Conventional Laser Doppler Velocimetry Research Articles

Thermographic laser Doppler velocimetry using the phase-shifted luminescence of BAM:Eu2+ phosphor particles for thermometry.

Simultaneous point measurements of gas velocity and temperature were recently demonstrated using thermographic phosphors as tracer particles. There, continuous wave (CW) excitation was used and the spectral shift of the luminescence was detected with a two-colour intensity ratio method to determine the gas temperature. The conventional laser Doppler velocimetry (LDV) technique was employed for velocimetry. In this paper, an alternative approach to the gas temperature measurements is presented, which is instead based on the temperature-dependence of the luminescence lifetime. The phase-shift between the luminescence signal and time-modulated excitation light is evaluated for single BaMgAl10O17:Eu2+ phosphor particles as they cross the probe volume. Luminescence lifetimes evaluated in the time domain and frequency domain indicate that in these experiments, interferences from in-phase signals such as stray excitation laser light are negligible. The dependence of the phase-shift on flow temperature is characterised. In the temperature sensitive range above 700 K, precise gas temperature measurements can be obtained (8.6 K at 840 K) with this approach.

Thermographic laser Doppler velocimetry.

We propose a point measurement technique for simultaneous gas temperature and velocity measurement based on thermographic phosphor particles dispersed in the fluid. The flow velocity is determined from the frequency of light scattered by BaMgAl10O17:Eu2+ phosphor particles traversing the fringes like in conventional laser Doppler velocimetry. Flow temperatures are derived using a two-color ratio method applied to the phosphorescence from the same particles. This combined diagnostic technique is demonstrated with a temperature precision of 4%-10% in a heated air jet during steady operation for flow temperatures up to 624 K. The technique provides correlated vector-scalar data at high spatial and temporal resolution.

The mean velocity profile of a smooth-flat-plate turbulent boundary layer at high Reynolds number

Smooth flat-plate turbulent boundary layers (TBLs) have been studied for nearly a century. However, there is a relative dearth of measurements at Reynolds numbers typical of full-scale marine and aerospace transportation systems (Reθ = Ueθ/ν > 105, where Ue = free-stream speed, θ = TBL momentum thickness and ν = kinematic viscosity). This paper presents new experimental results for the TBL that forms on a smooth flat plate at nominal Reθ values of 0.5 × 105, 1.0 × 105 and 1.5 × 105. Nominal boundary layer thicknesses (δ) were 80–90mm, and Karman numbers (δ+) were 17000, 32000 and 47000, respectively. The experiments were conducted in the William B. Morgan Large Cavitation Channel on a polished (k+ < 0.2) flat-plate test model 12.9m long and 3.05m wide at water flow speeds up to 20ms−1. Direct measurements of static pressure and mean wall shear stress were obtained with pressure taps and floating-plate skin friction force balances. The TBL developed a mild favourable pressure gradient that led to a streamwise flow speed increase of ~2.5% over the 11m long test surface, and was consistent with test section sidewall and model surface boundary-layer growth. At each Reθ, mean streamwise velocity profile pairs, separated by 24cm, were measured more than 10m from the model's leading edge using conventional laser Doppler velocimetry. Between these profile pairs, a unique near-wall implementation of particle tracking velocimetry was used to measure the near-wall velocity profile. The composite profile measurements span the wall-normal coordinate range from y+ < 1 to y > 2δ. To within experimental uncertainty, the measured mean velocity profiles can be fit using traditional zero-pressure-gradient (ZPG) TBL asymptotics with some modifications for the mild favourable pressure gradient. The fitted profile pairs satisfy the von-Kármán momentum integral equation to within 1%. However, the profiles reported here show distinct differences from equivalent ZPG profiles. The near-wall indicator function has more prominent extrema, the log-law constants differ slightly, and the profiles' wake component is less pronounced.

Highly spatially resolving laser Doppler velocity measurements of the tip clearance flow inside a hard disk drive model

The flow in the tip clearance of a hard disk drive model has been investigated with laser Doppler techniques. The flow was driven by co-rotating disks inside a cylindrical enclosure in order to simulate a hard disk drive used for data storage devices. The main focus of the investigation was on the understanding of complex flow behavior in the narrow gap region between the disk tip and the outer shroud wall, which is supposed to be one of the causes of flow induced vibration of the disks. Experiments in the past have never been able to examine this region because of the lack of the spatial resolution of sensors in the highly three-dimensional flow in the region. In the present investigation, the flow velocity in the tip clearance region was measured with optical measurement techniques for the first time. The flow behaviors are investigated for four different conditions with two different gap widths and two different shapes of the shroud walls with and without ribs. The velocity measurements were taken both with conventional laser Doppler velocimetry and using a laser Doppler velocity profile sensor with a spatial resolution in the micrometer range. The circumferential velocity component was measured along the axial and radial directions. The steep gradients of the circumferential mean velocity in both directions were successfully captured with a high spatial resolution, which was achieved by the velocity profile sensor. From the supplementary investigations, the existence of vortex structures in the tip clearance region was confirmed with a dependence on the shroud gap width and the shroud shape. The interactions of the two boundary layers seem to be the source of the complex three-dimensional behaviors of the flow in this region.

An advanced laser-Doppler velocimeter for full-vector particle position and velocity measurements

An advanced laser-Doppler measurement technique has been developed for fully resolved three-component particle position and velocity vector measurements in turbulent flows. The instrument deemed the ‘comprehensive’ laser-Doppler velocimeter employs a novel optical arrangement to measure multiple-component sub-measurement-volume-scale seed particle positions simultaneously with the velocity vector measurements of conventional laser-Doppler velocimetry (LDV). In the current paper, the effectiveness of the position resolution capabilities is considered, which allows for velocity statistics measurements at multiple locations within the measurement volume. Design estimates for the position vector uncertainties for a particle passing the measurement volume are about 3 µm root mean square in any direction, although in situ estimates indicate an uncertainty value closer to 14 µm with the possibility of further refinement through optimized alignment. To validate the operation of the instrument, measurements are presented in turbulent boundary layers previously examined with high-resolution conventional LDV. The flat-plate turbulent boundary layer is studied at two Reynolds numbers up to Reθ = 7500. Measurements are also presented in a pressure-driven three-dimensional turbulent boundary layer created beside a wing/body junction. These measurements illustrate the effectiveness of the technique for obtaining highly resolved velocity profiles within the measurement volume and give the highest spatial resolution velocity statistics published for Reynolds numbers of the magnitude studied.

Development of Miniaturized Fiber-Optic Laser Doppler Velocimetry Sensor for Measurement of Local Blood Velocity (Fabrication Method for Convex or Concave Lens-Like Fiber Tip and the Characteristics of Sensor Optical System)

A new miniaturized fiber-optic laser Doppler velocimetry (LDV) sensor has been developed, which is capable of measuring the local velocity in various semi-opaque and opaque fluid flows, particularly whole blood velocity in vessels. The sensor has a convex lens-like fiber tip as a pickup and an improved optical transmission system with markedly decreased stray light. This paper describes methods for fabricating fiber tips like concave and convex lens and the characteristics of the optical sensor system equipped with the fabricated fiber tip. Conventional fiber-optic LDV sensors developed up to now have not been capable of measuring such opaque fluids because scattered light from scattering particles as erythrocytes has very low intensity, which makes signal-to-noise ratio of Doppler signal received by a sensor pickup significantly decreased. To overcome these problems, convex lens-like fiber tips have been fabricated by chemical etching, in which quartz fibers of multimode graded refractive index have been etched in aqueous solutions of hydrogen fluoride and ammonium fluoride under the appropriately controlled condition of the concentration of the solution, the etching duration time and the etchant temperature to obtain the desired curvature radius of the lens-like surface of the fiber tip. In this fiber-optic sensor, a laser beam emitted from the fiber tip can be focused at any position from about 0.1 to 0.5 mm distant from the fiber tip according to its curvature radius. The convex lens-like etched tip totally reduced the intensity of undesired reflecting light at the fiber end by 1/2 to 1/6 compared with normal cut fiber tip. Consequently, this fiber-optic LDV sensor system is capable of measuring the local flow velocity in semi-opaque and opaque fluids, whose turbidity was about five times higher than by any kinds of previous sensors.

ＰＷＲ燃料ロッドバンドル内の乱流流速場に関する研究

Precise velocity measurement in a fuel rod bundle is required to improve the thermal hydraulic characteristics of Pressurerized Water Reactor (PWR) spacer grids. However, the use of conventional laser Doppler velocimetry (LDV) is restricted by the existence of invisible region in fuel rod bundles and insufficient spatial resolution for the narrow gaps in rod bundles. The new LDV was therefore developed to overcome these problems. The LDV is miniaturized with fiber optics embedded in a fuel cladding and can be inserted in an arbitrary grid cell instead of a fuel rod. The rod-embedded fiber LDV (rod LDV) is applied for cross-flow and axial-flow measurements at rod gaps and sub-channels to investigate turbulent velocity field in a rod bundle with mixing spacer grids. Distributions of mean velocity and turbulent intensity around the center rod of a 5×5 rod bundle are obtained. The measurements of this study are compared with particle image velocimetry (PIV) measurement for the same geometry and flow condition. Similarity of both measurements is confirmed qualitatively and some of data are in good agreement quantitatively.

Development of Rod-embedded Fiber LDV to Measure Velocity in Fuel Rod Bundles

Precise measurement of velocity in fuel bundles is required to improve the thermal hydraulic characteristics of Pressurerized Water Reactor (PWR) spacer grids. However, the use of conventional laser Doppler velocimetry (LDV) is restricted by the existence of invisible region in fuel rod bundles and insufficient spatial resolution for the narrow gaps in rod bundles. An LDV was therefore developed to overcome these problems. The new LDV is miniaturized with fiber optics embedded in a fuel cladding. The rod-embedded fiber LDV (rod LDV) can be inserted in an arbitrary grid cell instead of a fuel rod, and has the advantage of not disturbing the flow field since it is the same shape as a fuel rod. As there are a number of fuel rods in the flow field of a rod bundle, it is difficult to distinguish between Doppler signals and noise due to laser reflections from surrounding rods. To improve the data rate, a pedestal trigger was used to provide gate signals. It was shown that the rod LDV was capable of measuring the flow velocity in a rod bundle, and data for cross-flow and axial-flow velocity downstream of the spacer grid were obtained.

Development of Rod-embedded Fiber LDV to Measure Velocity in Fuel Rod Bundles

Precise measurement of velocity in fuel bundles is required to improve the thermal hydraulic characteristics of Pressurerized Water Reactor (PWR) spacer grids. However, the use of conventional laser Doppler velocimetry (LDV) is restricted by the existence of invisible region in fuel rod bundles and insufficient spatial resolution for the narrow gaps in rod bundles. An LDV was therefore developed to overcome these problems. The new LDV is miniaturized with fiber optics embedded in a fuel cladding. The rod-embedded fiber LDV (rod LDV) can be inserted in an arbitrary grid cell instead of a fuel rod, and has the advantage of not disturbing the flow field since it is the same shape as a fuel rod. As there are a number of fuel rods in the flow field of a rod bundle, it is difficult to distinguish between Doppler signals and noise due to laser reflections from surrounding rods. To improve the data rate, a pedestal trigger was used to provide gate signals. It was shown that the rod LDV was capable of measuring the flow velocity in a rod bundle, and data for cross-flow and axial-flow velocity downstream of the spacer grid were obtained.

A Two-Spot Sensor for Measurement of Dissipation by Means of Laser-Doppler Velocimetry*

Abstract An estimate of the dissipation rate for mechanical energy in a turbulent flow can be obtained by computing the variance of the temporal derivative of the fluid velocity measured at a single point. This technique is not suited to conventional laser-Doppler velocimetry because of inherent lower bounds on noise in the velocity measurements, which biases the dissipation estimates. A two-spot technique, described by George and Lumley, overcomes this difficulty. In this technique, laser-Doppler measurements of velocity are obtained at two points, which are separated in the spanwise direction by a distance larger than the size of the optical scatterers but smaller than the Kolmogorov scale. The covariance of the temporal derivative of the velocities measured at the two points provides an unbiased estimate of the dissipation, because the velocities at the two points are essentially identical, while the two noise records are uncorrelated. An oceangoing sensor based on this technique has been developed, an...

Image extraction of particle concentration at the plug front using 3D wavelets and comparison with LDV

Particle concentration distribution images of a dense two-phase solid/air (plug) flow have been obtained at 10-ms intervals in a horizontal pipeline using capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) were decomposed to wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time-dominant particle distribution with a specific time frequency level can be visualized in cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high time frequency levels 6 and 7, is centrally located above the stationary layer. The distribution image at the front is reasonable when compared with the particle velocity distribution by conventional laser Doppler velocimetry (LDV) that indicates the discharging particles toward the front air phase at the centerline.

Image extraction of particle concentration at the plug front using 3D wavelets and comparison with LDV

Particle concentration distribution images of a dense two-phase solid/air (plug) flow have been obtained at 10-ms intervals in a horizontal pipeline using capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) were decomposed to wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time-dominant particle distribution with a specific time frequency level can be visualized in cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high time frequency levels 6 and 7, is centrally located above the stationary layer. The distribution image at the front is reasonable when compared with the particle velocity distribution by conventional laser Doppler velocimetry (LDV) that indicates the discharging particles toward the front air phase at the centerline.

Self-Mixing Laser Diode Velocimetry: Application to Vibration and Velocity Measurement

A review of recent experimental and theoretical results concerning laser diode self-mixing velocimetry is presented, showing that this technique can be deployed to measure velocity and vibration of solid targets with an extremely simple optical setup. This technique reduces optical alignment problems and achieves results comparable to those obtained by the conventional laser Doppler velocimetry (LDV) approach. It is demonstrated that the self-mixing signal can be processed to recover the target velocity and vibration by applying the same analysis method used for LDV. An optimal signal processing method is then proposed to recover the target velocity with good accuracy, also in the presence of relevant speckle disturbance. Application to the measurement of sub-micron vibrations is also demonstrated, using a self-mixing vibrometer instrument capable of 5-nm accuracy. As an example, the characterization of response and hysteresis of piezoceramic transducers (PZTs) is carried out. These results illustrate the effectiveness of the self-mixing technique in the field of laser velocimetry, opening the way to new applications where compactness and low cost of the measuring apparatus are essential.

Optical Doppler velocimetry of red blood cells at different depths in retinal vessels by varying the coherence length of the source: feasibility study

To report on a novel approach to measure the velocity of red blood cells (RBCs) at different retinal vessel depths. The technique is an extension of conventional laser Doppler velocimetry using light sources of various coherence lengths (CL). Light scattered by the moving RBCs interferes with that reflected from the anterior vessel wall only if the optical path difference is shorter than CL. Therefore, using low coherence light sources, localized measurements of RBCs velocity can be performed. Measurements of RBCs velocity at different depths in a main retinal vein (diameter: 152 microns) of a volunteer has been performed using 4 different light sources with CLs of 14 microns, 21 microns, 32 microns and > m. Measured values are in good agreement with theoretically predicted values. This new approach permits to measure RBCs velocity at different depths of retinal vessels in the human retina.

Evanescent wave Doppler velocimetry for a wall’s near field

This study proposes a Doppler velocimetry utilizing evanescent waves that are induced by a laser beam. This velocimetry allows us to measure the flow velocity within the near field less than 1 μm from a dielectric surface. When evanescent waves propagate in opposite directions on a dielectric surface, the waves interfere with each other and produce a standing wave within the near field. When a tracer particle goes through the standing wave, the particle scatters the wave. The flow velocity within the near field can be determined by measuring the blinking frequency similar to conventional laser Doppler velocimetry. The validity of this velocimetry is empirically examined by applying it to a slip velocity measurement of a solid on a dielectric surface.

Particle size distribution technique using conventional laser Doppler velocimetry measurements

The difference between the particle velocity and the fluid velocity within a flowfield is a direct function of the particle size distribution. Utilizing classical particle behavior through an oblique shock, a numerical algorithm has been developed to extract a size distribution estimate from conventional laser Doppler velocimetry (LDV) measurements. As a specific application, the method is used to determine the size distribution of atomized silicon oil seed from LDV measurements through the leading-edge shock of a hypersonic forebody inlet at Mach 5.76. Knowledge of the particle sizes allows the measurement bias to be quantified and removed, thereby greatly enhancing the value of LDV as a nonintrusive flowfleld measurement technique.

Experimental study of the optical velocimeter using a double-exposure specklegram

An optical velocimeter using a double-exposure specklegram is studied experimentally. In the method, a double-exposure specklegram is used to produce Young's fringes over a probing area, instead of forming the fringes as the result of interference of two beams in the case of a differential-type laser Doppler velocimeter. The method has special advantages over conventional laser Doppler velocimetry such as simplicity in the optical arrangement together with high stability for mechanical shock. In the study, the velocities of the solid object having continuous surfaces as well as the fluid flow or particles in the fluid were measured to show the usefulness of the method. The velocity of the solid object having relatively small variation of the surface brightness was measured with high accuracy in the experiment.

Optical velocimeter using the double-exposure specklegram

A new type velocimeter using a double-exposure specklegram is proposed. In the method, a double-exposure specklegram is used to produce Young's fringes over a probing area, instead of forming the fringes as the result of interference of two beams in the case of a differential-type laser Doppler velocimeter. Compared to conventional laser Doppler velocimetry, the present method has special advantages such as simplicity in the optical arrangement and adaptability in the measuring range together with high stability. Preliminary experiments were performed to examine the validity of the method, especially under the existence of speckle noise. The results support the validity of the proposed method.