Zero-net Mass-flux Actuators Research Articles

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve substantial control over the separated flow by increasing the upstream suction pressure and downstream pressure recovery. Using the data available in the literature, the performance of the fluidic oscillators was compared to other active flow control (AFC) methods such as steady blowing, steady suction, and zero-net-mass-flux (ZNMF) actuators. Several integral parameters, such as the inviscid flow comparison coefficient, pressure drag coefficient, and modified normal force coefficient, were used as quality metrics in the performance comparison of the AFC methods. These quality metrics indicated the superiority of the steady suction method, especially at lower excitation amplitudes that is followed by the fluidic oscillators, steady blowing, and the ZNMF actuators, respectively. An aerodynamic figure of merit (AFM) was also constructed using the integral parameters and AFC power usage. The AFM results revealed that, for this study, steady suction was the most efficient AFC method at lower excitation amplitudes. The steady suction loses its efficiency as the excitation amplitude increases, and the fluidic oscillators become the most efficient AFC method. Both the steady suction and the fluidic oscillators have an AFM > 1 for the range tested in this study, indicating that they provide a net benefit when the AFC power consumption is also considered. On the other hand, both the steady blowing and ZNMF actuators were found to be inefficient AFC methods (AFM < 1) for the current configuration. Although they improved the flow field by controlling flow separation, the power requirement was more than their benefit.

Fluidic actuators for separation control at the engine/wing junction

PurposeThe area behind the engine/wing junction of conventional civil aircraft configurations with underwing-mounted turbofans is susceptible to local flow separation at high angles of attack, which potentially impacts maximum lift performance of the aircraft. This paper aims to present the design, testing and optimization of two distinct systems of fluidic actuation dedicated to reduce separation at the engine/wing junction.Design/methodology/approachActive flow control applied at the unprotected leading edge inboard of the engine pylon has shown considerable potential to alleviate or even eliminate local flow separation, and consequently regain maximum lift performance. Two actuator systems, pulsed jet actuators with and without net mass flux, are tested and optimized with respect to an upcoming large-scale wind tunnel test to assess the effect of active flow control on the flow behavior. The requirements and parameters of the flow control hardware are set by numerical simulations of project partners.FindingsThe results of ground test show that full modulation of the jets of the non-zero mass flux actuator is achieved. In addition, it could be shown that the required parameters can be satisfied at design mass flow, and that pressure levels are within bounds. Furthermore, a new generation of zero-net mass flux actuators with improved performance is presented and described. This flow control system includes the actuator devices, their integration, as well as the drive and control electronics system that is used to drive groups of actuators.Originality/valueThe originality is given by the application of the two flow control systems in a scheduled large-scale wind tunnel test.

Modification of vortex dynamics and transport properties of transitional axisymmetric jets using zero-net-mass-flux actuation

We study the near field of a zero-net-mass-flux (ZNMF) actuated round jet using direct numerical simulations. The Reynolds number of the jet ReD = 2000 and three ZNMF actuators are used, evenly distributed over a circle, and directed towards the main jet. The actuators are triggered in phase, and have a relatively low momentum coefficient of Cμ = 0.0049 each. We study four different control frequencies with Strouhal numbers ranging from StD = 0.165 to StD = 1.32; next to that, also two uncontrolled baseline cases are included in the study. We find that this type of ZNMF actuation leads to strong deformations of the near-field jet region that are very similar to those observed for non-circular jets. At the end of the jet's potential core (x/D = 5), the jet-column cross section is deformed into a hexagram-like geometry that results from strong modifications of the vortex structures. Two mechanisms lead to these modifications, i.e., (i) self-deformation of the jet's primary vortex rings started by distortions in their azimuthal curvature by the actuation, and (ii) production of side jets by the development and subsequent detachment of secondary streamwise vortex pairs. Further downstream (x/D = 10), the jet transforms into a triangular pattern, as the sharp corner regions of the hexagram entrain fluid and spread. We further investigate the global characteristics of the actuated jets. In particular when using the jet preferred frequency, i.e., StD = 0.33, parameters such as entrainment, centerline decay rate, and mean turbulent kinetic energy are significantly increased. Furthermore, high frequency actuation, i.e., StD = 1.32, is found to suppress the mechanisms leading to large scale structure growth and turbulent kinetic energy production. The simulations further include a passive scalar equation, and passive scalar mixing is also quantified and visualized.

Modeling of Electrodynamic Zero-Net Mass-Flux Actuators

electrodynamic zero-net mass-flux (ZNMF) actuator for flow control applications. The electrodynamic actuation schemeemploysamagneticassemblytoestablisharadialmagnetic fieldpatternwithanimbeddedcurrent-carrying coil winding. The coil is attached to a composite diaphragm having a rigid center boss and compliant outer annular region. The magnetic field acting on the coil assembly is modeled using a one-dimensional magnetic circuit. The composite diaphragm is modeled as a transversely isotropic, annular clamped plate. An overall lumped-element electromechanical system model is obtained by combining previously developed lumped models of the cavity/neck structures with lumped models for the composite diaphragm and electrodynamic actuation. The complete lumpedelement model is used to provide insight regarding performance characteristics and design limitations. The model is validated over a frequency range of 10–1000 Hz with hotwire anemometry measurements for both slot and orifice configurations for the jet. The model is useful as a design tool for electrodynamic ZNMF actuators. Nomenclature Amagnet = cross-sectional area of the permanent magnet, m 2 a = outer radius of the diaphragm, m aorif = axisymmetric orifice radius, m Bavg = average magnetic flux density acting on the coil placed in the magnetic assembly, T Bgap = magnetic flux density in the magnet gap predicted by magnetic circuit, T Br = remnant magnetic flux density of the permanent magnet, T b = inner radius of the compliant part of the diaphragm or the radius of the center boss, m CaC = acoustic compliance of the cavity, m 5 =N CaD = acoustic compliance of the diaphragm, m 5 =N CmD = mechanical compliance of the diaphragm, m=N c0 = speed of sound in air, m=s dC = diameter of the cavity, m dslot = diameter of the rectangular slot, m dT = overall diameter of the ZNMF actuator, m Ememb = Young’s modulus of the material of the compliant region of the diaphragm, Pa fN = undamped natural frequency of the diaphragm, Hz fr = steady-state resonant frequency of the diaphragm, Hz f1 = lower 3 dB cutoff frequency, Hz

Improving Safety and Performance of Small-Scale Vertical Axis Wind Turbines

Although horizontal axis wind turbines (HAWT) are considered more efficient in operation than their vertical axis wind turbine (VAWT) counterpart and are more commonly used in wind farms as large wind turbines, the VAWT may offer greater advantages in safety and operation when it comes to their application within the urban environment. Yaw control systems are an essential requirement for the safe operation of HAWT, which are costly and require high levels of maintenance, but are inherently unnecessary for VAWT. At low blade speed ratios, the performance of VAWT degrades owing to strong dynamic stall effects. This necessitates VAWT operation at high blade speed ratios to suppress them. However, the consequent large rotational speeds lead to hazardous operation especially in confined urban areas. Thus to improve the low blade speed performance, a preliminary experimental investigation has been carried out at the Aerodynamics Laboratory of the University of New South Wales on an H-type VAWT blade that employed zero-net mass flux actuation. This technique has traditionally been used for static stall delay and flow separation mitigation on aircraft wings. In the present study, large relative angles of incidence were simulated by sinusoidally oscillating the blade about its quarter-chord, and resulted in the formation of dynamic stall vortices. The application of zero-net mass flux actuation was found to have a beneficial effect on the blade aerodynamic performance by either suppressing dynamic stall or delaying its onset to higher angles of attack. This study, therefore, suggests that reduced oscillatory loads and more robust output power can be achieved with zero-net mass flux actuation on VAWT operating at low blade-speed ratios. Consequently, the findings have positive practical implications for the design of small-scale VAWT for widespread use in the urban environment.

Scaling of pressure drop for oscillatory flow through a slot

Reduced-order modeling and design of zero-net mass-flux actuators require an understanding of several fundamental flow mechanisms governing the performance of these devices. One such aspect is the pressure drop due to oscillatory flow through a slot. In this Brief Communication, we use numerical simulations and experiments to examine physics-based scaling laws for the various mechanisms that determine the net pressure drop, and critically evaluate a proposed phenomenological model for predicting the nonlinear component of the pressure drop in oscillatory slot flows.