On the mechanism of trailing vortex wandering

The mechanism of trailing vortex wandering has long been debated and is often attributed to either wind-tunnel effects or an instability. Using particle image velocimetry data obtained in the wake of a NACA0012 airfoil, we remove the effect of wandering from the measured velocity field and, through a triple decomposition, recover the coherent wandering motion. Based on this wandering motion, the most energetic structures are computed using the proper orthogonal decomposition (POD) and exhibit a helical mode with an azimuthal wavenumber of |m|=1 whose kinetic energy grows monotonically in the downstream direction. To investigate the nature of the vortex wandering, we perform a spatial stability analysis of a matched Batchelor vortex. The primary stability mode is found to be marginally stable and nearly identical in both size and structure to the leading POD mode. The strikingly similar structure, coupled with the measured energy growth, supports the proposition that the vortex wandering is the result of an instability. We conclude that the cause of the wandering is the non-zero radial velocity of the |m|=1 mode on the vortex centreline, which acts to transversely displace the trailing vortex, as observed in experiments. However, the marginal nature of the stability mode prevents a definitive conclusion regarding the specific type of instability

Turbulent separated shear flow control by surface plasma actuator: experimental optimization by genetic algorithm approach

The final publication is available at Springer via http://dx.doi.org/10.1007/s00348-015-2107-3The potential benefits of active flow control are no more debated. Among many others applications, flow control provides an effective mean for manipulating turbulent separated flows. Here, a nonthermal surface plasma discharge (dielectric barrier discharge) is installed at the step corner of a backward-facing step (U0 = 15 m/s, Reh = 30,000, Re¿ = 1650). Wall pressure sensors are used to estimate the reattaching location downstream of the step (objective function #1) and also to measure the wall pressure fluctuation coefficients (objective function #2). An autonomous multi-variable optimization by genetic algorithm is implemented in an experiment for optimizing simultaneously the voltage amplitude, the burst frequency and the duty cycle of the high-voltage signal producing the surface plasma discharge. The single-objective optimization problems concern alternatively the minimization of the objective function #1 and the maximization of the objective function #2. The present paper demonstrates that when coupled with the plasma actuator and the wall pressure sensors, the genetic algorithm can find the optimum forcing conditions in only a few generations. At the end of the iterative search process, the minimum reattaching position is achieved by forcing the flow at the shear layer mode where a large spreading rate is obtained by increasing the periodicity of the vortex street and by enhancing the vortex pairing process. The objective function #2 is maximized for an actuation at half the shear layer mode. In this specific forcing mode, time-resolved PIV shows that the vortex pairing is reduced and that the strong fluctuations of the wall pressure coefficients result from the periodic passages of flow structures whose size corresponds to the height of the step model.Peer ReviewedPostprint (author's final draft

Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow

A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing on the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier-Stokes simulations of this flow at a chord Reynolds number of 6 104 have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of lock-on states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 105. © 2010 Cambridge University Press

Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow

A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing on the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier-Stokes simulations of this flow at a chord Reynolds number of 6 104 have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of lock-on states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 105. © 2010 Cambridge University Press

Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow

A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing On the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier Stokes simulations of this flow at a chord Reynolds number of 6 x 10(4) have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of 'lock-on' states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 x 10(5).X112433sciescopu

Characterization of synthetic jet resonant cavities

The scientific and technical literature about synthetic jet actuators includes a very wide field of applications such as flow control, heat transfer from limited-size surfaces, general enhancement of mixing between fluid currents, generation of micro-thrust for propulsion or attitude control. The overall design of the actuators needs practical modeling tools. The present work is aimed at characterizing the frequency response of a resonant cavity based synthetic jet actuator driven by a thin piezoelectric disk and represents an alternative contribution to the Persoons’s work [1]; in particular, it approaches the frequency response analysis from the same perspective of Sharma [2], who proposed a frequency-response model directly based on fluid dynamics equations, whereas Persoons [1] follows more directly the Gallas et al. [3] perspective based on the equivalent electric circuit approach. A lumped element mathematical model [4] of the operation of a synthetic jet actuator is both analytically and numerically investigated in order to obtain information about the frequency response of the device; the oscillating membrane is considered as a single-degree-of-freedom mechanical system, while the resonant cavity and orifice components are described by means of proper forms of the continuity and Bernoulli’s unsteady equations. The non-linear governing equations system has been numerically integrated in MATLAB environment. From the analytical viewpoint, it is found that the device behaves as a two-coupled oscillators system and, by solving the relevant eigenvalues problem, simply analytical formulas are given in order to predict the two modified peak frequencies, as functions of the uncoupled first-mode structural and Helmholtz resonance frequencies. The model is also validated through systematic experimental tests carried out on three devices having different mechanical and geometrical characteristics leading to an increasing coupling factor; it is found a very strict agreement between exit flow velocity measurements and numerical simulations for any tested device, for different supply voltages; moreover, the analytical formulas yield predictions in close agreement with numerical and experimental findings as well. References [1] Persoons, T., “General Reduced Order Model to Design and Operate Synthetic Jet Actuators", AIAA Journal, Vol. 50, No. 4, 2012, pp. 916-927. [2] Sharma, R. N., “Fluid Dynamics Based Analytical Model for Synthetic Jet Actuators,” AIAA Journal, Vol. 45, No. 8, Aug. 2007, pp. 1841-1847. [3] Gallas, Q., Holman, R., Nishida, T., Carroll, B., Sheplak, M., and Cattafesta, L., “Lumped Element Modeling of Piezoelectric-Driven Synthetic Jet Actuators,” AIAA Journal., Vol. 41, No. 2, 2003, pp. 240-247. [4] de Luca, L., Girfoglio, M., and Coppola, G., “Modeling and Experimental Validation of the Frequency Response of Synthetic Jet Actuators,” under revision for AIAA Journal, 2013