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81.
《中国航空学报》2020,33(2):465-475
Mach reflection in steady supersonic flow is an important phenomenon having received extensive studies, among which simplified theoretical models to predict the size of Mach stem and other flow structure are of particular interest. Past efforts for such models were based on inviscid assumption while in real cases the flow is viscous. Here in this paper we consider the influence of wedge boundary layer on the Mach stem height. This is done by including a simplified boundary layer model into a recently published inviscid model. In this viscous model, the wedge angle and the trailing edge height, which control the Mach stem height, are replaced by their equivalent ones accounting for the displacement effect of the wedge boundary layer, with the boundary layer assumed to be laminar or fully turbulent. This viscous model is shown to compare well with numerical results by computational fluid dynamics and gives a Mach stem height as function of the Reynolds number and Mach number. It is shown that due to the viscous effect, the Mach stem height is increased, through increasing the effective wedge angle. 相似文献
82.
Bakul Das Shubham Sarkar Prabir Kumar Haldar Subrata Kumar Midya Sujay Pal 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(1):75-86
The D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analysed using Very Low Frequency (VLF) radio communication signals from three transmitters (VTX, NWC and JJI) received at two low latitude stations (Kolkata-CUB and Cooch Behar-CHB). The cyclone Fani formed from a depression on 26th April, 2019 over the Bay of Bengal (Northeastern part of the Indian Ocean) and turned into an extremely severe cyclone with maximum 1-min sustained winds of 250 km/h on 2 May, 2019 which made landfall on 3 May, 2019. Out of six propagation paths, five propagation paths, except the JJI-CHB which was far away from the cyclone track, showed strong perturbations beyond level compared to unperturbed signals. Consistent good correlations of VLF signal perturbations with the wind speed and cyclone pressure have been seen for both the receiving stations. Computations of radio signal perturbations at CUB and CHB using the Long Wave Propagation Capability (LWPC) code revealed a Gaussian perturbation in the D-region ionosphere. Analysis of atmospheric temperature at different layers from the NASA’s TIMED satellite revealed a cooling effect near the tropopause and warming effects near the stratopause and upper mesosphere regions on 3 May, 2019. This study shows that the cyclone Fani perturbed the whole atmosphere, from troposphere to ionosphere and the VLF waves responded to the disturbances in the conductivity profiles of the lower ionosphere. 相似文献
83.
《中国航空学报》2020,33(1):149-160
Accurate predictions of Shock Waves and Boundary Layer Interaction (SWBLI) and strong Shock Waves and Wake Vortices Interaction (SWWVI) in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes (RANS) model. In this work, the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane (NGV) are studied using a hybrid RANS/LES strategy. The Turbulence Kinetic Energy (TKE) budget and the Proper Orthogonal Decomposition (POD) method are used to analyze flow mechanisms. Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis. Strong shock waves induce boundary layer separation, while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon. The shock waves cause trailing-edge vortices to break clearly, and the wakes, in turn, can change the shocks intensity and direction. Furthermore, the Entropy Generation Rate (EGR) is used to analyze the irreversible loss. It turns out that the SWWVI can reduce the flow field loss. There are several weak shock waves in the NGV flow field, which can increase the irreversible loss. This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade. 相似文献
84.
P. Nenovski 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(2):731-738
Various mechanisms have been proposed for explanation of the global magnetospheric modes whose frequency peaks are in the frequency range 1–4 mHz. Recent papers claim: basic characteristics of the 1–4 mHz activity events observed on ground give evidences for an existence of MHD surface mode excited on the Earth magnetopause. The discrete frequencies of such MHD surface wave modes suggest an emergence of standing wave structures along the magnetic field lines lying on the magnetopause. Such discrete frequencies of MHD surface waves on magnetopause however, are not stable, at all. Contrariwise, MHD surface wave modes supported by the two plasma boundaries – the magnetopause and the plasmapause, are in accordance with existing experimental facts: discrete set of almost stable frequencies, field amplitude peaks and positions, energy dissipation, and field distribution from high to low latitudes. Mechanisms of the global magnetospheric mode resonance are pointed out as well as tools for their identification and discrimination. 相似文献
85.
SOHO/UVCS data indicate that minor ions in the corona are heated more than hydrogen, and that coronal heating results in T⊥ larger than T‖. Analogous behavior has been known from in situ measurements in solar wind for many years. Here we compare and contrast two
mechanisms which have been proposed to account for the above behavior: ion-cyclotron resonance and gravity damping.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
86.
Michael R. Collier Aaron Roberts Adolfo Viñas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(10):1704-1709
We describe a new wave mode similar to the acoustic wave in which both density and velocity fluctuate. Unlike the acoustic wave in which the underlying distribution is Maxwellian, this new wave mode occurs when the underlying distribution is a suprathermal κ-function and involves fluctuations in the power law index, κ. This wave mode always propagates faster than the acoustic wave with an equivalent effective temperature and becomes the acoustic wave in the Maxwellian limit as κ → ∞. 相似文献
87.
The review contains the important achievements in dynamics of the galactic disks. Among them there are I. New structures
discovered recently: • giant vortices (including giant anticyclone in the Solar vicinity); • slow bar; • inner oscillating
structure within spiral arms similar that of enveloped soliton; • chaotic streamlines in the velocity field of the gaseous
disk of a real galaxy. II. New collective phenomina discovered recently: • new overreflection instability initiating ‘mini-spiral’
in the innermost central parsec of Galaxy; • large-scale convection caused by nonlinear interaction of density wave with
disk gas; • non-kolmogorovian spectrum of weak turbulence corresponding to the observed one in the • Solar vicinity. III. New
methods worked out recently: • reconstruction of full three-dimensional vector field of gas velocity from the observed line-of-
• sight velocity field; • observational test for verification of the wave-nature of the spiral arms; • observational test
to distinguish two types of vertical motions: warp and z-motions in the • density wave; • derivation of correct system of two-dimensional dynamical equations from the initial three-
• dimensional one.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
88.
K. Andreov L. Pech 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2007,40(12):1871-1880
This paper is devoted to the study of propagation of disturbances caused by interplanetary shocks (IPS) through the Earth’s magnetosphere. Using simultaneous observations of various fast forward shocks by different satellites in the solar wind, magnetosheath and magnetosphere from 1995 till 2002, we traced the interplanetary shocks into the Earth’s magnetosphere, we calculated the velocity of their propagation into the Earth’s magnetosphere and analyzed fronts of the disturbances. From the onset of disturbances at different satellites in the magnetosphere we obtained speed values ranging from 500 to 1300 km/s in the direction along the IP shock normal, that is in a general agreement with results of previous numerical MHD simulations. The paper discusses in detail a sequence of two events on November 9th, 2002. For the two cases we estimated the propagation speed of the IP shock caused disturbance between the dayside and nightside magnetosphere to be 590 km/s and 714–741 km/s, respectively. We partially attributed this increase to higher Alfven speed in the outer magnetosphere due to the compression of the magnetosphere as a consequence of the first event, and partially to the faster and stronger driving interplanetary shock. High-time resolution GOES magnetic field data revealed a complex structure of the compressional wave fronts at the dayside geosynchronous orbit during these events, with initial very steep parts (10 s). We discuss a few possible mechanisms of such steep front formation in the paper. 相似文献
89.
90.
应用钠原子和中性大气分子的质量连续性方程来模拟突发纳层(SSL),垂直风场采用接近实际大气重力波的正弦行波模式,结果较好地反映了SSL的形成过程。SSL的形成时刻在5-15min之间并可持续到30min之后,形成高度大约在90-100km之间,峰宽为0.5-2km之间,这些都与实际观测SSL的特点相符,同时还进一步地研究了当重力波参数(主要指垂直波长和周期)、风速以及常态钠层半宽度发生变化时SSL的变化趋势。 相似文献