Effect Of Actuation Frequency Research Articles

Moving Surface Actuation, Effects of Frequency-based Shear Layer Excitation on the Response of a Bluff Body Wake

The effect of actuation frequency, using moving surface actuation, is investigated for a square cylinder bluff body wake. Pressure sensor data are used to optimize actuation characteristics through the implementation of an NSGA-II evolutionary algorithm. Velocity field data are obtained using Particle Image Velocimetry (PIV) for baseline and optimized actuation cases. A Proper Orthogonal Decomposition (POD) analysis shows that the vortex shedding frequency shifts between frequencies associated with the actuation, moving between regions of lock-on and quasi-periodicity. Additionally, the POD shows that the energy contained in the coherent shedding motion is reduced through actuation, while the total energy in the velocity field stays relatively constant. A reconstruction of the first 10 POD modes indicates that the coherent contribution to the Reynolds stresses significantly decreases compared to the non-actuated case. The mechanism for drag reduction is investigated using the shed circulation flux and Kochin’s drag formulation model. The drag obtained using PIV measurements and Kochin’s formulation is consistent with trends observed for the base pressure as a function of actuation frequency.

Experimental development of a Nusselt correlation for forced reciprocating oscillated vertical annular glycerol flow through a porous domain

The objective of this paper is to investigate the heat transfer to oscillating annular flow of a viscous fluid. The flow media includes stationary stainless steel wool porous domain and glycerol as the working fluid. The effects of actuation frequency and wall heat flux on the temperature field and resultant heat convection coefficient are studied. The temperature values at radial direction are close each other as porous media mixes the glycerol successfully. A correlation with a functional dependence to kinetic Reynolds number is recommended that can be used to acquire the averaged heat transfer for oscillating flows. Present experimental results with glycerol in a porous media are compared to the published experimental works with water. For the limited case of the two working fluids, Nusselt number is normalized well using the Prandtl number (Pr0.67). Results are also compared to non-porous media study and heat transfer is found to increase up to a factor of five in porous media. The recommended correlation is claimed to have a significant role for anticipating heat transfer of oscillating viscous fluid not only at low frequencies but also at low heat fluxes in a porous and permeable solid media.

Impact of ns-DBD plasma actuation on the boundary layer transition using convective heat transfer measurements

This paper demonstrates that the impact of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators on the structure of the boundary layer can be investigated using quantitative convective heat transfer measurements. For the experiments, the flow over a flat plate with a C4 leading edge thickness distribution was examined at low speed incompressible flow (6.6–11.5 m s−1). An ns-DBD plasma actuator was mounted 5 mm downstream of the leading edge and several experiments were conducted giving particular emphasis on the effect of actuation frequency and the freestream velocity. Local heat transfer distributions were measured using the transient liquid crystal technique with and without plasma activated. As a result, any effect of plasma on the structure of the boundary layer is interpreted by local heat transfer coefficient distributions which are compared with laminar and turbulent boundary layer correlations. The heat transfer results, which are also confirmed by hot-wire measurements, show the considerable effect of the actuation frequency on the location of the transition point elucidating that liquid crystal thermography is a promising method for investigating plasma-flow interactions very close to the wall. Additionally, the hot-wire measurements indicate possible velocity oscillations in the near wall flow due to plasma activation.

Effects of Actuation Frequency on Flow Control Applied to a Wall-Mounted Hump

Active flow control, and particularly unsteady mass injection (synthetic jets), promotes reattachment of naturally separated flows and can increase lift and decrease drag on airfoils at high angles of attack. Synthetic-jet actuators are typically characterized by the unsteady momentum coefficient =(ρ_s h_s)/(0.5ρ_(∞)U^2_(∞)c) and the actuation frequency F^(+)= fX_sep/U_∞, where ρ_s and u_s are the density and velocity of the injected fluid, respectively; h_s is the slot velocity; f is the actuation frequency; and X_sep is the separation distance or chord length, depending on the author or application. Actuation frequencies are often chosen to be on the order of the natural large-scale shedding frequency, or F^(+) ~ O(1). At this frequency, large-scale vortex shedding is induced, which increases the entrainment rate and promotes deflection of the separated shear layer toward the surface. Recent investigations have looked at a second regime of actuation frequencies, which are typically an order of magnitude higher than the most dominant natural frequency in the separated flow and are designed to excite Kelvin–Helmholtz instabilities in the boundary layer. In the airfoil experiments of Amitay and Glezer and Glezer et al., high-frequency excitation was found to be more effective in improving the aerodynamic performance than actuation at lower frequencies by increasing the suction force immediately after the leading edge actuation location. Simulations performed by Visbal and Visbal et al. have shown that plasma-based actuation pulsed at frequencies in the range of F^(+) = 4–8 are effective in promoting laminar-turbulent transition and suppressing separation. However, when applied to a fully turbulent boundary layer, the plasma actuation required significantly more power to achieve a reduction in separation bubble size. Other airfoil simulations at low or transitional Reynolds number have found that actuation close to the fundamental shear-layer frequency is ineffective, and forcing closest to the natural shedding frequency is more optimal; however, such two-dimensional simulations may not be accurately capturing the three-dimensional flow physics. Similar results are seen in the numerical experiments by Dandois et al. on a backward-facing step, in which the separation bubble length increased with high-frequency forcing. Here, we briefly report results for a compressible, large-eddy simulation (LES) of flow past a wall-mounted hump at high Reynolds number where oscillatory control is applied at F^(+) ~ O(10). The LES with F^(+) ~ O(1) was previously validated and found to compare favorably with extensive experimental results of this geometry. Many other computational results in the low-frequency regime were reported in the NASA Langley Computational Fluid Dynamics Validation workshop. We simulate the flow at Re_c = 500,000 and Mach numbers of 0.25 and 0.6, the latter of which is significantly higher than previous simulations of high-frequency actuation.

Multifield analysis of a piezoelectric valveless micropump: effects of actuation frequency and electric potential

Coupled multifield analysis of a piezoelectrically actuated valveless micropump device is carried out for liquid (water) transport applications. The valveless micropump consists of two diffuser/nozzle elements; the pump chamber, a thin structural layer (silicon), and a piezoelectric layer, PZT-5A as the actuator. We consider two-way coupling of forces between solid and liquid domains in the systems where actuator deflection causes fluid flow and vice versa. Flow contraction and expansion (through the nozzle and the diffuser respectively) generate net fluid flow. Both structural and flow field analysis of the microfluidic device are considered. The effect of the driving power (voltage) and actuation frequency on silicon-PZT-5A bi-layer membrane deflection and flow rate is investigated. For the compressible flow formulation, an isothermal equation of state for the working fluid is employed. The governing equations for the flow fields and the silicon-PZT-5A bi-layer membrane motions are solved numerically. At frequencies below 5000 Hz, the predicted flow rate increases with actuation frequency. The fluid–solid system shows a resonance at 5000 Hz due to the combined effect of mechanical and fluidic capacitances, inductances, and damping. Time-averaged flow rate starts to drop with increase of actuation frequency above (5000 Hz). The velocity profile in the pump chamber becomes relatively flat or plug-like, if the frequency of pulsations is sufficiently large (high Womersley number). The pressure, velocity, and flow rate prediction models developed in the present study can be utilized to optimize the design of MEMS based micropumps.

The effect of actuation frequency on the plasma synthetic jet

The flow induced by plasma synthetic jet actuator was simulated through solving the Reynolds-averaged Navier-Stokes equations augmented by body force phenomenological plasma model. The effect of actuation frequency on the plasma synthetic jet was examined by case study. The numerical results present that with the actuation frequency increasing, the stream-wise distance of the adjacent vortex pairs induced by the actuator decreases monotonically, which is the same as the situation of the velocity fluctuations field caused by the vortex pairs. When the actuation frequency is 60 Hz, the vortex pairs formed during the adjacent actuation periods merge together quickly, and the flow structure in the downstream region is more close to that of the steady case. The actuation frequency has no visible influence on the time-averaged flow field of plasma synthetic jet. However, when the actuation frequency is relatively low (f<40 Hz), the momentum flux close to the actuator increases with the actuation frequency increasing, which is contrary to the situation in the far field from the wall.

Generating Nanoparticles for Respiratory Drug Delivery

Generating liquid droplets is ideal for many applications including respiratory drug delivery because the droplets have uniform properties and can be easily controlled, sampled, and analyzed. In this study, a micropump-based droplet generator is proposed to produce the liquid droplets of micron to nano size. Numerical simulations were carried out to evaluate the ability of the proposed droplet generator device to produce liquid droplets. The velocity and diameter of the droplets generated by the droplet generator device were calculated, and the performance of the device’s flow rate and power consumption was evaluated. The effects of actuation frequency, actuation modes, and nozzle geometry on the performance of the device were investigated. Results showed that the proposed device can produce micron-/nano-sized liquid droplets with low power and the advantages of the proposed droplet generator device over traditional devices were discussed.

Drug Particle Delivery Investigation Through a Valveless Micropump

Micropumps with various types of actuations are being used in microfluidic transport for liquid drug delivery. Due to the complexity of the flow field, particle transport through a valveless micropump might be challenging in comparison to a pressure-driven flow micropumps. In order to better understand and develop an optimized design for the delivery of drug particles through valveless micropumps, computational simulations may be necessary. In this paper, the transport of drug particles through the valveless micropump is simulated through 3-D computational fluid dynamics combined with discrete particle transport methods. After computational validation, the effects of actuation frequency, particle size, and transporting style on the particle transport are investigated. Both the actuation frequency and transporting pattern have a strong relationship in terms of resident times and the spatial distribution of the transported particles through the designed micropump. The computational analysis results presented demonstrate that it is possible to optimize the proposed valveless micropump design through numerical simulations for specific delivery of drug particles.

Transport of drug particles in micropumps through novel actuation

Micropumps with various types of actuations have been used in lab-on-a-chip devices. In order to control the delivery of drug particles both in space and time and avoid clogging, other types of actuation mechanisms may be needed. In this study, a valveless micropump with novel actuation is proposed to transport particles for biomedical and environmental applications. The transport of drug particles through the designed valveless micropump is carried out through computational fluid dynamics combined with discrete particle transport methods. After convergence studies, the effects of actuation frequency, particle size and the resident times on the particle transport are investigated. Interestingly, both the actuation frequency and particle size have a strong effect in terms of resident times and the spatial distribution of the transported particles through the designed micropump. Based on the results obtained, the relationship between actuation frequency, fluid flow, and particle transport through the designed micropump is presented. The computational analysis presented demonstrates that it is possible to optimize the proposed valveless micropump design for specific delivery of drug particles for separation and sorting applications.