Session MW: Experimental Techniques II

Laser imaging measurements of flow dynamics and mixing in gel-phase flows

Gelled hypergolic propellants are interesting in rocket propulsion applications, in combining the stability of solid propellants and the controllability of liquid propellants. To exploit these advantages fully, we require an improved understanding of the flow and mixing properties of gel-phase fluids. In this work, we apply planar laser-induced fluorescence (PLIF) to investigate gel mixing in a mixing layer geometry, and particle image velocimetry (PIV) to measure flow velocities in and around two impinging gel streams. We consider both water-based gels (Ultrez 10) and mineral-oil based gels (Kraton G1650) of varying compositions (strengths). For the PLIF, these gels are doped with disodium fluorescein dye. We will discuss some of the issues attendant to the application of these laser diagnostic methods in the gel phase, and we will illustrate how these gel-phase flows differ from flows of Newtonian fluids in similar flow geometries.   

Low Pressure Seeder Development for PIV in Large Scale Open Loop Wind Tunnels

A low pressure seeding techniques have been developed for Particle Image Velocimetry (PIV) in large scale wind tunnel facilities was performed at the Subsonic Aerodynamic Research Laboratory (SARL) facility at Wright-Patterson Air Force Base. The SARL facility is an open loop tunnel with a 7 by 10 foot octagonal test section that has 56{\%} optical access and the Mach number varies from 0.2 to 0.5. A low pressure seeder sprayer was designed and tested in the inlet of the wind tunnel. The seeder sprayer was designed to produce an even and uniform distribution of seed while reducing the seeders influence in the test section. ViCount Compact 5000 using Smoke Oil 180 was using as the seeding material. The results show that this low pressure seeder does produce streaky seeding but excellent PIV images are produced.   

Large depth-of-field PIV in a in a narrow channel

The current work is motivated by a goal to obtain quantitative temporally-resolved velocity measurements of buoyant natural convection within a Hele-Shaw cell. In contrast with typical micro-PIV studies, PIV in a Hele-Shaw cell requires a large field-of-view in comparison to the channel gap spacing, precluding the use of a thin light sheet or a small depth-of-field that can isolate a narrow region of the local Poiseuille velocity profile across the gap. This necessitates imaging particles across the whole depth, causing the cross-correlation to become broadened by the velocity gradients across the gap. In addition to the velocity gradients, the finite Reynolds numbers associated with typical flow conditions may cause significant inertial migration of the seed particles, creating an evolving and non-uniform concentration distribution, which in turn will change the shape and relative peak location of the cross-correlation. In order to make a quantitative relationship to local mean flow within the gap, a uniform flow is studied experimentally and modeled using synthetic image generation at various positions along the flow. This information will then be used to optimize the conditions for reliable PIV interrogation, and sample results of buoyant convection will be given.   

Particle shadow velocimetry vs. LDV: measurements of a turbulent pipe flow

Image pre-processing was used to improve multi-color Particle Shadow Velocimetry (PSV) measurements of a near-wall turbulent pipe flow. These included corrections for color cross-talk, color aberration and image distortion. Multi-color PSV is a modification of DPIV which employs pulsed multicolor lights for illumination. The particle shadows are imaged to produce 2-D vector fields with traditional DPIV correlation methods. Multi-color PSV allows for higher temporal sampling rates than conventional DPIV, at lower cost. Aberrations were quantified by imaging a target with known geometry and generating a mapping function to correct the multicolor shadows for chromatic aberration shadow displacement shifts between colors. PSV images were spatially corrected with these functions. Color cross-talk was corrected for by subtracting average cross-talk intensity from the local intensity. Corrected mean velocity and second order statistics for turbulent pipe flow in the Penn State ARL glycerin tunnel showed favorable comparison to LDV and standard PIV measurements.   

A vision-based hybrid particle tracking velocimetry (PTV) technique using a modified cascade-correlation peak-finding method

In this talk we present new algorithms for particle identification and particle tracking velocimetry (PTV). The new particle identification algorithm uses both the Cascade Cross-correlation Method (CCM) and 2-D surface Gaussian fitting to overcome the issue of overlapping particles. Simulation with up to 5{\%} noise shows particles can be located with errors under 0.07 pixels for overlap ratios up to 50{\%}. The new PTV method, taking advantage from vision theory, is developed to map from a ``proximity'' matrix to a ``pairing'' matrix while inherently satisfying the exclusion principle. The validity of the results is ensured by hybridization with PIV data, and the reliability of the method is further tested by adding noise to synthetic data. The proposed method gives reliability values up to 99.9{\%} at particle densities up to 0.06 for a simulated moving wall flow, an oscillating wall flow and an Oseen vortex. Variations in particle intensity and diameter are also tested with simulated flow images. An experimental shear layer image pair is then tested, proving the robustness of the proposed method. Furthermore, it is shown that kriging interpolation in combination with PTV results can accurately resolve velocity gradients such as the wall region.   

A compact Self-Contained Underwater Velocimetry Apparatus (SCUVA) for in situ field measurements in daytime

In-field measurements at remote location present a challenge for the measurement systems involved. Not only do these systems have to be self-sufficient in regard to power supply and data acquisition but also robust and easy to handle. With the current level of miniaturization in electronics it becomes possible to construct PIV-systems, which meet these criteria and are even small enough to be used as hand held devices. Following the work of Katija and Dabiri\footnote{K. Katija and J.O. Dabiri, ``In situ field measurements of aquatic animal-fluid interactions using a Self-Contained Underwater Velocimetry Apparatus (SCUVA)''; Limnol. Oceanogr.: Methods 6, 162-171 (2008)} we present a PIV- system, which is designed for SCUBA divers to take in-field measurements of the flow around marine organisms in daytime. The fact that the system can be operated in daytime makes work for the divers considerably easier. On the other hand it presents an additional challenge due to the laser power, which can be installed in portable devices. A detailed description of the measurement setup will be given together with a discussion of some preliminary results.   

3D Synthetic Aperture Imaging for Fluid Flows

Three-dimensional and multiphase fluid flow environments demand advanced and innovative measurement systems in order to fully resolve the flow physics. We present implementations of synthetic aperture imaging techniques for 3D particle image velocimetry and bubble flow field extraction. This work lays the foundation for a comprehensive tool for measuring multiphase flows. Simulations have shown 3D synthetic aperture particle image velocimetry (SAPIV) to be a promising technique for resolving 3D velocity fields in densely seeded flows using an array of cameras. Here, we experimentally study a canonical vortex ring using a low-cost 8 camera array and benchmark the results with standard 2D PIV. Also, using a high speed camera array, we apply synthetic aperture imaging to the 3D bubble field entrained by a jet impinging on a free-surface.   

Modern quantitative schlieren techniques

Schlieren optical techniques have traditionally been used to qualitatively visualize refractive flowfields in transparent media. Modern schlieren optics, however, are increasingly focused on obtaining quantitative information such as temperature and density fields in a flow -- once the sole purview of interferometry -- without the need for coherent illumination. Quantitative data are obtained from schlieren images by integrating the measured refractive index gradient to obtain the refractive index field in an image. Ultimately this is converted to a density or temperature field using the Gladstone-Dale relationship, an equation of state, and geometry assumptions for the flowfield of interest. Several quantitative schlieren methods are reviewed here, including background-oriented schlieren (BOS), schlieren using a weak lens as a ``standard,'' and ``rainbow schlieren.'' Results are presented for the application of these techniques to measure density and temperature fields across a supersonic turbulent boundary layer and a low-speed free-convection boundary layer in air. Modern equipment, including digital cameras, LED light sources, and computer software that make this possible are also discussed.   

Megahertz rate Schlieren visualization of underexpanded, impinging jet using pulsed high power LED

Recent advances in light emitting diode (LED) technology has resulted in high power, single chip devices that provide luminous radiant fluxes exceeding several watts. Operated in pulsed current mode the instantaneous light emission of an LED can be further increased to levels comparable to that of photographic (xenon) flash units making it a suitable light source for Schlieren imaging. Compared to the commonly used xenon flash units an LED can be triggered within tens of nanoseconds at rise times on the order of 100 ns thereby enabling stroboscopic illumination at megahertz rates. In the present application the LED's driving electronics were synchronized to a high speed camera to provide time-resolved Schlieren images of an underexpanded free jet impinging on a flat plate (nozzle pressure ratio 2.0 to 5.2). The LED was pulsed in burst mode for 102 images at currents of up to 120 A at 500 ns per pulse. Compared to images obtained with a xenon white light flash the nearly monochromatic green light of the LED results in much crisper flow features with superior repeatability in intensity without any speckle artifacts commonly found with laser illumination.   

Direct Measurement of Turbulent Shear

Photon Correlation Spectroscopy (PCS) is used to directly measure the mean shear rate ${\overline s}$ in a turbulent soap film. A 5 mW 633 nm He-Ne laser is focused on the film at a point {\bf r}, the spot size being $w$ =100 $\mu$m. The scattered light intensity $I(t)$ is analyzed by a correlator that measures the average, over time $t$, of the correlation function $G(\tau) = \langle I(t)I(t + \tau) \rangle/\langle I(t)\rangle^2-1$. From $G(\tau)$, one extracts the shear ${\overline s}$ averaged over $w$ and the standard deviation of ${\overline s}$. Of special interest is the shear at points ${\bf r}$ near a solid boundary. The PCS measurements of ${\overline s}$ (in Hz) are compared with those obtained by laser Doppler velocimetry (LDV). The two techniques yield values of ${\overline s}$ that agree within a standard deviation. The PCS method has the advantage of compactness and rapid data collection, making it of potential use in biology and medicine. By changing the orientation of the incident and scattered beams, one can measure various components of the shear tensor. The implementation of the PCS method does not require the presence of a mean flow. It can also be applied to three-dimensional turbulence.