Time- and frequency-domain computations of broadband noise due to interaction between incident turbulence and rectilinear cascade of flat plates

Time-domain computational aeroacoustic (CAA) techniques are developed to investigate the broadband noise resulting from the interaction of a rectilinear cascade of flat plates with incident homogeneous, isotropic turbulence. The investigation is carried out by comparing the prediction results obtained by employing the time-domain CAA method with those using existing frequency-domain methods. A semi-analytic model (Wei & Cheong, 2010) and a full three-dimensional rectilinear cascade model (Lloyd & Peake, 2008; Lloyd, 2009) are adopted for the frequency-domain computations. By comparing these computation results, the three-dimensional characteristics of inflow turbulence noise are investigated; in particular, the effects of the wavenumber components of ingested turbulence in the spanwise direction are taken into consideration in the investigation. First, CAA results are compared with those from the semi-analytic model. The results for the acoustic modes of relatively low spanwise wavenumbers obtained using both methods show good agreement, but as the spanwise wavenumber increases, the results obtained by the two methods become increasingly different. To investigate in detail the reason for these differences, mode-decomposition analysis is performed by adopting a hybrid method as well as by employing the CAA and the semi-analytic method. The hybrid method involves the following two sequential computations: (i) the upwash velocities on the flat plate airfoils of the rectilinear cascade are first predicted using the frequency-domain method, and (ii) the acoustic wave propagation is subsequently analyzed using time-domain CAA techniques, with these upwash velocities applied as the boundary conditions on the flat plate. It is seen that the results of the time-domain CAA technique and the hybrid method show good agreement, irrespective of the wavenumber and frequency. However, comparisons of the acoustic solutions from three computations reveal that the prediction results of the semi-analytic model deviate more from the other two predictions as the spanwise wavenumber of the acoustic wave increases and the frequency decreases. On a basis of this observation, a formulation is derived for the error in the pressure jump across the flat-plate predicted by using the semi-analytic method. This formulation shows that the error is approximately inversely proportional to the sound speed in the spanwise direction of the concerned acoustic modes. This result quantitatively clarifies the limitations of applying the frequency-domain method of Wei & Cheong (2010) to the three-dimensional turbulence-cascade interaction problems. Secondly, the prediction results using the time-domain CAA method are compared with those from the full three-dimensional rectilinear model that is believed to be exact model for the cascade geometry considered in this paper. This comparison shows the good agreements between two predictions, which support the above arguments for the error and the successful application of the time-domain CAA methods. It is expected that these methods can be extended to the broadband noise problem in an annular cascade, including the nonlinear interaction of the real-airfoil cascade with the incident nonhomogeneous gust.     

On generalized matrix approximation problem in the spectral norm

In this paper theoretical results regarding a generalized minimum rank matrix approximation problem in the spectral norm are presented. An alternative solution expression for the generalized matrix approximation problem is obtained. This alternative expression provides a simple characterization of the achievable minimum rank, which is shown to be the same as the optimal objective value of the classical problem considered by Eckart–Young–Schmidt–Mirsky, as long as the generalized problem is feasible. In addition, this paper provides a result on a constrained version of the matrix approximation problem, establishing that the later problem is solvable via singular value decomposition.     

Spin wave measurements over the full Brillouin zone of multiferroic BiFeO3

Using the inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO(3) single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest-neighbor exchange interaction (J=4.38 meV), a next-nearest-neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO(3) provides a fundamental ingredient for understanding the novel magnetic properties of BiFeO(3).     

Spark-generated bubble collapse near or inside a circular aperture and the ensuing vortex ring and droplet formation

This paper aims to study the oscillation of a sparkgenerated submerged bubble located near or inside a circular aperture made in a flat plate using high-speed visualization technique. In the case of a bubble oscillating near an aperture the initial free surface of the water was set at the bottom surface of the plate. The effects of aperture size and bubblefree surface distance on the bubble behavior as well as on the ensuing droplet dynamics are investigated. It was found that the direction of the bubble reentrant jet was towards the aperture or away from it respectively when the normalized aperture size was smaller or greater than a certain critical value. In addition, a toroidal vortex ring was observed to form, which rotated inwards as it moved away from the aperture. It was also found that if the bubble was incepted at a distance sufficiently away from a supercritical size aperture a single droplet could be produced. In the case of a bub- ble initiated in the middle of a circular aperture submerged just beneath the water free surface, the bubble was found to take the shape of an ellipsoid during its expansion. Then a reentrant jet was initiated and pierced the bubble from its top side.     

A Supported Nickel Catalyst Stabilized by a Surface Digging Effect for Efficient Methane Oxidation

A surface digging effect of supported Ni NPs on an amorphous N‐doped carbon is described, during which the surface‐loaded Ni NPs would etch and sink into the underneath carbon support to prevent sintering. This process is driven by the strong coordination interaction between the surface Ni atoms and N‐rich defects. In the aim of activation of C?H bonds for methane oxidation, those sinking Ni NPs could be further transformed into thermodynamically stable and active metal‐defect sites within the as‐generated surface holes by simply elevating the temperature. In situ transmission electron microscopy images reveal the sunk Ni NPs dig themselves adaptive surface holes, which would largely prevent the migration of Ni NPs without weakening their accessibility. The reported two‐step strategy opens up a new route to manufacture sintering‐resistant supported metal catalysts without degrading their catalytic efficiency.     

Holographic microrheology of polysaccharides from <Emphasis Type="Italic">Streptococcus mutans</Emphasis> biofilms

We use three-dimensional holographic particle tracking to perform microrheological measurements of model gelled media, including the polysaccharide pellicle of dental biofilms created by the common cariogenic oral pathogen Streptococcus mutans. Nanometer-resolution video-rate holographic tracking of embedded colloidal spheres provides accurate measurements of the gels’ complex viscoelastic moduli, including insights into these properties’ heterogeneity. When applied to polysaccharides of S. mutans biofilms, these techniques promise quantitative microscopic assays for candidate therapeutic agents against cariogenic dental biofilms.     

Photoresponsive conductance switching of multi-walled carbon nanotubes bearing covalently linked spironaphthoxazine

Multi-walled carbon nanotubes functionalized with a photochromic spironaphthoxazines (MWCNTs-SPO) were synthesized by the reaction between the hydroxyl group of SPO and the carbonyl chloride groups on the surface of MWCNTs. The functional spirooxazine photochromic groups on the surface of MWCNTs were identified by the X-ray photoelectron spectroscopy. As a novel derivative of MWCNTs, photoresponsive conductance switching of the MWCNTs-SPO was demonstrated under a 365 nm UV light irradiation. A simple photoinduced resistance measurement following the cyclic irradiation of UV light shows a very reversible response. The effect of the electron injection in the SPO is rationalized in terms of the HOMO–LUMO energy levels of both spiro and merocyanine forms.     

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function $K$ (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function $K$ for plane Couette flow is derived. The result indicates that $K$ reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high $\operatorname{Re}$ for given same level of normalized perturbation in the domain. The critical value of $K$ at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of $K$ at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case).     

Nano-structured biogenic calcite: a thermal and chemical approach to folia in oyster shell

The thin sheets of calcite in oyster shell in Crassostrea gigas are termed folia and comprise much of the oyster shell. The folia are covered by a layer of discrete globules that has been proposed to consist of aggregations of an organic matrix and minerals. A continuous organic framework divides each tablet into nanograins. Their shape is globular with a mean extension from 30 to 40 nm. Chemical and thermal treatments to correlate between the organic matrix and the minerals are considered using spectrometers, thermal analyzers, and electron microscopes. After treatment, the nanograins of the foliar and organic matrix are clearly identified. The organic matrix plays a key role in the thermal stability and material properties of this biological composite. From analysis of the FT-IR results, it is identified that the organic matrix in folia is composed of proteins and polysaccharides.