Transition to turbulence in the bottom boundary layer under a solitary wave.

Se estudia la transición a la turbulencia en una capa límite oscilatoria.Se estudia la transición a la turbulencia en una capa límite oscilatoria.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Self-preservation in stratified momentum wakes

A general model is described for drag wakes in a linearly stratified fluid, based on the self-preservation of the flow. It is assumed that the buoyancy-controlled self-similar wake expands in the horizontal direction due to turbulent diffusion and in the vertical direction due to viscous diffusion. The mean characteristics of the wake (height, width and velocity defect) are analytically derived and show good agreement with existing data from experimental and numerical results. Moreover, the three regimes previously found in the literature that characterise different dynamical phases of the wake evolution are recovered, and two new regimes are found. The model allows for prediction of characteristic length and velocity scales at the high Reynolds numbers of large-scale applications of geophysical and naval origin

Nonlinear evolution of harmonically forced perturbations on a wingtip vortex

Wingtip vortices are created by flying airplanes due to lift generation. The vortex interaction with the trailing aircraft has sparked researchers’ interest to develop an efficient technique to destroy these vortices. Different models have been used to describe the vortex dynamics and they all show that, under real flight conditions, the most unstable modes produce a very weak amplification. Another linear instability mechanism that can produce high energy gains in short times is due to the non-normality of the system. Recently, it has been shown that these non-normal perturbations also produce this energy growth when they are excited with harmonic forcing functions. In this study, we analyze numerically the nonlinear evolution of a spatially, pointwise and temporally forced perturbation, generated by a synthetic jet at a given radial distance from the vortex core. This type of perturbation is able to produce high energy gains in the perturbed base flow (10^3), and is also a suitable candidate for use in engineering applications. The flow field is solved for using fully nonlinear three-dimensional direct numerical simulation with a spectral multidomain penalty method model. Our novel results show that the nonlinear effects are able to produce locally small bursts of instability that reduce the intensity of the primary vortex.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Automated Tracking of 3-D Overturn Patches in Direct Numerical Simulation of Stratified Homogeneous Turbulence

Abstract. Direct numerical simulation is a valuable tool for modeling turbulence, but like "wet lab" simulation, it does not solve the problem of how to interpret the data. Manual analysis, accompanied by visual aids, is a time consuming, error prone process due to the elaborate timedependent structures appearing in simulations. We describe a technique based on volume tracking, that enables the worker to identify and observe evolving coherent flow structures, eliminating uninteresting background data. Using our techniques we were able to investigate 3-D density overturns in stably stratified homogeneous turbulence, understand entangled physical structures and their dynamical behavior. We describe our technique, which improves on past work by incorporating application-specific knowledge into the identification process. Such knowledge was vital in filtering out spurious information that would have interfered with the experimental method. Representative results are shown which summarize the physical insight gained by the application of the above identification/tracking method

Design of 2-D FIR filters with nonuniform frequency samples

Acoustics, Speech, and Signal Processing, IEEE International Conference on ICASSP '87.The article of record may be found at http://dx.doi.org/10.1109/ICASSP.1987.1169469A method of designing two-dimensional (2-D) FIR filters, using nonuniform frequency samples, is presented. The method is based on an extension of the DFT method of design, which uses uniform frequency samples. The proposed method, is based on an extension of Newton's interpolation method to 2-D. The proposed procedure has the attractive properties of permanence and recursive computation of the design parameters. A design procedure is given for 2-D FIR linear phase filters and an example is given

High-order discontinuous element-based schemes for the inviscid shallow water equations: spectral multidomain penalty and discontinuous Galerkin methods

Two commonly used types of high-order-accuracy element-based schemes, collocationbased spectral multidomain penalty methods (SMPM) and nodal discontinuous Galerkin methods (DGM), are compared in the framework of the inviscid shallow water equations. Differences and similarities in formulation are identified, with the primary difference being the dissipative term in the Rusanov form of the numerical flux for the DGM that provides additional numerical stability; however, it should be emphasized that to arrive at this equivalence between SMPM and DGM requires making specific choices in the construction of both methods; these choices are addressed. In general, both methods offer a multitude of choices in the penalty terms used to introduce boundary conditions and stabilize the numerical solution. The resulting specialized class of SMPM and DGM are then applied to a suite of six commonly considered geophysical flow test cases, three linear and three non-linear; we also include results for a classical continuous Galerkin (i.e., spectral element) method for comparison. Both the analysis and numerical experiments show that the SMPM and DGM are essentially identical; both methods can be shown to be equivalent for very special choices of quadrature rules and Riemann solvers in the DGM along with special choices in the type of penalty term in the SMPM. Although we only focus our studies on the inviscid shallow water equations the results presented should be applicable to other systems of nonlinear hyperbolic equations (such as the compressible Euler equations) and extendable to the compressible and incompressible Navier-Stokes equations, where viscous terms are included.National Science FoundationONRCAREER award grant OCE-084555