Two-component Laser Doppler Velocimeter Research Articles

Physical and Numerical Modelling of Internal Aerodynamics of the Vortex Furnace with Distributed Tangential Input of Burner Streams

The work is devoted to experimental and numerical modelling of internal aerodynamics of a new type furnace for the thermal power plant steam generator with technology of pulverized-coal combustion in vortex flow. Measurements of aerodynamic features of the flow in the laboratory air isothermal model of the furnace have been performed with twocomponent laser Doppler velocimeter. Numerical simulation of three-dimensional turbulent isothermal flow in the laboratory model has been carried out with the use of CFD package FLUENT. Detailed 3-D flow structure in the studied model has been obtained. The agreement between numerical predictions and measurements on mean flow velocity distributions has been shown

Evolution of Gas Velocities Behind Flamelets in a Premixed Turbulent Bunsen Flame

In an earlier study of turbulent premixed Bunsen flames, we employed a two-component laser Doppler velocimeter (LDV) system and a three-element electrostatic probe to investigate influences of flamelets on local gas velocities in the reaction-sheet regime. Because the two-component LDV does not provide complete information about the gas velocity and the three-element probe does not provide complete information on the direction of flamelet motion, it was not possible to fully explain the results of the LDV measurements correctly. To remedy this difficulty so that the explanations can be completed, in the present work the two-component LDV is replaced by a three-component LDV and the three-element probe is replaced by a four-element probe, which provide full information on the direction of flamelet motion. The new LDV system was employed to measure the three components of velocity at a midflame point on the centerline of a propane–air flame of equivalence ratio 1.10 on a 26-mm-diameter Bunsen burner with fully developed turbulent pipe flow upstream at a Reynolds number of 6700. The LDV measurement volume was 1 mm below the leading electrode of the four-element electrostatic probe that provided the velocity and direction of motion of flamelets in the turbulent flame brush. It was found that when the coordinate system was rotated about a vertical axis to the plane containing the flamelet velocity vector, most of the burnt-gas motion following flamelet passage often remained largely in this plane, although in many cases a nearly constant rate of rotation about this plane was detected. The sequence of LDV data for gas motion in the burnt gas consistently showed movement beginning first in a direction different from that of flamelet passage, then gradually rotating until finally becoming the direction of flamelet passage. This behavior is explained qualitatively by considerations of momentum conservation and of the nature of the LDV measurements. Earlier speculations concerning the role of buoyancy are thereby resolved, now showing that buoyancy is not the principle controlling phenomenon, and additional information about turbulence structures in these flames is obtained.

Loss Production Mechanisms in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study, carried out by means of a two-component Laser Doppler Velocimeter, aimed at investigating the loss generation mechanisms induced by laminar separation bubble and transition process. Measurements have been performed along a flat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble. Results were detailed enough to allow calculating laminar and turbulent deformation works in the separated flow region. Normal and shear contributions of both viscous and turbulent deformation works have been analyzed and employed to explain the generation of total pressure losses in the separated flow region, where the generation and amplification of Kelvin–Helmholtz instability induces the separated shear layer roll-up, thus the bubble reattachment. Results obtained for different Reynolds number conditions have been employed for the formulation of a loss scaling procedure involving the separated shear layer thickness, which is directly correlated to the dynamics of Kelvin–Helmholtz roll-up vortices.

Effects of Acoustic Excitation on a Swirling Diffusion Flame

A swirling double concentric jet is commonly used for nonpremixed gas burner application for safety reasons and to improve the combustion performance. Fuel is generally spurted at the central jet while the annular coflowing air is swirled. They are normally separated by a blockage disk where the bluff-body effects further enhance the recirculation of hot gas at the reaction zone. This paper aims to experimentally investigate the behavior of flame and flow in a double concentric jet combustor when the fuel supply is acoustically driven. Laser-light sheet assisted Mie scattering method has been used to visualize the flow, while the flame lengths were measured by a conventional photography technique. The fluctuating velocity at the jet exit was measured by a two-component laser Doppler velocimeter. Flammability and stability at first fuel tube resonant frequency are reported and discussed. The evolution of flame profile with excitation level is presented and discussed, together with the reduction in flame length. The flame in the unforced reacting axisymmetric wake is classified into three characteristic modes, which are weak swirling flame, lifted flame, and transitional reattached flame. These terms reflect their primary features of flame appearances, and when the acoustic excitation is applied, the flame behaviors change with the excitation frequency and amplitude. Four additional characteristic modes are identified; e.g., at low excitation amplitudes, wrinkling flame with a blue annular film is observed because the excitation induces vortices in the central fuel jet and hence gives rise to the wrinkling of flame. The central jet vortices become larger with the increase in excitation amplitude and thus lead to a wider and shorter flame. If the excitation amplitude is increased above a certain value, the central jet vortices change the rotation direction and pacing with the annular jet vortices. These changes in the flow field induce large turbulent intensity and mixing and therefore make the flame looks blue and short. Further increase in the excitation amplitude would lift the flame because the flow field would be dramatically modified.

Investigations on the Effect of Backward-Facing and Forward-Facing Steps on Turbulent Mixed-Convection Flow Over a Flat Plate

The effects of backward-facing and forward-facing steps on a turbulent buoyancy-dominated mixed-convection flow over a flat plate are examined experimentally. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously by using a two-component laser-Doppler velocimeter and a cold wire anemometer, respectively. The experiment was carried out for a step (backward-facing/forward-facing) height of 22 mm, a temperature difference, ΔT, of 30°C between the heated walls and the free-stream air (corresponding to a local Grashof number Gr xi = 6.45 × 1010), and a free-stream velocity of 0.48 m/s. It was found that the introduction of backward- and forward-facing steps increases the turbulence intensity of the velocity and temperature fluctuations downstream of the step. The present results also reveal that the maximum local Nusselt number occurs in the vicinity of the reattachment zone, and it is approximately twice for the case of the backward-facing step and three times for the case of the forward-facing step than that of the flat plate value at similar flow and thermal conditions.

One-wavelength two-component laser Doppler velocimeter system for surface displacement monitoring

A two-component laser Doppler velocimeter (2D-LDV) prototype for surface-displacement measurement is presented. The system proposed is based on the Doppler differential technique with a backwardscattering configuration. A reference and two frequency-shifted probe beams are generated departing from a single laser beam by means of two beamsplitters and acousto-optic modulators. This configuration has the advantage of using a single-wavelength laser source and a single detector system, while permitting sign detection of the two vector components of the velocity in surface-displacement measurements. It also enables the implementation of a low-cost multitrigger data acquisition system, which is useful in low signal-to-noise ratio (SNR) situations, such as the velocity measurement of low-dispersion surfaces or in hydrodynamic applications. The prototype has been designed to measure velocities up to 3 m per second, both in industrial (solid target) and hydrodynamic applications, with an error well below 1% in the tests performed. The system works in real time with the use of an acquisition card and C++ based software on a PC.

Experimental study of flow patterns in deoiling hydrocyclone

Using a two-component laser Doppler velocimeter, the axial velocity, tangential velocity and fluctuation velocity were measured in a 35-mm deoiling hydrocyclone. Air core occurs in the hydrocyclone when inlet flow rate is more than 2.00 m 3/h. The fluctuation velocities with air core are greater than without air core. The velocity magnitudes are dependent on the inlet flow rate and the flow character is not change by changes in the inlet flow rate in hydrocyclone. The velocity fluctuations are greater near the core and near the wall. The results show that air core in the hydrocyclone will arose turbulence fluctuation. These studies are helpful to understand the separation mechanism of deoiling hydrocyclone.

Turbulence Characteristics of Open-Channel Flow with Bed Suction

The influence of bed suction on the characteristics of turbulent open channel flow is studied in a laboratory flume using a two-component laser Doppler velocimeter. The experimental results show how bed suction significantly affects the mean flow properties, turbulence levels, and Reynolds stress distributions. The data reveal the presence of a more negative vertical (downward) velocity. The results also show how the horizontal and vertical turbulence intensities and Reynolds shear stresses respond to suction. All these properties are found to reduce with increasing relative suctions: decreasing more rapidly around the bed region than that near the free surface. In the downstream direction, the flow structure in the suction zone undergoes a process of rapid readjustment within a transitional region. Beyond this region, the turbulence flow structures asymptotes toward an “equilibrium” region.

Spatially periodic reversing core in a twisted-fin generated swirling pipe flow

Experimental results for swirling turbulent flow in a pipe, generated by a 180° twisted fin inside the pipe, are presented. The results show that the core region undergoes a spatially periodic change in direction of rotation from counter-rotating to co-rotating and back to counter-rotating flow relative to the main swirling flow. Up to four transitions in direction of rotation have been recorded with a two-component laser-Doppler velocimeter downstream of the swirl generating fin. The Reynolds number is varied from 25 000 to 85 000 and the average swirl number varies from 0.5 to 0.25 downstream of the fin. The underlying cause of the periodicity in the direction of rotation of flow in the core region is conjectured to be based on the secondary flows generated by a pair of co-rotating helical vortices forming upstream at the spiral-shaped swirl generating fin.

Enhanced diffusion from a continuous point source in shallow free-surface flow with grid turbulence

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinder shoulders, and in the middle of the gap between cylinders. For the base case without grid turbulence, spreading rates agreed well with turbulent diffusion coefficients evaluated by others for wide channel flows. Eddy diffusivities for injections with grid turbulence were an order of magnitude greater than the base case; injections at the grid had larger diffusivities than for the injection downstream of the grid. For all experiments, the eddy diffusivity is shown to agree with Taylor’s theorem and to correlate with the size and turbulent intensity of the largest-scale coherent structures.

Wake-boundary layer interaction on the vaned diffuser of a centrifugal stage

An experimental investigation has been carried out by means of a two-component laser Doppler velocimeter (LDV) to survey the unsteady flow entering in the vaned diffuser of a centrifugal stage and to analyse into details the vane boundary layer development perturbed by the rotor flow. Instantaneous velocity data have been acquired in phase with the rotor blade passage and have been processed in ensemble-averaged form to separate periodic and random velocity fluctuations. Periodic incidence variations cause boundary layer transient separations near the vane leading edge, which alternate with opposition of phase on the two sides of the diffuser vane. Owing to the high turbulence energy conveyed by the wakes within the diffuser, which penetrates into the boundary layers, the periodic separation bubbles remain very short and the separated shear layers reattach early, giving rise to turbulent boundary layers capable of withstanding the adverse pressure gradients imposed by the diffuser.

Turbulent mixed convection flow over a forward-facing step—the effect of step heights

Measurements of heat transfer and fluid flow of turbulent mixed convection boundary-layer air flow over an isothermal two-dimensional, vertical forward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously by using, respectively, a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer. The present study examines the effect of forward-facing step heights on turbulent mixed convection flow along a vertical flat plate. The experiment was carried out for step heights of 0, 11, and 22 mm, at a free stream air velocity, u ∞ , of 0.55 m⋅s −1, and a temperature difference, Δ T, of 30 °C between the heated walls and the free stream air (corresponding to a local Grashof number Gr x i = 6.45 × 10 10 ). It was found that the turbulence intensity of the streamwise and transverse velocity fluctuations and the intensity of temperature fluctuations downstream of the step increase as the step height increases. Also, it was found that both the reattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height.

Investigation of Stator-Rotor Interaction in a Transonic Turbine Stage Using Laser Doppler Velocimetry and Pneumatic Probes

The current paper presents steady and unsteady flow data of a transonic test turbine stage operating under flow conditions similar to modern highly loaded gas turbines. Measurements were performed between stator and rotor as well as downstream of the rotor in planes perpendicular to the rotor axis. Time-resolved axial and tangential velocities were measured by a two-component laser doppler velocimeter (LDV) to investigate unsteady phenomena, while time-averaged flow properties were measured by means of a pneumatic seven-hole probe for all three spatial directions. The time-resolved investigation done by LDV allows to present velocity fields, flow angles and turbulence data at different stator-rotor positions during one blade passing period. Averaging these results enabled comparison with the pneumatic multihole probe measurement. LDV data and stage geometry can be obtained per email request and used for computational fluid dynamics (CFD) code verification.

EFFECTS OF FREE STREAM VELOCITY ON TURBULENT NATURAL CONVECTION FLOW OVER A VERTICAL FORWARD-FACING STEP

Measurements of heat transfer and fluid flow of turbulent boundary-layer air flow in natural and mixed convection over an isothermal two-dimensional, vertical forward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously using a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer, respectively. The present study treats buoyancy-dominated mixed convection over a vertical forward-facing step and examines the effect of a small free stream velocity on turbulent natural convection. The experiment was carried out for a step height of 22 mm, for a range of free stream air velocities 0 m/s ⋚ u∞ ⋚ 0.55 m/s (corresponding to a range of Reynolds numbers of 0 ⋚ Re\\abinf{s} ⋚ 712), and a temperature difference, ΔT, of 30°C between the heated walls and the free stream air (corresponding to a local Grashof number Grxi = 6.45 × 1010). It was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the small free stream velocity increases.

Flow and mixing characteristics of swirling wakes in blockage-effect regime

The flow structures and turbulence properties of swirling bluff-body wakes are studied via the velocity fields measured by a two-component laser Doppler velocimeter. The velocity vectors, streamline patterns, vorticities, velocity distributions, and turbulence properties are presented and discussed. The streamline patterns show two characteristic flow modes, single bubble and dual rings, in the bluff-body effective regime. The single bubble is a closed recirculation bubble with a stagnation point on the central axis. Unsteady large-structure flow motions in the bubble result in high turbulence intensities. The dual-ring flow structure is an open-top recirculation zone, in which a pair of large-vorticity-containing counter-rotating vortex rings exists. The dual rings induce a central swirling jet to stabilize on the circular disc and expel the flows in the dual rings to the downstream area. In order to obtain and compare the mixing capabilities of the single-bubble and dual-ring flow structures, the tracer gas is released into different regions of the wake, then the concentration distributions are measured. Both the flow characteristics and tracer-gas concentration distributions are analyzed and logically argued to elucidate the influences of the flow patterns and turbulence properties on the mixing capability.

Turbulent Boundary Layers Over Surfaces Smoothed by Sanding

Flat-plate turbulent boundary layer measurements have been made on painted surfaces, smoothed by sanding. The measurements were conducted in a closed return water tunnel, over a momentum thickness Reynolds number Reθ range of 3000 to 16,000, using a two-component laser Doppler velocimeter (LDV). The mean velocity and Reynolds stress profiles are compared with those for smooth and sandgrain rough walls. The results indicate an increase in the boundary layer thickness (δ) and the integral length scales for the unsanded, painted surface compared to a smooth wall. More significant increases in these parameters, as well as the skin-friction coefficient Cf were observed for the sandgrain surfaces. The sanded surfaces behave similarly to the smooth wall for these boundary layer parameters. The roughness functions ΔU+ for the sanded surfaces measured in this study agree within their uncertainty with previous results obtained using towing tank tests and similarity law analysis. The present results indicate that the mean profiles for all of the surfaces collapse well in velocity defect form. The Reynolds stresses also show good collapse in the overlap and outer regions of the boundary layer when normalized with the wall shear stress.

Turbulence quantification and sediment resuspension in an oscillating grid chamber

An oscillating grid chamber has been developed to study sediment suspension, desorption of compounds from the resuspended sediment, and air–water mass transfer. The chamber is designed to allow researchers to study desorption of contaminants from cohesive sediments and the flux of those contaminants to the vapor phase. The chamber uses a single vertically oscillating grid driven by a DC motor and closed-loop controller. Sediment to be studied is placed in the bottom of the chamber and entrained into the water column by the turbulence generated by the oscillating grid. A two-component laser Doppler velocimeter (LDV) was used to measure the turbulent velocity field inside the chamber. Detailed mapping of the turbulent kinetic energy (TKE) produced by this grid arrangement was compared with established grid-stirred systems. At distances closer to the grid than two grid bar spacings, large lateral gradients exist in the TKE. The suspension of cohesive sediments was also studied using this chamber. Steady-state suspended sediment concentrations were achieved within 30 min for a variety of turbulence levels. By adjusting the grid operating parameters, the TKE can be set to simulate the turbulence found either at the bed or free surface in open-channel flow systems. With some care, the oscillating grid chamber can be used as a simple laboratory analogue to study various environmental processes within the flow or at either the sediment–water or air–water interface.

Measurements in three-dimensional laminar separated flow

Velocity measurements are reported for three-dimensional laminar separated airflow adjacent to a backward-facing step using two-component laser Doppler velocimeter. The backward-facing step, with a height of S=1.0 cm, is mounted in a rectangular duct that has an upstream height of h=0.98 cm, downstream height of H=2 cm, and a width of W=8 cm. This geometry provides an aspect ratio of AR=8 and an expansion ratio of ER=2.02. The flow measurements covered a Reynolds number range between 98.5⩽ Re⩽525. Measurements of velocity distributions reveal that a swirling “jet-like” flow develops near the sidewall in the separating shear layer, and the impingement of that flow on the stepped wall causes a minimum to develop in the spanwise distribution of the reattachment region. Reverse and recirculation flow regions develop adjacent to both the sidewall and the step, and these regions increase in size as the Reynolds number increases. Velocity distributions that were measured at various planes downstream from the step are presented, and predictions compare favorably with these measurements. The results show some interesting flow behaviors that could not be deduced from two-dimensional studies.

Axisymmetric Swirling Vortical Wakes Modulated by a Control Disk

The effects of an above-exit circular disk on the flow structures, turbulence properties, and mixing characteristics of swirling jets separated by a large diameter blockage disk at jet exit are studied by using the smoke-wire flow-visualization technique and a two-component laser Doppler velocimeter. A carbon-dioxide tracer-gas experiment is conducted to examine the mixing capability of the flows modulated by the control disk. The smoke-streak flow patterns, characteristic lengths, velocity vectors, streamlines, velocity distributions, turbulence properties, and concentration distributions of the carbon dioxide are presented and discussed. Topological techniques are applied to analyze and interpret the flow patterns. With the variation of the distance between the control and blockage disks and the central jet Reynolds number, the bubble structures in the near field display complex behaviors of extension, escape, and encapsulation. Under the influence of the control disk, the central jet deflects radially, merges with the swirling annular flow, then turns downstream to form a recirculation bubble in the control-disk wake

Wake flow in adverse pressure gradient

In the interest of improving the predictability of high-lift systems at maximum lift conditions, a series of fundamental experiments were conducted to study the effects of adverse pressure gradient on a wake flow. Mean and fluctuating velocities were measured with a two-component laser-Doppler velocimeter. Data were obtained for several cases of adverse pressure gradient, producing flows ranging from no reversed flow to massively reversed flow. While the turbulent Reynolds stresses increase with increasing size of the reversed flow region, the gradient of Reynolds stress (− ∂ uv/ ∂y) does not. Computations using various turbulence models were unable to reproduce the reversed flow.