Actuation Amplitude Research Articles

Iterative Learning Active Flow Control Applied to a Compressor Stator Cascade With Periodic Disturbances

This paper presents the capability of iterative learning active flow control to decrease the impact of periodic disturbances in an experimental compressor stator cascade with sidewall actuation. The periodic disturbances of the individual passage flows are generated by a damper flap device that is located downstream of the trailing edges of the blades. The device mimics the throttling effect of periodically closed combustion tubes in a pulsed detonation engine (PDE). For the purpose of rejecting this disturbance, the passage flow is manipulated by fluidic actuators that introduce an adjustable amount of pressurized air through slots in the sidewalls of the cascade. Pressure sensors that are mounted flush to the suction surface of the middle blade provide information on the current flow situation. These data are fed back in real-time to an optimization-based iterative learning controller (ILC). By learning from period to period, the controller modifies the actuation amplitude such that, eventually, a control command trajectory is calculated that reduces the impact of the periodic disturbance on the flow in an optimal manner.

Integrated Line of Sight and Model Predictive Control for Path Following and Roll Motion Control Using Rudder

Roll motion control and path following are two representative marine control problems that have been traditionally treated separately. However, these two problems are closely coupled, as roll motion could cause negative effects on marine surface vessels during path following in seaways and path following actions could cause undesirable roll motion. In this article, an optimal controller is proposed for the integrated path following and roll motion control problem. The rudder, whose actuation amplitude and rate are both limited, is the only control input, while the cross-track error, heading angle, roll rate, and roll angle are the outputs that collectively define the performance of the system. This leads to a classic underactuated problem. Model predictive control is the natural choice for the solution, given its capability in dealing with constraints and multi-input‐multi-output system, and its design will be pursued in this article. Line of sight technique is used to extend the straight-line path following to arbitrary path following. A four degrees of freedom high-fidelity model is implemented as the simulation model, and the simulation results verify the effectiveness of the proposed controller. The influences of the control design parameters on system performance are investigated and the trade-offs between two key attributes, namely, the roll reduction and path following, are explored.

Integrated Line of Sight and Model Predictive Control for Path Following and Roll Motion Control Using Rudder

Roll motion control and path following are two representative marine control problems that have been traditionally treated separately. However, these two problems are closely coupled, as roll motion could cause negative effects on marine surface vessels during path following in seaways and path following actions could cause undesirable roll motion. In this article, an optimal controller is proposed for the integrated path following and roll motion control problem. The rudder, whose actuation amplitude and rate are both limited, is the only control input, while the cross-track error, heading angle, roll rate, and roll angle are the outputs that collectively define the performance of the system. This leads to a classic underactuated problem. Model predictive control is the natural choice for the solution, given its capability in dealing with constraints and multi-input-multi-output system, and its design will be pursued in this article. Line of sight technique is used to extend the straight-line path following to arbitrary path following. A four degrees of freedom high-fidelity model is implemented as the simulation model, and the simulation results verify the effectiveness of the proposed controller. The influences of the control design parameters on system performance are investigated and the trade-offs between two key attributes, namely, the roll reduction and path following, are explored.

Small-Scale Multiaxial Setup for Damage Detection Into the Very High Cycle Fatigue Regime

Micro electro mechanical systems, coatings, and thin films contain materials that are often subjected to complex loading conditions and high frequencies. Therefore, the implementation of a resonant multiaxial fatigue setup for small-scale samples is of high interest. Finite Element simulations have been used to design small-scale samples which possess very close bending and torsional resonant frequencies. The bending and torsional modes are excited by two piezo actuators working either in or out of phase with one another. The bending and torsional amplitudes are measured independently by a laser and the actuator amplitudes are controlled by a Field Programmable Gate Array. The fatigue setup can be used with a varying range of sample sizes from centimeters down to tens of micrometers. The novel multiaxial resonant micro fatigue setup as well as results (fatigue damage for Ni and lifetime for Cu) from bending small-scale fatigue tests are shown and discussed.

Active Control of Rotor–Stator Interaction Noise Using Stator-Mounted Actuators

Active control of rotor–stator interaction noise is studied numerically using a locally oscillating stator surface. A stator of 24 blades inside an infinite coaxial duct is considered. The governing equations are linearized three-dimensional Euler equations and solved with an explicit numerical technique based on a high-order approximation of spatial derivatives. A number of parameters governing actuation-induced sound field and control performance are investigated. It is found that the actuation-induced sound field and noise reduction achieved depend on the actuation amplitude and frequency. The side and position, where actuation is imposed, of the blade as well as actuator geometry are also crucial for the control performance. The downstream-radiated sound power may be reduced by 50% or more over a range of frequencies given an optimum combination of control parameters.

Second-order perturbation of global modes and implications for spanwise wavy actuation

AbstractSensitivity analysis has successfully located the most efficient regions in which to apply passive control in many globally unstable flows. As is shown here and in previous studies, the standard sensitivity analysis, which is linear (first order) with respect to the actuation amplitude, predicts that steady spanwise wavy alternating actuation/modification has no effect on the stability of planar flows, because the eigenvalue change integrates to zero in the spanwise direction. In experiments, however, spanwise wavy modification has been shown to stabilize the flow behind a cylinder quite efficiently. In this paper, we generalize sensitivity analysis by examining the eigenvalue drift (including stabilization/destabilization) up to second order in the perturbation, and show how the second-order eigenvalue changes can be computed numerically by overlapping the adjoint eigenfunction with the first-order global eigenmode correction, shown here for the first time. We confirm the prediction against a direct computation, showing that the eigenvalue drift due to a spanwise wavy base flow modification is of second order. Further analysis reveals that the second-order change in the eigenvalue arises through a resonance of the original (2-D) eigenmode with other unperturbed eigenmodes that have the same spanwise wavelength as the base flow modification. The eigenvalue drift due to each mode interaction is inversely proportional to the distance between the eigenvalues of the modes (which is similar to resonance), but also depends on mutual overlap of direct and adjoint eigenfunctions (which is similar to pseudoresonance). By this argument, and by calculating the most sensitive regions identified by our analysis, we explain why an in-phase actuation/modification is better than an out-of-phase actuation for control of wake flows by spanwise wavy suction and blowing. We also explain why wavelengths several times longer than the wake thickness are more efficient than short wavelengths.

Separated Flow Response to Single Pulse Actuation

The response of a separated flow over a two-dimensional wing to short-duration disturbances from a Lorentz force leading-edge actuator is presented. At a chord Reynolds number of and an angle of attack of , the flow is initially laminar but undergoes transition within the separated shear layer.The transient flow structures and lift force measurements were obtained with varying actuator pulse duration, pulse amplitude, and direction of actuation. The peak amplitude of the lift is shown to depend on the pulse duration when the pulse duration is less than 0.5 convective times, after which it saturates. Saturation of the lift amplitude also occurs when the effective actuator pulse amplitude exceeds . Detailed flow structures that develop in the separated shear layer were identified using the finite-time Lyapunov exponent method. The direction of the actuator pulse has a significant influence on the initial development of the shear layer, but the larger-scale envelope of the separated flow has essentially the same response, irrespective of the direction of actuation. A reconstruction of the velocity field using only three proper orthogonal decomposition modes is sufficient to reproduce the dominant features of the flow response to a single pulse. The second proper orthogonal decomposition mode correlates with the transient lift signal.

Out-of-plane and in-plane actuation effects on atomic-scale friction

The influence of out-of-plane and in-plane contact vibrations and temperature on the friction force acting on a sharp tip elastically pulled on a crystal surface is studied using a generalized Prandtl-Tomlinson model. The average friction force is significantly lowered in a frequency range determined by the ``washboard'' frequency of the stick-slip motion and the viscous damping accompanying the tip motion. An approximately linear relationship between the actuation amplitude and the effective corrugation of the surface potential is derived in the case of in-plane actuation, extending a similar conclusion for out-of-plane actuation. Temperature causes an additional friction reduction with a scaling relation in formal agreement with the predictions of reaction rate theory in the absence of contact vibrations. In this case the actuation effects can be described by the effective energy or, more accurately, by introducing an effective temperature.

Analysis of the cavitating flow induced by an ultrasonic horn – Experimental investigation on the influence of actuation phase, amplitude and geometrical boundary conditions

Till today, factors influencing the formation and collapse of densely distributed, interacting cavitation bubbles are only qualitatively understood. The aim of the present study is to investigate experimentally the influence of selected boundary conditions on the number and size distribution of cavitation bubbles created by an ultrasonic horn (sonotrode). Cavitation bubble clouds below the sonotrode were recorded by means of phase-locked shadowgraphy imaging. The time integrated number of cavitation bubbles was found to decrease exponentially with growing bubble radius. The number of bubbles was increased with growing actuation amplitude and gap width between the sonotrode tip and an opposing solid wall. Furthermore, it could be shown that the number of cavitation bubbles depends on the actuation phase. Future investigations will focus on establishing a statistical relation between the number and size distribution of cavitation bubbles in the near wall region and the resulting cavitation erosion on solid surfaces.

Electro-actuation of biocompatible Pluronic/methacrylic acid hydrogel in blood-plasma and in blood-mimicking buffers

The electro-responsiveness in blood plasma and in blood mimicking fluids of methacrylic acid modified Pluronic (P127) hydrogel is investigated. It is observed that the hydrogel's response to an applied potential, in all buffer solutions, is very high, and comparable in amplitude and time response to classic polyelectrolyte gels. The highest actuation amplitude is achieved in PBS, which is directly related to the largest current value obtained for that buffer. The electro-actuation achieved in blood plasma is comparable to that in KREBS and KCl buffers. The good performance in blood plasma is attributed to the low protein adhesion to hydrogel surface. Preliminary biocompatibility studies demonstrate that the investigated hydrogel can be considered biocompatible.

MoS2 nanoresonators: intrinsically better than graphene?

We perform classical molecular dynamics simulations to examine the intrinsic energy dissipation in single-layer MoS2 nanoresonators, where the point of emphasis is to compare their dissipation characteristics with those of single-layer graphene. Our key finding is that MoS2 nanoresonators exhibit significantly lower energy dissipation, and thus higher quality (Q)-factors by at least a factor of four below room temperature, than graphene. Furthermore, this high Q-factor endows MoS2 nanoresonators with a higher figure of merit, defined as frequency times Q-factor, despite a resonant frequency that is 50% smaller than that of graphene of the same size. By utilizing arguments from phonon-phonon scattering theory, we show that this reduced energy dissipation is enabled by the large energy gap in the phonon dispersion of MoS2, which separates the acoustic phonon branches from the optical phonon branches, leading to a preserving mechanism for the resonant oscillation of MoS2 nanoresonators. We further investigate the effects of tensile mechanical strain and nonlinear actuation on the Q-factors, where the tensile strain is found to counteract the reductions in Q-factor that occur with higher actuation amplitudes. Overall, our simulations illustrate the potential utility of MoS2 for high frequency sensing and actuation applications.

Liquids mixing enhanced by multiple synthetic jet pairs at low Reynolds numbers

In this paper, the effect of three synthetic jet pairs on the mixing between two fluid streams in a planar mixing channel is examined at a net flow Reynolds number of 2. They are mounted transversely to the incoming flow and operated 180° out-of-phase. The PLIF technique is used to provide a visual impression of the effect of synthetic jets on mixing as their operating condition varies, whereas PIV is used to provide the complementary information about the flow structures produced by the synthetic jets and their role in promoting mixing. Our experimental results demonstrate that this inline fluid mixer with a simple design produces an excellent mixing at low Reynolds numbers. It is found that such a good mixing is the result of a significantly increased interfacial area created by these synthetic jet pairs. Based on the degree of mixing deduced from the PLIF data at some distances downstream of the synthetic jets, a functional relationship between the synthetic jet actuation frequency and amplitude is also obtained. This functional relationship can be used for selecting the synthetic jet operating conditions to ensure a good mixing for a scaled version of this fluid mixer.

0305 プラズマアクチュエータによる角柱周り流れ制御の数値シミュレーション(OS3-1 流れの抵抗低減,OS3 流れの抵抗低減)

Flow around a square cylinder controlled by plasma actuators is numerically investigated by means of two-dimensional direct numerical simulation. The Reynolds number based on the cylinder diameter and the freestream velocity is set to be 100. Three different arrangements of plasma actuators on the cylinder surface and two different actuation amplitudes are investigated. The drag and the root-mean-square of lift fluctuations are found to reduce in some cases, accompanying a suppression of vortex shedding. Such an effect is most pronounced in the case where the plasma actuators are installed on the rear surface to induce inward flows.

Design and Fabrication of the Large Thrust Force Piezoelectric Actuator

This paper presents a novel piezoelectric actuator containing double pushers. By using finite element analysis software, this study simulated the vibration mode and amplitude of piezoelectric actuators. The Taguchi method was used to design the parameters of piezoelectric actuators including length, width, height, and electrodes setting. This paper also presents a discussion regarding the influence that the design parameters had on the actuator amplitudes. Based on optimal design parameters, a novel piezoelectric actuator containing double pushers is produced and some thrust tests are also carried out. From the experiment results, the piezoelectric actuator containing double pushers can provide a greater thrust force than that of traditional actuators containing a single pusher as the preload is greater. Comparing with the traditional actuators, the thrust force of new actuator can be increased by 48% with the double preload.

Active flow control applications with a jet and vortex actuator in a laminar cross flow

In this study the effect of an oscillatory, zero-net-mass flux device called a Jet and Vortex Actuator (JaVA) on the laminar boundary layer is investigated. The JaVA can be utilized to energize the boundary layer over a flat plate by creating jets and vortices thus it can delay or prevent boundary layer separation if it is used properly.It appears to produce qualitatively different flow regimes depending on its actuation parameters, e.g. frequency and amplitude or subtle changes in geometry such as the position of the actuator plate with respect to the JaVA-cavity. This latter effect was only discovered recently. Since little is known about the underlying fluid dynamics and because of a complete lack of unsteady data, a device is designed and built for experiments in water. Unsteady flow fields have been recorded for visualization and furthermore quantitative evaluation by means of Particle Image Velocimetry (PIV) has been carried out in addition to numerical simulations. Results show that the JaVA-induced vortices ejected into the flat plate boundary layer significantly enhance the velocity profiles and its characteristics such as the displacement thickness and the momentum thickness if the plate is oscillating at high frequencies as it is flush-mounted or inside the cavity. But if the plate is extracted out of the cavity then there is no improvement in the flow fields hence separation can be delayed or prevented for long downstream distances only if the actuation parameters and plate positions are selected properly.

Directed Drop Transport Rectified from Orthogonal Vibrations via a Flat Wetting Barrier Ratchet

We introduce the wetting barrier ratchet, a digital microfluidic technology for directed drop transport in an open air environment. Cyclic drop footprint oscillations initiated by orthogonal vibrations as low as 37 μm in amplitude at 82 Hz are rectified into fast (mm/s) and controlled transport along a fabricated ratchet design. The ratchet is made from a simple wettability pattern atop a microscopically flat surface consisting of periodic semi-circular hydrophilic features on a hydrophobic background. The microfluidic ratchet capitalizes on the asymmetric contact angle hysteresis induced by the curved features to drive transport. In comparison to the previously reported texture ratchets, wetting barrier ratchets require 3-fold lower actuation amplitudes for a 10 μL drop, have a simplified fabrication, and can be made optically flat for applications where transparency is paramount.

Numerical Investigation of Instabilities in Free Shear Layer Produced by NS-DBD Actuator

A numerical investigation of the effects of nanosecond barrier discharge on the stability of a two-dimensional free shear layer is performed. The computations are carried out using a compressible Navier-Stokes algorithm coupled with a thermodynamic model of the discharge. The results show that significant increases in the shear layer’s momentum thickness and Reynolds stresses occur due to actuation. Dependence on both frequency and amplitude of actuation are considered, and a comparison is made of the computed growth rates with those predicted by linear stability theory. Amplitude and frequency ranges for the efficient promotion of shear-layer instabilities are identified. Keywords—NS-DBD, plasma, actuator, flow control, instability, shear layer

Dynamics of Purcell’s three-link microswimmer with a passive elastic tail

One of the few possible mechanisms for self-propulsion at low Reynolds number is undulations of a passive elastic tail, as proposed in the classical work of Purcell (1977). This effect is studied here by investigating a variant of Purcell's three-link swimmer model where the front joint angle is periodically actuated while the rear joint is driven by a passive torsional spring. The dynamic equations of motion are formulated and explicit expressions for the leading-order solution are derived by using perturbation expansion. The dependence of the motion on the actuation amplitude and frequency is analyzed, and optimization with respect to the swimmer's geometry is conducted.

Enhancement of heat transfer coefficients by actuation against an impinging jet

Recent technological developments have lead to significant increase in the generated heat by electronic and optical components. The removal of high heat fluxes can be successfully treated by several methods, e.g. impinging jets. Further improvement is offered by incorporating arrays of jets or causing jets to pulsate. The research reported herein introduces a new method which is based on actuation of a slab against a two dimensional steady, impinging, laminar, liquid micro-jet. This leads to enhanced heat transfer in the wall region of the jet. An experimental setup which included a piezoelectric (PZT) actuator, a dedicated silicon chip and a steady, slot, impinging jet, was assembled. Using a high speed infrared (IR) radiometer, the cooling process of the chip was recorded and the heat transfer enhancement values were determined for normalized actuation amplitudes, Reynolds and Strouhal numbers in the ranges of 0.45<δ<0.75, 756<Re<1260 and 0<St<0.052, respectively. It was experimentally found that heat transfer coefficients were enhanced by up to 34%.

Hollow Square- and Ring-Plate MEMS Oscillators Embedded in a Phase-Locked Loop for Low Limit of Detection in Liquid

This letter demonstrates hollow plate microelectromechanical-system oscillators implemented into a digital phase-locked loop (PLL). The resonators feature innovative geometries: hollow square and ring plates, operating in Lamé or Kirkhope modes, respectively, with resonance frequencies comprised between 24 and 78 MHz. Advantageously, the ring-plate resonator presents better frequency stability when compared with its counterpart square plate. The PLL provides a real-time monitoring of the oscillators' frequency shift, which was tested by changing the electrostatic actuation amplitude. Ring-plate resonators show sensitivity between 38 and 232 Hz/V. Finally, the feedback loop was operated with a square-plate device filled with an aqueous solution demonstrating a sensitivity of up to 284 Hz/V.