Flow-sensing mechanisms and biomimetic potential of seal whiskers

Seals can accurately track fish trails as far as 180 m away solely using ultrasensitive whiskers. A high-aspect-ratio bluff body vibrates due to its shedding vortices when the oncoming flow is present, or it is being towed in still water. However, the undulating morphology of Phocid seal whiskers can reduce vortex-induced vibrations (VIV), rendering seals highly sensitive to flow stimuli. This thesis studied the form and function of seal whiskers, involving 1) geometric parameters (the formulated 3D whisker geometry, length and thickness distributions of all whiskers on the seal muzzle, and reshaped 3D distributions of whiskers), 2) mechanical responses (vibrations in the flow and natural frequencies of seal whiskers), and 3) biomimetic potentials (the bioinspired sensor design and the optimized whisker structure). Through fluid-structure interaction (FSI) studies and experimental investigations involving seal whiskers mounted on 3D-printed microelectromechanical systems (MEMS) sensors, results revealed that neighboring whiskers in an array influenced one another by resulting in greater flow vorticity fluctuation and distribution area, thus causing increased vibrations than an isolated whisker, indicating a vibration-strengthening effect in whisker arrays. Furthermore, it was measured that the ratio (λ) of undulation wavelength to mean diameter of undulating seal whiskers was around 4.36 and 4.63 for harbor and grey seal whiskers, respectively. VIVs of seal whiskers had troughs around λ ~ 4 – 5 for grey seal whiskers, which was around the measured λ value, indicating that the ratio (λ) has evolved to an optimized value and suggesting the biomimetic design of VIV-resistant underwater structures

An adjoint sensitivity study of chlorofluorocarbons in the North Atlantic

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C01007, doi:10.1029/2003JC002014.Adjoint sensitivities of CFC-11 concentrations and CFC-11/CFC-12 ratio ages in a North Atlantic general circulation model are analyzed. These sensitivities are compared with those of spiciness, T − (β/α) S, where α, β are the thermal and haline expansion coefficients, respectively. High-sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. In the dual (adjoint) solutions all three variables exhibit the major ventilation pathways and define the associated timescales in the model. Overall, however, spiciness shows the highest sensitivity to the flow field. In the North Atlantic Deep Water, sensitivities of CFC properties and spiciness to the isopycnal mixing and thickness diffusion are of the same order of magnitude. In the lower subtropical thermocline, sensitivities of CFC properties to the isopycnal mixing and thickness diffusion are higher. The utility of this sensitivity is undermined by the need to reconstruct their boundary conditions. Given the influence of T, S measurements on the density field, they produce the most powerful constraints on the model on the large scale. It still remains possible, however, that transient tracers can provide a larger relative information content concerning the mixing process between the near-surface boundary layer and the thermocline but dependent upon the ability to reconstruct accurate initial and boundary conditions.This work was supported by NSF Award OCE-9730071, OCE-9617570, NASA Award NAG5-7857 and NAG5- 11933

Smoothing analysis of two-color distributive relaxation for solving 2D Stokes flow by multigrid method

Smoothing properties of two-color distributive relaxation for solving a two-dimensional (2D) Stokes flow by multigrid method are theoretically investigated by using the local Fourier analysis (LFA) method. The governing equation of the 2D Stokes flow in consideration is discretized with the non-staggered grid and an added pressure stabilization term with stabilized parameters to be determined is introduced into the discretization system in order to enhance the smoothing effectiveness in the analysis. So, an important problem caused by the added pressure stabilization term is how to determine a suitable zone of parameters in the added term. To that end, theoretically, a two-color distributive relaxation, developed on the two-color Jacobi point relaxation, is established for the 2D Stokes flow. Firstly, a mathematical constitution based on the Fourier modes with various frequency components is constructed as a base of the two-color smoothing analysis, in which the related Fourier representation is presented by the form of two-color Jacobi point relaxation. Then, an optimal one-stage relaxation parameter and related smoothing factor for the two-color distributive relaxation are applied to the discretization system, and an analytical expression of the parameter zone on the added pressure stabilization term is established by LFA. The obtained analytical results show that numerical schemes for solving 2D Stokes flow by multigrid method on the two-color distributive relaxation have a specific convergence zone on the parameters of the added pressure stabilization term, and the property of convergence is independent of mesh size, but depends on the parameters of the pressure stabilization term

A Smoothing Process of Multicolor Relaxation for Solving Partial Differential Equation by Multigrid Method

This paper is concerned with a novel methodology of smoothing analysis process of multicolor point relaxation by multigrid method for solving elliptically partial differential equations (PDEs). The objective was firstly focused on the two-color relaxation technique on the local Fourier analysis (LFA) and then generalized to the multicolor problem. As a key starting point of the problems under consideration, the mathematical constitutions among Fourier modes with various frequencies were constructed as a base to expand two-color to multicolor smoothing analyses. Two different invariant subspaces based on the 2h-harmonics for the two-color relaxation with two and four Fourier modes were constructed and successfully used in smoothing analysis process of Poisson’s equation for the two-color point Jacobi relaxation. Finally, the two-color smoothing analysis was generalized to the multicolor smoothing analysis problems by multigrid method based on the invariant subspaces constructed

Effect of Copper Vapor on Radiation Properties of C4F7N Gas Mixtures

C4F7N and its mixture with buffer gases are regarded as the most promising SF6-alternative gases in gas circuit breakers. The switching arc can severely ablate the electrodes, producing copper metal vapor that combine with the C4F7N gas mixture to chang radiation characteristics. This paper compares the net emission coefficient of C4F7N mixtures at various mixing ratios and assesses the effect of 20% copper vapor. It is found that adding copper vapor can greatly enhance radiation