56,827 research outputs found
Heat transfer and fluid mechanics measurements in transitional boundary layer flows
Experimental results are presented to document hydrodynamic and thermal development of flat-plate boundary layers undergoing natural transition. Local heat transfer coefficients, skin friction coefficients and profiles of velocity, temperature and Reynolds normal and shear stresses are presented. A case with no transition and transitional cases with 0.68% and 2.0% free-stream disturbance intensities were investigated. The locations of transition are consistent with earlier data. A late-laminar state with significant levels of turbulence is documented. In late-transitional and early-turbulent flows, turbulent Prandtl number and conduction layer thickness values exceed, and the Reynolds analogy factor is less than, values previously measured in fully turbulent flows
The NANOGrav 11 yr Data Set: Limits on Gravitational Wave Memory
The mergers of supermassive black hole binaries (SMBHBs) promise to be incredible sources of gravitational waves (GWs). While the oscillatory part of the merger gravitational waveform will be outside the frequency sensitivity range of pulsar timing arrays, the nonoscillatory GW memory effect is detectable. Further, any burst of GWs will produce GW memory, making memory a useful probe of unmodeled exotic sources and new physics. We searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11 yr data set for GW memory. This data set is sensitive to very low-frequency GWs of ~3 to 400 nHz (periods of ~11 yr–1 month). Finding no evidence for GWs, we placed limits on the strain amplitude of GW memory events during the observation period. We then used the strain upper limits to place limits on the rate of GW memory causing events. At a strain of 2.5 × 10⁻¹⁴, corresponding to the median upper limit as a function of source sky position, we set a limit on the rate of GW memory events at <0.4 yr⁻¹. That strain corresponds to an SMBHB merger with reduced mass of ηM ~ 2 × 10¹⁰ M_⊙ and inclination of ι = π/3 at a distance of 1 Gpc. As a test of our analysis, we analyzed the NANOGrav 9 yr data set as well. This analysis found an anomolous signal, which does not appear in the 11 yr data set. This signal is not a GW, and its origin remains unknown
Free-stream turbulence and concave curvature effects on heated, transitional boundary layers, volume 1
An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-streem turbulence levels was performed. Where possible, sampling according to the intermittency function was made. Such sampling allowed segregation of the signal into two types of behavior: laminar-like and turbulent-like. The results from the investigation are discussed. Documentation is presented in two volumes. Volume one contains the text of the report including figures and supporting appendices. Volume two contains data reduction program listings and tabulated data
UK Foot and Mouth disease: a systemic risk assessment of existing controls
This article details a systemic analysis of the controls in place and possible interventions available to further reduce the risk of a foot and mouth disease (FMD) outbreak in the United Kingdom. Using a research-based network analysis tool, we identify vulnerabilities within the multibarrier control system and their corresponding critical control points (CCPs). CCPs represent opportunities for active intervention that produce the greatest improvement to United Kingdom's resilience to future FMD outbreaks. Using an adapted ‘features, events, and processes’ (FEPs) methodology and network analysis, our results suggest that movements of animals and goods associated with legal activities significantly influence the system's behavior due to their higher frequency and ability to combine and create scenarios of exposure similar in origin to the U.K. FMD outbreaks of 1967/8 and 2001. The systemic risk assessment highlights areas outside of disease control that are relevant to disease spread. Further, it proves to be a powerful tool for demonstrating the need for implementing disease controls that have not previously been part of the system
Bypass transition in boundary layers including curvature and favorable pressure gradient effects
Recent studies of 2-D boundary layers undergoing bypass transition were reviewed. Bypass transition is characterized by the sudden appearance of turbulent spots in boundary layer without first the regular, observable growth of disturbances predicted by linear stability theory. There are no standard criteria or parameters for defining bypass transition, but it is known to be the mode of transition when the flow is disturbed by perturbations of sufficient amplitude
GaBoDS: The Garching-Bonn Deep Survey VIII. Lyman-break galaxies in the ESO Deep Public Survey
Aims. The clustering properties of a large sample of U-dropouts are
investigated and compared to very precise results for B-dropouts from other
studies to identify a possible evolution from z=4 to z=3. Methods. A population
of ~8800 candidates for star-forming galaxies at z=3 is selected via the
well-known Lyman-break technique from a large optical multicolour survey (the
ESO Deep Public Survey). The selection efficiency, contamination rate, and
redshift distribution of this population are investigated by means of extensive
simulations. Photometric redshifts are estimated for every Lyman-break galaxy
(LBG) candidate from its UBVRI photometry yielding an empirical redshift
distribution. The measured angular correlation function is deprojected and the
resulting spatial correlation lengths and slopes of the correlation function of
different subsamples are compared to previous studies. Results. By fitting a
simple power law to the correlation function we do not see an evolution in the
correlation length and the slope from other studies at z=4 to our study at z=3.
In particular, the dependence of the slope on UV-luminosity similar to that
recently detected for a sample of B-dropouts is confirmed also for our
U-dropouts. For the first time number statistics for U-dropouts are sufficient
to clearly detect a departure from a pure power law on small scales down to ~2"
reported by other groups for B-dropouts.Comment: 10 pages, 11 figures, accepted by A&A, full resolution version
available at http://www.astro.uni-bonn.de/~hendrik/5880.pd
The non-Gaussianity of the cosmic shear likelihood - or: How odd is the Chandra Deep Field South?
(abridged) We study the validity of the approximation of a Gaussian cosmic
shear likelihood. We estimate the true likelihood for a fiducial cosmological
model from a large set of ray-tracing simulations and investigate the impact of
non-Gaussianity on cosmological parameter estimation. We investigate how odd
the recently reported very low value of really is as derived from
the \textit{Chandra} Deep Field South (CDFS) using cosmic shear by taking the
non-Gaussianity of the likelihood into account as well as the possibility of
biases coming from the way the CDFS was selected.
We find that the cosmic shear likelihood is significantly non-Gaussian. This
leads to both a shift of the maximum of the posterior distribution and a
significantly smaller credible region compared to the Gaussian case. We
re-analyse the CDFS cosmic shear data using the non-Gaussian likelihood.
Assuming that the CDFS is a random pointing, we find
for fixed . In a
WMAP5-like cosmology, a value equal to or lower than this would be expected in
of the times. Taking biases into account arising from the way the
CDFS was selected, which we model as being dependent on the number of haloes in
the CDFS, we obtain . Combining the CDFS data
with the parameter constraints from WMAP5 yields and for a flat
universe.Comment: 18 pages, 16 figures, accepted for publication in A&A; New Bayesian
treatment of field selection bia
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