Numerical and laboratory experiments in early wakes in stratified fluids
Abstract
ABSTRACT 1.1.1 Background For the canonical problem of a towed sphere of diameter D in a stratified ambient, with buoyancy frequency N, experimental observations can be related interchangeably to downstream distance, x/D, or dimensionless evolution times, Nt. The two are related through x/D = (Fr/2)Nt, where Fr = 2U/ND is a Froude number based on the sphere radius. Prior to our current study, the earliest data came from x/D = 6, Nt = 3 (for Fr = 4). At this time, anisotropy in both mean and turbulence profiles is fully established, and so the dynamics of the non-equilibrium regime, where initial turbulent motions adjust to the background density gradient, are already dominant. It is commonly argued that the turbulent wake evolves first as though independent of the stratification, and then readjusts as the available kinetic energy is no longer sufficient to lift increasingly small vertical length-scale parcels of fluid. Therefore, we usually begin models (and numerical simulations) with conditions that have been determined and established by experiment for non-stratified fluids. In current ONR-supported work, we have shown that this model is not correct, at any measurable time, for all Fr < 10. The validity at high Fr is not yet tested. • Since the early development of anisotropy is associated with the characteristic regular patterns in the vertical vorticity, the formation process for pancake eddy-dominated wakes is therefore dependent on initial conditions that may be hard to generalise and model. • The emergence of pattern and information from seemingly irregular initial conditions is a topic of rather broad importance. New tools need to be developed to interrogate experimental and numerical data that have unprecedented detail, so the necessary and sufficient conditions can be found. The proposed laboratory and numerical experiments will continue to contribute to both of these areas.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512506
Entities
People
- Robert Spedding
Organizations
- Office of Naval Research
- United States Navy
- University of Southern California