Measurements Of Velocity Components Research Articles

Calibration and Verification of Operation Parameters for an Array of Vectrino Profilers Configured for Turbulent Flow Field Measurement around Bridge Piers—Part II

High-frequency velocimeters used for flow measurements during laboratory experiments allow the user to select the range for several operation parameters to set up the instrument for optimal velocity measurement. The discrepancies between velocity measurements collected with different instrument configurations can be significant, depending on the flume bed configuration and boundary conditions. The aim of this paper is to quantify the differences in flow velocity profiles measured with Acoustic Doppler Velocimeter Profilers (ADVPs) configured using a combination of profiling parameters: Ping Algorithm (PA), Transmit Pulse Size (TPS), and Cell Size (CS). Whereas in Part I of this research, the goal was to identify the optimal probe configuration for downstream measurement of the complex hydraulic structure (pier protected with riprap) based on a match of the flow rate with measurements from other instruments, in this paper, effect of distinct probe configuration on velocity profile and turbulent kinetic energy (TKE) is demonstrated. Differences between ADVPs’ configurations were analyzed through sensitivity analysis with the intention to calculate and compare any discrepancies in the velocity measurements for all the three measured velocity components: streamwise u, spanwise v and vertical w collected on two characteristic flume cross-sections. The results show that each parameter change has a significant effect on the measured values of each velocity component when compared to the Target Configuration (TC). The largest root-mean-square-error (RMSE) is observed when TPS is changed, followed by CS and PA. Absolute RMSE calculated for TPS change from 4 mm to 1 mm is, on average, 6.30 cm/s, 0.90 cm/s, and 0.82 cm/s for velocity components u, v and w, respectively. Absolute RMSE calculated for CS change from 1 mm to 4 mm is, on average, 4.49 cm/s, 0.88 cm/s, and 0.71 cm/s for velocity components u, v and w, respectively. Absolute RMSE calculated for PA change from Adaptive to Max interval is, on average, 4.04 cm/s, 0.63 cm/s, and 0.68 cm/s for velocity components u, v and w, respectively. For a change in all parameters, RMSE is greater for the cross-section downstream of the pier than for the approach cross-section: on average, 90%, 57% and 54% for a change in the PA, TPS, and CS, respectively.

Turbine Vane Passage Experiments Documenting Evolution of Secondary Flows With Changes in Combustor Coolant Injection Flowrates

Abstract A secondary flow system with a dominant passage vortex pattern has been observed in many gas turbine vane passage studies in which there is no upstream coolant injection or only near-passage endwall coolant injection (no combustor cooling). However, it was shown in recent studies that combustor coolant introduced upstream of the vane passage changes secondary flow patterns in the passage. This results in a different secondary flow vortex system, called the “impingement vortex” system. It was discussed in recent literature having combustor coolant injection. Until now, there has been no study on how increases in combustor coolant momentum effect transition from the passage vortex system to the impingement vortex system. Such a study is presented in the present paper. Velocity component measurements are taken using a five-hole probe at three axial locations in the vane passage to document secondary flow development throughout the passage. Four combustor coolant flowrate cases are considered along with a comparison case having no coolant injection. It is shown that as the combustor coolant flowrate increases, the passage vortex system weakens and, at a sufficiently high combustor coolant flowrate, the impingement vortex system appears. Knowing the detailed flow physics of this transition between the two secondary flow systems is helpful for turbine thermal designers who wish to understand how secondary flows transport coolant within the turbine vane passage.

Experimental Design of Nature-Based-Solution Considering the Interactions between Submerged Vegetation and Pile Group on the Structure of the River Flow on Sand Beds

Designing correct engineering infrastructures to reduce land degradation processes and considering natural elements to achieve this goal are key to correctly managing potential natural hazards affecting human activities and natural ecosystems. This research investigated the scour depth and velocity vectors around bridge piles with and without upstream vegetation protection. A Doppler velocity meter was used to measure velocity components in a channel 90 cm wide, 16 m long, and 60 cm high. Variable parameters were the number of bridge piles, the height, density, and width of vegetation upstream, as well as the distance between bridge piles. Using a triple pile group with a distance between piles of 10 cm and overall vegetation across the channel, the depth of the scour hole upstream of the first pile decreased by 40% compared to the single pile with no vegetation. This result shows the significant impact of using vegetation and pile groups to reduce scour around piles. Lower vertical velocity gradients, more consistent velocity vectors, reducing the downstream flow range, and restraining horseshoe vortexes and wake vortices were observed in utilizing vegetation. We confirmed that vegetation is an essential factor in changing the flow, transportation of sediment, and conserving ecological services in rivers.

Сравнение результатов измерений уровней подводного шума источника, полученных прямым методом и с использованием векторно-фазовой обработки

Object and purpose of research. Moving broadband source underwater noise level estimations obtained by direct and indirect methods using a receiver for the measurement of sound pressure and sound particle velocity components combined. Materials and methods. Theoretical research using computer simulation, full-scale experiments in real measurement conditions, statistical analysis of experimental results. Main results. The noise level estimations obtained by the direct method, only through the sound pressure channel, and by the indirect method with recalculating the values of sound intensity were compared based on representative samples under ideal conditions and in real water area conditions. Conclusion. Under ideal measurement conditions a statistically significant difference between the two estimation methods occurs only at frequencies below 16 Hz. The results of a full-scale experiment in real conditions show that the estimate obtained on the basis of the spatial selective sound intensity vector is characterized by greater reliability and accuracy since it is less affected by reflections from the boundaries of the water area.

Experimental investigation of geometry effects and performance of five-hole probe in measuring jets in crossflow

Vortical and shear flows are common in turbomachinery. Multi-hole pressure probes are used in turbomachinery flows in order to provide robust and accurate measurements of both pressure and velocity components. In this study, two different miniature five-hole probes are designed and fabricated, both with a cobra shape. The probe tip was 1.45 mm and it was maintained in that size for the length of the cobra shape formation, providing very close proximity to the solid boundaries and reduced flow blockage. The difference among the probes corresponded to the head geometry, as the one probe was formed with a pyramid tip shape, while the other was maintained with a flat shape. The calibration process was carried out in an open-circuit suction wind tunnel for the range of ±32⁰ in yaw and pitch direction. The results showed that the pyramid probe exhibits a high flow angle spatial sensitivity and a reliable measurement range of ±28⁰ in yaw and pitch direction. The flat probe provided unexpected well angle sensitivity and reliable measurements data despite the fact that it is of a very simple form. The pyramid probe showed superior performance. In particular, the pyramid probe offers 12.5% wider operating range. In order to prove the effectiveness of the pyramid probe, measurements were obtained in a jet in cross flow. In order to evaluate the performance of the probe, further, a surface fit model was employed to produce ideal calibration coefficients. These were used to redefine the magnitude of the velocities in the measured flow domain. The accuracy in measurements was assessed, comparing the velocities produced by the two variants of pressure coefficients. The results indicate that the pyramid probe operates reliably in a very large range of constantly changing velocity vector, which occurs in jet in cross flow.

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.

Turbulent Momentum Flux Behavior above a Fire Front in an Open-Canopied Forest

Atmospheric turbulent circulations in the vicinity of wildland fire fronts play an important role in the transfer of momentum into and out of combustion zones, which in turn can potentially affect the behavior and spread of wildland fires. The vertical turbulent transfer of momentum is accomplished via individual sweep, ejection, outward interaction, and inward interaction events, collectively known as sweep-ejection dynamics. This study examined the sweep-ejection dynamics that occurred before, during, and after the passage of a surface fire front during a prescribed fire experiment conducted in an open-canopied forest in the New Jersey Pine Barrens. High-frequency (10 Hz), tower-based, sonic anemometer measurements of horizontal and vertical wind velocity components in the vicinity of the fire front were used to assess the relative frequencies of occurrence of the different types of momentum-flux events, their contributions to the overall momentum fluxes, and their periodicity patterns. The observational results suggest that the presence of surface fire fronts in open-canopied forests can substantially change the sweep-ejection dynamics that typically occur when fires are not present. In particular, sweep events resulting in the downward transport of high horizontal momentum air from above were found to be more prominent during fire-front-passage periods.

Velocity uncertainty quantification based on Riparian vegetation indices in open channels colonized by Phragmites australis

One of the main purposes of Ecohydraulics is to predict the effects of riparian vegetation on aquatic ecosystems within real water channels. The interaction between water flow and riparian plants significantly affects flow dynamics, hydraulic conveyance, and water quality of vegetated water bodies. This study aimed at quantifying analytically the uncertainty in flow average velocity estimations associated with the uncertainty of Leaf Area Index (LAI) of Phragmites australis (Cav.) Trin. ex Steudel covering a vegetated channel. The impacts of this species on the hydrodynamics of vegetated streams are far to be completely understood. The uncertainty in flow average velocity was assessed through the first-order second-moment statistical method, by comparing direct and indirect LAI measurements of mature Phragmites australis plants. Indirect LAI values were obtained using the LICOR® LAI-2000 Plant Canopy Analyzer device. The results of this study suggest that the uncertainties in flow average velocity estimations are comparable to those associated with experimental measurements of streamwise velocity components retrieved in real vegetated flows fully covered by mature Phragmites australis plants.

Empirical numerical model of tornadic flow fields and load effects

Tornadoes are the most devastating meteorological natural hazards. Many empirical and theoretical numerical models of tornado vortex have been proposed, because it is difficult to carry out direct measurements of tornado velocity components. However, most of existing numerical models fail to explain the physical structure of tornado vortices. The present paper proposes a new empirical numerical model for a tornado vortex, and its load effects on a low-rise and a tall building are calculated and compared with those for existing numerical models. The velocity components of the proposed model show clear variations with radius and height, showing good agreement with the results of field measurements, wind tunnel experiments and computational fluid dynamics. Normal stresses in the columns of a low-rise building obtained from the proposed model show intermediate values when compared with those obtained from existing numerical models. Local forces on a tall building show clear variation with height and the largest local forces show similar values to most existing numerical models. Local forces increase with increasing turbulence intensity and are found to depend mainly on reference velocity Uref and moving velocity Umov. However, they collapse to one curve for the same normalized velocity Uref / Umov. The effects of reference radius and reference height are found to be small. Resultant fluctuating force of generalized forces obtained from the modified Rankine model is considered to be larger than those obtained from the proposed model. Fluctuating force increases as the integral length scale increases for the modified Rankine model, while they remain almost constant regardless of the integral length scale for the proposed model.

Collective optical Thomson scattering in pulsed-power driven high energy density physics experiments (invited).

Optical collective Thomson scattering (TS) is used to diagnose magnetized high energy density physics experiments at the Magpie pulsed-power generator at Imperial College London. The system uses an amplified pulse from the second harmonic of a Nd:YAG laser (3J, 8ns, 532nm) to probe a wide diversity of high-temperature plasma objects, with densities in the range of 1017-1019cm-3 and temperatures between 10eV and a few keV. The scattered light is collected from 100 μm-scale volumes within the plasmas, which are imaged onto optical fiber arrays. Multiple collection systems observe these volumes from different directions, providing simultaneous probing with different scattering K-vectors (and different associated α-parameters, typically in the range of 0.5-3), allowing independent measurements of separate velocity components of the bulk plasma flow. The fiber arrays are coupled to an imaging spectrometer with a gated intensified charge coupled device. The spectrometer is configured to view the ion-acoustic waves of the collective Thomson scattered spectrum. Fits to the spectra with the theoretical spectral density function S(K, ω) yield measurements of the local plasma temperatures and velocities. Fitting is constrained by independent measurements of the electron density from laser interferometry and the corresponding spectra for different scattering vectors. This TS diagnostic has been successfully implemented on a wide range of experiments, revealing temperature and flow velocity transitions across magnetized shocks, inside rotating plasma jets and imploding wire arrays, as well as providing direct measurements of drift velocities inside a magnetic reconnection current sheet.

Fundamental flow measurement capabilities of optical Doppler and time-of-flight principles

In order to understand the fundamental measurement capabilities of different flow velocity measurement principles based on Mie scattering, a fundamental equation of how to calculate the shot noise limit for a respective signal model is derived. The derivation is based on the well-known rules of uncertainty propagation and yields the Cramér–Rao bound without the necessity to calculate the Fisher information. The derived equation is next applied to compare the shot noise limit for Doppler and time-of-flight principles including laser Doppler anemometry (LDA), planar Doppler velocimetry (PDV), laser-two-focus velocimetry (L2F), particle tracking velocimetry (PTV) and particle image velocimetry (PIV). The comparison is performed for an identical mean laser power, while two cases are studied in detail: measuring on a single seeding particle as well as measuring on multiple seeding particles and averaging. LDA, L2F and PTV/PIV obey a similar shot noise limit. For the case of a measurement on multiple seeding particles, the minimal achievable measurement uncertainty is directly proportional to the absolute value of the measured velocity component and inversely proportional to the spatial resolution. The respective shot noise limit for PDV is almost independent of the measured flow velocity component and the spatial resolution. Since PDV is sensitive with respect to a different flow velocity component depending on the observation direction, a comparison with the other principles is only reasonable to a certain extent. However, all shot noise limits in case of measuring on multiple seeding particles show the expected inverse proportionality to the square root of the total number of detected photons and thus also to the square root of the measurement time. Considering a comparable spatiotemporal resolution, an identical mean light power and typical measurement configurations, the PDV shot noise limit is the largest. As a final result, it is derived that each measurement principle obeys an uncertainty principle between position and the respective component of the wave vector, which is in agreement with Heisenberg’s uncertainty principle. Therefore, a common basis is provided to assess the fundamental measurement capabilities of Doppler and time-of-flight measurement systems on the basis of what is possible within the quantum mechanical constraints.Graphic abstract

Effect of electromagnetically driven liquid metal flows on the electric potential difference in a cuboid vessel

In this work, we study the effect on the electric potential difference (EPD) of a liquid metal (LM) flow confined in a cuboid vessel. The interaction between a magnetic field (MF) and an electric current density (j0) along the ascending axial direction generates a Lorentz force that stirs the LM producing an MHD flow. The MF is produced by either a single or a pair of NdFeB dipolar permanent magnets. j0 is in the range of 384 to 1153 A m-2. Experimental characterization is done by measuring velocity components using the ultrasound Doppler velocimetry (UDV) technique and measurements of the EPD in terminals of the vessel. Additionally, an MHD model is solved using the COMSOL Multiphysics software. A good agreement between measurements and simulations for the magnetic and velocity fields is obtained. Furthermore, an order of magnitude analysis for the characteristic velocity of the flow and EPD is presented. Comparing with a configuration without magnets, the EPD when one magnet is used reaches 16.8% higher values. In contrast, for a pair of magnets values are 22.9% smaller. This increasing/decreasing behavior for the EPD with the flow patterns could be of interest to applications such as liquid metal batteries.

Experimental and CFD analysis of MHD flow around smooth sphere and sphere with dimples in subcritical and critical regimes

An overview of previous researches related to the problem of flow around a bluff-body, using experimental and numerical methods, is presented in the paper. Experimental investigation was performed by a laser doppler anemometer, measuring velocity components of the water flow around a smooth sphere, and a sphere with dimples in square channels. Measurement results in subcritical velocity flow field, velocity fluctuation components, lift, drag and pressure coefficients, and 2-D Reynolds stress at quasi-stationary flow are conducted using 1-D laser doppler anemometer probe. The obtained experimental results are compared with numerical simulations, which are performed using the ANSYS-CFX software. For the numerical simulations of quasi-steady-state flow, k-? turbulent model was used, while for numerical simulation of unsteady fluid-flow and for the comparison of results related to the eddy structures, vortex shedding and Reynolds stresses, detached eddy simulation were used. Since the obtained results of experimental and numerical investigation of flow around smooth sphere and sphere with dimples showed good agreement, the considered flow problem was expanded by introducing the influence of a transverse magnetic field with a slight modification of the electrical conductivity of the working fluid. The other physical properties of the fluid remained the same, which also corresponds to realistically possible physical conditions. Numerical simulations were performed for three different values of Hartmann number and very small values of Reynolds magnetic number (inductionless approximation). Comparisons and analyzes of the results were made for the cases containing a magnetic field and those with an absence of a magnetic field.

Three-dimensional velocity and concentration measurements and simulations of a scaled Jack Rabbit II mock urban array

Abstract Magnetic resonance (MR) techniques provide non-invasive, three-dimensional measurements of velocity and concentration fields. Applying MR techniques to measure flows of contaminants through urban arrays provides a wealth of information that is difficult to obtain with large-scale field tests. In this project, a 1:188 scaled model of the phase 1 Jack Rabbit II field test was replicated and studied using a water tunnel with properties chosen to mimic field conditions. Three-dimensional, time-averaged flow data was measured using magnetic resonance velocimetry (MRV) and magnetic resonance concentration (MRC) techniques. The scaled flow was also modeled with large-eddy simulations (LES) to provide a dataset for comparison with the MR based measurements. Despite a complex, three-dimensional flow field, both velocity and concentration show good agreement between the experimental measurements and simulation data. Measurement uncertainty was estimated to be ± 5 % of each of the measured velocity components at each location for MRV and ± 4 % of the measured concentration at each location for MRC.

Combined measurement of velocity and temperature in liquid metal convection

Combined measurements of velocity components and temperature in a turbulent Rayleigh–Bénard convection flow at a low Prandtl number of $Pr=0.029$ and Rayleigh numbers of $10^{6}\leqslant Ra\leqslant 6\times 10^{7}$ are conducted in a series of experiments with durations of more than a thousand free-fall time units. Multiple crossing ultrasound beam lines and an array of thermocouples at mid-height allow for a detailed analysis and characterization of the complex three-dimensional dynamics of the single large-scale circulation roll in a cylindrical convection cell of unit aspect ratio which is filled with the liquid metal alloy GaInSn. We measure the internal temporal correlations of the complex large-scale flow and distinguish between short-term oscillations associated with a sloshing motion in the mid-plane as well as varying orientation angles of the velocity close to the top/bottom plates and the slow azimuthal drift of the mean orientation of the roll as a whole that proceeds on a time scale up to a hundred times slower. The coherent large-scale circulation drives a vigorous turbulence in the whole cell that is quantified by direct Reynolds number measurements at different locations in the cell. The velocity increment statistics in the bulk of the cell displays characteristic properties of intermittent small-scale fluid turbulence. We also show that the impact of the symmetry-breaking large-scale flow persists to small-scale velocity fluctuations thus preventing the establishment of fully isotropic turbulence in the cell centre. Reynolds number amplitudes depend sensitively on beam-line position in the cell such that different definitions have to be compared. The global momentum and heat transfer scalings with Rayleigh number are found to agree with those of direct numerical simulations and other laboratory experiments.

A method for the estimation of sideslip angle for a vehicle equipped with a one-antenna GPS measuring system

One-antenna GPS systems present no possibility for the direct determination of vehicle slip angle. This is an easy task for dual antenna systems; however, many users have this kind of apparatus. In this paper, a method of estimation of this parameter, which is important for the estimation of vehicle steerability factors, is proposed (e.g. TB factor calculated on the basis of data from input test [8]). The method is based on two parameters measured by a one-antenna GPS system; these are the heading angle created from the Doppler channel coming directly from the GPS engine, and the yaw rate measured by an IMU device integrated and cooperating with the GPS engine. The sideslip angle which was calculated according to the proposed method is compared with an equivalent angle calculated on the basis of data from a non-slip measurement of velocity components for selected point of vehicle acquired using. The presented method is illustrated with examples from real tests. In the author’s opinion, the sideslip angle calculated with the application of measurement data obtained from a one-antenna GPS device could be used in practice. From comparison with another upper mentioned method, it follows that the differences between average values of sideslip angles obtained from both considered methods is not greater than 8%.

FRANCIS G. PEASE AND THE FIRST GALACTIC ROTATION CURVES (1916‒1922)

With the 60-inch reflector at Mount Wilson Observatory, Francis G. Pease obtained the first measurements of the rotation curves of two nearly edge-on spiral galaxies, NGC 4594 (the Sombrero Galaxy) and NGC 224 (the Andromeda Galaxy) in 1916 and 1918, respectively. Pease aligned the spectrograph's slit with the major axes of the galaxies' foreshortened elliptical figures and obtained direct measurements of the Doppler-induced radial velocity components along their axes. This technique originally had been applied in 1895 by James E. Keeler to a rotational analysis of the rings of Saturn. By subtracting out the radial velocities of their centers, Pease obtained their rotational velocities as a function of distance away from the centers. This enabled him to determine that linear relationships existed in both cases, whereby these objects appeared to show evidence of solid-body rotations out to (and beyond) the limits that could be observed spectroscopically (about 2.5 arcminutes from their centers). These relationships contradicted the behaviors of planets that orbited our Sun, and indicated that the objects' mass distributions likewise increased in direct proportion away from the centers. Pease's dynamical results were applied in two independent cases (by Ernst J. Öpik and Knut Lundmark) to a greater understanding of the distance and nature of the 'spiral nebulae' in the aftermath of the 'Great Debate'. Yet, the remaining questions posed by Pease's data were not effectively answered for two decades (or more) until Horace W. Babcock extended spectroscopic investigations into the disk region of the Andromeda Galaxy and showed that those stellar velocities were largely independent of their distances from the center. Along with many other examples, this finding was later confirmed by Vera Rubin and her colleagues and has become one of the leading forms of evidence for the existence of dark matter in galaxies.

Three-dimensional flow structure, morphodynamics, suspended sediment, and thermal mixing at an asymmetrical river confluence of a straight tributary and curving main channel

Past field studies of confluence dynamics have focused mainly on experimental or small stream confluences and confluences of large rivers. Few studies have explored fluvial processes and forms at confluences of medium-size rivers, especially those where river planform varies from a straight alignment. This study examines changes in flow structure, bed morphology, sediment suspension, and mixing with changing incoming flow conditions at a river confluence where a straight tributary joins a curving main channel at the upstream end of a bend. Data include five sets of field measurements of three-dimensional velocity components, bed topography, acoustic backscatter intensity, and water-surface temperatures at the confluence of the curving Wabash River and its straight tributary, the Embarras River. Results show that, in contrast to large river confluences, channel-scale helical motion is a prominent feature of flow structure at this confluence. Dual counter-rotating helical cells, separated by a distinct shear layer, develop over a region of scour within the confluence and contribute to maintenance of scour by sweeping sediment away from this region. Helical motion develops through channel curvature and flow deflection within the confluence. The location and geometry of the scour hole are subject to change depending on the relative momentum fluxes of the two rivers with the zone of scour shifting outward and enlarging as the momentum flux of the tributary increases relative to the momentum flux of the main river. Patterns of backscatter intensity, used here as a surrogate for suspended sediment concentration, confirm that near-bed suspended sediment moves around the scour hole. The thermal mixing interface between the two rivers corresponds to the position of the shear layer, and thermal mixing at the surface within the immediate vicinity of the confluence is limited. The hydrodynamics, morphodynamics, and patterns of sediment suspension and thermal mixing at this river confluence reflect a combination of confluence and bend dynamics.

Unsteady velocity measurements of model-scale supersonic exhaust jets in military-relevant configurations

Of the utmost importance is the need to better understand the high temperature, high velocity flow fields generated by military tactical aircraft during “run up” and take-off that gives rise to extremely hazardous conditions for personnel and equipment within the vicinity of the aircraft. The present study aims to fill the need for high frequency, two velocity component measurements throughout the flow fields produced by university-scale supersonic jets exhausting from nozzles in configurations relevant to practical, full-scale application. Specifically, this work focuses on studying the supersonic jets operating in two basic configurations: horizontal, free jets and jets impinging normal to a ground plane reminiscent of current short-takeoff and vertical landing aircraft. Experiments are conducted at nozzle operating conditions similar to those of full-scale aircraft. Both mean velocities and turbulence components are measured in both flow fields using a laser Doppler velocimeter. Axial components of the mean flow and turbulence are measured in the free jet. In the single impinging jet flow field two-component mean velocity and turbulence components are measured in the jet plume, impingement region, and outwash flow. Free jet velocity measurements show good consistency with 50% increase in jet Reynolds number. Turbulence intensities up to 15% of the mean jet exit velocity are observed at the nozzle exit plane. Laser Doppler measurements in the outwash of an impinging jet show turbulent fluctuations produce unsteady velocities well above the mean value. Two-component impinging jet unsteady velocity spectra show a distinct peak at the same frequency as the impingement tone observed in prior impinging jet acoustic field measurements.

Measurement of acoustic velocity components in a turbulent flow using LDV and high-repetition rate PIV

The present study provides theoretical details and experimental validation results to the approach proposed by Minotti et al. (Aerosp Sci Technol 12(5):398–407, 2008) for measuring amplitudes and phases of acoustic velocity components (AVC) that are waveform parameters of each component of velocity induced by an acoustic wave, in fully turbulent duct flows carrying multi-tone acoustic waves. Theoretical results support that the turbulence rejection method proposed, based on the estimation of cross power spectra between velocity measurements and a reference signal such as a wall pressure measurement, provides asymptotically efficient estimators with respect to the number of samples. Furthermore, it is shown that the estimator uncertainties can be simply estimated, accounting for the characteristics of the measured flow turbulence spectra. Two laser-based measurement campaigns were conducted in order to validate the acoustic velocity estimation approach and the uncertainty estimates derived. While in previous studies estimates were obtained using laser Doppler velocimetry (LDV), it is demonstrated that high-repetition rate particle image velocimetry (PIV) can also be successfully employed. The two measurement techniques provide very similar acoustic velocity amplitude and phase estimates for the cases investigated, that are of practical interest for acoustic liner studies. In a broader sense, this approach may be beneficial for non-intrusive sound emission studies in wind tunnel testings.