34,233 research outputs found
Particle image velocimetry measurements of the interaction of synthetic jets with a zero-pressure gradient laminar boundary layer
Copyright @ 2010 American Institute of PhysicsAn experimental investigation of the interaction between a synthetic jet actuator and a zero-pressure gradient laminar boundary layer is reported. The aim of this study is to quantify the impact of synthetic jet vortical structures; namely, hairpin vortices, stretched vortex rings and tilted vortex rings on a boundary layer, and to assess their relative potential for flow separation control. Streamwise particle image velocimetry was employed in a water flume (free stream boundary layer thickness Reynolds number of 500 and boundary layer thickness-to-jet orifice diameter ratio of 4) to obtain phase- and time-averaged boundary layer profile information of the impact of synthetic jets near the wall. The potential for flow control was assessed by analyzing near wall fluid mixing, realized by the measure of increase in wall shear stress produced by a passing vortex. Hairpin vortices (produced at a jet-to-free stream velocity ratio, VR=0.32 and dimensionless stroke length, L=1.6) and stretched vortex rings (VR=0.27; L=2.7) exhibit characteristics akin to a streamwise vortex pair with a common upwash. Conversely, tilted vortex rings (VR=0.54; L=2.7) induce a streamwise vortex pair in the near wall region with a common downwash. Wall shear stress measurements show that synthetic jets composed of stretched vortex rings offer the best combination of near wall fluid mixing, persistency, and low rms fluctuations for potential applications of flow separation control.Financial support from the Engineering and Physical Sciences Research Council (EPSRC Grant No. AF566NEZ) was used for this work
The near wall effect of synthetic jets in a boundary layer
Copyright @ 2007 Elsevier Inc. All rights reserved.An experimental investigation to analyse the qualitative near wall effect of synthetic jets in a laminar boundary layer has been undertaken for the purpose of identifying the types of vortical structures likely to have delayed separation on a 2D circular cylinder model described in this paper. In the first instance, dye visualisation of the synthetic jet was facilitated in conjunction with a stereoscopic imaging system to provide a unique quasi three-dimensional identification of the vortical structures. Secondly, the impact of synthetic jet structures along the wall was analysed using a thermochromic liquid crystal-based convective heat transfer sensing system in which, liquid crystals change colour in response to the thermal footprints of a passing flow structure. Of the different vortical structures produced as a result of varying actuator operating and freestream conditions, the footprints of hairpin vortices and stretched vortex rings revealed a marked similarity with the oil flow pattern of a vortex pair interacting with the separation line on the cylinder hence suggesting that either of these structures was responsible in delaying separation. Conditions were established for the formation of the different synthetic jet structures in non-dimensional parameter space
Streaks to Rings to Vortex Grids: Generic Patterns in Transient Convective Spin-Up
We observe the transient formation of a ringed pattern state during spin-up
of an evaporating fluid on a time scale of order a few Ekman spin-up times. The
ringed state is probed using infrared thermometry and particle image
velocimetry and it is demonstrated to be a consequence of the transient balance
between Coriolis and viscous forces which dominate inertia, each of which are
extracted from the measured velocity field. The breakdown of the ringed state
is quantified in terms of the antiphasing of these force components which
drives a Kelvin-Helmholtz instability and we show that the resulting vortex
grid spacing scales with the ring wavelength. This is the fundamental route to
quasi-two dimensional turbulent vortex flow and thus may have implications in
astrophysics and geophysics wherein rotating convection is ubiquitous. sics.Comment: 4 pages, 5 figure
Flavor Evolution of the Neutronization Neutrino Burst from an O-Ne-Mg Core-Collapse Supernova
We present results of 3-neutrino flavor evolution simulations for the
neutronization burst from an O-Ne-Mg core-collapse supernova. We find that
nonlinear neutrino self-coupling engineers a single spectral feature of
stepwise conversion in the inverted neutrino mass hierarchy case and in the
normal mass hierarchy case, a superposition of two such features corresponding
to the vacuum neutrino mass-squared differences associated with solar and
atmospheric neutrino oscillations. These neutrino spectral features offer a
unique potential probe of the conditions in the supernova environment and may
allow us to distinguish between O-Ne-Mg and Fe core-collapse supernovae.Comment: 4 pages, 2 figures. Version accepted by PR
Neutrino Mass Hierarchy and Stepwise Spectral Swapping of Supernova Neutrino Flavors
We examine a phenomenon recently predicted by numerical simulations of
supernova neutrino flavor evolution: the swapping of supernova and
energy spectra below (above) energy \EC for the normal
(inverted) neutrino mass hierarchy. We present the results of large-scale
numerical calculations which show that in the normal neutrino mass hierarchy
case, \EC decreases as the assumed
effective vacuum mixing angle () is decreased.
However, these calculations also indicate that \EC is essentially independent
of the vacuum mixing angle in the inverted neutrino mass hierarchy case. With a
good neutrino signal from a future Galactic supernova, the above results could
be used to determine the neutrino mass hierarchy even if is too
small to be detected in terrestrial neutrino oscillation experiments.Comment: 4 pages, 2 figures. Version accepted by PR
Magnetoresistance due to Domain Walls in Micron Scale Fe Wires with Stripe Domains
The magnetoresistance (MR) associated with domain boundaries has been
investigated in microfabricated bcc Fe (0.65 to 20 m linewidth) wires with
controlled stripe domains. Domain configurations have been characterized using
magnetic force microscopy. MR measurements as a function of field angle,
temperature and domain configuration are used to estimate MR contributions due
to resistivity anisotropy and domain walls. Evidence is presented that domain
boundaries enhance the conductivity in such microstructures over a broad range
of temperatures (1.5 K to 80 K).Comment: 8 pages, 3 postscript figures, and 2 jpg images (Fig 1 and 2) to
appear in IEEE Transactions on Magnetics (Fall 1998
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