Student Projects

Wind blowing over the ocean surface energizes water waves on the surface and induces currents and turbulence in the water. This wind-water interaction regulates the transfer of gases, energy, and momentum between the atmosphere and the ocean, making it a critical component of Earth's climate system. ETH's new wind-water-wave tunnel has been constructed to enable experiments on the interaction between airflow and water flow. Among other capabilities, a fan circulates air over the water surface at speeds between approximately 3 and 20 m/s, causing surface waves to grow over the course of the tank length. The first studies being carried out in this facility are probing the characteristics of the turbulence generated just beneath the surface by the wind. The student will employ Fourier transform profilometry to measure the characteristics of the waves produced by the wind over a range of conditions. This experimental method involves projecting a pattern onto the surface of the wavy water and, using a high-speed camera, recording images of the pattern as viewed from above. The apparent modulation to the pattern in the images caused by the surface's waviness is then used to computationally infer the local surface elevation. Processing of the images will thus reveal time-resolved measurements of the local surface elevation over some spatial area. This elevation field will primarily consist of waves propagating in the wind-ward direction, but will also involve transverse movements. Understanding the three-dimensional nature of the wave field is critical to our interpretation of separate water velocimetry measurements, which in some cases resolve the flow and surface position in only a single plane. More fundamentally, the results will be used to understand the initial stages of wind-wave growth (in which the wind has only had enough time to produce disorganized ripples) and the waves' roles in generating three-dimensional turbulence in the water just beneath the surface.

The project will involve the following steps: - Consulting literature to understand (a) the current understanding of the physics of wind-driven wave growth and (b) the implementation of Fourier transform profilometry; - Selecting appropriate materials (projector, water dye, etc) for the project; - Developing a small-scale version of the projection and imaging system in a water tank (at ETH Zentrum) and writing Python code to process the data; - After initial testing, moving the system to the larger wind-water-wave tunnel at Empa in Dübendorf and acquiring data there under a range of wind speeds; - Using the measured results and existing literature to better understand the development of wind-driven surface waves. \end{itemize}

The project is especially well-suited for students with interests in experiment design, data analysis, and fluid mechanics. Interested students should apply with their their CV, full transcript, and if applicable, bachelor's thesis and semester project, submitted via SiROP or to daruth@ethz.ch.

Cavitation is defined as the formation of transient vapor bubbles in a liquid following a local pressure drop, below the liquid vapor pressure. In most practical applications, cavitation manifests as a multitude of interacting bubbles, often referred to as cavitation clouds. Accurately quantifying the dynamics of these clouds at the individual bubble level is difficult due to the high computational cost involved. Yet, if the bubbles are sufficiently spaced to maintain a spherical shape, their behavior can be approximated using coupled Rayleigh-Plesset type equations (1D ordinary differentla equations), which greatly reduces computational demands. These models enable the simulation of both the growth and collapse of the bubbles taking into account their interactions, and may be further developped to model the bubbles’ translation within the cloud (see for instance, Mettin et al., Bjerknes forces between small cavitation bubbles in a strong acoustic field, Physical Review E, 1997 and Doinikov, Mathematical model for collective bubble dynamics in strong ultrasound fields, JASA, 2004).

The objective of this project is to develop a numerical solver to simulate the dynamics of cavitation bubbles within a cloud, subjected to acoustic forcing. The first phase of the project will focus on implementing a solver that addresses only the coupled oscillatory dynamics of the bubbles and validating the results by comparing them with existing solutions available in the literature. In the second phase, the student will identify strategies (by reviewing the available literature) to model bubble translation within the cloud, and if time permits, implement this feature and then validate it using experimental data and/or literature sources.

For additional information, the candidates can contact Dr. Armand Sieber via e-mail (siebera@ethz.ch).

Gas bubbles undergo volumetric oscillations upon perturbation from their equilibrium state. Such oscillations can be observed by changing the pressure in the medium surrounding the bubble. Drop-weight impact testing systems typically study material response to impulsive loads. This project employs the “drop-tower” to impact a fluid-filled container causing pressure fluctuations, which mimic those induced by acoustic driving or hydrodynamic flows. The pressure experienced by a bubble inside the chamber can be measured using appropriate acceleration and pressure sensors for further characterization. Yield-stress fluids can trap small bubbles for a long time. The radial oscillations of a spherical bubble in a low-yield-stress fluid are comparable to those in a Newtonian medium. Carbopol is one such yield-stress fluid, which makes it a good candidate for controlled experiments.

The main goal of the project is to experimentally characterize impact-induced bubble dynamics through high-speed imaging. This can further be divided into the following sub-tasks that may be undertaken by the student to complete the project. 1. Improve the drop-tower setup and incorporate pressure sensors. 2. Characterize the typical response of the impact testing system. 3. Generate bubbles inside a Carbopol sample using a laser or by injecting air. 4. Measure bubble dynamics in the test sample using a high-speed camera and appropriate image processing. 5. Compare the experimental results with the Rayleigh-Plesset equation for a single bubble. 6. Extend the study for multiple bubble configurations.

Please send your CV/resume and transcript of records in PDF format via email to ppatel@ethz.ch. Feel free to contact me for more details! :)