Laser Doppler Anemometry Data Research Articles

Numerical modelling of stirred tank and its validation by Radioactive Particle Tracking (RPT) technique

ABSTRACT Computational Fluid Dynamics (CFD) model for Rushton Turbine (RT) stirred tanks has been developed. The CFD model has been validated with experimental (RPT) data. The RPT data were calibrated with Laser Doppler Anemometry (LDA) data from literature. The calibration of RPT setup was done in two conditions: (a) stationary rotor and (b) rotor rotating at 200 rpm. The radiation intensity data recorded from measurement was analysed by Analysis of Variance (ANOVA) test and found that intensity counts have statistically equal mean. Also, the percentage error between mean counts is less than 1.72. So calibration step in RPT setup can be done in rotor stationary condition which is useful for estimating fluid dynamical conditions inside the impeller region, which is not possible with any other instruments like PIV, LDA etc. Further, RPT experiments and CFD simulations were conducted and results were in good agreement with LDA data from literature. The classical flow pattern of a jet-like stream coming from impeller tip was observed from RPT and well predicted by the CFD model. CFD model predicted the mean velocities and the turbulent quantities in reasonably good manner in comparison with literature data.

Trade-off between number of constraints and primary-statement robustness in entropy models: the case of the open-channel velocity field.

In this research, the trade-off between the number of restrictions and the robustness of the primary formulation of entropy models was evaluated. The performance of six hydrodynamic models in open channels was assessed based on 1730 Laser-Doppler anemometry data. It was investigated whether it is better to use an entropy-based model with more restrictions and a weak primary formulation or a model with fewer restrictions, but with a strong formulation. In addition, it was also investigated whether the model performance improves with the insertion of restrictions. Three of the investigated models have a weak formulation (open-channel velocity field represented by Cartesian coordinates); while the other three models have a strong formulation, according to which isovels are represented by curvilinear coordinates. The results indicated that models with two restrictions performed better than those with one restriction, since the additional restriction includes information relevant to the system. Models with three restrictions perform worse than those with two restrictions, because the information lost due to the use of a numerical solution was more substantial than the information gained by the third restriction. In conclusion, a strong primary formulation brought more information to the system than the inclusion of a third constraint.

Measurements of local droplet velocities in horizontal gas-liquid pipe flow with low liquid loading

New data on streamwise droplet velocity profiles for low liquid loading pipe flows are reported. The fluids used in the experiments are SF6 (gas-phase) and Exxsol D60 (oil-phase). Experiments are conducted in a 10 cm pipe diameter high-pressure (∼780 kPa, absolute) flow loop to reproduce gas-condensate field conditions. Instantaneous streamwise velocity data, obtained using the non-intrusive Laser Doppler Anemometry (LDA) technique, are used to calculate mean and root-mean-squared (RMS) local velocities. Asymmetric droplet velocity profiles with respect to the pipe center-line are observed especially for the stratified low atomization flow conditions. However, as the flow momentum increases, the droplet velocity profiles seem to become more symmetric. Also, these data suggest that, irrespective of the conditions studied, the single-phase gas flow characteristics are preserved closer to the top pipe wall. The data from the LDA imply that the bottom pipe half region is highly influenced by the gas-liquid interfacial characteristics. This results in high streamwise turbulence intensity in the region influenced by the interfacial waves (interfacial turbulence). An isokinetic sampling instrument is also used to measure the local instantaneous dynamic pressure. The dynamic pressure and the locally extracted liquid (droplets) volume rate under isokinetic conditions are used to calculate local fluid velocity. Excellent agreement has been obtained when comparing this calculated local velocity from the isokinetic instrument to the LDA data.

Flow field measurements of a series of turbulent premixed and stratified methane/air flames

This paper presents flow field measurements for the turbulent stratified burner introduced in two previous publications in which high resolution scalar measurements were made by Sweeney et al. [1,2] for model validation. The flow fields of the series of premixed and stratified methane/air flames are investigated under turbulent, globally lean conditions (ϕg=0.75). Velocity data acquired with laser Doppler anemometry (LDA) and particle image velocimetry (PIV) are presented and discussed. Pairwise 2-component LDA measurements provide profiles of axial velocity, radial velocity, tangential velocity and corresponding fluctuating velocities. The LDA measurements of axial and tangential velocities enable the swirl number to be evaluated and the degree of swirl characterized. Power spectral density and autocorrelation functions derived from the LDA data acquired at 10kHz are optimized to calculate the integral time scales. Flow patterns are obtained using a 2-component PIV system operated at 7Hz. Velocity profiles and spatial correlations derived from the PIV and LDA measurements are shown to be in very good agreement, thus offering 3D mapping of the velocities. A strong correlation was observed between the shape of the recirculation zones above the central bluff body and the effects of heat release, stoichiometry and swirl. Detailed analyses of the LDA data further demonstrate that the flow behavior changes significantly with the levels of swirl and stratification, which combines the contributions of dilatation, recirculation and swirl. Key turbulence parameters are derived from the total velocity components, combining axial, radial and tangential velocities.

LDA measurements and turbulence spectral analysis in an agitated vessel

During the last years considerable improvement of the derivation of turbulence power spectrum from Laser Doppler Anemometry (LDA) has been achieved. The irregularly sampled LDA data is proposed to approximate by several methods e.g. Lomb-Scargle method, which estimates amplitude and phase of spectral lines from missing data, methods based on the reconstruction of the auto-correlation function (referred to as cor- relation slotting technique), methods based on the reconstruction of the time series using interpolation between the uneven sampling and subsequent resampling etc. These different methods were used on the LDA data mea- sured in an agitated vessel and the results of the power spectrum calculations were compared. The measurements were performed in the mixing vessel with flat bottom. The vessel was equipped with four baffles and agitated with a six-blade pitched blade impeller. Three values of the impeller speed (Reynolds number) were tested. Long time series of the axial velocity component were measured in selected points. In each point the time series were analyzed and evaluated in a form of power spectrum.

Numerical Simulation of the Effects of Mesoflaps in Controlling Shock/Boundary-Layer Interactions

This work utilizes an immersed-boundary method to simulate the effects of an array of aeroelastic mesoflaps in controlling oblique shock/turbulent boundary-layer interactions. A loosely coupled approach is adopted for the fluid-structure interaction problem, with separate solvers used for the fluid and structural domains. The mesoflaps are rendered as immersed objects for the fluid solver and modeled as cantilevered Euler-Bernoulli beams for the structural solver. Determination of the aerodynamic loads acting on the mesoflap surfaces from the surrounding fluid is done using a nearest neighbor approach combined with a bi-linear interpolation scheme. Simulations are performed for a Mach 2.46 shock / boundary layer interaction with and without control, based on experiments conducted at University of Illinois at Urbana-Champaign. Both Reynolds-averaged Navier-Stokes (RANS) and hybrid large-eddy/Reynolds-averaged Navier-Stokes (LES/RANS) turbulence closures are used. For the computations of the flow with mesoflap control, both 2-D quasi-steady and 3-D dynamic simulations of the fluid-structure interaction problem are performed. Comparisons made with experimental laser Doppler anemometry data and wall pressure measurements for flows with and without control show reasonable agreement, with better predictions away from the separation region. An analysis of the flow indicates that the mesoflap control system does not eliminate axial flow separation. Also, analysis of the frequency content of the mesoflap deflections suggests that a correlation might exist between the dominant frequency of the flap deflection and the low-frequency shock motion observed in separated flows.

Direct numerical simulation of fully developed turbulent and oscillatory pipe flows at

Shear flow turbulence in oscillatory fluid motions is of theoretical interest and practical relevance, since the onset of turbulence can drastically change the transport properties and mixing efficiency. To supplement former theoretical and experimental investigations on the transition to turbulence in Sexl–Womersley (SW) flows, we perform three-dimensional direct numerical simulations (DNS) of oscillatory pipe flows at three Womersley numbers (Wo∈{26, 13, 5}) and one constant Reynolds number () based on the friction velocity and the pipe diameter. For this, the incompressible Navier–Stokes equations are solved in cylindrical coordinates using a fourth-order-accurate finite-volume method on staggered grids, motivated by Schumann’s volume balance procedure. We generate a well-correlated high-Reynolds-number initial flow field for the oscillatory flows by means of a DNS of a statistically steady pipe flow at . To underline the reliability of the DNS results for the oscillatory pipe flows, we validate the finite-volume method, the spatial resolution of the computational grid and the length of the computational domain by comparing the results for the statistically steady pipe flow with experimental data obtained by laser Doppler anemometry (LDA). Comparing the statistical moments of the velocity components up to the fourth order shows good agreement with the corresponding LDA data. When started from the turbulent initial velocity field, the oscillatory flows relaminarise or reach a conditionally or fully turbulent state, depending on Wo. The peak flow rates decrease with increasing Wo, while the relaxation phase for the initially steady flow converging to a purely oscillating flow increases with increasing Wo. For the highest Wo considered, the flow completely relaminarises and we do not find any instabilities close to the wall, as expected from former stability analyses. On the other hand, we confirm the existence of turbulent bursts and increasing turbulence intensity during the deceleration phase of the flow and relaminarisation in the acceleration phase for Wo=13 in agreement with experimental results in the literature. By analysing selected terms of the transport equations for the mean and turbulent kinetic energy, we demonstrate the transport of turbulent kinetic energy from the axial to the radial and azimuthal velocity components during flow deceleration.

Numerical Simulations of Effects of Micro Vortex Generators Using Immersed-Boundary Methods

This work presents an immersed-boundary technique for compressible, turbulent flows and applies the technique to simulate the effects of micro vortex generators in controlling oblique-shock/turbulent boundary-layer interactions. The Reynolds-averaged Navier-Stokes equations, closed using the Menter k-ω turbulence model, are solved in conjunction with the immersed-boundary technique. The approach is validated by comparing solutions obtained using the immersed-boundary technique with solutions obtained on a body-fitted mesh and with experimental laser Doppler anemometry data collected at Cambridge University for Mach 2.5 flow over single micro vortex generators. Simulations of an impinging oblique-shock boundary-layer interaction at Mach 2.5 with and without micro vortex-generator flow control are also performed, considering the development of the flow in the entire wind tunnel. Comparisons are made with experimental laser Doppler anemometry data and surface-pressure measurements from Cambridge University and an analysis of the flow structure is performed. The results show that three dimensional effects initiated by the interaction of the oblique shock with the sidewall boundary layers significantly influence the flow patterns in the actual experiment. The general features of the interactions with and without the micro vortex-generator array are predicted to good accord by the Reynolds-averaged Navier-Stokes/ immersed-boundary model.

Effect of Tabs on Rectangular Jet Plume Development

The near-field mixing performance of a rectangular jet with and without solid tabs introduced at the nozzle lip was studied under supersonic (underexpanded) operating conditions. The effects of tab shape, tab location, and tab number were investigated. Flow visualization of the jet plume was accomplished via Schlieren imaging, and the velocity and turbulence fields quantified via laser Doppler anemometry measurements. Laser Doppler anemometry data were gathered along the jet centerline and on cruciform transverse and spanwise traverse lines downstream of the nozzle exit. Tab shape has only a minor effect, however tab location and number are sensitive parameters for jet plume development. Tabs located on the nozzle wide edge can lead to jet bifurcation, whereas tabs located on the narrow edge cause increased mixing primarily in the major axis direction. A nozzle with four tabs located on its wide edges was the most effective for overall enhancement of jet mixing.

Note on a Parametric Relation for Separating Flow over a Rough Hill

A parametrization method used to account for the effects of flow separation and wall roughness on the lower boundary condition for turbulent boundary layers is investigated against direct numerical simulation and laser Doppler anemometry data. The numerical simulation represents flow over a smooth, flat surface with a prescribed external adverse pressure gradient. The water-channel experiments cover flow over smooth and rough hills for two specified Reynolds numbers. Global optimization algorithms based on four different direct search methods are used to assess the parametrization function, C, in terms of local mean velocity profiles and the parametrization parameters u* (friction velocity), ∂xp (local pressure gradient), z0 (effective roughness) and d (zero-plane displacement). The study investigates regions of attached and reversed flows, and forty-two velocity profiles are compared with the proposed expression for the function C, including two profiles that satisfy the solution of Stratford.

Influence of swirl on the initial merging zone of a turbulent annular jet

This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S=0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of the CRZ significantly, increasing its complexity. The CRZ becomes toroidal and the jet fluid is entrained near the apex. At intermediate swirl numbers a vortex breakdown bubble appears downstream which moves upstream with increasing swirl. At high swirl, the CRZ and breakdown bubble merge which creates a complex and highly anisotropic flow field.

Transition on the T106 LP Turbine Blade in the Presence of Moving Upstream Wakes and Downstream Potential Fields

Boundary layer measurements were performed on a cascade of the T106 high lift low-pressure (LP) turbine blades that was subjected to upstream wakes and a moving downstream potential field. Tests were carried out at a low level of inlet freestream turbulence (0.5%) and at a higher (4.0%). It is found that perturbations in the freestream due to both disturbances are superposed on each other. This affects the magnitude of the velocity perturbations at the edge of the boundary layer under the wakes as well as the fluctuations in the edge velocity between the wakes. Furthermore, the fluctuations in the adverse pressure gradient on the suction surface depend on the relative phase of the upstream and downstream disturbances, providing an additional stimulus for clocking studies. Time-mean momentum thickness values calculated from laser Doppler anemometry (LDA) traverses performed near the suction surface trailing edge are used to identify the optimum relative phase angle of the combined interaction. Unsteady suction surface pressures, quasiwall shear stress and LDA data illustrate the resulting multimode process of transition, which is responsible for the observed clocking effects. The optimum relative phase angle of the upstream wake and the downstream potential field can produce 0.25% of efficiency improvement through the reduction of the suction surface boundary layer loss. This reduction is mainly related to the calmed region and the laminar flow benefits that can be more effectively utilized than when only the upstream wakes are present. During the remaining parts of the cycle, the features that are usually associated with the wake and the potential field effects are still present.

Analytical models for the mean flow inside dense canopies on gentle hilly terrain

AbstractSimplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments. Laser Doppler anemometry (LDA) measurements are conducted in a neutrally‐stratified boundary‐layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy. The hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy (H/L ≪ 1 and Lc/L ≈ 1, where H is the hill height, L the half‐length, and Lc the canopy adjustment length‐scale). The LDA data suggest that the often‐criticized linearizations of the advective terms, turbulent‐shear‐stress gradients and drag force appear reasonable except in the deep layers of the canopy. As predicted by a previous analytical model, the LDA data reveal a recirculation region within the lower canopy on the lee slope. Adjusting the outer‐layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model's performance. For the velocity perturbations in the deeper layers of the canopy, a new analytical model, which retains a balance between mean horizontal advection, mean pressure gradient and mean drag force but neglects the turbulent‐shear‐stress gradient, is developed. The proposed model reproduces the LDA measurements better than the earlier analytical model, which neglected advection but retained the turbulent‐shear‐stress gradient in the lower layers of the canopy and near the hill top. This finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small. In essence, the newly‐proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean‐momentum equation. We briefly discuss how to integrate all these findings with recent advances in canopy lidar remote‐sensing measurements of general topography and canopy height. Copyright © 2008 Royal Meteorological Society

An experimental investigation of the mean momentum budget inside dense canopies on narrow gentle hilly terrain

Recent theories and model calculations for flows inside canopies on gentle hilly terrain suggest that the impact of advection and pressure perturbations on the mean momentum budget remains problematic when the canopy adjustment length ( L c) is comparable to the hill half-length ( L) (referred to as narrow gentle hills). To progress on this problem, detailed laser Doppler anemometry (LDA) and water surface profile measurements were conducted in a large flume simulating a neutrally stratified boundary layer flow over a train of gentle hills covered by a dense canopy with L c/ L ≃ 1. The canopy was composed of an array of vertical cylinders with a frontal area index concentrated in the upper third to resemble a tall hardwood forest at maximum leaf area index. The data was presented in terms of component balance of the mean momentum equation decomposed into a background state and a perturbed state induced by topographic variation. We found that the measured and modelled pressure computed from the topographic shape function were not in phase, with the minimum pressure shifted downwind from the hill summit. We also showed that the recirculation region, predicted to occur on the lee side of the hill close to the ground, was sufficiently large to modify the mean streamlines both within the canopy sub-layer and just above the canopy. This adjustment in mean streamlines can be accounted for through an effective ground concept thereby retaining the usability of linear theory to model the mean pressure gradients. The LDA data suggested that the shear stress gradient remained significant at the bottom of the hill in the deeper layers of the canopy and was the leading term balancing the adverse pressure gradient in the recirculation region. The drag force was the leading contributor to the mean momentum balance near the canopy top and within the deeper layers of the canopy at the hill summit. However, we found that the drag force was not the primary term balancing the adverse pressure gradient within the recirculation zone. Advection was not only substantial above the canopy but remained significant in the deeper layers of the canopy near the hill summit as predicted by recent numerical simulations. In short, no one term in the mean momentum balance can be a priori neglected at all positions across a gentle narrow hill.

Estimation of cross-power spectra using sample-and-hold reconstruction of laser Doppler anemometry data

Measurement of turbulent spatial and temporal length scales is an increasingly important aspect of analysis in fluid mechanics. This information is derived either from the cross correlation or the cross spectrum between two measurements. The use of a laser Doppler anemometer (LDA) further complicates the determination of these properties because of the random data acquisition associated with LDA measurements. The calculation of the cross spectrum obtained for LDA data with equispaced data from a conventional instrument such as a hot wire anemometer (HWA) and for two LDA systems is considered. For each case, sample-and-hold reconstruction of the LDA data is used. The errors in cross spectrum estimates together with coherence function calculations (i.e. frequency domain cross covariance) are derived and procedures for correcting the cross spectrum and, hence, the phase and the coherence, are proposed. The results from numerical simulations with prescribed coherence and phase are shown to produce good results up to and beyond the equivalent Nyquist frequency based on the mean LDA data rate. Finally, the procedures are applied to two point LDA measurements in a jet and the results, using the proposed procedures, are shown to compare favourably with those obtained from the correlation slotting method.

Experimental and Numerical Investigation on the Effects of the Seeding Properties on LDA Measurements

Abstract The characteristics of the seeding particles, which are necessary to implement the laser Doppler anemometry (LDA) technique, may significantly influence measurement accuracy. LDA data were taken on a steady-flow rig, at the entrance of the trumpet of the intake system of a high-performance engine head. Five sets of measurements were carried out using different seeding particles: samples of micro-balloons sieved to give three different size ranges (25–63μm,90–200μm, and standard as received from the manufacturer 1–200μm), smoke from a “home-made” sawdust burner (particle size ⩽1μm), and fog from a commercial device (particle size around 1μm). The LDA data were compared with the results of two-phase computational fluid dynamics simulations. The comparison showed a very good agreement between the experimental and numerical results and confirmed that LDA measurements with particle dimensions in the order of 1μm or less represent the actual gas velocity. On the contrary, quite large particles, which are often used because of their cost and cleanliness advantages, introduce non-negligible errors.

Quantitative experimental study of shear stresses and mixing in progressive flow regimes within annular-flow bioreactors

Annular-flow bioreactors are normally operated under laminar Couette flow conditions in order to minimise shear-induced damage to cells. In this study, we computed the fluid shear stresses in model annular vessels over a range of laminar flow regimes, from Couette flow to Taylor-vortex flow, and at two geometric scales, using a shear rate model for freely suspended particles, together with experimental Laser Doppler Anemometry data for a 2-D velocity field. The shear stresses were greatest in the boundary layers adjacent to each wall in each case, with values typically 6 times higher than the mean stresses in the annular space; their respective magnitudes were significantly lower in the larger of the two vessels studied, however. Cell viability studies were also performed in which mammalian cells were cultured under dynamic conditions in a functional bioreactor having the same dimensions as the smaller vessel. The results of these studies demonstrated that a significantly greater number of cells remained in suspension in Taylor-vortex flows than in Couette flow, but at the expense of cell viability at higher Taylor numbers. Taken together, these findings suggest that the benefits of enhanced convective mass transport afforded by Taylor–Couette flows could be realised without risk of appreciable shear induced damage of cells and tissues in larger vessels operating under dynamically similar flow conditions.

An improved sample-and-hold reconstruction procedure for estimation of power spectra from LDA data

Techniques for deriving the auto or power spectrum (PSD) of turbulence from laser Doppler anemometry (LDA) measurements are reviewed briefly. The low pass filter and step noise errors associated with the sample-and-hold process are considered and a discrete version of the low pass filter for the resampled signal is derived. This is then used to develop a procedure by which the PSD estimates obtained from sample and hold measurements can be corrected. The application of the procedures is examined using simulated data and the results show that the frequency range of the analysis can be extended beyond the Nyquist frequency based on the mean sample rate. The results are shown to be comparable to those obtained using the method of Nobach et al. (1998) but the new procedures are more straightforward to implement. The technique is then used to determine the PSD of real LDA data and the results are compared with those from a hot wire anemometer.

Kinematics of extreme waves in deep water

The velocity profiles under crest of a total of 62 different steep wave events in deep water are measured in laboratory using particle image velocimetry. The waves take place in the leading unsteady part of a wave train, focusing wave fields and random wave series. Complementary fully nonlinear theoretical/numerical wave computations are performed. The experimental velocities have been put on a nondimensional form in the following way: from the wave record (at a fixed point) the (local) trough-to-trough period, T TT and the maximal elevation above mean water level, η m of an individual steep wave event are identified. The local wavenumber, k and an estimate of the wave slope, ϵ are evaluated from ω 2/( gk)=1+ ϵ 2, kη m =ϵ+ 1 2 ϵ 2+ 1 2 ϵ 3, where ω=2π/ T TT and g denotes the acceleration of gravity. A reference fluid velocity, ϵ g/k is then defined. Deep water waves with a fluid velocity up to 75% of the estimated wave speed are measured. The corresponding kη m is 0.62. A strong collapse of the nondimensional experimental velocity profiles is found. This is also true with the fully nonlinear computations of transient waves. There is excellent agreement between the present measurements and previously published Laser Doppler Anemometry data. A surprising result, obtained by comparison, is that the nondimensional experimental velocities fit with the exponential profile, i.e. e ky , y the vertical coordinate, with y=0 in the mean water level.

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.