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

https://www.egr.msu.edu/tmual/Papers_PDF/2009_Shafii%20et%20al-Experimental%20Thermal%20&%20Fluid%20Science.pdf

Using nucleators to control freckles in unidirectional solidification

We describe a new technique for characterizing fast solid state planar gas sensors. Using a moving gas outlet, we are able to change the gas around the sensor from one component to the next within one millisecond in a step-like manner. By simultaneously sampling at 10kHz, 100Hz and 1Hz, we can capture the fast component of the sensor response as well as the steady state value in a single experiment. The rate of exchange of the gas around the sensor was quantified using laser induced fluorescence (LIF) imaging. The sensor is characterized under standard industrial sensor operating conditions, including gas pressure and device temperature, while using flows typically found in a research laboratory. Since fast response times from planar gas sensors usually require elevated temperatures, we can heat the device to 630 degC

Fast solid state gas sensor characterization technique

The advent of the laser as a relatively common tool for flow visualization has stimulated the development of velocimetry techniques based on the use of photosensitive molecules. While a variety of different molecules have been employed for this purpose, they share the common attribute that laser excitation is used to produce a defined pattern of long-lived tracers which are embedded within the flow field. This process is often referred to as “tagging.” After a suitable time delay, a CCD (or other) camera is used to obtain an image of the displaced pattern (often termed “interrogation”). The observed displacement divided by the elapsed time is assumed to constitute a measurement of vector velocity field. Dependent upon the details of the optical processes and/or the nature of the tracer, the technique has alternately been referred to as Laser-Induced Photochemical Anemometry (Falco & Nocera, 1993), Flow Tagging Velocimetry (Lempert, 1995) and Molecular Tagging Velocimetry (Gendrich, Koochesfahani & Nocera, 1997). As will be shown, the properties of available photo-sensitive materials vary quite significantly, albeit it in somewhat subtle ways. The purpose of this chapter is to provide a framework sufficiently detailed to enable potential users to effectively match the diagnostic to their particular measurement environment. To avoid confusion, we shall adopt the terminology of Gendrich & Koochesfahani (1996), and use the term Molecular Tagging Velocimetry (MTV) to encompass

Molecular tagging velocimetry and thermometry

The focus of this work is on assessing the ability of an array of discrete finite-core vortices to model vorticity-dominated flows where vortex sheets that have not fully rolled up into isolated vo...

Structure-Based Model of a Forced Shear Layer

: A three-year coordinated experimental and computational investigation is carried out to study the fundamental aerodynamics of oscillating airfoils in the chord Reynolds number range of 2,000 - 60,000. The objective of the investigation is to establish the connection between the airfoil motion trajectory, the trailing edge flexure, the time history of vorticity flux at the trailing edge, the pattern of shed vorticity and its evolution, and the conditions for generation of thrust/drag and lift. The results show that the non-dimensional spacing and circulation of the wake vortices as well as the mean thrust coefficient acting on the airfoil are similar for both rigid and flexible airfoils at the same Strouhal number, if the latter is computed using the actual airfoil's trailing edge oscillation amplitude. Furthermore, it is discovered that the Reynolds number has a significant influence on the reduced frequency and Strouhal number at which the wake vortex arrangement switches to the reversed von Karman street configuration and the mean streamwise force changes from drag to thrust. This study has also resulted in the development of new measurement techniques that will make possible in the future measurements of the pressure and shear stress distribution on flexible and moving surfaces.

Manoochehr Koochesfahani

Papers

Using nucleators to control freckles in unidirectional solidification

Fast solid state gas sensor characterization technique

Molecular tagging velocimetry and thermometry

Structure-Based Model of a Forced Shear Layer

Investigation of Highly Unsteady Aerodynamics of Flapping Wings With/Without a Flexible Training Edge Using High Resolution MTV Measurements