Laser Doppler Velocimetry Measurements Research Articles

Understanding the Effect of Surface Terrain on Pollution Transport around Gebeng Industrial Area

Objectives:Air pollution dispersion is greatly affected by the wind speed and direction. Most often the pollution dispersion from industrial area is not fully understood. In the present work, the effect of surface topology on the wind speed and atmospheric turbulent around the Gebeng industrial area in Pahang was studied using a Computational Fluid Dynamics (CFD) method. Methods/Statistical Analysis: The typical southeastern wind with a speed of about 1.3003 m/s corresponding to the actual speed of approximately 5 km/h was considered for validation. The turbulent flow was modelled using k-e model. The model was compared with the Laser Doppler Velocimetry (LDV) measurement onthe scale-down rig. Findings:The CFD prediction showed good agreementwithin 10% deviation with the experimental measurement via LDV. It was found that the plume of noxious gas driven by the eastern wind can affect the nearby residential area R2, whereas the western wind is affecting the residential area R1. In the presence of hill obstacle, the gas plume moves southward into the flat terrain. Application/ Improvements: Results from this work may be useful to understand the risk of air pollutant dispersion around Gebeng industrial area.

Numerical investigation of turbulent flow and heat transfer over partially open cavities: Effect of opening ratio

Partially open cavities are encountered in various engineering systems such as electronic cooling devices and cooling for gas turbine blades, instead of conventional film cooling slots. Flow is to be imparted over the partially open cavity where it induces a shear layer and a shear driven vortex within the cavity, which is subjected to cooling effect at its wall. Depending on the opening ratio, heat and mass transfer occur between the main flow and the trapped vortex through the shear layer. In the present study, RANS simulations of such flow have been conducted for circular and square cavities to investigate the effect of opening ratio on the heat and mass transfer characteristics. The simulations were established on a rigorous numerical approach and proper validation with laser Doppler velocimetry measurements of turbulent flow in circular cavity. Based on the hydraulic diameter of the cavity, opening ratios ranging from 0.2 to 1.0 were investigated for a Reynolds number of 3?105. Generally, the maximum Nusselt number was achieved at higher opening ratios for both circular and rectangle cavities. On the other hand, the maximum dimensionless temperature gradient, ?, inside the cavity was achieved at L/D = 0.2 for both cavity configurations.

Acoustic and streaming velocity components in a resonant waveguide at high acoustic levels.

Rayleigh streaming is a steady flow generated by the interaction between an acoustic wave and a solid wall, generally assumed to be second order in a Mach number expansion. Acoustic streaming is well known in the case of a stationary plane wave at low amplitude: it has a half-wavelength spatial periodicity and the maximum axial streaming velocity is a quadratic function of the acoustic velocity amplitude at antinode. For higher acoustic levels, additional streaming cells have been observed. Results of laser Doppler velocimetry measurements are here compared to direct numerical simulations. The evolution of axial and radial velocity components for both acoustic and streaming velocities is studied from low to high acoustic amplitudes. Two streaming flow regimes are pointed out, the axial streaming dependency on acoustics going from quadratic to linear. The evolution of streaming flow is different for outer cells and for inner cells. Also, the hypothesis of radial streaming velocity being of second order in a Mach number expansion, is not valid at high amplitudes. The change of regime occurs when the radial streaming velocity amplitude becomes larger than the radial acoustic velocity amplitude, high levels being therefore characterized by nonlinear interaction of the different velocity components.

Micro-mixing measurement by chemical probe in homogeneous and isotropic turbulence

The chemical probe is commonly used to evaluate the performance of chemical reactors. By a localized injection of chemical reagents, it is possible to measure the local micro-mixing, which is readily related to the selectivity of chemical reactions, since mixing at the molecular scale is the limiting factor for a wide range of chemical systems. The raw result of the chemical-probe method is a segregation index that allows comparison of different situations (various locations in the reactor, various Reynolds numbers, various geometries, etc.). Beyond the qualitative assessment provided by this segregation index, it is possible to obtain the intrinsic micro-mixing time by means of a micro-mixing model, describing the temporal evolution of a chemical reaction whose rate is governed by the micro-mixing. Here a key step is the choice of micro-mixing model. Several micro-mixing models available in the literature have been used in some specific cases without evaluating their appropriateness for the problem in hand. The main difficulty in this evaluation is that the real flows often do not fully satisfy the basic model assumptions, in particular the condition of homogeneous and isotropic turbulence (HIT).The present work aims at assessing the validity of a micro-mixing model under “ideal” experimental conditions, i.e. HIT with no mean flow, to avoid the bias due to the flow gradients. The HIT is obtained here by a system of oscillating grids placed in a vessel. The chemical probe measurements carried out by the iodide/iodate reaction system are applied to the two most commonly used phenomenological models in the literature: the IEM (Interaction by Exchange with the Mean) and the EDD (Engulfment, Deformation and Diffusion) models. The benchmark for the micro-mixing models is based on comparison of the local turbulent kinetic energy (TKE) dissipation rate both drawn from the micro-mixing time by the theoretical model of Bałdyga and the reference direct experimental determination by laser Doppler velocimetry measurements. It is shown that the engulfment model EDD seems the more appropriate to analyze the chemical data and provide a quantitative characterization of micro-mixing.

Downstream relaxation of velocity profiles in pipe-flow with swirl disturbances

Abstract We study characteristic flow patterns downstream of a standardized swirl disturbance generator using laser-Doppler velocimetry (LDV). To investigate the spatial development of flow patterns, we conduct LDV measurements in cross-sections located at various distances downstream from the swirl disturbance generator. Focusing on velocity profiles, decay of swirl, and performance indicators used to describe the characteristic shape of the velocity profiles, we systematically compare the experimental results with available references and various theories for decay of swirl disturbances. We find that the standardized swirl disturbance generator provides exponentially decaying swirling flow that is best captured by the theory of Steenbergen and Voskamp Steenbergen W, Voskamp J (1998) The rate of decay of swirl in turbulent pipe flow. Flow Meas Instr 9:67–78 . In addition, deviations from the axial reference profile caused by the swirl disturber persist for long downstream distances. In particular, our results suggest that the peakness factor relaxes linearly towards the fully-developed state.

Monolayers of poly(amido amine) dendrimers on mica – In situ streaming potential measurements

The deposition of poly(amido amine) dendrimers on mica at various pHs was studied by the atomic force microscopy (AFM) and in situ streaming potential measurements. Bulk characteristics of dendrimers were acquired by using the dynamic light scattering (DLS) and the laser Doppler velocimetry (LDV). The hydrodynamic radius derived from DLS measurements was 5.2nm for the ionic strength of 10−2M and pH range 4–10. The electrophoretic mobility, the zeta potential and the number of electrokinetic charges per molecule were derived as a function of pH from the LDV measurements. It was revealed that the dendrimers are positively charged for pH up to 10. This promoted their deposition on negatively charged mica substrate whose kinetics was quantitatively evaluated by direct AFM imaging and streaming potential measurements interpreted in terms of the electrokinetic model. The desorption kinetics of dendrimers under flowing conditions from monolayers of various coverage was also studied. It was revealed that dendrimer deposition was partially reversible for pH above 5.8. The acid-base properties of the dendrimer monolayers deposited on mica were characterized.

A comparative experimental evaluation of uncertainty estimation methods for two-component PIV

Uncertainty quantification in planar particle image velocimetry (PIV) measurement is critical for proper assessment of the quality and significance of reported results. New uncertainty estimation methods have been recently introduced generating interest about their applicability and utility. The present study compares and contrasts current methods, across two separate experiments and three software packages in order to provide a diversified assessment of the methods. We evaluated the performance of four uncertainty estimation methods, primary peak ratio (PPR), mutual information (MI), image matching (IM) and correlation statistics (CS). The PPR method was implemented and tested in two processing codes, using in-house open source PIV processing software (PRANA, Purdue University) and Insight4G (TSI, Inc.). The MI method was evaluated in PRANA, as was the IM method. The CS method was evaluated using DaVis (LaVision, GmbH). Utilizing two PIV systems for high and low-resolution measurements and a laser doppler velocimetry (LDV) system, data were acquired in a total of three cases: a jet flow and a cylinder in cross flow at two Reynolds numbers. LDV measurements were used to establish a point validation against which the high-resolution PIV measurements were validated. Subsequently, the high-resolution PIV measurements were used as a reference against which the low-resolution PIV data were assessed for error and uncertainty. We compared error and uncertainty distributions, spatially varying RMS error and RMS uncertainty, and standard uncertainty coverages. We observed that qualitatively, each method responded to spatially varying error (i.e. higher error regions resulted in higher uncertainty predictions in that region). However, the PPR and MI methods demonstrated reduced uncertainty dynamic range response. In contrast, the IM and CS methods showed better response, but under-predicted the uncertainty ranges. The standard coverages (68% confidence interval) ranged from approximately 65%–77% for PPR and MI methods, 40%–50% for IM and near 50% for CS. These observations illustrate some of the strengths and weaknesses of the methods considered herein and identify future directions for development and improvement.

Investigation of an impinging heated jet for a small nozzle-to-plate distance and high Reynolds number: An extensive experimental approach

The present work aims at investigating a particular impinging jet configuration throughout a comprehensive experimental approach. A preheated air jet at 130°C issues a fully developed circular pipe at Reynolds number 60,000 and discharges in the laboratory room to impinge a flat plate located 3 diameters downstream. The description of the velocity field and the complete Reynolds stress tensor is provided by stereoscopic particle image velocimetry (S-PIV) and Laser Doppler Velocimetry (LDV) measurements. For the first time, data are reported for the mean and fluctuating temperature of an impinging jet configuration with the help of cold-wire thermometry (CWT) measurements. The heat transfer distribution on the impinged plate is determined through an inverse method based on infrared thermography measurements on the rear face of the plate. The agreement between S-PIV and LDV measurements is shown excellent over the whole flow field. The measured Nusselt number distribution exhibits a secondary maximum at r/D=2, as observed in previous experiments for short impinging distances. Flow dynamics is characterized through a spectral analysis of time-resolved measurements while flow topology features are identified through coherent structure detection based on S-PIV spatially-resolved instantaneous velocity fields. This analysis shows that the jet column mode, associated with a Strouhal number of 0.4, plays a key role in the primary structure dynamics.

Experimental and numerical investigation of coherent structure dynamics on mass transfer in a separated cavity flow

This study presents the experimental and numerical investigation of coherent structure dynamics on mass transfer in a separated cavity flow. The flow field dynamics of a cavity type stagnation zone with a length to width ratio of 2 were studied. The cavity was driven by a channel flow with a Reynolds number of 1.8E5. The study utilised flow visualisation, laser Doppler velocimetry (LDV) and electrical conductivity probe measurements. Measurements of mean velocity and turbulence intensity profiles across the separated flow field showed the development of the shear layer and the recirculating flow pattern in the cavity. The numerical simulations were also performed for comparison with the experimental results considering the solution of the RANS, LES model and two-layer turbulence model. LES produced the formation and dynamics of coherent structures within the separated shear layer. Flow visualisation revealed a pulsatile motion in the separated flow region associated with the formation and passage of coherent structures and was supported by measurements of tracer concentration. The study showed how coherent structures in the separated shear layer enhanced the mass transport process from the stagnation zone and how the recirculating flow in the cavity influenced the development of those structures.

Comparison of Flame Stability Under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame

The flame characteristics of a pilot-scale swirl burner for air and oxy-fuel combustion of pulverized coal are investigated. The local burner air (or oxygen) ratio λ and the oxygen concentration have been systematically varied. The investigated flames were characterized recording UV emissions originating from OH* chemiluminescence indicating the reaction zone in the gas phase, measuring the axial and tangential velocities using an laser Doppler velocimetry (LDV) system and analyzing the composition of the flue gas. A change of the flame structure was revealed from the conducted measurement: the “regular” flame for the investigated burner is characterized by a cone-shaped swirling combustion zone with a distinct inner recirculation zone. Reducing the oxidizer flows through the burner leads to a breakdown of the inner recirculation zone and a significant change of the flame pattern. This change was identified by the LDV measurements as well as from the chemiluminescence images, and it was found to be closely related to the momentum flow through the burner into the main combustion zone.

Experimental investigation of the near wake of a horizontal axis tidal current turbine

A scaled-model of an HATCT (Horizontal Axis Tidal Current Turbine) was tested in the CNR-INSEAN Circulating Water Channel. Two different analyses were performed during the experiments: preliminary investigation of the performances of the turbine in different working conditions and LDV (Laser Doppler Velocimetry) measurements in the near wake. Velocity measurements are performed in phase locking to the rotor, in order to resolve the blade wake structure at different working regimes. Indeed, the knowledge of all three velocity components (tangential, axial, radial) allows to evaluate through energy balance concerns, the transfer of inflow kinetic energy into mechanical work of the rotor and into different energy contributions in the wake. The LDV analysis of the near wake also contributes to understand some of the most important fluid dynamics aspects related to the design of the rotor and to the different working conditions, and can be used as a preliminary study for understanding the wake evolution and the interaction between two or more turbines in a farm.

Wall friction and velocity measurements in a double-frequency pulsating turbulent flow

Wall shear stress measurements employing a hot-film sensor along with laser Doppler velocimetry measurements of the axial and tangential velocity and turbulence profiles in a pulsating turbulent pipe flow are presented. Time-mean and phase-averaged results are derived from measurements performed at pulsation frequencies ${\it\omega}^{+}={\it\omega}{\it\nu}/\bar{u}_{{\it\tau}}^{2}$ over the range of 0.003–0.03, covering the low-frequency, intermediate and quasi-laminar regimes. In addition to the base case of a single pulsation imposed on the mean flow, the study also investigates the flow response when two pulsations are superimposed simultaneously. The measurements from the base case show that, when the pulsation belongs to the quasi-laminar regime, the oscillating flow tends towards a laminar state in which the velocity approaches the purely viscous Stokes solution with a low level of turbulence. For ${\it\omega}^{+}<0.006$, the oscillating flow is turbulent and exhibits a region with a logarithmic velocity distribution and a collapse of the turbulence intensities, similar to the time-averaged counterparts. In the low-frequency regime, the oscillating wall shear stress is shown to be directly proportional to the Stokes length normalized in wall units $l_{s}^{+}~(=\sqrt{2/{\it\omega}^{+}})$, as predicted by quasi-steady theory. The base case measurements are used as a reference when evaluating the data from the double-frequency case and the oscillating quantities are shown to be close to superpositions from the base case. The previously established view that the time-averaged quantities are unaffected by the imposition of small-amplitude pulsed unsteadiness is shown to hold also when two pulsations are superposed on the mean flow.

Wall-resolved Large Eddy Simulation in grid-free 5 × 5 rod bundle of MATiS-H experiment

In the present paper the WALE model has been implemented in OpenFOAM code and applied for Large Eddy Simulation (LES) of two wall-bounded turbulent flow cases at moderately high Reynolds numbers, i.e. Re=O(104). Firstly, the LES using the WALE model is verified against the direct numerical simulation (DNS) of the turbulent channel flow at Reτ=1020. An appropriate mesh discretization for a given numerical method is determined and the underresolution effects on first and second order moments of velocity are investigated. Then, the obtained best practices are used for accurate wall-resolved LES of a turbulent flow in the 5×5 rod fuel bundle. The velocities and fluctuations are compared with Laser Doppler Velocimetry measurements of the MATiS-H experiment, which was performed in 2011 at KAERI, South Korea. Experiments and simulations are compared in the bare fuel bundle section without spacer grids. LES using WALE model is found as a pretty accurate tool for simulations of the flow through the rod bundle, if sufficiently fine mesh is applied. Mean velocity profiles are predicted with a remarkable accuracy, while the second-order statistics exhibit slightly larger discrepancies.

Mechanism of immunoglobulin G adsorption on polystyrene microspheres

The adsorption of polyclonal immunoglobulin G (IgG) on negatively charged polystyrene microparticle suspension (latex) was studied by using the Laser Doppler Velocimetry (LDV) measurements. Using this technique, the dependence of the electrophoretic mobility of particles on the IgG concentration in the suspension was measured for various ionic strengths and pH 3.5. The increase in the electrophoretic mobility was quantitatively interpreted in terms of the 3D electrokinetic model. On the other hand, the maximum coverage of IgG on latex was determined using the depletion method based on AFM imaging. It was shown that IgG adsorption was irreversible and that its maximum coverage on the microspheres increased from 1.4mgm−2 for 0.001M NaCl to 2.0mgm−2 for 0.15M NaCl. This was interpreted in terms of reduced electrostatic repulsion among adsorbed molecules. The stability of IgG monolayers on the particles was confirmed in separate experiments where changes in its electrophoretic mobility were monitored over prolonged time periods. Additionally, the acid-base properties of the IgG monolayers on latex were determined in pH cycling experiments. The isoelectric point of the IgG monolayers on the microspheres was 4.8. The results obtained in this work indicate that basic physicochemical characteristics of IgG can be acquired via electrophoretic mobility measurements using microgram quantities of the protein.

Experimental study of the turbulent field behind a perforated vortex generator

The influence of the wake vortex arising behind a perforated tab on the mixing process in heat exchangers and chemical reactors is analyzed. The preliminary step of this study, i.e., investigation of the turbulent field generated by a single perforated tab, is presented here. For this aim, laser Doppler velocimetry measurements are conducted downstream from a perforated trapezoidal vortex generator placed in a wind tunnel. It is shown that two shear layers are generated by the tab. The first shear layer is located at the upper edge of the tab, and the other is ejected from the perforation edges. These shear layers are characterized by high turbulent kinetic energy levels, which are profitable for meso-mixing enhancement. Finally, a spectral study shows that the turbulent macro-scale is nearly the same for typical locations in the shear layers shed from the tab and perforation edges.

Velocity field investigation inside a bulb turbine runner using endoscopic PIV measurements

The flow in the inter-blade channels of a bulb turbine was measured using endoscopic cameras integrated to a stereoscopic particle image velocimetry (S-PIV) system. This paper presents results from the measurement campaign and also provides some key conclusions based on the dataset. The technical aspect of the measurement configuration is addressed. The main focus is on the novelties and challenges brought by the use of endoscopic cameras to achieve S-PIV measurements between the runner blades. For the first time in hydraulic rotating machinery, velocity measurements covered 62 % of a rotor inter-blade flow. After outlining the techniques used, comparison with laser Doppler velocimetry measurements allows assessing the intrusiveness of the endoscopes. Then, some velocity field analyses are shown. First, the rotor–stator interaction is outlined as the influence of the guide vane wakes on the runner flow. The size, localization, strength and dissipation of those structures are inferred from the information coming from measurements. Finally, the PIV data allow the identification of a vortex located near the suction side of the blades and originating from the corner between the leading edge and the hub when operating the bulb turbine at part-load.

An investigation of channel flow with a smooth air–water interface

Experiments and numerical simulation are used to investigate fully developed laminar and turbulent channel flow with an air–water interface as the lower boundary condition. Laser Doppler velocimetry measurements of streamwise and wall-normal velocity components are made over a range of Reynolds number based upon channel height and bulk velocity from 1100 to 4300, which encompasses the laminar, transitional and low Reynolds numbers turbulent regimes. The results show that the airflow statistics near the stationary wall are not significantly altered by the air–water moving interface and reflect those found in channel flows. The mean statistics on the water interface side largely exhibit results similar to simulated Poiseuille–Couette flow (PCF) with a solid moving wall. For second-order statistics, however, the simulation and experimental results show some discrepancies near the moving water surface, suggesting that a full two-phase simulation is required. A momentum and energy transport tubes analysis is investigated for laminar and turbulent PCFs. This analysis builds upon the classical notion of a streamtube and indicates that part of the energy from the pressure gradient is transported towards the stationary wall and is dissipated as heat inside the energy tubes, while the remainder is transmitted to the moving wall. For the experiments, the airflow energy is transmitted towards the water to overcome the drag force and drive the water forward; therefore, the amount of energy transferred to the water is higher than the energy transferred to a solid moving wall.

Advances in CFD Modelling of Multiphase Flows in Cyclone Separators

This paper evaluates the predictive performance of modern Computational Fluid Dynamics (CFD) methods in cyclone vortex-separation flows. Computer simulations are performed with the Eulerian-Eulerian (E-E) as well as Eulerian-Lagrangian (E-L) multiphase models, available is STAR-CCM+ CFD solver. Tested momentum closure strategies included typical two-equation isotropic turbulence models as well as high-order Reynolds Stress Models (RSM) and Large Eddy Simulation (LES) technique. The calculated results show satisfactory agreement compared to corresponding Laser Doppler Velocimetry (LDV) measurements and experimental values for the separation efficiency (η).

Cross-correlation Doppler global velocimetry (CC-DGV)

A flow velocimetry method, cross-correlation Doppler global velocimetry (CC-DGV), is presented as a robust, simplified, and high dynamic range implementation of the Doppler global/planar Doppler velocimetry technique. A sweep of several gigahertz of the vapor absorption spectrum is used for each velocity sample, with signals acquired from both Doppler-shifted scattered light within the flow and a non-Doppler shifted reference beam. Cross-correlation of these signals yields the Doppler shift between them, averaged over the duration of the scan. With presently available equipment, velocities from 0ms−1 to over 3000ms−1 can notionally be measured simultaneously, making the technique ideal for high speed flows. The processing routine is shown to be robust against large changes in the vapor pressure of the iodine cell, benefiting performance of the system in facilities where ambient conditions cannot be easily regulated. Validation of the system was performed with measurements of a model wind turbine blade boundary layer made in a 1.83m by 1.83m subsonic wind tunnel for which laser Doppler velocimetry (LDV) measurements were acquired alongside the CC-DGV results. CC-DGV uncertainties of ±1.30ms−1, ±0.64ms−1, and ±1.11ms−1 were determined for the orthogonal stream-wise, transverse-horizontal, and transverse-vertical velocity components, and root-mean-square deviations of 2.77ms−1 and 1.34ms−1 from the LDV validation results were observed for Reynolds numbers of 1.5 million and 2 million, respectively. Volumetric mean velocity measurements are also presented for a supersonic jet, with velocity uncertainties of ±4.48ms−1, ±16.93ms−1, and ±0.50ms−1 for the orthogonal components, and self-validation done by collapsing the data with a physical scaling.

Probing turbulence intermittency via autoregressive moving-average models.

We suggest an approach to probing intermittency corrections to the Kolmogorov law in turbulent flows based on the autoregressive moving-average modeling of turbulent time series. We introduce an index Υ that measures the distance from a Kolmogorov-Obukhov model in the autoregressive moving-average model space. Applying our analysis to particle image velocimetry and laser Doppler velocimetry measurements in a von Kármán swirling flow, we show that Υ is proportional to traditional intermittency corrections computed from structure functions. Therefore, it provides the same information, using much shorter time series. We conclude that Υ is a suitable index to reconstruct intermittency in experimental turbulent fields.