Preliminary Development of a Nearly-Instantaneous Three- Dimensional Imaging Technique for High-Speed Flow Fields

Recent advances in high-repetition rate laser and camera technology present a new opportunity to develop three-dimensional diagnostics for high-speed flows. The design of a three-dimensional imaging system based on a pulse burst laser, a high-speed laser scanner and a high speed camera is described here. The pulse burst laser system is the 5 th of its kind in the world and can produce high energy pulses at up to 10 MHz repetition rates. A highspeed optical deflector, such as a rotating mirror or acousto-optic deflector, can be used to rapidly deflect a laser sheet through the flow field. A high-speed camera can then be used to collect images at different planes in the flow field, from which a three-dimensional image can be reconstructed. The state-of-the-art of these technologies are described. The high-speed characteristics of an acousto-optic deflector were tested using an Nd:YAG laser where it was found that a full sweep through at least 32 resolvable spots could be completed in 10 μsec. Future work will include testing of a galvanometric scanning mirror and assembly of a complete system

Simultaneous Conventional and Plenoptic Background Oriented Schlieren Imaging

Plenoptic Background Oriented Schlieren (BOS) is an emerging schlieren technique that is capable of providing 3D qualitative and quantitative information about density gradients present in a wide range of fluid dynamics problems. In this work, the fundamental concepts of plenoptic BOS are reviewed before discussing an open-air experiment with a buoyant plume where both conventional BOS and plenoptic BOS measurements were acquired simultaneously. Both cameras had the same field-of-view for all experiments, and three different focal plane arrangements were explored: (1) the focal plane was set to the background positions and the plume varied between 11 different positions relative to this focal plane, (2) the focal plane was set to 635- millimeters in front of the background position, and (3) the nominal focal plane varied while the position of the plume remained fixed. Such discussion will provide insight on how the two techniques compare, and what additional work is required to better understand the results provided by these two imaging systems

Comparison of Stereo-PIV and Plenoptic-PIV Measurements on the Wake of a Cylinder in NASA Ground Test Facilities.

A series of comparison experiments have been performed using a single-camera plenoptic PIV measurement system to ascertain the systems performance capabilities in terms of suitability for use in NASA ground test facilities. A proof-of-concept demonstration was performed in the Langley Advanced Measurements and Data Systems Branch 13-inch (33- cm) Subsonic Tunnel to examine the wake of a series of cylinders at a Reynolds number of 2500. Accompanying the plenoptic-PIV measurements were an ensemble of complementary stereo-PIV measurements. The stereo-PIV measurements were used as a truth measurement to assess the ability of the plenoptic-PIV system to capture relevant 3D/3C flow field features in the cylinder wake. Six individual tests were conducted as part of the test campaign using three different cylinder diameters mounted in two orientations in the tunnel test section. This work presents a comparison of measurements with the cylinders mounted horizontally (generating a 2D flow field in the x-y plane). Results show that in general the plenoptic-PIV measurements match those produced by the stereo-PIV system. However, discrepancies were observed in extracted pro les of the fuctuating velocity components. It is speculated that spatial smoothing of the vector fields in the stereo-PIV system could account for the observed differences. Nevertheless, the plenoptic-PIV system performed extremely well at capturing the flow field features of interest and can be considered a viable alternative to traditional PIV systems in smaller NASA ground test facilities with limited optical access

Determination of Noise Sources within a high-speed Jet via Simultaneous Acoustic Measurements and real-time Flow Visualization,”

Much of the work in the field of jet aeroacoustics has focused on how to model noise sources without sufficient knowledge on what exactly constitutes a noise source. Further, the acoustics of a jet are often analyzed without any regard for the dynamic features of the noise producing events. The work presented in this paper examines the dynamic far field acoustic signature of an ideally expanded, Mach 1.3, high Reynolds number jet, and then attempts to relate outstanding features of the acoustic signature to interactions between large-scale structures within the jet mixing layer. A four-microphone array was used to determine the origin of individual sound pressure events while movies of the flow were taken simultaneously with a pulse burst laser/high speed camera system. Two different noise generation mechanisms have been observed and are presented here. The first involves the formation of large structures within the mixing layer via structure rollup, while the other mechanism involves the strong interaction between structures within the two sides of the mixing layer

Light-Field Imaging Toolkit

The Light-Field Imaging Toolkit (LFIT) is a collection of MATLAB functions designed to facilitate the rapid processing of raw light field images captured by a plenoptic camera. An included graphical user interface streamlines the necessary post-processing steps associated with plenoptic images. The generation of perspective shifted views and computationally refocused images is supported, in both single image and animated formats. LFIT performs necessary calibration, interpolation, and structuring steps to enable future applications of this technology. Keywords: Plenoptic, Light field, MATLA

Introduction to engineering experimentation

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