Blockage and sheltering effects of vegetation in turbulent flow

2020 ◽  
Author(s):  
Wei-Jie Wang
Keyword(s):  
Drag Coefficient ◽  
Flow Resistance ◽  
Aquatic Vegetation ◽  
Reynolds Numbers ◽  
Viscous Boundary Layer ◽  
Vortex Streets ◽  
Karman Vortex ◽  
Viscous Boundary ◽  
Sheltering Effect ◽  
Blockage Effect

<p>The interaction between aquatic vegetation and water flow is investigated here focusing on the drag coefficient. Compared with the standard drag coefficient of isolated cylinder, the phenomena of "blockage effect" and "sheltering effect" are put forward for vegetation clusters with different vegetation densities and Reynolds numbers. "Blockage effect" occurs when the drag coefficient of vegetation cluster is greater than the standard drag coefficient of isolated cylinder. The reason is that viscous boundary layer attached to the surface of vegetation items, resulting that the effective flowing width between adjacent vegetation items is less than the spacing of them, which brings a greater flow resistance and the drag coefficient of vegetation array is greater than the standard drag coefficient. On the other trend, "sheltering effect" is formed when the drag coefficient of vegetation array is less than the standard drag coefficient. This effect usually occurs for flow with large Reynolds numbers. In this case, Karman vortex streets forms and these vortexes are filled in the vegetation interval, thus causing the drag coefficient of vegetation cluster to be less than the standard drag coefficient of isolated cylinder.</p>

Vortex Formation in the Wake of a Vibrating, Flexible Cable

1974 ◽  
Vol 96 (4) ◽  
pp. 317-322 ◽  
Author(s):  
S. E. Ramberg ◽  
O. M. Griffin
Keyword(s):  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Hot Wire ◽  
Near Wake ◽  
Formation Region ◽  
Vortex Streets ◽  
Region Length ◽  
Karman Vortex ◽  
Flexible Cables

The von Karman vortex streets formed in the wakes of vibrating, flexible cables were studied using a hot-wire anemometer. All the experiments took place in the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the wake formation. Detailed measurements were made of the vortex formation flow for Reynolds numbers between 230 and 650. As in the case of vibrating cylinders, the formation-region length is dependent on a shedding parameter St* related to the natural Strouhal number and the vibrational conditions. Furthermore, the near wake configuration is found to be dependent on the local amplitude of vibration suggesting that the vibrating cylinder rseults are directly applicable in that region.

scholarly journals Prey detection in a cruising copepod

2011 ◽  
Vol 8 (3) ◽  
pp. 438-441 ◽  
Author(s):  
Sanne Kjellerup ◽  
Thomas Kiørboe
Keyword(s):  
Boundary Layer ◽  
Chemical Cues ◽  
Prey Capture ◽  
Reynolds Numbers ◽  
Prey Detection ◽  
Potential Prey ◽  
Viscous Boundary Layer ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Viscous Boundary

Small cruising zooplankton depend on remote prey detection and active prey capture for efficient feeding. Direct, passive interception of prey is inherently very inefficient at low Reynolds numbers because the viscous boundary layer surrounding the approaching predator will push away potential prey. Yet, direct interception has been proposed to explain how rapidly cruising, blind copepods feed on non-motile phytoplankton prey. Here, we demonstrate a novel mechanism for prey detection in a cruising copepod, and describe how motile and non-motile prey are discovered by hydromechanical and tactile or, likely, chemical cues, respectively.

The vortex street in the wake of a vibrating cylinder

1972 ◽  
Vol 55 (1) ◽  
pp. 31-48 ◽  
Author(s):  
Owen M. Griffin ◽  
Charles W. Votaw
Keyword(s):  
Vortex Breakdown ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Vortex Street ◽  
Fundamental Parameter ◽  
Formation Region ◽  
Von Karman ◽  
Vortex Streets ◽  
Karman Vortex ◽  
Von Kármán

The von Kármán vortex streets formed in the wakes of vibrating smooth cylinders and cables were studied using a hot-wire anemometer and flow visualization by fog injection in a wind tunnel. All the experiments took place in the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the formation of the wake. Since the flow in the vortex formation region is fundamental to further understanding of the interaction between a vibrating bluff obstacle and its wake, detailed measurements were made of the formation-region flow for Reynolds numbers between 120 and 350. The formationregion length is shown to be a fundamental parameter for the wake, and is dependent on a shedding parameterSt* related to the natureally occurring Strouhal number for the von Kármán street. The effects of vibration amplitude and frequency on the mean and fluctuating velocity fields in the wake become apparent when the downstream displacement is scaled with the formation length. The von Kármán vortex street behind a vibrating cylinder is divided into three predominant flow regimes: the formation, stable and unstable regions. Fundamental differences exist in the vortex streets generated behind stationary and vibrating cylinders, but many classical characteristics, including the manner of vortex breakdown in the unstable region, are shared by the two systems.

Velocity Correlation and Vortex Spacing in the Wake of a Vibrating Cable

1976 ◽  
Vol 98 (1) ◽  
pp. 10-18 ◽  
Author(s):  
S. E. Ramberg ◽  
O. M. Griffin
Keyword(s):  
Reynolds Numbers ◽  
Longitudinal Vortex ◽  
Velocity Correlation ◽  
Vortex Streets ◽  
Karman Vortex ◽  
Cross Correlations ◽  
Lock In ◽  
Flexible Cables ◽  
Synchronized Region ◽  
Approach Unity

The von Karman vortex streets formed in the wakes of vibrating, flexible cables were studied using hot wire anemometers. The experiments took place in or at the boundaries of the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the wake formation. Spacial cross-correlations of the wake velocity signals were made for Reynolds numbers between 400 and 1300. Within the synchronized region, the magnitude of the measured spanwise cross-correlation coefficient is seen to approach unity, being limited by turbulence but apparently independent of frequency, amplitude, and Reynolds number. The bounds of the lock-in regime are determined and compare remarkably well with previous vibrating, rigid cylinder results. Further, the downstream longitudinal vortex spacing and induced street velocity are also found to compare well with vibrating cylinder results.

Influence of the viscous boundary layer perturbations on single-mode panel flutter at finite Reynolds numbers

2018 ◽  
Vol 852 ◽  
pp. 578-601 ◽  
Author(s):  
Vsevolod Bondarev ◽  
Vasily Vedeneev
Keyword(s):  
Boundary Layer ◽  
Growth Rate ◽  
Single Mode ◽  
Reynolds Numbers ◽  
Instability Criterion ◽  
Panel Flutter ◽  
Arbitrary Form ◽  
Viscous Boundary Layer ◽  
Layer Effect ◽  
Viscous Boundary

Panel flutter is an aeroelastic instability of aircraft skin panels, which can lead to a reduction in service life and panel destruction. Despite the existence of many studies related to panel flutter, the influence of the boundary layer on the panel stability has been considered in only a few of them. Up to the present day, most papers on the boundary layer effect consider only a zero-pressure-gradient boundary layer over a flat plate. The only studies of a boundary layer of arbitrary form were conducted in our previous papers (Vedeneev, J. Fluid Mech., vol. 736, 2013, pp. 216–249 and Bondarev & Vedeneev, J. Fluid Mech., vol. 802, 2016, pp. 528–552), where the boundary layer was represented as an inviscid shear layer (the Reynolds number $R=\infty$). In this paper we investigate the problem, taking viscosity into account, at large but finite Reynolds numbers. As before, we assume that the panel length is large and use Kulikovskii’s global instability criterion to analyse the panel eigenmodes and consider two different types of boundary layer profiles: a generalised convex profile and a profile with a generalised inflection point. Results show that viscous perturbations can, in general, have both stabilising and destabilising effects on the system, depending on the velocity and temperature profiles of the boundary layer and on its thickness. However, surprisingly, we prove that if the boundary layer yields a significant growth rate in the inviscid approximation, then the viscosity always produces an even larger growth rate.

Experimental investigations of flow over NACA airfoils 0021 and 4412 of wind turbine blades with and without Tubercles

2021 ◽  
pp. 0309524X2110071
Author(s):  
Usman Butt ◽  
Shafqat Hussain ◽  
Stephan Schacht ◽  
Uwe Ritschel
Keyword(s):  
Wind Tunnel ◽  
Drag Coefficient ◽  
Wind Turbine ◽  
Critical Region ◽  
Lift Coefficient ◽  
Turbine Blades ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Wind Turbine Blades

Experimental investigations of wind turbine blades having NACA airfoils 0021 and 4412 with and without tubercles on the leading edge have been performed in a wind tunnel. It was found that the lift coefficient of the airfoil 0021 with tubercles was higher at Re = 1.2×105 and 1.69×105 in post critical region (at higher angle of attach) than airfoils without tubercles but this difference relatively diminished at higher Reynolds numbers and beyond indicating that there is no effect on the lift coefficients of airfoils with tubercles at higher Reynolds numbers whereas drag coefficient remains unchanged. It is noted that at Re = 1.69×105, the lift coefficient of airfoil without tubercles drops from 0.96 to 0.42 as the angle of attack increases from 15° to 20° which is about 56% and the corresponding values of lift coefficient for airfoil with tubercles are 0.86 and 0.7 at respective angles with18% drop.

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