: Lateral Shear and Strain Sensor for the Ocean Environment

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Sustainment OBJECTIVE: Develop a sensor to advance our capability of measuring finescale (1-10 m) lateral gradients in ocean velocity for environmental sensing of anisotropic turbulence. The sensor should be capable of direct measurement of lateral gradient fields within this scale range. While deployment via a ship is appropriate for the initial design phase, the design should consider future deployments on autonomous or moored platforms. DESCRIPTION: Direct measurement of ocean turbulence depends on sampling of quantities at length scales where universal scaling laws hold. These measurements are typically made in the vertical over regimes that are thought to be isotropic (invariant with rotation), homogeneous (invariant in space), and stationary (invariant in time). In the stratified ocean, the assumption of isotropy breaks down when the vertical scale of turbulent overturns is suppressed by the fluid's stabilizing stratification. In contrast, horizontal shear is unrestrained and turbulent eddies persist. This anisotropic turbulence regime resides in between spatial scales dominated by internal gravity waves in the open ocean and the inertial subrange of isotropic turbulence [Ref 1]. It is characterized by what have been termed pancake' eddies or vortical motion as turbulent vortices have a flattened' aspect ratio in the vertical. Direct measurement of velocity fluctuations over this spatial range are exceedingly rare as most existing Commercial off-the-shelf (COTS) sensors offer either a fixed volume or vertical profile of velocity. The Navy seeks development of a sensor capable of direct measurement of lateral velocity gradients over spatial ranges of order 1-10 meters. The Navy is agnostic to the approach, which may be acoustic [Ref 2], optical [Ref 3], or some alternative platform [Ref 4]. Proposed designs should either sample the lateral velocity vector while resolving differences over the scale of 1 m or measure lateral shear/strain directly on scales of at least 1 meter. The sensor should be capable of 1) resolving gradients of 0.0001 1/s (implying a noise floor below this value) and 2) distinguishing lateral motions from platform tilt either through motion correction or direct sensing. While deployment via a ship is appropriate for the initial design phase, the sensor should ultimately be deployable on autonomous or moored platforms. PHASE I: Identify a design concept for a sensor along with the hardware components that can meet the stated requirements. Develop a concept for onboard and offline software processing. Develop a detailed power budget for the sensor. A detailed analysis for strengths and weaknesses of the proposed design should be included in Phase I, considerations should include resolution and range tradeoffs, endurance determined by power and storage needs, and physical footprint of the sensor. After assessment of strengths and weaknesses a final design review should be completed. PHASE II: Develop and test a prototype system. Complete analysis of the performance of the system. Report on results. Perform multi-stage testing, allowing for redesign between tests with initial tests in a surrogate ocean environment (e.g., lake or tank) and final testing in the ocean under controlled conditions (e.g., coastal bay). Both hardware and software systems should be developed and tested during Phase II. Field testing in Phase II will constrain the parameter space under which the system is operationally capable for the Navy. PHASE III DUAL USE APPLICATIONS: Support the transition to Navy use. This technology has potential use in DoD and commercial applications that require current velocity information at small scales (1-10 m). Possible applications include monitoring of contaminants shed from wind/wave farms and other offshore structures, monitoring fluxes for marine carbon dioxide removal (mCDR) development, and navigation of UUVs. REFERENCES: Basic theory 1.Kunze, Eric. "A unified model spectrum for anisotropic stratified and isotropic turbulence in the ocean and atmosphere." Journal of Physical Oceanography, 49.2, 2019, pp. 385-407. Acoustic application 2. Smith, Jerome A.; Pinkel, Robert and Goldin, Michael A. "A vertical slice of acoustic intensity and velocity [2-D images of waves and turbulence]."2015 IEEE/OES Eleventh Current, Waves and Turbulence Measurement (CWTM), St. Petersburg, FL, 2015. Optical application 3. Steinbuck, Jonah V., et al. "An autonomous open-ocean stereoscopic PIV profiler." Journal of Atmospheric and Oceanic Technology, 27.8, 2010, pp. 1362-1380. Direct vorticity measurement 4. Sanford, Thomas B., et al. "An electromagnetic vorticity and velocity sensor for observing finescale kinetic fluctuations in the ocean." Journal of Atmospheric and Oceanic Technology, 16.11, 1999, pp. 1647-1667. Atmospheric application 5. Palmer, R et al. A Primer on Phased Array Radar Technology for the Atmospheric Sciences. Bull. Amer. Meteor. Soc., 103, 110, 2022, pp. E2391 E2416. https://doi.org/10.1175/BAMS-D-21-0172.1 KEYWORDS: stratified turbulence, anisotropic turbulence, ocean sensing, vorticity, lateral shear, lateral strain