Investigating Lubricated Friction Forces with a Singing Wineglass
Restricted (Penn State Only)
- Author:
- Orzolek, Megan
- Graduate Program:
- Acoustics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 18, 2024
- Committee Members:
- Michael Lester Jonson, Thesis Advisor/Co-Advisor
Alexander M Mertz, Thesis Advisor/Co-Advisor
Andrew Barnard, Program Head/Chair
Dan Russell, Committee Member - Keywords:
- Acoustics
Wineglass
Friction
Stick-slip
Structural vibrations - Abstract:
- The characterization of lubricated friction is a multiscale problem involving a balance between asperity contact and fluid film forces. This balance depends on the relative sliding speed and the applied load, which are both dependent on velocity. The velocity-dependent friction force can couple with an object’s structural modes, and create periodic stick-slip phenomena, which are often associated with the friction force that causes mechanical vibration. This type of vibration can cause noise to radiate via the structural modes of the system. Notable examples include brake squeal, violin bows, journal bearings, and wineglasses. Singing wineglasses have been used for centuries as musical instruments, so musical acousticians have studied the structural vibrations and acoustic output. Static, 1D forces have been measured, however, the dynamic force applied to the rim has not been measured. A successful measurement of this force is the most novel work of the project. During this project, a test mechanism was designed to measure the vertical, tangential, and radial dynamic forces applied to the spinning glass rim, and simultaneously capture the radiated noise. This rig was able to apply a known loads nearly normal to the wineglass rim, and also rotate the wineglass at various speeds. Feasibility tests were performed to provide a proof-of-concept for the test rig, and it was concluded that rubber was able to excite the wineglass singing. Calibration tests were performed first to determine if the wineglass mode shape rotated during singing, and second to estimate the difference between the force measured and the ground truth applied force. Next, an experimental modal analysis was performed on the wineglass alone and in contact with the test rig to identify frequencies that may appear in future measurements. The $n=2$ mode with two cycles around the rim circumference had the highest mobility of all wineglass modes, which confirms previous research that this is the structural mode most easily excited during singing. Finally, the test rig was used to excite wineglass singing. The rotation rate, near and far-field radiated pressure, and force on the wineglass rim were measured during a series of tests. An 11-minute record was taken and the strongest radiating mode and its harmonics in the pressure were found in the concurrently measured, nonlinear force. The presence of harmonics was confirmed with a comparison to the surface-averaged mobility. To capture the singing stability region, 121 measurements of force and pressure were conducted at different rotation speeds and applied loads. Singing quality and excitation success were observed to depend on the relative velocity at the rim and applied vertical load. Suggested improvements to the test setup include a tighter control on rotation rate and a centering mechanism for the wineglass. Further analysis of the measured forces may involve extended post-processing techniques to compute the force transfer function between the transducer and rim. Next, nonlinear, time domain modeling and/or complex eigenvalue analyses will be performed on the collected data and combined with existing contact and viscoelastic friction models. Finally, extended experiments may investigate the static load applied to the rim using the same test setup. Hopefully, this framework for investigating dynamic friction forces will be implemented into more complicated systems.