Idealized simulations of turbulence advected by surface waves: Implications for interpreting turbulence measurements in shallow water

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

In shallow water such as the inner shelf, bays, and estuaries, turbulence measurements are often made with fixed sensors. Surface waves, which are ubiquitous in these environments, confound turbulence measurement because wave orbital velocities are typically two orders of magnitude larger than turbulent fluctuations and they also advect turbulent eddies are advected past sensors. Turbulent flux measurements are usually biased by wave velocities and spectra are difficult to interpret because energy is distributed differently in frequency space than when turbulence is advected by a current alone. Methods that fit spectral shapes to frequency spectra to estimate turbulent fluxes, turbulent kinetic energy, and turbulent length scales are presently limited to conditions where wave orbital velocities are less than or equal to currents. In this study a series of idealized simulations will be performed to develop spectral fitting methods suitable for situations where wave orbital velocities exceed currents. Theoretical and semi-empirical turbulence wavenumber spectra from the literature, as well as fields from high resolution simulations of turbulence will be converted to frequency spectra that would be observed if the eddies were advected past a fixed location sensor by waves and a current. Preliminary idealized simulations of 1D advection of isotropic turbulence and anisotropic boundary-layer turbulence by monochromatic waves provide a framework for synthesizing the proposed simulations, as well as measurements and methods in the literature, in terms of important dimensionless parameters that relate advection of turbulence by waves to advection by currents. Idealized simulations will be extended to 2D (horizontal and vertical advection by waves) and 3D (waves propagating at an angle to the current), and to narrow- and broad-banded wave conditions typical of those on the inner shelf. Simulation results will be compared with existing laboratory and field measurements collected previously by the PI to assess the validity of model spectra (which were developed for turbulence in steady flows) for turbulence beneath waves, potentially providing new insights into wave-current boundary layers. The results of idealized simulations, along with the existing laboratory and field measurements, will be used to develop new tools that employ spectral fitting to estimate turbulence parameters and their uncertainties from fixed sensor measurements when wave orbital velocities exceed currents. Broader Impacts There has been much interest recently in measuring turbulent momentum and scalar fluxes beneath surface waves using fixed location sensors; however, these measurements can be difficult to interpret. The proposed work will improve understanding of the issues involved with making turbulence measurements beneath waves when wave orbital velocities exceed currents, and develop methods for estimating turbulence parameters and their uncertainties in these conditions that will be valuable to scientists studying the physics and chemistry of a range of shallow water environments. The synthesis of previous work in the context of idealized simulations will help guide the analysis of turbulence measurements from fixed sensors in wavy environments. A library of idealized simulation results, available on a website, will allow scientists to put their measurements in the context of a broad parameter space without repeating the simulations themselves. Matlab scripts for performing idealized simulations and applying new methods developed in this study will also be made available on the project website. This grant will provide salary support and a computer workstation for one early career scientist.