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Falls Church, VA, United States

Basovich A.,Cortana Corporation
Dynamics of Atmospheres and Oceans | Year: 2011

The interaction between non-uniform near-surface currents and long surface waves is shown to produce large-scale secondary circulations. The circulations are caused by the Craik-Leibovich vortex force imposed on the existing non-uniform current by the surface waves. The current could be produced by different types of sources, such as by ship wakes or by river and sewer outflows. In this paper the circulations are considered for three representative types of currents: a near-surface jet, a shear current, and an underwater jet. A model similar to the model of Langmuir circulations is formulated and studied numerically. The general model takes into account the effect of viscosity on the main current as well as the effect of the circulation-related advection on the main current and secondary flow itself. A simplified model that describes the initial stage of the development of circulations was used in order to demonstrate the strength of the phenomenon and its dependence on some parameters of the problem. At this initial stage, the effect of viscosity on the main current as well as the effect of advection caused by the circulations was neglected (under assumption that the perturbation velocity is small). The effect of the viscosity on the circulations was included in the solution, and it was shown that initial development of the circulations is practically independent of the viscosity. This fact simplifies the solution of the problem and removes the uncertainty related to the value of the turbulent viscosity at the initial stage of the circulations. The results obtained demonstrate that strong circulations are generated under very realistic assumptions regarding the parameters of the current and the surface waves. The maximum velocity at the surface produced by such circulations can easily reach several centimeters per second. A circulatory flow with this magnitude of velocity at the surface can significantly affect short surface waves and, correspondingly, radar and optical signatures produced by the initial currents on the sea surface. Some important conclusions about the nature of these signatures are made based on numerical results and simple qualitative arguments. Theoretical predictions include, for example, the asymmetry of centerline ship wakes and the difference in the width and length between images of two wakes of similar ships moving in opposite directions when ambient surface waves are present. © 2011 Elsevier B.V. Source


Basovich A.,Cortana Corporation
Journal of Physical Oceanography | Year: 2014

A new mechanism of instability leading to development of Langmuir circulations is proposed and studied based on the hypothesis that the turbulence and, correspondingly, the eddy viscosity are reduced in regions of higher than average contaminant concentration. Here, bubbles are considered as the contaminant, although it is known that surfactants and some particles also are capable of turbulence reduction. The analysis shows that only a very small local decrease in eddy viscosity is needed to initiate the instability. Simplifications to themomentum and bubble turbulence models, as well as neglect of vertical advection, make it possible to analytically solve the perturbation equations and determine the characteristic scale with the maximum growth rate. The scale of the fastest-growing Langmuir circulations is found to be a function of the concentration of bubbles in the nearsurface layer, the surface current shear (wind shear), the Stokes drift created by the surface waves, and the eddy viscosity. In contrast to the results of earlier models, the analysis predicts that the maximum change in the current velocity along the direction of the wind (the so-called jets and wakes) is at the surface, not below it. The ratio of perturbation of along-wind surface current and vertical velocity generated by circulations (the pitch) and the aspect ratio of the Langmuir rolls are in reasonable agreement with the experimental data. © 2014 American Meteorological Society. Source


Joy N.A.,Albany State University | Rogers P.H.,Cortana Corporation | Nandasiri M.I.,Western Michigan University | Nandasiri M.I.,Pacific Northwest National Laboratory | And 2 more authors.
Analytical Chemistry | Year: 2012

An optical plasmonic-based sensing array has been developed and tested for the selective and sensitive detection of H2, CO, and NO2 at a temperature of 500 °C in an oxygen-containing background. The three-element sensing array used Au nanoparticles embedded in separate thin films of yttria-stabilized zirconia (YSZ), CeO2, and TiO2. A peak in the absorbance spectrum due to a localized surface plasmon resonance (LSPR) on the Au nanoparticles was monitored for each film during gas exposures and showed a blue shift in the peak positions for the reducing gases, H 2 and CO, and a red shift for the oxidizing gas, NO2. A more in-depth look at the sensing response was performed using the multivariate methods of principal component analysis (PCA) and linear discriminant analysis (LDA) on data from across the entire absorbance spectrum range. Qualitative results from both methods showed good separation between the three analytes for both the full array and the Au-TiO2 sample. Quantification of LDA cluster separation using the Mahalanobis distance showed better cluster separation for the array, but there were some instances with the lowest concentrations where the single Au-TiO2 film had separation better than that of the array. A second method to quantify cluster separation in LDA space was developed using multidimensional volume analysis of the individual cluster volume, overlapped cluster volume, and empty volume between clusters. Compared to the individual sensing elements, the array showed less cluster overlap, smaller cluster volumes, and more space between clusters, all of which were expected for improved separability between the analytes. © 2012 American Chemical Society. Source


Grant
Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 149.44K | Year: 2010

This research will employ existing theories of the Debye Effect to estimate amplitudes of electromagnetic signals produced by a submarine operating in a realistic ocean environment. Estimates will incorporate known properties—mass, valence, volume, and friction coefficient—of each ionic species in seawater. Estimates will be made for a range of realistic acoustic sources associated with submarine operations, covering a range of frequencies and waveforms. The capabilities of electromagnetic signals to propagate in seawater independently of acoustic signals will be assessed. This research also will use existing theories of the Colloid Effect to estimate amplitudes of electromagnetic signals produced by a submarine operating in a realistic ocean environment. Estimates will incorporate known and estimated properties of organic detritus and other sources of colloids found in the ocean. Finally, this research will revise and adapt existing theories of electrokinetic interface conversion to apply to the case of charged surfactant layers at the air-water interface. Special attention will be given to the analysis of signals radiated directly into the atmosphere from the surface layer.


Grant
Agency: Department of Defense | Branch: Navy | Program: STTR | Phase: Phase II | Award Amount: 743.09K | Year: 2011

Phase II research will follow the most promising leads developed in Phase I, with the end goal of developing a viable physics-based model that can explain observations available to the Government. A dispersive focusing mechanism, supplemented by the theory of cylindrically-symmetric solitons, is the most promising mechanism identified in Phase I. Three related tasks will be performed: Detailed theoretical and numerical analysis of the process of linear dispersive focusing in cylindrical geometry. Formulation of correct non-linear solitary wave propagation equations in cylindrical coordinates, and theoretical and numerical analysis of the transition from linear to non-linear propagation. Modeling of the radar signature produced by the cylindrical wave packets propagating outward from the source. A weak shock mechanism was studied in Phase I, and some additional research related to this mechanism is proposed for Phase II. Two tasks are planned. Adaptation of the existing shock propagation model to a non-uniform environment. Laboratory measurements of the long-time effects of the Richtmyer-Meshkov instability at the air/water interface.

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