Upwellings, Downdrafts, and Whirlpools: Dominant Structures in Free Surface Turbulence

1994 ◽  
Vol 47 (6S) ◽  
pp. S166-S172 ◽  
Author(s):  
Sanjoy Banerjee
Keyword(s):  
Free Surface ◽  
High Speed ◽  
Wall Turbulence ◽  
Turbulence Structure ◽  
Normal Velocity ◽  
Two Dimensional ◽  
Near Surface ◽  
Surface Turbulence ◽  
Surface Patterns ◽  
Turbulence Generation

Fluid motion at flat, unsheared interfaces develops primarily due to impingement of coherent turbulent structures from the far field. On the other hand, when shear is imposed, alternating low-speed/high-speed regions are formed with ejection-sweep cycles qualitatively similar to those seen in wall turbulence. The transition to this “active” state depends on a shear rate non-dimensionalized by the Reynolds stress and dissipation rate. Turning back to the unsheared (or free) surface case, the bulk turbulence structures cause “upwellings” when they approach the interface. The regions between upwellings appear as stagnation lines on the surface plane—the surface-normal velocity being downwards. Whirlpool-like attached vortices also form at the edges of the upwellings. These attached vortices are remarkably persistent—the main annihilation mechanism being interaction with a subsequent upwelling. For situations where the surface patterns convect away from a region of turbulence generation, i.e. a decaying pattern, the attached vortices become the dominant structure since new upwellings and downdrafts are not formed. The attached vortices pair and decay in a manner such that the near-surface turbulence structure is essentially two-dimensional. Even in situations where turbulence generation occurs quite close to the free-surface, measures such as energy spectra indicate a quasi two-dimensional near-surface structure.

Spanwise turbulence structure over permeable walls

2017 ◽  
Vol 822 ◽  
pp. 186-201 ◽  
Author(s):  
Kazuhiko Suga ◽  
Yuka Nakagawa ◽  
Masayuki Kaneda
Keyword(s):  
Turbulent Flows ◽  
High Speed ◽  
Solid Wall ◽  
Measurement Data ◽  
Field Measurements ◽  
Wall Turbulence ◽  
Turbulence Structure ◽  
Mean Velocity ◽  
Permeable Walls ◽  
Plane Displacement

Spanwise flow field measurements are carried out for turbulent flows in channels with permeable bottom walls by particle image velocimetry (PIV) to understand the effects of the wall permeability on turbulence structure near porous walls. The porous media used are three kinds of foamed ceramics which have the same porosities (0.8) but different permeabilities. The turbulent flow fields in spanwise planes are discussed using instantaneous and statistical measurement data. At a small permeability Reynolds number ($Re_{K}$), low-speed and high-speed streaks, which are similar to those of solid-wall turbulence, are observed near the walls while at a large $Re_{K}$ the observed structure is very different from that of the solid-wall turbulence. It is found that the obtained spanwise scales of the structure can be reasonably correlated with the wall normal distance plus the zero-plane displacement which is estimated from the mean velocity profile. With the distribution profiles of the spanwise streak spacing and integral length scales, the transitional change of the turbulence structure over permeable walls is discussed.

Impact of terrain heterogeneity on near-surface turbulence structure

2009 ◽  
Vol 94 (2) ◽  
pp. 254-269 ◽  
Author(s):  
Clément Fesquet ◽  
Philippe Drobinski ◽  
Christian Barthlott ◽  
Thomas Dubos
Keyword(s):  
Turbulence Structure ◽  
Near Surface ◽  
Surface Turbulence

Waves on a stream of finite depth which has a velocity defect near the free surface

1970 ◽  
Vol 41 (3) ◽  
pp. 509-521 ◽  
Author(s):  
P. L. Betts
Keyword(s):  
Free Surface ◽  
High Speed ◽  
Surface Profile ◽  
Finite Depth ◽  
Simplified Model ◽  
Stationary Waves ◽  
Two Dimensional ◽  
Dimensional Theory ◽  
Velocity Defect ◽  
A Current

The conditions under which stationary waves may exist on a stream of water of finite depth are investigated theoretically for the case of a current which is uniform except for a constant defect in velocity in a region near the free surface. The analysis is extended to provide a two-dimensional theory for the surface profile induced by a simplified model of a hovering craft. The relevance of this work to the use of high speed flumes is discussed, and an example demonstrates the importance of the velocity distribution near the free surface.

On the structure of bow waves on a ship model

1997 ◽  
Vol 346 ◽  
pp. 77-115 ◽  
Author(s):  
RONALD R. DONG ◽  
JOSEPH KATZ ◽  
THOMAS T. HUANG
Keyword(s):  
Free Surface ◽  
Laser Sheet ◽  
Wave Crest ◽  
Two Dimensional ◽  
Characteristic Structure ◽  
Near Surface ◽  
Ship Model ◽  
Ship Wave ◽  
Bow Wave ◽  
Vorticity Production

Particle image velocitmetry (PIV) measurements and free-surface visualizations around a ship model focus on the flow within the attached liquid sheet, upstream of the point at which the bow wave separates from the model, the origin and structure of the bow wave and the flow downstream of the wave crest. The measurements are performed at Reynolds numbers ranging between 2.8×106 and 7.4×106 and Froude numbers between 0.17 and 0.45 (both are based on ship length L). Representative velocity and vorticity distributions at FrL=0.28 and FrL=0.45 demonstrate the characteristic structure of mild and steep waves, respectively. Very close to the bow the attached sheet is thin and quite unsteady. With increasing distance from the nose the sheet becomes thicker and its development involves considerable vorticity production. In the mild case this vorticity is originated at the free surface, whereas in the steep wave case, boundary layer separation occurs on the model, which also transports vorticity into the sheet. This vorticity and its associated induced lateral flow remain near the model downstream of the bow wave. By calculating the acceleration component tangent to the free surface of the sheet it is shown that the peaks in the near-surface vorticity appear in regions with high viscous flux of vorticity from the surface. Formation of a bow wave also involves considerable production of vorticity. Similar to two-dimensional breakers, the primary origin of this vorticity is at the toe of the breaker. However, unlike the two-dimensional cases, the region containing vorticity in the ship wave does not appear as an extended shear layer. Instead, this vorticity is convected out of the plane of the laser sheet in a series of distinct vortex filaments. The ship wave also has powerful counter-rotating vorticity concentrated near the wave crest that has been observed in two-dimensional waves, but not of the same strength. Breaking becomes weaker, i.e. there is less vorticity production, with increasing distance from the model, but it persists even at the ‘tail’ of the bow wave. The sites of vorticity entrainment of both signs are consistent with the computed near-surface acceleration. Estimates of the three-dimensional velocity distribution and head losses within the wave are also provided.

Katabatic Winds over Steep Slopes: Overview of a Field Experiment Designed to Investigate Slope-Normal Velocity and Near-Surface Turbulence

2021 ◽  
Author(s):  
Claudine Charrondière ◽  
Christophe Brun ◽  
Jean-Martial Cohard ◽  
Jean-Emmanuel Sicart ◽  
Martin Obligado ◽  
...  
Keyword(s):  
Field Experiment ◽  
Normal Velocity ◽  
Katabatic Winds ◽  
Near Surface ◽  
Surface Turbulence ◽  
Steep Slopes

Modeling Issues Related to the Hydrodynamics of Three-Dimensional Steady Planing

1995 ◽  
Vol 39 (01) ◽  
pp. 1-24
Author(s):  
Canhai Lai ◽  
Armin W. Troesch
Keyword(s):  
Free Surface ◽  
High Speed ◽  
Vortex Lattice ◽  
Hydrodynamic Force ◽  
Near Field ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Pressure Condition ◽  
General Shape ◽  
Pressure Distributions

A three-dimensional numerical model using vortex lattice methods is developed to solve the steady planing problem. This paper examines assumptions related to high-speed planing hydrodynamics, including those used by two-dimensional slender-body theories. Emphasis is placed upon the investigation of various modeling issues such as the zero pressure condition on the free surface and the treatment of the flow off the chines. Numerical results including hydrodynamic forces and pressure distributions are presented and compared with experiments. A simple model to include the effect of gravity in the near field is also examined. The models described here can be incorporated into design methodologies for predicting the hydrodynamic force and moment acting on planing hulls with general shape.

Wave Breaking and Near-Surface Turbulence

2002 ◽  
Author(s):  
David M. Farmer ◽  
Johannes Gemmrich
Keyword(s):  
Wave Breaking ◽  
Near Surface ◽  
Surface Turbulence

Load Spectrum Compiling and Fatigue Life Estimation of the Automobile Wheel Hub

2020 ◽  
Vol 13 ◽  
Author(s):  
Xintian Liu ◽  
Yang Qu ◽  
Xiaobing Yang ◽  
Yongfeng Shen
Keyword(s):  
Fatigue Life ◽  
High Speed ◽  
Two Dimensional ◽  
Load Spectrum ◽  
One Dimensional ◽  
Life Estimation ◽  
Load Spectra ◽  
Fatigue Life Estimation ◽  
Program Load Spectrum ◽  
Program Load

Background:: In the process of high-speed driving, the wheel hub is constantly subjected to the impact load from the ground. Therefore, it is important to estimate the fatigue life of the hub in the design and production process. Objective:: This paper introduces a method to study the fatigue life of car hub based on the road load collected from test site. Methods:: Based on interval analysis, the distribution characteristics of load spectrum are analyzed. The fatigue life estimation of one - dimensional and two - dimensional load spectra is compared by compiling load spectra. Results:: According to the S-N curve cluster and the one-dimensional program load spectrum, the estimated range fatigue life of the hub is 397,100 km to 529,700 km. For unsymmetrical cyclic loading, each level means and amplitude of load were obtained through the Goodman fatigue empirical formula, and then according to S-N curve clusters in the upper and lower curves and two-dimensional program load spectrum, estimates the fatigue life of wheel hub of the interval is 329900 km to 435200 km, than one-dimensional load spectrum fatigue life was reduced by 16.9% - 17.8%. Conclusion:: This paper lays a foundation for the prediction of fatigue life and the bench test of fatigue durability of auto parts subjected to complex and variable random loads. At the same time, the research method can also be used to estimate the fatigue life of other bearing parts or high-speed moving parts and assemblies.

Accelerating laser processes with a smart two-dimensional polygon mirror scanner for ultra-fast beam deflection

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Florian Roessler ◽  
André Streek
Keyword(s):  
Energy Distribution ◽  
Real Time ◽  
Laser Processing ◽  
High Speed ◽  
Beam Deflection ◽  
Two Dimensional ◽  
Heat Accumulation ◽  
Field Programmable ◽  
Drill Holes ◽  
Average Laser

Abstract In laser processing, the possible throughput is directly scaling with the available average laser power. To avoid unwanted thermal damage due to high pulse energy or heat accumulation during MHz-repetition rates, energy distribution over the workpiece is required. Polygon mirror scanners enable high deflection speeds and thus, a proper energy distribution within a short processing time. The requirements of laser micro processing with up to 10 kW average laser powers and high scan speeds up to 1000 m/s result in a 30 mm aperture two-dimensional polygon mirror scanner with a patented low-distortion mirror configuration. In combination with a field programmable gate array-based real-time logic, position-true high-accuracy laser switching is enabled for 2D, 2.5D, or 3D laser processing capable to drill holes in multi-pass ablation or engraving. A special developed real-time shifter module within the high-speed logic allows, in combination with external axis, the material processing on the fly and hence, processing of workpieces much larger than the scan field.

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