Role of secondary shear-layer vortices in the development of flow asymmetry on a cone–cylinder body at high angles of incidence

2020 ◽  
Vol 61 (10) ◽  
Author(s):  
Roopesh Kumar ◽  
Tufan Kumar Guha ◽  
Rajan Kumar
Keyword(s):  
Shear Layer ◽  
Flow Asymmetry

scholarly journals Unique Slow Crack Growth Behavior of Isotactic Polypropylene: The Role of Shear Layer-Spherulites Layer Alternated Structure

Polymers ◽  
2020 ◽  
Vol 12 (11) ◽  
pp. 2746
Author(s):  
Mingjin Liu ◽  
Jiaxu Luo ◽  
Jin Chen ◽  
Xueqin Gao ◽  
Qiang Fu ◽  
...  
Keyword(s):  
Shear Layer ◽  
Crack Growth ◽  
Isotactic Polypropylene ◽  
Slow Crack Growth ◽  
Crack Growth Behavior ◽  
Polymer Science ◽  
The Past ◽  
Propagation Behavior ◽  
Crack Propagation Behavior

With the development of polymer science, more attention is being paid to the longevity of polymer products. Slow crack growth (SCG), one of the most important factors that reveal the service life of the products, has been investigated widely in the past decades. Here, we manufactured an isotactic polypropylene (iPP) sample with a novel shear layer–spherulites layer alternated structure using multiflow vibration injection molding (MFVIM). However, the effect of the alternated structure on the SCG behavior has never been reported before. Surprisingly, the results showed that the resistivity of polymer to SCG can be enhanced remarkably due to the special alternated structure. Moreover, this sample shows unique slow crack propagation behavior in contrast to the sample with the same thickness of shear layer, presenting multiple microcracks in the spherulites layer, which can explain the reason of the resistivity improvement of polymer to SCG.

Free surface over a horizontal shear layer: vorticity generation and air entrainment mechanisms

2017 ◽  
Vol 813 ◽  
pp. 1007-1044 ◽  
Author(s):  
Matthieu A. André ◽  
Philippe M. Bardet
Keyword(s):  
Free Surface ◽  
Shear Layer ◽  
Air Entrainment ◽  
Horizontal Shear ◽  
Vortex Instability ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Spatio Temporal ◽  
The Waves

Two air entrainment mechanisms driven by vortex instability are reported in the unstable relaxation of a horizontal shear layer below a free surface. This flow is experimentally investigated by means of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) coupled with surface profilometry. PLIF identifies counter-rotating vortex pairs (CRVP) emanating from the surface following the growth of high steepness two-dimensional millimetre-size waves for Reynolds and Weber numbers based on the momentum thickness of 177 to 222 and 7.59 to 13.9, respectively. High spatio-temporal resolution PIV reveals the role of surface-generated vorticity and flow separation in the highly curved trough of the waves on the injection of a CRVP. Air bubbles are entrapped in the wake of these CRVPs at Reynolds number above 190. PIV data and spanwise PLIF images show two initiation mechanisms: primary vortex instability modulating the spanwise location where the flow separates, resulting in the pinch off of an air ligament, and secondary vortex instability turning a CRVP into$\unicode[STIX]{x1D6FA}$-shaped loops pulling the surface down. Instability wavelengths agree with linear stability analysis, and models for these new air entrainment mechanisms are proposed.

scholarly journals The role of gene flow asymmetry along an environmental gradient in constraining local adaptation and range expansion

2012 ◽  
Vol 25 (8) ◽  
pp. 1676-1685 ◽  
Author(s):  
K. M. FEDORKA ◽  
W. E. WINTERHALTER ◽  
K. L. SHAW ◽  
W. R. BROGAN ◽  
T. A. MOUSSEAU
Keyword(s):  
Gene Flow ◽  
Local Adaptation ◽  
Range Expansion ◽  
Environmental Gradient ◽  
Flow Asymmetry

The Role of Nonnormality in Overreflection Theory

2010 ◽  
Vol 67 (8) ◽  
pp. 2547-2558 ◽  
Author(s):  
Nikolaos A. Bakas ◽  
Brian F. Farrell
Keyword(s):  
Shear Layer ◽  
Gravity Waves ◽  
Finite Depth ◽  
Stimulated Emission ◽  
Incoming Wave ◽  
Propagating Waves ◽  
Three Stages ◽  
Orr Mechanism ◽  
Weak Stratification

Abstract The role of nonnormality in the overreflection of gravity waves is investigated. In the limit of weak stratification, wave packets having a critical level inside a shear layer of finite depth are reflected with amplified energy. This process, which exhibits the characteristics of stimulated emission, occurs in three stages: first, the incoming wave enters the shear layer and excites nonpropagating perturbations leaning with and against the shear. Subsequently, the energy of perturbations leaning against the shear grows in a manner similar to energy growth of perturbations in constant shear flows, indicating that the Orr mechanism that is slightly modified by stratification underlies the observed growth. Finally, the amplified perturbations excite propagating waves originating from the vicinity of the shear layer boundary. The role of nonnormality in this process is also investigated from the perspective of the associated nonorthogonality of the modes of the dynamical system. It is found that the incident wave packet projects on nonorthogonal analytic modes having the structure of a downward propagating wave in the far field below the shear layer and overreflection expressed by the interaction among these nonorthogonal modes.

Effects of Buoyancy on the Turbulent Wake of a Horizontal Cylinder in Cross-Flow

2012 ◽  
Author(s):  
Matthieu Boirlaud ◽  
Dominique Couton ◽  
Frédéric Plourde
Keyword(s):  
Mixed Convection ◽  
Shear Layer ◽  
Thermal Wave ◽  
Thermal Field ◽  
Cross Flow ◽  
Horizontal Cylinder ◽  
Transition To Turbulence ◽  
Cylinder Surface ◽  
Convection Regime

While heat transfer around bluff-bodies have been extensively studied in natural and forced convection regime, the mixed convection regime has not still yet brought so much attention; however the latter has direct interest either in various engineering applications or for fundamental point of views. Direct Numerical Simulation was applied in this paper to study the buoyancy effects in the wake of a horizontal cylinder in cross-flow for Re∞ = 1000 and Ri = 2.77. In the framework of mixed convection regime, results mainly focus on the role of thermal field and buoyancy effects. The main visible impact in the thermal field introduction is the asymmetry in the cylinder wake. In addition, typical mushroom-like structures driven by thermal field develop along the wake. From an unsteady point of view, a thermal wave develops from the bottom of the cylinder and the latter follows the cylinder surface. As a consequence, the upper shear-layer that occurs in isotherm case is strongly disturbed because of the interaction with the thermal wave and the lower shear-layer is stretched in the flow direction. Comparisons with the isotherm case help us to better understand the role of the thermal field and the effects of buoyancy in the transition to turbulence.

Effect of depth on flow over a fixed dune

2007 ◽  
Vol 34 (12) ◽  
pp. 1587-1599 ◽  
Author(s):  
Ram Balachandar ◽  
B.-S. Hyun ◽  
V. C. Patel
Keyword(s):  
Shear Layer ◽  
Outer Region ◽  
Vertical Component ◽  
Laser Doppler Velocimeter ◽  
Flow Depth ◽  
Flow Properties ◽  
Entire Depth ◽  
Degree Of Similarity ◽  
Turbulence Parameters

Laser Doppler velocimeter (LDV) measurements were carried out to study the effect of depth on the flow over a train of fixed two-dimensional dunes. Conventionally averaged velocity and turbulence parameters reveal large peaks in the streamwise and vertical components of turbulent intensities and shear stress, along the shear layer emanating from the dune crest. A secondary peak in the streamwise turbulence profiles some distance beyond the shear layer indicates maintenance of turbulence generated on the previous dune and convection of the flow history from one dune to the next. Analyses based on triple products and quadrant decomposition of velocity fluctuations reveals the central role of the shear layer in dictating the flow properties over the entire depth. The depth influences the flow in the near-bed region and the length of the separation zone is longer at a shallower depth. The streamwise mean profiles collapse onto a single curve in the outer region beyond the shear layer, indicating a degree of similarity and independence from the near-bed flow. The profiles of the vertical component of turbulence reveal a systematic dependence on flow depth, with lower turbulence intensity at larger depths. The quantitative effect of flow depth is evident in the measurements at all levels, including triple products and quadrant decomposition.

The Role of Streamwise Vorticity in the Control of Impinging Jets

2003 ◽  
Author(s):  
F. S. Alvi ◽  
H. Lou ◽  
C. Shih
Keyword(s):  
Velocity Field ◽  
Shear Layer ◽  
Acoustic Waves ◽  
High Speed ◽  
Feedback Loop ◽  
Large Scale ◽  
Field Measurements ◽  
Impinging Jets ◽  
Streamwise Vorticity

Supersonic impinging jets produce a highly unsteady flowfield leading to very high dynamic pressure loads on nearby surfaces. In earlier studies, we conclusively demonstrated that arrays of supersonic microjet, 400 μm in diameter, effectively disrupted the feedback loop inherent in high-speed impinging jet flows. This feedback disruption results in significant reductions in the adverse effects associated with such flows. In this paper, by primarily using detailed velocity field measurements, we examine the role of streamwise vorticity in order to better understand the mechanisms behind this control scheme. The velocity field measurements clearly reveal the presence of well-organized, streamwise vortices with the activation of microjets. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. We propose that the streamwise vorticity is mainly a result of the redirection of the azimuthal vorticity, which leads to a weakening of the large-scale structures in the primary jet. The appearance of strong vortices in the shear layer near the nozzle exit due to microjets further weakens the spatial coherence of the coupling between the acoustic waves and shear layer instability, while thickening the jet shear layer. All these effects are thought to be collectively responsible for the efficient disruption of the feedback loop using microjets.

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