# Browsing by Subject "Engineering, Aerospace"

###### Results Per Page

###### Sort Options

Item Open Access A Nonlinear Harmonic Balance Solver for an Implicit CFD Code: OVERFLOW 2(2009) Custer, Chad H.A National Aeronautics and Space Administration computational fluid dynamics code, OVERFLOW 2, was modified to utilize a harmonic balance solution method. This modification allows for the direct calculation of the nonlinear frequency-domain solution of a periodic, unsteady flow while avoiding the time consuming calculation of long physical transients that arise in aeroelastic applications.

With the usual implementation of this harmonic balance method, converting an implicit flow solver from a time marching solution method to a harmonic balance solution method results in an unstable numerical scheme. However, a relatively simple and computationally inexpensive stabilization technique has been developed and is utilized. With this stabilization technique, it is possible to convert an existing implicit time-domain solver to a nonlinear frequency-domain method with minimal modifications to the existing code.

This new frequency-domain version of OVERFLOW 2 utilizes the many features of the original code, such as various discretization methods and several turbulence models. The use of Chimera overset grids in OVERFLOW 2 requires care when implemented in the frequency-domain. This research presents a harmonic balance version of OVERFLOW 2 that is capable of solving on overset grids for sufficiently small unsteady amplitudes.

Item Open Access Coupled-Mode Flutter for Advanced Turbofans(2010) Clark, Stephen ThomasIn the vast majority of measured turbomachinery blade flutter occurrences, the response occurs predominately in a single mode. The primary reason for this single-mode flutter is that for turbomachinery applications the combination of high mass ratio, high solidity, and large natural frequency separation results in only slight mode coupling.

The increased importance of fuel efficiency is driving the development of improved turbofans and open-rotor fans. These new designs use fewer blades and will incorporate composite materials or hollowed airfoils in their fan blade designs. Both of these design changes result in lower mass ratio, lower solidity fan blades that may cause multi-mode flutter, rather than single-mode flutter as seen on traditional fan blades. Thus, a single mode flutter design analysis technique may not be adequate. The purpose of this study is to determine initial guidelines for deciding when a coupled-mode analysis is necessary.

The results of this research indicate that mass ratio, frequency separation, and solidity have an effect on critical rotor speed. Further, guidelines were developed for when a multi-mode flutter analysis is required. These guidelines define a critical mass ratio that is a function of frequency separation and solidity. For blade mass ratios lower than this critical value, a multi-mode flutter analysis is required. Finally, the limitations of aerodynamic strip-theory have been revealed in a three-dimensional coupled-mode flutter analysis.

Item Open Access Development of an Efficient Design Method for Non-synchronous Vibrations(2008-04-24) Spiker, Meredith AnneThis research presents a detailed study of non-synchronous vibration (NSV) and the development of an efficient design method for NSV. NSV occurs as a result of the complex interaction of an aerodynamic instability with blade vibrations. Two NSV design methods are considered and applied to three test cases: 2-D circular cylinder, 2-D airfoil cascade tip section of a modern compressor, and 3-D high pressure compressor cascade that encountered NSV in rig testing. The current industry analysis method is to search directly for the frequency of the instability using CFD analysis and then compare it with a fundamental blade mode frequency computed from a structural analysis code. The main disadvantage of this method is that the blades' motion is not considered and therefore, the maximum response is assumed to be when the blade natural frequency and fluid frequency are coincident. An alternate approach, the enforced motion method, is also presented. In this case, enforced blade motion is used to promote lock-in of the blade frequency to the fluid natural frequency at a specified critical amplitude for a range of interblade phase angles (IBPAs). For the IBPAs that are locked-on, the unsteady modal forces are determined. This mode is acceptable if the equivalent damping is greater than zero for all IBPAs. A method for blade re-design is also proposed to determine the maximum blade response by finding the limit cycle oscillation (LCO) amplitude. It is assumed that outside of the lock-in region is an off-resonant, low amplitude condition. A significant result of this research is that for all cases studied herein, the maximum blade response is not at the natural fluid frequency as is assumed by the direct frequency search approach. This has significant implications for NSV design analysis because it demonstrates the requirement to include blade motion. Hence, an enforced motion design method is recommended for industry and the current approach is of little value.Item Open Access Flutter and Forced Response of Turbomachinery with Frequency Mistuning and Aerodynamic Asymmetry(2008-04-25) Miyakozawa, TomokazuThis dissertation provides numerical studies to improve bladed disk assembly design for preventing blade high cycle fatigue failures. The analyses are divided into two major subjects. For the first subject presented in Chapter 2, the mechanisms of transonic fan flutter for tuned systems are studied to improve the shortcoming of traditional method for modern fans using a 3D time-linearized Navier-Stokes solver. Steady and unsteady flow parameters including local work on the blade surfaces are investigated. It was found that global local work monotonically became more unstable on the pressure side due to the flow rollback effect. The local work on the suction side significantly varied due to nodal diameter and flow rollback effect. Thus, the total local work for the least stable mode is dominant by the suction side. Local work on the pressure side appears to be affected by the shock on the suction side. For the second subject presented in Chapter 3, sensitivity studies are conducted on flutter and forced response due to frequency mistuning and aerodynamic asymmetry using the single family of modes approach by assuming manufacturing tolerance. The unsteady aerodynamic forces are computed using CFD methods assuming aerodynamic symmetry. The aerodynamic asymmetry is applied by perturbing the influence coefficient matrix. These aerodynamic perturbations influence both stiffness and damping while traditional frequency mistuning analysis only perturbs the stiffness. Flutter results from random aerodynamic perturbations of all blades showed that manufacturing variations that effect blade unsteady aerodynamics may cause a stable, perfectly symmetric engine to flutter. For forced response, maximum blade amplitudes are significantly influenced by the aerodynamic perturbation of the imaginary part (damping) of unsteady aerodynamic modal forces. This is contrary to blade frequency mistuning where the stiffness perturbation dominates.Item Open Access Nonlinear Aeroelastic Analysis of Flexible High Aspect Ratio Wings Including Correlation with Experiment(2009) Jaworski, JustinA series of aeroelastic analyses is performed for a flexible high-aspect-ratio wing representative of a high altitude long endurance (HALE) aircraft. Such aircraft are susceptible to dynamic instabilities such as flutter, which can lead to large amplitude limit cycle oscillations. These structural motions are modeled by a representative linear typical section model and by Hodges-Dowell beam theory, which includes leading-order nonlinear elastic coupling. Aerodynamic forces are represented by the ONERA dynamic stall model with its coefficients calibrated to CFD data versus wind tunnel test data. Time marching computations of the coupled nonlinear beam and ONERA system highlight a number of features relevant to the aeroelastic response of HALE aircraft, including the influence of a tip store, the sensitivity of the flutter boundary and limit cycle oscillations to aerodynamic CFD or test data, and the roles of structural nonlinearity and nonlinear aerodynamic stall in the dynamic stability of high-aspect-ratio wings.

Item Open Access Optimization of the Aerodynamics of Small-scale Flapping Aircraft in Hover(2008-06-27) Lebental, SidneyFlapping flight is one of the most widespread mean of transportation. It is a complex unsteady aerodynamic problem that has been studied extensively in the past century. Nevertheless, by its complex nature, flapping flight remains a challenging subject. With the development of micro air vehicles, researchers need new computational methods to design these aircrafts efficiently.

In this dissertation, I will present three different methods of optimization for flapping flight with an emphasis on hovering with each their advantages and drawbacks. The first method was developed by Hall et al. It is an extremely fast and powerful three-dimensional approach. However, the assumptions made to develop this theory limit its use to lightly loaded wings. In addition, it only models the motion of the trailing edge and not the actual motion of the wing.

In a second part, I will present a two-dimensional unsteady potential method. It uses a freely convected wake which removes the lightly loaded restriction. This method shows the existence of an optimal combination of plunging and pitching motion. The motion is optimal in the sense that for a required force vector, the aerodynamic power is minimal.

The last method incorporates the three-dimensional effects. These effects are especially important for low aspect ratio wings. Thus, a three-dimensional unsteady potential vortex method was developed. This method also exhibits the presence of an optimal flapping/pitching motion. In addition, it agrees really well with the two previous methods and with the actual kinematics of birds during hovering flapping flight.

To conclude, some preliminary design tools for flapping wings in forward and hovering flight are presented in this thesis.

Item Open Access Techniques to Assess Acoustic-Structure Interaction in Liquid Rocket Engines(2008-04-25) Davis, Robert BenjaminAcoustoelasticity is the study of the dynamic interaction between elastic structures and acoustic enclosures. In this dissertation, acoustoelasticity is considered in the context of liquid rocket engine design. The techniques presented here can be used to determine which forcing frequencies are important in acoustoelastic systems. With a knowledge of these frequencies, an analyst can either find ways to attenuate the excitation at these frequencies or alter the system in such a way that the prescribed excitations do result in a resonant condition. The end result is a structural component that is less susceptible to failure. The research scope is divided into three parts. In the first part, the dynamics of cylindrical shells submerged in liquid hydrogen (LH_{2}) and liquid oxygen (LOX) are considered. The shells are bounded by rigid outer cylinders. This configuration gives rise to two fluid-filled cavities: an inner cylindrical cavity and an outer annular cavity. Such geometries are common in rocket engine design. The natural frequencies and modes of the fluid-structure system are computed by combining the rigid wall acoustic cavity modes and the*in vacuo*structural modes into a system of coupled ordinary differential equations. Eigenvalue veering is observed near the intersections of the curves representing natural frequencies of the rigid wall acoustic and the*in vacuo*structural modes. In the case of a shell submerged in LH_{2}, system frequencies near these intersections are as much as 30% lower than the corresponding*in vacuo*structural frequencies. Due to its high density, the frequency reductions in the presence of LOX are even more dramatic. The forced responses of a shell submerged in LH_{2}and LOX while subject to a harmonic point excitation are also presented. The responses in the presence of fluid are found to be quite distinct from those of the structure*in vacuo*. In the second part, coupled mode theory is used to explore the fundamental features of acoustoelastic systems. The result is the development of relatively simple techniques that allow analysts to make informed decisions concerning the importance of acoustic-structure coupling without resorting to more time consuming and complex methods. In this part, a new nondimensional parameter is derived to quantify the fundamental strength of a particular acoustic-structure interaction irrespective of material and fluid properties or cavity size. It is be shown that, in some cases, reasonable approximations of the coupled acoustic-structure frequencies can be calculated without explicit knowledge of the uncoupled component mode shapes. Monte Carlo simulations are performed to determine the parameter values over which the approximate coupled frequency expressions are accurate. General observations concerning the forced response of acoustoelastic systems are then made by investigating the response of a simplified two mode system. The third part of this research discusses the implementation of a component mode synthesis (CMS) technique for use with geometrically complex acoustoelastic systems. The feasibility of conceptually similar techniques was first demonstrated over 30 years ago. Since that time there have been remarkable advancements in computational methods. It is therefore reasonable to question the extent to which CMS remains a computationally advantageous approach for acoustoelastic systems of practical interest. This work demonstrates that relative to the most recent release of the popular finite element software package, ANSYS, CMS techniques have a significant computational advantage when the forced response of an acoustoelastic system is of interest. However, recent improvements to the unsymmetric eigensolver available in ANSYS have rendered CMS a less efficient option when calculating system frequencies and modes. The CMS technique is then used to generate new results related to geometrically complex acoustoelastic systems.