Optimization of synthetic jet actuation by analytical modeling

Purpose The purpose of this paper is to analyze and optimize synthetic jet actuators (SJAs) by means of a literature-known one-dimensional analytical model. Design/methodology/approach The model was fit to a wide range of experimental data from in-house built SJAs with different dimensions. A comprehensive parameter study was performed to identify coupling between parameters of the model and to find optimal dimensions of SJAs. Findings The coupling of two important parameters, the diaphragm resonance frequency and the cavity volume, can be described by a power law. Optimal orifice length and diameter can be calculated from cavity height in good agreement with literature. A transient oscillation correction is required to get correct simulation outcomes. Originality/value Based on these findings, SJA devices can be optimized for maximum jet velocity and, therefore, high performance.

Ensemble Oscillation Correction (EnOC): Leveraging oscillatory modes to improve forecasts of chaotic systems

Oscillatory modes of the climate system are among its most predictable features, especially at intraseasonal time scales. These oscillations can be predicted well with data-driven methods, often with better skill than dynamical models. However, since the oscillations only represent a portion of the total variance, a method for beneficially combining oscillation forecasts with dynamical forecasts of the full system was not previously known. We introduce Ensemble Oscillation Correction (EnOC), a general method to correct oscillatory modes in ensemble forecasts from dynamical models. We compute the ensemble mean—or the ensemble probability distribution—with only the best ensemble members, as determined by their discrepancy from a data-driven forecast of the oscillatory modes. We also present an alternate method which uses ensemble data assimilation to combine the oscillation forecasts with an ensemble of dynamical forecasts of the system (EnOCDA). The oscillatory modes are extracted with a time-series analysis method called multi-channel singular spectrum analysis (M-SSA), and forecast using an analog method. We test these two methods using chaotic toy models with significant oscillatory components, and show that they robustly reduce error compared to the uncorrected ensemble. We discuss the applications of this method to improve prediction of monsoons as well as other parts of the climate system.

The crystal structure of hexam ethylenetetramine I. X -ray studies at 298, 100 and 34 °K

On account of its cubic molecular and crystal symmetry, hexamethylenetetramine is a key substance in the study of the lattice vibrations of molecular crystals; its anisotropic atomic-vibration amplitudes have been determined at 298, 100 and 34 °K from three-dimensional single crystal X-ray data. Two independent sets of experiments were done with Cu and Mo radiation; these led to results in very close agreement with one another (and with results derived from previous X-ray and neutron data at room temperature). Typical final discrepancy factors obtained from full-matrix least-squares refinements are 3.4 % for 95 Mo reflexions and 6.3 % for 44 Cu reflexions at 34 °K. The studies at the three temperatures provide direct support for the rotational-oscillation correction for atomic co-ordinates. The corrected weighted-mean dimensions from eight determinations are C—N = 1.476 ± 0.002 Å, ∠N—C—N = 113.6 ± 0.2°, ∠C—N—C = 107.2 ± 0.1°, and from the neutron study C—H = 1.088 ± 0.011 Å. The compact arrangement of molecules in the crystal is described.