD) analysis have been used to understand how spacer grids affect the water flow. This analysis is important to improve spacer grids thermal-hydraulic performance. This paper aims to investigate numerically and experimentally the water flow through PWR spacer grids. The numerical and experimental procedures have been developed for a 5x5 rod bundle with spacer grids at the Nuclear Technology Development Center (CDTN) in Belo Horizonte, Brazil. At CDTN, measurements of the velocity components are acquired with a 2D LDV (Laser Doppler Velocimetry) system and the numerical results are obtained using ANSYS CFX code. The measurements are obtained at one height downstream from a spacer grid and compared to CFD simulations for a flow rate at Reynolds number of 5.4x104 . Results show good agreement between both methodologies. The great repeatability and low experimental uncertainty evaluated (< 1.24%) in this work can be used to validate other CFD codes.

Cold flow characteristics of a novel high-hydrogen Micromix model burner based on multiple confluent turbulent round jets

As a promising commercial hydrogen-rich gas turbine combustion technology, micro-mixing combustion has been characterized for its excellent performance with low NOx emissions. New flame stabilization mechanism of micro-mixing flames may produce new design criteria. In order to explore that, cold flow characteristics of a novel Micromix model burner based on multiple confluent round jets has been studied experimentally and numerically, which is considered to be the basis for the exploration. A three-dimensional laser Doppler velocimetry system (3D-LDV) was used to measure the flow field of the model burner. It was found that the cold flow characteristics of the burner were different from the twin plane jets, the twin round jets, and the low Reynolds number confluent round jets. Compared to which, the interior of the micro-mixing nozzle is at a very high turbulence intensity level, and the jets merging point of the burner moved upstream; however, the position of the combined point of the burner was close to the confluent round jets. There is no recirculation region between jets near the burner outlet when the nozzle spacing was equal to 3 times the nozzle diameter and the Reynolds number was less than 16,702. The steady computational Reynolds averaged equations (RANS) model results were used to compare with the experimental results. It was found that the RANS results can match the experimental results well, and the three RANS models predict the spatial mixing deficiency less than 1% at the outlet, indicating that the fuel and air were almost completely premixed uniformly.

Interference of two intersecting Bessel beams and application to laser Doppler velocimetry

We study the interference of two intersecting similar Bessel beams. The front waves of the two beams interfere to form a distinct pattern of parallel fringes superimposed on the circular concentric fringes of the Bessel beams. We show that the two sets of fringes at the intersection region can be used in a simple laser Doppler velocimetry system to measure two velocity components. Simulations and measurements are presented to characterize the fringes and prove the concept of two-component measurement.

Measurement of a flow-velocity profile using a laser Doppler velocimetry coupled with a focus tunable lens

This paper reports on the measurement of a flow-velocity profile using a laser Doppler velocimetry (LDV) system having a focus tunable lens (FTL). In the system, the FTL is installed in the transmitting optics of the LDV; therefore, it can measure the flow velocity profile by changing the measurement position without any mechanical scanning system. To demonstrate the concept of the technique, the velocity profile measurement of Poiseuille flow was conducted, and the measured velocity profile showed good agreement with the theoretical value.

Investigation of the inner structure of gas–liquid flow in airlift pumps by laser‐high speed visualization technology

AbstractExperiments were conducted to investigate the inner structure of gas–liquid flow in airlift pumps by a laser Doppler velocimetry (LDV) system and a laser‐high speed photography. The measured central plane of the riser pipe was illustrated by a laser device, and the corresponding gas–liquid flow in this plane was recorded by a high speed camera. The results showed it was the wispy annular flow, rather than the annular flow, dominated the pump performance under a high gas flow rate. This wispy annular had a thick film with a maximal value 0.2 times of pipe diameter, and it kept pump working with a high output liquid flow rate at large gas flow rates. Moreover, new flow regime maps were presented with a function of accurately estimating the flow regime transition and presenting the airlift pump performance.

Re-examining log law velocity profile in smooth open channel flows

A comprehensive investigation of velocity distribution is presented, and the log law is re-examined using experimental data from a smooth uniform open channel flow. It is widely reported that the coefficients of the log law in channel flows deviate from those obtained from circular pipe flows by Nikuradse (Laws of flow in rough pipes, 1933), but the mechanism is not clear and no theoretical formulae are available to express these deviations. A Laser Doppler Velocimetry system was used to measure velocity profiles at the centre of the channel. The data obtained support previous conclusions that the additive constant B of the log law in channel flows can no longer be considered to be 5.5. Interestingly, the experimental data also support Tracy and Lester’s discovery that the shear velocities on both sides of the log law are different. Better agreement can be achieved if the global shear velocity (U*1) is used to normalize the measured velocity, and the local shear velocity (U*2) is used to normalize the distance from the wall. Other researchers’ data in the literature also validates this new relationship. Based on this new relationship, a theoretical value of B is obtained, which agrees well with the observed B, thus a new form of log law for channel flow is suggested. The new relationship developed was verified with present experimental and past literature data suggesting its universality irrespective of wide or narrow open channels, or subcritical or super critical flow conditions. By using the developed relationship the large scatter associated with the additive constant B associated with the log law has been explained. It is found that the additive constant B in the log law is a function of channel aspect ratio. The developed relationship for B is validated from a wide range of data from the literature to confirm its universality.

Signal denoising of double-beam and double-scattering laser doppler velocimetry based on wavelet layering

The noise contained in laser Doppler signal has a negative impact on signal processing of laser Doppler velocimetry (LDV). In order to reduce the noise and enhance the precision of signal processing, denoising methods based on wavelet layering are proposed. High frequency coefficient zeroing method and wavelet threshold denoising method were adopted for noise reduction after the wavelet layering of LDV signals. Processing results indicate that, signal to noise ratio (SNR) and root mean square error (RMSE) of simulated signals in low and high frequency ranges are satisfactory for both two kinds of approach. Overally speaking, high frequency coefficient zeroing has better performance than wavelet threshold denoising using heursure, sqtwolog, rigrsure and minimaxi rules. For measured signals obtained from double-beam and double-scattering LDV system built in laboratory, the minimum relative error of high frequency coefficient zeroing reaches only 0.087%. As shown in the processing and analysis of simulated and experimental signals, denoising methods based on wavelet layering have ability to restrain the noise in laser Doppler signal and improve the precision of signal processing.

Laser Doppler Velocimetry (LDV) measurements of airfoil surface flow on a Horizontal Axis Wind Turbine in boundary layer

This study investigated the flow characteristics of a Horizontal Axis Wind Turbine (HAWT) rotor surface during rotation with wind tunnel experiments. Firstly, the effect of Laser Doppler Velocimetry (LDV) system on the power coefficient of HAWT was measured. Then, the flow distributions near the blade surface were measured at the different radial positions (r/R = 0.3 and 0.7) in the cases of the low and optimal tip speed ratios. At last, the radial position effect on the flow field features in boundary layer were also investigated in detail. As a result, the boundary layer thickness decreased at the blade leading edge and then increased rapidly along the blade surface at the trailing edge region. Furthermore, in the case of r/R = 0.7, the two-dimensional resultant velocity in the acceleration region of boundary layer showed smaller value than that of r/R = 0.3. Meanwhile, the flow in the blade root showed a large expansion of the deceleration zone at the low tip speed ratio. The results acquired from this study was suitable for designing the airfoil and had a better understanding of the radial position effect on dynamic stall.

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.

An experimental investigation into the surface and hydrodynamic characteristics of marine coatings with mimicked hull roughness ranges

There are limited scientific data on contributors to the added drag of in-service ships, represented by modern-day coating roughness and biofouling, either separately or combined. This study aimed to gain an insight into roughness and hydrodynamic performance of typical coatings under in-service conditions of roughened ships’ hull surfaces. Comprehensive and systematic experimental data on the boundary layer and drag characteristics of antifouling coating systems with different finishes are presented. The coating types investigated were linear-polishing polymers, foul-release and controlled-depletion polymers. The data were collected through state-of-the-art equipment, including a 2-D laser Doppler velocimetry (LDV) system for hydrodynamic data in a large circulating water tunnel. Three coating systems were first applied on flat test panels with ‘normal’ finishes in the first test campaign to represent coating applications under idealised laboratory conditions. In order to address more realistic roughness conditions, as typically observed on ships’ hulls, ‘low’ and ‘high’ roughness densities were introduced into the same types of coating, in the second test campaign. The data collected from the first test campaign served as the baseline to demonstrate the effect on the surface roughness and hydrodynamic drag characteristics of these coating types as a result of ‘in-service’ or ‘severely flawed’ coating application scenarios. Data collected on coatings with a range of in-service surface conditions provided a basis to establish correlation between the surface roughness characteristics and hydrodynamic performance (roughness function). The findings of the study indicate that the estimations of drag penalties based on well-applied, relatively smooth coating conditions underestimate the importance of hull roughness, which although undesirable, is commonplace in the world’s commercial fleet.

A Flexible Hot-Film Sensor Array for Underwater Shear Stress and Transition Measurement.

A flexible hot-film sensor array for wall shear stress, flow separation, and transition measurement has been fabricated and implemented in experiments. Parylene C waterproof layer is vapor phase deposited to encapsulate the sensor. Experimental studies of shear stress and flow transition on a flat plate have been undertaken in a water tunnel with the sensor array. Compared with the shear stress derived from velocity profile and empirical formulas, the measuring errors of the hot-film sensors are less than 5%. In addition, boundary layer transition of the flat plate has also been detected successfully. Ensemble-averaged mean, normalized root mean square, and power spectra of the sensor output voltage indicate that the Reynolds number when transition begins at where the sensor array located is 1.82 × 105, 50% intermittency transition is 2.52 × 105, and transition finishes is 3.96 × 105. These results have a good agreement with the transition Reynolds numbers, as measured by the Laser Doppler Velocimetry (LDV) system.

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.

Bed shear stress and turbulence characteristics under unsteady flows

A series of laboratory experiments were conducted to investigate the turbulence and bed shear stress characteristics under solitary waves and dam break flows. The Laser Doppler velocimetry (LDV) systems were used to measure the velocity profile very near the bottom. A method was developed to find the wave-induced bed shear stress over a smooth bed from the measured velocities within or near the viscous sub-layer. A unified 2D numerical model was developed to simulate the entire flow field from bottom to free surface. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations, with the BSL k-ω turbulence closure model that can capture the turbulence inside and outside of boundary layer. The numerical results were compared to the experimental data and available theories in terms of free surface displacement, mean velocity, turbulence intensity, and bed shear stress. Discussions were made on the effects of the turbulence characteristics on changing the patterns of mean velocity profile and bed shear stress. Further efforts were made to revisit the existing boundary layer theories, trying to quantifying the role of free stream velocity and acceleration as well as eddy viscosity in modifying the response properties of the bed shear stress under unsteady flows. A formula was proposed to calculate the instantaneous bed shear using the free stream velocity and acceleration. The results using the present formula are compared to those from the earlier works for solitary wave, dam break wave, and saw-tooth wave.

Investigating the flow characteristics of air-lift pumps operating in gas–liquid two-phase flow

Abstract Experiments were conducted to investigate the flow characteristics in the riser pipe and the suction pipe of airlift pump at a series of air flow rates and submergence ratios by using a high speed camcorder and a Laser Doppler Velocimetry system (LDV). A modified model was developed to predict the performance of airlift pump operating in gas–liquid two-phase flow. The results show that an unstable flow structure composed by a water falling film, a bubbly mixture, a water ascending film appearing alternately in riser pipe dominates the performance of airlift pump at large air flow rates. The bubbly mixture with a strongest capacity for pumping water first increases to its maximum and then slightly decreases. In suction pipe, the average velocity shows a flat profile and increases with increasing submergence ratio. Moreover, the predicted results of modified model are in good agreement with the experimental data in a margin of ± 12%.