Fiber-optic Laser Doppler Anemometer Research Articles

Experimental and Numerical Investigation of Pre-Breaking and Breaking Vorticity within a Plunging Breaker

The characteristics of vorticity induced just prior and at the splash-down of a plunging breaker on a 1:10 planar slope have been studied using wave flume experiments and numerical simulations. Laboratory experiments involved detailed measurements in the outer surf zone of both fluid velocities below trough level, achieved by a fibre-optic laser-Doppler anemometer, and water surface elevations, obtained by an ultrasound probe. A Weakly-Compressible Smoothed Particle (WCSPH) model, coupled with a two-equation model for turbulent stresses, has been employed for the numerical simulations. A thorough calibration of the SPH’s numerical parameters has been first performed, through comparison between numerical and experimental wave elevation and velocity data. Then, considering that time-averaged laboratory data shows a significant vorticity beneath the free surface in the wave pre-breaking region, the vorticity generation mechanism has been thoroughly analyzed by means of the numerical model. In the attempt of explaining the generation of vorticity as induced by flow separation, we also inspected the role of the streamwise flow deceleration and surface-parallel vorticity flux. In analogy with the case of spilling breakers a cause-effect relation has been observed between streamwise flow deceleration and vorticity generation. Numerical findings are positively confirmed by the experimental results.

Performance Characteristics of Personal Samplers in Still and Moving Air

Personal samplers or in general blunt body samplers are widely used in occupational hygiene for collecting air contaminants in the work environments. This work is part of an ongoing research into the performance evaluation of personal samplers, particularly in terms of their aerodynamic properties. Velocity profiles have been measured around and within typical cylindrical sampling devices, placed inside of a wind tunnel. A two-component fibre optic laser Doppler anemometer (LDA) is employed to measure the flow field around as well as within the samplers under the test. A variety of conditions, i.e., suction flux, wind speed and orifice size, have been examined. Extensive tests have been carried out in order to verify the reproducibility and reliability of measurements. The results show that reproducibility of the measurements at upstream of samplers are within 1% of the mean velocity. However, analysis of extensive data revealed the presence of noticeable electrostatic effect within and around the sampling device. Attempts were made to eliminate the presence of the electrostatic effect by, for example, spraying the nonconductive sampler with an anti electrostatic liquid, and wire earthing of a sampler made from cooper, but unfortunately these measures were found to be fruitless in eliminating the electrostatic effects. It should nevertheless, be said that the observed velocity is found to be more stable for small inlet orifice size of 4 mm and high suction flux of 3 l/min and this may be because the charged particles have less time to interact with the field produced by the sampler.

A single-headed fibre optic laser Doppler anemometer probe for the measurement of flowangles

A single-headed fibre optic laser Doppler anemometer capable of measuring the angle of aflow is presented. The instrument relies upon the use of birefringent optical fibre and isconfigured to measure the three orthogonal velocity components, from which the flow anglesare calculated, using two Doppler difference channels and one reference beam channel. Thedesign of the probe head permits accurate measurement of the Cartesian velocity components,enabling measurement of the two angles used to define the flow with typical errors of ± 1° and ± 3°.

Effects of estrogen on the microcirculation and thrombus formation in pial vessels of the rat

Female Wistar/ST rats were ovariectomized at 14 weeks of age. Ethynylestradiol, 17β-estradiol (E 2β), medroxyprogesterone acetate (MPA) and 4-hydroxy-tamoxifen were administered subcutaneously every other day for 3 weeks from 1 week after ovariectomy. Closed cranial windows were created and the thrombotic potential was measured using the He–Ne laser-induced thrombosis method. The mean red cell velocity was measured with a fiber-optic laser-Doppler anemometer microscope. The plasma concentrations of nitrite/nitrate (nitric oxide (NO) metabolites) were determined using the Griess reagent. The diameters mean red cell velocity and blood flow in pial arterioles were reduced after ovariectomy, however, the thrombotic tendency was increased after ovariectomy. These parameters were reversed after E 2β treatment. In contrast, MPA had the opposite effects. The plasma concentrations of nitrite/nitrate were significantly decreased following ovariectomy and were increased after treatment with E 2β. MPA did not affect the concentration of NO metabolites. These results strongly indicated that estrogen in the female rat mediated beneficial effects on the cerebral microcirculation and moderated cerebral thrombotic mechanisms by enhancing the plasma levels of NO. The findings suggest that increased blood flow and inhibition of thrombosis might contribute to the prevention of cerebrovascular disease in a normal female.

Laser-Doppler measurements of velocities just downstream of a collapsible tube during flow-induced oscillations.

The flow field less than one diameter downstream of the end of a collapsible tube executing self excited oscillations was examined using a two-component fiber-optic laser-Doppler anemometer. The time-averaged Reynolds number of the flow was 11,000. With the tube oscillating periodically, results obtained during many cycles of oscillation were combined to yield surface plots of the axial component over the cross section at 16 phases of the cycle. By combining measurements obtained with the laser probe in two different orientations, secondary flow vectors over the cross section were likewise constructed for 16 phases. The measurements showed strongly phasic turbulence intensity, with the phase of high intensity coinciding with the time of maximal tube collapse. Reverse flow occurred during much of the cycle, at places in the cross section that agree with our previous observations of laminar and turbulent steady flow through a rigid simulated collapsed tube.

AN EXPERIMENTAL STUDY OF DISPERSED LIQUID/LIQUID TWO-PHASE UPFLOW IN A PIPE

This paper presents experimental data for dispersed liquid/liquid upflows. Water was the continuous phase and mineral oil was the dispersed droplet phase. For this flow regime reduced gravity bubbly flow phenomena was simulated because the mineral oil and water had almost the same density. The mean velocity and turbulence fields, the size distributions of the oil droplets, the volume fraction, and interfacial area density distribution were measured using fiber optic Laser Doppler Anemometer (LDA) and phase Doppler Anemometer (PDA) systems. Significantly, the results presented in this paper are similar to those for bubbly air/water flows in microgravity conditions (Kamp el at., 1995).

Maximal flow pulsation in the pial arterioles of rats at increased intracranial pressure.

Red cell velocity in the brain pial arterioles ( 12-58 microm in diameter) of rats was measured by a fiber-optic laser-Doppler anemometer microscope to study the effects of intracranial pressure (ICP) on flow pulsation. The amplitude of velocity pulsation in the pial arterioles was 24+/-9% (mean +/- SD) of the temporal mean velocity under normal physiological conditions (ICP = 5 mmHg). It increased gradually up to 40+/-11% with increasing ICP up to 50 mmHg on average. It also increased with the vessel diameter at fixed values of ICP. In each arteriole, the velocity amplitude increased steeply at a critical value of ICP as ICP increased. The critical value of ICP ranged between 20 and 40 mmHg,. increased with the vessel diameter, and decreased downstream along arterial trees. These results suggest a correlation between the critical ICP and the internal pressure of the pial arteriole.

TURBULENT STRUCTURE AND BED SHEAR STRESS NEAR THE JUNCTION BETWEEN MAIN-CHANNEL AND FLOOD-PLAIN IN SHALLOWER COMPOUND OPEN-CHANNEL FLOWS

In compound open-channel flows, it is very important to clarify the interaction between main-channel and flood-plain induced by coherent vortices, so-called the secondary currents and “large-scale horizontal vortices”. These turbulent structures and coherent vortices are characterized by high shear layers generated by the shear instability between main-channel and flood-plain flows. In this study, secondary currents and three-dimensional turbulent structures were experimentally revealed on shallower floodplain by making use of a two-component fiber-optic laser Doppler anemometer (FLDA). Next, the bed shear stress on shallower flood-plain was evaluated from the accurate measured velocity profile in the viscous sublayer by making use of one-component fiber-optic LDA.

ANALYSIS OF THE PROCESSES OF PRODUCTION AND TRANSPOTATION OF TURBULENCE IN CAVITY OPEN-CHANNEL FLOWS BY DNS

The processes of the production of turbulence statistics in cavity open-channel flows are investigated using the numerical flow field which is computed by Direct Numerical Simulation (DNS). The reliability of the results of DNS is confirmed by comparison which the results of turbulence measurements conducted by making use of a two component fiber-optic Laser Doppler Anemometer (LDA). The results indicate that turbulence structures in cavity open-channel flows are classified into six regions and these structures can not be reproduced by standard k-e turbulence model, but Reynolds Stress Model (RSM) has the ability to do them.

Multimode fiber optic laser Doppler anemometer with side-projected catheter

A new side-projected catheter for in vivo blood flow measure- ments has been developed and tested. This catheter is designed to mea- sure blood flow in both the forward (towards the catheter tip) and reverse (away from the catheter tip) flow directions. It consists of two multimode optical fibres with core diameter of 50mm and cladding diameter of 125 mm. One fibre transmits the laser beam into blood and the other receives the back-scattered light from the erythrocytes within the probe volume. The experimental plots of the dominant frequency (frequency with the highest amplitude) against the flow velocity are found to be linear in both the forward and reverse flow directions. This result was reaffirmed by the numerical flow simulation along the fibre catheter side wall. In the simu- lation, it was found that the flow within the boundary layer is indeed laminar, and hence the relationship between the Doppler shift frequen- cies and the flow velocities is linear, thereby making the linear calibration possible for predicting the free stream flow velocity. The analysis of the experimental and the computational results indicated that the boundary layer along the side wall of the fibre catheter varies with both the flow velocity and flow direction. Furthermore, using the fibre optic catheter with an increasing fibre core separation, it was observed experimentally and numerically that the percentage of the free stream flow velocity in- creases, suggesting that the projection of the measuring probe volume is indeed moving towards the free stream domain. © 1998 Society of Photo- Optical Instrumentation Engineers. (S0091-3286(98)02106-0)

Confined Coaxial Nozzle Flow with Central Lobed Mixer at Different Velocity Ratios

Measurements of turbulent, confined, coaxial nozzle flows with a central four-lobe forced mixer have been obtained by a two-component fibre-optic laser-Doppler anemometer at a Reynolds number of 5.1 x 104 (based on the baseline circular nozzle diameter, Dj = 30 mm, and bulk mean velocity, Ur> of the two streams at 1.7 m/s). Ratios of the annular mean to the core mean velocity (X) were kept at 0.4, 1.1 and 2.0 respectively. Results showed that the mixing processes were greatly enhanced by the formation and interaction of the normal and streamwise vorticity. In particular, the deformation and the subsequent stretching of the normal vortices shed at the nozzle walls by the streamwise vortices played an crucial role in the mixing processes. The higher speed stream appeared to determine the form of mixing downstream. At X = 2.0 and 1.1, the generation of the streamwise vorticity was found to originate from two sources: the geometry of the lobe and the gap between the lobe nozzle peaks and the inner annular wall. The former was important to enhance the mixing between the two co-flowing streams at both velocity ratios while the latter was more important for X, = 2.0 to the mixing between the annular flow and the surrounding still fluid. At X = 0.4, only the geometry of the lobe shed streamwise vorticity which was confined radially within each lobe as the flow traveled downstream. Mixing between the core and the surrounding fluid was better than that between the two co-flowing streams. For all the cases considered, streamwise vortices were dissipated within dDj from the nozzle exit plane. NOMENCLATURE

NUMERICAL SIMULATION ON TURBULENT STRUCTURES IN CAVITY FLOWS BY MEANS OF LARGE EDDY SIMULATION

This paper describes the numerical simulation results of cavity in open-channel flows by means of Large Eddy Simulation (LES). The comparison, between numerical simulation and turbulence measurements conducted by making use of a two component fiber-optic laser Doppler anemometer (FLDA), was carried out in two cases. The results indicate that LES can reproduce the variations of mean velocities and turbulent statistics by trench length changes, and these variations may be relevant to the cycle of separated vortices and the position and size of the large circulating vortex.

133 PIVを用いた開水路キャビティー流れの解析

This paper describes the results of the flow visualization of cavity in open-channel flows by means of a Particle Image Velocimetry(PIV). The comparison of mean values, between the results of flow visualization and the high-accurate turbulence measurements conducted by making use of a two component fiber-optic laser Doppler anemometer (LDA) was carried out intensively, and the comparison of coherent structures, between the PIV results and the results of Large Eddy simulation(LES) was also conducted. The coherent structures were investigated in mixing layer by time-space correlation analysis. As the result, the characteristics and effects of coherent structures were revealed and then discussed.

Space-time correlation structures of horizontal coherent vortices in compound open-channel flows by using particle-tracking velocimetry

In compound open-channel flows, it is very important to clarify an interaction mechanism between the mainchannel and flood-plains induced by coherent vortices, so-called the “intermittent upward secondary currents” and the “large-scale horizontal vortices”, as shown in Figure 1. In the present study, time-averaged structures of secondary currents were revealed experimentally by making use of a two-component fiber-optic Laser Doppler Anemometer (LDA). Next, a new quantitative visualization technique called the “Particle-Tracking Velocimetry (PTV)” was used to investigate evolutionary patterns of horizontal large-scale vortices, which are the ultimate interest in a shallow water flood-plain. By this technique, instantaneous velocity components in arbitrary horizontal plane were obtained at all grid points of laser-light-sheet (LLS) from continuous four images of one particle, and space-time correlation structures of coherent vortices between the main-channel and flood-plain were investigated by using a conditional sampling technique.

Experimental Study on Turbulent Structures of Partly Vegetated Open-channel Flows

Vegetation in rivers decreases the flow capacity and therefore has been removed from river basin. However, the importance of vegetated zone in rivers has recently been recognized not only for river management but also for water flow environment. Turbulent structures of vegetated open-channel flows and the associated sediment transport are investigated by some researchers.In this study, turbulent measurements in partly vegetated open-channel flows were accurately conducted by using a fiber-optic laser Doppler anemometer (FLDA). Basic turbulent structures such as mean velocity and turbulence were experimentally obtained and discussed as comparing compound open-channel flows.

Enhanced thrombogenicity and altered hemodynamics in the cerebral microvasculature of stroke-prone spontaneously hypertensive rats.

Thrombotic potential and hemodynamic changes were assessed in the cerebral microcirculation in normal rats (WKY), non-stroke-prone spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHRSP) at the age of 4, 16 and 32 weeks. Whole blood platelet aggregation revealed that the platelet aggregability in vitro was significantly depressed in SHRSP compared to WKY at 16 and 32 weeks using 2 microg/ml collagen but was similar or higher than WKY at 32 weeks using 20 or 40 microg/ml collagen. Platelet-rich thrombi were induced using a helium-neon (He-Ne) laser technique in vivo. The numbers of laser pulses required to induce an occlusive thrombus in arterioles were similar in the three strains at the age of 4 weeks. In contrast, the numbers of laser pulses needed to induce vascular occlusion in SHR (5.5 +/- 0.7; n = 4) and SHRSP (4.9 +/- 0.3; n = 4) were lower than in WKY (7.4 +/- 0.3; n = 5) at the age of 16 weeks. Similar differences were observed at 32 weeks (SHR 5.8 +/- 0.2; n = 6; SHRSP 4.3 +/- 0.1; n = 4; WKY 7.0 +/- 0.2; n = 7). Red blood cell velocities were measured in pial arterioles using a fiber-optic laser Doppler anemometer microscope. Red cell velocities and wall shear rates in SHR and SHRSP were significantly lower than those in WKY (p < 0.05) at the age of 16 weeks and were markedly lower in SHRSP than in either WKY or SHR at the age of 32 weeks. The plasma concentration of nitrite/nitrate determined by the Griess reaction was significantly reduced in SHRSP at 32 weeks compared with 4-week-old rats. These changes could contribute to the enhanced tendency to cerebrovascular stroke in SHRSP.

Fiberoptic laser-doppler anemometer microscope applied to the cerebral microcirculation in rats

We have applied our developed fiber-optic laser-Doppler anemometer microscope (FLDAM) for the study of the cerebral microcirculation in the rat. The red cell velocity in single pial microvessels was successfully measured through a closed cranial window for the vessel diameter range from 7.8 to 230 μm. The temporal resolution of the FLDAM was sufficiently high to detect the pulsation in the arterioles. Arterio-venous distributions of the temporal mean red cell velocity and wall shear rate are also described.

Towards an expert system for estimating wind loads on building attachments using detailed local velocity data

Abstract This paper presents a new working method for predicting design wind loads on attachments projecting from the walls of buildings. In the University of Oxford environmental wind tunnel, a two-component fibre-optic laser-Doppler anemometer has been used in a systematic study of highly turbulent wind flows at various locations close to walls and roofs of buildings. This includes instantaneous vertical and horizontal velocity components in planes normal to windward-facing walls. These have been analysed to start a database of local extreme gust vectors. Using these gust vectors, wind loads can be calculated at a measurement location for any object which has an appropriately defined aerodynamic force polar. However, the associated statistical analysis of extreme values is not trivial, and the paper provides a rigorous justification of the practical calculation steps required.

3-Dimensional Structure for Overbank Flow in Meandering Channels

3-dimensional flow structure in meandering channels during flood is discussed based on the velocity measurements results by 2-component fiber optic laser Doppler anemometer. Growth and decay processes of secondary flow cells in meandering channels and their differences between before and alter inundation are particularly highlighted. Interaction between the lower layer flow (the main channel), and the upper layer flow (the flood plain) is also stressed. The matters pointed out in the paper are: closely related to the energy loss mechanism in compound meandering flows and should therefore properly be estimated in mathematical or numerical methods for engineering purposes.

Turbulent Structure and Generation Mechanism of Coherent Vortex in Cavity Shear Layer of Open-channel Flows

Turbulence measurements in and over the cavity of open-channel flows were conducted by making use of a two component fiber-optic laser Doppler anemometer (FLDA). Mean Structures in the cavity shear layer were examined in detail and it was revealed that the turbulent structures depended on the length of cavity significantly.The visualization of coherent vortices was carried out by the use of dye injection techniques, and the characteristics of coherent and unstable vortices around separation point were clarified by means of unstable analysis. The fundamental frequency was determined and unstability of coherent vortices was examined.