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"turbulence"
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Air turbulence and its methods of detection
\"The book is a concise guide dealing with the subject of air turbulence and its methods of detection with particular applications to aviation turbulence. It begins with a general description of turbulence and provides a background into the nature and causes of atmospheric turbulence that affects aircraft motion, giving updates on the state-of-the-art research on clear air turbulence (CAT). Important physical processes leading to the Kelvin-Helmholtz instability, a primary producer of CAT, are also explained. The several categories of CAT along with its impact on commercial aviation are also presented in a separate chapter, with particular emphasis on the structural damages to planes and injury trends. The central part of the book describes both the earlier and the latest CAT detecting methods and techniques for remote and in situ sensing and forecasting. A concise presentation of new technologies for reducing aviation weather-related accidents is also offered. A chapter on the weather accident prevention project of the NASA aviation safety program is also included. Additionally, the book ends with a full description of the recent research activities on CAT and future challenges in turbulence detection, prediction and avoidance. Since atmospheric turbulence is a major hazard in the aviation industry that may cause serious injuries to passengers and crew, understanding the generation mechanisms of CAT is of fundamental importance for the development of new forecasting algorithms that will help reducing the impact of CAT on the aviation industry. Air Turbulence and Its Methods of Detection offers a concise presentation of the most recent advances in the understanding of air turbulence generation along with the more recent CAT detection and warning methods, making it a valuable tool for researchers and engineers working in the dynamics, detection and prediction of CAT\"-- Provided by publisher.
Book
Dynamics Near the Subcritical Transition of the 3D Couette Flow I: Below Threshold Case
The authors study small disturbances to the periodic, plane Couette flow in the 3D incompressible Navier-Stokes equations at high Reynolds number Re. They prove that for sufficiently regular initial data of size $\\epsilon \\leq c_0\\mathbf {Re}^-1$ for some universal $c_0 > 0$, the solution is global, remains within $O(c_0)$ of the Couette flow in $L^2$, and returns to the Couette flow as $t \\rightarrow \\infty $. For times $t \\gtrsim \\mathbf {Re}^1/3$, the streamwise dependence is damped by a mixing-enhanced dissipation effect and the solution is rapidly attracted to the class of \"2.5 dimensional\" streamwise-independent solutions referred to as streaks.
eBook
Wall turbulence control
Wall turbulence control is a major subject, the investigation of which involves significant industrial, environmental and fundamental consequences. This title addresses advances achieved in active and passive wall turbulence control over the past two decades.
Book
Effect of compressibility on the small-scale structures in isotropic turbulence
Using a simulated highly compressible isotropic turbulence field with turbulent Mach number around 1.0, we studied the effects of local compressibility on the statistical properties and structures of velocity gradients in order to assess salient small-scale features pertaining to highly compressible turbulence against existing theories for incompressible turbulence. A variety of statistics and local flow structures conditioned on the local dilatation – a measure of local flow compressibility – are studied. The overall enstrophy production is found to be enhanced by compression motions and suppressed by expansion motions. It is further revealed that most of the enstrophy production is generated along the directions tangential to the local density isosurface in both compression and expansion regions. The dilatational contribution to enstrophy production is isotropic and dominant in highly compressible regions. The emphasis is then directed to the complicated properties of the enstrophy production by the deviatoric strain rate at various dilatation levels. In the overall flow field, the most probable eigenvalue ratio for the strain rate tensor is found to be −3:1:2.5, quantitatively different from the preferred eigenvalue ratio of −4:1:3 reported in incompressible turbulence. Furthermore, the strain rate eigenvalue ratio tends to be −1:0:0 in high compression regions, implying the dominance of sheet-like structures. The joint probability distribution function of the invariants for the deviatoric velocity gradient tensor is used to characterize local flow structures conditioned on the local dilatation as well as the distribution of enstrophy production within these flow structures. We demonstrate that strong local compression motions enhance the enstrophy production by vortex stretching, while strong local expansion motions suppress enstrophy production by vortex stretching. Despite these complications, most statistical properties associated with the solenoidal component of the velocity field are found to be very similar to those in incompressible turbulence, and are insensitive to the change of local dilatation. Therefore, a good understanding of dynamics of the compressive component of the velocity field is key to an overall accurate description of highly compressible turbulence.
Journal Article
Dynamics Near the Subcritical Transition of the 3D Couette Flow II: Above Threshold Case
This is the second in a pair of works which study small disturbances to the plane, periodic 3D Couette flow in the incompressible
Navier-Stokes equations at high Reynolds number
eBook
Turbulent boundary layers at moderate Reynolds numbers: inflow length and tripping effects
A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows (Schlatter & Örlü, J. Fluid Mech., vol. 659, 2010, pp. 116–126) showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. While such differences are expected at very low Reynolds numbers and/or the immediate vicinity of the inflow or tripping region, it remains unclear whether inflow and tripping effects explain the differences observed even at moderate Reynolds numbers. This question is systematically addressed by re-simulating the DNS of a zero-pressure-gradient turbulent boundary layer flow by Schlatter et al. (Phys. Fluids, vol. 21, 2009, art. 051702). The previous DNS serves as the baseline simulation, and the new DNS with a range of physically different inflow conditions and tripping effects are carefully compared. The downstream evolution of integral quantities as well as mean and fluctuation profiles is analysed, and the results show that different inflow conditions and tripping effects do indeed explain most of the differences observed when comparing available DNS at low Reynolds number. It is further found that, if transition is initiated inside the boundary layer at a low enough Reynolds number (based on the momentum-loss thickness)
${\\mathit{Re}}_{\\theta } \\lt 300$
, all quantities agree well for both inner and outer layer for
${\\mathit{Re}}_{\\theta } \\gt 2000$
. This result gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments, assuming that the flow was not severely over- or understimulated. It is further shown that even profiles of the wall-normal velocity fluctuations and Reynolds shear stress collapse for higher
${\\mathit{Re}}_{\\theta } $
irrespective of the upstream conditions. In addition, the overshoot in the total shear stress within the sublayer observed in the DNS of Wu & Moin (Phys. Fluids, vol. 22, 2010, art. 085105) has been identified as a feature of transitional boundary layers.
Journal Article
Existence of Bolgiano–Obukhov scaling in the bottom ocean?
The seminal Bolgiano–Obukhov (BO) theory established the fundamental framework for turbulent mixing and energy transfer in stably stratified fluids. However, the presence of BO scalings remains debatable despite their being observed in stably stratified atmospheric layers and convective turbulence. In this study, we performed precise temperature measurements with 51 high-resolution loggers above the seafloor for 46 h on the continental shelf of the northern South China Sea. The temperature observation exhibits three layers with increasing distance from the seafloor: the bottom mixed layer (BML), the mixing zone and the internal wave zone. A BO-like scaling $\\alpha =-1.34\\pm 0.10$ is observed in the temperature spectrum when the BML is in a weakly stable stratified ($N\\sim 0.0018$ rad s$^{-1}$) and strongly sheared ($Ri\\sim 0.0027$) condition, whereas in the unstably stratified convective turbulence of the BML, the scaling $\\alpha =-1.76\\pm 0.10$ clearly deviated from the BO theory but approached the classical $-$5/3 scaling in isotropic turbulence. This suggests that the convective turbulence is not the promise of BO scaling. In the mixing zone, where internal waves alternately interact with the BML, the scaling follows the Kolmogorov scaling. In the internal wave zone, the scaling $\\alpha =-2.12 \\pm 0.15$ is observed in the turbulence range and possible mechanisms are provided.
Journal Article