Session AI: Experimental Techniques I

Three-dimensional in vivo blood flow imaging based on DDPIV defocusing concept

Three-dimensional microscale quantitative flow visualization is of considerable interest in fluid mechanical and biomedical research. In this study we present a high-speed three-dimensional microscopic system capable of in vivo microscale biofluid imaging, based on the defocusing digital particle image velocimetry (DDPIV) defocusing concept. A 3-aperture mask is attached to the back of an objective lens on an inverted microscope in order to generate defocused triangular image patterns. The system is capable of resolving spatial coordinate (Z) of a flow tracer from the separation between its corresponding defocused images. Capability of microscale imaging was validated by a calibration procedure. We demonstrated 3D blood flow imaging in an embryonic zebrafish using the developed system. Trajectories of injected 1-$\mu $m fluorescent tracer particles in the zebrafish yolk sac were obtained, with a measurement volume of 80x40x25 $\mu $m$^{3}$.   

Development of a 3D Digital Particle Image Thermometry and Velocimetry (3DDPITV) System

A novel 3D Digital Particle Image Thermometry and Velocimetry (3DDPITV) system has been designed and fabricated. By combining 3D Digital Particle Image Velocimetry (3DDPIV) and Digital Particle Image Thermometry (DPIT) into one system, this technique provides simultaneous temperature and velocity data in a volume of $\sim $1x1x0.5 in$^{3}$ using temperature sensitive liquid crystal particles as flow sensors. Two high-intensity xenon flashlamps were used as illumination sources. The imaging system consists of six CCD cameras, three allocated for measuring velocity, based on particle motion, and three for measuring temperature, based on particle color. The cameras were optically aligned using a precision grid and high-resolution translation stages. Temperature calibration was then performed using a precision thermometer and a temperature-controlled bath. Results from proof-of-concept experiments will be presented and discussed.   

Development and Application of a Modified Single-Camera 3DDPIV System

Three Dimensional Defocusing Particle Image Velocimetry (3DDPIV), as a true three-dimensional measurement system, allows for the measurement of three-dimensional velocities within a volume. Initially designed using a single CCD and 3-pinhole mask (Willert {\&} Gharib; 1993), it has evolved into a multi-CCD camera system in order to overcome the limitations of image saturation due to multiple exposures of each particle and as well as being able to properly image large areas (Pereira F, Gharib M, Dabiri D, et al.; 2000). We have modified the original single CCD implementation in order to allow for imaging of small areas without the above-mentioned limitation of multiple exposures of each particle. How these limitations are overcome towards achieving this modified single camera 3DDPIV system are presented and discussed. We apply this towards quantitatively visualizing the three-dimensional flow within a Rayleigh-Bernard convection cell (chamber height is 7 mm).   

Development of Small-Scale Submersible PIV System

A low-cost, small-scale submersible Particle Imaging Velocimetry (PIV) device has been developed to characterize unsteady flow in natural environments. PIV systems provide high accuracy, non intrusive, planar flow measurements of velocity and vorticity. Since this device is intended for the field, it is designed to be portable. This is accomplished using a powerful handheld laser, beam chopper, microprocessor, and the proper lenses, in conjunction with a one mega pixel CCD video camera. The system consists of two connected waterproof cases; one housing the camera and the other the laser/chopper system. The apparatus is fully self-contained and can be operated using a laptop computer on shore or on a floating platform. The system is also unique in that it was developed for under {\$}8000 USD. The PIV device was tested in a small creek in Michigan. Eddy diameter, circulation, orientation, and convective velocity were characterized. The design of a submersible PIV system like this one will lead to a better characterization of naturally occurring flows and a greater understanding of what conditions aquatic life find acceptable. This knowledge will prove most useful in river and shoreline restoration, as well as in the design of new coastal management plans to alleviate human impact on coastal regions.   

PIV Seeding Techniques in Large Scale Open Loop Wind Tunnel Facilities

Seeding techniques to validate the use of 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. Two seeding techniques were tested at Mach 0.2 and 0.3: a Rosco fogger line seeder system originally designed for flow visualization in the tunnel and a fluidized bed of aluminum oxide dispensing from a multi-port rod. Two and 3 component PIV images were taken in the streamwise plane over a semi hemispherical geometry that includes a shear layer and a 3D backward facing step. The results show the Rosco line seeder does produce excellent flow visualization images whereas the aluminum oxide seeder produces better seed dispersion resulting in excellent PIV images.   

Errors in PIV measurements of turbulence

As particle-image velocimetry is used to study complex fluid flows, especially turbulent ones, it becomes increasingly important to ensure the fidelity of the data produced by the method. When the purpose of a PIV experiment is to investigate the nature of turbulence itself, rather than the bulk properties of the flow, errors that were previously negligible become significant. From simply considering a flow where the velocity fluctuations are 5\% of the mean flow (for example the wall-normal fluctuations in a boundary layer or channel flow) it is apparent that any errors are twenty-times larger relative to the turbulent fluctuations than they are to the mean flow. The problem is exacerbated by the very nature of turbulence. Turbulent fluctuations can be superficially viewed as small, apparently random, disturbances about more manifest flow characteristics. The analogy to errors being small, apparently random, fluctuations about a manifest ``correct'' solution is obvious. It is possible that these similarities lead to errors being overlooked because they are adequately ``disguised'' as turbulent fluctuations. The nature of such errors and methods for minimising them will be discussed.   

Statistical Methods for Post-Correlation PIV Outlier Detection

PIV is a technique that has been used for many years for velocity measurements. However, this technique is susceptible to producing outliers due to a variety of reasons. Many methods of correcting these post-correlation outliers have been proposed but almost all of them rely on the use of thresholds that due to flow field variability are not constant. We propose to use a method based on statistical analysis to automatically identify outliers independent of flow field variability. We will discuss its robustness and effects on overdetection (i.e. identifying good vectors as spurious vectors), and undetection (i.e. identifying spurious vectors as good vectors) for different types of simulated outliers and flow fields.   

Experimental application of multilayer nano-particle image velocimetry

Nano-particle image velocimetry (nPIV) uses evanescent-waves with an intensity that decays exponentially along $z$ or the direction normal to the wall as the illumination source to measure the tangential velocity components within the first 300 nm next to the wall. Illuminated tracers in nPIV that are closer to the wall should therefore have images brighter than those farther from the wall. This variation in tracer intensity is the basis of ``multilayer nPIV,'' where the velocity parallel to the wall is estimated at a few distinct $z$-locations \underline {within} the region illuminated by the evanescent wave. The feasibility of this technique has already been demonstrated using synthetic images of plane Couette flow (Li \textit{et al.} (2006) \textit{Exp Fluids} DOI: 10.1007/s00348-006-0155-4). Initial experimental results will be presented where velocities at a few $z$ locations within 300 nm of the wall are extracted from experimental images of Poiseuille flow through a rectangular microchannel (cross-section dimensions $40\mbox{ }\mu \mbox{m}\times \mbox{300 }\mu \mbox{m})$. The experimental parameters are chosen based upon the results of Li \textit{et al}. to minimize bias due to the asymmetric nature of Brownian diffusion in the near-wall region.   

Quantum Dots for Velocity and Thermal Measurements in both Liquid and Gas Microflows

Micro/nano-scale velocity and temperature measurements are demonstrated using quantum dots (QDs).~ The small size and well-described temperature variation of QDs make them attractive thermal-fluid probes for micro and nanoscale systems. Particle tracking velocimetry (PTV) has been demonstrated previously using nanometer-sized QDs and in this talk we present results from both liquid and gas phase flows, demonstrating improved optical detection and statistical particle tracking techniques. QDs are also known to exhibit intensity variations with temperature due to changes in quantum efficiency. We present results on the measurement of two-dimensional temperature fields based on these intensity variations. Using a single intensified camera as a detector, coupled to an image-splitting two-color filter system to separate images by wavelength, we show that~it is possible to combine measurements of both velocity and temperature.   

Estimating Brownian diffusion directly from colloidal particle position

Brownian diffusion coefficients of colloidal spheres are measured using a statistical method, originally proposed for studying self-diffusion of concentrated high Peclet number suspensions (Breedveld \textit{et al}. (1998) \textit{J. Fluid Mech}. \textbf{375}, 297). Unlike techniques that estimate diffusion from changes in cross-correlation function of the particle images in interrogation windows, this method uses particle positions obtained directly from image pairs. For each particle in the first image, displacement vectors were calculated with respect to all particles in the second image. Obviously, at most one of these displacements represents the actual particle displacement. The actual displacements for about 10$^{3}$ distinct particle images are isolated from the histogram of all possible particle displacements using a symmetry argument. Finally, the diffusion coefficient is estimated by fitting the histogram of the actual particle displacements (which approximates the probability density function of these displacements) to a Gaussian function. The technique is verified using experimental data for volumetrically illuminated tracer particles of diameter up to 300 nm in Poiseuille flow. The results, which are in good agreement with Stokes-Einstein theory, demonstrate that the technique can measure diffusion coefficients even for images with low signal to noise ratio (SNR).   

Analysis of Array Motion in the Wake of a Submarine Model

High resolution stereo-PIV measurements were made on a long, small diameter cylinder towed from the control surface of a 1/18$^{th}$ scale submarine model. The experiments were performed in the Deep Water Tow Basin at NSWCCD at 5 kts. Three-dimensional velocity fields over ten body lengths downstream were obtained. The cylinders were approximately neutrally buoyant and towed through a stationary laser sheet oriented perpendicular to the tow direction. The objective of the study was to quantify the effect of the flow behind the submarine control surfaces and propeller on the boundary layer development and dynamics of a model towed array where $\delta >>$ the cylinder radius, a. Algorithms were developed to track the array motion through the field of view. This data is used to collocate subsequent images in order to average boundary layer velocity data along the array. Approximately 40 instantaneous vector fields were obtained for each location. The motion of the array is influenced by the wake of the submarine however a turbulent boundary layer still develops.   

3-D Digitization of Stereoscopic Jet-in-Crossflow Vortex Structure Images via Augmented Reality

Stereoscopic images of smoke-laden vortex flows have proven useful for understanding the topology of the embedded 3-D vortex structures. Images from two cameras allow a perception of the 3-D structure via the use of red/blue eye glasses. The human brain has an astonishing capacity to calculate and present to the observer the complex turbulent smoke volume. We have developed a technique whereby a virtual cursor is introduced to the perception, which creates an ``augmented reality.'' The perceived position of this cursor in the 3-D field can be precisely controlled by the observer. It can be brought near a characteristic vortex structure in order to digitally estimate the spatial coordinates of that feature. A calibration procedure accounts for camera positioning. Vortex tubes can be traced and recorded for later or real time supersposition of tube skeleton models. These models can be readily digitally obtained for display in graphics systems to allow complete exploration from any location or perspective. A unique feature of this technology is the use of the human brain to naturally perform the difficult computation of the shape of the translucent smoke volume. Examples are given of application to low velocity ratio and Reynolds number elevated jets-in-crossflow.