It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability (modes “A” and “B”; Williamson, 1988), depending on the regime of Reynolds number (Re), although almost no understanding of the physical origins of these instabilities, or indeed their effects on near wake formation, have hitherto been made clear. There is now some strong interest in this problem, coming not only from experiment, but also from Direct Numerical Simulation, where, in some cases, these modes A and B have been found clearly (Thompson & Hourigan, 1996; Zhang et al., 1995; Henderson, 1995; Mittal & Balachandar, 1996). Much of the recent surge of activity concerning the wake transition and development of turbulence in wakes has been addressed comprehensively in a review paper. Williamson (1996a).

Three-dimensional wake transition

: The performance of the dynamic subgrid-scale eddy-viscosity model and the suitability of high-order accurate, upwind-biased numerical methods for large eddy simulations of complex flows are investigated in the case of the turbulent wake behind a circular cylinder at Reynolds number 3,900, based on freestream velocity and cylinder diameter The numerical method consists of high-order upwind-biased finite difference techniques applied to the compressible Navier-Stokes equations written in generalized coordinates Integration in time is done using a fully implicit, second-order accurate iterative technique The results of three fifth-order accurate simulations performed on identical grids with the least-squares version of the dynamic model, the fixed-coefficient Smagorinsky model, and with no subgrid-scale model are compared in the first 10 diameters of the wake The impact of three-dimensionality is also examined via two and three-dimensional calculations without a subgrid-scale model The effect of numerical dissipation is investigated by comparing two simulations using upwind-biased schemes, the first being fifth-order, and the second seventh-order accurate It is found that the near-wake is highly three-dimensional at this Reynolds number It contains pairs of counter-rotating streamwise vortices, the effect of which cannot be reproduced in two-dimensional calculations Three-dimensional computations are essential for predicting flow statistics of engineering interest (AN)

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Numerical experiments on the flow past A circular cylinder at sub-critical reynolds number

By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of post-stall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Karman street.

Post-stall flow control on an airfoil by local unsteady forcing

We investigate the near wake of a cylinder at values of Reynolds number corresponding to the onset and development of shear-layer instabilities. By combining quantitative experimental imaging (particle image velocimetry, PIV) and direct numerical simulations at Re = 3900/4000 and 10000, we show that the flow structure is notably altered. At higher Reynolds number, the lengths of both the wake bubble and the separating shear layer decrease substantially. Corresponding patterns of velocity fluctuations and Reynolds stress contract towards the base of the cylinder. The elevated values of Reynolds stress at upstream locations in the separated layer indicate earlier onset of shear-layer transition. These features are intimately associated with the details of the shear-layer instability, which leads to small-scale vortices. The simulated signatures of the shear-layer vortices are characterized by a broadband peak at Re = 3900 and a broadband high spectral-density 'plateau' at Re=10000 in the power spectra. The shear-layer frequencies from the present direct numerical simulations study agree well with previous experimentally measured values, and follow the power law suggested by other workers.

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A combined direct numerical simulation-particle image velocimetry study of the turbulent near wake

A better understanding of the transition process in open flows can be obtained through identification of the possible asymptotic response states in the flow. In the present work, the asymptotic states in laminar wakes behind circular cylinders at low supercritical Reynolds numbers are investigated. Direct numerical simulation of the flow is performed, using spectral-element techniques. Naturally produced wakes, and periodically forced wakes are considered separately.It is shown that, in the absence of external forcing, a periodic state is obtained, the frequency of which is selected by the absolute instability of the time-average flow. The non-dimensional frequency of the vortex street (Strouhal number) is a continuous function of the Reynolds number. In periodically forced wakes, however, non-periodic states are also possible, resulting from the bifurcation of the natural periodic state. The response of forced wakes can be characterized as: (i) lock-in, if the dominant frequency in the wake equals the excitation frequency, or (ii) non-lock-in, when the dominant frequency in the wake equals the Strouhal frequency. Both types of response can be periodic or quasi-periodic, depending on the combination of the amplitude and frequency of the forcing. At the boundary separating the two types of response transitional states develop, which are found to exhibit a low-order chaotic behaviour. Finally, all states resulting from the bifurcation of the natural state can be represented in a two-parameter space inside ‘resonant horn’ type of regions.

Frequency selection and asymptotic states in laminar wakes

This paper mainly describes mechanisms of turbulence in the near wake of a circular cylinder through interaction of vertical motions of different scales.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112087002039

Nonlinear interaction and the transition to turbulence in the wake of a circular cylinder

Full coverage film cooling using compound angle

Numerical and experimental study of micro synthetic jets for active flow control

In order to complement a previous collaborative testing of eddy structure identification methods in free turbulent shear flows, a study about the influence of the transverse location of these large-scale vortical structures on their signatures for the same referenced data base is carried out. Using vorticity-based conditional sampling, events which are located on the same transverse locations are selected and averaged. Results show that a coherent structure has its proper identity contained in its core and keeps it whatever its trajectory. In this way, only its signature in comparison with the surrounding environment changes. The comparison with vorticity-based conditional sampling without this transverse selection brings to the fore a bias on the signature of coherent structures. This result throws back into question previous conclusions about the morphology of structures and related energy transfer scenarios.

Further characterization of large-scale coherent structure signatures in a turbulent-plane mixing layer

A synthetic jet is a time-averaged fluid motion generated by sufficient strong oscillatory flow downstream from a sudden expansion. The study of the interaction between the synthetic jet and an external flow is of great interest in particular for aeronautical applications. A network of such actuators could be used indeed on airplane wings for example to control, with a good energetic efficiency, the boundary layer separation in order to increase the lift or the laminar/turbulent transition for drag reduction. In this paper, the interaction of a sub-millimetric synthetic jet actuator with an external flow is experimentally studied. In these experiments, the actuation is ensured by acoustic excitations produced by a loud-speaker. Hot-wire anemometry is used for measurement of the velocity field in various functioning configurations (velocity of the transversal flow, actuation frequency, ...). In most of the tested configurations, the boundary layer of the transversal flow is significantly modified by the synthetic jet, which shows that these types of actuators could be efficiently used for flow control purposes.Copyright © 2007 by ASME

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Henri-Claude Boisson

Papers

Nonlinear interaction and the transition to turbulence in the wake of a circular cylinder

Full coverage film cooling using compound angle

Numerical and experimental study of micro synthetic jets for active flow control

Further characterization of large-scale coherent structure signatures in a turbulent-plane mixing layer

Behavior of a Mini Synthetic Jet in a Transverse Wall Flow: Experimental and Numerical Study