A transient laser Doppler anemometry for measurement of rapidly changing velocities of solid bodies

A transient laser Doppler anemometry (LDA) is developed to extract the velocity history of a solid body that is transient in nature. A digital storage oscilloscope was used in place of the conventional counter to allow postprocessing of the recorded Doppler signals. By sliding a rectangular window with a length on the order of 1μs, frequencies of the rapidly changing Doppler signals were extracted using a refined signal processing method. This method, which incorporates the chirp-Z transform algorithm to the short-time Fourier transform routine for frequency enhancement, has proved its superiority over the conventional short-time Fourier transform and the zero-crossing methods; the latter was used in the authors' previously published works. The transient LDA was then applied to record transient responses of projectiles with different masses during impact on plates of various thicknesses, and interesting impact phenomena were discovered. From these applications, it is found that this method is robust and reliable for extracting the velocity history of a solid body during a transient event.     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

LDA and PDA signal analysis using wavelets

A processing procedure is described that estimates the frequency and the arrival time from signals measured with a laser Doppler anemometer (LDA) and additionally, the phase difference and the time delay of signals measured with a phase Doppler anemometer (PDA). The basis of the estimation procedure is a wavelet transform. The procedure described works iteratively, alternating between the time and the frequency domain. The performance of the estimator is compared to other estimation procedures and the Cramér-Rao lower bound using numerical simulations. Received: 18 May 2000/Accepted: 8 September 2000     

Imaging laser Doppler velocimetry

Imaging laser Doppler velocimetry (ILDV) is a novel flow measurement technique, which enables the measurement of the velocity in an imaging plane. It is an evolution of heterodyne Doppler global velocimetry (HDGV) and may be regarded as the planar extension of the classical dual-beam laser Doppler velocimetry (LDV) by crossing light sheets in the flow instead of focused laser beams. Seeding particles within the flow are illuminated from two different directions, and the light scattered from the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of the illumination and the velocity of the particle. The superposition of the two different frequency-shifted signals on the detector creates interference and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity of the particle. This signal is detected using either a high-speed camera or alternatively a smart pixel imaging array. This detector array performs a quadrature detection on each pixel with a maximum demodulation frequency of 250 kHz. To demonstrate the feasibility of the technique, two experiments are presented: The first experiment compares the measured velocity distribution of a free jet using ILDV performed with the smart pixel detector array and a high-speed camera with a reference measurement using PIV. The second experiment shows an advanced setup using two smart pixel detector arrays to measure the velocity distribution on a rotating disk, demonstrating the potential of the technique for high-velocity flow measurements.     

Investigation of the acceleration of aluminum particles behind a shock wave using instantaneous Laser Doppler Velocimetry

The acceleration of aluminum particles with a 5μm diameter in the flow field behind an incident shock wave was investigated experimentally in a 10-m long and 70 mm inner diameter shock tube. By means of instantaneous Laser Doppler Velocimetry (LDV) the velocity of the particles was observed directly. The light scattered by the moving particles is Doppler shifted and sent to the laser Doppler velocimeter. The velocimeter essentially consists of a phase-stabilized Michelson interferometer used as a sensitive spectrometer. An electro-optical circuit ensures the phase stabilization that results in a voltage signal independent of the scattered light intensity and proportional to the mean velocity of the particles at the measurement point. Because of the very short response time (1μs) of the LDV system used here, the latter gives a continuous real-time signal of the particle acceleration. To avoid particle oxidation the particles were accelerated by a high-speed nitrogen gas flow. From the measured velocity the dimensionless drag coefficient was calculated. The drag coefficient is related to the fluid dynamic force exerted by the gas on the particles. The experimental data were compared to theoretical models from the literature. A significant deviation between the model and the experimental data was observed. This deviation is supposed to be induced by the shock wave, which hits the particles and breaks them into pieces of a smaller diameter. Further experiments will be carried out in the future to check the size distribution of the particles after the shock has gone past them.      

Laser generated and recorded transient bending waves in composite tubes

Propagating bending waves are studied in three different composite tubes by holographic interferometry. A conical mirror is placed axially inside the tubes. Axial illumination and observation directions make it possible to view the circumference of the tube, with a high sensitivity to radial deformation. It is shown how the deformation field can be numerically evaluated using a phase stepping and unwrapping technique. Transient bending waves in the tubes are both generated and recorded by the same pulsed laser, which makes the experiments easy to perform. Finite element simulations of the impacted tubes are compared to corresponding experiments. Both the geometry and the material properties of the tubes affect the wave propagation. For unidirectional composite tubes, the 0-deg and 90-deg directions have different dynamic behavior. The proposed method could be used in nondestructive testing of tubes.     

Simultaneous velocity and scalar measurements in premixed recirculating flames

 The use of a laser-Doppler velocimeter has been extended to the analysis of turbulent heat transfer in a strongly sheared disc-stabilised propane-air flame through its combination with either laser Rayleigh scattering or digitally-compensated fine-wire thermocouples. The laser velocimeter was based on a conventional forward scattering system from the green light of a 5W Argon-Ion laser, while the Rayleigh signals used the blue line of the same laser. The procedure for the numeric compensation of the thermocouple signals included analysis of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to allow combined velocity–temperature samples acquired by a purpose-built digital interference with a frequency up to 2000 Hz, without deterioration of the thermocouple by particle accretion. The maximum effective data rate for the combined Rayleigh/LDV system is shown to be around 130 Hz, which corresponds to a data rate of valid Doppler signals around 400 Hz and statistics based on more than 15 000 measurements is made possible. The results obtained with the two systems agree qualitatively, although the use of thermocouples attenuates the measured velocity-temperature correlations. The results are used to assess the extent to which turbulent mixing in flames is altered by the accompanying heat release and quantify the processes of non-gradient diffusion in a strongly recirculating premixed flame. Received: 15 November 1996/Accepted: 2 September 1997     

Phase validation criteria of size measurements for the phase Doppler technique

A validation procedure of the size measurements for the phase Doppler anemometer is proposed for a two and three photodetector system and is shown experimentally to reduce erroneous measurements due to unwanted Doppler signals caused by surface reflection rather than refraction through the particles. This problem known as trajectory ambiguity is caused by the Gaussian distribution of the incident laser beams when the particle diameter is similar to the diameter of the probe volume of the anemometer. The three photodetector system is shown to improve further the sizing accuracy of a two photodetector system by rejecting measurements from signals with phase ratio values lying outside the expected value for spherical scatterers and which would otherwise have been validated by the two photodetector system.Mr. J.R. Laker designed the electronic circuit of the phase Doppler counter. This work was partially supported by the Commission of the European Communities through contract JOUF-0040-C (TT).     

Measuring device for precise evaluation of torsional creep and recovery data

An apparatus has been designed and built up to determine the shear creep compliance and viscosity with high accuracy in a wide range of temperature and time. The characteristic feature of this apparatus is the possibility to measure directly the recoverable compliance and to determine the steady state recoverable complianceJ _e . Disturbing instrumental forces are minimized by use of a magnetic bearing. The torque is applied inductively by a modified three phase asynchronous motor. The torsional angle is measured with a laser beam reflected from a mirror to an electro-optical measuring device. Sample thermostating is performed by radiation in a heating chamber, which allows observation of the specimen during measurement.First results of creep and creep recovery measurements are reported, which were carried out on a technical polystyrene above the glass rubbery transition.Dedicated to Prof. Dr. H. Janeschitz-Kriegl on the occasion of his 60th birthday.     

Mean velocity and moments of turbulent velocity fluctuations in the wake of a model ship propulsor

Pod drives are modern outboard ship propulsion systems with a motor encapsulated in a watertight pod, whose shaft is connected directly to one or two propellers. The whole unit hangs from the stern of the ship and rotates azimuthally, thus providing thrust and steering without the need of a rudder. Force/momentum and phase-resolved laser Doppler anemometry (LDA) measurements were performed for in line co-rotating and contra-rotating propellers pod drive models. The measurements permitted to characterize these ship propulsion systems in terms of their hydrodynamic characteristics. The torque delivered to the propellers and the thrust of the system were measured for different operation conditions of the propellers. These measurements lead to the hydrodynamic optimization of the ship propulsion system. The parameters under focus revealed the influence of distance between propeller planes, propeller frequency of rotation ratio and type of propellers (co- or contra-rotating) on the overall efficiency of the system. Two of the ship propulsion systems under consideration were chosen, based on their hydrodynamic characteristics, for a detailed study of the swirling wake flow by means of laser Doppler anemometry. A two-component laser Doppler system was employed for the velocity measurements. A light barrier mounted on the axle of the rear propeller motor supplied a TTL signal to mark the beginning of each period, thus providing angle information for the LDA measurements. Measurements were conducted for four axial positions in the slipstream of the pod drive models. The results show that the wake of contra-rotating propeller is more homogeneous than when they co-rotate. In agreement with the results of the force/momentum measurements and with hypotheses put forward in the literature (see e.g. Poehls in Entwurfsgrundlagen für Schraubenpropeller, 1984; Schneekluth in Hydromechanik zum Schiffsentwurf, 1988; Breslin and Andersen in Hydrodynamics of ship propellers, 1996; Schneekluth and Bertram in Ship design for efficiency and economy, 1998), the co-rotating propellers model showed a much stronger swirl in the wake of the propulsor. The anisotropy of turbulence was analyzed using the anisotropy tensor introduced by Lumley and Newman (J Fluid Mech 82(1):161–178, 1977). The invariants of the anisotropy tensor of the wake flow were computed and were plotted in the Lumley–Newman-diagram. These measurements revealed that the anisotropy tensor in the wake of ship propellers is located near to the borders of the invariant map, showing a large degree of anisotropy. They will be presented and will be discussed with respect to applications of turbulence models to predict swirling flows.     

Photoelastometric recording of stress waves

A very sensitive, fast-responding photomultiplier tube and laser light source were used to record stress-optic data associated with a moving stress wave. By using a “gray-field” (which is halfway between a dark and light field) polariscope, optical-electrical recordings were obtained which were linearly proportional to strain. A discriminatory record results which exhibits a higher signal-to-noise ratio than semiconductor strain gages. Gage length can be varied from 0.005 in. upwards.     

Laser-dual-focus system for measuring the flow through narrow rod bundles

A non-disturbing optical instrument for the measurement of flow velocity in complicated flow channels, e.g. inclined flow through rod bundles is described. The measuring probe based on the laser-dual-focus principle is brought inside one of the plexiglass tubes forming the grid. A mirror which is situated inside the probe deflects the two laser beams producing the measuring volume into the flow field to be examined. An optical system for a thin probe has been developed which has to fit into one of the rods and which enables one both to produce the measuring volume as well as to receive the back-scattered light. The required laser power can be reduced to 2 mW. The photo-multiplier necessary for the evaluation of scattered light signals can be separated from the optical head by using light wave conductors. Examples are presented by which the applicability of the new probe to the described flow situation is demonstrated.     

Experimental testing of Taylor's hypothesis by L.D.A. in highly turbulent flow

A new configuration for the transmitting optics of a laser Doppler anemometer has been developed in order to measure the velocity at two different points at the same time. From the simultaneous measurements at two points along the mean flow direction it is possible to evaluate the spatial correlations and to compare them with the temporal correlation to verify the validity limits of Taylor's hypothesis also known as the frozen turbulence hypothesis. The transfer function between the velocity signals at two different points has been introduced to better explain the differences between Taylor's hypothesis and non frozen flow. The analysis is carried out in a flow with high turbulence levels.     

Time-resolved spectral-line-filtered velocimetry

Measurements were obtained with a velocimeter that simultaneously illuminates the particulate in a flow with argon and He–Ne lasers and the scattered light is passed through a carefully controlled iodine filter. The iodine cell absorbs the argon emission at a rate proportional to the velocity of the scattering particle, while the absorption of scattered He–Ne light is independent of velocity. A tunable acousto-optic filter is used to multiplex the argon and He–Ne signals collected from a single photodetector. The problem of detector alignment in conventional Doppler global velocimetry is thus avoided. Experimental results on a scattering disk and an axisymmetric jet are presented and compared with laser Doppler anemometry measurements. Received: 7 March 2000/Accepted: 5 January 2001     

Measurement of the velocity profile in liquid streams of large volumes using a laser Doppler velocimeter

The use of the laser Doppler velocimeter (LDVM) in fluid mechanics already has a solid past history and work on its development is proceeding rather intensively [1, 2], This method has been used successfully to measure various parameters of flows in channels or chutes of small size [3–6]. Fundamental difficulties due to possible local variations in the index of refraction along the path of the laser beam [7], in addition to technical difficulties, have not been eliminated for its use in other cases of practical importance, particularly in basins or reservoirs of large size. The aim of the present paper is the investigation of the velocity structure of streams in large volumes using an LDVM on the example of a turbulent submerged water jet. In order to estimate the effect of the thickness of the water layer on the applicability of the method we tested three schemes: based on direct [8] and back scattering [9] and a scheme with reflection of forward-scattered light from a mirror [10]. The results of the investigation of a submerged turbulent jet using an LDVM are presented.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 170–173, September–October, 1977.     

A high-resolution laser Doppler anemometer for three-dimensional turbulent boundary layers

 A new laser Doppler anemometer optimized for high spatial resolution near a wall is described. The instrument uses short focal length optics, a mirror probe in the flow, and side-scatter collection to produce a measuring volume 35 μm in diameter by 66 μm long. Data are presented for a two-dimensional boundary layer, demonstrating the instrument’s ability to measure Reynolds shear stresses as close to the wall as 0.1 mm, or y ⁺≈3. Received: 6 October 1995/Accepted: 9 April 1996     

Measuring the wind-induced water surface flow by laser Doppler velocimetry

The way to adjust the laser Doppler velocimeter for measuring the water motions near a moving air-water interface is described by specifying the experimental arrangement, the choice of the scattering configuration, the seeding problems and the preliminary tests which were performed. This technique is then used to explore the motions induced by the wind near the water surface. Illustrative results on the instantaneous longitudinal and vertical velocities, the mean profiles and the respective spectra are presented and discussed. It appears that the Eulerian measuring method when applied to such flows and the presence of dropouts on the signals may require significant corrections of the original data before any physical interpretation.     

Shadow Doppler velocimetry with double fiber-array sensors

Shadow Doppler velocimetry (SDV) systems with double fiber-array sensors were developed for the measurements of particle trajectory angles and for the stereoscopic investigation of particles. The parallel two-line fiber-array configuration improves the accuracy of the trajectory angle measurement in a plane perpendicular to the optical axis, which contributes to the high accuracy of the particle shape reconstruction process. It also provides information on the other trajectory angle in a plane parallel to the two laser beams. Furthermore, it realizes "time-of-flight" velocity measurement, which provides the possibility to simplify the original SDV setup by removing the laser Doppler velocimetry (LDV) components. On the other hand, stereoscopic SDV was also developed, which is effective in cases where three-dimensional characteristics of shape, orientation, or behavior of particles are important.     

A photoelectric relay for measuring voltages of low frequency

Summary A device is described, which can be used for amplifying and measuring the a.c. output of slow responding detectors (e.g. bolometers), receiving signals of constant but low frequency (∼1 Hz). This device, based on frequency conversion, which in principle is insensitive to drift-effects, consists of a primary galvanometer, a rotating disk, chopping the lightbeam reflected by the moving mirror of this galvanometer, and a phototube which receives the chopped lightbeam and which is coupled to an a.c. amplifier with an a.c. galvanometer as indicating instrument. The sensitivity of the instrument described, which had a response frequency of 1 2/3 Hz, proved to be limited by the Johnson noise of the input circuit only; drift effects proved to be greatly reduced.