Experimental Study on Flow Characteristics of Synthetic Jet with Circular Exit Array in Separated Flow

Piezo-electric diaphragm으로 작동되는 synthetic jet을 박리 유동 제어에 적용하기 위한 실험을 수행하여 전산 수치 해석 결과와 유동 현상을 비교 분석하였다. 외부 유동이 없는 조건에서 열선 유속계를 활용한 출구 속도 측정 실험을 통해 작동 주파수와 출구 지름이 출구 유속 변화에 중요한 역할을 하고 있음을 검증했다. 출구 단면적이 동일한 조건의 실험에서는 동일한 유량이 출구를 통해 분출되더라도 출구 둘레의 길이가 커질수록 점성의 영향이 증가하여 출구 유속이 감소함을 알 수 있었다. 외부 유입류가 있는 inclined flat plate에서 synthetic jet 작동 전/후의 압력 계수 분포를 비교하여 유동 제어 성능을 결정하는 핵심 파라미터의 역할을 검증했다. 그 결과 주어진 박리 발생 조건에서 최적의 유동 제어 성능을 발휘하는 작동 주파수, 출구 지름, 출구 간격을 발견할 수 있었다.The control of separated flow on an inclined flat plate using synthetic jet which actuated by piezo-electric diaphragm was investigated experimentally. Experimental results were compared with computational results in order to analyze flow physics. According to the result of jet velocity using hot wire anemometry with no cross flow, we found that actuation frequency and orifice diameter played important role in jet velocity. On the condition of same exit area, jet velocity was decreased because of exit perimeter which related with viscous effect. According to the result of measuring static pressure on the inclined flat plate with cross flow, we found that actuation frequency and orifice diameter and gap between orifices played important role in separation control performance.본 연구는 방위사업청 및 국방과학연구소 (UC100031JD), 슈퍼컴퓨팅센터(KSC-2010-C1- 0030), 항공 우주 신기술 연구소, BK 21의 지원 으로 수행되었음.OAIID:oai:osos.snu.ac.kr:snu2011-01/104/0000004648/24SEQ:24PERF_CD:SNU2011-01EVAL_ITEM_CD:104USER_ID:0000004648ADJUST_YN:NEMP_ID:A001138DEPT_CD:446CITE_RATE:0FILENAME:배열된_원형_출구_Synthetic_Jet의_박리_유동_제어_특성에_관한_실험적_연구.pdfDEPT_NM:기계항공공학부EMAIL:[email protected]:

Smart actuation for helicopter rotorblades

Successful rotorcrafts were only achieved when the differences between hovering flight conditions and a stable forward flight were understood. During hovering, the air speed on all helicopter blades is linearly distributed along each blade and is the same for each. However, during forward flight, the forward motion of the helicopter in the air creates an unbalance. The airspeed is increased for the blade passing in the advancing side of the helicopter, while it is reduced in the retreating side. Moreover, when each blade enters the retreating side of the helicopter, a reverse flow occurs around the profile where the blade speed is lower than the forward speed of the helicopter. The balance of a rotorcraft is solved by a cyclic pitch control, but trade-offs are made on the blade design to cope with the great variety of aerodynamic conditions. A smart blade that would adapt its characteristics to this large set of conditions would improve rotorcrafts energy efficiency while providing vibration and noise control.\ud Smart rotor blades systems are studied to adapt the aerodynamic characteristics of the blade during its revolution and to improve the overall performances. An increase in the lift over drag ratio on the retreating side has been studied to design a blade with better aerodynamic efficiency and better stall performances in the low-speed region. The maximum speed of a rotorcraft is limited by the angle of attack that the profile can sustain on the retreating side before stall. Therefore, increasing the maximum angle of attack that a profile geometry can sustain increases the rotorcraft flight envelope. Flow asymmetry and aerodynamic interaction between successive blades are also investigated to actively reduce vibrations and limit noise.\ud These improvements can be achieved by deploying flaps, by using flow control devices or by morphing the full shape of the profile at a specific places during the blade revolution. Each of the listed methods has advantages and disadvantages as well as various degrees of feasibility and integrability inside helicopter blades. They all modify the aerodynamic characteristics of the profile. Their leverage on the various aerodynamic effects depends on the control strategy chosen for actuation. Harmonic actuation is therefore studied for active noise and vibration control whereas stepped deployment is foreseen to modify the stall behaviour of the retreating side of the helicopter.\ud Helicopter blades are subjected to various force constraints such as the loads from the complex airflow and the centrifugal forces. Furthermore, any active system embedded inside a rotor blade needs to comply with the environmental constraints to which a helicopter will be subjected  during its life-span. Other concerns, like the power consumption and the data transfer for blade control, play an important role as well. Finally, such a system must have a life-time exceeding the life-time of a rotor blade and meet the same criteria in toughness, reliability and ease of maintenance.\ud Smart system is an interplay of aerodynamics, rotor-mechanics, material science and control, thus a multidisciplinary approach is essential. A large part of the work consists in building processes to integrate these domains for investigating, designing and testing smart components.\ud Piezoelectric actuators are a promising technology to bring adaptability to rotor blades. They can be used directly on the structure to actively modify its geometry, stiffness and aerodynamic behaviour or be integrated to mechanisms for the deployment of flaps. Their large specific work, toughness, reliability and small form factor make them suitable components for integration within a rotor blade. The main disadvantage of piezoelectric actuators is the small displacement and strain available. Amplification mechanisms must be optimised to produce sufficient displacement in morphing applications.\ud Smart actuation systems placed inside rotor blades have the potential to improve the efficiency and the performances of tomorrow's helicopters. Piezoelectric materials can address many of the challenges of integrating smart components inside helicopter blades. The key aspect remains the collaboration between various domains, skills and expertise to successfully implement these new technologies

Synthetic jet을 이용한 3차원 날개 능동 유동제어에 관한 수치적 연구

학위논문 (박사)-- 서울대학교 대학원 : 협동과정 계산과학전공, 2014. 8. 김종암.본 연구는 3차원 날개 형상의 공력 성능을 향상시키기 위하여 synthetic jet을 이용한 공력 특성 향상 메커니즘에 관하여 수치적 연구를 수행하였다. 본 연구를 통하여 synthetic jet의 출구 형상에 따른 유동 특성 파악을 통해 유동제어에 효과적인 출구 형상을 도출하고, 이로부터 얻은 synthetic jet의 형상을 3차원 날개에 적용하여 고 받음각에서의 박리유동 제어 특성을 분석하였다. Synthetic jet의 출구 형상은 jet vortex의 발달 과정 및 전체적인 jet momentum에 영향을 주기 때문에 유동제어 특성에 큰 영향을 미치는 요인 중 하나이다. 이에 유동제어에 효과적인 출구 형상을 도출하기 위해 평판에서 유입류가 존재할 경우 사각형과 원형 출구 형상에 대하여 jet vortex 유동 구조 및 유동제어 가능성을 분석하였다. 사각형 출구 형상의 경우, jet 출구 직후에서는 큰 와류를 발생시키지만 출구 끝에서 발생하는 회전 유동에 의해 jet에 의한 효과가 급격히 감소함을 확인하였다. 원형 출구 형상의 경우, 사각형 출구 형상보다 균일한 jet vortex를 생성하고 유입류 방향으로 보다 멀리까지 jet의 영향이 미치는 유동구조를 가지고 있어 사각형 출구 형상보다 유동제어에 효과적임을 확인하였다. 또한 원형 출구 형상의 hole gap과 hole diameter의 변화에 따른 유동 구조 및 유동 특성을 비교 분석함으로써 유동제어 효과를 극대화 할 수 있는 원형출구 형상을 도출하였다. 다양한 수치 해석 결과의 분석을 통해서 도출된 원형 출구 형상의 synthetic jet을 적용하여 동체-날개 혼합 형상의 유동제어를 수행하였다. 풍동 실험과 수치해석을 통해 받음각의 변화에 따른 3차원 날개의 유동 분석을 수행한 결과, 날개의 받음각 증가에 따라 앞전에서부터 발생한 와류유동은 붕괴하게 되며 날개 바깥쪽 부분에 서부터는 박리 유동이 발달함을 확인하였다. 이에 와류 붕괴 현상과 박리 유동을 제어하기 위하여 앞전 부근에 jet을 위치시켰다. 풍동 실험을 통하여 앞전에 위치한 jet을 모두 작동 시켜 유동제어 효과를 확인하였다. 또한 고성능, 저전력 구동을 위하여 jet의 개수에 따른 유동제어 성능을 평가하였다. 수치해석을 통하여 jet의 위치에 따른 유동제어 메커니즘을 확인하고 위치에 따라 유동제어를 수행할 경우 와류 붕괴 현상을 지연시키고 박리 유동을 제어할 수 있음을 확인하였다. 또한 고속에서의 유동제어 가능성을 확인하기 위하여 저속에서의 유동제어 전략을 고속에서 확장 적용하여 고속의 동체-날개 혼합형상에서도 효과적인 유동제어 방법을 통하여 고 받음각의 공력 성능을 향상 시킬 수 있음을 확인하였다. 본 연구에서 도출된 연구 결과는 유동제어에 효과적인 능동유동제어 시스템의 설계 및 무인 전투기 형상을 포함한 3차원 날개 형상의 공력 성능 향상 방안 수립에 활용될 수 있을 것이다.The present study deals with flow characteristics of synthetic jets for efficient flow control performance. It consists of two parts: flow characteristics of synthetic jets depending on exit configuration and flow control using synthetic jets over Blended Wing Body (BWB) configuration. In first part, flow characteristics of synthetic jets have been computationally investigated for different exit configurations under a cross flow condition. The exit configuration of a synthetic jet substantially affects the process of vortex generation and evolution, which eventually determines the mechanism of jet momentum transport. Two types of exit configurations were considered: one is a conventional rectangular exit, and the other is a series of circular holes. The interactions of synthetic jets with a freestream were performed by analyzing the vortical structure characteristics. The effectiveness of flow control was evaluated by examining the behavior of the wall shear stress. It was observed that the circular exit provides better performance than the rectangular exit in terms of sustainable vortical structure and flow control capability. According to a hole gap and a hole diameter of circular exit, comparative studies were then conducted with all the other parameters fixed. Detailed computations reveal that the hole gap yields a much more significant effect on flow characteristics than the hole diameter, which turned out to be relatively minor. Based on the strength and the persistency of jet vortices, the circular exit with a suitable hole gap formed critical jet vortices that beneficially affected separation control. This indicates that the flow control performance of circular exit array could be remarkably improved by applying a suitable dimensionless hole parameter. Based on the results of exit configuration, the second part deals with flow control strategy over BWB configuration. Flow structures were examined by analyzing the baseline characteristics of BWB configuration when synthetic jet was off. Based on the aerodynamic data and flow structure, a strategy for flow separation control on BWB configuration was established. Based on the aerodynamic data and flow structure, synthetic jet actuators were installed to prevent leading-edge stall at a relatively high angles of attack. All-actuators-on case and selective-actuators-on case were examined to find effective flow control method. Two types of exit locations are considered for analyzing flow mechanism: one is outboard array jets, and the other is inboard array jets. The interactions of synthetic jets with a free stream were performed by analyzing the vortical structure and the surface pressure characteristics. The effectiveness of flow control was evaluated by examining the aerodynamic coefficient and flow structures. As a result, the vortex breakdown point is moved toward the outboard section by synthetic jets, and the separation flow shows a stable structure. Based on the flow structure in overall speed rage, flow control strategy of low speed flight is applied to flow control of high speed flight. This shows effective flow control strategy applicable to all speed flight. Through numerical analyses on flow characteristics of synthetic jets, it is observed that the synthetic jets under suitable actuating conditions beneficially change the local flow feature and vortex structure to bring a significant improvement of the wing aerodynamics acting on the three-dimensional aircraft configuration in the stall angle.Chapter I Introduction 1 1.1 Literature Review 1 1.1.1 Synthetic Jet 1 1.1.2 Piezoelectirically-driven Synthetic Jet 3 1.1.3 Lambda Wing Aerodynamics 4 1.1.4 Flow Separation on Lambda Wing Flight Mechanics 5 1.1.5 1303 UCAV Configuration 7 1.2 Objectives and Contributions 10 1.3 Organization of Thesis 11 Chapter II Numerical Approaches 12 2.1 Governing Equations 12 2.2 Turbulence Models 14 2.2.1 The Standard Menters k-ω SST Model 14 2.2.2 The k-ω SST Model (Menter et al., 2003) 18 2.2.3 SST-DES Model (Strelets et al., 2001) 19 2.2.4 Zonal SST-DES Model (Menter et al., 2003) 21 2.3 Pseudo-Compressibility Method 22 2.4 Transformation of the Incompressible Navier-Stokes Equations with Turbulence Model 27 2.5 Space Discritizaion Method 31 2.5.1 Differencing of Inviscid Flux Terms 31 2.5.2 Upwind Differencing Method 33 2.5.3 Low Dissipative Upwind Differencing Method 38 2.5.4 Higher Order Spatial Accuracy 39 2.6 Time Integration Method 41 2.6.1 Dual Time Stepping 42 2.6.2 Pseudo-Time Discretization 44 2.6.3 LU-SGS Scheme 47 2.7 Synthetic Jet Boundary Condition 50 Chapter III Flow Characteristics of Synthetic Jets 51 3.1 Two Types of Synthetic Jet Exit 51 3.2 Code Validation 53 3.3 Characteristics of Rectangular and Circular Exits 55 3.3.1 Flow Structures 55 3.3.2 Flow Control Effectiveness 58 3.4 Characteristics of Circular Exits Depending on Hole Parameter 60 3.4.1 Variation of Hole Gap 60 3.4.2 Variation of Hole Diameter 62 Chapter IV Active Flow Control of Wing 63 4.1 Experimental Reference 63 4.1.1 BWB Configuration 63 4.1.2 Experimental Setup 64 4.2 Baseline Analysis 65 4.2.1 Code Validation 65 4.2.2 Flow Characteristics of BWB Configuration 67 4.3 Flow Control of BWB Configuration 69 4.3.1 Flow Control Depending on Jet Location 69 4.3.2 Application of Flow Control in High Speed Flight 72 Conclusion 75 References 77Docto