An introduction to heat transfer

One of the most challenging and time-consuming problems in experimental fluid mechanics is the measurement of the overall flow field properties, such as the velocity, vorticity, and pressure fields. Local measurements of the velocity field (i.e., at individual points) are now done routinely in many experiments using hot-wire (HW) or laser velocimetry (LV). However, many of the flow fields of current interest, such as coherent structures in shear flows or wake flows, are highly unsteady. HW or LV data of such flows are difficult to interpret, as both spatial and temporal information of the entire flow field are required and these methods are commonly limited to simultaneous measurements at only a few spatial locations.

Particle Image Velocimetry

Vortex formation in the wake of an oscillating cylinder

Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Karman street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.

Oscillating foils of high propulsive efficiency

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Simultaneous Velocity and Temperature Measurements in Electroosmotically Driven Flow in a Microchannel using Molecular Tagging Velocimetry and Thermometry

: The purpose of this work is to quantify the vorticity evolution in the flow field of the forced wake of a splitter plate inside a confining geometry. The interest in this flow stems from the fact that forcing a low Reynolds number 2-D wake can lead to a highly three-dimensional flow and a large increase in mixing. The authors' recent estimates, based on chemically reacting laser induced fluorescence (LIF) measurements, report the amount of molecularly mixed fluid in terms of mixed-fluid fraction to be 2.5 to 3 times larger than that in high Reynolds number natural two-stream mixing layers. Both reacting and non-reacting LIF data connect this increase in mixing to the downstream evolution of the streamwise vorticity, which is generated by the reorientation and stretching of spanwise vorticity near the side walls of the flow facility. It is believed that understanding the vorticity interaction with walls, its dynamics, and downstream evolution will be helpful to an overall strategy for mixing enhancement and control. The measurements were carried out by Molecular Tagging Velocimetry (MTV), a technique that takes advantage of molecules with long-lived excited states for nonintrusive, multi-point measurements of various fluid dynamical quantities. Small regions of the flow were tagged by a laser and their subsequent evolution was monitored over the luminescence lifetime of the molecule. A two-detector imaging system was used to acquire an image of the initially tagged regions and a subsequent image of these regions convected by the flow over a prescribed time delay. The Lagrangian displacement vectors from such image pairs were computed using a spatial correlation technique. The particular flow investigated here was highly three dimensional. This application highlights the capability of MTV to make measurements when strong out-of-plane motions are present. (5 refs.)

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Vorticity Field Evolution in a Forced Wake

We report recent progresses made in development of novel molecule-based flow diagnostic techniques, named as Molecular Tagging techniques, to achieve simultaneous measurements of multiple important flow variables (such as flow velocity and temperature) for micro-flows and micro-scale heat transfer studies. Instead of using tiny particles, specially-designed phosphorescent molecules, which can be turned into long-lasting glowing molecules upon excitation by photons of appropriate wavelength, are used as tracers for both velocity and temperature measurements. A pulsed laser is used to “tag” the tracer molecules in the regions of interest, and the movements of the tagged molecules are imaged at two successive times within the photoluminescence lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules between the two image acquisitions provides the estimate of the fluid velocity vector. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the two images. The implementation and application of the MTV&T technique are demonstrated by conducting simultaneous velocity and temperature measurements to qunatify the transient behavior of electroosmotic flow (EOF) inside a microchannel and to reveal the unsteady heat transfer, mass transfer and phase changing process inside micro-sized water droplets pertinent to wind turbine icing phenomena.Copyright © 2009 by ASME

Molecular Tagging Techniques for Micro-Flow and Micro-Scale Heat Transfer Studies

The present study is focused on obtaining the inviscid flow solution for a circular cylinder traversing steadily across an unbounded uniform-shear flow. The work is motivated by understanding the aerodynamics of bodies traversing across a shear flow, as well as by developing an understanding of fundamental concepts that are well understood for uniform free stream but not in the presence of shear. The results show that, in general, the presence of shear makes it impossible to define a unique approach-stream incidence angle , respectively.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/22A32C92AAE09A7E631224E01EF44B6A/S0022112019008206a.pdf/div-class-title-inviscid-flow-analysis-of-a-circular-cylinder-traversing-across-an-unbounded-uniform-shear-stream-div.pdf

Inviscid flow analysis of a circular cylinder traversing across an unbounded uniform-shear stream

: Even though vorticity is one of the most important variables in fluid dynamics, we do not currently have a practical way to measure it directly and in a non-intrusive fashion. In this exploratory research effort we demonstrate the first direct and localized non-intrusive measurement of vorticity in afluid flow using the Rotational Doppler Effect (RDE) and Laguerre-Gaussian (LG) light beams that possess orbital angular momentum (OAM).The approach has been implemented in the flow field of solid body rotation where the flow vorticity is known precisely. Experimental results arefound to be in excellent agreement with the expected values.

/pdf/direct-optical-measurement-of-vorticity-in-fluid-flow-1w9bkiu0xv.pdf

Manoochehr Koochesfahani

Papers

Simultaneous Velocity and Temperature Measurements in Electroosmotically Driven Flow in a Microchannel using Molecular Tagging Velocimetry and Thermometry

Vorticity Field Evolution in a Forced Wake

Molecular Tagging Techniques for Micro-Flow and Micro-Scale Heat Transfer Studies

Inviscid flow analysis of a circular cylinder traversing across an unbounded uniform-shear stream

Direct Optical Measurement of Vorticity in Fluid Flow