Steady-state computational fluid dynamics (CFD) simulations are an essential tool in the design process of centrifugal compressors. Whilst global parameters, such as pressure ratio and efficiency, can be predicted with reasonable accuracy, the accurate prediction of detailed compressor flow fields is a much more significant challenge. Much of the inaccuracy is associated with the incorrect selection of turbulence model. The need for a quick turnaround in simulations during the design optimisation process also demands that the turbulence model selected be robust and numerically stable with short simulation times. In order to assess the accuracy of a number of turbulence model predictions, the current study used an exemplar open test case, the centrifugal compressor “Radiver”, to compare the results of three eddy-viscosity models and two Reynolds stress type models. The turbulence models investigated in this study were: (i) Spalart-Allmaras (SA), (ii) Shear Stress Transport (SST), (iii) a modification to the SST model denoted the SST-curvature correction (SST-CC), (iv) Reynolds stress model of Speziale, Sarkar and Gatski (RSM-SSG), and (v) the turbulence frequency formulated Reynolds stress model (RSM-ω). Each was found to be in good agreement with the experiments (below 2% discrepancy), with respect to total-to-total parameters at three different operating conditions. However, for the near surge operating point P1, local flow field differences were observed between the models, with the SA model showing particularly poor prediction of local flow structures. The SST-CC showed better prediction of curved rotating flows in the impeller. The RSM-ω was better for the wake and separated flow in the diffuser. The SST model showed reasonably stable, robust and time efficient capability to predict global performance and local flow features.
Abstract. FEFO is a liquid explosive with a density of 1.60 g/cm 3 and an energy output similar to that of trinitrotoluene (TNT), making it one of the more energetic liquid explosives. Here we describe shock initiation experiments that were conducted using a two-stage gas gun using magnetic gauges to measure the wave profiles during a shock-to-detonation transition. Unreacted Hugoniot data, time-to detonation (overtake) measurements, and reactive wave profiles were obtained from each experiment. FEFO was found to initiate by the homogeneous initiation model, similar to all other liquid explosives we have studied (nitromethane, isopropyl nitrate, hydrogen peroxide). The new unreacted Hugoniot points agree well with other published data. A universal liquid Hugoniot estimation slightly under predicts the measured Hugoniot data. FEFO is very insensitive, with about the same shock sensitivity as the triamino-trinitro-benzene (TATB)-based explosive PBX9502 and cast TNT.
Prior investigations by Plomp and Mimpen have concluded that the ear can hear and identify separately only the first five to seven particles of a tone, in agreement with limitations deduced from the critical band theory. With somewhat different equipment and procedure, a set of experiments was conducted for the purpose of discovering whether such variations of timbre as are typical of musical performance may affect a replication of the conclusions of Plomp and Mimpen. Oscillators were phase locked to a common fundamental frequency and adjusted to uniform sound levels, which were checked by a wave analyzer; the resultant complex tone could be deprived of any partial (except its fundamental partial) by means of interruption by an electronic switch; an observer was asked to duplicate the frequency of the interrupted partial, and the chosen frequency was checked with a frequency counter. Results indicate a less categorical limitation of the ear's capability, in which, at least in the middle frequencies, partials as high as the 11th and the 12th may be separately heard with some consistency. (Research sponsored by North Texas State University.)
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