Naval Postgraduate School

About: Naval Postgraduate School is a education organization based out in Monterey, California, United States. It is known for research contribution in the topics: Tropical cyclone & Nonlinear system. The organization has 5246 authors who have published 11614 publications receiving 298300 citations. The organization is also known as: NPS & U.S. Naval Postgraduate School.

Effect of ambient turbulence on trailing vortices

Abstract: The effects of ambient turbulence (generated by a biplanar grid) on the migration and lifespan of trailing vortices are investigated in a towing tank through the use of two NACA-0012 foils moving at a constant angle of attack. The results show that the rise and demise of the vortices are controlled primarily by the rate of dissipation of the background turbulence. The integral scale of turbulence plays only a minor role. In both a quiescent or weakly-turbulent fluid the sinusoidal instability and in a fluid with stronger turbulence, the vortex bursting precedes the subsequent instability events which brings about the eventual destruction of the vortices. Both forms of the large scale instability are often accompanied by the roll of the vortex pair onto its side. Shear is not necessary for the roll but may enhance it under atmospheric conditions. Nomenclature & - aspect ratio of the wing B = wing span 60 = initial vortex core spacing c = chord length of the wing d = width of a square bar in the grid d0 = initial depth of the vortex pair H = vortex rise height H* -H/b0y normalized migration height Ln = integral scale of turbulence M =mesh size of the grid Re = Reynolds number re = effective core radius T* - V0t/bQ, normalized time t = time U = model velocity u =x component of velocity v =y component of velocity K0 =r0/2?rZ?0, initial mutual induction velocity w = z component of velocity x,y = coordinate axes a = model angle of attack F0 = initial circulation of vortex A =d0/b0 e =rate of decay of turbulence energy e* = (eZ?0)1/3 / V0, turbulence parameter v = kinematic viscosity of water p = density of water

Many-body embedded-atom potential for describing the energy and angular distributions of Rh atoms desorbed from ion-bombarded Rh\{111\}

Abstract: In this paper, we show that many-body interactions are important for describing the energy- and angle-resolved distributions of neutral Rh atoms ejected from keV-ion-bombarded Rh{111}. We compare separate classical-dynamics simulations of the sputtering process assuming either a many-body potential or a pairwise additive potential. The many-body potential is constructed using the embedded-atom method to describe equilibrium properties of the crystal, parameters from the Moliere potential to describe close encounters between energized atoms, and parameters from a Rh2 potential to aid the description of the desorption event. The most dramatic difference between the many-body potential and the pair potential is in the predicted kinetic energy distributions. The pair-potential kinetic energy distribution peaks at ∼2 eV, whereas the many-body potential predicts a broader peak at ∼4 eV, giving much better agreement with experiment. This difference between the model potentials is due to the predicted nature of the attractive interaction in the surface region through which all ejecting particles pass. Variations of the many-body-potential parameters are examined in order to ascertain their effect on the predicted energy and angular distributions. A specific set of parameters has been found which leads to excellent agreement with recent experimental trajectory measurements of desorbed Rh atoms.

Development, implementation and evaluation of a data-assimilative ocean forecasting system off the central California coast

Abstract: The development and implementation of a real-time ocean forecast system based on the Regional Ocean Modeling System (ROMS) off the coast of central California are described. The ROMS configuration consists of three nested modeling domains with increasing spatial resolutions: the US West coastal ocean at 15-km resolution, the central California coastal ocean at 5 km, and the Monterey Bay region at 1.5 km. All three nested models have 32 vertical sigma (or terrain-following) layers and were integrated in conjunction with a three-dimensional variational data assimilation algorithm (3DVAR) to produce snapshots of the ocean state every 6 h (the reanalysis) and 48-h forecasts once a day. This ROMS forecast system was operated in real time during the field experiment known as the Autonomous Ocean Sampling Network (AOSN-II) in August 2003. After the field experiment, a number of improvements were made to the ROMS forecast system: more data were added in the reanalysis with more careful quality control procedures, improvements were made in the data assimilation scheme, as well as model surface and side boundary conditions. The results from the ROMS reanalysis are presented here. The ROMS reanalysis is first compared with the assimilated data as a consistency check. An evaluation of the ROMS reanalysis against the independent measurements that are not assimilated into the model is then presented. This evaluation shows the mean differences in temperature and salinity between reanalysis and observations to be less than 1 °C and 0.2 psu (practical salinity unit), respectively, with root-mean-square (RMS) differences of less than 1.5 °C and 0.25 psu. Qualitative agreement is found between independent current measurements and the ROMS reanalysis. The agreement is particularly good for the vertically integrated current along the offshore glider tracks: the ROMS reanalysis can realistically reproduce the poleward California Undercurrent. Reasonably good agreement is found in the spatial patterns of the surface current as measured by high-frequency (HF) radars. Preliminary results concerning the ROMS forecast skill and predictability are also presented. Future plans to improve the ROMS forecast system with a particular focus on assimilation of HF radar current measurements are discussed.