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Development and Optimization of Synthetic Jets to Control Active Flow

Development and Optimization of Synthetic Jets to Control Active Flow

The primary goal of this project and this manuscript has been to investigate the effect of active flow control on the wake of a circular cylinder at low Re. To that end, numerical investigations were performed in Fluent, based on a DNS approach, and results were analysed through Matlab, Tecplot and VIP_R. The active flow control tested consisted of 2D and 3D forcing. These forcing brought altogether new insights on the active control field. First the vortex formation process was dramatically altered at Re=300 when the exact mode A instability wavelength forcing was applied. As a consequence the drag and lift coefficient exhibits no more oscillation. Secondly synthetic jets were proved to
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A Comparison between steady jets and synthetic jets at low Reynolds numbers

A Comparison between steady jets and synthetic jets at low Reynolds numbers

In the above graphs at an H/D of 1 the magnitude of the steady jet heat transfer rate is up to 3 times that of the synthetic jet; this is largely due to confinement and recirculation effects on the synthetic jet. Recirculation effects can be seen in figure 2a reaching a radial distance of approximately 2.2, this was shown by McGuinn et al. (2008) using Particle Image Velocimetry to confirm that these recirculation zones do exist for synthetic jets at an H/D = 1.

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Active control of continuous air jet with bifurcated synthetic jets

Active control of continuous air jet with bifurcated synthetic jets

Abstract: The synthetic jets (SJs) have many significant applications and the number of applications is increasing all the time. In this research the main focus is on the primary flow control which can be used effectively for the heat transfer increasing. This paper deals with the experimental research of the effect of two SJs worked in the bifurcated mode used for control of an axisymmetric air jet. First, the control synthetic jets were measured alone. After an adjustment, the primary axisymmetric jet was added in to the system. For comparison, the primary flow without synthetic jets control was also measured. All experiments were performed using PIV method whereby the synchronization between synthetic jets and PIV system was necessary to do.
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Visualization of synthetic jets at higher Stokes numbers

Visualization of synthetic jets at higher Stokes numbers

Abstract: Visualization of synthetic jets at higher Stokes numbers (S = 90 and 127) by the phase-locked smoke-wire technique is presented and discussed. The working fluid is air. The Reynolds numbers are quantified using hot-wire anemometry. Although our method of visualization essentially provides only qualitative results, the present study also demonstrates some quantitative results, namely the behavior of the zero-net-mass-flux jet near its critical stage. Visualization of the sub-critical stage is also shown.

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Experimental investigation into flow in an ejector with four synthetic jets

Experimental investigation into flow in an ejector with four synthetic jets

Abstract: The article deals with experimental investigation into flow in an ejector with four synthetic jets. The aim of the synthetic jets is to excite the mixing layer in the ejector and intensify the mixing process. The cavities of the synthetic jet actuators are hidden in the mixing chamber wall and the synthetic jets are perpendicular to the ejector axis. CTA and pneumatic measuring method were used to investigate the influences of synthetic jets on flow inside the ejector.

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Flow control in axial fan inlet guide vanes by synthetic jets

Flow control in axial fan inlet guide vanes by synthetic jets

Abstract. Tested high pressure axial flow fan with hub/tip ratio of 0.70 and external diameter of 600 mm consisted of inlet guide vanes (IGV), rotor and stator blade rows. Fan peripheral velocity was 47 m/s. Air volume flow rate was changed by turning of rear part of the inlet guide vanes. At turning of 20 deg the flow was separated on the IGV profiles. The synthetic jets were introduced through radial holes in machine casing in the location before flow separation origin. Synthetic jet actuator was designed with the use of a speaker by UT AVCR. Its membrane had diameter of 63 mm. Excitation frequency was chosen in the range of 500 Hz – 700 Hz. Synthetic jets favourably influenced separated flow on the vane profiles in the distance of (5 – 12) mm from the casing surface. The reduction of flow separation area caused in the region near the casing the decrease of the profile loss coefficient approximately by 20%.
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Numerical and experimental studies of a channel flow with multiple circular synthetic jets

Numerical and experimental studies of a channel flow with multiple circular synthetic jets

Laminar flow and heat transfer in ducts has been frequently studied in the past [1, 2]. Recently, an importance of this topic arose in the case of microchannels of the microelectromechanical systems (MEMS) [3, 4]. An important consequence is that the transport processes under low Reynolds numbers are limited by low rate of laminar (Fickian) gradient diffusion processes. An intensification of these processes can be achieved by an excitation of flow fields by superimposed oscillations. In the present study, an active flow control of the main laminar channel flow by means of synthetic jets (SJs) [4–7] is used. It is known fact that the SJs can be used in various applications typically aimed at the flow control or at heat and mass transfer enhancement [8, 9]. In order to investigate the effects of synthetic jet interaction with cross flow in micro- channel for the cooling of microchips, a three-dimensional computational model was recently developed by Timchenko et al. [10, 11] and Lee et al. [12]. To account for the deflection of the membrane located at the bottom of the actuator cavity, a novel moving mesh algorithm has been adopted to solve the flow and heat transfer. On the other hand,
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Experimental Study of Synthetic Jets

Experimental Study of Synthetic Jets

The early research of Smith and Swift [25] showed a resemblance in characteristic of low Reynolds number synthetic jets and high Reynolds number continuous jets. Later, they compared synthetic jets and continuous jets and explored the effect of dimensionless parameters on synthetic jets [1]. It was discovered that although synthetic jets and continuous jets have the same profile, synthetic jets grow more rapidly in terms of jet width and volume flux. Their research is the motivation of this work. A rectangular slot with cross-stream width of 75 mm was chosen for the experiment. Measurements were performed in air using a single hot-wire. Oscillations were generated using a set of eight loudspeakers in the frequency range of 10 to 100 Hz. Velocity amplitudes of up to 50 m/s were achieved at the exit plane. U o was used to define Reynolds number for synthetic jets ( Re 4 R S T
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Enhancement of synthetic jets by means of an integrated valve-less pump Part II. Numerical and experimental studies

Enhancement of synthetic jets by means of an integrated valve-less pump Part II. Numerical and experimental studies

The name “synthetic jets” was derived from the idea of their being produced or synthesized from the train of individual vortical structures (often of the character of vortex rings) generated by periodic alternating flows into and out from a nozzle. This inflow and outflow is produced by periodic alternating displacement and ingestion of fluid from and into the nozzle. In a typical layout, a sealed displacement cavity is provided with the nozzle at its one end and an actuating piston or diaphragm at the other end, the periodic movements of which result in periodic variations of the available volume inside the cavity. There is no time-mean mass flux of fluid into the cavity and hence also through the nozzle. This is why another common description of the generated flowfield is zero-net-mass-flux jet. The positive jet-like flow away from the nozzle is generated due to the non-linearity of the governing hydrodynamic equations, which makes the ingestion part of the flow cycle different from the outflow part. The time-mean mass flow rate is, of course, zero only in the nozzle exit cross section. With progressing distance from the nozzle the time-mean mass flux increases in a similar manner as in standard steady jets. The difference between the inflow and outflow phases (the outflow reaching farther because of the inertia gained inside the nozzle) may be described as a form of rectification and in fact it is related to the rectification phenomena used in fluidic pumping where it is used to replace the conventional suction and displacement one-way valves.
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Valves with flow control by synthetic jets

Valves with flow control by synthetic jets

Characteristic feature of present-day fluid mechanics is shifting the emphasis of analysed problems. Earlier, the efforts were directed towards understanding the fluid flows. Today, the task is increasingly often to control the flow and change its properties. Another characteristic aspect of the development is a change in the means by which the control action is applied. Traditionally, the flow control was made by mechanical actuators, usually inserted into the flow – such as various flaps or agitators. Recently, preference is typically given – because of lower price, higher reliability, long lifetime, and robustness with which the actuator device withstand adverse conditions - to actuators acting on the controlled boundary layer by fluid jets. Available comparisons show unequivocal effectiveness advantage of periodic jets over their steady counterparts and this development towards higher unsteady component of the jet flow has resulted in the recent interest concentrating on the case of synthetic jets [2], with the purely alternating nozzle flow – i.e. zero time-mean flow component, Fig. 1. The idea is not altogether new. In association, e.g., with the attempts at power transfer by alternating flow it was found possible to perform the rectification of the alternating flow into the steady output by the innate nonlinear properties of jet flows [3]. It is this rectification effect that produces the synthetic jet (in the sense of being synthesized from the individual vortex ring generated at each outflow part of the period) as a flow applicable for the action suppressing boundary layer separation or control transition into turbulence [1, 4] or perhaps even decrease turbulent friction drag by suppressing the hairpin vorteces in turbulence. Nevertheless, the real impetus behind the current popularity of the studies of synthetic jets was by the work of Professor Glezer as discussed in [2].
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Vortex flow generator utilizing synthetic jets by diaphragm vibration

Vortex flow generator utilizing synthetic jets by diaphragm vibration

A millimeter scale fully packaged device which generates a vortex flow of high velocity is reported. The synthetic jet flow actuated by a PZT diaphragm whose velocity increases after each circulation forms a vortex in a desired chamber. The design of the device is firstly conducted by a numerical analysis whose results are referred as the base of our experiment. Experimental results are in good agreement with our numerical prediction and a flow vortex is observed by a high-speed camera. Both the numerical and experimental results demonstrate the potential of the device in various applications related to inertial sensing, fluidic amplifier and micro/nano particle trapping and mixing.
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Experimental study of impinging flow generated by synthetic jets actuators

Experimental study of impinging flow generated by synthetic jets actuators

Abstract: The paper describes experimental research of the flow impinging on heated plate.. The flow was generated by synthetic jet actuators.[r]

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No-moving-part hybrid-synthetic jet actuator

No-moving-part hybrid-synthetic jet actuator

Synthetic jets are formed by a periodically alternating inflow into and outflow from a nozzle. They were investigated by the present principal author already more than a quarter of a century ago [4], [6], [7] and [9], mainly with the perspective of using their rectification properties in fluidic pumping [4], [6] and [23]. They were later called “synthetic jets” by Glezer et al. [14], Smith and Glezer [15], Smith and Glezer [18], and Glezer and Amitay [19] the term suggesting their being “synthesised” from individual vortex rings, although this character is there only in a certain range of operating conditions. Synthetic jet actuators are a particularly attractive idea in the context of small scale, microelectromechanical systems (MEMS) – e.g. [17] and [29] – because scaling down is usually associated with decreasing Reynolds number, which decreases efficiency of mixing as well as of convective heat transfer in steady flows. The pulsation associated with of the synthetic jet flows agitates the flowfield and produces effects similar to those of turbulent convection. There are, in fact, some experimental data suggesting that synthetic jets are capable of achieving extreme magnitudes of the thermal power transfer density, unobtainable by any other means, because the pulsation can destroy or reduce the thin insulating layer of stagnant fluid, held at the wall in steady flows. In spite of its minute thickness, this layer has the essential limiting influence on the achievable heat transfer rate, because heat has to cross it by the very ineffective conduction mechanism [24] and [30].
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Active Stall Control System on NACA0012 by Using Synthetic Jet Actuator

Active Stall Control System on NACA0012 by Using Synthetic Jet Actuator

DOI: 10.4236/jfcmv.2019.71005 68 Journal of Flow Control, Measurement & Visualization It is seen from Figure 10(a) that the leading edge vortices are not attached to the upper surface of the airfoil. A periodic excitation produced by the synthetic jets occurs in the separated shear layer under control (Figure 10(b)), and vortex growth occurs in the separated shear layer from the leading edge under control during the blowing phase. In Figure 10(b), with synthetic jets acting for control, the flow has less of tendency to separate, and Figure 10(b) shows a reduction in the tendency toward stall when compared with the no control case. The instabil- ity in the separated shear layer is promoted, and a large-scale vortex attaches to the airfoil surface (Figure 10(c) and Figure 10(d)). After that time, the attached vortex is elongated toward the orifice during the suction phase, and the attach- ing region expands (Figure 10(e)). These results suggest that vortex formation and evolution is responsible for enhanced boundary layer mixing of the airfoil upper surface.
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Experimental investigation of a control synthetic jet

Experimental investigation of a control synthetic jet

Synthetic jet (SJ) is generated using an oscillating element in a cavity. Fluid is sucking in and pushing out of the cavity through an orifice – Smith and Glezer [3], Cater and Soria [4]. This is a special case of pulsating flow with zero net mass flux in the actuator exit. Oscillating element could be a diaphragm or piezoelectric material. Following these characteristics we can point out the advantages of synthetic jets – it is possible to control them rather easily using an electronic system and we do not need any supply pipe. For these reasons a flow control by means of SJs can be utilized in many promising applications.
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A general correlation for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet

A general correlation for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet

This is considered to be an appropriate dimensionless group to identify flow and heat transfer regimes in impinging synthetic jets, since the stroke length along with the Reynolds number[r]

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Exploration into the feasibility of underwater synthetic jet propulsion

Exploration into the feasibility of underwater synthetic jet propulsion

Smith and Glezer characterized formation and evolution of synthetic jets [25] with a rectangular orifice geometry (using air as the working fluid). They determined the near field (close to the orifice) evolution of synthetic jet flow to be dominated by the formation, expulsion, and advection of discrete vortices. These vortex rings eventually transition to turbulence, slow down, and lose their coherence. During the outstroke of the membrane, fluid rolls up into a vortex pair (or ring in the case of a circular orifice) which travels away from the orifice at a self-induced velocity. They found that the in-stroke of the membrane seemed to trigger the transition of the vortex pairs to turbulence, possibly due to the core instabilities associated with the reversal of the streamline velocity near the orifice plane [25, 39]. They also noted the formation of secondary vortical structures wrapped around the cores of the primary vortices. They suggested that these lead to the breakdown of the primary vortices [25].
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CFD Simulation of Impinging Jet Flows and Boiling Heat Transfer

CFD Simulation of Impinging Jet Flows and Boiling Heat Transfer

Impinging jets have many practical applications in cooling, heating, metal cutting and industrial cleaning. They include different types of flows such as free jet flow, stagnating flow and a wall jet (see Fig. 1.1). In an impinging jet, flow exiting from the nozzle interacts with the ambient flow and due to the Kelvin- Helmholtz instabilities a street of roll-up vortices is generated. There is a frequency for the generation of these vortices which is dependent on different parameters such as boundary conditions, nozzle geometry and Reynolds number. While traveling towards the plate, these vortices interact, break up, pair and coalesce with neighbouring vortices and their symmetrical shape is lost. This results in an unsteady three-dimensional behaviour for pressure and shear stresses in the impingement zone and a vorticity field in the entire domain. The heat transfer from the plate is also influenced by these unsteady three- dimensional structures approaching the plate.
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The Innermost Regions of Relativistic Jets: Wrapping Up the Enigma

The Innermost Regions of Relativistic Jets: Wrapping Up the Enigma

Our knowledge of the positively charged particles inside jets is quite limited. Positrons might greatly outnumber protons, or vice versa. If protons dominate, do they hold ∼ 100 times the energy density as electrons, are they in equipartition, or do they even have less energy than the electrons? In his review, Markus Böttcher reported that the SEDs of blazars can be fit as well by hadronic models as by leptonic models. There is a high cost, though: a factor of ∼ 100 in energy. This seems too much if blazars have an energy crisis, but could be reasonable in objects with high accretion rates. And the factor of ∼ 100 is similar to the ratio of the proton to electron energy densities in Galactic cosmic-rays. On the other hand, Apostolos Mastichiadis and Maria Petropoulou expressed difficulty in their efforts to produce light curves similar to PKS 2155-304 with a hadronic model, although they will continue to try.
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Standard Model measurements with the ATLAS detector

Standard Model measurements with the ATLAS detector

The successful operation of the Large Hadron Collider (LHC) during the past three years allowed to explore particle interaction in a new energy regime. Various Standard Model measurements have been performed in proton-proton collisions at a centre-of-mass energy of √ s = 7 and 8 TeV using the ATLAS detector [1] at the LHC. These allow for precision tests of the electroweak dynamics of the Standard Model, but also challenge next-to-next-to-leading-order predictions. Di ff erences between measurements and Standard Model predictions could prove evidence for new phenomena. Recent Quantum Chromo Dynamics (QCD) results highlight the importance of precision QCD measurements for improving state-of-the-art theoretical tools and searching for new physics. A selection of the latest results of electroweak measurements, W/Z production in association with jets, jet physics and soft QCD is reported. These consist of the measurement of the WW cross section, first evidence of the electroweak production of W ± W ± j j, and electroweak Z j j production. Measurements are in general found to be well described by the Standard Model predictions. Measurements of differential produc- tion cross sections of a Z boson in association with b-jets, the production cross section of the W boson in association with jets as well as the ratio of the production cross sections for W and Z bosons in association with jets are presented. The measurements are compared to next-to-leading-order pertur- bative QCD calculations and to predictions from different Monte Carlo (MC) generators implement- ing leading-order matrix elements supplemented by parton showers. Various inclusive, differential or double differential jet measurements are presented. The dominant experimental uncertainty on these measurements comes from the jet energy scale. Next-to-leading order QCD calculations corrected to account for non-perturbative e ff ects are compared to the measurements. A good agreement between the data and the theoretical predictions based on most of the global parton distribution functions is found over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values. A measurement of charged-particle distributions sensitive to the properties of
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