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

The main motivation behind this study is to understand the susceptibility of the suspension lines of Precision Airdrop Systems (PADS) to flow-induced vibrations by galloping instability. Such vibrations are undesirable as they negatively affect the controllability of PADS. Suspension cables of PADS have a cross-section of a roundcorner rectangle with surface geometrical features (topology) produced by the braided structure of the cables. Therefore, an experimental study is conducted to provide fundamental understanding of the effects of surface topology on the instability of round-corner rectangular cylinders to galloping. The study utilizes a baseline cylinder with smooth (topology-free) surface, a side (chord-to-width) ratio of 2.5 and fully-round corners. Results from the baseline model are compared against those from cylinders with the same geometry but with added surface topology. Fourier modes are employed to define different surface topologies with an amplitude of 5% of the cylinder width d along the perimeter only (2D geometry), as well as along the span (3D geometry). Boundary-layer-resolved measurements are conducted using single-component molecular tagging velocimetry at Reynolds numbers based on d of Red = 1, 100 and 2, 500, and cylinder angle of attack of 0°, 2° and 5°. A complementary investigation using direct force measurements is also carried out to determine the galloping stability characteristics of the cylinders examined. Results suggest that the topology could lead to destabilization to galloping when the associated geometrical features cause the flow to separate along the corner region of the cylinder, promoting earlier separation relative to the baseline smooth geometry. On the other hand, reattachment of the separated shear layer, which has a stabilizing effect on galloping, is promoted by the presence of spanwise topology variation (3D geometry) and increasing Red.

Characterization of the Aerodynamics of Rectangular Cylinders with Surface Topology

The performance of an internal combustion engine is optimized utilizing an electrically controlled driver (40) which is located relative to the intake port (28) to excite the intake charge at a selected frequency, duration and power level which depends on the engine operating condition. The purpose of this excitation is to control combustion by optimizing large scale recirculation patterns of the intake charge within the combustion chamber (18). In particular, this device will provide the effect of optimizing volumetric efficiency at wide open throttle and minimize fuel consumption and emissions at part throttle. An electronic engine controller varies the operation of the drive device as a function of engine operating conditions. The driver excites the intake charge at a frequency of about 5 to 1000 times of the engine speed.

Method and apparatus for active control of the combustion processes in an internal combustion engine

On Using Shaped Honeycombs for Experimental Generation of Arbitrary Velocity Profiles in Test Facilities

Single-component force balance for the measurement of low-magnitude mean aerodynamic loads

Wind tunnel experiments were used to investigate the effects of geometry on the transverse galloping behavior of nominally rectangular cylinders at Reynolds numbers from 1000 to 10,000. Static measurements of the lift and the drag forces were used to determine the variation of the normal force coefficient with angle-of-attack, in accordance with the typical quasi-steady description of galloping. Cylinders with unity chord-to-thickness ratio (side ratio) were found to vary from unstable, to neutrally stable, to stable as the corner radius was increased from sharp, to half-round, to fully-round. Cylinders with side ratios of 2 or 3 demonstrated either stability over the entire Reynolds number range, or a transition from unstable to stable with increasing Reynolds number, depending on corner radius. The results demonstrated that in general, increasing the corner radius had a stabilizing effect.

Manoochehr Koochesfahani

Papers

Characterization of the Aerodynamics of Rectangular Cylinders with Surface Topology

Method and apparatus for active control of the combustion processes in an internal combustion engine

On Using Shaped Honeycombs for Experimental Generation of Arbitrary Velocity Profiles in Test Facilities

Single-component force balance for the measurement of low-magnitude mean aerodynamic loads

Geometry effects on the galloping instability of rectangular cylinders at low Reynolds number