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Digital holography experiment of 3D detection of underwater bubble fields
Abstract
Detection of underwater bubbles is one of the key issues of the research of ocean-atmosphere flux exchange. Digital holographic experiment is carried out based on Mach-Zehnder digital holographic system, to detect the distribution of bubbles. Holographic images of the dynamical bubble fields are recorded by the chargecoupled device (CCD) video system and the tomographic images at different depth are reproduced. The distribution of sizes and densities of the bubbles is obtained through following steps as denoising, edgedetection, and bubble-recognition using Hough transform. Through the experiments, the efficiency and applicability of the digital holographic detection of underwater bubble fields are tested and verified.
... Recently, digital inline holography (DIH) was introduced as a low cost and compact solution for measuring bubble size distribution in 3D (Tian et al. 2010, Talapatra et al. 2012, Liu et al. 2013, and Sentis et al. 2018. DIH employs a single beam light source to illuminate the objects and uses a digital sensor to record the holograms generated from the interference between the scattered light from the objects and un-scattered portions of the beam. ...
... In addition, multiple investigations have attempted to incorporate additional segmentation criterions such as shapes or intensity gradients to improve the accuracy of segmentation and localization of the bubbles from holograms (e.g. Talapatra et al. 2012, Liu et al. 2013. Particularly, Talapatra et al. (2012) combined intensity metric and intensity gradient with circular Hough transform to segment bubbles from the holograms. ...
... Over 8000 holograms are captured through the span of the experiments corresponding to a time scale of 2.71 s. The bubble sizes are in a range of 2 to 180 pixels, which correspond to a bubble diameter of 0.1 mm to 9 mm. of challenges in the processing of holograms from real water tunnel experiments, which has not been adequately addressed by the previous techniques yet (Tian et al. 2010, Talapatra et al. 2012, Liu et al. 2013, and Sentis et al. 2018). These challenges include strong background noise, high dynamic range of the bubble size and shape, and bubble clustering as shown in the supplemental video S1. ...
The paper presents a hybrid bubble hologram processing approach for measuring the size and 3D distribution of bubbles over a wide range of size and shape. The proposed method consists of five major steps, including image enhancement, digital reconstruction, small bubble segmentation, large bubble/cluster segmentation, and post-processing. Two different segmentation approaches are proposed to extract the size and the location of bubbles in different size ranges from the 3D reconstructed optical field. Specifically, a small bubble is segmented based on the presence of the prominent intensity minimum in its longitudinal intensity profile, and its depth is determined by the location of the minimum. In contrast, a large bubble/cluster is segmented using a modified watershed segmentation algorithm and its depth is measured through a wavelet-based focus metric. Our processing approach also determines the inclination angle of a large bubble with respect to the hologram recording plane based on the depth variation along its edge on the plane. The accuracy of our processing approach on the measurements of object size and 3D distributions are assessed through synthetic bubble holograms and oil droplet holograms from an experiment separately. In addition, we evaluate the ability of this algorithm to estimate the bubble inclination with respect to the hologram recording plane through measuring a 3D-printed physical target of pillars with different inclination angles. The holographic measurement technique is further implemented to capture the fluctuation of instantaneous gas leakage rate from a ventilated supercavity generated in a water tunnel experiment. Overall, our paper introduces an inexpensive and compact solution for high resolution characterization of bubbles and other particles in multiphase flows from a broad range of applications.
... Recently, digital inline holography (DIH) was introduced as a low cost and compact solution for measuring bubble size distribution in 3D (Tian et al. 2011, Tapapatra et al. 2012, Liu et al. 2013, Sentis et al. 2018. DIH employs a single beam light source to illuminate the objects and uses a digital sensor to record the holograms generated from the interference between the scattered light from the objects and un-scattered portions of the beam. ...
... In addition, multiple investigations have attempted to incorporate additional segmentation criterions such as shapes or intensity gradients to improve the accuracy of segmentation and localization of the bubbles from holograms (e.g. Tapapatra et al. 2012, Liu et al. 2013. Particularly, Tapapatra et al. (2012) combined intensity metric and intensity gradient with circular Hough transform to segment bubbles from the holograms. ...
... Over 8000 holograms are captured through the span of the experiments corresponding to a time scale of 2.71 s. The bubble sizes are in a range of 2 to 180 pixels, which correspond to a bubble diameter of 0.1 mm to 9 mm. of challenges in the processing of holograms from real water tunnel experiments, which has not been adequately addressed by the previous techniques yet (Tian et al. 2011, Tapapatra et al. 2012, Liu et al. 2013, Sentis et al. 2018). These challenges include strong background noise, high dynamic range of the bubble size and shape, and bubble clustering as shown in the supplemental video S1. ...
The paper presents a hybrid bubble hologram processing approach for measuring the size and 3D distribution of bubbles over a wide range of size and shape. The proposed method consists of five major steps, including image enhancement, digital reconstruction, small bubble segmentation, large bubble/cluster segmentation, and post-processing. Two different segmentation approaches are proposed to extract the size and the location of bubbles in different size ranges from the 3D reconstructed optical field. Specifically, a small bubble is segmented based on the presence of the prominent intensity minimum in its longitudinal intensity profile, and its depth is determined by the location of the minimum. In contrast, a large bubble/cluster is segmented using a modified watershed segmentation algorithm and its depth is measured through a wavelet-based focus metric. Our processing approach also determines the inclination angle of a large bubble with respect to the hologram recording plane based on the depth variation along its edge on the plane. The accuracy of our processing approach on the measurements of bubble size, location and inclination is assessed using the synthetic bubble holograms and a 3D printed physical target. The holographic measurement technique is further implemented to capture the fluctuation of instantaneous gas leakage rate from a ventilated supercavity generated in a water tunnel experiment. Overall, our paper introduces a low cost, compact and high-resolution bubble measurement technique that can be used for characterizing low void fraction bubbly flow in a broad range of applications.
... Digital holography (DH) [1] provides access to the quantitative measurement of in-depth information of an object. Due to its advantages, DH has been applied widely in various fields, such as microscopic three-dimensional (3D) measurement and imaging, including 3D cell imaging [2,3] , 3D micro-structure measurement [4] , super-resolution measurement [5] , 3D micro-particles tracking [6,7] , etc. Although DH has many advantages compared with conventional holography, its lateral resolution and field of view (FOV) are its major deficiencies due to digital recording devices. ...
A long working distance digital holographic system is developed to study burning coal particles, which are less than 200 μm in diameter, in a methane-air flame. A general model for the lensed digitial holography system is introduced first. An optimal lateral resolution of 3.9 μm is achieved while the object is located at a safe distance (>15 cm) from the optical apparatus. The evolutions of particle morphology and devolatilization products are observed. Sizes and three-dimensional (3D) velocities of particles are measured and compared at the burner outlet, 10 cm and 24 cm above the burner. At the ignition, condensed phase cloud is observed surrounding the coal particle due to the ejection of devolatilization products. Then the volatile cloud accumulates into stringy objects after detaching from the parent particle. The surface of the coal particle has rough morphology due to formation of porous structure after the reaction. Statistical size distributions at different heights above the burner show a 35% increase of peak size after combustion, and the fraction of the larger particles also significantly increased. Particles spread in the radial direction meanwhile particle velocities along the axial direction decrease with the height because of the drag force and gravity.
We propose a method for simultaneous 3D temperature and velocity measurement of a micro-flow field. The 3D temperature field is characterized with two-color laser-induced fluorescence particles which are tracked with micro-digital holographic particle tracking velocimetry. A diffraction-based model is applied to analyze defocused particles to determine the intensity ratio of two fluorescent dyes on the particle. The model is validated with experimental images. As the result shows that the intensity ratio nearly remains unchanged with respect to depth positions, defocused particles can be used as 3D temperature sensors. Numerical work is carried out to check the method, and 3D temperature and velocity field in a 120 μm×120 μm×80 μm test volume are retrieved.
The rate of dissipation of turbulent kinetic energy has been measured with airfoil probes mounted on an autonomous vehicle. Autosub, on constant-depth legs at 2-10 m below the surface in winds up to 14 m s-1. The observations are mostly in an area limited by fetch to 26 km where the pycnocline depth :s about 20 m. At the operational depths of 1.55-15.9 times the significant wave height Hs, and in steady winds of about 11.6 m s-1 when the wave age is 11.7-17.2, dissipation is found to be lognormally distributed with a law-of-the-wall variation with depth and friction velocity. Breaking waves, leaving clouds of bubbles in the water, are detected ahead of the Autosub by a forward-pointing sidescan sonar, and the dissipation is measured when the clouds are subsequently reached. Bands of bubbles resulting from the presence of Langmuir circulation are identified by a semiobjective method that seeks continuity of band structure recognized by both forward- and sideways-pointing sidescan sonars. The times at which bands are crossed are determined and are used to relate dissipation rates and other measured parameters to the location of Langmuir bands. Shear-induced "temperature ramps" are identified with large horizontal temperature gradients. The turbulence measurements are consequently related to breaking waves, the bubble clouds. Langmuir circulation, and temperature ramps, and therefore to the principal processes of mixing in the near-surface layer of the ocean, all of which are found to have associated patterns of turbulent dissipation rates. A large proportion of the highest values of dissipation rate occur within bubble clouds. Dissipation is enhanced in the convergence region of Langmuir circulation at depths to about 10 m, and on the colder, bubble containing, side of temperature ramps associated with water advected downward from near the surface. Near the sea surface, turbulence is dominated by the breaking waves; below a depth of about 6Hs the local vertical mixing in stronger Langmuir circulation cells exceeds that produced on average by the shear-induced eddies that form temperature ramps.
We present a digital in-line holographic imaging system for measuring the size and three-dimensional position of fast-moving bubbles in air–water mixture flows. The captured holograms are numerically processed by performing a two-dimensional projection followed by local depth estimation to quickly and efficiently obtain the size and position information of multiple bubbles simultaneously. Statistical analysis on measured bubble size distributions shows that they follow lognormal or gamma distributions.
We examined the effect of individual bubble clouds on remote-sensing reflectance of the ocean with a 3-D Monte Carlo model of radiative transfer. The concentrations and size distribution of bubbles were defined based on acoustical measurements of bubbles in the surface ocean. The light scattering properties of bubbles for various void fractions were calculated using Mie scattering theory. We show how the spatial pattern, magnitude, and spectral behavior of remote-sensing reflectance produced by modeled bubble clouds change due to variations in their geometric and optical properties as well as the background optical properties of the ambient water. We also determined that for realistic sizes of bubble clouds, a plane-parallel horizontally homogeneous geometry (1-D radiative transfer model) is inadequate for modeling water-leaving radiance above the cloud.
We describe a simple holographic method that has enabled us to capture as a single data set the trajectories of micrometer-sized objects suspended in water. By subtracting consecutive holograms of a particle suspension and then adding these difference holograms, we constructed a final data set that contains the time evolution of the particle trajectories free from spurious background interference effects. The method is illustrated by a recording of the motion of 5 – 10 ‐ µ m diameter algae in water.
An in-line holographic system for in situ detection of atmospheric cloud particles [Holographic Detector for Clouds (HOLODEC)] has been developed and flown on the National Center for Atmospheric Research C-130 research aircraft. Clear holograms are obtained in daylight conditions at typical aircraft speeds of 100 m s⁻¹. The instrument is fully digital and is interfaced to a control and data-acquisition system in the aircraft via optical fiber. It is operable at temperatures of less than -30 °C and at typical cloud humidities. Preliminary data from the experiment show its utility for studies of the three-dimensional spatial distribution of cloud particles and ice crystal shapes.
Holographic and Coulter Counter detection techniques were jointly used to measure the concentration density distribution of cavitation nuclei in the ocean. Comparison of the two techniques indicates that Coulter Counter analysis measures particulate contents up to an order of magnitude smaller than indicated by the holographic method and may also produce a distorted concentration density distribution. Several possible explanations of the observed discrepancies are proposed and discussed, including fundamental differences between the in situ holographic samples and the collected samples examined with the Coulter Counter, differences between the unknown electrical conductivity of the measured particles in the sea water samples and the non-conductive polystyrene spheres used to calibrate the Coulter Counter, the rupture of aggregate particles in the flow through the Coulter Counter orifice, the effect of electronic noise on the Coulter Counter signal, and the influence of statistical sampling error.
A photographic analysis of bubbles generated in coastal seas by breaking waves and general turbulence has allowed the number and spectrum of sizes of bubbles greater than 17 ..mu..m in radius to be counted and observed. A distribution of numbers and sizes is presented for bubbles at 1.5-m depth arising from wave activity driven by winds of from 8 to 10 m sâ»Â¹; under these conditions the number of bubbles was 2.7 x 10â´ mâ»Â³. In winds of 11--13 m sâ»Â¹ the numbers of bubbles determined from photographs were 4.8 x 10âµ mâ»Â³ at 0.7-m depth, 1.6 x 10âµ mâ»Â³ at 1.8-m depth, and 1.6 x 10â´ mâ»Â³ at 4-m depth. The data acquired by this technique enable us and others to calculate the rate of the invasion of atmospheric gas into and out of the sea and to investigate the production of nonliving organic particulate matter by the processes of adsorption and bubble dissolution. The numbers of bubbles that do not dissolve completely but rise to the sea surface and burst are also calculable and are fundamental to quantifying the production of marine aerosols. A comparison of this technique with classical acoustic methods is now imperative.
Recently we have developed an underwater holographic camera for the analysis of plankton and other marine organisms. This camera (HoloMar) was unique in that it was able to record simultaneous in-line and off-axis holograms to cover a range of size of marine organisms from a few microns to tens of millimetres and at concentrations from a few particles per cubic centimetre to dense aggregates. However, HoloMar suffered for being physically large and heavy and difficult to deploy. It also was based on the use of photographic emulsions to record the holograms. To overcome some of these difficulties we have started to develop a new holographic camera (eHoloCam) based on digital holography. In electronic or digital holography (eHolography) an electronic hologram is directly recorded onto a CCD or CMOS sensor and then numerically reconstructed by simulation of the optical hologram reconstruction. In this paper, we discuss some of the possible optical layouts and algorithms under consideration. We present some eHolograms produced in the laboratory prototypes of the eHoloCam