Heterodyne Doppler global velocimetry

Doppler Global Velocimetry (DGV) is an imaging flow measurement technique which allows the measurement of the velocity distribution in a plane. In DGV the frequency shift of scattered light from moving particles within the flow is used to determine the local flow velocity. Heterodyne Doppler Global Velocimetry (HDGV) is a new approach which combines the imaging and geometrical characteristics of DGV with the measurement principles of reference beam laser Doppler anemometry. The frequency shifted scattered light from the flow tracers is heterodyned with a reference beam from the same light source. Due to interference the result of this superposition is a harmonic intensity modulated signal. This signal is detected using a smart pixel detector array to obtain the velocity distribution. Two different experiments are presented. The first experiment compares the measured velocity distribution of a rotating disk with its actual velocity. The second experiment demonstrates the capability of the technique to measure a real flow.     

Single-camera Doppler global velocimetry based on frequency modulation techniques

The main advantage of the described Doppler global velocimeter (DGV) systems based on frequency modulation (FM) or frequency shift keying (FSK) is that no reference detector is required. The frequency variation of the laser light during one modulation period additionally allows an on-line calibration of the complete DGV system. Thus, the new method has the potential to reduce the uncertainty of conventional DGV velocity measurements since time resolved velocity field measurements on a spinning disc have shown standard deviations down to 0.02 m/s. On investigating flow fields, velocity components notably less than 0.5 m/s were resolved.     

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.     

Application of oil-film interferometry skin-friction measurement to large wind tunnels

The oil-film interferometry skin-friction technique is described and applied to flows in some of the NASA Ames large wind tunnel facilities. Various schemes for applying the technique are discussed. Results are shown for tests in several wind tunnels that illustrate the oil film's ability to measure a variety of flow features such as shock waves, separation, and three-dimensional flow.     

Ultrasonic Doppler velocimetry in liquid gallium

For the first time, flow velocity is measured in a vortex of liquid gallium, using the pulsed Doppler shift ultrasonic method. At the top of a copper cylinder filled with liquid gallium, we spin a disk and create a turbulent vortex with a dominant nearly axisymmetric velocity field with little variation in the axial direction. The velocity profiles are shown to be well resolved and in quantitative agreement with earlier observations. Reliable velocity measurements in liquid gallium could be obtained only after serious problems due to the formation of oxides were solved. This work opens the way to performing accurate velocity measurements in other liquid metals; preliminary results for liquid sodium are shown. Received: 14 January 2000 / Accepted: 12 January 2001     

Application of digital speckle pattern interferometry for fluid velocimetry in wind tunnel flows

This paper shows the feasibility of using digital speckle pattern interferometry (DSPI) as a fluid velocimetry technique in high speed gaseous flows. The light scattered from an illuminated plane was recorded with a CCD camera at the same time as a uniform reference beam. A fibre optic was used to bring this reference beam from the laser cavity to the CCD camera. The comparison of two subsequent frames gives information about the velocity field. DSPI was applied to a Von Karman street flow set up in a wind tunnel. Particle image velocimetry (PIV) measurements were also obtained for comparison with the information provided by DSPI. A system for increasing the measurement region when using short coherence length lasers is proposed. Received: 15 June 2000/Accepted: 8 September 2000     

Production of oscillatory flow in wind tunnels

A method for producing oscillatory flow in open-circuit wind tunnels driven by centrifugal fans is described. Performance characteristics of a new device installed on two such tunnels of greatly differing size are presented. It is shown that sinusoidal variations of the working section flow, having peak-to-peak amplitudes up to at least 30% of the mean flow speed and frequencies up to, typically, that corresponding to the acoustic quarter-wave-length frequency determined by the tunnel size, can be obtained with negligible harmonic distortion or acoustic noise difficulties. A brief review of the various methods that have been used previously is included and the advantages and disadvantages of these different techniques are highlighted. The present technique seems to represent a significant improvement over many of them.     

Application of nonlinear spectral analysis and signal reconstruction to laser Doppler velocimetry

Spectral analysis is a useful tool in engineering applications but many practical problems are encountered in computing power spectral densities (PSD), particularly in laser Doppler velocimetry where data are irregularly sampled. Methods to compute PSD are usely based on discrete Fourier transforms such as FFT. A non linear spectral analysis approach is tested and found to give better results, particularly with short data records. Results are also quite promising when non linear spectral analysis is combined with signal reconstruction to process irregularly sampled data.A preliminary version of this paper has been presented at LDA specialist meeting on the use of computers, May 1987, Institut Saint Louis, France     

A wind driven dynamic model manipulator for wind tunnels

Experimental Techniques -     

Suppression of oscillations of piston-aided wind tunnels

The method chosen to compensate for the forces acting on the barrel in the course of compression and confinement of the test gas in the settling chamber of a hypersonic wind tunnel with a free locking piston is justified. A method is proposed to compensate for the effect of adverse factors by optimizing the mass of an auxiliary piston introduced into the system. The effectiveness of the method is validated experimentally. As a result, for the case of gas compression to 200 MPa, the displacement of the center of mass of the barrel is reduced from 50 to 0.25 mm, which is smaller than the amplitude of elastic axial extension corresponding to the maximum pressure of the gas. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 5, pp. 212–220, September–October, 2008.     

Measurements of velocity spectra using time-resolving Doppler global velocimetry with laser frequency modulation and a detector array

Measuring velocity spectra in turbulent flows requires methods providing a high temporal resolution and a low measurement uncertainty. Hot-wire anemometry is often used, but it is intrusive. Laser Doppler anemometry is non-intrusive, but due to the statistical arrival of individual tracers provides no constant measurement rate. We therefore propose the use of Doppler global velocimetry (DGV), which is a contactless method allowing temporally equidistant measurements of continuous signals. Additionally, 2d measurements are possible instead of single point measurements. The commonly applied slow cameras are substituted by a fibre coupled detector array consisting of 25 avalanche photo diodes, which increases temporal resolution up to 10 μs. Contrarily to conventional DGV, a sinusoidal laser frequency modulation enables omitting the reference detector array. A correction of beam splitting and image misalignment errors is thus not necessary, but disturbances due to temporal fluctuations of the scattered light can occur and have to be reduced by increasing the modulation frequency. We validate the proposed system capability of synchronously measuring velocity spectra at multiple points in turbulent flows by presenting experimental results. The acquired velocity spectra in a wind tunnel experiment show good agreement with hot-wire comparison measurements within 0.1 m/s. An uncertainty analysis is given, which allows the achievable measurement uncertainty to be estimated as a function of the desired temporal resolution. An uncertainty down to 0.2 m/s can, for example, be achieved assuming a desired temporal resolution of 1 ms. These promising results open new perspectives for turbulence and correlation studies in flows such as to investigate the turbulence characteristics behind a truncated cylinder attached to a plate or the inlet of an aircraft turbine for flow characterisation in industry.     

Progress in Doppler picture velocimetry (DPV)

The special wide-field Michelson interferometer designed at ISL transforms the Doppler frequency shift of light scattered by tracer particles crossing a light sheet into a shift of luminous intensity at the output of the Michelson interferometer, yielding information about the particle velocity. To overcome former disadvantages, the optical set-up as well as the Doppler picture-processing algorithm were further improved. The present status of Doppler picture velocimetry (DPV) is explained by means of measurements carried out at Mach 6 in the ISL shock tunnel STA. The vertical velocity distribution around several bodies, such as a wedge, a sphere and a cylinder was visualized and measured.     

A century of wind tunnels since Eiffel

Fly higher, faster, preserve the life of test pilots and passengers, many challenges faced by man since the dawn of the twentieth century, with aviation pioneers. Contemporary of the first aerial exploits, wind tunnels, artificially recreating conditions encountered during the flight, have powerfully contributed to the progress of aeronautics.But the use of wind tunnels is not limited to aviation. The research for better performance, coupled with concern for energy saving, encourages manufacturers of ground vehicles to perform aerodynamic tests. Buildings and bridge structures are also concerned.This article deals principally with the wind tunnels built at ONERA during the last century. Somme wind tunnels outside ONERA, even outside France, are also evocated when their characteristics do not exist at ONERA.     

Assessment of turbulent spectral bias in laser Doppler velocimetry

A critical evaluation is made of the spectral bias which occurs in the use of a laser Doppler velocimeter (LDV). In order to accommodate the randomly sampled LDV data, statistical treatments of particle arrival times are needed. This is modeled as a doubly stochastic Poisson process which includes the intensity function of the velocity field. Three processing algorithms are considered for spectral estimates: the sample and hold method (SH), the modified Shannon sampling technique (SR), and the direct transform (RG). Assessment is made of these for varying data densities (0.05 ≤ d.d ≤ 5) and turbulence levels (t.i.=30%, 100%). The effects of the values of the Reynolds stress coefficients and the transversal standard deviation on the spectral contents were examined. As an improved version of the spectral estimator, the utility of POCS (the projection onto convex sets) has been tested in the present study. This algorithm is found useful to be in the region when d.d. ? 3.     

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.     

Unsteady flow in rotating drums using laser Doppler velocimetry

Non-destructive measurements by laser Doppler velocimetry is employed to study unsteady flow in a hollow drum filled with liquid. The drum is suddenly accelerated from rest or is suddenly decelerated from a steady rotation to rest. Pure water and glycerin-water mixtures are used as the test liquid in which polyethylenelatex particles are mixed as the light scattering tracer. The boundary layer formation, the time history of velocity, momentum and kinetic energy of the liquid, the wall-to-fluid force transfer, and the transient response time are determined. Also determined are the effects of side walls and fluid viscosity on the transient flow response. Of importance is the disclosure of Ekman layer instability near the inner radial wall of the test drum. It is actuated by the centripetal acceleration-induced buoyancy force.List of symbols A wetted surface area of test drum, cm² - a reciprocal of characteristic velocity, = t _sH, s/cm - B width of test drum, cm - b axial coordinate of test drum, cm - D diameter of test drum, cm; D ₁, inner diameter; D ₂, outer diameter - d diameter of laser beam, mm - d _p particle diameter, mm - E kinetic energy of liquid, kg · cm²/s²; E _s, steady value - F force transferred from drum walls to liquid, N - f focal length of lens, mm - G one-half of spacing between two parallel split beams, Fig. 1 - H characteristic length of test drum, cm; = V/A - M momentum of liquid, kg·cm/s; M _s, steady value - m mass of control volume, kg - r radial coordinate of test drum, cm - S fringe spacing, mm - t time, s - t _p time for particle to travel through fringe spacing, s - t _s transient time, s - u liquid velocity, cm/s - V liquid volume in test drum, cm³ - V _s effective volume of sample volume, mm³ - v velocity of tracer particle, cm/s; = S/t - W waist diameter of parabola in Fig. 2, mm - (x, y, z) coordinates for paraboloid in Fig. 2, mm - crossing angle of splitting beams, degrees - wavelength of laser length, cm - v kinematic viscosity, cm²/s - liquid density, kg/cm³ - Doppler frequency, l/s - s at steady state - 1 outer - 2 inner On leave from the Dept. of Mechanical Engineering, Musashi Institute of Technology, Tokyo, Japan     

Six-component strain-gage balances for large wind tunnels

ONERA, the French Aerospace Research Institute, currently design, manufacture and calibrate the many dynamometers and balances, measuring one or several components, necessary for the tests in the large wind tunnels and other installations of its Modane-Avrieux and Le Fauga-Mauzac Centers, as well as those requested by external organizations, in France or abroad. The number of dynamometric sensors realized since 1955 amounts to several hundreds. Their capacities range from 0.01 N to 1000 kN. Their precision is usually about 10^?3, but may reach 10^?4. This paper describes the problems concerning the metallic architecture of the multicomponent balances, their strain-gage and electric equipment, thermal effects and their compensation it presents the general principles and the means used for the calibration of these balances.     

Rheology of pulp suspensions using ultrasonic Doppler velocimetry

Conventional rheometry coupled with local velocity measurements (ultrasonic Doppler velocimetry) are used to study the flow behaviour of various commercial pulp fibre suspensions at fibre mass concentrations ranging from 1 to 5 wt.%. Experimental data obtained using a stress-controlled rheometer by implementing a vane in large cup geometry exhibits apparent yield stress values which are lower than those predicted before mainly due to existence of apparent slip. Pulp suspensions exhibit shear-thinning behaviour up to a high shear rate value after which Newtonian behaviour prevails. Local velocity measurements prove the existence of significant wall slippage at the vane surface. The velocimetry technique is also used to study the influence of pH and lignin content on the flow behaviour of pulp suspensions. The Herschel–Bulkley constitutive equation is used to fit the local steady-state velocity profiles and to predict the steady-state flow curves obtained by conventional rheometry. Consistency between the various sets of data is found for all suspensions studied, including apparent yield stress, apparent wall slip and complete flow curves.