Heterodyne Doppler global velocimetry

Doppler Global Velocimetry (DGV) is an imaging flow measurement technique which allows the measurement of the velocity distribution in a plane. In DGV the frequency shift of scattered light from moving particles within the flow is used to determine the local flow velocity. Heterodyne Doppler Global Velocimetry (HDGV) is a new approach which combines the imaging and geometrical characteristics of DGV with the measurement principles of reference beam laser Doppler anemometry. The frequency shifted scattered light from the flow tracers is heterodyned with a reference beam from the same light source. Due to interference the result of this superposition is a harmonic intensity modulated signal. This signal is detected using a smart pixel detector array to obtain the velocity distribution. Two different experiments are presented. The first experiment compares the measured velocity distribution of a rotating disk with its actual velocity. The second experiment demonstrates the capability of the technique to measure a real flow.     

Measurements of velocity spectra using time-resolving Doppler global velocimetry with laser frequency modulation and a detector array

Measuring velocity spectra in turbulent flows requires methods providing a high temporal resolution and a low measurement uncertainty. Hot-wire anemometry is often used, but it is intrusive. Laser Doppler anemometry is non-intrusive, but due to the statistical arrival of individual tracers provides no constant measurement rate. We therefore propose the use of Doppler global velocimetry (DGV), which is a contactless method allowing temporally equidistant measurements of continuous signals. Additionally, 2d measurements are possible instead of single point measurements. The commonly applied slow cameras are substituted by a fibre coupled detector array consisting of 25 avalanche photo diodes, which increases temporal resolution up to 10 μs. Contrarily to conventional DGV, a sinusoidal laser frequency modulation enables omitting the reference detector array. A correction of beam splitting and image misalignment errors is thus not necessary, but disturbances due to temporal fluctuations of the scattered light can occur and have to be reduced by increasing the modulation frequency. We validate the proposed system capability of synchronously measuring velocity spectra at multiple points in turbulent flows by presenting experimental results. The acquired velocity spectra in a wind tunnel experiment show good agreement with hot-wire comparison measurements within 0.1 m/s. An uncertainty analysis is given, which allows the achievable measurement uncertainty to be estimated as a function of the desired temporal resolution. An uncertainty down to 0.2 m/s can, for example, be achieved assuming a desired temporal resolution of 1 ms. These promising results open new perspectives for turbulence and correlation studies in flows such as to investigate the turbulence characteristics behind a truncated cylinder attached to a plate or the inlet of an aircraft turbine for flow characterisation in industry.     

On the spatio-temporal structure of cylinder wakes

The wake of a short aspect ratio cylinder placed in a uniform flow is experimentally investigated. After having characterized the temporal behavior of the Bénard–Von Kàrmàn vortex shedding by the use of a classical hot-wire anemometer, an ultrasound anemometry technique is applied to study the spatial critical behavior of the envelope of the transverse velocity of the wake. It is shown that this envelope which represents the spatial form of the global mode of the wake, follows universal scaling laws which are in agreement with a second order phase transition. In a second set of experiments, the behavior of the longitudinal velocity fluctuations is also investigated. It is discovered that there is a special point several diameters behind the cylinder, which plays a role of a wave maker. Finally, for very small aspect ratio cylinders, symmetric vortex shedding is reported and interpreted using a system of coupled oscillators. Received: 15 May 1997/Accepted: 20 January 1998     

Experimental analysis of the precessing vortex core in a free swirling jet

An experimental analysis of the precessing vortex core (PVC) instability in a free swirling jet of air at ambient pressure and temperature is performed by means of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Two parametric studies are considered, varying the swirl parameter and the Reynolds number. The range of parameters considered allowed to study conditions of strong precession as well as the inception and settlement of the instability. Mean velocity and standard deviation profiles, power spectral density functions and probability density functions for the axial and tangential velocity components are presented. Average as well as instantaneous PIV maps are considered in the characterization of the flowfield structure and detection of the instantaneous position of the vortex center. Joint analysis of velocity PDFs and power spectra shows that the PVC contribution to the global statistics of the velocity field can be properly separated from the contribution of the true flow turbulence, giving additional insight to the physics of the precession phenomenon. The results obtained in the explored range of conditions indicate that the true turbulence intensity is not dependent on the swirl parameter. An erratum to this article can be found at     

Slip velocity measurements in an upward bubbly flow by combined LDA and electrodiffusional techniques

 An experimental technique for the measurement of the local slip velocity of spherical bubbles is reported. It is based on the measurement of the local liquid velocity by an electrodiffusional method, and the bubble velocity by a specially adapted LDA (Laser Doppler anemometer) with a short measuring volume. The bubble velocity is measured taking into account the shift between the bubble centre and the centre of the LDA measuring volume. The slip velocity is obtained by subtracting the liquid velocity from the bubble velocity at the point corresponding to the bubble centre. The technique is applicable for flows with high velocity gradients. Results of the slip velocity measurements in an upward bubbly flow at laminar pipe Reynolds numbers are presented. Received: 25 July 1996/Accepted: 13 April 1998     

Obstacle-induced spiral vortex breakdown

An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability.     

Dynamics of heavy particles in two swirling jets

We report flow visualisations and laser Doppler anemometry (LDA) velocity measurements in the near field of two swirling jets. The Reynolds number based on jet diameter and bulk velocity at the nozzle exit is 1.4 × 10⁵. In the first jet, a small recirculation region is formed around the jet axis, while, in the second, the streamwise velocity remains positive and overshoots near the jet centre. In both cases, flow visualisations show that the vortex core of the jets is depleted of seeding particles. By using time-averaged distributions of the streamwise and tangential velocities measured at the nozzle outlet, the dynamics of the particles is simulated, by integrating their simplified equations of motion. The particles trajectory thus computed agrees well with that observed in the flow visualisations. Although the turbulence intensity is substantially different in the core of the two jets, its effect on the seeding concentration is localised near the edge of the core.     

Measured turbulent entropy production with large eddy particle image velocimetry

In this paper, statistical post-processing of measured velocity, dissipation rate and turbulence data is performed to establish whole-field distributions of entropy production within a channel. Thermal irreversibilities arising from temperature variations were not included in the study, as the experiments were conducted between unheated plexiglass plates in an essentially isothermal water tunnel. Unlike velocity or temperature, the measurement of entropy cannot be performed directly, so entropy production is measured indirectly through spatial differencing of measured velocities in large eddy PIV. In contrast to single-point methods of anemometry, large eddy PIV enables whole-field, time-varying measurements of the velocity field, which can be post-processed to yield entire spatial variations of the entropy production rate. An uncertainty analysis is performed to estimate measurement uncertainties with the new experimental technique. The uncertainties are decomposed into systematic and random components, including a propagated uncertainty, due to spatial differencing of the velocity field. Close comparisons between measured results of turbulence dissipation and direct numerical simulations provide useful verification of the formulation, before post-processed results of dissipation rates are used to determine entropy production within a channel.     

Distortion of LDA fringe pattern by tracer particles

For precise flow velocity measurements laser Doppler anemometry (LDA) is wide-spread in use in the laboratories of industry and universitarian research institutions. The LDA method has the advantage of being not intrusive and able to discriminate between forward and reverse velocities. So far, laser Doppler anemometry is believed to be one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not constant within an individual burst. This can induce an additional scatter in the signal frequency and in the determination of the flow velocity. Until now, the reason for the period variations has not been investigated in detail although the problem was observed before. This paper describes experimental investigations which show that the particle passage through the laser beams shortly before the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern. Thus, the signal period length from an individual particle, passing the center of the measuring volume at the same time, varies according to the distortion of the fringe spacing.     

An LDA technique for in situ simultaneous velocity and size measurement of large spherical particles in a two-phase suspension flow

A relatively simple optical scheme using the reference-mode laser Doppler anemometry for the in situ measurement of flow properties of a dilute particle-fluid two-phase suspension having a predominant flow direction is hereby proposed. It is an extension of the established technique of optical gating for particle sizing which is fully integrated into the established technique of laser Doppler anemometry for velocity measurement. Particles that can be measured by this scheme are limited to those with sizes greater than the smaller dimension of the optical measuring volume. Inherent in the methodology is a procedure for providing information on the local particle number density and velocity distributions for each size range of the particles and the local velocity distribution of the continuous phase. The accompanying electronics and interfaces are also established for data processing and analysis in a mini computer. Validation of the scheme has been accomplished by controlled experiments using stainless steel balls and water droplets of 1 mm and greater in diameter.     

Global Doppler frequency shift detection with near-resonant interferometry

The recent development in measuring 2D Doppler shift distributions for flow velocimetry using the dispersive properties of atomic line filters is presented. On the basis of velocity field measurements on a subsonic jet flow and a tip vortex flow in a medium-sized wind tunnel, the technique was assessed. Atomic line filters near a resonant transition combine imaging capabilities with a sharp frequency cutoff and an associated region of strong anomalous dispersion. While conventional Doppler global velocimetry relies on the absorption of the filter to convert frequency shifts to intensity variations, near-resonant interferometry uses its dispersion to detect frequency shifts as phase changes in an interference pattern. In the present setup, an iodine vapor cell in an imaging Michelson interferometer is used. With the illuminating laser frequency tuned near a resonant transition, the cell’s dispersion converts the frequency content of the field of view into a distortion of the carrier-fringe pattern recorded at the image plane of the interferometer. The phase distribution in the fringe images is reconstructed by filtering the individual images with a 2D Gabor filter pair tuned to the spatial frequencies of the basic carrier-fringe pattern. The post-processing is concluded with subsequent phase-unwrapping and subtraction of the carrier reference fringe phase. The method and the setup were demonstrated and calibrated experimentally on a rotating disc. The capability of the technique to operate in a real experimental environment was validated in a free subsonic jet and a tip vortex flow behind a wing section in a medium-sized wind tunnel facility. The measurements were found to be in generally good agreement with the theoretically predicted system characteristics and the reference measurements. As with other Doppler global techniques, the stability of the pulsed laser system and the secondary scattering in the test volume were identified as the main error sources.     

Application of ultrasonic velocity profile meter to vortex shedding and empirical eigenfunctional analysis

 The vortex shedding behind a cylinder in a fully developed pipe flow is investigated by applying new measurement and data-analysis techniques. An ultrasound anemometry technique, providing the multipoint information of the flow, is employed. The measured instantaneous velocity fluctuations are decomposed by the empirical eigenfunctional analysis known as the proper orthogonal decomposition. The predominant modes of fluid motions are well identified by this technique, and their time–frequency features are discussed. Received: 11 April 2000/Accepted: 20 March 2001     

How to get spatial resolution inside probe volumes of commercial 3D LDA systems

In laser Doppler anemometry (LDA) it is often the aim to determine the velocity profile for a given fluid flow. The spatial resolution of such velocity profiles is limited in principal by the size of the probe volume. The method of using time of flight data from two probe volumes allows improvements of the spatial resolution by at least one order of magnitude and measurements of small-scale velocity profiles inside the measuring volume along the optical axis of commercial available 3D anemometers without moving the probe. No change of the optical set-up is necessary. An increased spatial resolution helps to acquire more precise data in areas where the flow velocity changes rapidly as shown in the vicinity of the stagnation point of a cuboid. In the overlapping region of three measuring volumes a spatially resolved 3D velocity vector profile is obtained in the direction of the optical axis in near plane flow conditions. In plane laminar flows the probe volume is extended by a few millimetres. The limitation of the method to a plane flow is that it would require a two-component LDA in a very special off-axis arrangement, but this arrangement is available in most commercial 3D systems.     

Kinematic properties of monopolar vortices in a strain flow

The advection properties of monopolar vortices subjected to background strain were investigated both experimentally and numerically. Dye-visualization studies in a stratified fluid demonstrated the deformation of the vortex core and the shedding of passive tracers from the edge of the vortex. The main kinematic characteristics of the vortex evolution could be well captured by a simple model, in which the monopole was represented by a (strength-varying) point vortex surrounded by a contour of passive tracers. Full numerical simulations of the vortex evolution showed an excellent agreement with the observed tracer distributions and also revealed that the spatial distribution of vorticity must be taken into account in order to explain the final tearing of the laboratory vortex.     

Identification and analysis of the meandering of a fin-tip vortex using Proper Orthogonal Decomposition (POD)

The meandering of a vortex exists in a broad range of engineering applications and can lead to flow instability and other undesirable characteristics. Compared to a static vortex, measurement of a meandering vortex can result in a ‘smeared’ mean-flow field and increased levels of turbulence at the centre of the vortex. A case study was performed here on the meandering nature of a fin-tip vortex generated by a manoeuvring submarine. From stereoscopic particle image velocimetry (SPIV) measurements, it is possible to remove the meandering by shifting each instantaneous velocity field so as to produce a common centre for the vortex. In this paper, a snapshot Proper Orthogonal Decomposition (POD) technique is used to capture the dominant large-scale coherent structures (from inspection of eigenvalue or energy distributions) and to improve vortex centre identification. The POD reconstructed velocity field using only the most energetic modes enabled the coherent structures of the flow to be clearly visualised, providing improved identification of the vortex centre and subsequent evaluation of the meandering effect on the turbulent statistics. The present findings suggest that the vortex meandering only has a small impact on the ensemble-averaged resultant velocity, while contributing up to a maximum of 28% for the fluctuating component. The meandering correction also leads to an overall decrease of turbulence intensity in the peak fluctuating region of the vortex core.     

A new laser vorticity probe — LAVOR: Its development and validation in a turbulent boundary layer

Vorticity measurements, which are scarce at the present time, can provide valuable dynamical information, particularly in unsteady and separated flows. Advances in laser Doppler anemometry and optical techniques have furnished the opportunity for the development of a non-intrusive vorticity probe with very fine spatial and temporal resolutions. The laser vorticity probe (LAVOR), which makes use of minimal laser beams and optical components, is capable of measuring velocity gradients with a separation distances as small as 0.3 mm. Velocity gradients are measured using two points on the same probe volume. However, unlike other techniques, the LAVOR also provides the instantaneous velocity at each point in the probe volume, and so provides additional dynamical information. The LAVOR probe was used in a fully turbulent two-dimensional boundary layer, and the data obtained are compared with the existing hot-wire vorticity data obtained in the same wind tunnel facility and with data obtained in other facilities. The spatial resolution is of the order of three Kolmogorov microscale units. Published online: 19 October 2002     

PIV investigation of the shear layer vortices in the near wake of a circular cylinder

Particle image velocimetry measurements are performed in the near wake of a circular cylinder at a Reynolds number of 12,500. Attention is focused on the shear layer that develops just downstream of the separation point from the cylinder surface to investigate the possible existence of a preferred spatio-temporal organization in this flow region and the possible occurrence of the vortex pairing phenomenon. Eddy structures are identified in instantaneous velocity maps in order to investigate their spatial relationships. For that purpose a vortex extraction procedure is designed, based on the wavelet transform of instantaneous maps of the swirling strength. This algorithm allows not only the detection of the vortical structures from the instantaneous velocity fields, giving access to their instantaneous location, but also the estimation of their main characteristics such as their radius, intensity and convection velocity. The vortex population detected in the shear layer is found to be of small diameter compared to that of the von Kármán vortex and of rather high intensity, in agreement with the existence of a thin shear layer. The strong flapping motion of the shear layer and its complex spatial development is also confirmed. By employing conditional analysis of the computed data and their proper scaling, the surrounding of the detected vortex cores is investigated. A preferred spatial vortex separation is detected and is shown to vary with the longitudinal distance from the origin of the shear layer, in agreement with the qualitative behavior of a turbulent plane mixing layer. Evidence of the occurrence of the vortex pairing or amalgamation mechanisms in the shear layer is also demonstrated.     

Bifurcation of an elliptic vortex ring

Time-dependent vorticity fields of elliptic vortex rings of aspect ratios 2, 3 and 4 were measured by means of hot-wire anemometry. The time evolution of their vorticity fields was analyzed and the processes of vortex ring formation, advection, interaction and decay, and the mechanism of vortex bifurcation are studied. The following crosslinking model is proposed: A thick vortical region composed of many equivalent vortex filaments with distributed cores is initially formed at the orifice and they behave as inviscid filaments. The elliptic ring deforms and the end parts of its major axis get closer. Then, the vortex filaments interact at the touching point and the ring partially bifurcates. Almost simultaneously, turbulent spot appears at this point, and propagates around the ring cross section, thus preventing further bifurcation. And it becomes a turbulent blob. This model is also supported by numerical simulation by a high-order vortex method and the Navier-Stokes solution.     

An inclined wall jet: Mean flow characteristics and effects of acoustic excitation

 The mean velocity field of a 30° inclined wall jet has been investigated using both hot-wire and laser Doppler anemometry (LDA). Provided that the nozzle aspect ratio is greater than 30 and the inclined wall angle (β) is less than 50°, LDA measurements for various β show that the reattachment length is independent of the nozzle aspect ratio and the nozzle exit Reynolds number (in the range 6670–13,340). There is general agreement between the reattachment lengths determined by LDA and those determined using wall surface oil film visualisation technique. The role of coherent structures arising from initial instabilities of a 30° wall jet has been explored by hot-wire spectra measurements. Results indicate that the fundamental vortex roll-up frequency in both the inner and outer shear layer corresponds to a Strouhal number (based on nozzle exit momentum thickness and velocity) of 0.012. The spatial development of instabilities in the jet has been studied by introducing acoustic excitation at a frequency corresponding to the shear layer mode. The formation of the fundamental and its first subharmonic has been identified in the outer shear layer. However, the development of the first subharmonic in the inner shear layer has been severely suppressed. Distributions of mean velocities, turbulence intensities and Reynolds shear stress indicate that controlled acoustic excitation enhances the development of instabilities and promotes jet reattachment to the wall, resulting in a substantially reduced recirculation flow region. Received: 24 November 1998/Accepted: 24 August 1999     

Error estimation of laser-Doppler anemometry measurements in fluids with spatial inhomogeneities of the refractive index

We report a combined experimental and theoretical investigation of the influence of spatial non-uniformities of the refractive index on the accuracy of laser Doppler anemometry (LDA) measurements in transparent fluids. One LDA beam is guided through heated air of a thermal boundary layer near a heated vertical flat plate. It is found that the hot air is deflecting the beam because of a modification of the refractive index n in the fluid. This deflection causes three effects: (1) spatial displacement of beam intersection, (2) waist mismatch in the measurement volume and (3) variation in interference fringe distance. With the help of a rotating disk calibration system the resulting displacement of the LDA measurement volume and the Doppler frequency variation is systematically studied at different temperatures. Using a simple model of beam propagation under the influence of well-defined temperature inhomogeneities, the displacement of measurement volume and change in Doppler frequency are calculated and are found to be in agreement with the experimental observations. The results provide a rational framework for an assessment of the accuracy of LDA data in arbitrary transparent fluids with non-uniform refractive index.