Optimization of composite structures

Structural optimization is introduced and examples which illustrate potential problems associated with optimized structures are presented. Optimized structures may have very low load carrying ability for an off design condition. They tend to have multiple modes of failure occurring simultaneously and can, therefore, be sensitive to imperfections. Because composite materials provide more design variables than do metals, they allow for more refined tailoring and more extensive optimization. As a result, optimized composite structures can be especially susceptible to these problems

Performance Analysis in Off-design Condition of Gas Turbine Air-bottoming Combined System☆

Abstract Nowadays, the gradual depletion of fossil fuels associated with constraints on emissions of greenhouse gases leads to valorize their wasted heat from power plant. One of the technologies adopted for improvement is the utilization of combined cycles. For this purpose, the steam cycle is used most frequently. These systems are highly efficient, but they are very complex and water is requested, moreover they are very heavy, so they cannot always be used. In this context, Air Bottoming Cycles (ABC) become attractive for potential use in future plants and repowering because they are light, compact and they have flexible-use and no water consumption. An application of an Air Bottoming Cycle (ABC) is composed of a gas turbine powered by natural gas, an air compressor and an air turbine coupled to the system by means of a heat exchanger, referred to as the AHX (Air Heat Exchanger). The aim of this paper is to study an Air Bottoming Cycle (ABC) that uses a medium power industrial gas turbine as topper cycle. A thermodynamic optimization is realized, determining the best pressure ratio and air mass flow rate of bottomer cycle. Then, an off-design analysis varying ambient temperature and FAR (Fuel Air Ratio) is shown, in fact, in this case, the exhaust gas conditions from topper gas turbine and inlet air of bottoming joule cycle change

Shape Optimization of Busemann-Type Biplane Airfoil for Drag Reduction Under Non-Lifting and Lifting Conditions Using Genetic Algorithms

The focus of this chapter is on the shape optimization of the Busemann-type biplane airfoil for drag reduction under both non-lifting and lifting conditions using genetic algorithms. The concept of biplane airfoil was first introduced by Adolf Busemann in 1935. Under design conditions at a specific supersonic flow speed, the Busemann biplane airfoil eliminates all wave drag due to its symmetrical biplane configuration; however, it produces zero lift. Previous research has shown that the original Busemann biplane airfoil shows a poor performance under off-design conditions. In order to address this problem of zero lift and to improve the off-design-condition performance, shape optimization of an asymmetric biplane airfoil is performed. The commercially available computational fluid dynamics (CFD) solver ANSYS FLUENT is employed for computing the inviscid supersonic flow past the biplane airfoil. A single-objective genetic algorithm (SOGA) is employed for shape optimization under the non-lifting condition to minimize the drag, and a multi-objective genetic algorithm (MOGA) is used for shape optimization under the lifting condition to maximize both the lift and the lift-to-drag ratio. The results obtained from both SOGA and MOGA show a significant improvement in the design and off-design-condition performance of the optimized Busemann biplane airfoil compared to the original airfoil

LES 및 URANS를 이용한 원심펌프 내부의 난류 유동해석

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부, 2021.8. 최해천.원심펌프는 가장 널리 이용되는 펌프로서 저압에서 다양한 산업 분야에서 많은 유량을 이송하기 위해 사용되고 있다. 원심펌프는 다양한 범위의 압력 상승 및 유량 조건을 만족하기 위해 설계 조건뿐 아니라 탈설계 조건에서도 흔히 작동한다. 탈설계 조건에서는 펌프 내부에서 더 복잡한 난류 유동 특성이 발달하여 널리 사용되는 비정상 Reynolds 평균 Navier-Stokes 난류 모델(unsteady Reyonolds Navier-Stokes: URANS)가 부정확한 결과를 도출할 수 있다는 것이 잘 알려져 있다. 따라서 이러한 탈설계 조건 해석을 위해서는 큰에디모사(large eddy simulation: LES)와 같은 보다 정확한 수치 해석 방법이 요구된다. 1장에서는 원심펌프 내부 난류 유동 해석을 위해 LES를 수행하고 설계 조건 및 탈설계 조건에서의 유동 특성을 분석하였다. 임펠러 블레이드의 압력 및 흡입면 모두에서 유동 박리가 발생하였고, 특히 탈설계 조건에서는 블레이드 압력면의 박리 기포가 비정상 특성을 나타내며 볼루트 혀 부근에서 더 크게 발달하였다. 블레이드 회전에 따라 블레이드 후단으로부터 와류 구조가 발생하였고 탈설계 조건에서는 이들이 다음 블레이드 후단 와류와 강하게 상호 작용하여 볼루트 내부에서 더 강한 와도장을 생성하였다. 임펠러 주변 압력 섭동을 살펴보기 위해서 삼중 분해를 수행하였다. 압력의 난류 섭동은 볼루트 혀 부근에서 크게 증가하였고 특히 탈설계 조건에서는 주기적 섭동보다 강하게 발달하였다. 볼루트 혀에서는 유동 박리가 발생하였다. 특히 탈설계 조건에서는 많은 유량이 출구 파이프로 흐르지 않고 임펠러-볼루트 간극을 따라 볼루트 상류로 누설되었다. 이는 블레이드 압력면에 강한 역압력 구배를 형상하여 박리기포의 재부착을 지연시키고 비정상적인 유동박리 현상을 생성하였다. 또한 볼루트에서의 높은 압력은 축 방향으로의 압력 구배를 형성하고 반경 방향 간극으로 누설 유동을 야기하였다. 탈설계 조건에서는 볼루트 내부에서 더 높은 압력이 형성되어 누설 유동이 강하게 발달하며 볼루트 내부에서의 이차 유동의 발달에 기여하였다. 이러한 원심펌프 내부의 다양한 손실유동은 임펠러-볼루트 상호작용에 의해 영향을 받아 탈설계 조건에서 더 크게 발달하였다. 2장에서는 URANS 해석을 수행하고 이를 LES에서의 유동 특성과 비교하였다. LES는 두 유량 조건에서 펌프의 압력 상승 및 효율을 잘 예측하였지만, URANS는 이들을 과다 예측하였다. URANS의 블레이드 후단 와류 구조는 LES의 순간 유동 구조를 잘 나타내지 못 하였고, 오히려 LES의 상평균 유동 구조와 유사한 와도장을 나타내었다. 설계점에서 LES 및 URANS는 블레이드 표면을 따라 유사한 압력 및 마찰항력 분포를 나타내었다. 하지만 탈설계점에서는 LES는 볼루트 혀 부근 블레이드 압력면을 따라 박리기포의 재부착이 지연되어 더 큰 박리기포가 형성되었는데, URANS는 이를 예측하지 못 하였다. 임펠러 주변에서의 압력 섭동은 URANS가 주기적 섭동 성분은 비교적 잘 예측하지만, 난류 섭동을 잘 예측하지 못 하는 것을 보여주었다. 따라서 난류 섭동이 중요해지는 탈설계점에서는 URANS를 통한 압력 섭동 예측이 부정확한 결과를 나타내었다. 또한 설계점에서는 LES 및 URANS가 볼루트 내부를 따라 유사한 전압 분포를 나타내었다. 하지만 탈설계점에서는 볼루트 혀에서 발생하는 유동박리가 손실을 야기하고 볼루트 상류에서 전압이 크게 감소하였다. URANS는 이러한 손실을 잘 예측하지 못 하였지만 이외 영역에서는 전압 분포를 잘 예측하였다. 또한 본 연구에서의 펌프에서는 토출 파이프의 곡률 및 면적 증가로 인해 강한 와류 유동이 발생하였다. 이러한 와류 유동은 큰 손실을 야기하고 이는 설계 조건에서 높은 유량으로 인해 더 크게 발달하였다. LES는 이러한 와류 유동 및 손실을 잘 예측하여 실험에서의 압력 상승 및 효율을 잘 예측하였지만, URANS는 이를 예측하지 못 하여 펌프 성능을 과다예측하였다.Centrifugal pumps, which are the most common type of pumps, are widely used in various industrial applications. Centrifugal pumps often operate at off-design conditions as well as at the design condition to meet various ranges of pressure rise and flow rates required. At off-design conditions, more complex and unsteady flows develop inside pumps making an accurate prediction of the flow challenging. Commonly used Reynolds-averaged Navier-Stokes (RANS) turbulence model often inaccurately predict turbulent flow inside centrifugal pumps at off-design conditions. Therefore, more accurate numerical method like large eddy simulation (LES) is demanding. In part 1, we perform LES to investigate turbulent flow inside a volute-type centrifugal pump for the design and off-design condition. Along the pressure and suction sides of impeller blades, separation bubbles are generated. At the off-design condition, the blade pressure side near the tongue contains a larger separation bubble with highly unsteady characteristics due to the impeller-volute interaction. The trailing vortices shed from rotating blades strongly interact with those from the following blade at the off-design condition, generating stronger vorticity field in a wider region inside the volute. On the other hand, this mutual interaction of vortices shed from consecutive blades is weak at the design condition. Triple decomposition of pressure fluctuations along the impeller periphery demonstrates that turbulent fluctuations are small at the design condition, whereas they become significant at the off-design condition especially near the tongue. Flow separation also occurs at the volute tongue. At the off-design condition, a large amount of volute flow does not follow the main stream to the discharge pipe but re-enters into the volute upstream near the tongue. This pressurized fluid forms a high adverse pressure gradient on the blade pressure side, resulting in strong unsteady separation there. Also, a high pressure gradient in the axial direction at the radial gaps is formed especially near the tongue, creating the leakage into the cavities. Inside the volute, secondary vortices grow along the volute passage. A secondary motion induced by these vortices also significantly affects the leakage to the cavities. All of these flow losses show unsteady features that are strongly influenced by impeller-volute interactions, especially at the off-design condition. In part 2, we conduct unsteady Reynolds-averaged Navier-Stokes (URANS) simulation and compare the flow characteristics with that by LES. URANS overpredicts the head coefficient and efficiency of the pump, whereas LES shows very good agreement with experiments. Vorticity fields inside the impeller and volute show that URANS does not resolve the instantaneous nature of turbulent flows. Rather, URANS displays similar magnitude and distribution of vorticity to phase-averaged fields by LES, indicating it provides phase-averaged flow features to some degree. Inside the impeller passage, for the design condition, LES and URANS show homogeneous characteristics for pressure and skin friction between five blades. On the other hand, along the blade pressure side for the off-design condition, LES reveals that higher pressure is induced near the tongue than the other four blades, delaying the reattachment of the separation bubble there. However, URANS does not show this larger separation bubble near the tongue. Along the impeller periphery, pressure fluctuations by LES and URANS are compared. For both flow conditions, URANS predicts periodic fluctuations well, whereas turbulent fluctuations are largely underestimated. Therefore, total fluctuations by LES and URANS exhibit satisfactory agreement at the design condition, whereas those at the off-design condition shows significant difference because of increased turbulent fluctuations for the latter condition. The time-averaged total pressure coefficient by LES and URANS shows good agreement at the design condition inside the volute. However, at the off-design condition, total pressure decreases at the volute upstream due to flow separation at the tongue. URANS does not predict these losses, overestimating total pressure there. Inside the discharge pipe for the present pump, strong flow separation and swirls are observed by LES due to the curvature and area expansion of the pipe. The losses by these swirls are larger for the design condition because of the higher mean flow rate. However, URANS does not capture strong swirls and subsequent losses, resulting in overprediction of the pressure rise and efficiency.Part I Large eddy simulation of turbulent flow in a centrifugal pump 1 1 Introduction 2 2 Numerical details 5 2.1. Pump specifications and operating conditions 5 2.2. Governing equations and computational setup 6 2.3. Resolution studies and comparison to experiments 8 3 Results and discussions 15 3.1. Overall flow structures in a centrifugal pump 15 3.2. The flow characteristics inside the impeller 17 3.2.1. Relative eddy and equations of motion in a noninertial frame 17 3.2.2. Qualitative analysis of flow structures inside the impeller 18 3.2.3. Quantitative analysis of flow characteristics inside the impeller 21 3.3. Impeller-volute interaction 23 3.3.1. Trailing vortices shed from blades and flow separation at the tongue 23 3.3.2. Pressure fluctuations along the impeller periphery 25 3.3.3. Leakage through the radial gaps 27 4 Summary and concluding remarks 43 Part II LES vs. URANS: turbulent flow in a centrifugal pump 46 1 Introduction 47 2 Numerical details of URANS 53 3 Results and discussions 55 3.1. Resolution studies and comparison to experiments 55 3.2. Flow near the interface between the impeller blade and volute 57 3.3. Flow characteristics inside the impeller 59 3.4. Radial thrust and pressure fluctuations along the impeller periphery 62 3.5. Flow features inside the volute 65 3.6. Flow characteristics inside the discharge pipe 68 4 Summary and concluding remarks 86 References 90 A Modified volute casing to reduce the leakage to cavities 98 B Circumferential grooves to reduce the leakage to the pump inlet 101 Abstract (in Korean) 104박

Shape Optimization of Busemann-Type Biplane Airfoil for Drag Reduction under Nonlifting and Lifting Conditions Using Genetic Algorithms

The focus of this thesis is on the shape optimization of the Busemann-type biplane airfoil for drag reduction under both nonlifting and lifting conditions using genetic algorithms. The concept of the Busemann-type biplane airfoil was first introduced by Adolf Busemann in 1935. Under its design condition at a specific supersonic flow speed, the Busemann biplane airfoil eliminates all wave drag due to its symmetrical biplane configuration; however it produces zero lift. Previous research has shown that the original Busemann biplane airfoil design has a poor performance under off-design conditions as well. In order to solve this problem of zero lift and to improve the off-design-condition performance of the biplane airfoil, shape optimization of the asymmetric biplane airfoil is performed to minimize the drag while maximizing the lift. In this thesis, the commercially available CFD solver ANSYS FLUENT is employed for computing the inviscid flow past the biplane airfoil. An unstructured mesh is generated using ICEM software. A second-order accurate steady density-based solver is employed to compute the supersonic flow field. A single-objective genetic algorithm (SOGA) is employed to optimize the Busemann biplane airfoil shape under nonlifting condition to minimize the drag coefficient and a multi-objective genetic algorithm (MOGA) is employed to optimize the Busemann biplane airfoil shape under lifting condition to maximize both the lift coefficient and the lift to drag ratio simultaneously. Both results obtained by using SOGA and MOGA show significant improvement in the design and off-design-condition performance of the optimized Busemann biplane airfoil compared to the original one

Simulation and Analysis of Cavitating Flow in the Draft Tube of the Francis Turbine with Splitter Blades at Off-Design Condition

Hydraulic plants are required to operate with wider operating range to cover the variants of power outputs into the electrical grid. Although there have been many studies of off-design conditions, studies of cavitating draft tube vortices at the Francis turbine with splitter blades are limited, and the cavitating property is not yet well comprehended. This study presents a prediction of the cavitating characteristics in the Francis-99 draft tube obtained by numerical analysis using the Zwart mass transfer model and shear-stress transport (SST) model. The shape characteristics of two types of cavitating vortex rope (spiral and columnar) and its influence on the cavitation development in the runner blades are analyzed. The link between cavitation with the vorticity is further highlighted by the vorticity transport equation (VTE). The result indicates that the runner cavitating is symmetric for both types of cavitating vortex ropes, and cavitation is significantly improved when a runner with splitter blade is used

Preliminary Design of a 2D Supersonic Inlet to Maximize Total Pressure Recovery

Presented at the AIAA 5th Aviation, Technology, Integration, and Operations Conference (ATIO), 26 - 28 September 2005, Arlington, Virginia.This paper provides a method of preliminary design for a two-dimensional, mixed compression, two-ramp supersonic inlet to maximize total pressure recovery and match the mass flow demand of the engine. For an on-design condition, the total pressure recovery is maximized according to the optimization criterion, and the dimensions of the inlet in terms of ratios to the engine face diameter are calculated. The optimization criterion is defined such that in a system of (n-1) oblique shocks and one normal shock in two dimensions, the maximum shock pressure recovery is obtained when the shocks are of equal strength. This paper also provides a method to estimate the total pressure recovery for an off-design condition for the specified inlet configuration. For an off-design condition, conservative estimation of the total pressure recovery is given so that performance of the engine at the off-design condition can be estimated. To match the mass flow demand of the engine, the second ramp angle is adjusted and the open/close schedule of a bypass door is determined. The effects of boundary layer are not considered for the supersonic part of the inlet, however friction and expansion losses are considered for the subsonic diffuser

Combustion Dynamics Characteristics and Fuel Pressure Modulation Responses of a Three-Cup Third-Generation Swirl-Venturi Lean Direct Injection Combustion Concept

This paper presents the combustion dynamic data and fuel modulation response of a three-cup Lean Direct Injection combustor developed by Woodward, FST. The test was conducted at the NASA Glenn Research Center CE-5 flame tube test facility. The facility provided inlet air up to 922 K and pressure up to 19.0 bar. At the low-power configuration, the combustion noise was quiet. Large combustion pressure oscillations were observed with the High-power configuration at an off design condition, with low inlet air temperature and pressure conditions and a high equivalence ratio (about T3=600 K, P3 = 800 kPa, and ER =0.46). The noise amplitude was as high as 1.5 psi at around 220 Hz. As inlet air pressure and temperature increased, this combustion instability decreased. Fuel modulated signals were produced with the WASK fuel modulator located in the fuel line upstream of the center cup pilot fuel-air mixer. The amplitudes of the modulated signals detected in the combustor were low. Only less than 0.13% (0.06 psi) of the input energy was detected, and the signal amplitudes decreased as the modulated frequencies increased. Interaction between the modulated signals and the combustion noise varied with operating conditions. At a condition with low combustion noise around 150 hz, modulating a signal at around the same frequency would increase the combustion noise from 0.2 psi to as high as 0.6 psi, whereas at a condition with a high combustion instability around 250 hz, the modulated signal did not seem to have much effect on the combustion noise

Implicit large-eddy simulation of compressible flows using the Interior Embedded Discontinuous Galerkin method

We present a high-order implicit large-eddy simulation (ILES) approach for simulating transitional turbulent flows. The approach consists of an Interior Embedded Discontinuous Galerkin (IEDG) method for the discretization of the compressible Navier-Stokes equations and a parallel preconditioned Newton-GMRES solver for the resulting nonlinear system of equations. The IEDG method arises from the marriage of the Embedded Discontinuous Galerkin (EDG) method and the Hybridizable Discontinuous Galerkin (HDG) method. As such, the IEDG method inherits the advantages of both the EDG method and the HDG method to make itself well-suited for turbulence simulations. We propose a minimal residual Newton algorithm for solving the nonlinear system arising from the IEDG discretization of the Navier-Stokes equations. The preconditioned GMRES algorithm is based on a restricted additive Schwarz (RAS) preconditioner in conjunction with a block incomplete LU factorization at the subdomain level. The proposed approach is applied to the ILES of transitional turbulent flows over a NACA 65-(18)10 compressor cascade at Reynolds number 250,000 in both design and off-design conditions. The high-order ILES results show good agreement with a subgrid-scale LES model discretized with a second-order finite volume code while using significantly less degrees of freedom. This work shows that high-order accuracy is key for predicting transitional turbulent flows without a SGS model.Comment: 54th AIAA Aerospace Sciences Meeting, AIAA SciTech, 201

Periodic report, design of a strut supported turbine vane with a wire-form porous shell

Thermal and structural design analysis of strut supported, porous wall turbine van