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31.
32.
This work is an experimental study of mechanisms for transition to turbulence in the boundary layer on a rotating disk. In
one case, the focus was on a triad resonance between pairs of traveling cross-flow modes and a stationary cross-flow mode.
The other was on the temporal growth of traveling modes through a linear absolute instability mechanism first discovered by
Lingwood (1995, J Fluid Mech 314:373–405). Both research directions made use of methods for introducing controlled initial
disturbances. One used a distributed array of ink dots placed on the disk surface to enhance a narrow band of azimuthal and
radial wave numbers of both stationary and traveling modes. The size of the dots was small so that the disturbances they produce
were linear. Another approach introduced temporal disturbances by a short-duration air pulse from a hypodermic tube located
above the disk and outside the boundary layer. Hot-wire sensors primarily sensitive to the azimuthal velocity component, were
positioned at different spatial (r,θ) locations on the disk to document the growth of disturbances. Spatial correlation measurements were used with two simultaneous
sensors to obtain wavenumber vectors. Cross-bicoherence was used to identify three-frequency phase locking. Ensemble averages
conditioned on the air pulses revealed wave packets that evolved in time and space. The space–time evolution of the leading
and trailing edges of the wave packets were followed past the critical radius for the absolute instability, r
c
A
. With documented linear amplitudes, the spreading of the disturbance wave packets did not continue to grow in time as r
c
A
was approached. Rather, the spreading of the trailing edge of the wave packet decelerated and asymptotically approached a
constant. This result supports the linear DNS simulations of Davies and Carpenter (2003, J Fluid Mech 486:287–329) who concluded
that the absolute instability mechanism does not result in a global mode, and that linear-disturbance wave packets are dominated
by the convective instability. In contrast, wave-number matching between traveling cross-flow modes confirmed a triad resonance
that lead to the growth of a low azimuthal number (n = 4) stationary mode. At transition, this mode had the largest amplitude. Signs of this mechanism can be found in past flow
visualization of transition to turbulence in rotating disk flows. 相似文献
33.
David Mills 《Granular Matter》2004,6(2-3):173-177
For fine powdered materials, capable of being conveyed in dense phase, a conveying region exists in which instability in flow and pipeline blockage can occur if the pressure gradient available is insufficient to maintain a minimum value of solids loading ratio. A reduction in material flow rate can also lead to pipeline blockage in this region. Conveying trials under-taken with cement are used to illustrate the nature of the problem. 相似文献
34.
35.
部分样本雷达信号的参数估计 总被引:1,自引:0,他引:1
线性调频信号参数检测是SAR对抗的一个重要问题 ,其估计参数的方法有多种 ,但对于分析部分样本LFM信号而言 ,绝大多数估计方法的误差很大。在用快速相关解线调估计和分析部分样本LFM信号存有误差的基础上 ,提出了一种改进的快速相关解线调算法运用于部分样本LFM信号的起始频率和调频斜率参数的估计 ,减小了误差。最后对改进的算法进行了仿真 ,证实了改进后的有效性。 相似文献
36.
通过荻港海螺水泥有限公司(2×2500t/d)工程设计、建成投产的实践,阐述了水泥厂建在山坡上的厂址选择,总体规划布局,厂内道路最大坡度确定,高台段划分及边坡爆破,场地整平压实,护坡挡墙与厂房土建基础埋深等技术问题的处理方法,总结其设计成功的经验和不足之处,为同类型工厂总图布置提供借鉴. 相似文献
37.
Dynamic elastic-plastic buckling phenomena which might develop in a rod from an axial impact loading are studied in order to identify the conditions for quasi-static behaviour. A discrete model for dynamic elastic-plastic buckling, which retains the axial and the lateral inertia forces, is proposed, and the relationship between the model parameters and the characteristics of an actual structure is given. Examples of different external loadings and boundary conditions are considered in order to clarify the influence of elastic-plastic axial wave propagation on the buckling process. The critical time for the initiation of buckling is obtained and the post-buckling behaviour of the model is analysed. Particular attention is paid to the role of the striking mass on the characteristics of the buckling process and on the development of the buckling shape. The numerical study reveals that the inertia of the striking mass affects considerably the development of the buckling shape causing different patterns of axial strain distributions at the initiation of buckling. The comparisons which are made between the model predictions and some previously published experimental data show that the buckling process is governed by the impact velocity as well as by the external loading history provided by the experimental technique. 相似文献
38.
通过工程地质调查、室内围岩物化分析和力学试验 ,深入研究了围岩变形力学机制和破坏机理 .根据新奥法施工原理 ,采取了有针对性的综合治理措施 ,将围岩复杂的变形力学机制转化为单一的变形力学机制 ,使该工程得到了有效治理 .取得的技术经验推广应用于该矿井其它失稳破坏工程 ,均获得了圆满成功 相似文献
39.
40.
列举了在径向水压力与温升或温降两种荷载组合下圆弧拱与5种非圆弧拱的拱坝多拱梁法分析成果,从位移、应力、岸坡稳定性等方面作了比较。对于两种荷载组合下了解、掌握各类拱的拱坝特性提供了充实的资料,在此基础上,提出了温降组合一般起控制作用的见解 相似文献