The experiment was conducted in a 57.4 m wave tank located in the Ocean Engineering Research Center at Memorial University of Newfoundland’s Faculty of Applied Science and Engineering. This tank is 57.4 m long, 4.5 m wide, and a maximum depth of 𝑝𝑝𝑡𝑡 = 3.04 m. It has a single piston board-style wave-maker, which is driven by a
single 3,000 PSI, 125 hp hydraulic drive. The other end contains a wave damping beach that serves as a wave energy absorber and reflection reducer. The water depth is kept
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at H = 1.9 m. A tow carriage supports the simulated hull models and is located approximately at the tank’s midway point. Despite the significant effect of wind on breakup process, air surrounding the model kept calm at all time.
Due to the high wave energies required to create a spray, strength and rigidity were primary concerns in model development. The other priority was to create a laboratory- scale model that had the ability to let the water experience breakup in the same way as full-scale vessels. A height of h = 1 m for the models was considered adequate to balance these two requirements. The first model was a flat-shaped plate with a height of hf=
1 m, a width of bf= 0.7 m, and 0.2 m wide side plates. The shape of the second model
approximated that of a ship’s bow. It had a narrower front face and side plates with variable flare angles, which can be mounted on the carriage with different stem angles with respect to the vertical axis. The dimensions of this model were hb= 1 m in height,
bb= 0.15 m in width and 0.3 m wide side plates. Additionally, 0.2 m of each model was
submerged in the water, and the intersection between the free surface level and middle of the models is set as origin O(0, 0, 0).
The free surface variations due to the wave height passing several positions were measured with capacitive wave probes. Three probes were used to measure the far-field wave height, the wave height at the point of impact, and the wave reflection after impact. The far-field probe (WP1) was located at a 1 m distance from the model to avoid impact- wave reflection in the negative x-direction, and the point of impact probe (WP2) was located exactly in front of the model to measure the wave height at the moment of impact
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and the wave reflection. The reflection probe (WP3) was located 0.6 m from the model but in the same plane (Fig. 2-1b). It should be noted that the wave probes are not located in front of the camera, and are located such that they do not conflict with the image capturing.
Wave breaking processes in the laboratory for comparisons with a real scale are challenging to predict numerically. Several authors demonstrated similarity between a breaking wave in the laboratory and the actual scale. Philips (1985) presented a model for the distribution of the breaker front length, which leads to the momentum flux from the wave field into the upper ocean and the total gravity wave energy dissipated by breaking waves. Several other past studies such as Melville (1994) and Drazen et al. (2008) improved the equations by Philips (1985) and compared the breaking wave in the laboratory with a real scale ocean breaking wave. Further, recent work of Sutherland and Melville (2013) introduced a dimensional analysis for predicting the distribution of the breaker front length. In the current study, a non-dimensional study by Sutherland and Melville (2013) was used to determine the wave characteristics that are compatible with the wave data presented by Ryerson (1995).
A qualitative measurement of spray characteristics was acquired with a high-speed camera. A Phantom V611 high-speed camera was placed in two different positions in relation to the test model: a side view through the tank windows for lateral and vertical spray, and a front view for spray distribution along the width of the models. The distances between the camera and center of both objects for the side view and front view are kept
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at 2.30 𝑚𝑚 and 1.7 𝑚𝑚, respectively. For BIV technique several LEDs were used on one side, and these lights were diffused by an acrylic plastic sheet. Several other LED and DC light fixtures were used to increase the brightness and quality of the captured videos. The BIV method was used to analyze the aerated area and the backlighting technique leads to sharp images. The camera frame rate was fixed at 1000 fps with a fixed resolution of 1024 × 768 pixels. Due to restrictions on the Field of View (FOV), the camera was aimed either at the lower position of the sheet formation or the higher position looking at the spray heights. Adequate lighting was used for the camera to obtain sharp and clear images.
Nova Sensor model NPI − 19 medium pressure sensors were used. These sensors were located on the model at 27.5 cm (A), 30 cm (B), 32.5 cm(C), 35 cm (D), and 37.5 cm (E) from the bottom of the model to capture the distribution of wave impact pressure. Each sensor contains a piezoresistive sensor chip inside a hermetically sealed diaphragm housing. External threading allowed the sensors to be flush-mounted to the model test rig. The justification for using these particular sensors is found in Fullerton et
al. (2010). The schematics of the experiment, which demonstrate different FOV (side and
front), are shown in Figs. (2-1a) and (2-1b), respectively. As is shown in Fig. (2-1a), two different FOV were considered in this experiment: FOV1 for studying the spray and measuring droplet characteristics, and FOV2 for analyzing the behavior of a wave at the time of impact, as well as water sheet production arising from the impact, in which both FOV have dimensions of 600 × 450 mm.
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Fig. 2-1 Model structure and FOV of the experiment for both (a) side view, (b) 3-D schematic view
and the BIV apparatus.
The DPIV technique was used to calculate velocity and droplet size and to track droplet detached from aerated regions. This method of computationally analyzing digital video
(a)
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images removes both the photographic and optomechanical processing steps inherent in PIV and Laser Speckle Velocimetry (LSV). The method, first introduced by Willert and Gharib (1991), has been upgraded by various researchers.
DPIV and BIV techniques are restricted and controlled by a narrow Depth of Field (DOF). Ray (2002) suggested adjusting the camera aperture and the distance from the camera lens to the center of measurement in front of the object. DOF is defined as the distance near the object captured by the camera, which is well focused and sharp. Due to the narrow DOF, errors related to displacement in the image, which is further correlated to velocity, were reduced. Detailed correlation and a discussion regarding finding the DOF can be found in Ryu et al. (2005). However, Ryu et al. (2005) reported velocity measurements in both sharp and blurred images as independent from DOF methodology.
Several sample images of DPIV and BIV studies are shown in Fig. (2-6). These show the several stages of image filtering, morphological steps, and noise reduction as well as droplet detection. Due to the algorithms for image filtering, thresholding, and background subtraction in this study, only some samples are shown in Fig. (2-6). In this study, boundaries that are out of the focal plane of the camera and appeared blurry in the image were removed from the ensemble averaging. Droplet detection from these images was used for further post-processing such as size and velocity detection.
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Fig. 2-2 Sample raw images of side and front views of the water droplet and jet formation using
several algorithms and image filtering for noise reduction, morphological steps, background subtraction, and droplet detection as well as velocimetry.
The waves in the tow tank were calibrated for ensuring that the wave board span and frequency set points were accurate and that wave amplitudes were generated accordingly. Wave probes and pressure sensors were calibrated separately and prior to use to ensure proper measurements. The camera calibration was conducted by taking images from a square checkerboard pattern panel of known and uniform width. These checkerboard images were placed in the focal plane of the camera and the midpoint of the FOV.
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