Measurements of roughness noise

Experiments have been performed on the roughness noise produced by a two-dimensional turbulent wall jet boundary layer flowing over short fetches of sandpaper roughness. A range of rough surface sizes were studied from hydrodynamically smooth through fully rough. Velocity measurements were made to document the form of the wall jet boundary layer and the influence of the roughness upon it. Acoustic measurements showed background noise levels to be very low so that the sound produced by the rough surfaces could be clearly detected with signal to noise ratios as large as 20 dB. Even hydrodynamically smooth roughness was found to produce noise, conclusively indicating the presence of scattering as a source mechanism. Variations of the roughness noise spectra with flow speed and roughness size are found to be inconsistent with any simple parameter scaling. Boundary layer wall pressure fluctuation measurements made within the roughness fetches reveal a spectral form quite different than the roughness noise, and fluctuation levels some 50-70 dB higher. Despite these differences the wall pressure and roughness noise are found to be very simply related, at least at lower frequencies (<6 kHz). The roughness noise spectrum varies closely as the product of the wall pressure spectrum, the frequency squared, and the mean-square roughness height. This is the scaling predicted by scattering theory and implies a major simplification to the problem of roughness noise prediction for stochastic surfaces.     

Asymmetric synthesis of methylenetetrahydrofurans by palladium-catalyzed [3 + 2] cycloaddition of trimethylenemethane with aldehydes--a novel ligand design

The palladium-catalyzed [3 + 2] cycloaddition of trimethylenemethane (TMM) with aldehydes is a direct and efficient route to methylenetetrahydrofurans. Herein we describe the first asymmetric synthesis of methylenetetrahydrofurans utilizing a palladium-TMM complex in the presence of a novel phosphoramidite ligand possessing a stereogenic phosphorus. The method allows for the formation of chiral disubstituted tetrahydrofurans in good yields and enantioselectivities.     

Computational design of the sequence and structure of a protein-binding peptide

The de novo design of protein-binding peptides is challenging because it requires the identification of both a sequence and a backbone conformation favorable for binding. We used a computational strategy that iterates between structure and sequence optimization to redesign the C-terminal portion of the RGS14 GoLoco motif peptide so that it adopts a new conformation when bound to Gα(i1). An X-ray crystal structure of the redesigned complex closely matches the computational model, with a backbone root-mean-square deviation of 1.1 ?.     

Perfect imaging of hypersurfaces via transformation optics

Conventional optical imaging systems suffer from the presence of many imperfections, such as spherical aberrations, astigmatism, or coma. If the imaging system is corrected for spherical aberrations and fulfills the Abbe sine condition, perfect imaging is guaranteed between two parallel planes but only in a small neighborhood of the optical axis. It is therefore worth asking for optical systems that would allow for perfect imaging between arbitrary smooth surfaces without restrictions in shape or extension. In this Letter, we describe the application of transformation optics to design refractive index distributions that allow perfect, aberration-free imaging for various imaging configurations in R(n). A special case is the imaging between two extended parallel lines in R(2), which leads to the well-known hyperbolic secant index distribution that is used for the fabrication of gradient index lenses.     

Boron trifluoride facilitated transesterification of dioxaborolanes

The direct transesterification of dioxaborolanes (alkane-1,2-diol based boronate esters) was explored. Using BF₃·OEt₂, alkane-1,2-diol based mono- and bis-boronate esters (i.e., pinacol and ethylene glycol) have been converted quantitatively to either benzene-1,2-diol or alkane-1,3-diol based boronate esters. In the case of pinacol esters, esterification is facilitated by the accompanying pinacol rearrangement, thus shifting the reaction equilibrium.     

An electronic Venturi-based pressure microregulator

Microfluidic systems often use pressure-driven flow to induce fluidic motion, but control of pumps and valves can necessitate numerous external connections or an extensive external control infrastructure. Here, we describe an electronically controlled pressure microregulator that can output pressures both greater and less than atmospheric pressure over a range of 2 kPa from a single pressurized air input of 110 kPa. Multiple independently controlled microregulators integrated in one device can potentially share the same air input. The microregulator operates by using embedded resistive heaters to vary the temperature of a gas flowing through a converging-diverging Venturi nozzle between 25 degrees C and 85 degrees C with a resolution of 33 Pa degrees C(-1). We established the switching speed of the microregulator by accurately moving 1 microL droplets of water in a microchannel via pneumatic propulsion. Droplet deceleration from approximately 1 cm s(-1) to zero velocity required less than 0.8 s. The component is readily integrable into most device designs containing fluidic channels and electronics without introducing additional fabrication complexity.