Laser Doppler Anemometer Research Articles

On the vorticity characteristics of lobe-forced mixer at different configurations

Lobe-forced mixer is one typical example of the passive flow controllers owing to its corrugated trailing edge. Besides the spanwise Kelvin–Helmholtz vortex shedding, streamwise vortices are also generated within its mixing layer. The geometrical configuration of the lobe significantly affects these two types of vortices, which in turn affects the mixing performance of the mixer. In the present investigation, characteristics of mixers with five different configurations were examined and evaluated for two velocity ratios (r = 1, 0.4). The mixers have only one lobe in order to eliminate any possible interactions between neighboring vortices generated by the adjacent lobes. Hot-wire anemometer was used to examine the Kelvin–Helmholtz vortices via the spectrum analysis while laser Doppler anemometer was employed to examine the streamwise vortices. It was found that there were two main frequencies for the Kelvin–Helmholtz vortices in the wake of the mixer; and the Strouhal numbers approached their respective maximum values at high Reynolds number. The rectangular mixer had similar mixing performance with the semicircular one; and both of them were better than the triangular mixer. The scalloping modification enhanced mixing by generating additional streamwise vortices while the scarfing modification could not improve the mixing performance.

Effect of Reynolds number, near-wall perturbation and turbulence on smooth open-channel flows

Three series of experiments were conducted to study the effect of Reynolds number, near-wall perturbation and turbulence on the velocity characteristics of smooth open-channel flows. Measurements were carried out using a laser Doppler anemometer. The variables of interest include mean velocity, turbulence, velocity probability density functions and Gram-Charlier series coefficients. For the range of depth Reynolds number studied (23×103 < Re h <72×103), the turbulence intensity and Reynolds shear stress profiles show that the effect of Reynolds number can be significant in open-channel flows. However, at identical distances from the bed, the velocity probability density functions are relatively insensitive to Reynolds number effects. The coefficients of the Gram-Charlier series expansion are also independent of Reynolds number for wall normal distances y+ <300. In the case of the flow with the near-wall perturbation, the mean velocity profile at the farthest downstream station more-or-less recovers to the undisturbed state, whereas the turbulence intensity profiles do not completely recover. From the various profiles obtained downstream of the perturbation, the wall-normal distance corresponding to 0.01<y/h<0.1 requires a greater longitudinal distance to recover to the undisturbed state. As noticed from the velocity defect profiles, the effects of near-wall disturbance begin to penetrate into the flow with increasing distance from the perturbation. The presence of higher levels of turbulence influences the skin friction coefficient, the extent of collapse with the log–law and the wake parameter.

The effect of floodplain roughness on flow structures, bedforms and sediment transport rates in meandering channels with overbank flows: Part I

Studies were carried out to understand the effect of floodplain roughness on flow structures, sediment transport rates and bedforms in a mobile meandering channel with overbank flows. Three floodplain roughnesses were examined in this study. Flow structures and bedforms were measured using a three component laser Doppler anemometer (LDA) system and digital photogrammetry, respectively. Comparisons of flow structures and bedforms between different floodplain rougheness are made. Considerable changes in the flow structure and bed form were observed. In particular, multiple secondary flow cells along the meandering channel occur at deeper water depths as the floodplain roughness increases. These cells also cause a series of wavy bedforms along the meandering channel. This paper is separated into two parts. Part I concentrates, in detail, on the flow structure with bedforms in the meandering channel for overbank flow as to change of floodplain roughness. The sister paper, Part II, concentrates on bed form formation during flood, sediment transport rates and flow resistance in the main channel and floodplain.

Effects of varying submergence and channel width on local scour by plane turbulent wall jets

An experimental program was carried out to understand the role of channel width and varying tailwater depth on the scour caused by a plane wall jet. To this end, four groups of tests, denoted as A, B, C and D, were conducted. In group A, during each test the tailwater depth was gradually increased from 2bo to 20bo in a systematic manner. Here, bo is the nozzle height at the exit. In group B, the tailwater depth was gradually decreased from 20bo to 2bo. Group C tests were conducted at fixed tailwater depths based on inferences made from groups A and B tests but correspond to low tailwater conditions. Group D tests were conducted at fixed tailwater conditions corresponding to a large submergence.Within each group, the jet exit velocity was varied from 0.75 to 1.16 m/s while the channel width was varied from 100 to 400 mm. A two-dimensional laser Doppler anemometer was used to characterize the velocity field. Present observations indicate that the velocity-time history can be used to characterize various regimes of flow. Distinct velocity signature patterns have been identified for the digging and refilling phases. These patterns also serve to distinguish between high and low submergence flow. The complete absence of the refilling phase indicates high submergence flow. Most previous studies made the distinction between high and low submergence based on the tailwater ratio. However, the present results show that the distinction also depends on the jet exit velocity. Though the channel width does not have a significant effect on distinguishing the flow as high or low submergence, the scour pattern is affected by the channel width. Narrower channels tend to provide for a two-dimensional (2D) scour pattern in a nominally 2D flow, whereas, wider channels provide for a three-dimensional mound region. These aspects should be borne in mind while conducting future scour experiments and in developing empirical equations that are commonly used to describe scour geometry.

All semiconductor laser Doppler anemometer at 155 μm

We report to our best knowledge the first all semiconductor Laser Doppler Anemometer (LIDAR) for wind speed determination. We will present the design and first experimental results on a focusing coherent cw laser Doppler anemometer for measuring atmospheric wind velocities in the 10 meters to 300 meters distance range. Especially, we will demonstrate that both the output power as well as the demanding coherence properties required from the laser source can be accomplished by an all semiconductor laser. Preliminary tests at a distance of 40 meters indicate a typical signal to noise ratio of 9 dB. This result is obtained at a clear day with an up-date rate of 12 Hz.

Control of Post-stall Airfoil AerodynamicsBased on Surface Perturbation

This paper presents an experimental investigation on the control of airfoil aerodynamics at poststall angle of attack α using a surface perturbation technique. Piezoceramic actuators were deployed to create a local surface perturbation on a NACA0012 airfoil, along with an open-loop control system, to manipulate flow around the airfoil. Two different control signals were examined, i.e., square and sine waves. Whereas the lift and drag forces were measured using a load cell, the flow was documented using a particle image velocimetry, laser Doppler anemometer and a single hot wire. The surface perturbation significantly improved the airfoil aerodynamics for 12 ≤ α ≤ 20 deg. The control effect with the square-wave excitation was found to be much more effective than that with the sine wave. The control was most effective at α = 14 deg: the mean lift coefficient, lift-to-drag ratio, and figure of merit (i.e., the ratio of the power to aerodynamic efficiencies) were enhanced by 35, 64, and 44%, respectively, whereas the mean drag coefficient dropped by 23%. Furthermore, the airfoil stall was postponed by 3 deg. The physics behind the observations were discussed in detail.

Physical Modeling of Flow Field inside Urban Street Canyons

Abstract The flow characteristics inside urban street canyons were studied in a laboratory water channel. The approaching flow direction was horizontal and perpendicular to the street axis. The street width was adjusted to form street canyons of aspect ratios 0.5, 1.0, and 2.0. The velocity field and turbulent intensity were measured with a laser Doppler anemometer at various locations within the street canyons, which were used to elucidate the flow pattern inside the street canyons. It was found that the previous numerical modeling results are in good agreement with the current experimental results at most locations. For the street canyon of aspect ratio 0.5, which belongs to the wake interference flow regime, the mean and fluctuating velocity components were more difficult to measure as compared with the other two cases because of its more complicated flow pattern. Some guidelines for numerical modeling were developed based on the measurement results. The data presented in this paper can also be used as a comprehensive database for the validation of numerical models.

Tailwater effects on the characteristics of a square jet near a free-surface

Characteristics of a square jet near a free surface were experimentally studied at four different tailwater depths. Velocity measurements were made with a two-component laser Doppler anemometer at five stations along the jet streamwise axis. A qualitative assessment was made of the location of jet impingement on the free surface. The results reveal that the tailwater depth has little influence on the mean velocity distribution beyond a certain depth. Turbulence intensities and shear stress profiles indicate jet anisotropy and higher spread rates than an axi-symmetric free jet. The analysis of third-order moments divulged a marked increase in the turbulent ejection and entrainment events for the jet as the tailwater depth increased and the effect of free surface confinement was reduced. A quadrant analysis confirmed that the confinement effect due to the proximity of the free surface acted to curtail the ability of the jet to eject or entrain fluid.

An experimental study of pulsating turbulent flow in a pipe

An experimental study of pulsating turbulent flow in a pipe is reported in which measurements of instantaneous velocity were made using a two-component Laser Doppler Anemometer system. Local values of ensemble-averaged axial velocity, and radial and axial components of root-mean-square turbulent velocity fluctuation were obtained from the measurements. The frequency of the imposed pulsation of flow rate was varied systematically over a wide range covering inner scale dimensionless frequency ω ν / u τ 2 from 0.004 to 0.04. In terms of outer scale frequency ω D / u τ the corresponding values varied from 1.8 to 18. In addition, effects of changing the mean flow rate and the amplitude of flow rate pulsation were studied. Radial distributions of the amplitude of the modulation of ensemble-averaged axial velocity and the axial and radial components of RMS turbulent fluctuation, and their phase shifts relative to the imposed flow pulsation, are presented for conditions which include the low, intermediate and high frequency ranges. These add to and reinforce the body of information available from earlier experimental work and have enabled useful progress to be made in evaluating and validating approaches used for correlating such data. By relating observed behaviour to the fundamental processes of turbulence production, redistribution of turbulence energy between its components and radial propagation of turbulence, a good understanding of the results has been obtained.

Experimental study of the flow field with spilling type breaking

This paper shows the results of an experimental work on the spilling type breaking flow field produced by a hydrofoil positioned in a uniform current. From the examination of the Laser Doppler Anemometer results downstream of the hydrofoil a flow field similar to a mixing layer, close to the breaking zone, and a flow field similar to a wake were found. The conclusions that can be drawn by the studies conducted can be summarized as follows: (i) downstream of the hydrofoil it is possible to distinguish two regions of flow, namely one immediately downstream of the hydrofoil, including spilling characterized by a mixing of water and air, and one downstream of the same spilling; (ii) in first of the aforementioned regions the flow is similar to that of a mixing layer, while, in the downstream region, it shows conditions similar to a wake; (iii) the experiments furnish information on the turbulence characteristics in the two regions, and extract the zone where the wake effect can be considered negligible; (iv) the time-averaged free surface height presents a decreasing trend from the wake origin.

Wind velocity measurements using a pulsed LIDAR system: first results

Wind velocity measurements were taken using a Leosphere Windcube LIDAR system, which operates as a pulsed laser Doppler anemometer. Here we report on first results, which show typical characteristics of atmospheric wind velocities.

Laser measurements of flow over a forest

It is estimated that 20-30% of the total European wind energy growth takes place in areas where the wind flow is affected by forests. The description of the wind conditions near and above forests poses a challenge, since assumptions of classical boundary-layer theory are violated. Turbines are designed for maximal turbulence intensity and wind profile gradient. In forested areas, these limits are often violated possibly leading to reduced turbine life-time. In this paper we investigate the mean wind profile and turbulence statistics above an 85 years old dense beech forest by use of a laser Doppler anemometer and compare the profiles with a CFD model specifically made for the modelling of flow over vegetation canopies.

Laser measurements of flow over a forest

It is estimated that 20-30% of the total European wind energy growth takes place in areas where the wind flow is affected by forests. The description of the wind conditions near and above forests poses a challenge, since assumptions of classical boundary-layer theory are violated. Turbines are designed for maximal turbulence intensity and wind profile gradient. In forested areas, these limits are often violated possibly leading to reduced turbine life-time. In this paper we investigate the mean wind profile and turbulence statistics above an 85 years old dense beech forest by use of a laser Doppler anemometer and compare the profiles with a CFD model specifically made for the modelling of flow over vegetation canopies.

Lois analytiques pour les écoulements en cavité rotor–stator

The evolution of the entrainment coefficient K of the rotating fluid in a rotor-stator cavity with an axial throughflow is studied according to the flow parameters. Following the analysis performed by Schlichting [1] in the single disk case, we determine analytical laws for the prediction of K, which depends in each case on a different local flow rate coefficient. The influence of all parameters (geometry of the cavity, rotation rate, throughflow, prerotation level, rugosity, blades) are investigated. The behavior of the entrainement coefficient is confirmed by extensive measurements realized in water for a Batchelor type of flow with separated boundary layers by the means of a laser Doppler anemometer. The results are also compared to those performed by pressure transducers.

Influence of tailwater depth, sediment size and densimetric Froude number on scour by submerged square wall jets

In this paper, an effort is made to better understand the scour caused by submerged square jets in a cohesionless sand bed. The variables of interest are the densimetric Froude number, the tailwater depth and sediment grain size. Experiments were carried out at three different values of densimetric Froude number, a range of tailwater conditions varying from low to very high submergence, and two different sand bed grain sizes. Velocity measurements were conducted using a laser Doppler anemometer. At a given densimetric Froude number, progressing from the start of the test towards asymptotic conditions, the geometric parameters used to describe scour are found to be sensitive to tailwater conditions and the ratio of the nozzle size-to-grain size. The present results indicate that the effect of nozzle size-to-grain size ratio can be important and needs to be incorporated in the interpretation of scour. The effect is reduced as asymptotic conditions are reached. At a given tailwater depth, the scour geometry tended to increase with increasing densimetric Froude number and the effect was prominent at all tailwater depths. Turbulent bursts have been noted to have an important role in the scour process, and are more distinguishable with the finer bed material. At low values of the relative tailwater depth, the flow and the corresponding scour pattern tends to be nonsymmetrical. Moreover, at low values of tailwater depth and higher densimetric Froude number, the scour pattern is quite different from the other test conditions. Empirical equations describing the scour geometry are proposed for the asymptotic state. These relations are found to be valid for a wide range of test conditions.

Numerical characterisation and experimental validation of AusIron top submerged multi-injection system

The present paper presents a numerical study of the fluid flow structure inside an elliptical liquid bath, agitated by a top submerged gas injection system. Using the Eulerian–Eulerian approach, a three-dimension gas–liquid transient numerical model coupled with the standard k-ϵ model is developed. Various combinations of operating parameters are investigated and the numerical predictions are validated against the laser doppler anemometer (LDA) experimental data. The results indicate that at a higher gas flowrate a better mixing at the bottom of the bath is achieved. On the effect of the lance submergence a deeper submergence level gives highly improved agitation and mixing within the bath. The comparisons of the numerical predictions with experimental data under various operating conditions show a reasonable agreement for both mean velocity and turbulent fluid flow characteristics. Furthermore, the development of a new empirical correlation based on Froude number is presented.

Modeling of the microconvective contribution to wall heat transfer in subcooled boiling flow

In the present work, the two-phase turbulent boundary layer in subcooled boiling flow is investigated. The bubbles in the near-wall region have a significant effect on the dynamics of the underlying liquid flow, as well as on the heat transfer. The present work develops a single-fluid model capable of accounting for the interactions between the bubbles and the liquid phase, such that the two-phase convective contribution to the total wall heat transfer can be described appropriately even in the framework of single-fluid modeling. To this end, subcooled boiling channel flow was experimentally investigated using a laser-Doppler anemometer to gain insight into the bubble-laden near-wall velocity field. It was generally observed that the streamwise velocity component was considerably reduced compared to the single-phase case, while the near-wall turbulence was increased due to the presence of the bubbles. Since the experimentally observed characteristics of the liquid velocity field turned out to be very similar to turbulent flows along rough surfaces, it is proposed to model the near-wall effect of the bubbles on the liquid flow analogously to the effect of a surface roughness. Incorporating the proposed approach as a dynamic boundary condition into a well-established mechanistic flow boiling model makes it possible to reflect adequately the contribution of the microconvection to the total wall heat transfer. A comparison against the experimental data shows good agreement for the predicted wall shear stress as well as for the wall heat flux for a wide range of wall temperatures and Reynolds numbers.

CFD simulation of bubble columns incorporating population balance modeling

A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51, 1311–1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k – ε model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas hold-up, liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485–1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201–9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas–liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071–5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359–6365] also show a good agreement for liquid velocity and gas hold-up profiles.

Laser Doppler Anemometer Measurements of the Flow in the Impeller Blade Channels of a Centrifugal Blood Pump

Hemodynamics in the rotary blood pumps is of great importance for it is highly related to the blood hemolysis and thrombus generation in the blood pumps. In the present study, the flow in the impeller passages of Kyoto-NTN centrifugal blood pump with 16 forward-bending blades was measured using Laser Doppler Anemometer (LDA) on a 1:1 size pump model. The measurement results were compared with those of the numerical simulation study and the measurement results on a 5:1 enlarged pump model. The comparison indicated a reasonable agreement between the results of the present measurement and the numerical simulation. The comparison between the measurement results on the 1:1 and 5:1 enlarged pump models revealed, however, significant differences in the flow patterns. The main cause of the differences between the 1:1 and 5:1 pump model should be due to the incomplete similarity of the later, which has produced 31% higher dimensionless flow rate than the prototype. Therefore, it is important to require more elabor...

Reducing Head-Gimbal-Assembly Windage for High-Track-Density Hard Disk Drives

The windage of a head gimbal assembly (HGA) is the flow-induced vibration affecting head slider positioning in the off-track direction. Reducing the windage is important for increasing the track density of hard disk drives (HDDs). To develop HGAs with a windage smaller than that of conventional HGAs, we examined the characteristics of HGA windage. We used a 12-mm (from the swage hole center to the dimple) HGA that had a maximum windage peak at 11 kHz. We put it in a spin stand and in a HDD and measured its windage with a laser Doppler vibrometer. The windage amplitude correlated well with the mean airflow velocity in the HDD (measured with a laser Doppler anemometer). Windage amplitude was also affected by fluctuation of the airflow velocity. Our results show that HGA windage can be reduced by increasing the distances between the rotating disk and the cover and the base.