Dual-beam Laser Doppler Velocimeter Research Articles

载体振动对差动激光多普勒测速仪的影响

载体振动对差动激光多普勒测速仪的影响

Research on laser Doppler velocimeter for vehicle self-contained inertial navigation system

An idea of using laser Doppler velocimeter (LDV) to measure the velocity for the vehicle inertial navigation system was put forward. The principle of measuring its own velocity with laser Doppler technique was elaborated and reference-beam LDV was designed. Then Doppler signal was processed by tracking filter, frequency spectrum refinement and frequency spectrum correction algorithm. The result of theory and experiment showed that the reference-beam LDV solved the problem that dual-beam LDV cannot be used for measuring when the system was out of focus. Doppler signal was tracked so that signal-to-noise ratio was improved, and the accuracy of the system was enhanced by the technology of frequency spectrum refinement and correction. The measurement mean error was less than 1.5% in velocity range of 0–30 m/s.

Influence of the configuration of the optical setup on the characteristics of Doppler signals in a dual-beam laser velocimeter: evolution of signal visibility

A method is presented to compute signal characteristics in a dual-beam laser Doppler velocimeter of any configuration and, as a first application, the influence of the optical configuration on signal visibility is also investigated. A simplified model of signal generation is also introduced by which the behavior of signal properties can be physically explained. This model shows that signal visibility is due to a coupling effect between the spatial period of the scattered interference pattern and the angle subtended by the detection aperture. Closed analytical expressions are derived for this period for the scattering components of reflection and refraction without internal reflections. A similar study concerning the evolution of signal phase shift is now in progress.

Fiber-optic dual-cylindrical wave sensor for measurement of wall velocity gradient in a fluid flow

A Young's fringe pattern created by two light waves emanating from a pair of slits has been used as the basis of a fiber-optic device for measuring the velocity gradients of flows near solid surfaces. This device is a variant of a dual-beam laser Doppler velocimeter, specialized to direct measurements of wall velocitygradients. A compact version of the device is described in which a fiber-polishing technique is employedfor fabrication of a two-point optical source consisting of two fiber cores separated by 18 µm. The light from this source is collimated and focused on two slits that are prepared through electron-beam lithography and that are 1 µm wide and 10 µm apart. Preliminary testing of the compact probe has been successful.

前方散乱差動型ＬＤＶによる粒径，速度の測定

A method of in situ particle size and velocity measurement in parti-cle laden flow by forward-scatter dual-beam Laser Doppler Velocimeter (LDV) has been investigated both analytically and experimentally. The analysis is based on Mie's electromagnetic scattering theory for spherical particles to relate the collected scattered light power for the LDV optical arrangement to the particle diameter. The measured de-pendence of the pedestal height in the Doppler burst signal to the particle diameter agrees well with calculations from Mie's theory. It should be emphasized that the in-creasing dependence assumption of collected scattered power on the square of particle diameter is not satisfied for the LDV collection optics geometry and the calibration with numerical calculations based on Mie's theory is indespensable. Although the mask has to be attached on the collecting lens to block the unscattered beams, it may be noticed that the monotonic dependence of collected scattered power on the particle size can not be sustained for too small collection opening. The selective data sampling scheme to set the threshold of the certain Doppler cycle number for each burst signal is proved to re-duce significantly the error resulting from particle trajectry through nonuniform incident intensity distribution in the measurement volume, however, overall measurement error at most approximately 30% is evaluated.

Flow mechanism of .LAMBDA.-type pseudoshock waves in a straight-square duct.

This paper is concerned with the internal structure of λ-type pseudoshock waves in a straight-square duct. The experiments were carried out for M1=1.77, where M1 Was the flow Mach number just ahead of the pseudoshock wave, using a two-component, dual-beam laser Doppler velocimeter (LDV) and double exposure laser holographic interferograms. The velocity distributions in the pseudoshock wave were measured in detail. The variations of velocity and boundary layers displacement thickness were clarified. Especially, the displacement thickness increased at the shock wave locations and decreased in the regions between the shock waves. The velocity distributions calculated by the density distributions, which were measured by using the holographic technique, were compared with the results from LDV. It was found that the central flow in the pseudoshock wave was closely related to the isentropic flow, while the flow near the wall was closely related to the adiabatic flow.

On principal noise of the laser Doppler velocimeter

The principal noise of the Laser Doppler Velocimeter is shown to be due to transit broadening rather than the most commonly suggested signal shot; the phase ambiguity noise is also shown as less serious for the dual-beam Laser Doppler Velocimeter. On the basis of optical geometry and Fourier analysis of electric fields interacting with moving particles, the transit broadening noise has been calculated; some supporting experimental evidence is also discussed along with means for reducing this principal noise.

Velocity field of convergent flow into a rectangular slit for a polymeric fluid

Three-dimensional velocity distributions in the entry region of a rectangular slit contraction were investigated using a dual-beam laser Doppler velocimeter. The flow of a silicone oil (a Newtonian fluid) and a solution of silicone rubber in the same silicone oil (a viscoelastic fluid) was studied at low Reynolds numbers (Re < 0.5). In contrast to the usual velocity distribution of a Newtonian fluid, the viscoelastic fluid showed the following characteristic features: (1) a pronounced axial velocity overshoot immediately after the slit entrance and a maximum before the slit exit; (2) appearance of an axial flow deceleration region just before the sharp acceleration near the slit entrance. Even more remarkably, a saddle form of velocity profile was found in the entrance region. This flow pattern is completely different from that found for Newtonian fluids and has not yet been explained using existing rheological analysis.

A correction lens for laser Doppler velocimeter measurements in a cylindrical tube

A lens was designed using analytical ray tracing techniques to correct aberrations caused by a cylindrical tube wall when measurements are made off the plane of symmetry with a dual-beam laser Doppler velocimeter. The lens keeps the probe volume stationary when the measurement axis is rotated and reduces aberrations in the image of the probe volume on the pinhole. A planoconcave cylindrical lens with a radius of curvature of 1195 mm (47 in.) was fabricated and tested for use with a 101.6 mm (4 in.) o.d., 3.2 mm (1/8 in.) thick acrylic plastic tube with air as the flow medium. Good results were obtained out to a normalized tube radius of r/R = 0.83.

Remote measurement of wind speed using a dual beam backscatter laser Doppler velocimeter

Measurements of atmospheric wind speeds at distances up to 60 m using a dual beam laser Doppler velocimeter with digital autocorrelation analysis, an unfocused argon ion laser (0.10-0.35 W) at 5145 A, and a 20-cm collecting telescope are reported. Criteria affecting the field performance of this system and an expression giving the power and range dependence of the data accumulation rate are presented. Major improvement of the system's performance can be made by focusing the laser beams and properly restricting the field of the collecting telescope. The potential of this technique for remote wind measurement at ranges of several hundred meters is discussed.

A geometric analysis of the probe volume in a dual beam laser Doppler velocimeter

The probe volume of the dual beam laser Doppler velocimeter is analyzed from a geometrical standpoint. The volume is graphically reconstructed and the existence of the characteristic dual beam laser Doppler velocimeter fringes is demonstrated. The fringes are analyzed and are shown to be composed of moving tubes or pockets of coherent radiation. The composition of the light is analyzed and the extent of the coherence of the laser radiation within the light pockets is considered. The scattered light from the tubes is analyzed and the coherence enhancement of the fringe structure is shown. This information is used in an attempt to resolve the controversy as to whether the dual beam laser Doppler velocimeter is governed by a fringe counting phenomena or by a beating phenomena of light from two sources of coherent radiation of different frequencies.