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. . 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,
More recently it has been shown by Kercher et al. (2003) that synthetic jets can deliver similar cooling effects without the need for an external air supply system. In a review article by Glezer and Amitay (2002), it was noted that impinging synthetic jets are proving to be an extremely promising technology for use in electronics cooling and also have excellent potential for cooling in manufacturing processes.
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.
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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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.
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%.
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 , 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 . 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 .
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Practically all the above-mentioned synthetic jets applications may be applied in devices of macro- as well as micro-size. In fact, in the microscale or microelectromechanical systems (MEMS) many advantages become more pronounced because of the basic physical principles: scaling down is usually associated with decreasing Reynolds numbers, which complicates mixing as well as convective heat transfer, typically poor in the laminar flow regime. It should be noted that typical development strategy followed by many researchers begins at macro-size and scaled-up laboratory models, the subsequent step (which, of course, may lead to practical micro- fabrication problems and crucial difficulties of operation at small Reynolds numbers) being the scaling down to the final micro-size. Characteristically, early investigations of synthetic jet used macro-size actuators , , , , , , , , , , ,  and , and only recently micro-fabrication in silicon was used by Mallinson et al. .
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Aspects of formation, velocity profiles and applications of synthetic jets have been studied both experimentally and numerically in the last 40 years. They are also referred to as Zero-Net-Mass-Flux (ZNMF) devices due to their unique nature of a zero net mass injection across their system boundary. This makes them a desirable and affordable choice for a variety of applications such as flow control, fluid mixing, mass transfer, thermal management and cooling as indicated in the next paragraphs. Simplistic design is another attractive feature for industrial researchers to examine and explore the behavior of synthetic jets.
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Abstract: This paper develops a millimeter scale fully packaged device in which a vortex flow of high velocity is generated inside a chamber. Under the actuation by a lead zirconate titanate (PZT) diaphragm, a flow circulates with increasing velocity after each actuating circle to form a vortex in a cavity named as the vortex chamber. At each cycle, the vibration of the PZT diaphragm creates a small net air flow through a rectifying nozzle, generates a synthetic jet which propagates by a gradual circulation toward the vortex chamber and then backward the feedback chamber. The design of such device is firstly conducted by a numerical analysis whose results are considered as the base of our experimental set-up. A vortex flow generated in a chamber of the device (named as the votex chamber) was observed by a high-speed camera. The present approach which was illustrated by both the simulation and experiment is potential in various applications related to the inertial sensing, fluidic amplifier and micro/nano particle trapping and mixing. Keywords: vortex flow, synthetic jet, confined space, PZT diaphragm, OpenFOAM
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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
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 , Cater and Soria . 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.
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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Smith and Glezer characterized formation and evolution of synthetic jets  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 .
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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 , ,  and , mainly with the perspective of using their rectification properties in fluidic pumping ,  and . They were later called “synthetic jets” by Glezer et al. , Smith and Glezer , Smith and Glezer , and Glezer and Amitay  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.  and  – 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  and .
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As can be seen in Fig. 2, where the distributions for number of jets in semileptonic and dileptonic events are shown, the signal over background ratio increases requiring a higher number of jets (and also b-jets), according to the ttH topology, where two b-jets are always present and additional jets come from W decays.
Figure 3. Cross-sections for (left) Z+ ≥ 1 b-jet, and (right) Z+ ≥ 2 b-jets. The measurement is shown as a vertical blue line with the inner blue shaded band showing the corresponding statistical uncertainty and the outer green shaded band showing the sum in quadrature of statistical and systematic uncertainties. Comparison is made to NLO predictions from MCFM interfaced to diﬀerent PDF sets and aMC@NLO interfaced to the same PDF set in both the 4FNS and 5FNS. The statistical (inner bar) and total (outer bar) uncertainties are shown for these predictions, which are dominated by the theoretical scale uncertainty. Comparisons are also made to LO multi-legged predictions from ALPGEN+HERWIG+JIMMY and SHERPA; in this case the uncertainty bars are statistical only, and smaller than the marker .
Circular jets impinging vertically on flat surfaces have many practical applications such as in heating, cooling, metal cutting, fabric weaving and cleaning. Most of the experiments on impinging jets have been performed for short stand-off distances, i.e., with an impingement height (H) to nozzle diameter (D) ratio of less than six. Cooper et al. (1993) carried out experiments on a jet impinging on a large plane surface and measured mean and turbulence quantities in different regions of the jet. They considered two Reynolds numbers, 23,000 and 70,000, while the H/D ratio varied from two to ten, with particular focus between two and six. For H/D < 6, researchers have found that the core of the jet is still developing when reaching the plate surface (Nishino et al. 1996; Hadziabdic and Hanjalic 2008, Shademan et al. 2013).
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Thus Corollary 3.14 gives a characterization of good bases and sections. The finite-dimensional Frobenius type structures obtained via Corollary 3.14 are only approximate, that is they are order p jets of families of Frobenius type structures on the bundle K (ζ). Before we may apply Hertling’s result (Theorem 3.1) we need to consider the problem of lifting them to genuine Frobenius type structures. This problem will be solved in the next section. Remark 3.16. The structure on K(ζ ) specified by (∇ r,ζ s , C| K(ζ) , U| K(ζ) , V s ζ ,
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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.