Study of mechanisms and factors that influence the formation of vortical wake of a heaving airfoil

This is the published version. Copyright © 2012 American Institute of PhysicsA two-dimensional numerical study is performed to investigate the relation between the direction of a deflected wake and the vortex pairing mechanisms. The deflection angle can be correlated with two effective phase velocities defined to represent the trends of symmetry breaking and symmetry holding, respectively. The deflection angle increases with the strength of the vortex pairs, which is associated with the heaving amplitude, frequency, and the free stream Reynolds number. Furthermore, not only the influence of Strouhal number but also those of the two heaving motion components – amplitude and frequency – are studied individually under different Reynolds numbers. The study shows that the deflection angle consistently increases with the difference between the symmetry-breaking phase velocity and symmetry-holding phase velocity

The Influence of Reynolds Number and Atmospheric Effects on Aircraft Wake Vortices Near the Ground

Aircraft wakes represent potential hazards which can control aircraft spacing and thus limit airport capacity. Wake vortex trajectories and strengths are altered radically by interactions with the ground plane and by atmospheric conditions. This work has been concerned with developing more accurate numerical predictions. A two-dimensional, unsteady numerical-theoretical study is presented which has included viscous effects, the influence of stratification, crosswind and turbulence on vortex behavior near the ground plane, using a vorticity-streamfunction formulation. A two-parameter perturbation procedure has been developed which uses analytic solutions for the initial flow field to accommodate the ground effect region in the numerical simulation. Using an order of magnitude analysis, it was possible to justify the Boussinesq approximations for turbulent wake vortex predictions, including ground effects and atmospheric stratification. It has been shown that the eddy-viscosity turbulence models were not effective in predicting wake vortex flows and a Reynolds stress transport model was implemented. The numerical solutions to the Navier-Stokes equations have been compared with experimental results for the laminar, unstratified cases and good agreement has been obtained. The computational simulations show that the vortex rebound near the ground plane is caused by ground boundary-layer separation. High stratification levels can confine the motion of the vortex system and alleviate the primary vortex strength. Vortex turbulence influences vortex trajectories more strongly than it influences the rate of change in vortex strength. Weak crosswinds cause the upstream primary vortex to rebound less strongly than the downstream vortex. Finally, suggestions are made for future research

Initialization and Simulations of Three-Dimensional Aircraft Wake Vortices

The research period under the sponsorship of NAG1-1911 is two years: from Feb. 1, 1997 to Jan. 31, 1999 (extended no-cost to April 30, 1999). During the two-year research period, deliverables have included brief monthly progress statements, annual reports and subroutines of computational programs. Several technical papers have been published and are attached to this final report. This Summary of Research contains a brief description of those research results and refers the details to each attached paper

Reynolds number effects on flow/acoustic mechanisms in spherical windscreens

This is the published version. Copyright 2003 Acoustical Society of AmericaThere is a practical need to fully understand the mechanisms involved in the flow/pressure fluctuations around a screened microphone. A stream of uniform flow with low-frequency turbulence encountering a rigid, impermeable spherical windscreen is considered in this study. Pressure distributions on the surface of the sphere are determined by the flow structure. Pressure fluctuations at the center of the sphere are then calculated based on the integration of surfacepressure distributions. Because of the low-frequency assumption, results from steady-state laminar flows can be used to investigate the Reynolds numbereffects on wind noise reduction. Three types of flow have been studied in this paper: an inviscid case, a low-Reynolds-number Stokes flow, and intermediate- and high-Reynolds-number flows. A Reynolds-number/wind-noise-reduction correlation shows that the wind noise reduction increases with decreasing Reynolds number

Proper orthogonal decomposition and recurrence map for the identification of spatial–temporal patterns in a low-Re wake downstream of two cylinders

Flow decomposition methods provide systematic ways to extract the flow modes, which can be regarded as the spatial distribution of a coherent structure. They have been successfully used in the study of wake, boundary layer, and mixing. However, real flow structures also possess complex temporal patterns that can hardly be captured using the spatial modes obtained in the decomposition. In order to analyze the temporal variation of coherent structures in a complex flow field, this paper studies the recurrence in phase space to identify the pattern and classify the evolution of the flow modes. The recurrence pattern depends on the time delay and initial condition. In some cases, the flow system will revisit a previous state regardless of the initial state, and in other cases, the system’s recurrence will depend on the initial state. These patterns are determined by the arrangement and interactions of coherent structures in the flow. The temporal order of the repetition pattern reflects the possible ways of flow evolution

Proper Orthogonal Decomposition and Recurrence Map for the Identification of Spatial–Temporal Patterns in a Low-Re Wake Downstream of Two Cylinders

Flow decomposition methods provide systematic ways to extract the flow modes, which can be regarded as the spatial distribution of a coherent structure. They have been successfully used in the study of wake, boundary layer, and mixing. However, real flow structures also possess complex temporal patterns that can hardly be captured using the spatial modes obtained in the decomposition. In order to analyze the temporal variation of coherent structures in a complex flow field, this paper studies the recurrence in phase space to identify the pattern and classify the evolution of the flow modes. The recurrence pattern depends on the time delay and initial condition. In some cases, the flow system will revisit a previous state regardless of the initial state, and in other cases, the system’s recurrence will depend on the initial state. These patterns are determined by the arrangement and interactions of coherent structures in the flow. The temporal order of the repetition pattern reflects the possible ways of flow evolution

Nonlinear spacing and frequency effects of an oscillating cylinder in the wake of a stationary cylinder

This is the published version. Copyright 2010 American Institute of PhysicsNonlinear responses to a transversely oscillating cylinder in the wake of a stationary upstream cylinder are studied theoretically by using an immersed-boundary method at Re=100. Response states are investigated in the three flow regimes for a tandem-cylinder system: the “vortex suppression” regime, the critical spacing regime, and the “vortex formation” regime. When the downstream cylinder is forced to oscillate at a fixed frequency and amplitude, the response state of flow around the two cylinders varies with different spacing between the two cylinders, while in the same flow regime, the response state can change with the oscillating frequency and amplitude of the downstream cylinder. Based on velocity phase portraits, each of the nonlinear response states can be categorized into one of the three states in the order of increasing chaotic levels: lock-in, transitional, or quasiperiodic. These states can also be correlated with velocity spectral behaviors. The discussions are conducted using near-wake velocity phase portraits, spectral analyses, and related vorticity fields. A general trend in the bifurcation diagrams of frequency spacing shows the smaller the spacing, frequency, or amplitude, the less chaotic the response state of the system and more likely the downstream and upstream wakes are in the same response state. The system is not locked-in in any case when the spacing between the cylinders is larger than the critical spacing. The near-wake velocity spectral behaviors correspond to the nonlinear response states, with narrow-banded peaks shown at the oscillation frequency and its harmonics in the lock-in cases. High frequency harmonic peaks, caused by interactions between the upstream wake and the downstream oscillating cylinder, are reduced in the near-wake velocity spectra of the upstream cylinder when the spacing increases

Research on energy extraction characteristics of an adaptive deformation oscillating-wing

Oscillating foil machines represent a type of flow energy harvesters which perform pitching and plunging motions simultaneously to harness the energy from incoming stream. In this paper, a new adaptive deformation oscillating wing was proposed and the theoretical performance of such a concept was studied here through unsteady two-dimensional simulations using an in-house developed computational fluid dynamics code. During operation, the proposed oscillating foil whose initial shape is symmetric can be deformed into a cambered foil, which aims to produce large lift force. Our numerical results suggest that the power efficiency of the proposed oscillating foil can be about 16.1% higher than the conventional oscillating foil without deformation. In addition, the effects of the maximum bending displacement and effective angle of attack on the efficiency of proposed oscillating foil were also discussed in this work