Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

This paper highlights steady and unsteady measurements and flow control results obtained on an Ahmed model with slant angle of 25° in wind tunnel. On this high-drag configuration characterized by a large separation bubble along with energetic streamwise vortices, time-averaged and time-dependent results without control are first presented. The influence of rear-end periodic forcing on the drag coefficient is then investigated using electrically operated magnetic valves in an open-loop control scheme. Four distinct configurations of flow control have been tested: rectangular pulsed jets aligned with the spanwise direction or in winglets configuration on the roof end and rectangular jets or a large open slot at the top of the rear slant. For each configuration, the influence of the forcing parameters (non-dimensional frequency, injected momentum) on the drag coefficient has been studied, along with their impact on the static pressure on both the rear slant and vertical base of the model. Depending on the type and location of pulsed jets actuation, the maximum drag reduction is obtained for increasing injected momentum or well-defined optimal pulsation frequencies.     

Fluidic control of wake-flow behind a two-dimensional square back bluff body

This experimental study deals with wake-flow fluidic control behind a two-dimensional square back geometry positioned close to the ground. The fluidic control system is made of pulsed jets positioned at the upper edge of the model base. The objective of the fluidic action is to modify the wake-flow development, and as a consequence the static pressure distribution over the model base and hence the pressure drag. The main concern of this study is to determine to what extent the presence of a flow confined between the model and the floor influences the effectiveness of the control. Static pressure measurements at the model base and wake-flow characteristics derived from PIV measurements at a high acquisition frequency indicate global similarities between a case where an underbody flow exists and a case where this underbody flow is absent. For low actuation frequencies, discrepancies in the way the coherent structures due to the control develop in the shear layer appear.     

Investigation on the generation process of impact-driven high-speed liquid jets using a CFD technique

High-speed liquid jets have been applied to many fields of engineering, science and medicine. It is therefore of benefit to all these areas to investigate their characteristics by modern and inexpensive methods using a computational fluid dynamics (CFD) technique. Previously, high-speed liquid jets have been studied experimentally using a momentum exchange method, called the “impact driven method (IDM)”, by which the impact of a high-velocity projectile on the liquid package contained in the nozzle cavity produced the jet. The shock pulse reflections in the cavity caused by the impact then drove a multiple pulsed jet from the nozzle exit. In this study, a two-fluid simulation consisting of liquid and air can be successfully calculated by using a two-phase flow mixture model and a moving mesh for the projectile motion. The CFD results show good agreement to the results of previous experimental studies, both quantitatively and qualitatively. For the first time, the wave propagation within the liquid in the nozzle has been captured and analyzed, thereby demonstrating the dynamic characteristics of multiple pulsed high-speed liquid jets initiated by the IDM. This provides a breakthrough in the simulation of the supersonic injection of a liquid into air by using a well-known and user-friendly CFD software. It is useful fundamental knowledge for future studies of high-speed injection with applications in all its related fields.     

Flow and mixing characteristics of pulsed confined opposed jets in turbulent flow regime

A numerical study is performed on a two-dimensional confined opposed-jet configuration to gain basic understanding of the flow and mixing characteristics of pulsed turbulent opposed-jet streams. The sinusoidal pulsating flows with different temperature are imposed at opposed-jet inlets, which are mixed with each other in a confined flow channel. The current mathematical model taking the effect of temperature-dependent thermo-physical properties of fluid into account can present a good prediction for opposed-jet streams compared with experimental data. The numerical results indicate that introduction of temperature difference between opposed jet flows can lead to an asymmetric flow field immediately after jet impact, and the sinusoidal flow pulsations can effectively enhance mixing rate of opposed jets. Parameter studies are conducted for optimization of pulsed opposed jets. The effect of Reynolds number and flow pulsation as well as the configuration geometry on the mixing performance are discussed in detail. Examination of the flow and thermal field shows that the mixing rate is highly dependent on the vortex-induced mixing and residence time of jet fluid in the exit channel.     

Unsteady effects in mechanically-excited turbulent plane jets

Two methods of mechanically exciting a plane turbulent free jet are described; periodic perturbatin of the nozzle exit velocity, and forced oscillation of a small vane located in the het potential core. Hot-wire measurements obtained by conditional sampling techniques indicated that the flow fields of the two jets are substantially different although they have the same Strouhal number of 0.0032. While the mean flow development of the pulsed jet can be described adequately by a quasi-steady model, the vane-excited jet exhibits unsteady effects which depart significantly from quasi-steady approximations such as increased entrainment, amplification of excitation and non-linear effects in the form of the presence of high harmonics. The constancy of momentum flux has been examined in both the steady and unsteady jets     

Development of localized arc filament RF plasma actuators for high-speed and high Reynolds number flow control

Recently developed localized arc filament plasma actuators (LAFPAs) have shown tremendous control authority in high-speed and high Reynolds number flow for mixing enhancement and noise mitigation. Previously, these actuators were powered by a high-voltage pulsed DC plasma generator with low energy coupling efficiency of 5–10%. In the present work, a new custom-designed 8-channel pulsed radio frequency (RF) plasma generator has been developed to power up to 8 plasma actuators operated over a wide range of forcing frequencies (up to 50 kHz) and duty cycles (1–50%), and at high energy coupling efficiency (up to 80–85%). This reduces input electrical power requirements by approximately an order of magnitude, down to 12 W per actuator operating at 10% duty cycle. The new pulsed RF plasma generator is scalable to a system with a large number of channels. Performance of pulsed RF plasma actuators used for flow control was studied in a Mach 0.9 circular jet with a Reynolds number of about 623,000 and compared with that of pulsed DC actuators. Eight actuators were distributed uniformly on the perimeter of a 2.54-cm diameter circular nozzle extension. Both types of actuators coupled approximately the same amount of power to the flow, but with drastically different electrical inputs to the power supplies. Particle image velocimetry measurements showed that jet centerline Mach number decay produced by DC and RF actuators operating at the same forcing frequencies and duty cycles is very similar. At a forcing Strouhal number near 0.3, close to the jet column instability frequency, well-organized periodic structures, with similar patterns and dimensions, were generated in the jets forced by both DC and RF actuators. Far-field acoustic measurements demonstrated similar trends in the overall sound pressure level (OASPL) change produced by both types of actuators, resulting in OASPL reduction up to 1.2–1.5 dB in both cases. We conclude that pulsed RF actuators demonstrate flow control authority similar to pulsed DC actuators, with a significantly reduced power budget.     

Pulsed hot-wire measurements in two- and three-dimensional wall jets

Pulsed Hot-Wire Anemometry (PHWA) measurements are performed in well defined two- and three-dimensional turbulent wall jets. For the two-dimensional wall jet the objective is to study reported differences between conventional Hot-Wire Anemometry (HWA) and Laser Doppler Anemometry (LDA) results. In the three dimensional wall jet, new improved data are provided, employing a measuring technique suitable for highly turbulent flows. This, since only hot-wire results previously have been published for this flow. The pulsed wire results show good agreement with existing Laser Doppler anemometer data in the two-dimensional wall jet, both reporting significantly higher turbulence levels in the outer region of the flow than hot-wires do. The hot-wire anemometer errors generally increase with increasing local turbulence intensity and since the three-dimensional wall jet has a higher turbulence level than its two-dimensional equivalent, the new pulsed hot-wire results improve the information available for the turbulence field in this flow significantly. Received: 29 January 1998/ Accepted: 19 February 1999     

Development and structure of a gravity current head

A new approach for 2-dimensional flow field investigation by PIV has been developed for measurements with high spatial resolution without the well known directional ambiguity. This feature of the technique is especially important for measurements in flows with reversal regions or strong turbulent motion as in-cylinder engine measurements. The major aim of the work was to achieve the benefits of cross correlation PIV image evaluation at reasonable cost and under application of common single wavelength double pulsed laser systems as they are mainly used for PIV experiments. The development of the technique is based on polarization rotation of the light scattered by the seeding particles by means of a ferroelectric liquid crystal half wave plate (FLC).Measurement samples from low turbulent jets and the flow in the wake of a cylinder are being presented.     

Characteristics of flow from an oxy-fuel burner with separated jets: influence of jet injection angle

This paper presents an experimental study of flow development and structure on a separated jet burner in reacting and non-reacting flows. Effects of deflection jets in an aligned configuration of three round jets are emphasized. The idea is based on the confinement of a central jet of fuel by two side jets of oxygen to improve mixing, to control flame stability, and to reduce pollutant emissions. The fields of mean velocity and fluctuation intensity were measured using Particle Image Velocimetry. The deflection of jets has a considerable effect on the dynamic behavior and on the flame characteristics. Results showed that the deflection of jets favors mixing and accelerates merging and combining of jets to a single one. Measurements in reacting flow showed a high influence of combustion on dynamic fields. Compared to non-reactive case, in combustion, larger radial expansion and higher velocity were observed, particularly, above the stabilization point of the flame.     

Coherent contribution to turbulent mixing of a jet in cross flow

In this work, an experimental study of a jet issuing in cross flow is reported. Support is offered to a theory on the origin of Karman-like vortices, in the wake of jets issuing in boundary layers. Further, a coherent structure with strong contribution to the energetics of the flow field is identified here. This structure has not been reported earlier, according to the present authors' knowledge, and it is shown to be related to engulfment of external fluid at the bottom of the jet, and to interesting stochastic and spectral characteristics of the flow field. Three-dimensional plots of the coherent quantities, based on experimental data, reveal a double-helical morphology of the coherent structure. The same morphology has recently been proposed for the far field of jets issuing in stagnant fluid (i.e., without cross flow). The results of this study are expected to support theoretical and numerical work on jets issuing in cross flow.     

Promotion of Turbulent Thermal Mixing of Hot and Cold Airflows in T-junction

An experimental study has been conducted on the promotion and control of turbulent thermal mixing of hot and cold airflows in a T-junction with rectangular cross sections, which simulates the HVAC unit for automobile air-conditioning system. In order to promote the turbulent thermal mixing, small jets have been blown into the main channel at the upstream edge of the T-junction in the direction of 45° against the main flow. Turbulence intensity in the upper part of the thermal mixing layer can be increased with these jets, and consequently the turbulent mixing of hot and cold airflows is promoted effectively. Moreover, it has been found that the degree of thermal mixing can be controlled by changing the jet velocity.     

定常吹气对倾转旋翼机向下载荷的影响

为进一步提高倾转旋翼机悬停状态下的有效载重,开展了定常吹气流动控制对向下载荷的影响研究。首先应用延迟脱体涡模拟(DDES)方法对翼型-90°迎角下非定常大范围分离流动结构进行了数值分析;然后分别开展了前缘吹气、后缘吹气降载措施研究,揭示了吹气降载的机理,并对不同吹气口位置和吹气动量系数的影响进行了定量分析,最后开展了前、后缘同时吹气作用下降载数值模拟研究。计算结果表明:前缘最佳吹气位置在翼型的前缘点,而后缘吹气最佳位置位于襟翼弦长的15%处;前缘吹气的降载效果要优于后缘吹气,而且吹气动量系数对向下载荷的影响较小;相对于初始未施加流动控制构型,阻力系数减小量可达到32.72%。     

Fluidic oscillators for flow control

Fluidic oscillators are based on the bi-stable states of a jet (or a pair of jets) of fluid inside a specially designed flow chamber. These produce sweeping or pulsing jets of high exit velocity (~sonic exit velocities) extending the control authority achievable to high subsonic flows. Sweeping and pulsing jets with frequencies ranging from 1 to 20 kHz have been obtained with meso-scale (nozzle sizes in the range of 200 μm–1 mm) fluidic oscillators with very low mass flow rates of the order of 1 g/s. Such actuators have been recently used in laboratory scale experiments for separation control and cavity noise control with significant promise to be implemented in full-scale systems. In this paper, we provide a historical background of fluidic oscillators and methods to produce either sweeping or pulsing jets, their typical frequency, flow rate, and scaling characteristics. Some challenges in detailed characterization of such actuators through measurement will be presented. We will also discuss some of the system integration issues of translating this technology into practice. This is followed by a brief discussion of the need for further development of such actuators and the understanding of the mechanism by which flow control is achieved by these sweeping jets.     

Characteristics of strongly-forced turbulent jets and non-premixed jet flames

Previous researchers have demonstrated that strong pulsations of the fuel flow rate can significantly reduce the flame length and luminosity of laminar/transitional non-premixed jet flames. The physical mechanisms responsible for these changes are investigated experimentally in acoustically-forced jet flows where the peak velocity fluctuations are up to eight times the mean flow velocity. Both reacting and non-reacting flows were studied and Reynolds numbers, based on the mean flow properties, ranged from 800 to 10,000 (corresponding to peak Reynolds numbers of 1,450–23,000), and forcing frequencies ranged from 290 to 1,140 Hz. Both the first and second organ-pipe resonance modes of the fuel delivery tube were excited to obtain these frequencies. An analysis of the acoustic forcing characteristics within the resonance tube is provided in order to understand the source of the high amplitude forcing. Flow visualization of jets with first resonant forcing confirms the presence of large-scale coherent vortices and strong reverse flow near the exit of the fuel tube. With second-resonant forcing, however, vortices are not emitted from the tube as they are drawn back into the fuel tube before they can fully form. Increased fine-scale turbulence is associated with both resonant cases, but particularly at second resonance. The power spectra of the velocity fluctuations for a resonantly pulsed jet show the presence of an inertial subrange indicating that the flow becomes fully turbulent even for mean-Reynolds-number jets that are nominally laminar. It is shown that these pulsed jet flows exhibit strong similarities to synthetic jets and that the Strouhal number, based on the maximum velocity at the fuel tube exit, is the dominant parameter for scaling these flows. The Strouhal number determines the downstream location where the coherent vortices breakdown, and is found to provide better collapse of flame length data (both current and previous) than other parameters that have been used in the literature.     

Breakup of liquid jets from non-circular orifices

The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.     

Magnetic Field Effects on Axis-Switching and Instabilities in Rectangular Plasma Jets

The effect of an external magnetic field on the evolution of rectangular plasma jets is examined. Specifically investigated is the influence of a primarily axial magnetic field on the uniquely characteristic axis-switching phenomenon of rectangular jets and flow instabilities. The results indicate that the magnetic field decelerates the jet (more rapid spreading), prevents axis-switching and inhibits instabilities. The key physical mechanisms underlying the changes are (1) the ability of the magnetic field to reverse the direction of vorticity and (2) transfer of energy from kinetic to magnetic forms. This study has important implications for magneto-hydro-dynamic flow control and propulsion applications.     

Étude d'une ligne de jets impactant une paroi concave par PIV stéréoscopiqueStudy of a row of jets impinging a concave wall by stereoscopic PIV

The aerodynamic study of a row of axisymmetric jets impinging a concave wall is carried out from velocity measurements obtained by the standard and stereoscopic Particle Image Velocimetry. The principle and the specific aspects of the stereoscopic PIV set up, a recent technique of three-dimensional velocimetry, are explained. After a statistical data processing, the three-dimensional structure and the characteristics of multiple jets impinging a concave wall are described with the mean velocity fields and the turbulent values in several planes of the flow. To cite this article: V. Gilard, L.-E. Brizzi, C. R. Mecanique 334 (2006).     

Passive mixing control of plane parallel jets

An experimental study on the mixing of two plane, unventilated, parallel jets reveals an instability characterized by sinuous flapping of the jets and enhanced mixing of the jets with the ambient fluid. The frequency and amplitude of the instability is shown to be a function of the jets spacing and momentum flux ratios, with the maximum mixing occurring for cases with matched momentum flux. When the momentum flux of the two jets is mismatched by as much as a factor of three, the flow becomes steady. Schlieren flow visualization and hot-wire anemometry demonstrate and quantify the large-scale mixing. The instability has a strong frequency and amplitude dependence on the momentum ratio of the jets. The Strouhal number is also found to decrease with the spacing between the jets. The instability described provides a means to passively control the jet mixing with the ambient.     

Experiments on free and impinging supersonic microjets

The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental study examines supersonic microjets of 100–1,000 μm in size with exit velocities in the range of 300–500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (short takeoff and vertical landing) aircraft, cavity flows, and flow separation. Flow properties of free as well as impinging supersonic microjets have been experimentally investigated over a range of geometric and flow parameters. The flowfield is visualized using a micro-schlieren system with a high magnification. These schlieren images clearly show the characteristic shock cell structure typically observed in larger supersonic jets. Quantitative measurements of the jet decay and spreading rates as well as shock cell spacing are obtained using micro-pitot probe surveys. In general, the mean flow features of free microjets are similar to larger supersonic jets operating at higher Reynolds numbers. However, some differences are also observed, most likely due to pronounced viscous effects associated with jets at these small scales. Limited studies of impinging microjets were also conducted. They reveal that, similar to the behavior of free microjets, the flow structure of impinging microjets strongly resembles that of larger supersonic impinging jets.