Statistics and tensor analysis of polymer coil–stretch mechanism in turbulent drag reducing channel flow

2017 ◽  
Vol 824 ◽  
pp. 135-173 ◽  
Author(s):  
Anselmo S. Pereira ◽  
Gilmar Mompean ◽  
Laurent Thais ◽  
Roney L. Thompson
Keyword(s):  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Tensor Analysis ◽  
Turbulent Structures ◽  
Significant Alignment ◽  
Turbulent Drag ◽  
Polymer Coil ◽  
Conformation Tensor ◽  
Elastic Fluids ◽  
Nonlinear Elastic

The polymer coil–stretch mechanism in turbulent drag reducing flows is analysed using direct numerical simulations of viscoelastic finitely extensible nonlinear elastic fluids with the Peterlin approximation. The study is carried out taking into account low and high drag reduction regimes. The polymer stretching and the alignment between the conformation tensor and other relevant entities are investigated using statistical and tensor analysis. The significant alignment between the former and the velocity fluctuations product tensor indicates that the initial polymer stretching due to the mean shear is increased by the flow stress fluctuations, providing a supplementary polymer extension. In addition, interactions between the turbulence and the polymer are evaluated from an instantaneous turbulent energy exchange perspective by considering streamwise work fluctuating terms in elliptical and hyperbolic flow regions separately. Near the wall, polymers not only release energy to the streaks, but also to the elliptical (or vortical) and hyperbolic (or extensional) structures. However, polymers can also be dragged around near-wall vortices, passing through hyperbolic regions and experiencing a significant straining within both these turbulent structures and storing their energy. Hence, polymers weaken elliptical and hyperbolic structures leading to a tendency toward relaminarization of the flow. Polymer release of energy occurs primarily in the streamwise direction, which is in agreement with the enhanced streamwise velocity fluctuation observed in drag reducing flows. A detailed polymer coil–stretch mechanism is provided.

Characterization of Geometrical and Temporal Properties of Large-scale Motions in Turbulent Flows

2021 ◽  
Author(s):  
Christina Tsai ◽  
Kuang-Ting Wu
Keyword(s):  
Turbulent Flow ◽  
Turbulent Flows ◽  
Coherent Structures ◽  
Large Scale ◽  
Streamwise Velocity ◽  
Turbulent Structures ◽  
Temporal Properties ◽  
Turbulent Statistics ◽  
Spatial Locations

<p>It is demonstrated that turbulent boundary layers are populated by a hierarchy of recurrent structures, normally referred to as the coherent structures. Thus, it is desirable to gain a better understanding of the spatial-temporal characteristics of coherent structures and their impact on fluid particles. Furthermore, the ejection and sweep events play an important role in turbulent statistics. Therefore, this study focuses on the characterizations of flow particles under the influence of the above-mentioned two structures.</p><div><span>With regard to the geometry of turbulent structures, </span><span>Meinhart & Adrian (1995) </span>first highlighted the existence of large and irregularly shaped regions of uniform streamwise momentum zone (hereafter referred to as a uniform momentum zone, or UMZs), regions of relatively similar streamwise velocity with coherence in the streamwise and wall-normal directions.  <span>Subsequently, </span><span>de Silva et al. (2017) </span><span>provided a detection criterion that had previously been utilized to locate the uniform momentum zones (UMZ) and demonstrated the application of this criterion to estimate the spatial locations of the edges that demarcates UMZs.</span></div><div> </div><div>In this study, detection of the existence of UMZs is a pre-process of identifying the coherent structures. After the edges of UMZs are determined, the identification procedure of ejection and sweep events from turbulent flow DNS data should be defined. As such, an integrated criterion of distinguishing ejection and sweep events is proposed. Based on the integrated criterion, the statistical characterizations of coherent structures from available turbulent flow data such as event durations, event maximum heights, and wall-normal and streamwise lengths can be presented.</div>

scholarly journals Active control of multiscale features in wall-bounded turbulence

2019 ◽  
Vol 36 (1) ◽  
pp. 12-21 ◽  
Author(s):  
Xiaotong Cui ◽  
Nan Jiang ◽  
Xiaobo Zheng ◽  
Zhanqi Tang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Discrete Wavelet ◽  
Wall Region ◽  
Mean Velocity ◽  
Pzt Actuator ◽  
The Impact ◽  
Near Wall

Abstract This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales’ velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.

Microstructures of fiber suspensions in complex geometry

e-Polymers ◽  
2010 ◽  
Vol 10 (1) ◽  
Author(s):  
Chunlei Ruan ◽  
Jie Ouyang
Keyword(s):  
Fiber Orientation ◽  
Molecular Conformation ◽  
Complex Geometry ◽  
Fluid Model ◽  
Transversely Isotropic ◽  
Fiber Suspensions ◽  
Conformation Tensor ◽  
Planar Contraction ◽  
Volume Method ◽  
Nonlinear Elastic

AbstractEvolutions of molecular conformation and fiber orientation in fiber suspensions are investigated by collocated finite volume method on unstructured triangular meshes. FENE-P (Finite Extensible Nonlinear Elastic Dumbbell model with Peterlin’s approximation) model which is microstructure-based is chosen to describe the polymeric matrix and TIF (transversely isotropic fluid) model is used to calculate the fiber contribution in order to realize the coupling of flow and fiber orientation. Microstructures of molecule and fiber are obtained by analyzing the information of molecular conformation tensor and second-order fiber orientation tensor respectively. Two numerical examples are considered, namely, a planar contraction flow and a planar flow past a confined cylinder. Present results are hoping to give more insight into the microscopic details of complex flows and thus be more helpful for industrial application.

scholarly journals Flow–Sediment Turbulent Ejections: Interaction between Surface and Subsurface Flow in Gravel-Bed Contaminated by Fine Sediment

Water ◽  
2020 ◽  
Vol 12 (6) ◽  
pp. 1589
Author(s):  
Bustamante-Penagos N. ◽  
Niño Y.
Keyword(s):  
Subsurface Flow ◽  
Power Spectra ◽  
Surface Current ◽  
Streamwise Velocity ◽  
Fine Sediment ◽  
Surface Flow ◽  
Subsurface Water ◽  
Turbulent Structures ◽  
Before And After ◽  
Scatter Plots

Several researchers have studied turbulent structures, such as ejections, sweeps, and outwards and inwards interactions in flumes, where the streamwise velocity dominates over vertical and transversal velocities. However, this research presents an experimental study in which there are ejections associated with the interchange between surface and subsurface water, where the vertical velocity dominates over the streamwise component. The experiment is related to a surface alluvial stream that is polluted with fine sediment, which is percolated into the bed. The subsurface flow is modified by a lower permeability associated with the fine sediment and emerges to the surface current. Quasi-steady ejections are produced that drag fine sediment into the surface flow. Particle image velocimetry (PIV) measured the velocity field before and after the ejection. The velocity data were analyzed by scatter plots, power spectra, and wavelet analysis of turbulent fluctuations, finding changes in the distribution of turbulence interactions with and without the presence of fine deposits. The flow sediment ejection changes the patterns of turbulent structures and the distribution of the turbulence interactions that have been reported in open channels without subsurface flows.

Statistical analysis of local turbulent energy fluctuations

1999 ◽  
Vol 382 ◽  
pp. 1-26 ◽  
Author(s):  
G. GUJ ◽  
R. CAMUSSI
Keyword(s):  
Wavelet Coefficient ◽  
Strong Dependence ◽  
Turbulent Energy ◽  
Characteristic Size ◽  
Distribution Functions ◽  
Statistical Properties ◽  
Turbulent Structures ◽  
Time Frequency ◽  
Mean Delay ◽  
Amplitude Fluctuations

Time–frequency energy fluctuations of turbulent experimental velocity signals for Reλ≃10 and 800, are analysed using orthogonal wavelet transform. Some statistical properties of the energy bursts are analysed and discussed. The probability distribution functions (PDFs) of the energy amplitude fluctuations are investigated at different scales. Such PDFs show that the so-called non-intermittent and intermittent regions are characterized by quite different behaviour. Analysis of the wavelet coefficient scaling relations, averaged under suitable conditioning, reveals that the most energetic events localized in time and scale are responsible for the structure function (or wavelet coefficients) scaling anomalies related to intermittency. It is shown that the statistical properties which are correlated with the mechanism of the energy cascade from large to small scales are characterized by a universal behaviour. On the other hand, when the chosen statistical indicators are related to the characteristic size of turbulent structures, no universality is achieved, and a strong dependence upon the turbulent generator and Reλ is observed. This is demonstrated by analysis of the statistics of time delays between successive events which show non-universal PDFs. The mean delay between successive intermittent events is also Reλ dependent and increases for increasing Reλ.

Study of the Reduction of Turbulent Drag by Photon Correlation Spectroscopy

1991 ◽  
Vol 248 ◽  
Author(s):  
P. Tong ◽  
W. I. Goldburg ◽  
J. S. Huang
Keyword(s):  
Large Scale ◽  
Functional Form ◽  
Photon Correlation Spectroscopy ◽  
Turbulent Energy ◽  
Correlation Spectroscopy ◽  
Small Scale ◽  
Concentric Cylinder ◽  
Photon Correlation ◽  
Suppression Effect ◽  
Turbulent Drag

AbstractTurbulent drag reduction in a dilute polymer solution has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences in a concentric cylinder cell, in which the inner cylinder rotates. A large anisotropic suppression of turbulent velocity differences is found in the bulk region of the turbulent fluid. The suppression effect occurs at various length scales up to ∼ 1 mm, which is far beyond the Kolmogorov dissipation length ℓd (∼ 0.04 mm). The large-scale velocity fluctuations are suppressed, but their statistical properties remain unchanged. The small-scale fluctuations, on the other hand, are damped out much more strongly, resulting in a different functional form for the velocity density function. The latter observation is consistent with the notion that the polymer-turbulence interaction causes a truncation of the turbulent energy cascade at small scales.

scholarly journals Destructiveness of pyroclastic surges controlled by turbulent fluctuations

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Ermanno Brosch ◽  
Gert Lube ◽  
Matteo Cerminara ◽  
Tomaso Esposti-Ongaro ◽  
Eric C. P. Breard ◽  
...  
Keyword(s):  
High Pressure ◽  
Large Scale ◽  
Dynamic Pressure ◽  
Low Frequency ◽  
Turbulent Energy ◽  
Turbulent Structures ◽  
Mean Values ◽  
Flow Energy ◽  
Pyroclastic Surges ◽  
Pressure Pulses

AbstractPyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 kPa inside flows capable of obliterating reinforced buildings. However, to date, there are no measurements inside these currents to quantify the dynamics of this important hazard process. Here we show, through large-scale experiments and the first field measurement of pressure inside pyroclastic surges, that dynamic pressure energy is mostly carried by large-scale coherent turbulent structures and gravity waves. These perpetuate as low-frequency high-pressure pulses downcurrent, form maxima in the flow energy spectra and drive a turbulent energy cascade. The pressure maxima exceed mean values, which are traditionally estimated for hazard assessments, manifold. The frequency of the most energetic coherent turbulent structures is bounded by a critical Strouhal number of ~0.3, allowing quantitative predictions. This explains the destructiveness of real-world flows through the development of c. 1–20 successive high-pressure pulses per minute. This discovery, which is also applicable to powder snow avalanches, necessitates a re-evaluation of hazard models that aim to forecast and mitigate volcanic hazard impacts globally.

Direct Numerical Simulations of Turbulent Channel Flow With Polymer Additives

2020 ◽  
Vol 36 (5) ◽  
pp. 691-698
Author(s):  
Che-Yu Lin ◽  
Chao-An Lin
Keyword(s):  
Drag Reduction ◽  
Numerical Simulations ◽  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Weissenberg Number ◽  
Direct Numerical Simulations ◽  
Polymer Additives ◽  
Boundary Treatments ◽  
Conformation Tensor ◽  
Nonlinear Elastic

ABSTRACTDirect numerical simulations have been applied to simulate flows with polymer additives. FENE-P (finite-extensible-nonlinear-elastic-Peterlin) dumbbell model solving for the conformation tensor is adopted to investigate the influence of the polymer on the flowfield. Boundary treatments of the conformation tensor on the flowfield are examined first, where boundary condition based on the linear extrapolation scheme provides more accurate results with second-order accurate error norms. Further simulations of the turbulent channel flow at different Weissenberg numbers are also conducted to investigate the influence on drag reduction. Drag reduction increases in tandem with the increase of Weissenberg number and the increase saturates at Weτ~200, where the drag reduction is close to the maximum drag reduction (MDR) limit. At the regime of y+ > 5, the viscous layer thickens with the increase of the Weissenberg number showing a departure from the traditional log-law profile, and the velocity profiles approach the MDR line at high Weissenberg number. The Reynolds stress decreases in tandem with the increase of Weτ, whereas the levels of laminar stress and polymer stress act adversely. However, as the Weissenberg number increases, the proportion of the laminar stress in the total stress increases, and this contributes to the drag reduction of the polymer flow.

scholarly journals Derivative moments in stationary homogeneous shear turbulence

2001 ◽  
Vol 441 ◽  
pp. 109-118 ◽  
Author(s):  
JÖRG SCHUMACHER
Keyword(s):  
Reynolds Numbers ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Turbulent Shear ◽  
Shear Direction ◽  
Turbulent Shear Flow ◽  
Local Isotropy ◽  
Apparent Violation ◽  
Shear Turbulence ◽  
Stress Free Boundary

A statistically stationary and nearly homogeneous turbulent shear flow is established by an additional volume forcing in combination with stress-free boundary conditions in the shear direction. Both turbulent energy and enstrophy are stationary to a much better approximation than in previous simulations that use remeshing. The temporal fluctuations decrease with increasing Reynolds number. Energy spectra and shear-stress cospectra show that local isotropy is satisfactorily obeyed at the level of second-order moments. However, derivative moments of high order up to n = 7 yield increasing moments for n [ges ] 4 for the spanwise vorticity and the transverse derivative of the streamwise velocity in the range of Taylor Reynolds numbers 59 [les ] Rλ [les ] 99. These findings, which are in apparent violation of local isotropy, agree with recent measurements.

Structure of Turbulence in an Incipient-Separating Axisymmetric Flow

1995 ◽  
Vol 117 (3) ◽  
pp. 433-438 ◽  
Author(s):  
Rakesh K. Singh ◽  
Ram S. Azad
Keyword(s):  
Pipe Flow ◽  
Energy Production ◽  
Reynolds Shear Stress ◽  
Axisymmetric Flow ◽  
Adverse Pressure Gradient ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Wall Layer ◽  
Diffuser Flow ◽  
Conical Diffuser

The relative intensity, skewness, and flatness of fluctuating streamwise velocity along the centerline of an 8 deg included angle conical diffuser show dramatic rapid growth in the final stages of the flow under the increasing influence of growing instantaneous reversals in the wall-layer. Pulsed-wire anemometry was effectively used for the measurement of quantitative instantaneous reversals and the turbulent flow field. In the severe adverse pressure gradient of the diffuser flow, the maxima of the streamwise and transverse fluctuating velocities, Reynolds shear stress, and turbulent energy production coincide and move away from their near-wall position in the pipe, also the velocity triple products show completely opposite nature as compared to the pipe flow. These measurements reveal the strong influence of instantaneous backflow on the structure of turbulence. The present results further corroborate the ability of the “structural” turbulence model of Nagano and Tagawa (1990) to predict velocity triple products in an axisymmetric diffuser flow.

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