Shock velocity measurements using microwave Doppler shift

A commercially available 10.587 GHz microwave Doppler module is used for the measurement of shock velocity in a conventional shock tube. With proper electronic circuits the Doppler frequency obtained is found to be quite noisefree and consistent for shock velocities in the range of 1.8 mm/sec to 2.0 mm/sec.     

空间激光干涉引力波探测

为印证广义相对论和开拓引力波天文学窗口,引力波探测是当前国际研究热点. 本文围绕空间激光干涉引力波探测,对其科学意义、发展状况、关键技术等进行了回顾. 与地面激光干涉引力波探测相比,空间探测的工作频段更低,从10^-4»10 Hz,在工作距离为百万公里量级上,预计能探测到双致密星系统、超大质量比双黑洞绕转系统、中等质量比双黑洞绕转系统,以及星系合并引起的超大质量黑洞并合等波源. 为此,测距精度须达到皮米的量级,并且保证测距技术有效工作的无拖曳航天技术亦有很高的要求. 本文以欧洲的空间激光引力波探测计划为例,主要对上述两项技术进行分析和阐述,并展望了空间引力波探测在我国的发展趋势和前景.     

Two component LDV system with dual-differential acousto-optical frequency shift and its applications

This paper describes the main properties of the two component LDV system developed with the dual-differential frequency shift technique. It has the advantages of multiple functions, better SNR and high utilization of information. So it offers as a complete technique and method for measuring 2-D complex flows. By applying this system, 2-D turbulent separated flow measurements have been made in a duct with an asymmetrical sudden expansion and in a wind tunnel flow over a 2-D rib. The Reynolds numbers were at 5900 and 4500 respectively.     

Model interferometry with air-holograms

An “air-hologram” is formed by the superposition of collimated object and reference light beams in a standard transmission holographic bench. Since no model is initially placed in the system, the reconstruction is simply the collimated beam that had passed through the model space. Insertion of transparent model material changes the optical path-length profile causing interference between the existing beam, which traverses the model, and the reconstructed beam which had traversed air only—hence, the designation “air-hologram”. Using this method, model material can be inspected and selected for optical flatness. Interference in the unloaded models can then be eliminated by simple rotation of the hologram. Real-time holographic interferometry is performed in the same manner as is photoelasticity. Further, it is shown how errors caused by large model displacements can be minimized.     

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.     

Dynamics of the shift in resonance frequency in a triple pendulum

We propose a general model for pendular systems with an arbitrary number of links arranged sequentially. The form of this model is easily adaptable to different settings and operating conditions. The main subject of analysis is a system obtained as a specific case taken from the general analysis, a three-links pendulum with damping subject to periodic perturbation. We performed a theoretical analysis of the frequency response and compared it with results from temporal integration. Moreover, a law was obtained explaining the behavior of the shift of the resonant frequencies due to a change in a parameter.     

Comparative holographic interferometry: A nondestructive inspection system for detection of flaws

Comparative holographic interferometry addresses a key problem of nondestructive evaluation: comparison of the resistance to strength in real time of two nominally identical specimens. After outlining the basis of comparative holography in flaw detection, the present paper reports on some new and complementary developments in comparative holographic interferometry. The proposed systems stand out by their ability to store the master displacement field in the interferometric setup. The integration of the storage capacity in the instrument considerably augments its potential in nondestructive routine inspection tasks. The visual display of the fringes contouring the difference in mechanical response is shown to improve considerably with the addition of auxiliary phase difference satisfying certain conditions. Methods for the generation of corresponding fringes are considered and their localization investigated in brief. Particular attention is devoted to the formation of the holographic moiré fringes. The influence of system misalignment on the moiré fringe interpretation is examined. A potential application of comparative holography to the quantitative evaluation of fatigue is described. Experimental evidence supporting the operational feasibility of the technique along with the results obtained in application to flaw detection are finally presented.     

Resolution enhancement of Doppler frequency measurements using a centroid technique

 A centroid-based technique for locating peaks in the frequency spectrum is demonstrated to have the potential for substantially enhancing the resolution of Fourier-based laser Doppler velocimetry measurements. The effect of data windowing and signal-to-noise ratio on the potential resolution improvement is also analyzed. Received: 10 January 2001/Accepted: 19 November 2001     

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.     

Moiré interferometry with ±45-deg gratings

Specimen gratings with rulings oriented +45 deg and ?45 deg to they axis can be used to determine theu andv displacement fields, i.e., the displacements in thex andy directions. The fringe patterns are identical to those obtained with (X) and (Y) specimen gratings. The analysis leading to this conclusion is presented. A vector quantity related to the fringe gradient in the interference pattern is defined and found effective for visualizing and describing the image-forming rays. Advantages relate to construction of apparatus for moiré interferometry.     

Thermal expansion measurement of composites with optical heterodyne interferometry

A dilatometer based on an optical heterodyne laser interferometer for high-precision measurements of thermal expansion coefficient (α) has been developed. The instrument is mainly intended for measuring low-expansion composites at room temperature. Compared with already existing instruments, the proposed system is characterized by a higher thermal expansion resolution (30 nm/m). In the case of low-expansion composites, intervals of uncertainty of α lower than 0.1 μm/mK have been obtained.     

Thermal bending associated with heat conduction by holographic interferometry

An experimental technique using real-time holographic interferometry combined with digitized image processing has been developed to measure the thermal diffusivity of polymers. This technique uses a cantilever beam or an annular disk with one side subjected to a pulse of radiant energy from a photographic flash. The resulting thermally induced deflection is measured by holographic interferometry. The observed deflection is due to a resultant thermal moment induced by a temperature gradient through the thickness. As time goes on, the heat conducts from the exposed surface through the thickness, resulting in a decrease of the bending moment and transverse deflection. It is shown that the deflection is proportional to the thermal moment, and the thermal diffusivity can be retrieved by moment analysis without deriving the analytical solution to the thermomechanical problem.     

Moiré interferometry with embedded grids-effect of optical refraction

Optical refraction resulting from refractive index gradients in nonhomogeneously stressed models is investigated as a limitation on the usefulness of moiré interferometry employing embedded grids. Apparent displacements caused by internal refraction are estimated for static and dynamic cases; Boussinesq's problem, and the problem of a spherically expanding pulse, respectively. Refraction effects are found to be far larger in the dynamic case. Moiré observations of motions in models loaded by pellet impact are compared with independently measured motion histories. Anomalies in the moiré data are found to agree fairly well with predicted refraction effects for a spherically expanding pulse. It is concluded that optical refraction effects are potentially quite detrimental to observation of transient internal motions using moiré interferometry.     

Measuring velocity fields in wind tunnels with holographic interferometry

This paper investigates the feasibility of using holographic interferometry in wind tunnel flows for measuring velocity fields rather than density or temperature fields. First results were obtained in a vortex street behind a cylinder at Re=190(U _∞=0.7 m/s). The light scattered from an illuminated fluid plane was holographically recorded twice with the same reference beam. Using a time interval of 10 μs, local fluid displacements smaller than a few microns were recorded. The holographic plate was placed in front and as close as possible to the fluid plane. The interferograms obtained from the hologram reconstruction give information about one velocity component, at 45° with the illuminated plane. The alignment of the cylinder axis with this 45° direction provided definite confirmation about the vortex street having a non-negligible axial velocity. The constant velocity fluid region has proven to be very useful for quantifying the velocity information contained in the interferogram. Received: 8 November 1999/Accepted: 14 March 2000     

Instantaneous phase-stepping interferometry using polarization imaging with a micro-retarder array

An instantaneous phase-stepping and subsequent phase analysis method, using a CCD camera with a form-birefringent micro-retarder array, is proposed for interferometry. An optical setup of a polarization interferometry using a Twyman-Green interferometer with two polarizers is constructed to analyze the distribution of out-of-plane displacement. Light emerging from the interferometer is recorded using a CCD camera that has micro-retarder array on the CCD plane. This micro-retarder array has four different principal directions. That is, an image obtained by the CCD camera contains four types of data corresponding to four different optical axes of the retarder. The four images separated from the image recorded by the CCD camera are reconstructed using gray-level interpolation. Then, the distributions of the Stokes parameters that represent the state of polarization are calculated from the four images. The phase distribution of the interference fringe pattern produced by the Twyman-Green interferometer is then obtained from these Stokes parameters. This method is applicable to time-dependent phenomena because multiple exposures are unnecessary for sufficient data acquisition in the completion of phase analysis.     

Global detection of detached periodic solution branches of friction-damped mechanical systems

Nonlinear Dynamics - Two novel methods to determine detached periodic solution branches of low-dimensional and large-scale friction-damped mechanical systems have been developed. The approach for...