Laser Doppler Velocity Profile Sensor Research Articles

Measurements in a Turbulent Channel Flow by Means of an LDV Profile Sensor

Spatially and temporally resolved velocity measurements in wall-bounded turbulent flows remain a challenge. Contrary to classical laser Doppler velocimetry (LDV) measurements, the laser Doppler velocity profile sensor (LDV-PS) allows the combined measurement of tracer particle position and velocity, which makes it a promising tool. To assess its feasibility a commercial LDV-PS is employed in a turbulent channel flow at Reτ=350\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Re_\ au =350$$\\end{document}. Additionally, the measurement and signal-processing accuracies of velocity and location are evaluated for various tracer-object sizes and velocities. On this basis, the turbulent channel flow measurements are evaluated and compared to reference data from direct numerical simulations. Thus, potentials of the LDV-PS are investigated for different regions of the flow and various data processing routines as well as the experimental practice are discussed from an application perspective.

Application of optical velocity measurements including a novel calibration technique for micron-resolution to investigate the gas flow in a model experiment for crystal growth

An important step in the fabrication of many modern semiconductor materials and devices is the crystal growth process. These processes take place in high-temperature furnaces using inert gas and other atmospheres. The flow in the gas phase influences the transport of crystal components, dopants and impurities and hence has a significant impact on the grown crystals. In this work, we study the flow in a simplified model of a crystal growth furnace. This model is made of PMMA filled with a Diesel mixture as a model fluid to match the refractive index of PMMA and to allow for measurements in the complex geometry. The comparability to the flow in a real furnace is ensured by matching the Reynolds number. Two optical measurement methods, Particle Image Velocimetry (PIV) and the Laser Doppler Velocity Profile Sensor (LDV-PS) are used to investigate the global flow field as well as the small-scale flow structures. A calibration model is developed for the LDV-PS to reduce systematic measurement errors caused by the refractive index of the model fluid from up to 1% to less than 0,1%. The results obtained in this study improve the understanding of the gas flow behavior inside a crystal growth furnace and provide reference data for simulation. The first analysis shows a highly unsteady flow with unexpected flow direction along the crystal and melt surfaces. The near-surface flow patterns are of particular relevance in crystal growth because of their large influence on the local heat and mass transport during the crystallization process.

Comparative Accuracy Study Of LDV Profile-Sensor Acquisition Modes And Particle Diameters

The laser Doppler velocity profile sensor (LDV-PS) system offers the possibility to measure flow velocities with high spatial resolution and is therefore a promising method for the investigation of flows with high velocity gradients. The present experimental study revolves around the interplay between a broad range of chosen LDV-PS acquisition parameters, the dynamic velocity range within the observed measurement volume, and the accuracy of the velocity and position estimation from an application perspective. In the chosen set-up, thin wires of different diameters rotate at varying velocity levels and are captured instead of tracer particles by the applied system. It is observed that constant sample rate and number for the Fast Fourier Transform (FFT) evaluation of the detected burst signal allow straightforward measurements of unknown velocity profiles, but imply a velocity-dependent spatial measurement uncertainty. In contrast, velocity-adjusted acquisition settings allow to acquire particle bursts with equal signal resolution and length effecting a constant measurement accuracy at all velocity levels when adjusted appropriately. The position standard deviation is furthermore observed to increase with the wire diameter.Hence, the size of the scattering object should be chosen appropriately small during calibration and measurement. The gained knowledge offers the possibility to adapt the evaluation parameters more specifically to a given measurement problem.Consequently, measurements can be conducted in flows with small velocities with a high spacial resolution, when the FFT parameters and processing routines are adjusted accordingly, small particles are chosen and vibrations in the set-up are avoided.

Simultaneous velocity profile and temperature profile measurements in microfluidics

Simultaneous non-intrusive temperature and velocity measurements in flows are of high technological interest, e.g. to study the heat transfer in microfluidic environments. However, a measurement system that offers a low velocity uncertainty and micrometre spatial resolution as well as highly accurate temperature measurements in a single device has not been demonstrated so far. In this work, this problem is solved by combining a Laser Doppler Velocity Profile Sensor (LDV-PS) with Laser-Induced Fluorescence (LIF). Seeding particles are employed, that contain the fluorescent dyes uranine and rhodamine B. The multiple dye approach eliminates the influence of the droplet size. Relative velocity uncertainties of down to 0.4% and a temperature uncertainty of down to 0.24∘C with a spatial resolution of 10μm are achieved in a demonstration air flow experiment. The method has the potential to be optimised for different temperature ranges and uncertainty requirements, making it applicable on a wide range of thermal flows like fuel cells or microbioreactors. A better understanding of heat exchange processes can improve the energy efficiency of microfluidic devices.

Measurement and Numerical Simulation of the Velocity Profile in the Thin Film of an Impinging Water Jet

Abstract Impinging circular free-surface water jets are used in challenging cooling and cleaning tasks. In order to develop simulation models for process optimization, validation data are required, which are currently not available. Therefore, the flow field of these jets is studied for the first time with the novel laser Doppler velocity profile sensor. The mean velocity field and fluctuations are measured within the stagnation and adjacent redirection region for radial coordinates up to three times the nozzle diameter. In the examined parameter range with jet velocities up to 17 m/s and nozzle diameters up to 5.2 mm, i.e., Reynolds numbers up to 69 500, thin films of a few hundred micrometers are formed, which hinder the measurement with common optical measuring systems. Based on the measurement results, a comparatively low-cost volume of fluid simulation model is developed and validated that presumes a relaminarized film flow. The profiles measured and the simulated flow show very good agreement. In the future, the simulation model provides a basis for process optimization and the innovative measurement technology used will prospectively provide further detailed insights into other flows with high velocity gradients.

Investigation and equalisation of the flow distribution in a fuel cell stack

The possibility to use fuel cells as an electrical power source makes them interesting for a wide range of applications. In this work, computational fluid dynamics (CFD) simulations and optical measurements are performed to predict the flow distribution in a flow setup resembling the parallel flow circuits in fuel cell stacks. For the first time it is shown that by an adaptation of the port sizes in the inlet manifold to the individual fuel cells, the average global deviation between the flow rates can be reduced from 10.1% to 4.0% by means of a model experiment. The measurements are performed with a high resolution laser Doppler velocity profile sensor (LD-PS) specifically developed for measurements in small-scale channels, in this work 4×1mm2, allowing for a spatial resolution below 2 μm and relative velocity uncertainties below 0.1%, helping to resolve installation effects possibly occurring in fuel cells to improve their efficiency. The presented results can be used by manufacturers to increase the efficiency of their fuel cell stacks.

Aeroacoustic near-field measurements with microscale resolution

In order to analyse aeroacoustic phenomena at near-fields, e.g. the sound–flow interaction at aircraft engine liners, measurements of the flow velocity and the acoustic particle velocity (APV) with microscale resolution are required. To this end, the APV measurement with a high spatial resolution of 10 µm was conducted by means of a laser Doppler velocity profile sensor. For validation of the APV measurements using the profile sensor in a superposed flow, a good agreement with indirect microphone measurements as a reference was achieved, up to a maximum Mach number of 0.25. Aeroacoustic measurements at a minimum distance of 350 µm to the perforation of a bias flow liner were performed using the profile sensor. As a result, acoustically induced velocity oscillations near the rim of the orifice were detected with microscale resolution. The phase-resolved oscillation field indicates vortex shedding from the perforation, which is initiated by the sound–flow interaction. Thus, it is demonstrated that the profile sensor is a valuable tool for analysing aeroacoustic phenomena at near-fields, down to the Kolmogorov scale.

An experimental study on the flow behavior near the read-and-write arm in a hard disk drive model with a shroud opening

Complex flow inside a hard disk drive (HDD) was investigated using a simplified 3.5″ model for clarifying the mechanism causing flow-induced vibration. In contrast to the authors’ related study in the past, our model had a non-axisymmetric geometry equipped with a shroud opening and a read/write arm (RWA). The model is designed to serve as a benchmark to study HDD flows both in experiments and in numerical simulations. The complex flow behavior in the disk-to-disk space was investigated with the RWA inserted into the inter-disk space. Flow measurements were carried out with a test rig which consisted of transparent disks, RWA and covers. The measurements were performed at the disk Reynolds number Re d = 1.2 × 105 which corresponds to the rotation speed of 7,700 rpm of a real 3.5″ HDD. Two sets of the flow measurements were performed—the first Reynolds stress components measured along four different lines with the RWA inserted at a shallow angle (experiment I), and the other mean and rms velocity statistics along several selected lines with two different RWA insertion angles (experiment II). The mean velocity and velocity variance were obtained at a spatial resolution of 30 μm along eight different lines perpendicular to the disk surfaces. The high spatial resolution of the results was achieved using a laser Doppler velocity profile sensor with a physical resolution in micrometers and a velocity uncertainty of 0.2 %. In the experiment I, the mean velocity and velocity variance statistics were mostly consistent with the common findings in other studies using axisymmetric models except for the flow behavior in the radial direction at the shroud opening. The secondary flow behavior was likely caused by the shroud opening which was not included in most of the models in the past. In experiment II, the mean velocity and velocity variance were successfully measured through examination of the flow above and below the RWA in the space between the rotating disks. The resulting velocity statistics exhibit turbulent Couette-like flow in the narrow 1 mm space between the disks and the RWA.

Analysis of the Electrolyte Convection inside the Concentration Boundary Layer during Structured Electrodeposition of Copper in High Magnetic Gradient Fields

To experimentally reveal the correlation between electrodeposited structure and electrolyte convection induced inside the concentration boundary layer, a highly inhomogeneous magnetic field, generated by a magnetized Fe-wire, has been applied to an electrochemical system. The influence of Lorentz and magnetic field gradient force to the local transport phenomena of copper ions has been studied using a novel two-component laser Doppler velocity profile sensor. With this sensor, the electrolyte convection within 500 μm of a horizontally aligned cathode is presented. The electrode-normal two-component velocity profiles below the electrodeposited structure show that electrolyte convection is induced and directed toward the rim of the Fe-wire. The measured deposited structure directly correlates to the observed boundary layer flow. As the local concentration of Cu(2+) ions is enhanced due to the induced convection, maximum deposit thicknesses can be found at the rim of the Fe-wire. Furthermore, a complex boundary layer flow structure was determined, indicating that electrolyte convection of second order is induced. Moreover, the Lorentz force-driven convection rapidly vanishes, while the electrolyte convection induced by the magnetic field gradient force is preserved much longer. The progress for research is the first direct experimental proof of the electrolyte convection inside the concentration boundary layer that correlates to the deposited structure and reveals that the magnetic field gradient force is responsible for the observed structuring effect.

Optical velocity measurements of electrolytic boundary layer flows influenced by magnetic fields

Magnetic fields are applied to electrically conducting fluids in order to influence electrochemical processes through the magnetohydrodynamic effect. Various phenomena, e.g. on electrodeposited metal layers, which can be attributed to forced convections were observed. To provide information about acting forces, the laser Doppler velocity profile sensor was applied to measure the transition layer of a Lorentz force influenced flow over a backward-facing step and the velocity boundary layer during copper deposition. With this sensor, the electrolyte convection within < 500 μm of the front of an electrode is measured with a spatial resolution down to 15 μm. The interaction of buoyancy, Lorentz and magnetic field gradient forces is studied by measuring the velocities down to 10 μm in front of the cathode. Inside the concentration boundary layer, complex electrolyte convection is induced, which varies not only in time but also in its structure, depending on the forces present and their influence over time. In inhomogeneous magnetic field configurations, the magnetic field gradient force dominates the velocity boundary layer at steady state and transports electrolyte toward regions of high magnetic gradients, where maximum deposit thicknesses are found. In this way, the measurements confirm the predicted influence of the magnetic field gradient force on the structuring of copper deposits.

Determination of the phase-resolved body force produced by a dielectric barrier discharge plasma actuator

Dielectric barrier discharge (DBD) plasma actuators are a promissing tool for active flow control applications. In order to determine the time-resolved body force induced by a DBD plasma actuator, which was driven by a 9.5 kHz sinusoidal signal, the flow has to be captured with a high spatial and temporal resolution. For this purpose we applied a laser Doppler velocity profile sensor for the measurements of the two-component velocity field and Lagrangian acceleration. A temporal and spatial resolution of 7.3 µs and 40 µm was achieved. We present the time-resolved local flow behaviours induced by the DBD actuator. Based on these measurement data the time-resolved body force generated by the plasma actuator was derived. Experiments revealed a defined dependence of the force direction on the phase angle of the ac operating voltage. This result is a contribution to the controversy on the temporal behaviour of the body force of plasma actuators.

Phase resolved measurements of the statistical flow behavior of a backward‐facing step flow controlled by a sinusoidal Lorentz‐force

AbstractIn order to gain deeper inside into the behavior of a Lorentz‐force controlled flow behind a backward‐facing step, measurements with a laser Doppler velocity profile sensor were undertaken. By varying the frequency of the sinusoidal Lorentz‐force the influence on the turbulence statistics was studied. Moreover, phase resolved measurement data is presented. The results confirm the characteristic frequency determined on basis of the momentum thickness beforehand. The aim of this study is to understand the influence of the different incitation signals and to reduce the reattachment length. The statistical results show a dependency of the turbulence degree on the excitation frequency as well as a deformation of the flow profile behind the step. (© 2011 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Spot of Turbulence

AbstractMeasuring technique: An innovative technique enables non‐intrusive, high‐precision measurements of microscopic and macroscopic fluid flows. The laser Doppler velocity profile sensor has huge potential for applications in science and technology.

Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor

The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 μm of a vertically aligned cathode in an aqueous 0.05 M CuSO 4-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 μm. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, δ max and δ v = 0 , where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, v max , in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density.

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.

Optic simulation and optimization of a laser Doppler velocity profile sensor for microfluidic applications

The application of laser Doppler velocimetry to microfluidic flows is usually prevented due the large size of the measurement volume of typically 100 µm. A significantly higher spatial resolution can be achieved with the concept of the laser Doppler velocity profile sensor. However, the resolution is limited by wavefront distortions caused by aberrations of the optical setup. In this work, optical simulation software was employed to investigate the aberrations caused by the profile sensor setup by going from ideal to real lenses step by step. As the main source for the aberrations, the aspheric lens for collimating the laser beams was identified. On the basis of this simulation the setup of the velocity profile sensor could be optimized. The sensor was employed to measure the velocity profile in a microchannel. A spatial resolution of 1.2 µm and a relative uncertainty of the velocity of 0.25% result, confirming the findings of the simulation. This demonstrates the potential of the optimized velocity profile sensor for flow measurements at the microscale.

Precise micro flow rate measurements by a laser Doppler velocity profile sensor with time division multiplexing

This paper presents the measurement of flow rate inside a microchannel by using a laser Doppler technique. For this application a novel laser Doppler velocity profile sensor has been developed. Instead of parallel fringe systems, two superposed fan-like fringe systems with opposite gradients are employed to determine the velocity distribution inside the microchannel directly. The sensor utilizes the time division multiplexing technique to discriminate both fringe systems. A velocity uncertainty of 0.18% and a spatial resolution of 960 nm are demonstrated in the flow, which is the highest spatially resolved measurement by a laser Doppler technique published to date. Flow rate measurements, in the range of 30 µl min−1, with a statistical uncertainty of 5 × 10−4 are further presented. In comparison to a reference, by precise weighing, the mean deviation between both measurement principles amounts to 1%. With the advantage of high spatial resolution with simultaneous low velocity uncertainty, the laser Doppler velocity profile sensor offers a new tool for microfluidic diagnostics, e.g. in lab-on-a-chip systems or for drug delivery, which requires very small flow rates.

Two-point correlation estimation of turbulent shear flows using a novel laser Doppler velocity profile sensor

In this paper we describe, for the first time, a new method of two-point correlation estimations of turbulent flows using a laser Doppler velocity profile sensor. For the spatial correlation estimations the laser Doppler velocity profile sensor offers unique opportunities since a high spatial resolution of approximately 20 micron within the measurement volume is achieved. Furthermore, the low relative velocity measurement uncertainty of about 0.1% yields a high resolution of small velocity fluctuations and, therefore, allows correlation investigations where such high resolution is required. Moreover, a new virtual detection volume technique is presented which is only applicable in conjunction with the laser Doppler velocity profile sensor and offers the potential to achieve highly precise spatial correlation estimations. Measurements have been carried out in the turbulent wake of a circular. Both temporal as well as spatial correlation estimations have been calculated from the acquired velocity data yielding a longitudinal Taylor microscale of 3.53 mm and a transverse Taylor microscale of 1.84 mm.

Precise flow rate measurements of natural gas under high pressure with a laser Doppler velocity profile sensor

This paper reports about the first application of a laser Doppler velocity profile sensor for precise flow rate measurements of natural gas under high pressure. The profile sensor overcomes the limitations of conventional laser Doppler anemometry (LDA) namely the effect of spatial averaging and the effect of fringe spacing variation (virtual turbulence). It uses two superposed, fan-like interference fringe systems to determine the axial position of a tracer particle inside the LDA’s measurement volume. Consequently, a spatial resolution of about 1 μm can be achieved and the effect of virtual turbulence is nearly eliminated. These features predestine the profile sensor for flow rate measurements with high precision. Velocity profile measurements were performed at the German national standard for natural gas, one of the world′s leading test facilities for precision flow rate measurements. As a result, the velocity profile of the nozzle flow could be resolved more precisely than with a conventional LDA. Moreover, the measured turbulence intensity of the core flow was of 0.14% mean value and 0.07% minimum value, which is significantly lower than reference measurements with a conventional LDA. The paper describes the performed measurements, gives a discussion and shows possibilities for improvements. As the main result, the goal of 0.1% flow rate uncertainty seems possible by an application of the profile sensor.

Measurement of acceleration and multiple velocity components using a laser Doppler velocity profile sensor

For the investigation of turbulent flows, the measurement of Lagrangian acceleration is of great interest as it represents a direct component of the Navier–Stokes equations. The presented sensor is based on the common laser Doppler technique, but offers in addition combined high spatial resolution in the micrometre range and the possibility of measuring the velocity component along the optical axis. As a result of the sensor setup, signals of particles with inclined trajectories show frequency modulation within a single burst similar to the signal of an accelerated particle. A model-based approach to distinguish between both quantities is presented, and a signal processing technique based on the Hilbert transform has been developed. The processing is comparatively fast and showed good agreement with preset values, even for signals of poor quality. The variance of velocity and acceleration measurements nearly reaches the Cramér–Rao lower bound. Experimental verification is done by the measurement of a harmonic oscillator with known parameters and a stagnation flow within a free jet.