scholarly journals Low- and High-Drag Intermittencies in Turbulent Channel Flows

Entropy ◽  
2020 ◽  
Vol 22 (10) ◽  
pp. 1126 ◽  
Author(s):  
Rishav Agrawal ◽  
Henry C.-H. Ng ◽  
Ethan A. Davis ◽  
Jae Sung Park ◽  
Michael D. Graham ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Numerical Simulations ◽  
Channel Flow ◽  
Reynolds Shear Stress ◽  
Frequency Of Occurrence ◽  
Time Duration ◽  
Wall Shear ◽  
Conditional Events ◽  
Duration Criterion

Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at Reτ=uτh/ν between 70 and 100, where uτ, h and ν are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed “hibernating” and “hyperactive”, respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for Reτ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at Reτ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk’s maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for Reτ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.

Groyne spacing role on the effective control of wall shear stress in open-channel flow

2018 ◽  
Vol 57 (2) ◽  
pp. 167-182 ◽  
Author(s):  
Theofano I. Koutrouveli ◽  
Athanassios A. Dimas ◽  
Nikolaos Th. Fourniotis ◽  
Alexander C. Demetracopoulos
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Channel Flow ◽  
Open Channel ◽  
Open Channel Flow ◽  
Effective Control ◽  
Wall Shear

Evolution of wall shear stress with Reynolds number in fully developed turbulent channel flow experiments

2019 ◽  
Vol 4 (7) ◽  
Author(s):  
Pierre-Alain Gubian ◽  
Jordan Stoker ◽  
James Medvescek ◽  
Laurent Mydlarski ◽  
B. Rabi Baliga
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Wall Shear ◽  
Flow Experiments

Measurement of the Wall Shear Stress With Sublayer Thin Plate of Thermochromic Liquid Crystal

2015 ◽  
Author(s):  
Takashi Kodama ◽  
Shinsuke Mochizuki
Keyword(s):  
Shear Stress ◽  
Liquid Crystal ◽  
Friction Coefficient ◽  
Wall Shear Stress ◽  
Skin Friction ◽  
Channel Flow ◽  
Skin Friction Coefficient ◽  
Local Skin ◽  
Local Skin Friction ◽  
Wall Shear

New optical method for measurement of the local wall shear stress has been developed by using thermo-chromic liquid crystal temperature measurement based on hue [1], [2] of the camera view. The flow field is the fully developed turbulent channel flow. Thin film made of thermo-chromic liquid crystal is placed on the wall. A rectangular shaped obstacle is glued on the film. The obstacle is within a region of buffer layer with height from the wall. Temperature of the film and the obstacle are slightly raised by a heater below the wall. The air flow makes non-uniform temperature distribution and non-uniform color distribution appears on the surface of the film. Relations between hue and local skin friction coefficient were examined in a turbulent air channel flow. It is indicated that a certain hue of a point is varying linearly against the corresponding local skin friction coefficient.

scholarly journals Acute Stent-Induced Endothelial Denudation: Biomechanical Predictors of Vascular Injury

2021 ◽  
Vol 8 ◽  
Author(s):  
Claire Conway ◽  
Farhad R. Nezami ◽  
Campbell Rogers ◽  
Adam Groothuis ◽  
James C. Squire ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Numerical Simulations ◽  
Vascular Injury ◽  
Ex Vivo ◽  
Circumferential Stress ◽  
Arterial Injury ◽  
Stent Design ◽  
Endothelial Denudation ◽  
Wall Shear

Recent concern for local drug delivery and withdrawal of the first Food and Drug Administration-approved bioresorbable scaffold emphasizes the need to optimize the relationships between stent design and drug release with imposed arterial injury and observed pharmacodynamics. In this study, we examine the hypothesis that vascular injury is predictable from stent design and that the expanding force of stent deployment results in increased circumferential stress in the arterial tissue, which may explain acute injury poststent deployment. Using both numerical simulations and ex vivo experiments on three different stent designs (slotted tube, corrugated ring, and delta wing), arterial injury due to device deployment was examined. Furthermore, using numerical simulations, the consequence of changing stent strut radial thickness on arterial wall shear stress and arterial circumferential stress distributions was examined. Regions with predicted arterial circumferential stress exceeding a threshold of 49.5 kPa compared favorably with observed ex vivo endothelial denudation for the three considered stent designs. In addition, increasing strut thickness was predicted to result in more areas of denudation and larger areas exposed to low wall shear stress. We conclude that the acute arterial injury, observed immediately following stent expansion, is caused by high circumferential hoop stresses in the interstrut region, and denuded area profiles are dependent on unit cell geometric features. Such findings when coupled with where drugs move might explain the drug–device interactions.

scholarly journals Investigating channel flow using wall shear stress signals at transitional Reynolds numbers

2020 ◽  
Vol 82 ◽  
pp. 108525 ◽  
Author(s):  
Rishav Agrawal ◽  
Henry C.-H. Ng ◽  
David J.C. Dennis ◽  
Robert J. Poole
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Channel Flow ◽  
Reynolds Numbers ◽  
Wall Shear ◽  
Stress Signals

scholarly journals Simultaneous Stereo PIV and MPS3 Wall-Shear Stress Measurements in Turbulent Channel Flow

Optics ◽  
2020 ◽  
Vol 1 (1) ◽  
pp. 40-51
Author(s):  
Esther Mäteling ◽  
Michael Klaas ◽  
Wolfgang Schröder
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Channel Flow ◽  
Inclination Angle ◽  
Large Scale ◽  
Turbulent Channel Flow ◽  
Two Dimensional ◽  
Velocity Fluctuations ◽  
Wall Shear ◽  
Near Wall

An extended experimental method is presented in which the micro-pillar shear-stress sensor (MPS 3 ) and high-speed stereo particle-image velocimetry measurements are simultaneously performed in turbulent channel flow to conduct concurrent time-resolved measurements of the two-dimensional wall-shear stress (WSS) distribution and the velocity field in the outer flow. The extended experimental setup, which involves a modified MPS 3 measurement setup and data evaluation compared to the standard method, is presented and used to investigate the footprint of the outer, large-scale motions (LSM) onto the near-wall small-scale motions. The measurements were performed in a fully developed, turbulent channel flow at a friction Reynolds number R e τ = 969 . A separation between large and small scales of the velocity fluctuations and the WSS fluctuations was performed by two-dimensional empirical mode decomposition. A subsequent cross-correlation analysis between the large-scale velocity fluctuations and the large-scale WSS fluctuations shows that the streamwise inclination angle between the LSM in the outer layer and the large-scale footprint imposed onto the near-wall dynamics has a mean value of Θ ¯ x = 16.53 ∘ , which is consistent with the literature relying on direct numerical simulations and hot-wire anemometry data. When also considering the spatial shift in the spanwise direction, the mean inclination angle reduces to Θ ¯ x z = 13.92 ∘ .

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