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

Progress in shock wave/boundary layer interactions

Detailed instantaneous velocity fields of a jet in crossflow have been measured with stereoscopic particle image velocimetry (PIV). The jet originated from a fully developed turbulent pipe flow and entered a crossflow with a turbulent boundary layer. The Reynolds number based on crossflow velocity and pipe diameter was 2400 and the jet to crossflow velocity ratios were R=3.3 and R=1.3. The experimental data have been analysed by proper orthogonal decomposition (POD). For R=3.3, the results in several different planes indicate that the wake vortices are the dominant dynamic flow structures and that they interact strongly with the jet core. The analysis identifies jet shear-layer vortices and finds that these vortical structures are more local and thus less dominant. For R=1.3, on the other hand, jet shear-layer vortices are the most dominant, while the wake vortices are much less important. For both cases, the analysis finds that the shear-layer vortices are not coupled to the dynamics of the wake vortices. Finally, the hanging vortices are identified and their contribution to the counter-rotating vortex pair (CVP) and interaction with the newly created wake vortices are described.

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A turbulent jet in crossflow analysed with proper orthogonal decomposition

Modern developments in shear-stress measurement ☆

Transverse jets and their control

An experimental investigation of three-dimensional boundary-layer separation on a flat plate ahead of a circular cylinder at Mach 2.36 was made. Emphasis was given to the laminar flow regime and to the flow region upstream of the cylinder. The heights and diameters of the cylinders used in the study were larger than the undisturbed boundary-layer thickness at the cylinder location. Data were obtained by oil flow visualization, Schlieren observations, static pressure measurements, and laser anemometry. Oil flow visualization revealed three separation lines on the flat plate ahead of the cylinder. A postulated flowfield structure, which was suggested by this skin-friction pattern, could not be confirmed by the velocity measurements. Velocity measurements indicated an unsteady flow structure.

Supersonic Separated Flow past a Cylindrical Obstacle on a Flat Plate

Mean velocity data obtained by PIV (Particle Image Velocimetry) around the head of a real-life size breathing thermal manikin are presented for two cases of 'no breathing' and 'continuous exhalation through nose'. Experiments were conducted in a special chamber which provided stationary convective flows around the seated manikin. Results are limited to the plane of symmetry. The paper aims to describe the physical structure of the turbulent flow field by presenting velocity and vorticity data in color graphics.

A Visual Description of the Convective Flow Field around the Head of a Human

The experimental documentation of a three-dimensional shock-wave/boundary-layer interaction in a nominal Mach 3 flow is presented. The model consisted of a sting-supported cylinder, aligned with the freestream flow, and a 20-deg half-angle conical flare offset 1.27 cm from the cylinder centerline. Surface oil flow, laser light sheet illumination, and spark schlieren photography were used to document the flow topology. Extensive surface-pressure and skin-friction measurements were made throughout the interaction region. A laser interterometric skin-friction instrument was employed to acquire the skin-friction data. Resolved skin-friction measurements of C fx and C fz were made within the highly swept three-dimensional separated regions. The skin-friction data will be of particular value for turbulence modeling and computational fluid dynamics validation

Skin-friction measurements in three-dimensional, supersonic shock-wave/boundary-layer interaction

Stereoscopic Particle Image Velocimetry (PIV) has been used to make a three-dimensional flow mapping of a jet in crossflow. The Reynolds number based on the free stream velocity and the jet diameter was nominally 2400. A jet-to-crossflow velocity ratio of 3.3 was used. Details of the formation of the counter rotating vortex pair found behind the jet are shown. The vortex pair results in two regions with strong reversed velocities behind the jet trajectory. Regions of high turbulent kinetic energy are identified. The signature of the unsteady shear layer vortices is found in the mean vorticity field.

Oktay Ozcan

Papers

A turbulent jet in crossflow analysed with proper orthogonal decomposition

Supersonic Separated Flow past a Cylindrical Obstacle on a Flat Plate

A Visual Description of the Convective Flow Field around the Head of a Human

Skin-friction measurements in three-dimensional, supersonic shock-wave/boundary-layer interaction

Flow Mapping of a Jet in Crossflow with Stereoscopic PIV