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

Reported herein is an investigation of the influence of the structural flexibility of sinusoidally pitching airfoils on the pattern of vorticity shed into the wake. For rigid airfoils, it is well known that, depending on the frequency of oscillation, this pattern takes the form of the classical or reverse von Karman vortex street. The latter may be characterized by the vortex circulation () and vortex-to-vortex streamwise and cross-stream spacing (a and b respectively). In the present work, these three parameters are obtained from particle image velocimetry measurements in the wake of airfoils consisting of a rigid ‘head’ and flexible ‘tail’, fabricated using 3D printing methods. Data are obtained at chord Reynolds number Rec = 1950 for different tail flexibilities (obtained by varying the thickness of the tail) as well as for a fully rigid airfoil. The results show that using the Strouhal number based on the actual trailing edge oscillation amplitude yields a common description of the dependence of a and b on the motion parameters for all airfoils. These results are complemented with analyses using a vortex array model to clarify the relationship between variation in the three wake-vortex pattern parameters and the net mean thrust acting on the airfoil.

Investigation of the Wake Vortex Pattern of Rigid and Flexible Airfoils Undergoing Harmonic Pitch Oscillation

MTV Measurements of the Flow Structure Downstream of an Oscillating Airfoil

Measurements and computations of the separation and reattachment locations are reported for the SD7003 airfoil with the chord Reynolds number in the range 2 – 4 ×10. Measurements are based on multi-line Molecular Tagging Velocimetry. Comparisons of experimental results with current computations and previous experiments point to challenges involved in the experimental determination of separation bubble characteristics. Results also bring into focus the facility dependent issues, specifically freestream turbulence, that impacts the experimental data. Preliminary results indicate a downstream/upstream movement of separation/reattachment, respectively, with added freestream turbulence. This observation may explain some of the discrepancies among the various experimental and computational results.

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An Investigation of the effect of Freestream Turbulence on the Laminar Separation Bubble on an SD7003 Airfoil

Influence of geometry on the galloping instability of rectangular cylinders in the Reynolds number range 1,000–10,000

This work explores the aerodynamics of an airfoil oscillating in a uniform-shear flow at chord Reynolds number of approximately 1.2×104 using complementary two-dimensional Navier–Stokes computation...

Manoochehr Koochesfahani

Papers

Investigation of the Wake Vortex Pattern of Rigid and Flexible Airfoils Undergoing Harmonic Pitch Oscillation

MTV Measurements of the Flow Structure Downstream of an Oscillating Airfoil

An Investigation of the effect of Freestream Turbulence on the Laminar Separation Bubble on an SD7003 Airfoil

Influence of geometry on the galloping instability of rectangular cylinders in the Reynolds number range 1,000–10,000

Joint Computational-Experimental Investigation of Harmonically Pitching Airfoil Aerodynamics in Uniform-Shear Approach Flow