A vortex interaction mechanism for generating energy and enstrophy fluctuations in high-symmetric turbulence

2019 ◽  
Vol 874 ◽  
pp. 639-676 ◽  
Author(s):  
Tatsuya Yasuda ◽  
Genta Kawahara ◽  
Lennaert van Veen ◽  
Shigeo Kida
Keyword(s):  
Periodic Motion ◽  
Large Amplitude ◽  
Large Scale ◽  
Interaction Mechanism ◽  
Wall Turbulence ◽  
Cyclic Process ◽  
High Symmetry ◽  
Dynamical Interaction ◽  
Decaying Turbulence ◽  
Axial Waves

Turbulent vortex dynamics is investigated in triply periodic turbulent flow with Kida’s high symmetry (Kida, J. Phys. Soc. Japan, vol. 54, 1985, pp. 2132–2136) by means of unstable periodic motion representing both the statistical and dynamical properties of turbulence (van Veen et al., Fluid Dyn. Res., vol. 38, 2006, pp. 19–46). In the periodic motion, the large-scale columnar vortices, the smaller-scale vortices and the large-amplitude axial waves on the large-scale columnar vortices are detected. In terms of mutual dynamical interaction between the large-scale columnar vortices and smaller-scale vortices, we demonstrate a cyclic process of excitation of the axial waves, which leads to large-amplitude fluctuations of the total kinetic energy and enstrophy. This cyclic process is characterised by three distinct phases and is therefore reminiscent of the regeneration cycle of near-wall turbulence structures (Hamilton et al., J. Fluid Mech., vol. 287, 1995, pp. 317–348). Notably, such oscillatory behaviour is observed even in freely decaying turbulence as a consequence of the instantaneous energy transfer from smaller to larger scales.

scholarly journals On the dynamical interaction between overshooting convection and an underlying dipole magnetic field – I. The non-dynamo regime

2021 ◽  
Vol 503 (1) ◽  
pp. 362-375
Author(s):  
L Korre ◽  
NH Brummell ◽  
P Garaud ◽  
C Guervilly
Keyword(s):  
Magnetic Field ◽  
Turbulent Diffusion ◽  
Large Scale ◽  
Molecular Diffusion ◽  
Mean Field ◽  
Stable Region ◽  
Turbulent Regime ◽  
Poloidal Magnetic Field ◽  
Dynamical Interaction ◽  
Large Scale Field

ABSTRACT Motivated by the dynamics in the deep interiors of many stars, we study the interaction between overshooting convection and the large-scale poloidal fields residing in radiative zones. We have run a suite of 3D Boussinesq numerical calculations in a spherical shell that consists of a convection zone with an underlying stable region that initially compactly contains a dipole field. By varying the strength of the convective driving, we find that, in the less turbulent regime, convection acts as turbulent diffusion that removes the field faster than solely molecular diffusion would do. However, in the more turbulent regime, turbulent pumping becomes more efficient and partially counteracts turbulent diffusion, leading to a local accumulation of the field below the overshoot region. These simulations suggest that dipole fields might be confined in underlying stable regions by highly turbulent convective motions at stellar parameters. The confinement is of large-scale field in an average sense and we show that it is reasonably modelled by mean-field ideas. Our findings are particularly interesting for certain models of the Sun, which require a large-scale, poloidal magnetic field to be confined in the solar radiative zone in order to explain simultaneously the uniform rotation of the latter and the thinness of the solar tachocline.

scholarly journals Empirically Bounding of Space Booms with Tape Spring Hinges

2013 ◽  
Vol 20 (3) ◽  
pp. 503-517 ◽  
Author(s):  
A.L. Jennings ◽  
J. Black ◽  
C. Allen
Keyword(s):  
Large Amplitude ◽  
Large Scale ◽  
Free Fall ◽  
Free Condition ◽  
Elliptical Motion ◽  
Scale Behavior

Self-deploying structures seek to provide a compact launch package for large, lightweight satellite booms. One self-deploying method is a foldable tape spring. This paper examines the large scale behavior of a boom attached by a tape spring hinge during mock deployments. A boom attached by tape spring to a rigid stand was released and the boom bounced up to 60° before coming to rest (as opposed to snap-through behavior). These large amplitude bounces can cause the boom to collide with sensors, other booms or arrays causing damage or preventing full deployment. Results show the first bounce of deployment is nearly bounded by a four parameter ellipse. The ellipses of similar folds are similar also, suggesting that a model can be developed. Free-fall tests simulating the free-free condition found in microgravity also show similar elliptical motion. Envelopes that bound the extents of the boom motion allow for collisions to be prevented by adjustment of the design.

scholarly journals Reynolds number trend of hierarchies and scale interactions in turbulent boundary layers

2017 ◽  
Vol 375 (2089) ◽  
pp. 20160077 ◽  
Author(s):  
W. J. Baars ◽  
N. Hutchins ◽  
I. Marusic
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Wall Turbulence ◽  
Small Scale ◽  
Velocity Fluctuations ◽  
Scale Interaction ◽  
High Reynolds ◽  
Near Wall

Small-scale velocity fluctuations in turbulent boundary layers are often coupled with the larger-scale motions. Studying the nature and extent of this scale interaction allows for a statistically representative description of the small scales over a time scale of the larger, coherent scales. In this study, we consider temporal data from hot-wire anemometry at Reynolds numbers ranging from Re τ ≈2800 to 22 800, in order to reveal how the scale interaction varies with Reynolds number. Large-scale conditional views of the representative amplitude and frequency of the small-scale turbulence, relative to the large-scale features, complement the existing consensus on large-scale modulation of the small-scale dynamics in the near-wall region. Modulation is a type of scale interaction, where the amplitude of the small-scale fluctuations is continuously proportional to the near-wall footprint of the large-scale velocity fluctuations. Aside from this amplitude modulation phenomenon, we reveal the influence of the large-scale motions on the characteristic frequency of the small scales, known as frequency modulation. From the wall-normal trends in the conditional averages of the small-scale properties, it is revealed how the near-wall modulation transitions to an intermittent-type scale arrangement in the log-region. On average, the amplitude of the small-scale velocity fluctuations only deviates from its mean value in a confined temporal domain, the duration of which is fixed in terms of the local Taylor time scale. These concentrated temporal regions are centred on the internal shear layers of the large-scale uniform momentum zones, which exhibit regions of positive and negative streamwise velocity fluctuations. With an increasing scale separation at high Reynolds numbers, this interaction pattern encompasses the features found in studies on internal shear layers and concentrated vorticity fluctuations in high-Reynolds-number wall turbulence. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.

scholarly journals A preventative approach to plagiarism: An empirical study of a first-year unit for undergraduates

2006 ◽  
Vol 2 (2) ◽  
Author(s):  
Sandy Darab
Keyword(s):  
Empirical Study ◽  
Large Scale ◽  
Cyclic Process ◽  
First Year ◽  
Academic Skill ◽  
Academic Values ◽  
First Semester ◽  
Assessment Tasks ◽  
Unit Design ◽  
Preventative Approach

This paper reports on an empirical study into the communications of academic values, codes and conventions within a large-scale foundation studies unit for firstyear undergraduates at a regional Australian university in first semester 2005. In this unit, one of the foci was teaching students about issues of plagiarism and assessing how students reflected upon and took up those ethics. The unit’s content and its assessment were conducted online. Students engaged in experiential learning within multiple online tasks associated with plagiarism and of direct relevance to unit assessment. Unit design, delivery and assessment involved a cyclic process of action research, which facilitated insights into students’ value communications and academic skill development over the semester. A qualitative analysis of students’ communications within sequential online assessment tasks and anonymous surveys, as well as the value discourses articulated by students and staff, reveals most students were highly receptive to information on plagiarism and intent upon avoiding it through developing academic skills.

Estimating large-scale structures in wall turbulence using linear models

2018 ◽  
Vol 842 ◽  
pp. 146-162 ◽  
Author(s):  
Simon J. Illingworth ◽  
Jason P. Monty ◽  
Ivan Marusic
Keyword(s):  
Linear Model ◽  
Large Scale ◽  
Eddy Viscosity ◽  
Linear Models ◽  
Systems Approach ◽  
Wall Turbulence ◽  
Linear Estimator ◽  
Small Scale ◽  
Spanwise Direction ◽  
Time Resolved

A dynamical systems approach is used to devise a linear estimation tool for channel flow at a friction Reynolds number of $Re_{\unicode[STIX]{x1D70F}}=1000$. The estimator uses time-resolved velocity measurements at a single wall-normal location to estimate the velocity field at other wall-normal locations (the data coming from direct numerical simulations). The estimation tool builds on the work of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) by using a Navier–Stokes-based linear model and treating any nonlinear terms as unknown forcings to an otherwise linear system. In this way nonlinearities are not ignored, but instead treated as an unknown model input. It is shown that, while the linear estimator qualitatively reproduces large-scale flow features, it tends to overpredict the amplitude of velocity fluctuations – particularly for structures that are long in the streamwise direction and thin in the spanwise direction. An alternative linear model is therefore formed in which a simple eddy viscosity is used to model the influence of the small-scale turbulent fluctuations on the large scales of interest. This modification improves the estimator performance significantly. Importantly, as well as improving the performance of the estimator, the linear model with eddy viscosity is also able to predict with reasonable accuracy the range of wavenumber pairs and the range of wall-normal heights over which the estimator will perform well.

Vibration Suppression of Rotating Machinery Utilizing Discontinuous Spring Characteristics

2005 ◽  
Author(s):  
Yukio Ishida ◽  
Jun Liu
Keyword(s):  
Periodic Motion ◽  
Large Amplitude ◽  
Critical Speed ◽  
Vibration Suppression ◽  
Rotating Machinery ◽  
Almost Periodic ◽  
Suppression Method ◽  
Rotor Stiffness ◽  
Parameter Values ◽  
Almost Periodic Motion

In rotating machinery, resonance phenomena occur with large amplitude in the vicinities of the major critical speeds. In this paper, a new vibration suppression method utilizing a discontinuous spring characteristic is proposed. This spring characteristic is made by additional springs with preload. This method has the following advantages: In designing these additional springs, we need not adjust their parameter values to the rotor stiffness and the system damping. The amplitude of vibration can be suppressed to any desired level. Although this method has a disadvantage that an almost periodic motion occurs above the major critical speed, two countermeasures are proposed to diminish it. We clarified these phenomena theoretically and experimentally.

Active Control of Near-Wall Coherent Structures

2002 ◽  
Author(s):  
P. Konieczny ◽  
A. Bottaro ◽  
V. Monturet ◽  
B. Nogarede
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Coherent Structures ◽  
Large Scale ◽  
Travelling Wave ◽  
Friction Reduction ◽  
Wall Turbulence ◽  
Integral Control ◽  
Macroscopic Simulation ◽  
Near Wall

This work aims at finding efficient means to reduce skin friction drag in a turbulent boundary layer. The argument on which the study is based is that turbulence exists near a wall because of the presence of an autonomous cycle which is maintained even in the absence of forcing from the free-stream. The central elements of this cycle are the near-wall coherent structures whose dynamics control the turbulence production. It is postulated that an action at the wall capable of disrupting the turbulent wall-cycle can yield a significant skin friction reduction. A model cycle is produced by embedding artificial, large scale streamwise vortices and streaks in a Blasius boundary layer. A control is then conceived, meant to produce an agglomeration of the streaks to hamper the cycle. The action envisaged consists in a movement of the wall, in the form of a spanwise standing or travelling wave of sufficiently long wavelength. The controllers in the present macroscopic simulation are simply cantilever beams whose movement is driven by ceramic piezo-actuators. Piezoelectric fibers realizing the same action (properly rescaled) provide, possibly, the answer to the technological challenge of the integral control of near-wall turbulence.

scholarly journals Simultaneous longitudinal and transverse oscillations in filament threads after a failed eruption

2019 ◽  
Vol 633 ◽  
pp. A12 ◽  
Author(s):  
Rakesh Mazumder ◽  
Vaibhav Pant ◽  
Manuel Luna ◽  
Dipankar Banerjee
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Large Amplitude ◽  
Large Scale ◽  
Radius Of Curvature ◽  
Solar Dynamics Observatory ◽  
Solar Prominences ◽  
Solar Dynamics ◽  
Transverse Oscillations

Context. Longitudinal and transverse oscillations are frequently observed in the solar prominences and/or filaments. These oscillations are excited by a large-scale shock wave, impulsive flares at one leg of the filament threads, or due to any low coronal eruptions. We report simultaneous longitudinal and transverse oscillations in the filament threads of a quiescent region filament. We observe a large filament in the northwest of the solar disk on July 6, 2017. On July 7, 2017, it starts rising around 13:00 UT. We then observe a failed eruption and subsequently the filament threads start to oscillate around 16:00 UT. Aims. We analyse oscillations in the threads of a filament and utilize seismology techniques to estimate magnetic field strength and length of filament threads. Methods. We placed horizontal and vertical artificial slits on the filament threads to capture the longitudinal and transverse oscillations of the threads. Data from Atmospheric Imaging Assembly onboard Solar Dynamics Observatory were used to detect the oscillations. Results. We find signatures of large-amplitude longitudinal oscillations (LALOs). We also detect damping in LALOs. In one thread of the filament, we observe large-amplitude transverse oscillations (LATOs). Using the pendulum model, we estimate the lower limit of magnetic field strength and radius of curvature from the observed parameter of LALOs. Conclusions. We show the co-existence of two different wave modes in the same filament threads. We estimate magnetic field from LALOs and suggest a possible range of the length of the filament threads using LATOs.

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