LINK FELLOWSHIP AWARDEES FOR 2009
OCEAN ENGINEERING AND INSTRUMENTATION
Department of Physics and Ocean Engineering
Florida Atlantic University
Research Project: Experimental Study of Synchronization and Phase Dynamics in Flapping Foil Propulsion
Cyndee earned her BS degree in Physics from Florida Atlantic University and her MS in Physics from the University of Central Florida. She is currently pursuing her PhD in Physics from Florida Atlantic University.
Cyndee’s project is the Experimental Study of Synchronization and Phase Dynamics in Flapping Foil Propulsion.
Underwater vehicles have had a recent surge of interest due to their many commercial, military and scientific applications. Of the three types of motion studied—fixed wing (airplane---like), bird/insect like ornithopter (flapping wing), and helicopter---like rotary wing models—flapping wings offer the most potential for miniaturization due to their high propulsive efficiency and maneuverability. This has re---stimulated interest in the dynamics of natural flight and swimming. The imitation of natural motions for the locomotion of man---made vehicles has been termed ‘biomimetics’ and has spawned novel ideas in the areas of propulsion and active flow control . Since natural fliers use oscillating tails to produce propulsive and maneuvering forces, heaving foils have been extensively studied . However, the dynamics of these free flying systems have yet to be explored.
If flapping foil propulsion is a limit cycle process, then it may represent the synchronization of multiple coupled oscillators. Generally speaking, flow actuation seems to be most effective when the frequency of the control forcing approaches the frequencies of the dominant natural instabilities of a flow. Examples of this are seen in flutter---induced heaving and pitching of two dimensional foils, flow in a Rijke tube, mean---flow---driven cavity oscillations in a slightly compressible flow, and the flow behind an oscillating foil. It is suspected that controlled fluid systems effectively behave as systems of coupled oscillators. Support for this idea includes the flow around a stationary two dimensional cylinder and the general review of synchronization in physical systems .
Experiments will be conducted in a re---circulating water channel, located in the FAU Center for Hydrodynamics and Physical Oceanography. A D---shaped cylinder is selected to generate a von---Karman vortex street that yields a fixed wake width which provides good vortex---formation phase control  . A two---dimensional NACA0012 airfoil, spans the width of the tank bounded by two vertical Plexiglas end plates to reduce three---dimensional end effects. The system is suspended from a set of rails, with pillow block air---bearings, at the top of the water tunnel. The airfoil is subjected to a sinusoidal heave---only motion oscillating at zero angle of attack in the direction perpendicular to the plane.
The synchronization between vortex shedding and foil forcing will be studied for the isolated 2D foil oscillating in a uniform freestream. Here, the system is operated with the foil only (D---shaped cylinder removed). Synchronization occurs when the foil driving frequency approaches natural shedding frequency of the wake, the resulting flow behind the wing is mode---locked (synchronized). Output from strain gauges mounted on foil will measure the force response of the airfoil as a function of frequency. Synchronization is predicted to be peaks in the force response of the airfoil as a function of frequency. With this, we can see if previous models where the freestream velocity between the fluid and flyer are imposed accurately represent those occurring in a freely flying system.
Next, the flapping foil mechanism will be allowed to freely translate in the freestream where a vortex street will be introduced ahead of the oscillating foil by positioning a D--- shaped cylinder upstream of the wing. The interaction of the vortex street with the oscillating airfoil will simulate the interaction between body shed vortices and wing generated vortices that occur in natural flight.
1. Triantafyllou, M. S., Triantafyllou, G. S., and Yue, D. K. P. Hydrodynamics of Fishlike Swimmers. Annu. Rev. Fluid Mech., 32 (2000), 33-53.
2. von Ellenrieder, K. D., Parker, K., and Soria, J. Fluid Mechanics of Flapping Wings. Exp. Therm. Flu. Sci., 32 (2008), 1578-1589.
3. Vittori, G. and Blondeaux, P. Quasiperiodicity and phase locking route to chaos in the 2-D oscillatory flow around a circular cylinder. Phys. Fluids A, 5 (1993), 1866.
4. Stalnov, O., Palei, V., Fono, I., Cohen, K., and Seifert, A. Experimental estimation of a D-shaped cylinder wake using body-mounted sensors. Experiments in Fluids, 42, 531-542.
5. Gopalkrishnan, R., Triantafyllou, M. S., Triantafyllou, G. S., and Barrett, D. Active vorticity control in a shear flow using a flapping foil. J. Flu. Mech., 274 (1994), 1-21.
Jeremy Alan Dillon
Memorial University, St. John’s
Research Project: Turbulence Measurement with Coherent Doppler Sonar
Jeremy earned his Bachelor of Engineering degree in Aerospace from Carleton University, Ottawa, Ontario, where he was ranked 1st in his class and was awarded the University Medal upon graduation. He received his MS degree in Aeronautics from the California Institute of Technology, and a MS degree in Mathematics from Carleton University, Ottawa, Ontario. He is currently pursuing his PhD in Physical Oceanography from Memorial University, St. John’s, Newfoundland.
Jeremy’s project, Turbulence Measurement with Coherent Doppler Sonar, is described below.
Coherent Doppler sonar is an acoustic method for measuring the velocity of particles suspended in a fluid. Instruments of this type are widely used for current measurement in oceanography. However, the performance of these systems in turbulent conditions is not well understood. Coherent Doppler sensors measure the pulse-to-pulse evolution of backscatter phase to estimate the mean velocity. Turbulence causes a rapid rearrangement of particles that decreases the required coherence between pulses. Thus, decoherence potentially limits the accuracy of measurements in turbulent flows .
In many applications, physical processes such as mixing, transport, and dispersion are directly related to properties of the turbulent flow field. In the ocean, turbulence statistics vary in time, e.g. due to tidal, diurnal and seasonal effects. Shear probes  provide high spatial resolution of turbulence microstructure; however the resolution in time is limited. Acoustic sensors are attractive due to the effective propagation of sound in turbid water, the non-intrusive nature of remote measurements, and the high temporal resolution of the data. Acoustic sensors can also log data autonomously for extended periods of time. Efforts have have been made to measure turbulence with acoustic Doppler current profilers . This method, however, tends to over-estimate turbulence statistics since it is difficult to separate velocity measurement noise from turbulent fluctuations. Various correction methods have been proposed, such as . However, existing methods make assumptions about either the turbulence structure or the backscatter correlation properties. Therefore, there exists an opportunity to develop coherent Doppler sonar into a more effective tool for measuring turbulence in the ocean.
The long-term goal of this work is to develop improved instrumentation for studying the evolution of sandy sea floor topography. For this doctoral project, the specific goals are to develop and validate a model that predicts sonar performance, to quantify the achievable accuracy of Doppler sonar in turbulent flows, and to develop a new algorithm for estimating turbulence parameters using a multi-static (i.e. multiple receiver), multi-frequency sonar configuration.
A turbulence model will be developed and incorporated into the Doppler sonar model described in . Particle motion due to small scale turbulent fluctuations will be modeled as a stochastic diffusion process , with parameters to represent flows with different turbulence intensities, correlation properties, and kinetic energy spectra. The multi-frequency coherent Doppler profiler (mfCDP) described in  will be used to measure flux and dispersion of suspended sand in a turbulent wall jet. The experiment will be conducted in a jet tank facility at Dalhousie University while simultaneously recording particle image velocimetry (PIV) measurements for model validation. A new algorithm for extracting turbulence parameters from the mfCDP data will be derived using optimal estimation theory and validated with the PIV data set.
This research will lead to a better understanding of the accuracy of coherent Doppler sensors in turbulent conditions, which will benefit the entire oceanographic research community. A validated model will be available to improve the design of next-generation sensors by minimizing the adverse effects of turbulence on sonar performance. Optimal processing of multi-static, multi-frequency sonar data will be of interest to scientists who study transport processes in the benthic boundary layer. If this method for measuring turbulence is successful, it can be commercialized relatively quickly through an existing partnership between Memorial University, Dalhousie University, and the Doppler sonar manufacturer Nortek.
 Hay, A.E. (2008) Near-bed turbulence and relict waveformed sand ripples: Observations from the inner shelf. J. Geophys. Res. 113, C04040, doi:10.1029/2006JC004013.
 Osborn, T.R. (1974) Vertical profiling of velocity microstructure. J. Phys. Oceanogr. 4:109-115.
 Lu, Y. and Lueck, R.G. (1999) Using a broadband ADCP in a tidal channel. Part II: Turbulence. J. Atmos. Oceanic Tech. 16:1568-1579.
 Hurther, D. and Lemmin, U. (2001) A correction method for turbulence measurements with a 3D acoustic Doppler velocity profiler. J. Atmos. Oceanic Tech. 18(3):446-458.
 Zedel, L. (2008) Modeling pulse-to-pulse coherent Doppler sonar. J. Atmos. Oceanic Tech. 25(10):18341844.
 Pope, S.B. (1994) Lagrangian PDF methods for turbulent flows. Ann. Rev. Fluid Mech. 26:23-63.
 Hay, A.E., Zedel, L., Craig, R. and Paul, W. (2008) Multi-frequency, pulse-to-pulse coherent Doppler sonar profiler. In Proc. IEEE/OES 9th Working Conference on Current Measurement Technology. 25-29.