Simulation of the starting flow in a wedge–like nozzle

The starting process of the flow in a wedge-like expansion nozzle of a shock tunnel is simulated by an unsplit 2-D GRP scheme on an unstructured grid. The scheme is briefly outlined and results are presented and discussed in comparison to the experimental (shadowgraph) findings obtained by Amann. The simulated pattern of reflected and transmitted shock waves in the nozzle inlet region and inside the nozzle is found to agree well with the experimental data. Received 5 April 1996 / Accepted 16 June 1997     

窄禁带光子晶体能态密度的研究

利用平面波展开法研究了窄禁带光子晶体材料的能态密度分布情况,得到了窄禁带材料构成的三角晶格光子晶体能态密度随着介电常数差值的增大而增大,并且分析了氮化镓在f=0.3时,能态密度分布最小,出现最大光子带隙。研究结果为窄禁带光子晶体器件的构造提供理论依据。     

Diode laser spectroscopy of the ν7a band of fluorobenzene

The diode laser absorption infrared spectrum of fluorobenzene has been recorded near 1230 cm⁻¹ after cooling the molecules in a supersonic pulsed jet. Spectral lines have been assigned to the ν_7a fundamental band. Transitions of J between 32 and 49 have been recorded, that show characteristic line groupings in the P branch. Analysis of the spectrum gives the vibrational band origin and rotational and centrifugal distortion constants of the molecule in the ν_7a = 1 state.     

On some coarse shape invariants

The author and N. Ugleši? have recently introduced a new classification of topological spaces, strictly coarser than the shape type classification. The corresponding coarse shape theory is founded on the coarse shape category. In this paper several topological and shape invariants are considered with respect to the coarse shape. It is shown that the coarse shape domination preserves connectedness, shape dimension, movability, n-movability and strong movability. Further, stability is a coarse shape invariant. Moreover, the coarse shape and shape coincide on the class of stable spaces.     

Expansions and penultimate distributions of maxima of bivariate normal random vectors

It is well known that the marginal maxima of n$n$ standard normal random vectors with correlation coefficient ρ<1$\rho <1$ are asymptotically independent. In this article, the residual dependence will be captured by asymptotic expansions and certain penultimate distributions including the case where ρ(n)↑1$\rho \left(n\right)\uparrow 1$ at a certain rate.     

On the use of two classical series expansion methods to determine the vibration of harmonically excited pure cubic oscillators

An analytical approach to determine the steady-state response of a damped and undamped harmonically excited oscillator with no linear term and with cubic non-linearity is presented. The governing equation is transformed into a form suitable for the application of a classical series expansion technique. The Linstedt–Poincaré method and the method of multiple scales are then used to determine the amplitude-frequency response and approximate solution for the response at the excitation frequency. The results obtained are compared with numerical solutions and analytical solutions found in the literature for the case when there is strong non-linearity.     

Application of RANS-based method to predict acoustic noise of chevron nozzles

Passive noise control devices for jet flows, such as chevron nozzles, have been studied for a long time due to their large application in turbofan engines. The main purpose of their usage is the reduction of low-frequency noise generation and thus decreasing the noise perceived at the far field. This work is a numerical study of acoustic noise generated by jet flow operating at Mach number 0.9 and Reynolds number 1.38 × 10⁶, considering two chevron nozzle geometries that differ from each other by the penetration angle into the flow. The main aim was to demonstrate that Reynolds averaged Navier Stokes (RANS)-based methods are reliable means to obtain acoustical noise predictions for the industry with a considerably low computational cost. In order to achieve this objective, computational fluid dynamics (CFD) RANS simulations were performed with a cubic k-ɛ model and the acoustic noise spectrum for different angles of radiation was obtained via the Lighthill ray-tracing (LRT) method. The numerical results for the acoustic and flow fields were seen to be in reasonable agreement with the experimental data, suggesting that this methodology can be used as a fast and useful option to predict acoustic noise of jet flows from chevron nozzles.     

Investigation of the effect of nozzle shape on supersonic/hypersonic impactors designed for size discrimination of nanoparticles

In this study the flow field and the nanoparticle collection efficiency of supersonic/hypersonic impactors with different nozzle shapes were studied using a computational modeling approach. The aim of this study was to develop a nozzle design for supersonic/hypersonic impactors with the smallest possible cut-off size d₅₀ and rather sharp collection efficiency curves. The simulation results show that the changes in the angle and width of a converging nozzle do not alter the cut-off size of the impactor; however, using a conical Laval nozzle with an L/D_n ratio less than or equal to 2 reduced d₅₀. The effect of using a cap as a focuser in the nozzle of a supersonic/hypersonic impactor was also investigated. The results show that adding a cap in front of the nozzle had a noticeable effect on decreasing the cut-off size of the impactor. Both flat disks and conical caps were examined, and it was observed that the nozzle with the conical cap had a lower cut-off size.