Engineering
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Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions
Chapters
Summary February 17th, 2019
This protocol demonstrates the basic experimental configuration for water entry experiments with free-falling spheres. Methods for the alteration of liquid surface with penetrable fabrics, the preparation of chemically non-wetting spheres, and steps for splash visualization and data extraction are discussed.
Transcript
This protocol demonstrate water inject experiments with free-fall spheres. The alternations of liquid surfaces with penetrated fabrics, the preparation of chemical in emerging spheres, splash visualization, and data extraction are discussed. These techniques provide insight into fundamental fluid dynamics research geared towards understanding interfacial interactions between solid objects and fluid bodies.
Demonstrating the procedure will be Joshua Bom and Jeremy Stephen, undergraduate students from Alberta. Wear industrial grade nitrile gloves throughout this procedure to avoid contaminating the water and impactors with skin oils. To begin thoroughly clean transparent tank with tap water and detergent.
Then fill it with a 32 liters of tap water. Clean two metric rulers with 99%isopropyl alcohol and let them dry for one minute. Mount one ruler facing the front of the tank with the base just touching the water.
Fix the second ruler vertically underwater. Then secure an hinged platform over the tank so that the platform is within the same depth plane as the above water ruler with respect to the front of the tank. Adjust the platform to the maximum desired drop height then attached a multi LED light to an articulating arm and position the light above and in front of the tank to fully illuminate the splash zone under the platform.
Set a black screen at the back of the tank to help with splash and cavity visualization. Lay a clean oil free shock absorber such as a closed cell sponge at the bottom of the tank and hold it in place with clean weights. Next, mount a high speed camera facing the tank in line with the surface of the fluid.
Connect a trigger switch to the camera if desired. Connect the camera to a computer and configure the camera for a frame rate of at least 1, 000 frames per second. Choose the desired resolution and set the shutter speed to one per framed second.
If applicable set the trigger mode to End. Next, clean a small scoop and polyoxide methanol spheres with 99%isopropyl alcohol to remove skin oils and other contaminates. Allow the spheres and scoop to dry for one minute.
Preform test trials and adjust the camera position, focus, and resolution to achieve the optimal visualization quality for the splash event. Then retrieve the spheres with a scoop and clean the spheres and scoop with isopropyl alcohol. When ready to begin the experiment start filming the tank.
Lift the hinge platform into place and rest a clean dry sphere on the platform. Release the base of the platform to drop the sphere into the tank. Then stop filming, save the video file, and trim the video to show only the splash event.
When ready to proceed to the next test retrieve the sphere with the scoop. Clean the sphere with isopropyl alcohol and let it dry for one minute before repeating the test. When the experiment is finished empty the tank and let it dry.
To prepare penetrable fabrics for the test separate the desired fabrics into square or round plies and use Vernier calipers to measure the compressed thickness of the dry fabric. When setting up the experiment mount a second high speed camera over the tank to provide a top down view of the sphere impacting the fabric. Connect the horizontal camera outputs to the overhead camera input terminals with BNC cables.
Connect a trigger switch to the horizontal camera. Then use Ethernet cables to connect both cameras to an offline router connected to the camera computer. Focus and configure both cameras as usual.
When ready to preform the experiment gently rest the dry fabric on the surface of the liquid in the tank. Insure that the fabric does not start sinking before the trial begins. Use a clean, oil free scoop to position the fabric below the hinged platform.
Use the trigger switch to start simultaneously filming the splash zone with both cameras before setting up the collision. Remove the fabric immediately after the collision. If the fabric fragments on impact and the scrapes can not be manually collected empty and clean the tank after every trial to insure that all fabric is removed.
To begin preparing chemically hydrophobic spheres fit a polyoxide menthol sphere into an acrylic holder. Positioning spheres for a surface coating is the most difficult aspect of this protocol. Positioning is controlled with layer coat acrylic sheets that have holes slightly wider then the spheres.
Spray the sphere with an hydrophobic base coat from about 15 to 30 centimeters away without soaking the surface of the sphere. Next, spray the sphere with the hydrophobic top coat from about 15 to 30 centimeters away without soaking the surface and let it dry for one to two minutes. Apply two or three more top coatings in the same way.
Let the sphere dry for 30 minutes for light use or about 12 hours for full use. When preforming impact tests let the sphere dry completely after each trial rather then rinsing it with isopropyl alcohol. The hydrophobic coating will degrade after about 20 trials.
To reuse this sphere remove the coating with 99%isopropyl alcohol and reapply the base and top coats as previously described. To begin the analysis import a high speed video of splash event into a video analysis software. Use the scaling tools to set the scaling value in centimeters based on the visible area of the ruler.
Advance through the video to find a suitable frame showing splash crown ascension and air entrapment. Measure the splash crown height, the cavity width, and the cavity depth. Use the angle measurement tool to determine the separation angle.
Advance further in the video to find a suitable frame showing Worthington Jet formation and measure the Worthington Jet height. Lastly, use the auto tracking feature to extract temporal position and velocity date for the impactor. If tracking is interrupted use manual tracking until automatic tracking can resume.
Here this procedure was used to evaluate the effects of fabric on splashing from vertical impacts. The splash height with one ply was amplified relative to the splash height without fabric. The splash height with two plies was roughly equivalent to the splash height without fabric.
The splashing was heavily attenuated with three and four plies. Minimal variation was observed across Weber numbers for cavity depth, splash ground height, and cavity width from spheres impacting a single ply of fabric. The spheres vertical positions were tracked underwater allowing the observation of greater decreases in underwater velocity with each added ply of fabric.
Investigators must pay keen attention to the maintenance of sanitary experimental conditions during the experiment to obtain consistent and valid results. This technique is not restricted to the materials presented here and can be undertaking with materials of similar functionalities and capabilities. With introduction of an elevated reservoir such as a syring, this procedure can be upped for investigations of the vertical impacts of liquid droplets onto solid or a liquid surfaces.
This protocol can also be preformed on an larger scale to generic greater impact velocities and an increase range of dimensionless parameters which all goes well for a Naval and industrial applications.
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