共查询到20条相似文献,搜索用时 358 毫秒
1.
The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel
flow are studied using large eddy simulation (LES) of compressible turbulent channel flows with spanwise wall oscillation
(SWO). The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other
researches. The high consistency of coherent velocity structures and temperature structures is found based on the analyses
of the turbulent flow field. When the coherent velocity structures are suppressed, the transportations of momentum and heat
are reduced simultaneously, demonstrating the same trend. This shows that the turbulent coherent structures have the same
effects on the transportations of momentum and heat. The averaged wall heat flux can be reduced with appropriate oscillating
parameters.
Supported by the Key Subjects of National Natural Science Foundation of China (Grant No. 10732090), the National Natural Science
Foundation of China (Grant No. 50476004), and the 111 Project (Grant No. B08009) 相似文献
2.
Phase-averaged waveforms of Reynolds stress in wall turbulence during the burst events of coherent structures 总被引:1,自引:0,他引:1
The time sequence signals of instantaneous longitudinal and normal velocity components at different vertical locations in
the turbulent boundary layer over a smooth flat plate have been finely measured by constant temperature anemometry of model
IFA-300 and X-shaped hot-wire sensor probe in a wind tunnel. The longitudinal and normal velocity components have been decomposed
into multi-scales by wavelet transform. The upward eject and downward sweep motions in a burst process of coherent structure
have been detected by the maximum energy criterion of identifying burst event in wall turbulence through wavelet analysis.
The relationships of phase-averaged waveforms among longitudinal velocity component, normal velocity component and Reynolds
stress component have been studied through a correlation function method. The dynamics course of coherent structures and their
effects on statistical characteristics of turbulent flows are analyzed.
Supported by the National Natural Science Foundation of China (Grant No. 10472081), the Program for New Century Excellent
Talents in Universities of Ministry of Education of China, and Tianjin Science and Technology Development Plan (Grant No.
06TXTJJC13800) 相似文献
3.
The inflow conditions for spatial direct numerical simulation (SDNS) of turbulent boundary layers should reflect the characteristics
of upstream turbulence, which is a puzzle. In this paper a new method is suggested, in which the flow field obtained by using
temporal direct numerical simulation (TDNS) for fully developed turbulent flow (only flow field for a single moment is sufficient)
can be used as the inflow of SDNS with a proper transformation. The calculation results confirm that this method is feasible
and effective. It is also found that, under a proper time-space transformation, all statistics of the fully developed turbulence
obtained by both temporal mode and spatial mode DNS are in excellent agreement with each other, not only qualitatively, but
also quantitatively. The normal-wise distributions of mean flow profile, turbulent Mach number and the root mean square (RMS)
of the fluctuations of various variables, as well as the Reynolds stresses of the fully developed turbulence obtained by using
SDNS, bear similarity in nature.
Supported by the National Natural Science Foundation of China (Grant No. 90205021), the China Postdoctoral Science Foundation
(Grant No. 20060400707), and the Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant No.
200328), and partially supported by Liu-Hui Center of Applied Mathematics, Nankai University and Tianjin University 相似文献
4.
Dynamic model of coherent structure in the wall region of a turbulent boundary layer 总被引:1,自引:0,他引:1
Based on the theoretical model for a single coherent structure in the wall region of a turbulent boundary layer, we studied the interaction of two coherent structures by direct numerical simulation in order to explain the mechanism for the formation of low-speed streaks. 相似文献
5.
Transition prediction for boundary layers has always been one of the urgent problems waiting for a solution for the development
of aero-space technology, yet there is no reliable and effective method due to the complexity of the problem. The e
N
method has been regarded as an effective method for the transition prediction of boundary layers. However, it heavily relies
on experiment or experience. And in cases with three-dimensional base flow, for instance, the boundary layer on a cone with
angle of attack, the result of its application is not satisfactory. The authors have found its cause and proposed the method
for its improvement, which did yield the fairly satisfactory result for a given test case, and also did not rely so much on
experiment or experience. However, before people can really apply this method to practical problems, more test cases have
to be studied. In this paper, more test cases for the application of the improved e
N
method to problems of transition prediction of supersonic and hypersonic boundary layers on cones with angle of attack will
be studied. The results are compared with those obtained by experiments and/or direct numerical simulations, confirming that
the improved e
N
method is effective and reliable. We also find that there may be more than one ZARF for each meridian plane, and which one
should be chosen for the e
N
method has been clarified.
Supported by the National Natural Science Foundation of China (Grant Nos. 10632050 and 90716007), the Special Foundation for
the Authors of National Excellent Doctoral Dissertations (Grant No. 200328), and the Foundation of Liu-Hui Center of Applied
Mathematics of Nankai University and Tianjin University Contributed by ZHOU Heng 相似文献
6.
Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate
turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved
2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized
as legs of quasi-streamwise vortices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic
shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding
to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is implemented to get the
ensemble-averaged inclination angle θ
R of typical LCS. θ
R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y
+=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to
the head is located around y
+=100. The ensemble- averaged convection velocity U
c of typical LCS is finally calculated from temporal-spatial correlation analysis of FTLE field. It is found that the wall-normal
profile of the convection velocity U
c(y) accords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the downstream convection of hairpins determines the transportation properties
of the turbulent boundary layer in the log-region and beyond.
Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001) 相似文献
7.
The problem of transition prediction for hypersonic boundary layers over a sharp cone has been studied in this work. The Mach
number of the oncoming flow is 6, the cone half-angle is 5°, and the angle of attack is 1°. The conventional e
N
method is used, but the transition location so obtained is obviously incorrect. The reason is that in the conventional method,
only the amplifying waves are taken into account, while in fact, for different meridians the decay processes of the disturbances
before they begin to grow are different. Based on our own previous work, new interpretation and essential improvement for
the e
N
method are proposed. Not only the amplification process but also the decay process is considered. The location, where by
linear stability theory, the amplitude of disturbance wave is amplified from its initial small value to 1%, is considered
to be the transition location. The new result for transition prediction thus obtained is found to be fairly satisfactory.
It is also indicated that for the calculation of base flow, boundary layer equations can be used for a small angle of attack.
Its computational cost is much smaller than those for DNS.
Supported by the National Natural Science Foundation of China (Grant Nos. 10632050 and 90716007), the Special Foundation for
the Authors of National Excellent Doctoral Dissertations (Grant No. 200328), and the Foundation of Liu-Hui Center of Applied
Mathematics of Nankai University and Tianjin University 相似文献
8.
Results of an experimental study of turbulent breakdown in gradient boundary layer at high freestream turbulence are reported.
For the first time it is shown that, like the flat-plate boundary layer, the wing boundary layer at high freestream turbulence
is modulated with streaky structures. One of possible mechanisms underlying the generation of turbulence spots in wing boundary
layer is modelled assuming the interaction of streaky structures with high-frequency waves. Qualitative and quantitative data
concerning the evolution of streaky structures in swqpt-wing boundary layer and in swept-wing boundary layer are presented.
Certain differences between the evolution of streaky structures in wing boundary layer and in flat-plate boundary layer are
revealed.
This work was supported by the President of the Russian Federation (Grants NSh-454.2008.1, MK- 101.2007.1), by the Russian
Foundation for Basic Research (Grant No. 05-01-00034), by the Ministry of Education and Science of the Russian Federation
(Grant No. RNP 2.1.2.3370), and by the Russian Government (State Contract No. 02.513.12.0043). 相似文献
9.
Two multiparty simultaneous quantum identity authentication (MSQIA) protocols based on secret sharing are presented. All the
users can be authenticated by a trusted third party (TTP) simultaneously. In the first protocol, the TTP shares a random key
K with all the users using quantum secret sharing. The ith share acts as the authentication key of the ith user. When it is necessary to perform MSQIA, the TTP generates a random number R secretly and sends a sequence of single photons encoded with K and R to all the users. According to his share, each user performs the corresponding unitary operations on the single photon sequence
sequentially. At last, the TTP can judge whether the impersonator exists. The second protocol is a modified version with a
circular structure. The two protocols can be efficiently used for MSQIA in a network. They are feasible with current technology.
Supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB311100), the National High Technology
Research and Development Program of China (Grant Nos. 2006AA01Z419 and 20060101Z4015), the Major Research Plan of the National
Natural Science Foundation of China (Grant No. 90604023), the Scientific Research Common Program of Beijing Municipal Commission
of Education (Grant No. KM200810005004), the Scientific Research Foundation for the Youth of Beijing University of Technology
(Grant No. 97007016200701), the National Research Foundation for the Doctoral Program of Higher Education of China (Grant
No. 20040013007), the National Laboratory for Modern Communications Science Foundation of China (Grant No. 9140C1101010601),
and the Doctor Scientific Research Activation Foundation of Beijing University of Technology (Grant No. 52007016200702) 相似文献
10.
The coherent-entangled state |α, x; λ> with real parameters λ is proposed in the two-mode Fock space, which exhibits the properties of both the coherent and entangled states. The completeness relation of |α, x; λ> is proved by virtue of the technique of integral within an ordered product of operators. The corresponding squeezing operator is derived, with its own squeezing properties. Furthermore, generalized P-representation in the coherent-entangled state is constructed. Finally, it is revealed that superp... 相似文献
11.
《中国科学:物理学 力学 天文学(英文版)》2015,(6)
In the present paper,we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure-gradient turbulent boundary layer.An array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky structures.With both qualitative visualization and quantitative measurement,we found that the regularization only occurs in the region below the height of the roughness elements.Statistical analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements;however,the latter's effect is in competition with the intrinsic streak generation mechanisms of smooth wall turbulence.Below the top of the roughness elements,local streamwise turbulent fluctuation intensity can be reduced by about 10%.We used POD analysis to depict such regularization effect in terms of near-wall structure modulation.We further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence,there is no streak-regularization effect in the buffer region,so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce. 相似文献
12.
According to the characteristics of coherent structures in near-wall turbulence, an accurate extraction and verification method is developed based on wavelet transform (WT) and correlation analysis in this paper. At first, the fluid field of a turbulent boundary layer is measured precisely in a gravitational low-speed water tunnel. On the basis of the distribution of the coherent structures, velocity data of three test points are selected and analyzed, whose dimensionless heights are 20.8, 33.5, and 42.6. According to the frequency range of power spectrum density (PSD), coherent and incoherent structures are both extracted from the original signals using continuous and orthogonal wavelet transforms. To confirm the validity of the extracted signals, the probability density function (PDF) of each extracted signal is calculated. The result demonstrates that the incoherent structures obey the Gaussian distribution, while the coherent structures deviate from the Gaussian distribution. The PDFs of the coherent structures and the original signals are similar, which shows that the coherent structures make most contributions to the turbulence. For further verification, a correlation parameter between coherent and incoherent structures is defined, which evidently proves the validity of the extraction method in this paper. 相似文献
13.
14.
A threshold proxy quantum signature scheme with threshold shared verification is proposed. An original signer could authorize
a group as its proxy signers. Then only t or more of n persons in the proxy group can generate the proxy signature on behalf of the original signer and any t − 1 or fewer ones cannot do that. When the proxy signature needs to be verified, any t or more of n persons belonging to the verification group can verify the message and any t − 1 or fewer ones cannot verify the validity of the proxy signature.
Supported by the National Basic Research Program of China (973 Program)(Grant No. 2007CB311100), the National High-Technology
Research and Development Program of China (Grant Nos. 2006AA01Z419 and 20060101Z4015), the Major Research Plan of the National
Natural Science Foundation of China (Grant No. 90604023), the Scientific Research Common Program of Beijing Municipal Commission
of Education (Grant No. KM200810005004), the Scientific Research Foundation for the Youth of Beijing University of Technology
(Grant No. 97007016200701), the Doctoral Scientific Research Activation Foundation of Beijing University of Technology (Grant
No. 52007016200702), and the National Laboratory for Modern Communications Science Foundation of China (Grant No. 9140C1101010601) 相似文献
15.
The steady-state optical bistability (OB) and optical multistability (OM) behavior in the quasi-Λ-type atomic system driven
by a probe field and a coherent coupling field inside a unidirectional ring cavity are shown, and the effects of coupling-field
detuning and coupling-field intensity on the OB and OM behavior are investigated. The transition from OB to OM or vice versa is found by varying the detuning of the coherent coupling field or by adjusting the intensity of the coupling field. The
influence of the atomic cooperation parameter on the OM behavior is also discussed.
Supported by the Natural Science Foundation of Jiangxi of China (Grant No. 2007GZW0819), the National Basic Research Program
of China (Grant No. 2005CB724508), and the Foundation of Talent of Jinggang of Jiangxi Province (Grant No. 2008DQ00400) 相似文献
16.
Frequency Response of Near-Wall Coherent Structures to Localized Periodic Blowing and Suction in Turbulent Boundary Layer 下载免费PDF全文
We experimentally investigate the frequency response of near-wall coherent structures to localized periodic blow- ing and suction through a spanwise slot in a turbulent boundary layer by changing the frequency of periodic disturbance at similar velocities of free stream. The effects of blowing and suction disturbance on energy redistri- bution, turbulent intensity U^+ rums over y^+ and waveforms of phase-averaged velocity during sweeping process are respectively discussed under three frequencies of periodic blowing and suction in near-wall region of turbulent boundary layer, compared with those in a standard turbulent boundary layer. The most effective disturbance frequency is figured out in this system. 相似文献
17.
ZHAO YuXin YI ShiHe TIAN LiFeng HE Lin & CHENG ZhongYu College of Aerospace Material Engineering National University of Defense Technology Changsha China 《中国科学G辑(英文版)》2008,51(8):1134-1143
Flow visualization of supersonic mixing layer has been studied based on the high spatiotemporal resolution Nano-based Planar Laser Scattering(NPLS) method in SML-1 wind tunnel. The corresponding images distinctly reproduced the flow structure of laminar,transitional and turbulent region,with which the fractal measurement can be implemented. Two methods of measuring fractal dimension were introduced and compared. The fractal dimension of the transitional region and the fully developing turbulence region of supersonic mixing layer were measured based on the box-counting method. In the transitional region,the fractal dimension will increase with turbulent intensity. In the fully developing turbulent region,the fractal dimension will not vary apparently for different flow structures,which em-bodies the self-similarity of supersonic turbulence. 相似文献
18.
Fabrication of metal oxide nanostructures based on Atomic Force Microscopy lithography 总被引:1,自引:0,他引:1
XiaoYang Zhu Gang Cheng ShuJie Wang ShuXi Dai ShaoMing Wan XingTang Zhang ZuLiang Du 《中国科学G辑(英文版)》2008,51(10):1448-1454
Atomic Force Microscopy (AFM) mechanical lithography is a simple but significant method for nanofabrication. In this work,
we used this method to construct nanostructures on Pt/Cu bilayer metal electrodes under ambient conditions in air. The influence
of various scratch parameters, such as the applied force, scan velocity and circle times, on the lithography patterns was
investigated. The Pt-Cu-Cu
x
O-Cu-Pt nanostructure was constructed by choosing suitable scratch parameters and oxidation at room temperature. The properties
of the scratched regions were also investigated by friction force microscopy and conductive AFM (C-AFM). The I–V curves show symmetric and linear properties, and Ohmic contacts were formed. These results indicate that AFM mechanical lithography
is a powerful tool for fabricating novel metal-semiconductor nanoelectronic devices.
Supported by the National Natural Science Foundation of China (Grant No. 90306010), the Program for New Century Excellent
Talents in University of China (Grant No. NCET-04-0653), the National Basic Research Program of China (Grant No. 2007CB616911),
and the Science and Technology Department of Henan Province (Grant No. 072300420100) 相似文献
19.
Based on Reynolds-averaged Navier-Stokes approach,a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows.The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy,ωthe specific dissipation rate and the intermittency factorγ.The particular features of the model are that:1)k includes the non-tu... 相似文献
20.
In most situations, the signer is generally a single person. However, when the message is written on behalf of an organization,
a valid message may require the approval or consent of several persons. Threshold signature is a solution to this problem.
Generally speaking, as an authority which can be trusted by all members does not exist, a threshold signature scheme without
a trusted party appears more attractive. Following some ideas of the classical Shamir’s threshold signature scheme, a quantum
threshold group signature one is proposed. In the proposed scheme, only t or more of n persons in the group can generate the group signature and any t − 1 or fewer ones cannot do that. In the verification phase, any t or more of n signature receivers can verify the message and any t − 1 or fewer receivers cannot verify the validity of the signature.
Supported by the National Basic Research Program of China (973 Program)(Grant No. 2007CB311100), the National High-Technology
Research and Development Program of China (Grant Nos. 2006AA01Z419 and 2006AA01Z440), the Major Research Plan of the National
Natural Science Foundation of China (Grant No. 90604023), the Scientific Research Common Program of Beijing Municipal Commission
of Education (Grant No. KM200810005004), the Scientific Research Foundation for the Youth of Beijing University of Technology
(Grant No. 97007016200701), the Doctoral Scientific Research Activation Foundation of Beijing University of Technology (Grant
No. 52007016200702), the ISN Open Foundation, and the National Laboratory for Modern Communications Science Foundation of
China (Grant No. 9140C1101010601) 相似文献