Mark Jabbal

Particle image velocimetry measurements of the interaction of synthetic jets with a zero-pressure gradient laminar boundary layer

An experimental investigation of the interaction between a synthetic jet actuator and a zero-pressure gradient laminar boundary layer is reported. The aim of this study is to quantify the impact of synthetic jet vortical structures; namely, hairpin vortices, stretched vortex rings and tilted vortex rings on a boundary layer, and to assess their relative potential for flow separation control. Streamwise particle image velocimetry was employed in a water flume free stream boundary layer thickness Reynolds number of 500 and boundary layer thickness-to-jet orifice diameter ratio of 4 to obtain phase- and time-averaged boundary layer profile information of the impact of synthetic jets near the wall. The potential for flow control was assessed by analyzing near wall fluid mixing, realized by the measure of increase in wall shear stress produced by a passing vortex. Hairpin vortices produced at a jet-to-free stream velocity ratio, VR=0.32 and dimensionless stroke length, L=1.6 and stretched vortex rings VR=0.27; L=2.7 exhibit characteristics akin to a streamwise vortex pair with a common upwash. Conversely, tilted vortex rings VR=0.54; L=2.7 induce a streamwise vortex pair in the near wall region with a common downwash. Wall shear stress measurements show that synthetic jets composed of stretched vortex rings offer the best combination of near wall fluid mixing, persistency, and low rms fluctuations for potential applications of flow separation control.

Active Flow Control Systems Architectures for Civil Transport Aircraft

This paper considers the effect of choice of actuator technology and associated power systems architecture on the mass cost and power consumption of implementing active flow control systems on civil transport aircraft. The research method is based on the use of a mass model that includes a mass due to systems hardware and a mass due to the system energy usage. An Airbus A320 aircraft wing is used as a case-study application. The mass model parameters are based on first-principle physical analysis of electric and pneumatic power systems combined with empirical data on system hardware from existing equipment suppliers. Flow control methods include direct fluidic, electromechanical-fluidic, and electrofluidic actuator technologies. The mass cost of electrical power distribution is shown to be considerably less than that for pneumatic systems; however, this advantage is reduced by the requirement for relatively heavy electrical power management and conversion systems. A tradeoff exists between system power efficiency and the system hardware mass required to achieve this efficiency. For short-duration operation the flow control solution is driven toward lighter but less power-efficient systems, whereas for long duration operation there is benefit in considering heavier but more efficient systems. It is estimated that a practical electromechanical-fluidic system for flow separation control may have a mass up to 40% of the slat mass for a leading-edge application and 5% of flap mass for a trailing-edge application.

The performance of round synthetic jets in quiescent flow

PIV measurements in the near-field region of a jet flow emanating from a round synthetic jet actuator into quiescent air were conducted over a range of operating conditions. The primary purpose of this work was to investigate the nature of synthetic jets at different operating conditions and to examine the jet flow parameters that dictate the behaviour of synthetic jet actuators. The effects of varying diaphragm displacement and oscillatory frequency for fixed actuator geometry were studied. It was observed that the characteristics of synthetic jets are largely determined by the Reynolds number and stroke length. An increase in the former is observed to increase the strength of consecutive vortex rings that compose a synthetic jet, whereas an increase in the latter results in an increase in relative vortex ring spacing and for further increases in stroke length, shedding of secondary vortices. Correlations were also made between the operating parameters and the performance parameters most effective for flow control and which therefore determine the impact of a synthetic jet on an external flow. Relations of time-averaged dimensionless mass flux, momentum flux and circulation with the jet flow conditions were established and found to widely support an analytical performance prediction model described in this paper. It is anticipated that the experimental data obtained in this study will also contribute towards providing a PIV database for macro-scale synthetic jet actuators.

Development of design methodology for a synthetic jet actuator array for flow separation control applications

This article documents the development of synthetic jet actuator array hardware to augment high-lift system effectiveness in a wind tunnel model. The study involved the design, manufacture and bench test of a synthetic jet actuator array based on an inclined actuator configuration to reduce volume installation requirements without a loss in jet velocity relative to a non-inclined baseline model; incorporation of proper synthetic jet actuator systems wiring and internal synthetic jet actuator chamber pressure-sensing for actuator health monitoring. The peak velocity obtained from the inclined synthetic jet actuator array was 100 m/s, which favourably compares to the baseline array (∼90 m/s), while reducing the usable depth requirements by 50%. The final outcome of this study has been to apply the design lessons learned to develop a methodology for designing a synthetic jet actuator array with the constraints of using piezoelectric-based actuator technology for flow separation control applications.

Systems and certification issues for civil transport aircraft flow control systems

The use of flow control (FC) technology on civil transport aircraft is seen as a potential means of providing a step change in aerodynamic performance in the 2020 time frame. There has been extensive research into the flow physics associated with FC. This paper focuses on developing an understanding of the costs and design drivers associated with the systems needed and certification. The research method adopted is based on three research strands: 1. Study of the historical development of other disruptive technologies for civil transport aircraft, 2. Analysis of the impact of legal and commercial requirements, and 3. Technological foresight based on technology trends for aircraft currently under development. Fly by wire and composite materials are identified as two historical examples of successful implementation of disruptive new technology. Both took decades to develop, and were initially developed for military markets. The most widely studied technology similar to FC is identified as laminar flow control. Despite more than six decades of research and arguably successful operational demonstration in the 1990s this has not been successfizlly transitioned to commercial products. Significant future challenges are identified in cost effective provision of the additional systems required for environmental protection and in service monitoring of FC systems particularly where multiple distributed actuators are envisaged. FC generated noise is also seen as a significant challenge. Additional complexity introduced by FC systems must also be balanced by the commercial imperative of dispatch reliability, which may impose more stringent constraints than legal (certification) requirements. It is proposed that a key driver for future successful application of FC is the likely availability of significant electrical power generation on 787 aircraft forwards. This increases the competitiveness of electrically driven FC systems compared with those using engine bleed air. At the current rate of progress it is unlikely FC will make a contribution to the next generation of single-aisle aircraft due to enter service in 2015. In the longer term, there needs to be significant movement across a broad range of systems technologies before the aerodynamic benefits ofFC can be exploited.

Measurements of Synthetic Jets in a Boundary Layer

PIV measurements along the centerline of a synthetic jet embedded in a flat plate boundary layer were conducted for three types of jet vortex structures identified by the authors in previous flow visualization studies, namely hairpin vortices, stretched vortex rings and tilted vortex rings. The primary purpose of this work was to quantify the near wall effect of these structures in terms of their manipulation of the boundary layer velocity profile. In the near field region, synthetic jets composed of stretched vortex rings, which remain within the boundary layer and tilted vortex rings, which rapidly penetrate the boundary layer produced fuller velocity profiles in comparison to the jet off case. Further downstream, only the velocity profiles manipulated by the hairpin vortices and stretched vortex rings continued to fill out close to the wall, thus suggesting that these embedded structures may offer potential as an optimal configuration for flow separation control.

A comparative study of hybrid flow control system architectures for an A320 aircraft

Due to fast-growing air traffic and the increasing cost pressure, the introduction of flow control technologies for the reduction of fuel consumption and direct operating costs in the next generation aircraft is highly likely. The successful application of new technologies depends on the economic and system integration feasibility. Much research has been done in the past on important flow control technologies such as Active Flow Separation Control (AFSC) and Hybrid Laminar Flow Control (HLFC). The system architecture for HLFC and AFSC systems are different and also the needs and purpose of both the systems are different. Considering the application of these two technologies on an A320 aircraft vertical tail plane, a total drag reduction of 3.01% was estimated, with fuel savings of up to 2.02% for a 3000NM range. This paper also analyzed and compared various flow control system architectures and the associated economic benefits and flight performance. A trade-off study is performed to compare and analyze the benefits of various systems architecture and to determine the flow control system with most benefits.

Development of a Smoke Visualisation System for Wind Tunnel Laboratory Experiments

The design, build and test of a smoke visualisation system for a vertical wind tunnel at Brunel University are described. The smoke visualisation system utilised a fog machine for smoke generation and required the design and manufacture of a smoke rake to produce smoke lines inside the wind tunnel test section. The application of these smoke lines over several test objects, including wing sections and bluff body shapes, demonstrated the functionality of the smoke system in producing high-quality visualisations. The facility has proven to be an economic addition in supporting other research projects and is anticipated to be a valuable ‘hands on’ addition to existing aerospace laboratory teaching.

Investigation of passive flow control techniques to enhance the stall characteristics of a microlight aircraft

This report investigates the enhancement of aerodynamic stall characteristics of a Skyranger microlight aircraft by the use of passive flow control techniques, namely vortex generators and turbulators. Each flow control device is designed and scaled to application conditions. Force balance measurements and surface oil flow visualisation are carried out on a half-model of the microlight to further investigate the nature of the flow on the aircraft with and without the flow control devices. The results indicate a clear advantage to the use of turbulators compared with vortex generators. Turbulators increased the maximum lift coefficient by 2.8%, delayed the onset of stall by increasing the critical angle by 17.6% and reduced the drag penalty at both lower (pre-stall) and higher angles of attack by 8% compared to vortex generators. With vortex generators applied, the results indicated a delayed stall with an increase in the critical angle by 2% and a reduced drag penalty at higher angles of attack.