Hydro/aerodynamic laboratory experiment aimed at the laboratory modelling of the physical processes marine atmospheric boundary layer is one of the most complicated. Especial features as spray of droplets, the bubbles in the water and foam generated during the breaking of the waves should be taken into account when modelling extreme weather conditions caused by severe winds. Thus, in the experiment we are dealing with a multiphase turbulent flow with a free boundary. This investigation describes developing approaches to the use of optical methods based on visualization for performing these investigations. Presented results were obtained in experiments carried out on wind-wave facilities. To study the processes of fragmentation of the water surface leading to the formation of droplets and foam, high-speed multi-angle video taking is used in combination with the shadow imaging method.
The boundary layer between the atmosphere and hydrosphere (first of all the world ocean is
meaning) of our planet is characterized by a large number of different processes physical processes (see
review [
2021 Copyright for this paper by its authors. the need for measurements in air and water close to the interface, which varies significantly in space and time depending on the characteristics of the wind and waves 2) the need for measurements with high spatial and temporal resolution to obtain the necessary ensemble of data for subsequent statistical processing 3) the multiphase nature of the air and water flows due to the formation of sprays and underwater bubbles in strong winds with regular breaking waves. Optical methods based on visualization of flows compares favorably point measurements. It allows simultaneous observations in significant volumes of continuous media with high spatial resolution, and also do not introduce disturbances into the medium under study. The present work is devoted to the processes of interaction of the air flow with a rough water surface under laboratory modeling of extreme weather conditions, primarily associated with strong winds. Field measurements in such conditions are very difficult. That is why this study with laboratory modeling focused on the measurements of sprays and foam coverage which are the features of the boundary layer under severe conditions, the measurements of which are often more complicated than the measurements of the characteristics of flows in the surface water layer. At high winds regular wave breaking begins with the fragmentation of the water surface, leading to the generation of sprays and the formation of foam on the surface. The first part of the work is devoted to the study of the processes of generation of spray droplets from the rough surface, and their characteristics. The second is to study the characteristics of the foam and methods for its artificial creation in laboratory experiments (the results of recent studies are presented).
Sea spray is a very important environmental effect and a common feature of the marine atmospheric
boundary layer. The size distribution of droplets injected in a unit of time from a unit of water surface
is quantitively characterized by sea spray generation function (SSGF). Problems of direct measurements
of droplets parameters in severe conditions and lack of understanding about the ways of the spray
formation are the main causes for insufficient knowledge on SSGF leading to significant uncertainties
in estimations of sea sprays influence on the marine atmospheric boundary layer [
The main feature of the studies performed by our group in the last three years ([
The obtained statistics data allowed us to offer a fundamentally new approach to obtaining the necessary SSGF. It can be obtained as a combination (convolution) of the statistics of phenomena leading to the sprays formation and statistics of droplets occurred in a single phenomenon (see [10]). At the same time, it is rather difficult to study in detail a single phenomenon under natural conditions of wind-wave interaction, even in laboratory modeling, given its sporadic nature and the difficulty of performing measurements, especially at high wind speeds. Therefore, the idea was proposed to artificially initiate these phenomena under controlled conditions for detailed study. In our last study, we succeeded in realizing the artificial initiation of the bag-breakup phenomenon in the modified TSWiWaT. Arterially produced bag-breakup phenomenon turned out to be close to that observed under natural conditions of the wind-wave interaction (see the comparison on Figure 2).
diffuser screen
Foam (or whitecapping), like spray, is almost always present on the sea surface, with the exception of only very weak waves and wind. In its characteristics, it is very different from both water and air, which determines its special role in the processes of exchange between the atmosphere and the sea surface. The main mechanism of the foam coverage formation is concerned with intensive breaking of the wave crests which leads to underwater air involvement and formation of the bubbles and subsequent whipping into the foam. Large-scale near-surface currents, for example, Langmuir circulation also can lead to the foam coverage formation. By combining various factors, the relative area of the foam coverage increases sharply in hurricane winds (U10 more than 30 m/s) and exceeds 70% (the results presented in [11]). It should be noted that field data are characterized by a large scatter. Uncertainties complicate use of field data for development of theoretical models. Therefore, in the present work, a system for the artificial generation of foam was first developed, as well as a technique for measuring the characteristics of the foam (relative coverage area and bubble sizing) specifically for research in laboratory modeling.
Experiments to study the characteristics of the foam in the framework of laboratory modeling were carried out also at the TSWiWaT (see [12]). In conditions of clean water, as well as small accelerations, it is difficult to ensure regular generation of foam. Investigations of the foam parameters in the framework of laboratory experiments were also carried out on TSWiWaT (see [12]). In clear water conditions, as well as short fetches, it is difficult to ensure regular foaming. Therefore, to create a stable foam coverage of the required density on the water surface, a special device (foam generator) was developed. It includes two diffusers (tubes 1 cm in diameter and 350 mm long and with side holes 2 mm in diameter with a pitch of 7 mm) connected by side surfaces to each other, covered with foam rubber. The device was positioned horizontally, perpendicular to the flume, at a depth of less than 5 mm, on the distance from the airflow inlet is 1.2 m. The foaming agent solution is supplied through one of the diffusers under a pressure of 1.5 m high level, compressed air (1.5 Barr) is supplied through the second diffuser to push the foaming agent solution through the foam layer. As a result, finely dispersed foam forms on the surface of the water. The result is finely dispersed foam coverage on the water surface. The level of the foam solution agent and the compressed air pressure were maintained on the fix level for all experiments. It is ensured a constant level of the foam coverage formation rate regardless of the air flow rate and wave parameters
The foam coverage parameters were measured on the distance of 6.5 m from the inlet for six different wind speeds U10 from 12 to 34 m/s. We used the shadow method of visualizing processes on the rough surface of the water with backlight under the water, which has proven itself in studying the statistics of spray generation phenomena (see previous paragraph).
The number of pixels which of lower light intensity than a certain threshold was used to estimate the proportion of surface area under the foam coverage. The original images were divided into regions, and for each region average intensity was calculated. Such approach of averaging was used eliminate the influence of noise and small optical inhomogeneities.
One background image was calculated as the average for each pixel of the image, using control records taken without wind. After subtracting the background image for each frame, histogram common for all frames was constructed. It is the distribution of the areas of all images in terms of brightness (intensity). The histogram allows us to estimate the proportion of areas with intensity below the threshold, which depends on the level of threshold set.
To determine the threshold, a direct calculation of the foam coverage area using morphological image processing was performed. It was carried out in several stages shown on Figure 3 a-f. The background was subtracted from the original image and cropping was carried out; the image was binarized with a fixed threshold; the morphological opening with the structural element "disk" of 1 px was performed followed by the morphological closure with a "disk" of 21 px; closed regions of the image were filled with flood-fill operation. The ratio of the number of pixels found to the total number of pixels was taken as the fraction of the surface area covered with foam.
Based on this analysis, a relative coverage was found for sequences of frames from several records in experiments with artificial foam at U10 = 12 m/s. For each record, a relative brightness threshold is selected for analysis by histograms, which gives the corresponding coverage. Then this threshold value averaged over several over all realizations for U10 = 12 m/s was used for all cases of wind speeds. Information about the typical size of the foam bubbles was obtained additionally to data on the area of foam coverage. For this purpose, several frames were selected (for each record), on which the foam is clearly visible. Areas were identified on which the bubbles merge (located in several layers) and others unsuitable for automatic determination of the area. For the remaining areas of the images after the preliminary filtering, all circles were found (see Figure 3 g) by two-stage circular Hough transform (see [13]) and their distribution in radius was calculated.
(a) (b) (c) (d)
Investigations were perforned out under support of Russian Science Foundation project 19-17-00209 (development of an optical system) and project 21-19-00755 (development of software for image processing and carrying out experiments), participation of A. Kandaurov was supported by the President's grant for young scientists МК-5503.2021.1.5, participation of A.Suvorov was supported within state assignment of IAP RAS (Theme No. 0030-2019-0018). The experiments were carried out at unique scientific facility "Complex of large-scale geophysical facilities of IAP RAS". 5. References [10] Y. I. Troitskaya, Y.I. Troitskaya, O.S. Ermakova, A.A. Kandaurov, D.A. Sergeev, D.S. Kozlov, S.S. Zilitinkevich, Non-monotonous dependence of the ocean surface drag coefficient on the hurricane wind speed due to the fragmentation of the ocean-atmosphere interface, Doklady Earth Sciences 477 (2017) 1373–1378. doi.org/10.1134/S1028334X17110265 [11] L. H. Holthuijsen, M. D. Powell, J. D. Pietrzak, Wind and waves in extreme hurricanes, J. Geophys.
Res. 117 (2012). C09003, doi:10.1029/2012JC007983. [12] Yu Troitskaya, D. Sergeev, A. Kandaurov, M. Vdovin, S.S. Zilitinkevich, J. Physical. Ocean., The Effect of Foam on Waves and the Aerodynamic Roughness of the Water Surface at High Winds 49(4) (2019), 959-981. doi.org/10.1175/JPO-D-18-0168.1 [13] H.K. Yuen, J. Princen, J. Illingworth, and J. Kittler. Comparative study of Hough transform methods for circle finding, Image and Vision Computing. 8(1) (1990), 71-77. doi:10.1016/02628856(90)90059-E