Browsing by Subject "Modal analysis"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Compressible Shear Flow Transition and Turbulence: Enhancement of GKM Numerical Scheme and Simulation/Analysis of Pressure Effects on Flow Stabilization(2012-12-03) Kumar, Gaurav 1984-Despite significant advancements in the understanding of fluid flows, combustion and material technologies, hypersonic flight still presents numerous technological challenges. In hypersonic vehicles turbulence is critical in controlling heat generation in the boundary layer, mixing inside the combustor, generation of acoustic noise, and mass flow in the intake. The study of turbulence in highly compressible flows is challenging compared to incompressible due to a drastic change in the behavior of pressure and a relaxation of the incompressibility constraint. In addition fluid flow inside a flight vehicle is complicated by wall-effects, heat generation and complex boundary conditions. Homogeneous shear flow contains most of the relevant physics of boundary and mixing layers without the aforementioned complicating effects. In this work we aim to understand and characterize the role of pressure, velocity-pressure interaction, velocity-thermodynamics interaction in the late-stage transition-to-turbulence regime in a high speed shear dominated flow by studying the evolution of perturbations in in a high Mach number homogeneous shear flow. We use a modal-analysis based approach towards understanding the statistical behavior of turbulence. Individual Fourier waves constituting the initial flow field are studied in isolation and in combination to understand collective statistical behavior. We demonstrate proof of concept of novel acoustic based strategies for controlling the onset of turbulence. Towards this goal we perform direct numerical simulations (DNS) in three studies: (a) development and evaluation of gas kinetic based numerical tool for DNS of compressible turbulence, and perform detailed evaluation of the efficacy of different interpolation schemes in capturing solenoidal and dilatational quantities, (b) modal investigation in the behavior of pressure and isolation of linear, non-linear, inertial and pressure actions, and (c) modal investigation in the possible acoustic based control strategies in homogeneously sheared compressible flows. The findings help to understand the manifestation of the effects of compressibility on transition and turbulence via the velocity-pressure interactions and the action of individual waves. The present study helps towards the design of control mechanisms for compressible turbulence and the development of physically consistent pressure strain correlation models.Item Frequency response computation for complex structures with damping and acoustic fluid(2004-12) Kim, Chang-wan, 1969-; Bennighof, Jeffrey Kent, 1960-Modal frequency response analysis is a very economical approach for large and complex structural systems since there is an enormous reduction in dimension from the original finite element frequency response problem to the number of modes participating in the response. When damping does not exist, the modal frequency response problem is inexpensive to solve because it becomes uncoupled. However, when damping exists, the modal damping matrices can become fully populated, making the modal frequency response problem expensive to solve at many frequencies. The conventional approach to solve the modal frequency response problem with damping is to use either direct methods with O(n 3 ) operations at each frequency, or iterative methods with O(n 2 ) operations per iteration and numerous iterations at each frequency, where n is the number of modes used to represent the response. Another approach is to use a state space formulation and an eigensolution to uncouple the damped modal frequency response problem, but this doubles the dimension of the problem. All of the existing traditional methods are very expensive for systems with many modes. In this dissertation, a new algorithm to solve the modal frequency response problem for large and complex structural systems with structural and viscous damping is presented. The newly developed algorithm, fast frequency response analysis (FFRA), solves the damped modal frequency response problem with O(n 2 ) operations at each frequency. The FFRA algorithm considers both structural damping and viscous damping for structural systems. When only structural damping exists, the modal frequency response problem is uncoupled by applying the eigensolution of the complex symmetric modal stiffness matrix. A complex symmetric matrix eigensolver (CSYMM) has been developed to solve the complex symmetric matrix eigenvalue problem efficiently. If a viscous damping matrix is also present, the algorithm handles viscous damping by noting that the rank of the viscous damping matrix is typically very low for the problems of interest in the automobile industry because of the small number of viscous damping elements such as shock absorbers and engine mounts. This algorithm has also been applied to the coupled response of systems consisting of a light acoustic fluid and structure, and systems with enforced motion. Also, the algorithm is implemented in parallel on shared memory multiprocessor machines for performance improvement. The FFRA algorithm is evaluated for several industry finite element models which have millions of degrees of freedom. The FFRA algorithm produces outstanding performance compared to the methods available in the commercial finite element software MSC.Nastran or NX.Nastran in terms of analysis time, since the new algorithm is many times faster while obtaining almost the same accuracy as MSC.Nastran. Therefore, the new FFRA algorithm makes inexpensive high frequency analysis possible and extends the capability of solving modal frequency response analysis to higher frequencies.Item Modal Analysis of Deepwater Mooring Lines Based on a Variational Formulation(2013-04-22) Martinez Farfan, Jose AlbertoPrevious work on modal analysis of mooring lines has been performed from different theoretical formulations. Most studies have focused on mooring lines of a single homogeneous material, and the effect of added mass and damping produced by the water has not been examined deeply. The variational formulation approach, employed in this research to perform a modal analysis, has been useful to study the behavior of several realistic mooring lines. The cases presented are composed from segments of materials with different mechanical characteristics, more similar to those in current offshore projects. In the newly proposed formulation, damping produced by transverse motion of the mooring line through the surrounding water has been added to the modal analysis. The modal analysis formulation applied in this work has been verified with calculations from commercial software and the results are sufficiently accurate to understand the global behavior of the dynamics of mooring lines with the damping produced by the sea water. Inclusion of linearized drag damping in the modal analysis showed that the modal periods of the mooring systems studied depend on the amplitude of the transverse motion of the mooring line. When more amplitude in the motion is expected more damping is obtained. Two realistic designs of mooring lines were compared: one made up with a main insert of steel rope, called ?Steel System?, and one composed by a main insert of polyester, named ?Polyester System?. Comparing the natural periods of both systems, the Steel System appears to be safer because its fundamental natural period is more distant from the wave excitation periods produced by storms. The same happens considering the wave excitation periods produced by prevailing seas. In this case the natural periods of the Polyester System are nearer to the wave excitation periods causing fatigue loads. The transverse mode shapes for lateral motions of the mooring lines are observed to be continuous and smooth across material transitions, such as transitions between chain and wire rope and transitions between chain and polyester rope. This behavior is not always observed in the tangential mode shapes for the Polyester System where significant differences in dynamic tension seem to be present in the specific cases studied.Item Modal analysis of long wave equations(2002) Socha, Katherine Sue; Vishik, Mikhail; Bona, J. L.This work studies the use of modal expansion approximations of solutions of model long wave equations. Such model equations are of interest to oceanographers and engineers because they describe the propagation of surface water waves, used in near-shore models of sandbar formation. General theoretical results are derived for standard long wave models in the form of dispersive, nonlinear partial differential equations. Particular numerical results are computed for such model equations, including the Korteweg-de Vries equation, the Benjamin-Bona-Mahony equation, and the Benjamin-Ono equation, among others.Item Operational modal analysis of a rotating cantilever beam using high-speed digital image correlation(2015-12) Rizo-Patron, Sergio Sebastian; Sirohi, Jayant; Ravi-Chandar, KrishnaswamyA novel procedure to perform an operational modal analysis on a rotating cantilever beam is described. This procedure uses Digital Image Correlation (DIC) to measure the deformation of a beam from images captured with a pair of high-speed digital cameras. Modal parameters including natural frequencies and mode shapes are determined from the deformation data through application of the Ibrahim Time Domain method. The procedure was validated on a 2 m diameter, Mach-scale helicopter rotor, excited by a jet of compressed air. Images of the rotor blade were captured at a sampling rate of 1000 Hz at rotational speeds up to 900 RPM. The out-of-plane deformation of the rotor was measured with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006% of the rotor radius. The first three flap bending modes were identified at each rotational speed and compared to an analytical model of the system. It was found that the analytical model over-predicted the natural frequencies due to differing boundary conditions between the model and the experiment, and so the analytical frequencies were scaled to the results of a rap test using traditional frequency domain analysis. The scaled analytical and experimental natural frequencies agreed to within 0.2% in the best case and 10.0% in the worst case. The experimental mode shapes were also found to closely match the analytical model. The results of this test demonstrate the ability of this procedure to determine the modal parameters of rotating cantilever beams.Item Screening procedure to identify power system events of the Texas Synchrophasor Network(2012-05) Sant, Aprajita; Grady, W. M.; Santoso, SuryaThis work presents a method for screening synchrophasor data to search for power system events of interest. The method employs prony algorithm to perform modal analysis and estimate mode amplitude, frequency, and damping ratio on the data obtained from the Texas Synchrophasor Network. The procedure uses seven different Linear Prediction Model (LPM) orders, plus a 10 second window width that slides in steps of 1 second, to minimize the possibility of overlooking events of interest. Further, the algorithm is extended to include user defined modal characteristics thresholds, window length and step size to capture specific power system events.