Ajay K. Prasad

Effect of resolution on the speed and accuracy of particle image velocimetry interrogation

Particle image velocimetry incorporates a process by which an image of a flow field, bearing double images of seeding particles, is analyzed in small regions called “interrogation spots.” Each spot is imaged onto a photodetector array whose digitized output is evaluated computationally using the auto-correlation technique. This paper examines the effects of resolving the spot using arrays of various resolutions, motivated primarily by a gain in speed. For this purpose, two specially created test photographs representing (i) uniform flow and (ii) solid body rotation, were interrogated using array sizes ranging from 32 × 32 to 256 × 256. Each reduction in resolution by a factor of two gains a factor of four in interrogation speed, but this benefit is counteracted by a loss in accuracy. The particle image diameter strongly influences accuracy through two distinct error mechanisms. When the particle image is small compared to the pixel size, mean bias error becomes significant due to finite numerical resolution of the correlation function. Conversely, when the particle image is large, random error due to irregularities in the electronic images predominates. The optimum image size, therefore, lies not at either extreme but at an intermediate value such that the particle image is small in an absolute sense, and yet large relative to the pixel size.

Stereoscopic Particle Image Velocimetry Applied to Liquid Flows

A twin-camera stereoscopic system has been developed to extend conventional high image-density Particle Image Velocimetry (PIV) to three-dimensional vectors on planar domains. The stereoscopic velocimeter performs with extremely high accuracy. Translation tests have yielded errors (rms) of 0.2% of full-scale for the in-plane displacement, and 0.8% of full-scale for the out-of-plane component, both of which agree with the errors predicted by an uncertainty analysis. In addition, modified techniques in hardware and software have enabled the stereoscopic system to perform successfully when acquiring images through a thick liquid layer, wherein previously the aberrations arising due to the liquid-air interface have restricted the use of such systems. With these techniques, the stereoscopic system, in combination with a simple method for image-shifting, is able to accurately measure threedimensional velocity fields in liquids. This is demonstrated by measurements of the helical, three-dimensional flow induced by a rotating disk in glycerine.

Reynolds number and end-wall effects on a lid-driven cavity flow

A series of experiments has been conducted in a lid‐driven cavity of square cross section (depth=width=150 mm) for Reynolds numbers (Re, based on lid speed and cavity width) between 3200 and 10 000, and spanwise aspect ratios (SAR) between 0.25:1 and 1:1. Flow visualization using polystyrene beads and two‐dimensional laser‐Doppler anemometer (LDA) measurements have shed new light on the momentum transfer processes within the cavity. This paper focuses on the variation, with Re and SAR, of the mean and the rms velocities profiles, as well as the ∼(U’V’) profile, along the horizontal and vertical centerlines in the symmetry plane. In addition, the contribution of the large‐scale ‘‘organized structures,’’ and the high‐frequency ‘‘turbulent’’ velocity fluctuations to the total rms is examined. At low Re, the organized structures account for most of the energy contained in the flow irrespective of SAR. As the Re increases, however, so does the energy content of the higher frequency fluctuations. This trend is ...

SCHEIMPFLUG STEREOCAMERA FOR PARTICLE IMAGE VELOCIMETRY IN LIQUID FLOWS

A novel stereocamera has been developed based on the angular-displacement method, wherein the two camera axes are oriented in a nonorthogonal manner toward the object plane. The stereocamera satisfies the Scheimpflug condition such that the image plane, the object plane, and the lens plane are nominally colinear. A unique feature of the stereocamera is the introduction of a liquid prism between the object plane and the recording lens, which significantly reduces the radial distortions that arise when imaging through a thick liquid layer. The design of the camera and its computer optimization with geometric modeling are described. Results indicate that the use of a liquid prism reduces the amount of radial distortion by an order of magnitude. The results have been shown to agree very well with experiments.

Combined forced and natural convection heat transfer in a deep lid-driven cavity flow

Abstract In this study, we describe the combined forced and natural convection (also known as mixed-convection) heat transfer process within a recirculating flow in an insulated lid-driven cavity of rectangular cross section (150 mm × 450 mm) and depth varying between 150 mm and 600 mm. The forced convection is induced by a moving lid, which shears the surface layer of the fluid in the cavity, thereby setting up a recirculating flow, while the natural convection flow is induced by heating the lower boundary and cooling the upper one. By appropriately varying the lid speed, the vertical temperature differential, and the depth, we obtained Gr Re 2 ratios for these flows from 0.1 to 1000. Flow visualization using liquid crystals and heat flux measurements at specific locations over the lower boundary provided an insight into the nature of the heat transfer process under different flow and temperature conditions. The mean heat flux values over the entire lower boundary were analyzed to produce Nusselt number and Stanton number correlations which should be useful for design applications.

Integral Solution for the Mean Flow Profiles of Turbulent Jets, Plumes, and Wakes

Integral methods are used to derive similarity solutions for several quantities of interest including the cross-stream velocity, Reynolds stress, the dominant turbulent kinetic energy production term, and eddy diffusivities of momentum and heat for axisymmetric and planar turbulent jets, plumes, and wakes. A universal constant is evaluated for axisymmetric and planar plumes

Optimization procedure for pulse separation in cross-correlation PIV

In PIV, the optimal time separation (Δt) between successive laser pulses is influenced by a number of parameters. In the present paper, only two kinds of error affecting the choice of Δt are studied: (i) random error arising from noise during recording of the flow seeded with tracer particles and subsequent interrogation of the particle images, and (ii) acceleration error arising from approximation of the local Eulerian velocity based on small (but non-zero) particle displacements. These two kinds of error place conflicting requirements on Δt. A model to optimize Δt with respect to these errors is described, and the model is confirmed by the results of a Monte Carlo simulation. This model for optimal Δt is extended to various acceleration distributions. An estimate for the spatial resolution of the velocity field resulting from cross-correlation PIV is proposed.

A parallel digital processor system for particle image velocimetry

A high-speed, high-resolution, digital computer-based interrogation system for particle image velocimetry (PIV) has been developed. It utilizes advanced parallel array processing technology to achieve computational speeds comparable to those of current supercomputers, and high-resolution image acquisition equipment to achieve the data input rates necessary for high-speed analysis of PIV images. Advanced cross correlation techniques are incorporated into the algorithm, which take advantage of the high-speed capabilities and improve the quality of the measurements. The architecture, algorithms, and performance characteristics of the system are described.

In situ characterization of the catalyst layer in a polymer electrolyte membrane fuel cell

The catalyst layer CL in the polymer electrolyte membrane fuel cell PEMFC has not been explored in detail due to its complexities and difficulties associated with accessing it. It is important to understand water ingress and egress at the CL as it offers potential for water management by manipulating the evaporation rate. A technique for in situ visual characterization of the CL is presented. This is accomplished by designing and fabricating a catalyst-visible operational fuel cell and developing a microvisualization system. The dynamics of microdroplets on the CL surface including formation, growth, coalescence, and removal were visualized in an operating PEMFC. The liquid water behavior at the interface of the CL and the gas diffusion layer GDL were shown to promote the periodic droplet reemergence on the GDL surface in the flow channels. Mechanisms of water condensation and transport within the CL pores are discussed with respect to pore architecture and wetting properties. It has been shown that reduction of pore size and CL thickness alleviates flooding therein and promotes better catalyst utilization. Evaporation was identified as one of the distinguishing mechanisms of the CL, and one of the future challenges will be to control this mechanism. The experimental results should prove useful in clarifying the role of the CL in water management, and in refining models used to optimize PEMFC performance.

Modeling of Airflow in the Pharynx With Application to Sleep Apnea

A three-dimensional numerical modeling of airflow in the human pharynx using an anatomically accurate model was conducted. The pharynx walls were assumed to be passive and rigid. The results showed that the pressure drop in the pharynx lies in the range 200-500 Pa. The onset of turbulence was found to increase the pressure drop by 40 percent. A wide range of pharynx geometries covering three sleep apnea treatment therapies (CPAP, mandibular repositioning devices, and surgery) were modeled and the resulting flow characteristics were investigated and compared. The results confirmed that the airflow in the pharynx lies in the laminar-to-turbulence transitional flow regime and thus, a subtle change in the morphology caused by these treatment therapies can significantly affect the airflow characteristics.