Stratified turbulent wakes: The strongly stratified regime at very high Reynolds numbers and machine-learning-based analysis of internal wave radiation
Abstract
Submerged naval wakes in stratified waters operate at values of body-based Reynolds number Re = O(10�^8) and internal Froude number Fr = O(10) to O(10^3). Previous and ongoing ONR-funded research by the PI, based on high-resolution/accuracy parallel implicit Large Eddy Simulations (ILES) of stratified towed-sphere wakes has pushed Re to values as high as 10^5 and 4 x 10^5 while spanning values of Fr=4, 16 and 64. At this sufficiently high Re, an order of magnitude above that attained by laboratory experiments, the analysis of the resulting datasets has provided a first probe into of the emergence of the Strongly Stratified Regime (SSR) in the intermediate-to-late wake (ITLW) where buoyancy controls the flow field to leading order. Within the SSR, turbulence in the wake core continues to overturn intermittently within high-aspect ratio layers. Moreover, the power released by the SSR-wake into internal waves(IWs) has been found to constitute a major fraction of the wake energy budget. Nonetheless, the two higher Re examined by the PI may be viewed as operating at a Re-cusp in terms of accessing the duration of SSR and associated turbulence dynamic range required to reveal the richness of physics typical of navally relevant Re. A push to an additional value of larger Re is imperatively needed. Linked to this push, with its elevated computational cost, is the introduction of additional structural and dynamical complexity within the ITLW: a) a non-patchy, wake-core-filling, highly layered turbulence will be established with an internal scale separation of two decades between largest overturning and viscous scales and b) the momentum/energy released into IWs will be further enhanced. Most importantly, both of these phenomena should support adequately strong vertical fluxes of momentum which cause non-trivial modifications of the mean wake profile. Current operational (e.g. low-resolution LES or unsteady RANS) models do not account for SSR turbulence and IW radiation, posing concerns about the reliability of their predictions for the ITLW. The insights gained by the proposed research enables more robust and reliable parameterization of this regime in operational tools.The ILES proposed will revisit the previously considered range of Re and Fr, augmenting it with the critically needed value of Re = 1.6 x 10^6, in longer computational domains. Central to this objective is a redesigned Fourier/Modal-Spectral-Element flow solver. This code has been built to facilitateefficient parallel performance, on state-of-the-art DoD HPC platforms of simulations which can support the resolution (35 billion grid points) and core count (8,000) required for an objective and deep examination of the SSR at Re = 1.6 x 10^6. Statistical analysis of wake core turbulence will quantify vertical fluxes of momentum due to the SSR. A similar quantification of momentum/energy fluxes due to IWs, as a function of increasing offset from the wake edge, will be pursued along witha characterization of radiated IW frequency content and its correlation to that within the wake core. In parallel, building on recent work on a machine learning (ML)-based taxonomy of wake radiated IWs, the proposed work will establish an automated technique for detecting and tracking individual IW events. As such, a quantitative description of the evolution of the momentum/energy content of individual members of the IW population will be pursued, along with corresponding population-wide estimates. Such findings complement the above statistical analysis towards building a first-order parameterization of IW-driven fluxes. Finally, a preliminary investigation of the modification of high Rephysics, and namely SSR development, by more realistic background conditions will be pursued by simulating low Re wakes in a two-layer continuous stratification with either a broad or narrow pycnocline.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 06, 2023
- Source ID
- N000142312172
Entities
People
- Peter J Diamessis
Organizations
- Cornell University
- Office of Naval Research
- United States Navy