Whitecap coverage measurements in laboratory modeling of wind-wave interaction in presence of induced wave breaking

Whitecap coverage were retrieved from high-speed video recordings of the water surface obtained on the unique laboratory faculty The Large Thermostratified Test Tank with wind-wave channel (cross-section from 0.7&#215;0.7 to 0.7&#215;0.9 m2 at the end, 12 m fetch, wind velocity up to 35 m/s, U10 up to 65 m/s). The wind wave was induced using a wave generator installed at the beginning of the channel (a submerged horizontal plate, frequency 1.042 Hz, amplitude 93 mm) working in a pulsed operation (three periods). Wave breaking was induced in working area by a submerged plate (1.2&#215;0.7 m2, up to 12 depth, AOA -11,7&#176;). Experiments were carried out for equivalent wind velocities U10 from 17.8 to 40.1 m/s. Wire wave gauge was used to control the shape and phase of the incident wave.To obtain the surface area occupied by wave breaking, we used two Cygnet CY2MP-CL-SN cameras with 50 mm lenses. The cameras are installed above the channel at a height of 273 cm from the water surface, separated by 89 cm. The image scale was 302 &#956;m/px, the size of the image obtained from each camera is 2048x1088 px2, which corresponds to 619x328 mm2 (the long side of the frame along the channel). The shooting was carried out with a frequency of 50 Hz, an exposure time of 3 ms, 250 frames were recorded for each wave train. To illuminate the image areas to the side of the measurement area, a diffuse screen was placed on the side wall, which was illuminated by powerful LED lamps to create a uniform illumination source covering the entire side wall of the section.Using specially developed software for automatic detection of areas of wave breaking, the values of the whitecap coverage area were obtained. Automatic image processing was performed using morphological analysis in combination with manual processing of part of the frames for tweaking the algorithm parameters: for each mode, manual processing of several frames was performed, based on the results of which automatic algorithm parameters were selected to ensure that the resulting whitecap coverage corresponded. Comparison of images obtained from different angles made it possible to detect and exclude areas of glare on the surface from the whitecap coverage.The repeatability of the created wave breakings allows carrying out independent measurements for the same conditions, for example the parameters of spray generation will give estimations of the average number of fragmentation events per unit area of the wave breaking area.The work was supported by the RFBR grants 21-55-50005 and 20-05-00322 (conducting an experiment), President grant for young scientists &#1052;&#1050;-5503.2021.1.5 (software development) and the RSF grant No. 19-17-00209 (data processing).

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Whitecap coverage measurements in laboratory modeling of wind-wave interaction

10.5194/egusphere-egu2020-15621 ◽

2020 ◽

Author(s):

Alexander Kandaurov ◽

Yuliya Troitskaya ◽

Daniil Sergeev ◽

Dmitry Kozlov

Keyword(s):

High Speed ◽

Wind Wave ◽

Surface Condition ◽

Vertical Direction ◽

Water Volume ◽

Small Scale ◽

Unit Surface Area ◽

Mode Water ◽

Whitecap Coverage ◽

Manual Processing

Whitecap coverage were retrieved from high-speed video recordings of the water surface obtained on the unique laboratory faculty Heidelberg Small-Scale Air-Sea Interaction Facility, the Aeolotron (annular wind-wave facility, 60 cm width, 2.4 m height, circumference of 27.3 m at the inner wall; water depth during experiments 1.0 m, water volume 18.0 m&#179;, air space volume 24 m&#179;; wind was generated by two axial fans mounted into the ceiling).Records were made in the vertical direction (from top to bottom) in a shadowgraph configuration with backlight located under the channel. On the annular channel, regimes with an abrupt start of wind under an unperturbed surface condition were implemented, including the case of butanol presence in water simulating salinity. At the same time, the wave parameters varying depending on the time elapsed after the wind was turned on, made it possible to study the characteristics of the generation of spray at various effective fetches. As a result of semi-automatic processing of image sequences using specially developed software that allows marking the moment and position of the bag-breakup formation on the videos, the dependences of the frequency of occurrence of these phenomena per unit surface area versus time after turning on the wind were obtained. From the same images, using the developed software for automatic detection of areas of wave breaking, the values of the whitecap coverage area were obtained. In this case, automatic image processing was performed using morphological analysis in combination with manual processing of part of the frames for tweaking the algorithm parameters: for each mode (water characteristics and wind speed), manual processing of several frames was performed, based on the results of which automatic algorithm parameters were selected to ensure that the resulting whitecap coverage corresponded. Since the same high-speed surface images were used to study the statistics of occurrence of events leading to the spray generation and the dependences of the whitecap coverage on time after turning on the wind for each regime were obtained, we were able to estimate the average number of fragmentation events per unit area of the collapse area.The work was supported by the RFBR grant 18-35-20068 (conducting an experiment), President grant for young scientists MK-3184.2019.5 (software development) and the RSF grant No. 18-77-00074 (data processing).

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First air–sea gas exchange laboratory study at hurricane wind speeds

Ocean Science Discussions ◽

10.5194/osd-10-1971-2013 ◽

2013 ◽

Vol 10 (6) ◽

pp. 1971-1996

Author(s):

K. E. Krall ◽

B. Jähne

Keyword(s):

Wave Breaking ◽

High Speed ◽

Wind Wave ◽

Breaking Waves ◽

Gas Transfer ◽

Transfer Velocity ◽

Wind Speeds ◽

Hurricane Wind ◽

Wide Range ◽

Enhanced Surface

Abstract. In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique High-Speed Wind-Wave Tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10=67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available dataset at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment. It was found that tracers spanning a wide range of solubilities and diffusivities are needed to separate the effects of enhanced surface area and turbulence due to breaking waves from the effects of bubble and spray mediated gas transfer.

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First laboratory study of air–sea gas exchange at hurricane wind speeds

Ocean Science ◽

10.5194/os-10-257-2014 ◽

2014 ◽

Vol 10 (2) ◽

pp. 257-265 ◽

Cited By ~ 20

Author(s):

K. E. Krall ◽

B. Jähne

Keyword(s):

Wave Breaking ◽

High Speed ◽

Wind Wave ◽

Gas Transfer ◽

High Wind ◽

Transfer Velocity ◽

Data Set ◽

Wind Speeds ◽

Bubble Cloud ◽

Hurricane Wind

Abstract. In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique high-speed wind-wave tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10 =67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind-induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available data set at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment.

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SOME CONTRIBUTIONS TO HTDRAULIC MODEL EXPERIMENTS IN COASTAL ENGINEERING

Coastal Engineering Proceedings ◽

10.9753/icce.v10.72 ◽

1966 ◽

Vol 1 (10) ◽

pp. 72 ◽

Cited By ~ 1

Author(s):

Shoitiro Hayami ◽

Tojiro Ishihara ◽

Yuichi Iwagaki

Keyword(s):

High Speed ◽

Wind Wave ◽

Coastal Engineering ◽

Experimental Results ◽

Prevention Research ◽

Disaster Prevention ◽

View Point ◽

Model Experiments ◽

Wave Channel ◽

Disaster Prevention Research Institute

This paper presents some aspects of the hydraulic model experiments in coastal engineering made at the Ujigawa Hydraulic Laboratory, Disaster Prevention Research Institute, Kyoto University, including the experiments performed by using an estuary model basin and a high speed wind wave channel. In particular, the problems to which attentions should be paid from view point of similitude between model and prototype will be discussed in addition to the presentation of experimental results.

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Modeling the Excess Velocity of Low-Viscous Taylor Droplets in Square Microchannels

Fluids ◽

10.3390/fluids4030162 ◽

2019 ◽

Vol 4 (3) ◽

pp. 162 ◽

Cited By ~ 2

Author(s):

Thorben Helmers ◽

Philip Kemper ◽

Jorg Thöming ◽

Ulrich Mießner

Keyword(s):

High Speed ◽

Process Intensification ◽

Continuous Phase ◽

Channel Cross Section ◽

Optimization Approach ◽

Mean Flow ◽

Bypass Flow ◽

Wall Film ◽

Proposed Model ◽

The Mean

Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to determine the excess velocity of Taylor droplets in square microchannels. This velocity difference between the droplet and the total superficial velocity of the flow has a direct influence on the droplet residence time and is linked to the pressure drop. Since the droplet does not occupy the entire channel cross-section, it enables the continuous phase to bypass the droplet through the corners. A consideration of the continuity equation generally relates the excess velocity to the mean flow velocity. We base the quantification of the bypass flow on a correlation for the droplet cap deformation from its static shape. The cap deformation reveals the forces of the flowing liquids exerted onto the interface and allows estimating the local driving pressure gradient for the bypass flow. The characterizing parameters are identified as the bypass length, the wall film thickness, the viscosity ratio between both phases and the C a number. The proposed model is adapted with a stochastic, metaheuristic optimization approach based on genetic algorithms. In addition, our model was successfully verified with high-speed camera measurements and published empirical data.

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Suppression of Wind Ripples and Microwave Backscattering Due to Turbulence Generated by Breaking Surface Waves

Remote Sensing ◽

10.3390/rs12213618 ◽

2020 ◽

Vol 12 (21) ◽

pp. 3618

Author(s):

Stanislav Ermakov ◽

Vladimir Dobrokhotov ◽

Irina Sergievskaya ◽

Ivan Kapustin

Keyword(s):

Remote Sensing ◽

Surface Waves ◽

Wave Breaking ◽

Wave Train ◽

Wind Waves ◽

Breaking Waves ◽

Doppler Spectrum ◽

Strong Wave ◽

Wave Maker ◽

Radar Backscattering

The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for the development of theories and methods on ocean remote sensing, in particular for oil spill remote sensing. Recently it has been shown that microwave radar return is determined by both Bragg and non-Bragg (non-polarized) scattering mechanisms and some evidence has been given that the latter is associated with wave breaking, in particular, with strong breaking such as spilling or plunging. However, our understanding of mechanisms of the action of strong wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. In this paper an effect of suppression of radar backscattering after strong wave breaking has been revealed experimentally and has been attributed to the wind ripple suppression due to turbulence generated by strong wave breaking. The experiments were carried out in a wind wave tank where a frequency modulated wave train of intense meter-decimeter-scale surface waves was generated by a mechanical wave maker. The wave train was compressed according to the gravity wave dispersion relation (“dispersive focusing”) into a short-wave packet at a given distance from the wave maker. Strong wave breaking with wave crest overturning (spilling) occurred for one or two highest waves in the packet. Short decimeter-centimeter-scale wind waves were generated at gentle winds, simultaneously with the long breaking waves. A Ka-band scatterometer was used to study microwave backscattering from the surface waves in the tank. The scatterometer looking at the area of wave breaking was mounted over the tank at a height of about 1 m above the mean water level, the incidence angle of the microwave radiation was about 50 degrees. It has been obtained that the radar return in the presence of short wind waves is characterized by the radar Doppler spectrum with a peak roughly centered in the vicinity of Bragg wave frequencies. The radar return was strongly enhanced in a wide frequency range of the radar Doppler spectrum when a packet of long breaking waves arrived at the area irradiated by the radar. After the passage of breaking waves, the radar return strongly dropped and then slowly recovered to the initial level. Measurements of velocities in the upper water layer have confirmed that the attenuation of radar backscattering after wave breaking is due to suppression of short wind waves by turbulence generated in the breaking zone. A physical analysis of the effect has been presented.

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Splash formation by spherical drops

Journal of Fluid Mechanics ◽

10.1017/s0022112000002500 ◽

2001 ◽

Vol 427 ◽

pp. 73-105 ◽

Cited By ~ 91

Author(s):

LIOW JONG LENG

Keyword(s):

High Resolution ◽

Quantitative Data ◽

Water Surface ◽

High Speed ◽

Scaling Laws ◽

Capillary Wave ◽

Water Drop ◽

Qualitative And Quantitative ◽

High Resolution Images ◽

The Impact

The impact of a spherical water drop onto a water surface has been studied experimentally with the aid of a 35 mm drum camera giving high-resolution images that provided qualitative and quantitative data on the phenomena. Scaling laws for the time to reach maximum cavity sizes have been derived and provide a good fit to the experimental results. Transitions between the regimes for coalescence-only, the formation of a high-speed jet and bubble entrapment have been delineated. The high-speed jet was found to occur without bubble entrapment. This was caused by the rapid retraction of the trough formed by a capillary wave converging to the centre of the cavity base. The converging capillary wave has a profile similar to a Crapper wave. A plot showing the different regimes of cavity and impact drop behaviour in the Weber–Froude number-plane has been constructed for Fr and We less than 1000.

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Visual Observation of Fragmentation Behavior on Molten Material Jet Surface in Coolant

Volume 2: Fuel Cycle and High Level Waste Management; Computational Fluid Dynamics, Neutronics Methods and Coupled Codes; Student Paper Competition ◽

10.1115/icone16-48359 ◽

2008 ◽

Author(s):

Yuta Uchiyama ◽

Yutaka Abe ◽

Akiko Fujiwara ◽

Hideki Nariai ◽

Eiji Matsuo ◽

...

Keyword(s):

Water Surface ◽

High Speed ◽

Internal Flow ◽

Core Material ◽

Generation Process ◽

High Speed Camera ◽

Sodium Coolant ◽

Molten Material ◽

Fragmentation Behavior ◽

Jet Breakup

For the safety design of the Fast Breeder Reactor (FBR), it is strongly required that the post accident heat removal (PAHR) is achieved after a postulated core disruptive accident (CDA). In the PAHR, it is important that the molten core material is solidified in sodium coolant which has high boiling point. Thus it is necessary to estimate the jet breakup length which is the distance that the molten core material is solidified in sodium coolant. In the previous studies (Abe et al., 2006), it is observed that the jet is broken up with fragmenting in water coolant by using simulated core material. It is pointed out that the jet breakup behavior is significantly influenced by the fragmentation behavior on the molten material jet surface in the coolant. However, the relation between the jet breakup behavior and fragmentation on the jet surface during a CDA for a FBR is not elucidated in detail yet. The objective of the present study is to elucidate the influence of the internal flow in the jet and fragmentation behavior on the jet breakup behavior. The Fluorinert™ (FC-3283) which is heavier than water and is transparent fluid is used as the simulant material of the core material. It is injected into the water as the coolant. The jet breakup behavior of the Fluorinert™ is observed by high speed camera to obtain the fragmentation behavior on the molten material jet surface in coolant in detail. To be cleared the effect of the internal flow of jet and the surrounding flow structure on the fragmentation behavior, the velocity distribution of internal flow of the jet is measured by PIV (Particle Image Velocimetry) technique with high speed camera. From the obtained images, unstable interfacial wave is confirmed at upstream of the jet surface, and the wave grows along the jet-water surface in the flow direction. The fragments are torn apart at the end of developed wave. By using PIV analysis, the velocity at the center of the jet is fast and it suddenly decreases near the jet surface. This means that the shear force acts on the jet and water surface. From the results of experiment, the correlation between the interfacial behavior of the jet and the generation process of fragments are discussed. In addition, the influence of surface instability of the jet induced by the relative velocity between Fluorinert™ and coolant water on the breakup behavior is also discussed.

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