2013 Summer Fellowship Award Recipients (Select the blue text below to expand the project description)
LT Victoria Futch, Science Department, Marine Science section (More...)
“Quantifying Dispersion Patterns in Island-Induced Flows as a Means to Improve Drift Prediction Accuracy.”
As the North Equatorial Current (NEC) passes the southernmost point of the island of Hawaiʾi, it separates into a shear layer which is unstable, forming vigorous anticyclonic vortices recurrently observed west of the island. To fully understand the dynamics of vortex formation and growth, the areas of the separation point and downstream must be observed in three dimensions.
Three High Frequency Doppler Radios will be deployed, one configured at high resolution to map the origin of the shear layer, and two configured for long range to map the subsequent growth of the vortices. Seventy surface drifters will be deployed in lines crossing the shear layer, and in clusters seeding emerging vortices, to obtain Lagrangian sampling of the flow. Two shipboard cruises using ADCP and hydrography, and three ADCP moorings, will provide the vertical structure of the vortices.
Strong negative wind stress curl in the lee of the island provides a competing mechanism for anticyclonic vortex formation, resulting in a Stommel-gyre extending west of the island into the interior, and a predicted counter-current. It is hypothesized that this Hawaii Lee Countercurrent (HLCC) is forced mostly by the transfer of zonal momentum by the instability vortices, wind stress curl playing a secondary, and possibly preconditioning, role.
The proposed observations will help determine which vortex formation mechanism dominates, a step to understand the dynamics of the HLCC. Improved statistics on seasonal patterns of the strength and location of the HLCC, a current influencing a large area of the subtropical pacific, will be obtained. The results will help constrain models and improve drift prediction in the region. In addition, a better understanding of the vortices will reveal characteristics such as thermocline depth, mixing, and related biological productivity effects.
Dr. Elisha Garcia, Engineering Department, Naval Architecture and Marine Engineering section (More...)
“Effect of Damping on VIV and Galloping of Arrays of Circular Cylinders when Analyzed with Variable Added Mass.”
Collaborator: Dr. Michael M. Bernitsas, Professor of VIVACE research group at the University of Michigan.
Description: Vortex-Induced Vibrations (VIV) and galloping are two vibration phenomena that have been known to exist for hundreds of years, but neither is fully understood. VIV and galloping are international fields, encompassing analytical, numerical, and experimental studies. The reason for this vast interest in VIV and galloping is the large forces that can be experienced by structures (offshore structures, bridges, towers, etc.) exposed to flow can be catastrophic. There are countless references to VIV or galloping leading to the fatigue and failure of structures. A better understanding of these phenomena can lead to better mitigation of failures.
For many decades, the idea that VIV can be successfully modeled as a lock-in phenomenon of a mass-spring-dashpot system with an ideal added mass term has persisted. In 2000, it was suggested by Vikestad et al. that VIV may be modeled better as a resonance phenomenon with variable natural frequency due to a variable added-mass term. Recently, Vandiver suggested a new dimensionless damping parameter c* that requires further vetting.
In this research project, the variable added-mass approach will be used for analysis of VIV and galloping of arrays of multiple cylinders. I have found for a single cylinder that a variable natural frequency due to variable added mass will be equal to the forcing frequency (and the frequency of oscillation) when the cylinders are in VIV, and the expectation is that the same will hold for multiple cylinder arrangements. This hypothesis can be further tested when the cylinders are in galloping triggered by passive turbulence control (PTC) in the form of sand strips. Additionally, experimental data on VIV and galloping of PTC-cylinders with variable damping will be analyzed in order to assess the effect of damping on variable added-mass and the validity of the new dimensionless damping parameter c*.
Experimental time-displacement data for VIV and galloping with varying stiffness and damping coefficient will be analyzed to extract a time-averaged added mass over each oscillation period of the data. Time-derivatives, corrected for phase-shift filtering, provide velocity and acceleration used to reconstruct the lift force as a function of time from the mass-spring-dashpot system. From this, the added mass can then be calculated for each specific test by extracting the term in phase with the acceleration. That is, by integrating the force multiplied by the acceleration. A time-averaged variable added mass will then be calculated by averaging all periods of oscillation for a given data set.
2012 Summer Fellowship Award Recipients
- Dr. Susan B. Swithenbank, Engineering Department, Naval Architecture and Marine Engineering section
- CDR Michael Corl, Engineering Department, Naval Architecture and Marine Engineering section
- Dr. Tooran Emami, Engineering Department, Electrical Engineering section
- Dr. Sam Wainwright, Science, Marine Science section
For more information on the CAS fellowships, contact Dr. Ali Reza.