Developmental design, fabrication, and test of acoustic suppressors for fans of high bypass turbofan engines

An analysis procedure was developed for design of acoustically treated nacelles for high bypass turbofan engines. The plan was applied to the conceptual design of a nacelle for the quiet engine typical of a 707/DC-8 airplane installation. The resultant design was modified to a test nacelle design for the NASA Lewis quiet fan. The acoustic design goal was a 10 db reduction in effective perceived fan noise levels during takoff and approach. Detailed nacelle designs were subsequently developed for both the quiet engine and the quiet fan. The acoustic design goal for each nacelle was 15 db reductions in perceived fan noise levels from the inlet and fan duct. Acoustically treated nacelles were fabricated for the quiet engine and quiet fan for testing. Performance of selected inlet and fan duct lining configurations was experimentally evaluated in a flow duct. Results of the tests show that the linings perform as designed

Mixing rates across the Gulf Stream, Part 1: On the formation of Eighteen Degree Water

Microstructure profiles taken in February 2007 across the Gulf Stream (GS) measured the temporal and spatial variability of the intense mixing that forms Eighteen Degree Water (EDW). Strong winds, gusting to 30 m s–1, and heat fluxes up to 1000 W m–2 produced moderate-to-strong mixing in the surface mixed layer and the entrainment zone, as well as in the thermocline. In the limit of a vertically balanced heat budget, EDW formation is driven primarily by surface heat loss to the atmosphere across a region extending O(100) km south from the GS core, where entrainment heat fluxes based on dissipation rates were relatively low, O(10) to O(100) W m–2. Near the GS core, much larger entrainment fluxes, O(100) to O(1000) W m–2, contribute significantly to cooling the mixed layer, but less so to overall EDW formation due to its smaller volume. Relationships between observed dissipation rates and the atmospheric and local shear forcing scales are examined for this limited data set and compared with empirical scalings both within the mixed layer and in the entrainment zone. Below the mixed layer near the GS, diapycnal diffusivities in the thermocline averaged about O(10–4) m2 s–1, and are approximately 10 times levels previously observed in the GS during other seasons. Horizontally coherent shear structures, with shoaling phase and clockwise rotation, indicate that downward-propagating near-inertial waves are responsible for much of this enhanced subsurface mixing

Mixing rates across the Gulf Stream, Part 2: Implications for nonlocal parameterization of vertical fluxes in the surface boundary layers

The turbulent kinetic energy (TKE) budget of the surface mixed layer is evaluated at wintertime stations occupied in the vicinity of the strong Gulf Stream (GS) jet. The nonlocal K-profile parameterization (KPP) of vertical fluxes is combined with observed hydrography and meteorology to diagnose TKE production. This KPP-based production is averaged over the surface mixed layer and compared with corresponding averages of observed TKE dissipation rate from microstructure measurements, under assumptions of a homogeneous steady-state balance for the layer-averaged TKE budget. The KPP-based TKE production estimates exceed the mean observed boundary layer dissipation rates at occupied stations by up to an order of magnitude. In cases with strong upper ocean shear, the boundary layer depths predicted by the bulk Richardson number criteria of KPP tend to be deeper than indicated by observed dissipation rates, and thereby including strong entrainment zone shear contributes excessively to the KPP-based diagnosis of TKE production. However, even after correcting this diagnosis of mixed layer depth, the layer-averaged production still exceeds observed dissipation rates. These results have several possible implications, including: (1) KPP tends to overestimate vertical momentum flux in cases with strong shear due to geostrophically balanced thermal wind, unbalanced submesoscale dynamics, or entrainment driven by mixed layer inertial oscillations; (2) a mean local TKE balance does not hold in baroclinic mixed layers due to radiation of inertial waves, divergence in horizontal TKE flux or an inverse cascade to larger scales; and (3) both the boundary layer depth and the remaining TKE budget discrepancies indicate the limited validity of mixed layer models in the simulation of submesoscale ocean phenomena