Project 1: Smart Modulation of Flow Separation On Streamlined Composite Structure Using Embedded Shape Memory Alloy Actuators

Funding Body:
Competitive Earmarked Research Grant, Research Grants Council of HKSAR Government

Principal Investigator:
Dr Randolph C. K. Leung

Co-Investigators:
Dr Alan K. T. Lau, Prof. Ronald M. C. So and Dr L. M. Zhou

Research Personnel:
Miss Elizabeth W. S. Kam (Research Student)
Miss Angela O. K. Leung (Research Assistant)

Abstract of the Project:
Flow past streamlined structures separates under certain conditions. The separated boundary layer, depending on the flow Reynolds number and pressure gradient dictated by structure surface geometry, will either reattach to the structure surface or rolls up to form vortices that are shed into the structure wake. Such series of vortex dynamical processes are detrimental to the primary aerodynamic characteristics, such as lift and drag, of the structure as well as such secondary consequence as aerodynamic noise generation. In fixed- and rotary- aircrafts, not only will this phenomenon cause severe wing/rotor blade vibration; it will also generate noise. The rapidly changing induced velocity field due to the separating unsteady vortical flow could cause large and time varying fluctuations in wing/blade loading. Although there are control techniques based on momentum-injection and use of flaps/spoilers to alter the separation on the wing/blade surface, their applications may be difficult to integrate with existing wing/blade design since additional structures will be installed and/or redesign of the entire wing/blade structure may be needed. An alternative approach to control flow separation is to employ smart materials and structures technology to give desirable shape and strength profiles for the streamlined structures. Shape memory alloys (SMA) actuator is one of the major elements of smart materials and structures. The SMA actuators can be embedded into a structure made of composite material and be controlled individually over a structure span. The controls can modify aerodynamic loading distribution along the structure span in addition to its natural frequency. In this project, the control of flow separation using an advanced composite streamlined structure with integrated SMA actuators is studied. The optimization of the control of structure surface deformations and strength variations through the use of a different number of actuators, different interval spacings of the actuators and different locations of embedment is also attempted.

Project 2: Nonlinear Fluid-Structure Interaction of an Elastic Lifting Surface Undergoing Dynamic Stall and Its Control

Funding Body:
Competitive Earmarked Research Grant, Research Grants Council of HKSAR Government

Principal Investigator:
Dr Randolph C. K. Leung

Co-Investigators:
Prof. Ronald M. C. So and Prof. L. Cheng

Research Personnel: 
Dr W. Q. Qian (Research Associate)

Abstract of the Project:
Dynamic stall of lifting surface has been known to be a factor that limits the performance and operation envelope of rotors designed for helicopters, wind turbines, and axial-flow compressors. Multiple factors conspire to render dynamic stall a physically complex and challenging phenomenon to comprehend. Viewed as an essentially two-dimensional flow process, dynamic stall commences when the angle of attack of the lifting surface dynamically exceeds the static stall limit. When this happens, the unsteady boundary layer separates near the leading edge from the suction surface, rolls-up to form a tiny, but energetic dynamic stall vortex. This vortex rapidly intensifies, convects quickly downstream, and sheds from the lifting surface. During this process, the low pressure region generated by the vortex results in significant lift amplification beyond static levels, which is followed by the occurrence of abrupt deep stall at vortex shedding. The lift amplification characteristics is certainly beneficial as far as the rotor operation is concerned; however, the highly fluctuating aerodynamic loads, the strong stresses and subsequently high fatigue imposed on the lifting structure hinder the full utilization of the lift enhancement. Despite tremendous efforts in understanding the dynamic stall physics and suppressing its adversity, past studies appear to have ignored a very important physical parameter arising from practical reality; that is the aero-elastic behavior of the lifting surface due to its long span. The aeroelasticity of the lifting surface provides a feedback to the flow excitation forces via its motion responses and the highly non-linear interaction between the flowing fluid and the moving structure. The coupled fluid-structure interaction would alter the development of dynamic stall since the structural responses of the lifting surface are now important. If these coupled responses are ignored, existing knowledge of the physics of dynamic stall and the proposed control methods will most likely become deficient. The aim of the proposed research is to attempt to bridge this knowledge gap and to suggest a dynamic stall control scheme that is more applicable to practical situations.

Unsteady flow past a pitching NACA 0012 airfoil

Project 3: Aeroacoustic Resonance of In-Duct Cascade

Funding Body:
Internal Competitive Research Grant, The Hong Kong Polytechnic University

Principal Investigator:
Dr Randolph C. K. Leung

Co-Investigators: 
Dr S. K. Tang (BSE, HKPolyU)

Abstract of the Project:
Noise from the air-conditioning and ventilation system has long been a problem in the building industry and its control is always a challenge to the acoustical and building services engineers. The problem becomes acute in a modernized city like Hong Kong where many high-rise and heavily serviced buildings are required. Owing to the adverse effects of over-exposure to noise, a good acoustical indoor environment is important for human health. Seldom the air inside the air-conditioning and ventilation ductworks can flow through without encountering any obstructions or restrictions. In reality because of design requirements and space limitations, the flow needs to change direction, thus leading to branching and the necessity to introduce internal guide vanes to smooth transition, bends, and different types of junctions. In addition, high-static-pressure air flows need to be vented or throttled. As a result, devices/elements are introduced into the flow and they will invariably affect the flow structure and behavior inside the air-conditioning and ventilation ductworks. Turbulence is also generated at these duct devices. One of the most popular system devices is the in-duct silencer with an absorbing splitter plates amid the duct flow. Splitter silencers are commonly used in ducts for absorbing the noise produced by flow handling equipment, such as ventilation system fans and ground based gas turbine installations. Such splitter plates not only absorb the noise incident on their absorptive surfaces, but also regenerate noise resulting from the unsteady flow and turbulence in the splitter wakes. The major objectives of the proposed research are:

(i) to identify and understand an important aeroacoustical physics, namely acoustic resonance, of cascade structures residing in air-conditioning and ventilation duct flow. The resonance is related to the sound generation and amplification within the duct. This part is to identify the important governing parameters for acoustic resonance in an in-duct cascade setting.

(ii) to verify the findings in (i) using an experimental ventilation duct.

(iii) to derive, based on the results of (i) and (ii), the optimal design guidelines for cascade parameters that suppress in-duct cascade resonance in practical air-conditioning and ventilation ductworks.

Calculated unsteady flow and acoustic wave in a duct with expansion chamber