647 research outputs found
Analysis of dynamic stall development on a cross-flow turbine blade
This research computationally investigates the complex dynamic stall
phenomena of a cross-flow turbine blade utilizing modal analysis to identify
pertinent events within the cycle. The blade rotation perpendicular to the
freestream generates a curved relative flow, a non-sinusoidal variation of
relative flow speed and angle of attack, and the necessity of travelling
through its own wake. These complexities have challenged traditional predictors
of dynamic stall such as pitch rate, pitching moment, or relative angle of
attack. To investigate these phenomena, aerodynamic loads and flow fields on
the blade from large-eddy simulations are examined across two tip speed ratios.
Proper orthogonal decomposition of the velocity fields is employed to analyze
the spatio-temporal evolution of the dominant flow features. The modes' time
development coefficients reveal a stronger representation of the flow at the
higher rotation rate, capturing the trend of relative flow velocity magnitude
and lift generation on the blade, along with critical events such as vortex
formation and detachment. Additionally, mean power generation is enhanced by
40\% by applying a non-constant rotation rate (intracycle control or angular
velocity control). The flow fields, supported by corresponding changes in the
modal analysis, demonstrate that a delayed stall behavior is responsible for
the additional power extraction. Finally, flow curvature, history effects, and
induced flow are identified as significant factors that modify the dynamic
stall onset and resulting force and moment curves as compared to non-rotating
pitching or plunging foils
Large-Eddy Simulation of Separation Control for Compressible Flow Over a Wall-Mounted Hump
Compressible large-eddy simulations of turbulent flow over a wall-mounted hump with active flow control are performed and compared to previous experiments. We consider a range of Mach numbers from 0.1 to 0.6. Control is applied just before the natural separation point via steady suction and zero-net mass flux oscillatory forcing. Compared with the baseline flow, control shortens the separation bubble length, but is generally found to be less effective at compressible Mach numbers. The LES matches well to the available experimental data for the baseline and steady suction cases. With oscillatory forcing, the LES captures the major flow physics of the large scale shedding of vortical structures, but over-predicts the separation bubble length at low Mach numbers
Metallicity Mapping with gri Photometry: The Virgo Overdensity and the Halos of the Galaxy
We describe the methodology required for estimation of photometric estimates
of metallicity based on the SDSS gri passbands, which can be used to probe the
properties of main-sequence stars beyond ~ 10 kpc, complementing studies of
nearby stars from more metallicity-sensitive color indices that involve the u
passband. As a first application of this approach, we determine photometric
metal abundance estimates for individual main-sequence stars in the Virgo
Overdensity, which covers almost 1000 square degrees on the sky, based on a
calibration of the metallicity sensitivity of stellar isochrones in the gri
filter passbands using field stars with well-determined spectroscopic metal
abundances. Despite the low precision of the method for individual stars,
internal errors of in [Fe/H] ~ +/- 0.1 dex can be achieved for bulk stellar
populations. The global metal abundance of the Virgo Overdensity determined in
this way is = -2.0 +/- 0.1 (internal) +/- 0.5 (systematic), from
photometric measurements of 0.7 million stars with heliocentric distances from
~ 10 kpc to ~ 20 kpc. A preliminary metallicity map, based on results for 2.9
million stars in the northern SDSS DR-7 footprint, exhibits a shift to lower
metallicities as one proceeds from the inner- to the outer-halo population,
consistent with recent interpretation of the kinematics of local samples of
stars with spectroscopically available metallicity estimates and full space
motions.Comment: 4 pages, 2 figures, to appear in IAU Symp. 26
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