Influence of the Reynolds number on the vortical structures in the logarithmic region of turbulent boundary layers

2013 ◽  
Vol 716 ◽  
pp. 5-50 ◽  
Author(s):  
Sophie Herpin ◽  
Michel Stanislas ◽  
Jean Marc Foucaut ◽  
Sebastien Coudert
Keyword(s):  
Reynolds Number ◽  
Coherent Structures ◽  
Flow Region ◽  
Wall Turbulence ◽  
Statistical Characteristics ◽  
Present Contribution ◽  
Vortical Structures ◽  
Stereo Particle Image Velocimetry ◽  
Logarithmic Region ◽  
The Subject

AbstractNear-wall turbulence is a subject of prime importance for turbulence modelling. Coherent structures were hypothesized early by Theodorsen in this flow region and have been the subject of intensive research ever since. The overall organization of these coherent structures has now been well assessed. Vortical structures appear to play a key role in this organization. Their characteristics and scaling have been studied by many authors as listed in the Introduction. The present contribution to the subject relies on high-resolution stereo particle image velocimetry (PIV) to study these structures. High-quality measurements are performed in a thick turbulent boundary layer at different values of the Reynolds number. The data quality is first assessed by comparing the statistics to those of hot-wire anemometry and direct numerical simulation data. The agreement between the two appears satisfactory. The PIV data are then processed in order to extract the vortex characteristics in a streamwise plane and in a spanwise plane. The statistical characteristics of these vortices are analysed in detail as a function of wall distance. The scaling of the data appears to be universal when the Kolmogorov scales are used. These results are analysed and discussed in terms of their probability density functions. This leads to a question regarding the Kolmogorov cascade in this region of the flow.

scholarly journals Wall-attached structures of velocity fluctuations in a turbulent boundary layer

2018 ◽  
Vol 856 ◽  
pp. 958-983 ◽  
Author(s):  
Jinyul Hwang ◽  
Hyung Jin Sung
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
High Reynolds Number ◽  
Wall Turbulence ◽  
Velocity Fluctuations ◽  
Moderate Reynolds Number ◽  
Wide Range ◽  
Logarithmic Region ◽  
High Reynolds ◽  
Attached Eddies

Wall turbulence is a ubiquitous phenomenon in nature and engineering applications, yet predicting such turbulence is difficult due to its complexity. High-Reynolds-number turbulence arises in most practical flows, and is particularly complicated because of its wide range of scales. Although the attached-eddy hypothesis postulated by Townsend can be used to predict turbulence intensities and serves as a unified theory for the asymptotic behaviours of turbulence, the presence of coherent structures that contribute to the logarithmic behaviours has not been observed in instantaneous flow fields. Here, we demonstrate the logarithmic region of the turbulence intensity by identifying wall-attached structures of the velocity fluctuations ($u_{i}$) through the direct numerical simulation of a moderate-Reynolds-number boundary layer ($Re_{\unicode[STIX]{x1D70F}}\approx 1000$). The wall-attached structures are self-similar with respect to their heights ($l_{y}$), and in particular the population density of the streamwise component ($u$) scales inversely with $l_{y}$, reminiscent of the hierarchy of attached eddies. The turbulence intensities contained within the wall-parallel components ($u$ and $w$) exhibit the logarithmic behaviour. The tall attached structures ($l_{y}^{+}>100$) of $u$ are composed of multiple uniform momentum zones (UMZs) with long streamwise extents, whereas those of the cross-stream components ($v$ and $w$) are relatively short with a comparable width, suggesting the presence of tall vortical structures associated with multiple UMZs. The magnitude of the near-wall peak observed in the streamwise turbulent intensity increases with increasing $l_{y}$, reflecting the nested hierarchies of the attached $u$ structures. These findings suggest that the identified structures are prime candidates for Townsend’s attached-eddy hypothesis and that they can serve as cornerstones for understanding the multiscale phenomena of high-Reynolds-number boundary layers.

Effects of polymer stresses on eddy structures in drag-reduced turbulent channel flow

2007 ◽  
Vol 584 ◽  
pp. 281-299 ◽  
Author(s):  
KYOUNGYOUN KIM ◽  
CHANG-F. LI ◽  
R. SURESHKUMAR ◽  
S. BALACHANDAR ◽  
RONALD J. ADRIAN
Keyword(s):  
Reynolds Number ◽  
Drag Reduction ◽  
Buffer Layer ◽  
Outer Region ◽  
Wall Turbulence ◽  
Channel Height ◽  
Large Contribution ◽  
Streamwise Vortices ◽  
Vortical Structures ◽  
Near Wall

The effects of polymer stresses on near-wall turbulent structures are examined by using direct numerical simulation of fully developed turbulent channel flows with and without polymer stress. The Reynolds number based on friction velocity and half-channel height is 395, and the stresses created by adding polymer are modelled by a finite extensible nonlinear elastic, dumbbell model. Both low- (18%) and high-drag reduction (61%) cases are investigated. Linear stochastic estimation is employed to compute the conditional averages of the near-wall eddies. The conditionally averaged flow fields for Reynolds-stress-maximizing Q2 events show that the near-wall vortical structures are weakened and elongated in the streamwise direction by polymer stresses in a manner similar to that found by Stone et al. (2004) for low-Reynolds-number quasi-streamwise vortices (‘exact coherent states: ECS’). The conditionally averaged fields for the events with large contribution to the polymer work are also examined. The vortical structures in drag-reduced turbulence are very similar to those for the Q2 events, i.e. counter-rotating streamwise vortices near the wall and hairpin vortices above the buffer layer. The three-dimensional distributions of conditionally averaged polymer force around these vortical structures show that the polymer force components oppose the vortical motion. More fundamentally, the torques due to polymer stress are shown to oppose the rotation of the vortices, thereby accounting for their weakening. The observations also extend concepts of the vortex retardation by viscoelastic counter-torques to the heads of hairpins above the buffer layer, and offer an explanation of the mechanism of drag reduction in the outer region of wall turbulence, as well as in the buffer layer.

Investigation of Flow and Mass Transfer on a Flat Plate Downstream of a Shear Inducing Moving Wall

2010 ◽  
Author(s):  
Kalyanjit Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Energy Content ◽  
Turbulent Energy ◽  
Wall Turbulence ◽  
Freestream Velocity ◽  
Moving Wall ◽  
Wall Boundary Layer ◽  
Logarithmic Region

The effects of an opposing (upstream-moving) wall-shear on a two-dimensional turbulent boundary layer are investigated. The shear at the boundary is imparted by a moving belt, flush with the wall. Boundary layer measurements are reported for four surface-to-freestream velocity ratios (0, −0.38, −0.51, −0.63) with the Reynolds number (based on the momentum thickness) between 922 and 1951. Velocity profiles downstream of the moving surface show an increased velocity deficit near the wall, which is more pronounced at higher (negative) belt velocity. Streamwise turbulence values downstream of the belt show the growth of a second peak in the logarithmic region of the boundary layer in addition to the normally-observed peak in the buffer region. This suggests the presence of larger length-scale turbulent eddies at locations away from the wall in the boundary layer. Spectral measurements indicate that the turbulent energy content is distributed over a wide portion of the logarithmic region. Mass transfer measurements using naphthalene sublimation provide the variation of Stanton with Reynolds number on the plate downstream of the moving belt. It shows little difference from the stationary belt case, which suggests that increased wall turbulence is balanced by an increase in the boundary layer thickness.

scholarly journals The relationship between free-stream coherent structures and near-wall streaks at high Reynolds numbers

2017 ◽  
Vol 375 (2089) ◽  
pp. 20160078 ◽  
Author(s):  
K. Deguchi ◽  
P. Hall
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Coherent Structures ◽  
Free Stream ◽  
Stokes Equations ◽  
Numerical Approach ◽  
Wall Turbulence ◽  
Large Reynolds Number ◽  
Equilibrium Solutions ◽  
New Family

The present work is based on our recent discovery of a new class of exact coherent structures generated near the edge of quite general boundary layer flows. The structures are referred to as free-stream coherent structures and were found using a large Reynolds number asymptotic approach to describe equilibrium solutions of the Navier–Stokes equations. In this paper, first we present results for a new family of free-stream coherent structures existing at relatively large wavenumbers. The new results are consistent with our earlier theoretical result that such structures can generate larger amplitude wall streaks if and only if the local spanwise wavenumber is sufficiently small. In a Blasius boundary layer, the local wavenumber increases in the streamwise direction so the wall streaks can typically exist only over a finite interval. However, here it is shown that they can interact with wall curvature to produce exponentially growing Görtler vortices through the receptivity process by a novel nonparallel mechanism. The theoretical predictions found are confirmed by a hybrid numerical approach. In contrast with previous receptivity investigations, it is shown that the amplitude of the induced vortex is larger than the structures in the free-stream which generate it. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.

Commands in Wolaitta

2017 ◽  
Author(s):  
Azeb Amha
Keyword(s):  
Sentence Type ◽  
Positive Polarity ◽  
Present Contribution ◽  
Grammatical Categories ◽  
The Subject ◽  
Leave Taking ◽  
And Gender ◽  
Syntactic Properties

This chapter examines expressions of commands (imperatives) in Wolaitta and the ways in which the imperative is distinguished from statements and questions. Although each sentence type is formally distinct, imperatives and questions share a number of morpho-syntactic properties. Similar to declarative and interrogative sentences, imperatives in Wolaitta involve verbal grammatical categories such as the distinction of person, number, and gender of the subject as well as negative and positive polarity. In contrast to previous studies, the present contribution establishes the function of a set of morphemes based on -árk and -érk to be the expression of plea or appeal to an addressee rather than politeness when issuing a command. Instead, politeness in commands is expressed by using plural (pro)nominal and verbal elements. The imperative in Wolaitta is a robust construction which is also used in formulaic speeches such as leave-taking as well as in blessing, curses, and advice.

scholarly journals Turbulence Intensity Modulation by Micropolar Fluids

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 195
Author(s):  
George Sofiadis ◽  
Ioannis Sarris
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Fluid Flows ◽  
Turbulent Channel Flow ◽  
High Volume ◽  
Reynolds Numbers ◽  
Intensity Modulation ◽  
Wall Turbulence ◽  
Volume Fractions ◽  
Physical Phenomena

Fluid microstructure nature has a direct effect on turbulence enhancement or attenuation. Certain classes of fluids, such as polymers, tend to reduce turbulence intensity, while others, like dense suspensions, present the opposite results. In this article, we take into consideration the micropolar class of fluids and investigate turbulence intensity modulation for three different Reynolds numbers, as well as different volume fractions of the micropolar density, in a turbulent channel flow. Our findings support that, for low micropolar volume fractions, turbulence presents a monotonic enhancement as the Reynolds number increases. However, on the other hand, for sufficiently high volume fractions, turbulence intensity drops, along with Reynolds number increment. This result is considered to be due to the effect of the micropolar force term on the flow, suppressing near-wall turbulence and enforcing turbulence activity to move further away from the wall. This is the first time that such an observation is made for the class of micropolar fluid flows, and can further assist our understanding of physical phenomena in the more general non-Newtonian flow regime.

Ultra mensuram tendere. Zur Maßlosigkeit mönchischer Askese (David von Augsburg, García Jiménez de Cisneros, Bernardino de Laredo)

2018 ◽  
Vol 23 (1) ◽  
pp. 29-47
Author(s):  
Christina Johanna Bischoff
Keyword(s):  
Early Modern ◽  
General Problem ◽  
Present Contribution ◽  
The Subject

AbstractThe present contribution deals with excess in medieval and early modern monastic asceticism. It claims that excess, in this context, is not an individual option, but a general problem inherent to Christian asceticism. By giving priority to self-denial, this type of asceticism promotes a self-regulated, autonomous subjectivity which is clearly an unintentional by-product of the asceticism. Therefore individual asceticism oscillates between prohibited self-fashioning and, as a response, intensified self-denial. The problem is outlined in an introductory chapter about ‘De exterioris et interioris hominis compositione’ by David of Augsburg. Following this, two early modern Spanish books are used to illustrate how spiritual texts deal with the ambivalences of asceticism. The ‘Exercitatorio de la vida spiritual’ by García Jiménez de Cisneros regulates asceticism in detail, while the ‘Subida del monte Sión’ by Bernardino de Laredo allows the subject more room for interpretation; this however comes at the price of a fictionalisation of asceticism.

The structure of internal intermittency in turbulent flows at large Reynolds number: experiments on scale similarity

1973 ◽  
Vol 59 (3) ◽  
pp. 537-559 ◽  
Author(s):  
C. W. Van Atta ◽  
T. T. Yeh
Keyword(s):  
Reynolds Number ◽  
Probability Density ◽  
Streamwise Velocity ◽  
Statistical Characteristics ◽  
Large Reynolds Number ◽  
Higher Moments ◽  
Scale Similarity ◽  
Probability Densities ◽  
Scale Ratio ◽  
Velocity Derivative

Some of the statistical characteristics of the breakdown coefficient, defined as the ratio of averages over different spatial regions of positive variables characterizing the fine structure and internal intermittency in high Reynolds number turbulence, have been investigated using experimental data for the streamwise velocity derivative ∂u/∂tmeasured in an atmospheric boundary layer. The assumptions and predictions of the hypothesis of scale similarity developed by Novikov and by Gurvich & Yaglom do not adequately describe or predict the statistical characteristics of the breakdown coefficientqr,lof the square of the streamwise velocity derivative. Systematic variations in the measured probability densities and consistent variations in the measured moments show that the assumption that the probability density of the breakdown coefficient is a function only of the scale ratio is not satisfied. The small positive correlation between adjoint values ofqr,land measurements of higher moments indicate that the assumption that the probability densities for adjoint values ofqr,lare statistically independent is also not satisfied. The moments ofqr,ldo not have the simple power-law character that is a consequence of scale similarity.As the scale ratiol/rchanges, the probability density ofqr,levolves from a sharply peaked, highly negatively skewed density for large values of the scale ratio to a very symmetrical distribution when the scale ratio is equal to two, and then to a highly positively skewed density as the scale ratio approaches one. There is a considerable effect of heterogeneity on the values of the higher moments, and a small but measurable effect on the mean value. The moments are roughly symmetrical functions of the displacement of the shorter segment from the centre of the larger one, with a minimum value when the shorter segment is centrally located within the larger one.

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