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

This work demonstrates the viability of a novel transverse-gust generator that is capable of producing a controllable time-varying gust without increasing the turbulence level within a large region...

Development of a Low-Turbulence Transverse-Gust Generator in a Wind Tunnel

A new non-intrusive surface pressure and shear stress diagnostics technique is introduced in the present work. The technique is enabled by the unique ability of one-component molecular tagging velocimetry (1c-MTV) to provide measurements at very high spatial resolution (at every pixel along a tagged line of molecules) in the imme- diate proximity of surfaces. The utility of the new method to provide accurate measurements of the wall pressure and shear stress is demonstrated in an experiment involving a cylinder in cross flow at a Reynolds number of 6000. These measurements agree very well with experimental and com- putational data published in literature. The new technique is particularly advantageous in situations where embed- ding sensor arrays in the wall is impractical or cost/time prohibitive.

/pdf/measurement-of-the-wall-pressure-and-shear-stress-oi936anto4.pdf

Measurement of the wall pressure and shear stress distribution using molecular tagging diagnostics

We present results for first molecular tagging velocimetry (MTV) measurements in water under resonant femtosecond excitation/emission process of a phosphorescent supramolecule. Both two-photon and three-photon absorption processes are examined, and the feasibility of measurements is demonstrated by single component velocimetry in a simple jet flow. The new capabilities enabled by FemtoMTV include elimination of the need for short wavelength UV excitation source and UV optical access in flow facilities, and potential for high rep-rate flow imaging.

/pdf/multi-photon-molecular-tagging-velocimetry-with-femtosecond-12p002pm8r.pdf

Multi-photon molecular tagging velocimetry with femtosecond excitation (FemtoMTV)

A previous application of the advanced diagnostic method, Molecular Tagging Velocimetry (MTV), showed promise with respect to providing an improved understanding of velocity fields in the complex thermo-solutal flows associated with solidification in the presence of significant buoyancy. The present study extends the earlier results (Wirtz et. al, 1998) in an ammonium-water alloy analog by demonstrating the capability of quantifying two components of the instantaneous velocity vectors of such flows. The model alloy was directionally-solidified by chilling from below. Velocity vectors were measured near the solidification front at early phases (~3minutes) of the solidification when fine structures exist. Well-defined convective flow patterns were documented with velocities on the order of 1 mm/s measured for both velocity components. Velocity vectors within and around a plume were also measured further away from the solidification front later in the solidification process (~100 minutes). The optical grid used for tagging must be refined to provide adequate spatial resolution within the plumes. The capabilities and limitations of the MTV method in its present realization are described. The results demonstrate that the MTV method is a powerful tool that can be expected to provide valuable data related to complex thermo-solutal flows associated with solidification in the presence of significant buoyancy.

/pdf/measurements-of-the-velocity-field-with-mtv-during-the-opelch3hsi.pdf

Measurements of the velocity field with mtv during the solidification of an alloy analog with mushy region

Outlined in this paper are the theoretical foundation, implementation framework and experimental demonstration of a new diagnostic technique for non-intrusive, whole-field measurement of pressure within gasses. The new technique, which is referred to as molecular tagging manometry (MTM), relies on oxygen quenching of phosphorescence emission from photo-excited tracers in oxygen-containing gases. As the pressure increases, the density of oxygen becomes larger, leading to a shorter emission lifetime: a working principle that is similar to pressure sensitive paint but applied within the body of the flow rather than on the wall. Using an experimental apparatus that is built around a pressure vessel, the viability of MTM is demonstrated for the first time using acetone as a tracer. Furthermore, the experimentally recorded response is compared to theoretical predictions, and the sensitivity of MTM’s response to the uncertainty of various parameters is analyzed.

/pdf/feasibility-study-of-whole-field-pressure-measurements-in-2ycxnhlofr.pdf

Manoochehr Koochesfahani

Papers

Development of a Low-Turbulence Transverse-Gust Generator in a Wind Tunnel

Measurement of the wall pressure and shear stress distribution using molecular tagging diagnostics

Multi-photon molecular tagging velocimetry with femtosecond excitation (FemtoMTV)

Measurements of the velocity field with mtv during the solidification of an alloy analog with mushy region

Feasibility study of whole-field pressure measurements in gas flows: Molecular tagging manometry