Flow Characteristics Of Synthetic Jet Research Articles

Experimental investigation of flow and thermal characteristics of synthetic jetissuing from sharp-edged orifices

ABSTRACT The present experimental study reports the flow and heat transfer characteristics of a synthetic jet issuing from a sharp-edged orifice (diverging-shaped orifice). The experiments are carried out for a varied range of opening angles of sharp-edged orifices (θ = 0°, 30°, 60°, 90°, and 120°), Reynolds number (Re = 3243–8143), different jet-to-surface spacings (z/d = 1–16), and for two different values of orifice thicknesses, t = 5 mm (t/d = 0.33) and 10 mm (t/d = 0.66). The hot-wire anemometry is used to study the flow characteristics of synthetic jet, while heat transfer characteristics are studied by using a thermal imaging technique. The time-averaged flow fields associated with sharp-edged orifices reveal that orifices with t = 5 mm and 10 mm exhibit saddle-backed and top-hat velocity profile shapes, respectively. The results show that for a square-edge orifice (θ = 0°), the heat transfer rate decreases with an increase in orifice plate thickness from 5 to 10 mm, while the opposite trend in heat transfer is observed with sharp-edged orifice. The heat transfer rate with a 10 mm thick sharp-edged orifice is higher than the 5 mm thick sharp-edged orifice for all the tested opening angles. Furthermore, the results also show that for sharp-edged orifices, the heat transfer rate increases with the increase in opening angle from θ = 0° to 60°, while it decreases with further increasing from θ = 60° to 120°. The maximum value of average Nusselt number (Nuavg) is obtained for θ = 60° for both the orifice thicknesses (t = 5 and 10 mm), and this effect is found to be more pronounced for t = 10 mm orifice. For sharp-edged orifice (θ = 60°), the maximum enhancement in Nuavg is found to be 12.66% and 23% higher for t = 5 mm and 10 mm, respectively, compared to the equivalent square-edged orifice (θ = 0°). The cause for variation in heat transfer rate with sharp-edged orifices is interpreted due to the effect of flow recirculation and mass flow rate. A correlation has been proposed for Nuavg as a function of different opening angles. The present finding is useful for the optimization of the synthetic jet geometrical parameters for the effective heat transfer rate.

Flow characteristics of two-dimensional synthetic jets under diaphragm resonance excitation

Purpose This paper aims to experimentally study the external flow characteristic of an isolated two-dimensional synthetic jet actuator undergoing diaphragm resonance. Design/methodology/approach The resonance frequency of the diaphragm (40 Hz) depends on the excitation mechanism in the actuator, whereas it is independent of cavity geometry, excitation waveform and excitation voltage. The velocity response of the synthetic jet is influenced by excitation voltage rather than excitation waveform. Thus, this investigation selected four different voltages (5, 10, 15 and 20 V) under the same sine waveform as experiment parameters. Findings The velocity field along the downstream direction is classified into five regions, which can be obtained by hot-wire measurement. The first region refers to an area in which flow moves from within the cavity to the exit of orifice through the oscillation of the diaphragm, but prior to the formation of the vortex of a synthetic jet. In this region, two characteristic frequencies exist at 20 and 40 Hz in the flow field. The second region refers to the area in which the vortices of a synthetic jet fully develop following their initial formation. In this region, the characteristic frequencies at 20 and 40 Hz still occur in the flow field. The third region refers to the area in which both fully developed vortices continue traveling downstream. It is difficult to obtain the characteristic frequency in this flow field, because the mean center velocities (ū) decay downstream and are proportional to (x/w)−1/2 for the four excitation voltages. The fourth region reveals variations in both vortices as they merge into a single vortex. The mean center velocities (ū) are approximately proportional to (x/w)0 in this region for the four excitation voltages. A fifth region deals with variations in the vortex of a synthetic jet after both vortices merge into one, in which the mean center velocities (ū) are approximately proportional to (x/w)−1 in this region for the four excitation voltages (x/w is the dimensionless streamwise distance). Originality/value Although the flow characteristics of synthetic jets had reported for flow control in some literatures, variations of flow structure for synthetic jets are still not studied under the excitation of diaphragm resonance. This paper showed some novel results that our velocity response results obtained by hot-wire measurement along the downstream direction compared with flow visualization resulted in the classification of five regions under the excitation of diaphragm resonance. In the future, it makes valuable contributions for experimental findings to provide researchers with further development of flow control.

Experimental investigation on thermal and flow characteristics of synthetic jet with multiple-orifice of different shapes

The impingement heat transfer characteristics of a multiple-orifice synthetic jet of circular, oval and diamond orifice shapes are investigated experimentally at different Stokes number, normalized axial distance and pitch ratio. In this study, particle image velocimetry (PIV) based measurements are also undertaken to better understand the underlying flow characteristics of multiple-orifice synthetic jet and to support the associated heat transfer data. The average Nusselt number (Nuavg) obtained with multiple-orifice synthetic jet is up to 75% higher than a single orifice synthetic jet using the same input power to synthetic jet actuator. For larger pitch ratio (R > 1.25), the maximum value of Nuavg is observed to be reduced and obtained closer to heated surface. Multiple-orifice configurations with diamond shape exhibited better thermal performance as compared to other shapes examined in the study. The multiple-orifice configurations with one and two satellite orifices demonstrated two local peaks in Nuavg and are observed only with smaller values of pitch ratio. The location of these local peaks illustrated very little or no dependence on Stokes number (S). Configurations with no center orifice exhibited significantly lower heat transfer as compared to the configurations with center orifice as well as the case with single center orifice. The secondary maxima in Nuavg are entirely missing in the configurations with no center orifice. The PIV data reveals interactions between the adjacent synthetic jets in multiple-orifice case to form a combined synthetic jet with better mixing characteristics as compared to a single orifice synthetic jet and therefore the former exhibits higher heat transfer rate. The PIV data further illustrated the individual behavior of synthetic jets resulting from satellite orifices in multiple-orifice configurations with no center orifice. In addition to heat and flow characteristics of multiple-orifice synthetic jet, the heat transfer correlations are also developed from the experimental data for different orifice shapes. The study of multiple-orifice synthetic jet presented here is expected to be useful in designing the cooling modules for electronic devices.

Heat transfer enhancement of a heated cylinder with synthetic jet impingement from multiple orifices

This study proposed and investigated a method to enhance the heat transfer performance of a heated cylinder with synthetic jet impingement from multiple orifices. A synthetic jet has a flow with alternately changing orientations at its exit and eventually generates a periodic flow in constant orientation in the area far from the exit. First, the flow characteristics of synthetic jets generated from the multiple orifices on a piccolo tube were investigated in detail with a hot-wire anemometer. Next, the heat transfer of a heated cylinder was enhanced with multiple synthetic jets. The Nusselt number of the heated cylinder was calculated using the temperature variation of the cylinder with time. The heat transfer rate enhanced with multiple synthetic jets was 2–3 times higher than that without jets. The results also showed that the heat transfer rate with upward-oriented jets from the piccolo tube located below the heated cylinder was higher than that with downward-oriented jets from the tube located above the cylinder. We also compared synthetic jets to continuous jets generated with a high-pressure air source. The results showed that the effects of the synthetic jets were valid at greater distances than those of the continuous jets.

합성제트를 이용한 풍력터빈 에어포일의 유동 제어에 관한 수치적 연구

In the present study, a numerical study about flow control of a wind turbine airfoil was performed using computational fluid dynamics (CFD). Two dimensional unsteady Reynolds-Averaged Navier-Stokes (RANS) solver was used to solve the flow around a wind turbine airfoil. Synthetic jets were applied to the DU 35 airfoil used in the inboard section of the NREL 5MW wind turbine. In order to simulate the synthetic jets, suction/blowing boundary condition was applied. The flow characteristics of synthetic jets were investigated according to the injection speed and injection angle. The injection velocity is 1, 2, and 3 times the freestream and the injection angle is 45 degrees. As the injection speed increases, more energy is transferred into the boundary layer. Therefore, the performance of the DU 35 airfoil is most improved when the injection speed is 3 times the freestream. In the case of the injection angle, the flow analysis was performed at 0 degrees, 45 degrees, and the direction perpendicular to the surface. The injection speed at this time is 3 times. The size and shape of the vortices generated by the synthetic jets depend on the injection angle. When the injection angle is 45 degrees, the performance enhancement of the DU 35 airfoil is the greatest.

シンセティックジェットの流動特性に及ぼす振動条件の影響について

シンセティックジェットの流動特性に及ぼす振動条件の影響について

円柱壁面シンセティックジェットの流動特性の解明

円柱壁面シンセティックジェットの流動特性の解明

Flow Study of Synthetic Jet: A Review

Synthetic jet is an immersing technology; it has proved very effective for cooling purpose. Synthetic jet has been looking as one of the challenging technology which can replace almost all traditional electronic cooling system very effectively. Huge amount of work has been done yet and still in progress to understand the flow of synthetic jet and to improve its performance. The review of experimental and computational study has been discussed. The experimental study consists of the formation and evolution with different stages in flow field. Also, it includes the parametric study of synthetic jet; in which effect of most of the geometric and operational parameters on the performance of the synthetic jet has studied. The computational study using commercial software’s has been explained in this paper. It has been observed that the flow characteristics of synthetic jet are different in the near field as compared to continuous jet.

합성제트 기반의 유량 공급 장치에 대한 수치적 연구

Flow characteristics of synthetic jet based flow supplying devices have been computationally investigated for different device shapes. Jet momentum was produced by the volume change of a cavity by two piezoelectric-driven diaphragms. The devices have additional flow path compared with the original synthetic jet actuator, and these flow path changes the flow characteristics of synthetic jet actuator. Four non-dimensional parameters, which were functions of the shapes of the additional flow path, were considered as the most critical parameters in jet performance. Comparative studies were conducted to compare volume flow rate and jet velocity. Computed results were solved by 2-D incompressible Navier-Stokes solver with k-w SST turbulence model. Detailed computations revealed that the additional flow path diminishes suction strength of the synthetic jet actuator. In addition, the cross section area of the flow path has more influence over the jet performances than the length of the flow path. Based on the computational results, the synthetic jet based flow supplying devices could be improved by applying suitable shape of the flow path.

0220 非対称スロットによって生成される二次元シンセティックジェットの流動特性

0220 非対称スロットによって生成される二次元シンセティックジェットの流動特性

Study on the fundamental flow characteristics of synthetic jets(Behavior of free synthetic jets)

Study on the fundamental flow characteristics of synthetic jets(Behavior of free synthetic jets)

10701 離散渦法によるシンセテイツクジェツト流れのシミュレーション(OS5-1【流れの計測と制御(1)】)

The present study attempts to clarify some of the fundamental flow characteristics of synthetic jet and continuous jet, based on method numerical simulations using the vortex. The flow pattern and the unsteady flow characteristics of the synthetic jet are compared with those of a continuous jet The numerical distributions of dimensionless mean velocities agree quantitatively with the experimental ones, which indicate the numerical simulation is valid. It is found that a vortex pair flowing downstream causes the synthetic jet. The accelerating domain of the jet which was confirmed in the synthetic jet experimentally is also observed in the numerical results.

사각형 및 원형 출구 Synthetic Jet의 유동 특성에 대한 수치적 연구

사각형 및 원형 출구 형상 synthetic jet의 수치적 연구를 통하여 유입류가 존재할 경우 평판에서의 유동 구조 및 유동 제어 효과를 분석하였다. 사각형 출구 형상의 경우, jet 직후에 강한 vortex가 생성되지만 주변에 적은 momentum을 공급하기 때문에 유입류 방향으로 갈수록 유동제어 효과가 감소하게 된다. 원형 출구 형상의 경우, 규칙적인 vortex의 형태가 slot 중앙부터 끝까지 나타나고 보다 멀리까지 jet vorticity의 영향이 미치게 된다. 유동제어 효과를 예상하기 위하여 위치 별 wall shear stress를 비교하였다. 이에 원형 출구 형상이 사각형 출구 형상보다 유동제어 효과가 더 클 것으로 판단된다. 또한 최적의 원형 출구 형상을 도출하기 위하여, hole gap과 diameter의 변화에 따른 유동 구조 및 유동 제어 효과를 비교 분석하였다. 그 결과, hole diameter와 gap를 고려하여 원형 출구 형상을 설계할 경우 유동제어 효과를 극대화 할 수 있음을 밝혔다. The flow characteristics of synthetic jet depending on rectangular and circular jet exit configuration are investigated using numerical computation with cross flow. In rectangular slot, synthetic jet generates the strong vortex but supplies fewer momentum and effectiveness of flow control is reduced along flow direction. In circular slot, regular vortex is formed from slot center to end. It affects the wider region than rectangular slot. The distribution of wall shear stress is considered in order to indicate the effectiveness of flow control device for flow separation delay. Consequently, circular slot is a more suitable candidate for delaying flow separation. In order to derive the optimal shape of a circular slot exit, hole gap and diameter that affect the flow structure and flow control were analyzed. As a result, consider the hole diameter and gap of circular slot exit design, effectiveness of the flow control can be increased.

Flow Characteristics of Synthetic Jet Induced by Plasma Actuator

The flowfield induced by a plasma synthetic jet actuator was investigated by solving Reynolds-averaged Navier- Stokes equations, augmented with a phenomenological model representing the plasma-induced body force imparted by a plasma actuator on the fluid. Flow characteristics of an unsteady plasma synthetic jet in quiescent air were presented. The results indicate that the time-averaged flowfield is quite similar to the steady plasma synthetic jet, the traditional blowing synthetic jet, and the plane jet. The time-averaged streamwise velocity is a little smaller than that of the steady case, but the jet half-width is much larger. This makes the volume and momentum flux of an unsteady plasma synthetic jet larger. With unsteady actuation, a plasma synthetic jet does not exhibit the suction effect of the cavity during the second half-period, as does the traditional blowing synthetic jet. The vortex pair, formed from the wall jets that are produced by plasma actuators, has a strong unsteady effect on the plasma synthetic jet close to the wall. In the far field, the vortex pair coalesces and eventually develops to a continuous jet.

Study on the Fundamental Flow Characteristics of Synthetic Jets (1st Report : Behavior of Free Synthetic Jets)

Jet flows have been applied in a number of fields in order to control flow separation. Over the last decade, several studies have been carried out on the production of synthetic jets. However, there are still a large number of unknown quantities, including details of the structure and the formation mechanism of such jets. The present study attempts to clarify some of the fundamental flow characteristics of free synthetic jets, based on experiments and numerical simulations. Experimental velocity measurements and flow visualizations are performed using the hot-wire anemometer and the smoke wire method, respectively. It is found that both the temporal change in flow pattern and time-averaged velocity distribution at the centerline depend on K=ReU/S2 (the ratio of the Reynolds number to the square of the Stokes number). The unsteady downstream flow characteristics are discussed, in addition to the relation between the formative point of the synthetic jet and the value of K. Furthermore, the flow pattern and the unsteady flow characteristics of the synthetic jet are compared with those of a continuous jet.

Experimental and Numerical Study on the Flow Characteristics of Synthetic Jets

Over the last decade, several studies have investigated synthetic jets. However, there are still many clarifications needed, including details of the structure and Coanda effect of synthetic jets. The present study clarifies some fundamental flow characteristics of free synthetic jets and synthetic jets near a rigid boundary by conducting an experiment and numerical simulations. As the main results, it is found that the velocity distribution of free synthetic jets depends on K = Re/S2 (the ratio of the Reynolds number to the square of the Stokes number) and can be identified by the maximum velocity at the centerline and the jet half-width. In addition, it is confirmed that the flow characteristics of the synthetic jet near a rigid boundary and the re-attachment length of the synthetic jet are determined not only by H1/b0 (normalized step height) but also K.

B41 壁面近傍におけるシンセティックジェットの基本流動特性に関する研究(B4噴流II)

B41 壁面近傍におけるシンセティックジェットの基本流動特性に関する研究(B4噴流II)