Turbulence modelling of the shear layers in the trailing edge region of a modern rear-loaded airfoil at incidence

After carefull analysis in a turbulent zero-pressure gradient flow, various simple algebraic turbulence models were applied to the almost separated flow on the upperside of an airfoil at incidence. The Johnson-King and Horton non-equilibrium (or rate equation) models give clearly improved results.     

Wavelength conversion in GaAs micro-ring resonators

Tightly confined, low-loss waveguides in highly nonlinear materials permit nonlinear optical interactions to occur over much shorter distances than do fibers. The nonlinear interactions are further enhanced in resonators. Both theory and experiment of enhanced four-wave mixing in micro-ring resonators are presented that can be used for many applications. A conversion efficiency of 14% achievable with only 10-mW peak pump power is predicted under realizable conditions. The experiment, the first one to the authors' knowledge in nonlinear optics performed in micro-rings, shows, even in a lossy GaAs/AlGaAs ring, a 26-dB improvement in the conversion efficiency compared with that of an equivalent straight waveguide, in agreement with theory.     

Filter synthesis for periodically coupled microring resonators

The theory of filter synthesis using periodically coupled microring resonators is developed as a means to overcome the fabrication sensitivities inherent in conventional higher-order filters, while still achieving desirable spectral characteristics. Each resonator in the array can compensate for deficiencies in any of the others. These filters exhibit a boxlike shape and very high extinction ratios.     

The measurement of ?U?, ?V? and ?uv? inside the turbulent spot using conditionally sampled hot-wire anemometer signals

By means of a tripping wire turbulent spots are generated in a conditionally stable laminar boundary layer along a flat plate in a low-turbulence windtunnel. At several vertical and lateral positions the streamwise (U) and vertical (V) velocity components of the spot are measured using anX-shaped hot wire anemometer. The spot's leading edge (LE) and trailing edge (TE) are determined by a high frequency detection criterion.The derived ensemble averaged shear stress uv shows a remarkable plateau over the last 3/4 of the spot length and over about 1/2 of the spot width, approximately independent of the height above the plate. The measurements suggest that the velocities of the LE and the TE can be explained by simple advection of the active turbulent region. Other mechanisms seem to be responsible for the lateral spread of the spot, as is suggested by the measurements outside the plane of symmetry.     

Two-sided Grassmann–Rayleigh quotient iteration

The two-sided Rayleigh quotient iteration proposed by Ostrowski computes a pair of corresponding left–right eigenvectors of a matrix C. We propose a Grassmannian version of this iteration, i.e., its iterates are pairs of p-dimensional subspaces instead of one-dimensional subspaces in the classical case. The new iteration generically converges locally cubically to the pairs of left–right p-dimensional invariant subspaces of C. Moreover, Grassmannian versions of the Rayleigh quotient iteration are given for the generalized Hermitian eigenproblem, the Hamiltonian eigenproblem and the skew-Hamiltonian eigenproblem.     

Time and spatial correlation measurements in the turbulent wake of a circular cylinder using Laser Doppler Anemometry

The present paper reports Laser Doppler Anemometry (LDA) measurements in the turbulent (R _d =1300) wake of a circular cylinder (d=2 mm). On several typical locations in the wake, at cross-section x/d=125, temporal and lateral space velocity correlations have been measured. Temporal autocorrelation functions were determined from the randomly sampled LDA data by means of the Slotting Technique. The autocorrelation functions which were obtained were too low, the discrepancy giving an indication of the influence of noise in the measurements. The influence of noise generated by the photomultiplier tubes, which appeared to be the most important noise source, could be reduced by a cross-correlation technique. A partly new LDA system for two-point velocity correlation measurements was developed, consisting of an elongated measuring volume and a two-point detection system including a semireflecting mirror to divide the scattered light. The results of this investigation clearly demonstrate that LDA is a powerful tool for measuring time and spatial correlations and related properties of turbulence.     

The measurement of 〈U〉, 〈V〉 and 〈uv〉 inside the turbulent spot using conditionally sampled hot-wire anemometer signals

By means of a tripping wire turbulent spots are generated in a conditionally stable laminar boundary layer along a flat plate in a low-turbulence windtunnel. At several vertical and lateral positions the streamwise (U) and vertical (V) velocity components of the spot are measured using anX-shaped hot wire anemometer. The spot's leading edge (LE) and trailing edge (TE) are determined by a high frequency detection criterion. The derived ensemble averaged shear stress 〈uv〉 shows a remarkable plateau over the last 3/4 of the spot length and over about 1/2 of the spot width, approximately independent of the height above the plate. The measurements suggest that the velocities of the LE and the TE can be explained by simple advection of the active turbulent region. Other mechanisms seem to be responsible for the lateral spread of the spot, as is suggested by the measurements outside the plane of symmetry.     

Reactions of Podophyllotoxin with DDQ

Introduction 2, 3-Dichloro-5, 6-dicyanobenzoquinone (DDQ) can react with lignans of the mono- arylidene-butyrolactone1, aryltetralin2, dibenzylbutane3 and aryltetralin-butyrolactone4,5 series. We have studied the reactions of this reagent with podophyllotoxin 1, which is a well-known natural product on account of its long history of use in folk medicine and the biological activity of its many derivatives6. In particular, derivatives of 4-demethyl epipodophyllotoxin are used in cancer chemo…