4,376 research outputs found
Experimental study of boundary layer transition on a heated flat plate
A detailed investigation to the document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were done in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate, and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for momentum thickness Reynolds number, Re(sub theta) less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length and uniform heat flux. A small dependence of turbulence results on the freestream turbulence intensity was observed
Turbulent heat flux measurements in a transitional boundary layer
During an experimental investigation of the transitional boundary layer over a heated flat plate, an unexpected result was encountered for the turbulent heat flux (bar-v't'). This quantity, representing the correlation between the fluctuating normal velocity and the temperature, was measured to be negative near the wall under certain conditions. The result was unexpected as it implied a counter-gradient heat transfer by the turbulent fluctuations. Possible reasons for this anomalous result were further investigated. The possible causes considered for this negative bar-v't' were: (1) plausible measurement error and peculiarity of the flow facility, (2) large probe size effect, (3) 'streaky structure' in the near wall boundary layer, and (4) contributions from other terms usually assumed negligible in the energy equation including the Reynolds heat flux in the streamwise direction (bar-u't'). Even though the energy balance has remained inconclusive, none of the items (1) to (3) appear to be contributing directly to the anomaly
Climate dynamics experiments using a GCM simulations
The study of surface-atmosphere interactions has begun with studies of the effect of altering the ocean and land boundaries. A ten year simulation of global climate using observed sea surface temperature anomalies has begun using the NCAR Community Climate Model (CCM1). The results for low resolution (R15) were computed for the first 8 years of the simulation and compared with the observed surface temperatures and the MSU (Microwave Sounding Unit) observations of tropospheric temperature. A simulation at higher resolution (T42) was done to ascertain the effect of interactive soil hydrology on the system response to an El Nino sea surface temperature perturbation. Initial analysis of this simulations was completed
Chemically encoded self-organized quantum chain supracrystals with exceptional charge and ion transport properties
Artificially grown superstructures from small building blocks is an intriguing subject in ‘bottom-up’ molecular science and nanotechnology. Although discrete nanoparticles with different morphologies and physicochemical properties are readily produced, assembly them into higher-order structure amenable to practical applications is still a considerable challenge. This report introduces a stepwise heterogeneous approach for coupling colloidal quantum dots (QDs) synthesis with self-organization to directly generate quantum chains (QCs). By using vulcanized sulfur precursors, QDs are interdigitated into microscale chainlike supracrystals associated with oleylamine and oleic acid as structure directing agents. The cooperative nature of the QD growth and assembly have been extended to fabricate binary (PbS) and ternary metal chalcogenides (CuInS2) QC superstructures over a range of length scales. In addition, enhanced ion and charge transfer performance have been demonstrated which are determined to originate from the minimum interparticle distance and nearly bare nanocrystal surface. The process reported here is general and can be readily extended to the production of many other metal chalcogenide QD superstructures for energy storage applications
Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results
Using one month of the cloud-resolving Nonhydrostatic Icosahedral
Atmospheric Model (NICAM) simulations, we examined the impact of different
definitions of clear-sky flux on the determination of longwave cloud
radiative forcing (CRF). Because the satellite-like cloud-free composite
preferentially samples drier conditions relative to the all-sky mean state,
the conventional clear-sky flux calculation using the all-sky mean state in
the model may represent a more humid atmospheric state in comparison to the
cloud-free state. The drier bias is evident for the cloud-free composite in
the NICAM simulations, causing an overestimation of the longwave CRF by
about 10% compared to the NICAM simulated longwave CRF. Overall, water
vapor contributions of up to 10% of the total longwave CRF should be
taken account for making model-generated cloud forcing comparable to the
satellite measurements
Ballistic dynamics of a convex smooth-wall billiard with finite escape rate along the boundary
We focus on the problem of an impurity-free billiard with a random
position-dependent boundary coupling to the environment. The response functions
of such an open system can be obtained non-perturbatively from a supersymmetric
generating functional. The derivation of this functional is based on averaging
over the escape rates and results in a non-linear ballistic -model,
characterized by system-specific parameters. Particular emphasis is placed on
the {}``whispering gallery modes'' as the origin of surface diffusion modes in
the limit of large dimensionless conductance.Comment: 12 pages, no figure
The Non-Destructive and Nano-Microstructural Characterization of Thermal-Barrier Coatings
The durability of thermal barrier coatings (TBCs) plays an important role in the service reliability and maintainability of hot-section components in advanced turbine engines for aerospace and utility applications. Photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) are being concurrently developed as complimentary nondestructive evaluation (NDE) techniques for quality control and liferemain assessment of TBCs. This paper discusses recent achievements in understanding the residual stress, phase constituents, and electrochemical resistance (or capacitance) of TBC constituents—with an emphasis on the thermally grown oxide. Results from NDE by PSLS and EIS are correlated to the nano- and microstructural development of TBCs
Fermi Edge Singularities in the Mesoscopic Regime: I. Anderson Orthogonality Catastrophe
For generic mesoscopic systems like quantum dots or nanoparticles, we study
the Anderson orthogonality catastrophe (AOC) and Fermi edge singularities in
photoabsorption spectra in a series of two papers. In the present paper we
focus on AOC for a finite number of particles in discrete energy levels where,
in contrast to the bulk situation, AOC is not complete. Moreover, fluctuations
characteristic for mesoscopic systems lead to a broad distribution of AOC
ground state overlaps. The fluctuations originate dominantly in the levels
around the Fermi energy, and we derive an analytic expression for the
probability distribution of AOC overlaps in the limit of strong perturbations.
We address the formation of a bound state and its importance for symmetries
between the overlap distributions for attractive and repulsive potentials. Our
results are based on a random matrix model for the chaotic conduction electrons
that are subject to a rank one perturbation corresponding, e.g., to the
localized core hole generated in the photoabsorption process.Comment: 10 pages, 8 figures, submitted to Phys. Rev.
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