Laser Doppler Anemometry Measurements Research Articles

Distributed reactions in highly turbulent premixed methane/air flames: Part I. Flame structure characterization

Simultaneous planar laser-induced fluorescence (PLIF) measurements of a series of reactive scalars and Rayleigh scattering measurements of temperature, i.e. CH/CH2O/OH, HCO/CH2O/OH and T/CH2O/OH, and laser Doppler anemometry (LDA) measurements are carried out to characterize the flame/turbulence interaction in various regimes of turbulent combustion, including the laminar flamelet regime, the thin reaction zone (TRZ) regime, and the distributed reaction zone (DRZ) regime. A series of turbulent premixed methane/air jet flames with different jet speeds and equivalence ratios are studied. The jet Reynolds number ranges from 6000 to 40,000 and the Karlovitz number (Ka) of the studied flames varies from 25 to 1470. It is shown that in the TRZ regime CH/HCO layer remain thin but the layer of CH2O and temperature gradient are broadened owing to the rapid turbulence transport. In the DRZ regime the CH and HCO layers are also broadened owing to the rapid transport of reactive species such as OH radicals from the high temperature regions where these radicals are formed to the low temperature region. In the DRZ regime CH and HCO are found to coexist with OH or CH2O owing to the rapid turbulence eddy interaction, which differs fundamentally from that in the TRZ regime and the laminar flamelet regime. For the present investigated flames, the temperature range for the distributed reaction to occur is found to be between 1100K and 1500K. It is shown that the structures of flames in different regimes can affect the turbulence field differently. In the DRZ regime the temperature gradient is lower than that in the laminar flamelet and the TRZ regimes, which results in a lower peak of turbulence intensity owing to the retarded velocity gradient across the flames and thereby a lower rate of turbulence production.

Semianalytical characterization of turbulence from radial impellers, with experimental and numerical validation

Based on conventional turbulent jet theory and the general theoretical framework of scalar dispersion in turbulent shear flows, a novel formulation of the radial impeller's jet in stirred tanks is introduced. Whereas previous studies considered the impeller's jet as developed, it is now comprised of two separate spatial regions along the radial axis: the zone of flow establishment (ZFE) and the zone of established flow (ZEF). This formulation is accompanied with semi‐analytical expressions for the prediction of turbulent key parameters including the random part of k and ε in the ZFE. The new theoretical framework is validated both with laser Doppler anemometry measurements and with 3‐D numerical simulations using the standard turbulent model. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1413–1426, 2015

Determining the discharge rate from a submerged oil leak jet using ROV video

With expanded deep sea drilling in the Gulf of Mexico, and possibly the Arctic, it is imperative to have a technology available to quickly and accurately measure the discharge rate from a submerged oil leak jet. This paper describes an approach to measure the discharge rate using video from a Remotely Operated Vehicle (ROV). ROV video can be used to measure the velocity of visible features (turbulent eddies, vortices, entrained particles) on the boundary of an oil leak jet, from which the discharge rate can be estimated. This approach was first developed by the Flow Rate Technical Group (FRTG) Plume Team, of which the authors Savaş and Shaffer were members, during the response to the Deepwater Horizon (DWH) oil leak. Manual tracking of visible features produced the first accurate government estimates of the oil discharge rate from the DWH. However, for this approach to be practical as a routine response tool, software is required that automatically measures the velocity of visible features. To further develop this approach, experiments were conducted to simulate a submerged oil leak jet using a dye-colored water jet in the U.C. Berkeley Tow Tank facility. Jet exit diameters were 10.2cm and 20.3cm. With flow rates up to 11gal/s, Reynolds numbers in the range of the DWH oil leak jets (up to 500,000) were achieved. The dye-colored water jets were recorded with high speed video and radial profiles of velocity were mapped with Laser Doppler Anemometry (LDA). Particle Image Velocimetry (PIV) software was applied to measure the velocity of visible features. The velocities measured with PIV software were in good agreement with the LDA measurements. Finally, the PIV software was applied to ROV video of the DWH oil leak jet. The measured velocities were 10–50% lower than manual measurements of velocity. More research is required to determine the reasons why PIV software produced much lower velocities than manual tracking for the DWH oil leak jet.

Dead time effects in laser Doppler anemometry measurements

We present velocity power spectra computed by the so-called direct method from burst type laser Doppler anemometer (LDA) data, both measured in a turbulent round jet and generated in a computer. Using today's powerful computers we have been able to study more properties of the computed spectra than was previously possible, and we noted some unexpected features of the spectra that we now attribute to the unavoidable influence of a finite measurement volume (MV). The most prominent effect, which initially triggered these studies, was the appearance of damped oscillations in the higher frequency range, starting around the cut-off frequency due to the size of the MV. Using computer generated data mimicking the LDA data, these effects have previously been shown to appear due to the effect of dead time, i.e., the finite time during which the system is not able to acquire new measurements. These dead times can be traced back to the fact that the burst-mode LDA cannot measure more than one signal burst at a time. Since the dead time is approximately equal to the residence time for a particle traversing a measurement volume, we are dealing with widely varying dead times, which, however, are assumed to be measured for each data point. In addition, the detector and processor used in the current study introduce a certain amount of fixed processing and data transfer times, which further contribute to the distortion of the computed spectrum. However, we show excellent agreement between a measured spectrum and our modeled LDA data thereby confirming the validity of our model for the LDA burst processor.

Underexpanded Jet Development from a Rectangular Nozzle with Aft-Deck

An experimental study is reported of underexpanded supersonic jet plumes issuing from a high-aspect-ratio convergent rectangular nozzle. Schlieren visualization, Pitot probe, and Laser Doppler Anemometry measurements are carried out to capture the plume development in the near field, and in particular the effect on the plume flow of a finite-length extended shelf or aft-deck attached to the lower nozzle wall. This creates asymmetry in the inviscid shock cell pattern and the entrainment characteristics, both of which influence shear-layer growth and plume trajectory. A net pressure force is induced on the aft-deck wall, which leads to transverse deflection of the jet plume once it leaves the aft-deck, both upward and downward, depending on aft-deck length and nozzle pressure ratio. For sufficiently high nozzle pressure ratio and a sufficiently long aft-deck, separation and reattachment of the plume from the aft-deck is observed. Detailed mapping of both mean velocity and turbulence in the plume near field has been carried out, enabling comparison of flow behavior for a clean nozzle and a nozzle with aft-deck. The data provided are proposed as a suitable benchmark validation test case for computational fluid dynamics studies of rectangular nozzle plumes with aft-deck interaction effects.

Experimental study on the flow characteristics of unstructured block ramps

ABSTRACTUnstructured block ramps (UBR) are fish-friendly structures to stabilize a river bed. They are increasingly used to replace drops and represent large macro-roughness elements randomly placed on the river bed. Laser Doppler Anemometry measurements were conducted to determine the mean flow characteristics in a laboratory channel covered by block ramps. The resulting local time-averaged flow quantities allow for considerations of the flow field heterogeneity and for the identification of migration corridors for a certain target fish. Double-averaging (in time and in space) was applied to characterize the flow conditions taking into account the strongly heterogeneous bed of an unstructured block ramp. By applying this method, the characteristic vertical profiles describing the general flow conditions are presented. Based on geometrical and physical considerations, a suggestion is made to sub-divide the interfacial sublayer of macro-rough beds into upper and lower parts. The aim of this research is to present not only the flow field, but also to highlight the ecological value of UBR.

CFD investigation of flow in the MATIS-H test facility

Abstract In this paper steps of calculations for a bare rod bundle and so-called swirl type spacer grid are showed and the effects of choice of model details are examined. The modeled pin bundle and spacer grid geometry are related to rectangular western type PWR fuel assemblies. First a CFD model has been developed for a subchannel of the MATIS-H test facility in order to carry out a mesh independence study. Based on the results a model for the half cross-section of the test bundle has been developed and calculations are carried out with different turbulence models. Translational periodicity is applied in axial direction in order to calculate a fully developed flow, which will be used as inlet boundary condition for spacer grid calculations. Results are compared with LDA (Laser Doppler Anemometry) measurements published in the frame of the OECD NEA MATIS-H benchmark. As a last step a model for rod bundle incorporating swirl type spacer grid is developed and calculations are carried out with different turbulence models using the results of the bare rod bundle simulations as inlet boundary conditions.

Prediction of Noise Sources in Axial Compressor from URANS Simulation

As part of a larger aeroacoustic study focused on rotor–stator interaction noise, the unsteady flow in a single-stage axial compressor is achieved by a numerical Reynolds-averaged Navier–Stokes simulation. The flowfield is precisely analyzed and assessed by comparing it with available experimental and numerical data. Specifically, the rotor wakes correctly agree with previous laser Doppler anemometry measurements. The unsteady flow, especially the potential effects created by the stator vanes, is then investigated, showing that the stator potential field is responsible for a major secondary tonal noise source on the rotor blades. The rotor wake velocity and turbulent content correspond to the excitations for tonal and broadband noise, respectively. They are greatly influenced by the vane potential effects and, to a lesser extent, by their convection with the mean flow. Two methods of extraction are surveyed along the whole span, showing the need to consider the blockage induced by the stator on the wakes. Finally, the realistic tonal acoustic sources of rotor–stator interaction extracted in time on the stator vane surfaces show a noise mechanism mainly driven by the leading edge and some significant variations from hub to tip.

Validation of in-cylinder flow of a controlled auto-ignition engine

Numerical calculations have been carried out to investigate the in-cylinder transient flow structure of a controlled auto-ignition (CAI) engine running at speeds of 1500 rpm and 2000 rpm. The calculated turbulent flow structure and velocities are validated against published laser doppler anemometry (LDA) experimental data (Pitcher et al., 2003). The experimental data was reprocessed to represent the time dependent mean velocities for all measured points. The actual geometry configuration of the engine is imported into the computational fluid dynamics (CFD) code used in this study. The simulations take into account the movement of the inlet, exhaust valves and the piston. The CFD simulations replicate the experimental work where only air was inserted into a driven optical engine. Also, to simulate an engine in controlled auto-ignition (CAI) mode, the same valve timing that allows 36% internal exhaust gas recirculation (IEGR) was applied for the air intake. The calculated results found to agree well with the LDA measurements with an overall agreement of 75.06% at 1500 rpm and 73.42% at 2000 rpm.

Favre- and Reynolds-averaged velocity measurements: Interpreting PIV and LDA measurements in combustion

Previous studies using particle image velocimetry (PIV) and laser Doppler anemometry (LDA) have raised the question of how these measurements should be compared. This study reports on the difference between Favre-averaged and Reynolds-averaged velocity statistics for a turbulent burner using PIV and LDA for unconditional and conditional velocity measurements. The experimental characterization of flow fields of premixed and stratified methane/air flames is carried out under globally turbulent lean conditions (global equivalence ratio at 0.75), over a range of stratifications and swirl numbers. Unconditioned velocity data was acquired using aluminium oxide to seed the flow field. Conditioned measurements were performed using vegetable oil aerosol as seed, which burns through the flame front, thus allowing only the non-reacting flow velocities to be obtained. A critical comparison of unconditioned velocity profiles measured using both PIV and LDA, including axial, radial, and tangential components is made against conditioned and reconstructed mean velocities at different cross-sections of the flame. The comparison reveals how the differences between the Favre-averaged (unconditioned) and the Reynolds-averaged (conditioned) velocity measurements in the flame brush region can be accounted for using the mean progress of reaction, and highlights the limits of the accuracy and agreement between PIV and LDA measurements.

Measuring turbulence in a flotation cell using the piezoelectric sensor

Turbulence and its distribution are of great importance in flotation processing and has been the subject of much research. However, there is no mature technique to measure turbulence in three phase (liquid–solid–gas) systems. In this research, the Piezoelectric Vibration Sensor (PVS) was developed, based on previous research, as a promising tool for turbulence measurements in industrial flotation environments. A frequency response model was established to calculate force applied to the sensor. Experimental results and comparison with Laser Doppler Anemometry (LDA) measurement data showed that the PVS can measure intensity of kinetic energy fluctuation (σv2), which has been found in experiments to correlate with turbulent kinetic energy (TKE), a parameter often related to flotation performance in the literature. The sensor was then applied to a 60l laboratory batch cell running at different impeller speeds and air flow rates to obtain turbulence profiles. Results showed that the piezoelectric sensor is fully capable of measuring turbulence in a multi-phase environment.

Turbulent swirling flow in a dynamic model of a uniflow-scavenged two-stroke engine

It is desirable to use computational fluid dynamics for optimization of the in-cylinder processes in low-speed two-stroke uniflow-scavenged marine diesel engines. However, the complex nature of the turbulent swirling in-cylinder flow necessitates experimental data for validation of the used turbulence models. In the present work, the flow in a dynamic scale model of a uniflow-scavenged cylinder is investigated experimentally. The model has a transparent cylinder and a moving piston driven by a linear motor. The flow is investigated using phase-locked stereoscopic particle image velocimetry (PIV) and time-resolved laser Doppler anemometry (LDA). Radial profiles of the phase-locked mean and rms velocities are computed from the velocity fields recorded with PIV, and the accuracy of the obtained profiles is demonstrated by comparison with reference LDA measurements. Measurements are carried out at five axial positions for 15 different times during the engine cycle and show the temporal and spatial development of the swirling in-cylinder flow. The tangential velocity profiles in the bottom of the cylinder near the end of the scavenge process are characterized by a concentrated swirl resulting in wake-like axial velocity profiles and the occurrence of a vortex breakdown. After scavenge port closing, the axial velocity profiles indicate that large transient swirl-induced structures exist in the cylinder. Comparison with profiles obtained under steady-flow conditions shows that the scavenge flow cannot be assumed to be quasi-steady. The temporal development of the swirl strength is investigated by computing the angular momentum. The swirl strength shows an exponential decay from scavenge port closing to scavenge port opening corresponding to a reduction of 34 %, which is in good agreement with theoretical predictions.

A regular Strouhal number for large-scale instability in the far wake of a rotor

AbstractThe flow behind a model of a wind turbine rotor is investigated experimentally in a water flume using particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The study performed involves a three-bladed wind turbine rotor designed using the optimization technique of Glauert (Aerodynamic Theory, vol. IV, 1935, pp. 169–360). The wake properties are studied for different tip speed ratios and free stream speeds. The data for the various rotor regimes show the existence of a regular Strouhal number associated with the development of an instability in the far wake of the rotor. From visualizations and a reconstruction of the flow field using LDA and PIV measurements it is found that the wake dynamics is associated with a precession (rotation) of the helical vortex core.

Near-field mixing of jets issuing from an array of round nozzles

This article presents results of an experimental study of the confluence of low Reynolds number jets in the near field of a 6×6 in-line array of round nozzles. Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) were employed to measure mean velocities and turbulence statistics. The comparison of the results from PIV and LDA measurements along different cross-sectional profiles and geometrical centerlines showed good agreement. However, LDA enabled more accurate results very close to the nozzle exits. The evolution of all the individual jets in the array into a single jet showed flow regions similar to twin jets (i.e., initial, converging including mixing transition, merging and combined regions). The lateral displacements play an important role for a confluent jet, where all jets to some degree are deflected towards the center of the nozzle plate. The jet development in terms of velocity decay, length of potential core and lateral displacement varies significantly with the position of the jet in the array. A comparison with single jet and twin jets flow showed considerable differences in velocity decay as well as location and velocity in the combined point. The flow field of confluent jets showed asymmetrical distributions of Reynolds stresses around the axis of the jets and highly anisotropic turbulence. Additionally, the lateral displacement as well as the turbulence development in the proximal region of the studied confluent jet was shown to be dependent on Reynolds number.

Study of the Turbulent Flow Structure around a Standard Rushton Impeller

Abstract The velocity field around the standard Rushton turbine was investigated by the Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) measurements. The mean ensembleaveraged velocity profiles and root mean square values of fluctuations were evaluated at two different regions. The first one was in the discharge stream in the radial direction from the impeller where the radial flow is dominant and it is commonly modelled as a swirling turbulent jet. The validity range of the turbulent jet model was studied. The second evaluated region is under the impeller where flow seems to be at first sight rather rigorous but obtained results show nonnegligible values of fluctuation velocity.

LDA measurements in the Francis-99 draft tube cone

Velocity measurements were performed in the draft tube cone of a 1:5.1 scaled model of the Tokke hydropower plant, Norway; also known as the Francis-99 model. Results from the laser Doppler anemometry measurements undertaken at three operating points will be used as validation data for an upcoming workshop on the state of the art of Francis turbine numerical simulation. With the turbine operating at the best efficiency point, a sensitivity analysis of the flow parameters head, flow rate and runner rotational speed shows that the effects on the dimensionless velocity profiles are small as long as nED and QED are held constant. The results indicate a well-functioning turbine at the best efficiency point and high load. At the part load operating point, a vortex breakdown occurs which distorts the velocity profiles and significantly lowers the turbine's hydraulic efficiency. Frequency spectrums of each LDA signal at part load reveals a peak which is asynchronous to that of the runner angular speed. The peaks might be related to the precession of a rotating vortex rope but the characteristics of the LDA signals are different compared to previous studies involving rotating vortex ropes.

Performance characterisation of AG/SAG mill pulp lifters using CFD techniques

The pulp lifter is an integral component of autogenous (AG) and semi-autogenous (SAG) grinding mills as it controls the throughput, performance and efficiency of mills. The slurry transport from the AG/SAG mill through grate holes into the discharge trunnion is the main function of the pulp lifter. This process develops complex flow behaviour in the region of the grate and pulp lifter. Efficient and effective removal of pulp/slurry from the mill is the key objective of the pulp lifter design.This work aims to understand slurry flow behaviour in pulp lifter sections and its contribution to mill performance using computational fluid dynamics (CFD) modelling and Laser Doppler Anemometry (LDA) measurements applied to a laboratory scale mill. The CFD model is validated against the LDA measurements, and then used to build a cohesive computational framework for modelling industrial pulp lifters, to investigate unique problems associated with their design and performance.

CFD Simulation of the Discharge Flow from Standard Rushton Impeller

The radial discharge jet from the standard Rushton turbine was investigated by the CFD calculations and compared with results from the Laser Doppler Anemometry (LDA) measurements. The Large Eddy Simulation (LES) approach was employed with Sliding Mesh (SM) model of the impeller motion. The obtained velocity profiles of the mean ensemble-averaged velocity and r.m.s. values of the fluctuating velocity were compared in several distances from the impeller blades. The calculated values of mean ensemble-averaged velocities are rather in good agreement with the measured ones as well as the derived power number from calculations. However, the values of fluctuating velocities are obviously lower from LES calculations than from LDA measurements.

Evaluation and accuracy of the local velocity data measurements in an agitated vessel

Velocity measurements of the flow field in an agitated vessel are necessary for the improvement and better understanding of the mixing processes. The obtained results are used for the calculations of the impeller pumping capacity, comparison of the power consumption etc. We performed various measurements of the local velocities in an agitated vessel final results of which should be processed for several purposes so it was necessary to make an analysis of the obtained data suitability and their quality. Analysed velocity data were obtained from the LDA (Laser Doppler Anemometry) and PIV (Particle Image Velocimetry) measurements performed on a standard equipment where the flat bottomed vessel with four baffles was agitated by the six-blade Rushton turbine. The results from both used methods were compared. The frequency analyses were examined as well as the dependency of the data rates, time series lengths etc. The demands for the data processed in the form of the ensemble-averaged results were also established.

LATTICE-BOLTZMANN SIMULATIONS OF FLOWS OVER BACKWARD-FACING INCLINED STEPS

The lattice-Boltzmann method (LBM) is used in conjunction with a very large-eddy simulation (VLES) turbulence modeling approach to compute separated flows over backward-facing steps at different wall inclination angles. The Reynolds number Re H based on the step height H and center-line velocity at the channel inlet ucl is 64 000. The expansion ratio of the outlet section to the inlet section of the channel is 1.48. Wall inclination angles α considered include 10°, 15°, 20°, 25°, 30° and 90°. The computed flow fields for different inclination angles of the step are assessed against the laser Doppler anemometry (LDA) measurements of Makiola [B. Makiola, Ph.D. Thesis, University of Karlsruhe (1992); B. Ruck and B. Makiola, Flow separation over the step with inclined walls, in Near-Wall Turbulent Flows, eds. R. M. C. So, C. G. Speziale and B. E. Launder (Elsevier, 1993), p. 999.]. In addition to validating the lattice-Boltzmann solution with the experiments, this study also investigates the effects of three dimensionality, the proximity of the inlet to the step, and the grid resolution on the quality of the predictions.