Metastability and vortex pairing in the kolmogorov flow

We show that a system of very elongated vortices, such as appear in the Kolmogorov flow near threshold can go through a succession of quasi-equilibrium vortex solutions with n, n−1, n−2, … vortices. The result applies more generally to the competition between cellular solutions in “zig-zag” instabilities.     

Theory of pairing symmetry in the vortex states

We investigate pairing symmetry in an Abrikosov vortex and vortex lattice. It is shown that the Cooper pair wave function at the center of an Abrikosov vortex with vorticity m has a different parity with respect to frequency from that in the bulk if m is an odd number, while it has the same parity if m is an even number. As a result, in a conventional vortex with m = 1, the local density of states at the Fermi energy has a maximum (minimum) at the center of the vortex core in an even (odd)-frequency superconductor. In the vortex lattice of s-wave superconductor, we find that only odd-frequency pairing is present at the core centers, while at the midpoint of the vortex lines, only even-frequency pairing exists. Thus, the odd and even-frequency pairings also form the lattice in the vortex lattice state. We also propose a scanning tunneling microscope experiment using a superconducting tip to explore odd-frequency superconductivity.     

Kinetics of vortex formation in superconductors with anisotropic pairing

The evolution of the vortex state in superconductors with anisotropic pairing is investigated on the basis of the time-dependent Ginzburg-Landau equations. Fiz. Tverd. Tela (St. Petersburg) 39, 35–38 (January 1997)     

脉动喷注噪声与稳态喷注噪声的关系

在假设气流脉动基本不影响湍流性质的基础上,求得了脉动喷注噪声的声压级公式(90°方向,1m远)。稳态喷注噪声的声压级,相当于脉动频率为零的特例,从而将脉动喷注噪声和稳态喷注噪声统一了起来,实验结果与理论符合。     

Non-monotonic driven vortex noise in HTSC

We present noise measurements on YBCO thin films in different conditions of magnetic field and driving current. Noise spectra for non-driven and driven cases (in the flux-creep region) evidence deep differences in vortex dynamics between these two regimes. For the driven case, the effect of applying magnetic field is a reduction in noise, which can be explained by the increase in the fraction of vortices that undergo flux-flow. For the non-driven case, magnetic field has no significative influence on noise, probably due to the absence of Lorentz force that causes coherent movement of vortices. For all magnetic fields studied in this work (0-154 mT) the effect of increasing current is an increase of noise, which is in contrast to the results from other authors. This behavior can be explained by an increase of current induced vortex-antivortex annihilation events. We propose that driven noise has a non-monotonic behavior due to the competition between annihilation events and driving force which causes opposite effects on noise.     

The life of a vortex in an axisymmetric jet

Graphical Abstract  

     

Bicoherence analysis of model-scale jet noise

Bicoherence analysis has been used to characterize nonlinear effects in the propagation of noise from a model-scale, Mach-2.0, unheated jet. Nonlinear propagation effects are predominantly limited to regions near the peak directivity angle for this jet source and propagation range. The analysis also examines the practice of identifying nonlinear propagation by comparing spectra measured at two different distances and assuming far-field, linear propagation between them. This spectral comparison method can lead to erroneous conclusions regarding the role of nonlinearity when the observations are made in the geometric near field of an extended, directional radiator, such as a jet.     

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.     

二维约瑟夫森结阵列中的相变及噪声频谱研究

采用电阻分路(RSJ)模型，运用数值计算方法研究二维约瑟夫森结阵列的涡旋度,涡旋密度涨落随温度的变化，表明了超导阵列中存在KTB型相变.还研究了相变点附近的涡旋噪声频谱随温度、驱动电流变化的特性.计算结果与最近实验报导定性一致，并能用涡旋的运动图像来解释噪声频谱的变化规律. 关键词： 高温超导 噪声频谱 约瑟夫森结阵列 KTB相变     

Unsteady surface signature of a pulsed vortex generator jet

Abstract  

Porous pressure-sensitive paint (PSP) is employed as a visualization technique for unsteady flow features on a low-pressure turbine blade. Recognizing that the measurement of high-frequency pressure fluctuations in unsteady flows—especially in turbomachinery—has proven to be difficult, recent advancements in the development of porous PSP have enabled the high-resolution measurement of pressure fields with frequency content of at least 20 kHz. In this work, PSP is applied to an L1A low-pressure turbine blade section (Re = 20,000 based on axial chord) to visualize the surface dynamics of a vortex generator jet (VGJ) pulsed at 10.6 Hz with nitrogen gas. Intensity-based, time-resolved PSP measurements reveal the development and the surface structure of the VGJ as well as the spanwise variation in the blowing profile.     

Adaptive noise cancelling and condition monitoring

When mechanical signature analysis methods are applied to the detection of faults within a complex machine, one is often confronted with a situation in which the diagnostic signal is embedded in a background noise. Coherent filtering techniques are of help in improving the signal to noise ratio (SNR) only when a synchronizing signal is available; on the other hand, the adaptive noise cancelling (ANC) technique can be successfully applied to increase the signal to noise ratio even in those situations where a synchronization signal is not available. Adaptive noise cancelling is a form of optimal filtering in which use is made of an auxiliary or a reference signal. In the work reported here it has been shown that the statistical and spectral analyses techniques which fail to detect and diagnose faults because of a poor signal to noise ratio can be made effective by using an adaptive noise cancelling technique. The expression for the signal to noise density ratio at the output of the noise canceller is derived for a simplified model of a machine.     

Power spectral density of subsonic jet noise

The scaling of the power scaling density (PSD) of the far field noise for subsonic, unheated, axisymmetric jets has been examined based on data obtained from the literature as well as from new experiments. It has been demonstrated that the PSD scales as the Strouhal number (St_D) alone for most locations, except at shalloow angles (θ<30°) where the best scaling results with the Helmholtz number times the Doppler factor, H_D(1?M_c cos θ). In contrast to the common assumption, scaling with St_D times the Doppler factor only degraded the collapse of the data. Also, a “source convection Mach number”, M_c = 0·5 M, rather than the commonly used value of 0·67 M, yielded the best collapse, M_c = 0·5 M agreeing with the recent measurement of jet large-scale structure convection velocity. At θ = 90°, the amplitude of the PSD varies as U^6·5, which agrees with the observed U^7·5 variation of the overall intensity; a similar power law assumed at θ ～- 30° yields an exponent 8·5 for the PSD. Contrary to claims in the literature, the PSD amplitude was not observed to vary sharply about any critical Reynolds number. For shallow angles, the scaling on H_D (1 ? M_c cos θ) also contrasts simple H_D scaling previously observed on the basis of one-third octave spectra.     

Counter rotating vortex pair structure in a reacting jet in crossflow

This paper analyzes the time averaged flow structure of a reacting jet in cross flow (RJICF), emphasizing the structure of the counter-rotating vortex pair (CVP) by using simultaneous tomographic particle image velocimetry (TPIV) and hydroxyl radical planar laser induced fluorescence (OH-PLIF). It was performed to determine the extent to which heat release, and the associated effects of gas expansion and baroclinic vorticity production, impact the structure of the CVP. These results show the clear presence of a CVP in the time averaged flow field, whose trajectory lies below the jet centerline on either side of the centerplane. Consistent with other measurements of high momentum flux ratio JICF in nonreacting flows, there is significant asymmetry in strength of the two vortex cores. The strength and structure of the CVP was quantified with vorticity and swirling strength (λ_ci), showing that some regions of the flow with high shear are not necessarily accompanied by large scale bulk flow rotation and vice-versa. The OH PLIF measurement allows for correlation of the flame position with the dominant vortical structures, showing that the leeward flame branch lies slightly above, as well as, in the region between the CVP cores.     

Euler evolution for singular initial data and vortex theory

We study the evolution of a two dimensional, incompressible, ideal fluid in a case in which the vorticity is concentrated in small, disjoint regions of the physical space. We prove, for short times, a connection between this evolution and the vortex model.     

Adjoint-based optimization for understanding and suppressing jet noise

Advanced simulation tools, particularly large-eddy simulation techniques, are becoming capable of making quality predictions of jet noise for realistic nozzle geometries and at engineering relevant flow conditions. Increasing computer resources will be a key factor in improving these predictions still further. Quality prediction, however, is only a necessary condition for the use of such simulations in design optimization. Predictions do not themselves lead to quieter designs. They must be interpreted or harnessed in some way that leads to design improvements. As yet, such simulations have not yielded any simplifying principals that offer general design guidance. The turbulence mechanisms leading to jet noise remain poorly described in their complexity. In this light, we have implemented and demonstrated an aeroacoustic adjoint-based optimization technique that automatically calculates gradients that point the direction in which to adjust controls in order to improve designs. This is done with only a single flow solutions and a solution of an adjoint system, which is solved at computational cost comparable to that for the flow. Optimization requires iterations, but having the gradient information provided via the adjoint accelerates convergence in a manner that is insensitive to the number of parameters to be optimized. This paper, which follows from a presentation at the 2010 IUTAM Symposium on Computational Aero-Acoustics for Aircraft Noise Prediction, reviews recent and ongoing efforts by the author and co-workers. It provides a new formulation of the basic approach and demonstrates the approach on a series of model flows, culminating with a preliminary result for a turbulent jet.