LINK FELLOWSHIP AWARDEES FOR 2009
OCEAN ENGINEERING AND INSTRUMENTATION
Cyndee Finkel
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.
Jeremy Alan Dillon
Physical Oceanography
Memorial University, St. John’s
Newfoundland
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 [1].
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 [2] 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 [3]. 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 [4]. 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 [5]. Particle motion due to small scale turbulent fluctuations will be modeled as a stochastic diffusion process [6], with parameters to represent flows with different turbulence intensities, correlation properties, and kinetic energy spectra. The multi-frequency coherent Doppler profiler (mfCDP) described in [7] 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.
References:
[1] 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.
[2] Osborn, T.R. (1974) Vertical profiling of velocity microstructure. J. Phys. Oceanogr. 4:109-115.
[3] 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.
[4] 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.
[5] Zedel, L. (2008) Modeling pulse-to-pulse coherent Doppler sonar. J. Atmos. Oceanic Tech. 25(10):18341844.
[6] Pope, S.B. (1994) Lagrangian PDF methods for turbulent flows. Ann. Rev. Fluid Mech. 26:23-63.
[7] 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.