축소 초음속 제트로부터 발생하는 공력-음향 압력 하중에 의한 구조 응답 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 항공우주공학과, 2021. 2. 신상준.The noise generated by the supersonic jet is a principal source of structural vibration and internal noise during the launching event or static-firing operations. Complex phenomena are observed due to the interaction between the aerodynamic, acoustic and vibratory loads. In this dissertation, linearized aero-acoustics and structural analyses are performed via the numerical simulation and further validated with experimental results obtained by the small-scale article test for a supersonic free-jet. For the numerical simulation, three-dimensional computational fluid dynamics(CFD), especially Reynolds averaged Navier-Stokes simulation(RANS) and delayed detached eddy simulation(DDES), are conducted. It is in order to explain the generation and propagation mechanism of the acoustic wave and reasonably calculate acoustic variables such as static pressure and its spatial/transient derivatives. Kirchhoff surface is placed at a distance to avoid the nonlinear turbulent sound-producing region. Helmholtz-Kirchhoff(H-K) method is employed in order to predict far field acoustic noise by using stored calculation results obtained by CFD at Kirchhoff surface. The Acceleration power spectral density(APSD) is predicted by employing a finite element method. The Optimal membrane element and discrete Kirchhoff triangle plate bending element(OPT-DKT) and Newmark-β time integration scheme are utilized in order to reduce discrepancies in lower to mid frequency response of the structure. Using the present CFD and H-K method, sound pressure level(SPL)s are quantitatively compared with experimental results measured by 12 and 15 microphones at near and far field, respectively. Moreover, APSD's are compared with the experimental results obtained by an accelerometer at three different locations. The objective of this dissertation is to establish an estimation procedure for vibratory responses of structure due to the aero-acoustic pressure generated from the supersonic jet noise in lower to mid frequencies.음향 하중에 의한 구조 진동은 초고속비행체 또는 우주발사체가 발사될 때 발생하는 대표적인 현상이다. 음향 하중에 의한 구조 진동 현상을 예측하기 위해서는 유동 하중, 음향 하중 및 구조 응답 등 다물리 해석이 요구된다. 본 논문에서는 공력-음향-구조 진동 해석을 수행하여 축소 초음속 제트로부터 발생하는 공력-음향 압력에 의한 구조 응답 연구를 수행하였으며 직접 실험을 수행하여 결과를 비교하였다. 첫째, RANS 및 DDES 전산유체해석을 수행하여 축소 초음속 제트의 소음 생성 원리 및 음향 변수를 예측하였다. 음향 변수를 이용하여 근거리장 소음의 크기를 예측하였고Kirchhoff 평면을 이용한 Helmholtz-Kirchhoff 기법을 이용하여 원거리장 소음 크기를 예측하였다. 둘째, 유한요소 해석을 이용하여 구조물의 응답 연구를 수행하였다. OPT-DKT 쉘 요소 및 Newmark-β 기법을 이용하여 구조물의 가속도 스펙트럼을 예측하였다. 또한 축소 초음속 제트에 대한 실험을 수행하였다. 마이크로폰 및 가속도계를 이용하여 근거리/원거리장에서의 소음의 크기 및 구조물의 가속도 스펙트럼을 측정하였다. 예측된 근거리장에서의 소음의 크기는 실험 결과와 평균 4dB, 원거리장에서의 소음의 크기는 평균 2dB 이내의 정확성을 확인하였다. 또한 공력-음향 압력 하중에 의해 진동하는 구조물은 3,000[Hz]까지 가속도 스펙트럼 신뢰 주파수 범위가 있음을 확인하였다. 본 논문의 목적은 축소 초음속 제트에서 생성되는 공력-음향 압력 하중에 의해 진동하는 구조물의 저주파~중주파 응답을 예측하는 것이다.Abstract i Contents iii List of Tables vi List of Figures viii Chapter 1 Introduction 1 1.1 Background 1 1.1.1 Aero-acoustic loads 1 1.1.2 Vibro-acoustic loads 4 1.2 Literature survey 5 1.2.1 Review of the aero-acoustic prediction for supersonic jet 5 1.2.2 Review of the vibro-acoustic prediction for launch vehicle structures 13 1.3 Aims and Scope 15 1.4 Outline of Dissertation 18 Chapter 2 Aero-acoustic prediction for supersonic jet 20 2.1 Governing equation for three-dimensional fluid dynamics 20 2.1.1 Reynolds averaged Navier Stokes equation 20 2.1.2 Large eddy simulation 23 2.1.3 Delayed detached eddy simulation 25 2.2 Boundary element method for CFD near field to the acoustic far field 27 2.2.1 Helmholtz-Kirchhoff method 27 Chapter 3 Experimental setup for a small-scale supersonic jet 29 3.1 Configuration of the small-scale supersonic jet 29 3.2 Experimental configuration for a small-scale supersonic jet 32 3.2.1 Near-field microphone array 34 3.2.2 Fear-field microphone array 37 Chapter 4 Aero-acoustic prediction and validation for the supersonic jet noise 40 4.1 Computational approach for the supersonic jet noise prediction 42 4.2 Validation of RANS and DDES 50 4.3 Near-field noise prediction and validation 54 4.4 Far-field noise prediction and validation 58 4.5 Discussion for the supersonic jet noise prediction and validation 63 4.5.1 Comparison of the numerical and experimental results 64 4.5.2 Effects of Kirchhoff surface location 68 4.5.3 Possibility of the crackle phenomena for the small-scale supersonic jet 71 Chapter 5 Vibro-acoustic analysis for a clamped thin plate structure 73 5.1 OPT-DKT shell element 75 5.2 Modal analysis of a clamped thin plate 80 5.3 Frequency response function of a clamped thin plate 83 5.4 Mesh convergence examination for the frequency response function 87 Chapter 6 Structural responses due to the aero-acoustic pressure 93 6.1 Computational approach for the vibro-acoustic analysis 94 6.2 Experiments for a clamped thin plate with an accelerometer 96 6.3 Equivalent modeling for the computational analysis 98 6.4 Validation of the present vibro-acoustic analysis 100 6.5 Discussion for structural responses predicting capability due to the aero-acoustic pressure 103 6.5.1 Natural frequencies of a clamped thin plate obtained by the experimental results 104 6.5.2 Maximum reliable frequency and the shifting effects 105 Chapter 7 Conclusion 109 7.1 Summary 109 7.2 Contributions of the present thesis 111 7.3 Future suggestions 113 Reference 114Docto