Digital holographic particle image velocimetry is a promising technique for the measurement of the instantaneous flow velocity within a volume of a flow. It poses, however—problems of illumination and recording—the major problem of how to extract the information about the location of the particles within the flow and their displacement between two illuminations from the storage medium, i.e., the digital hologram. Various recording parameters, such as the recording distance, the particle size, the wavelength, the size of the CCD chip, the pixel size, and the particle concentration, will considerably affect the results of the reconstruction, and may even determine the success or failure of a measurement. This paper discusses the influence of these parameters and presents a formula for the critical recording distance. Numerical simulations indicate that when the recording distance is near to the critical value, the results of the reconstruction are much better than those obtained far away from it, especially for high particle concentration. An experiment has also been made to validate the analysis.
The objective of this paper is the description of the Projected Pattern Correlation method for measuring surface velocities and to present results of a feasibility study. Similar to the Moire technique the local surface velocities of a large area are determined simultaneously, which replace a time consuming point wise scanning as it is necessary in e.g. Laser Doppler Vibrometry. Furthermore, the dynamics of non-periodic processes can be resolved temporally and spatially. In difference to the Moire or grid projection techniques the evaluation step is fast (real-time measurements are possible) more robust and provides a high spatial resolution. The measurement precision is assessed using a simple test arrangement. Vibration measurements are performed on a satellite model structure and a honeycomb sandwich plate.
A high-speed digital camera based on video technology for application of particle image velocimetry in wind tunnels is described. The camera contains two independently triggerable interline CCD sensors which are mounted on two faces of a cube beam splitter permitting the use of a single lens. Each of the sensors has a minimal exposure time of 0.8 microsecond(s) with a trigger response time of less than 1 microsecond(s) . The asynchronous reset capability permits the camera to trigger directly off a pulsed laser with a repetition rate differing from the standard 25 Hz CCIR video frame rate. Captured images are digitized within and stored in RAM the camera which can be read through the parallel port of a computer. The camera is software configurable with the settings being non-volatile. Technical aspect such as sensor alignment and calibration through software are described. Close-up PIV measurements on a free jet illustrated that, in the future, the camera can be successfully utilized at imaging high-speed flows over a small field of view covering several cm2, such as the flow between turbine blades. Further, the electronic shutter permits its use in luminous environments such as illuminated laboratories, wind tunnels or flames.
In its 'classical' form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double-pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross-correlation methods, are used to determine the projections of the velocity vectors onto the plane between both light sheets. In the manner described below, the amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light sheet planes. The mathematical background of this method is described in the paper. Numerical simulations show the influence of the main parameters (e.g. light sheet thickness, light sheet displacement and out-of-plane component) on the resolution and reliability of the new method. Two different recording procedures and their results will be shown to demonstrate the ease of operation when applying this technique to liquid flows.
Quantitative and visual data of artificially forced instabilities are acquired in a flat plate boundary layer in air. For the first time, particle image velocimetry (PIV) allows the instantaneous recording of a complete velocity field. With this technique, quantitative data are obtained within the boundary layer and very close to the wall. By using a light sheet technique it is possible furthermore to obtain visual data. Reproducible and constant conditions in the development of the instabilities are achieved by forcing the boundary layer with well-controlled disturbances. These disturbances are generated with an arrangement for acoustic excitation.
Particle Image Velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. At DLR an experimental setup for PIV has been developed which can be operated in transonic flows under the rough environmental conditions (noise, vibrations) of a large high speed wind tunnel. This PIV-system has been successfully applied to flow field measurements in the velocity range from U equals 10 to 500 m/s. The evaluation and post processing of the PIV recordings runs fully automatical on a workstation. Two different aerodynamic investigations of instantaneous transonic flow fields around a bluff cylinder and around a NACA 0012 airfoil have been performed in a blow-down wind tunnel. The experience gained during these experiments and some technical improvements of the PIV technique which are necessary to enable its application in transonic flows are described.
Quantitative and visual data of artificially forced instabilities have been acquired in a flat plate boundary layer in air. For the first time the Particle Image Velocimetry (PIV) allows the recording of a complete velocity field instantaneously. With this technique quantitative data have been obtained within the boundary layer also very close to the wall. By using a light sheet technique it was possible furthermore to obtain visual data. Reproducible and constant conditions in the development of the instabilities are achieved by forcing the boundary layer with well controlled disturbances. These disturbances are generated with an arrangement for acoustic excitation.
The instantaneous flow field above a NACA 0012 airfoil has been investigated by means of Particle Image Velocimetry (PIV). Two different flow conditions were realized in the high speed blow down wind tunnel of DLR Gottingen in order to obtain a flow field similar to that above a rotor blade of a helicopter. In the first part of the experiment the angle of incidence varied from (alpha) equals 5 degree(s) to (alpha) equals 20 degree(s) at a freestream velocity of Ma equals 0.25 in order to simulate the flow above the retreating blade of the rotor. Typical values for the second measuring sequence were (alpha) equals 0 degree(s) to (alpha) equals 5 degree(s) and Ma equals 0.75 (advancing blade). Due to some technical improvements of the DLR PIV-System, such as a high speed rotating mirror system and an optimized seeding device, it was possible to obtain the velocity data of these instantaneous flow fields with an accuracy high enough to allow a later comparison with data of numerical calculations.
A method for evaluating particle image velocimetry (PIV) recordings by means of an analog optical processor utilizing a spatial light modulator is proposed. A prototype of such a setup has been tested and the successful evaluation of PIV recordings has been demonstrated.
A fast analog optical method for evaluating Doppler bursts is presented. The method uses an optical 2D Fourier processor and a spatial light modulator. Two different setups for evaluation of Doppler bursts are described. The method reduces considerably the time needed for evaluation and the setup cost is reduced compared to computer-based burst analyzers.
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