Showing papers by "Gilead Tadmor published in 2009"

A generalized mean-field model of the natural and high-frequency actuated flow around a high-lift configuration

Abstract: A low-dimensional Galerkin model is proposed for the flow around a high-lift configuration, describing natural vortex shedding, the high-frequency actuated flow with increased lift and transients between both states. The form of the dynamical system has been derived from a generalized mean-field consideration. Steady state and transient URANS (unsteady Reynolds-averaged Navier–Stokes) simulation data are employed to derive the expansion modes and to calibrate the system parameters. The model identifies the mean field as the mediator between the high-frequency actuation and the low-frequency natural shedding instability.

System reduction strategy for Galerkin models of fluid flows

Abstract: We propose a system reduction strategy for spectral and Galerkin models of incompressible fluid flows. This approach leads to dynamic models of lower order, based on a partition in slow, dominant and fast modes. In the reduced models, slow dynamics are incorporated as non-linear manifold consistent with mean-field theory. Fast dynamics are stochastically treated and can be lumped in eddy-viscosity approaches. The employed interaction models between slow, dominant and fast dynamics respect momentum and energy balance equations in a mathematically rigorous manner—unlike unsteady Reynolds-averaged Navier–Stokes models or Smagorinsky-type reductions of the Navier–Stokes equation. The proposed system reduction strategy is employed to the cylinder wake benchmark. Copyright © 2009 John Wiley & Sons, Ltd.

Lift Response of a Stalled Wing to Pulsatile Disturbances

Abstract: The transient lift response of a low-Reynolds-number wing subjected to small amplitude pulsatile disturbances is investigated. The wing has a small aspect ratio and a semicircular planform, and it is fully stalled at a 20 deg angle of attack. Microvalve actuators distributed along the leading edge of the wing produce the transient disturbance. It is shown that the lift response to a single pulse increases with increasing actuator supply pressure and that the lift response curves are similar to each other when scaled by the total impulse. Furthermore, for fixed actuator supply pressure, the amplitude and total impulse of the transient lift response curve increases with increasing external flow speed. In this case, the lift response curves are similar when scaled by the dynamic pressure. The lift response to a single pulse can be treated as a filter kernel, and it can be used to predict the lift time history for the arbitrary actuator input signals. The kernel is similar in shape to transient measurements obtained by other investigators on two-dimensional wings and flaps. Comparisons between the model predictions and the experiments using multiple pulse inputs and square-wave modulated input signals at low frequencies are presented.

Low-Reynolds Number Wing Response to an Oscillating Freestream with and without Feed Forward Control

Abstract: The unsteady lift of a low Reynolds number wing in an oscillating freestream is documented in terms of its amplitude and phase. The phase variation of the lift relative to the freestream velocity shows a larger phase difference than predicted by classical unsteady flow theory. A constant time delay between the lift and the actuator was observed to be τ + =tdelayU/c = 5.3 when normalized by the freestream speed and chord. Feed forward control of pulsed-jet actuators is used to modulate the lift coefficient of the wing, in an attempt to suppress the lift oscillations. Suppression of the fluctuating lift at the fundamental frequency was partially successful, but additional “noise” was added to harmonics of the lift signal by the controller.

Reduced-Order Modelling of Turbulent Jets for Noise Control

Abstract: A reduced-ordermodelling (ROM) strategy is pursued to achieve a mechanistic understanding of jet flow mechanisms targeting jet noise control. Coherent flow structures of the jet are identified by the proper orthogonal decomposition (POD) and wavelet analysis. These techniques are applied to an LES data ensemble with velocity snapshots of a three-dimensional, incompressible jet at a Reynolds number of Re=3600. A low-dimensionalGalerkin model of a three-dimensional jet is extracted and calibrated to the physical dynamics. To obtain the desired mechanistic understanding of jet noise generation, the loudest flow structures are distilled by a goal-oriented generalisation of the POD approach we term ’most observable decomposition’ (MOD). Thus, a reduction of the number of dynamically most important degrees of freedom by one order of magnitude is achieved. Capability of the presented ROM strategy for jet noise control is demonstrated by suppression of loud flow structures.

Imposing FIR Structure on $H^2$ Preview Tracking and Smoothing Solutions

Abstract: This paper studies continuous-time $H^2$ feedforward tracking and estimation problems with FIR (finite impulse response) solutions. Both causal and noncausal solutions are considered. It is shown that these problems can be treated as special cases of static problems in the lifted domain, which, in turn, can be solved in terms of a two-point boundary value (Hamilton-Jacobi) system. This results in fixed-dimension closed-form solutions. A numerically stable form of these solutions, which uses matrix exponentials of Hurwitz matrices only, is also derived.

Implementing a Highly Parameterized Digital PIV System on Reconfigurable Hardware

Abstract: This paper presents PARPIV the design and prototyping of a highly parameterized digital Particle Image Velocimetry (PIV) system implemented on reconfigurable hardware. Despite many improvements to PIV methods over the last twenty years, PIV post-processing remains a computationally intensive task. It becomes a serious bottleneck as camera acquisition rates reach 1000 frames per second. In this research, we aim to substantially speed up PIV processing by implementing it in reconfigurable hardware. Furthermore, this implementation is highly parameterized, supporting adaptation to a variety of setups and application domains. The circuit is parameterized by the dimensions of the captured images as well as the dimensions of the interrogation windows and sub-areas, pixel representation, board memory width, displacement and overlap. Through this work a parameterized library of different VHDL components was built. To the best of the authors’ knowledge, this is the first highly parameterized PIV system implemented on reconfigurable hardware reported in the literature. For a typical PIV configuration with images of 512×512 pixels, 40×40 pixel interrogation windows and 32×32 pixel sub-areas, we achieved about 65\ times speedup in hardware over a standard software implementation.

Parametric modeling of the beating heart with respiratory motion extracted from magnetic resonance images

Abstract: In atrial fibrillation ablation procedures on-line measurement of catheter position is often displayed to the clinician against a static anatomy from pre-procedure scans. However the heart is moving due to both contraction and respiratory motion. Thus both small-scale and large-scale inaccuracies are introduced into the visualization. As part of a larger project to improve delivery of ablation, we are developing parametric models to animate static three-dimensional pre-procedure anatomical models to include the dynamics. To make our heart model “beat” we combine image processing methods with Fourier and polynomial representations, and combine global and local smoothing. The result is an efficient parameterization of the moving surface over both space and time. The steps for making the heart move due to respiration are only partially complete. Here we concentrate on registering a sequence of ungated MR slice sequences. The approach involves parameterizing motion of curves representing anatomical landmarks and enforcing consistency in the cross-slice direction.

4D Cardiac Segmentation of the Epicardium and Left Ventricle

Abstract: We present an efficient pipeline for the segmentation and tracking of the Epicardium (Epi) and Left Ventricle (LV) from a set of 3D cardiac MRI sequences. The surface structure is handled as an array of planar active contours, interconnected between adjacent slices and frames, providing spatial and temporal consistency. In a given cardiac phase, the stacking of slice contours constitute a 3D mesh with a cylindrical topology. In a first stage the heart region is automatically localized by making use of the time variance of the heart region. Extraction of heart border is performed by means of energy minimization. Finally, we construct a 3D tensor array from all time surfaces in cylindrical coordinates and time and space consistency of the segmentation is enforced by fitting an m-variate tensor smoothing spline to the final 3D array.

Bifurcation analysis of the axisymmetric vortex breakdown in a pipe

Abstract: The paper outlines a numerical investigation of the dynamics of a high Reynolds number axisymmetric vortex o w in a pipe with high levels of swirl. A global picture of bifurcations in the o w is revealed by mapping the branches of equilibrium xed points. These branches characterize a domain of local attractiveness where the o w tends to align itself along a low dimensional subspace. We further study the transient dynamics of the o w along this attractive domain and describe it within the framework of Galerkin Mean Field Theory.

The Effect of Parameterization on a Reconfigurable Implementation of PIV.

Abstract: This paper presents PARPIV, the design and prototyping of a highly parameterized digital Particle Image Velocimetry (PIV) system implemented on reconfigurable hardware. Despite many improvements to PIV methods over the last twenty years, PIV postprocessing remains a computationally intensive task. It becomes a serious bottleneck as camera acquisition rates reach 1000 frames per second. Besides, in different engineering applications, different PIV parameters are required. Up to now, there is no PIV system that combines both flexible parameterization and high computational performance. In our research we are creating such a system. This implementation is highly parameterized, supporting adaptation to a variety of setups and application domains. The circuit is parameterized by the dimensions of the captured images as well as the dimensions of the interrogation windows and sub-areas, pixel representation, board memory width, displacement and overlap. Through this work a parameterized library of different VHDL components was built. To the best of the authors’ knowledge, this is the first highly parameterized PIV system implemented on reconfigurable hardware reported in the literature. We report the speedup in hardware over a standard software implementation of 10 different implementations with different parameters.