Separation control using synthetic vortex generator jets in axial compressor cascade

An experimental investigation conducted in a high-speed plane cascade wind tunnel demonstrates that unsteady flow control by using synthetic (zero mass flux) vortex generator jets can effectively improve the aerodynamic performances and reduce (or eliminate) flow separation in axial compressor cascade. The Mach number of the incoming flow is up to 0.7 and most tested cases are at Ma = 0.3. The incidence is 10° at which the boundary layer is separated from 70% of the chord length. The roles of excitation frequency, amplitude, location and pitch angle are investigated. Preliminary results show that the excitation amplitude plays a very important role, the optimal excitation location is just upstream of the separation point, and the optimal pitch angle is 35°. The maximum relative reduction of loss coefficient is 22.8%.The project supported by the National Natural Science Foundation of China (10477002 and 50476003) and the Ph.D. Innovative Foundation of Beihang University. The English text was polished by Yunming Chen.     

Physical factors of primary jet vectoring control using synthetic jet actuators

A primary jet vectoring using synthetic jet actuators with different exit con- figurations was investigated,and the main physical factors influencing jet vectoring were analyzed and summarized.The physical factors of the pressure difference,the location and area of the lower pressure region,the component of the synthetic jet momentum and the entrainment ratio of the synthetic jet flow to primary jet flow directly control the vectoring force and the vectoring angle.Three characteristic parameters of the syn- thetic jet contribute to the pressure difference and the area of the lower pressure region Both the extension step and slope angle of the actuator exit have functions of regulating the location of the lower pressure region,the area of the lower pressure region,and the entrainment ratio of the synthetic jet flow to primary jet flow.The slope angle of the actuator exit has additional functions of regulating the component of the synthetic jet momentum.Based upon analyzing the physical factors of jet vectoring control with syn- thetic jets,the source variables of the physical factors were established.A preparatory control model of jet vectoring using synthetic jet actuator was presented,and it has the benefit of explaining the efficiency of jet vectoring using synthetic jet actuator with source variables at different values,and it indicates the optimal actuator is taking full advantage of the regulating function.     

Numerical simulation on micromixer based on synthetic jet

This paper studied a concept of micromixer with a synthetic jet placed at the bottom of a rectangular channel. Due to periodic ejections from and suctions into the channel, the fluids are mixed effectively. To study the effects of the inlet velocity, the jet intensity and frequency, and the jet location on the mixing efficiency, 3-D numerical simulations of the micromixer have been carried out. It has been found that when the jet intensity and the frequency are fixed, the mixing efficiency increases when Re 〈 50, and decreases when Re 〉 50 with the best mixing efficiency achieved at Re = 50. When the ratio of the jet velocity magnitude to the inlet velocity is taken as 10 and the jet frequency is 100 Hz, the mixing index reaches the highest value. It has also been found that to get better mixing efficiency, the orifice of the synthetic jet should be asymmetrically located away from the channel＇s centerline.     

Active manipulation of a particle-laden jet

The interaction of a particle-laden jet (_ReUe=6600)$({\mathit{Re}}_{U_{\text{e}}}=6600)$ with a single synthetic jet or a continuous control jet located upstream of the main jet exit (i.e., within the main jet nozzle) was examined experimentally using PIV and PTV. A reduction technique was used to calculate 3-D flow fields from multiple 2-D measurement planes to study the complex 3-D interactions. The synthetic jet was shown to influence the particles both directly and indirectly through the manipulation of the carrier fluid’s drag force on the particles. The synthetic jet impulse directly vectors the particles away from the synthetic jet, while the formation of large vortical structures indirectly affects the particles, spreading throughout the measurement domain. By comparison, a continuous control jet only vectors the particles away from itself. The lowest Stokes number particles respond similarly to the carrier fluid, while higher Stokes number particles are less responsive to the control and only follow the strong vortical structures (i.e., higher circulation), which suggests that the preferential concentration concept depends on both the Stokes number as well as the strength of the coherent structures.     

Active control of flow separation over an airfoil using synthetic jets

We perform large-eddy simulation of turbulent flow separation over an airfoil and evaluate the effectiveness of synthetic jets as a separation control technique. The flow configuration consists of flow over an NACA 0015 airfoil at Reynolds number of 896,000 based on the airfoil chord length and freestream velocity. A small slot across the entire span connected to a cavity inside the airfoil is employed to produce oscillatory synthetic jets. Detailed flow structures inside the synthetic-jet actuator and the synthetic-jet/cross-flow interaction are simulated using an unstructured-grid finite-volume large-eddy simulation solver. Simulation results are compared with the 2005 experimental data of Gilarranz et al., and qualitative and quantitative agreements are obtained for both uncontrolled and controlled cases. As in the experiment, the present large-eddy simulation confirms that synthetic-jet actuation effectively delays the onset of flow separation and causes a significant increase in the lift coefficient. Modification of the blade boundary layer due to oscillatory blowing and suction and its role in separation control is discussed.     

Experimental investigation of axisymmetric coaxial synthetic jets

Measurements were made in the near field of piston driven axisymmetric coaxial synthetic jets emanating from an orifice and a surrounding annulus of equal exit areas and cavity volumes. Piston velocity, amplitude, radial spacing between the orifice and the annulus, and exit angles had a strong influence on the dominant features of the flow. Flow visualization revealed three distinct topologies of the jet consisting of expanding, contracting and recirculating regions and doubling of the number of foci inside of the cavity compared to jet from the orifice alone. The direction of the swirl/rotation imposed on the mean flow was also dependent on the direction of the rotation of dominant foci. Interaction between flow from the orifice and the annulus amplified the azimuthal instability of ring vortices due to the periodic axial stretching and compression of the streamwise vortex filaments. Bifurcation of ring vortices into elliptical lobes reported earlier [S.V. Gaimella, V.P. Schroeder, Local heat transfer distributions in confined multiple air impingement, ASME Journal of Electronic Packaging 123 (3) (2001) 165–172] for single cavity jet was also observed in the coaxial jet. The number of cellular structures however was considerably larger than the single jet case. Large excursions of the jets from the plane of symmetry were observed. Power spectra exhibited sub-harmonic distribution of energy due to coalescence of the vortices. Growth of jet width and decay of centerline velocity were strongly influenced by the spacing and forcing frequency.     

Towards the Design of Synthetic-jet Actuators for Full-scale Flight Conditions

Despite the proven capability of synthetic-jet actuators in delaying boundary-layer separation in laboratory experiments, a capability that allows the geometry and operating conditions of these devices to be designed and selected for maximum flow-control effectiveness in full-scale flight conditions has yet to be developed. In this two-part paper, the key results obtained during a 3-year research programme aiming at establishing such a capability based on a better understanding of the fluid mechanics of synthetic jets and an improved modelling capacity are reported. In Part 1 of this paper, the experimental studies of the behaviour of synthetic jets in both quiescent flow and a boundary layer are described. The work has led to an improved understanding of the dimensionless parameters that determine the formation and development of vortex rollup and how the strength of rollup can be enhanced by optimizing the geometry and operating condition. Based on the study of the nature of vortical structures produced as the result of the interaction with a boundary layer and their impact in the near-wall region where flow control is desired, the conditions for producing effective vortical structures for delaying flow separation were established. The finding from this work forms the basis of a number of criteria used for designing synthetic jet actuators for full-scale flight condition to be presented in Part 2.     

Performance of a flapping foil flow energy harvester in shear flows

Through numerical simulations, we investigate the energy harvesting performance of a heaving/pitching foil in shear flow. With two-dimensional Navier–Stokes simulations, we examined the energy harvesting efficiencies of such a system in linear shear flows and compared the results with those in uniform flows. It is found that in low shear rates, the performance of the system in linear shear flow is slightly higher than that in uniform flow, whereas the energy harvesting efficiency is greatly diminished if the shear rate is sufficiently high (this effect is more pronounced in higher frequencies). This is attributed to the effects of linear shear on the vorticity generation and the synchronization between fluid forcing and foil motion – when a strong shear flow is introduced the lift force induced by the leading edge vortex that is in phase with the heaving motion of the foil is diminished. Furthermore, by studying the instability of the wake behind the foil, we confirm that the optimal performance of the foil in linear shear flows is associated with the same physical mechanism that controls the performance of the foil in uniform flows, i.e. the excitation of the most unstable modes in the wake when the oscillation frequency of the foil is close to the frequencies of these modes.     

Flow field around the flapping flag

The flapping flag is a canonical fluid–structure interaction problem that describes a cantilever plate with flow along its elastic axis. When the flapping flag loses stability it enters a large amplitude Limit Cycle Oscillation (LCO). While theoretical models can accurately predict the flutter velocity and frequency, there are still discrepancies between the experimental observations and the theoretical predictions of the post-critical LCO response. This note provides recent flow field visualizations in a single longitudinal plane for a cantilevered aluminum plate in axial flow during its LCO. Particle Image Velocimetry (PIV) techniques are used to show that the flow over the midspan of the plate is attached even during the violent LCO motion. This observation suggests that potential flow aerodynamic models may be able to capture the essential features in the flow field.     

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k–ε turbulence model or a 2-D and 3-D realisable k–ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k–ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.     

Turbulent jet initiation of detonation in hydrogen-air mixtures

Large scale experiments (50 m³) have been carried out on the initiation of detonation by means of a jet of hot combustion products. The effects of hydrogen concentration (18–30% vol.), jet orifice diameter (100–400 mm), and the mixture composition in constant volume explosion chamber (25–50%) were investigated. Both high enough hydrogen concentration and large enough jet size are necessary for detonation initiation. The minimum values are within the ranges of 20 to 25% vol. H₂, and of 100 to 200 mm correspondingly. A minimum ratio of jet size and mixture cell width 12–13 is required for detonation initiation.     

Self-sustained oscillations of a confined impinging jet

The present paper investigates the dynamics of a laminar plane jet impinging on a flat plate in a channel. An experimental parametric study is carried out to determine the flow regimes at different levels of confinement and Reynolds numbers. For very confined jets, the flow is steady whatever the Reynolds number. The overall structure of the flow is symmetric with respect to the jet axis and is characterized by the presence of recirculation zones at the channel walls. The dynamics is radically different for less confined jets. Above a critical Reynolds number, the flow bifurcates in the form of an oscillating flapping mode of the impinging jet. Analyses of the experimental results provide with a quantitative characterization of this regime in terms of amplitude, wavelength and frequency. This self-oscillating bifurcated flow induces strong sweepings of the target plate by the jet and intense vortex dipole ejections from the impacted wall. Such a regime is expected to be particularly useful in the enhancement of the local heat transfer at relatively low cost in terms of flow rate.     

Numerical simulation of compressible plane jets

Two- and three-dimensional spatial direct numerical simulations of a compressible plane jet exhausting into a parallel stream are described. These simulations reveal the inadequacy of a two-dimensional model in capturing the totality of the flow physics. In two dimensions, instabilities evolve into highly organized large-scale mixing events; two-dimensional time-averaged turbulence quantities also suffer from artificial vortex organization. Mean normal velocity profiles show a significant reduction in entrainment with increased compressibility, while the effect is much less pronounced in three dimensions. While streamwise and spanwise turbulence intensities exhibit no change with increased compressibility, normal intensity and shear stress are significantly reduced.     

Coherent structures and wavepackets in subsonic transitional turbulent jets

A large eddy simulation (LES) is performed for two subsonic jets with a Reynolds number of Re = 105, which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL) and noise spectra are well validated against previous exper-imental results. It is found that the OASPL is raised by heating at shallow angles. The most energetic coherent struc-tures are extracted with specified frequencies using the filter based on the frequency domain variant of the snapshot method of proper orthogonal decomposition (POD). The m = 0, 1 modes have high coherence of near-field pres-sure for both jets, while the coherence of m = 0 modes is enhanced greatly by heating. Based on the coherent struc-tures, spatial wavepackets are educed and the characteristics of growth, saturation and decay are analyzed and compared between the two jets in detail. The results show that heat-ing would enhance the linear growth rate for high frequency components, and nonlinear growth rates for low frequency components in general, which are responsible for higher OASPL in the hot jet. The far-field sound generated by wavepackets is computed using the Kirchhoff extrapolation, which matches well with that of LES at shallow angles. This indicates that the wavepackets associated with coherent structures are dominant sound sources in forced transitional turbulent jets. Additionally, the present POD method is proven to be a robust tool to extract the salient features of the wavepackets in turbulent flows.     

Numerical analysis of a synthetic jet using an automated adaptive method

An automated adaptive remeshing methodology is applied to a synthetic jet. A set of two‐dimensional, axisymmetric, time‐dependent Computational Fluid Dynamics analyses are performed. Grid independence is achieved via successive levels of adaptive refinement using a novel methodology. The method employs adaptive remeshing, performed in an automated fashion. Adaptation criteria are based upon the undivided differences in select field variables. Sensors are placed at strategic locations within the flow field, which are used to aid in judging grid independence. The resulting analytical predictions are compared to an experimental dataset. The automated methodology yields both a verified and validated set of analysis results for the synthetic jet. Copyright © 2011 John Wiley & Sons, Ltd.