Structural dynamics

Several modifications have been implemented to numerical simulation codes based on
blade element momentum theory (BEMT), for application to the design of ocean current
turbine (OCT) blades. The modifications were applied in terms of section modulus and
include adjustments due to core inclusion, buoyancy, and added mass. Hydrodynamic loads
and mode shapes were calculated using the modified BEMT based analysis tools. A 3D
model of the blade was developed using SolidWorks. The model was integrated with
ANSYS and several loading scenarios, calculated from the modified simulation tools, were
applied. A complete stress and failure analysis was then performed. Additionally, the
rainflow counting method was used on ocean current velocity data to determine the loading
histogram for fatigue analysis. A constant life diagram and cumulative fatigue damage
model were used to predict the OCT blade life. Due to a critical area of fatigue failure being
found in the blade adhesive joint, a statistical analysis was performed on experimental
adhesive joint data.

This dissertation will consider the sound radiation from forward-facing steps and a three dimensional cylindrical embossment of very low aspect ratio mounted on a plate. Glegg et al (2014) outlined a theory for predicting the sound radiation from separated flows and applied the method to predicting the sound from forward-facing steps. In order to validate this theory it has been applied to the results of Catlett et al (2014) and Ji and Wang (2010). This validation study revealed that the original theory could be adjusted to include a mixed scaling which gives a better prediction. RANS simulations have been performed and used to support the similarities between the forward-facing step and the cylindrical embossment. The simulations revealed that the cylindrical embossment exhibits a separation zone similar to that of the forward-facing step. This separation zone has been shown to be the dominant source of noise on the forward-facing step in previous works and therefore was expected to be the major source of sound from the cylindrical embossment. The sensitivity of this separation zone to the different parameters of the flow has been investigated by performing several simulations with different conditions and geometries. The separation zone was seen to be independent of Reynolds number based on boundary layer thickness but was directly dependent on the height of the cylinder. The theory outlined in Glegg et al (2014) was then reformulated for use with a cylindrical embossment and the predictions have been compared with wind tunnel measurements. The final predictions show good agreement with the wind tunnel measurements and the far-field sound shows a clearly defined directionality that is similar to an axial dipole at low frequencies.

This research is aimed at investigating and analyzing the rain-windinduced cable vibration phenomena experienced in cables of cable-stayed bridges and also the countermeasures employed by engineers to mitigate the large-amplitude vibration problem reported by various researchers around the world. In order to investigate the problem of the water rivulet creation at the top of the cable surface, a single-degree-of-freedom (SDOF) analytical model was developed and analyzed. This thesis studies the aerodynamic instability of cables in cable-stayed bridges by doing literature review of a typical in-situ test, developing a single degree-of-freedom (SDOF) analytical model, and an ANSYS finite element model. Furthermore, a linear viscous damper that acts as a
countermeasure to the large amplitudes of vibration is reported and analyzed. The suppression characteristics and damper effectiveness of such countermeasure are summarized.

In the field of machine prognostics, vibration analysis is a proven method for
detecting and diagnosing bearing faults in rotating machines. One popular method
for interpreting vibration signals is envelope demodulation, which allows a technician
to clearly identify an impulsive fault source and its severity. However incipient faults -faults in early stages - are masked by in-band noise, which can make the associated impulses difficult to detect and interpret. In this thesis, Wavelet De-Noising (WDN) is implemented after envelope-demodulation to improve accuracy of bearing fault diagnostics. This contrasts the typical approach of de-noising as a preprocessing step.
When manually measuring time-domain impulse amplitudes, the algorithm
shows varying improvements in Signal-to-Noise Ratio (SNR) relative to background
vibrational noise. A frequency-domain measure of SNR agrees with this result.

A finite element tool has been developed to design and investigate a multi-hull
composite ship structure, and a hybrid hull of identical length and beam. Hybrid hull
structure is assembled by Titanium alloy (Ti-6Al-4V) frame and sandwich composite
panels. Wave loads and slamming loads acting on both hull structures have been
calculated according to ABS rules at sea state 5 with a ship velocity of 40 knots.
Comparisons of deformations and stresses between two sets of loadings demonstrate that
slamming loads have more detrimental effects on ship structure. Deformation under
slamming is almost one order higher than that caused by wave loads. Also, Titanium
frame in hybrid hull significantly reduces both deformation and stresses when compared
to composite hull due to enhancement of in plane strength and stiffness of the hull.
A 73m long hybrid hull has also been investigated under wave and slamming loads in time
domain for dynamic analysis.

The research presented in this thesis utilizes Blade Element Momentum (BEM) theory with a
dynamic wake model to customize the OrcaFlex numeric simulation platform in order to allow
modeling of moored Ocean Current Turbines (OCTs). This work merges the advanced cable modeling
tools available within OrcaFlex with well documented BEM rotor modeling approach creating a
combined tool that was not previously available for predicting the performance of moored ocean
current turbines. This tool allows ocean current turbine developers to predict and optimize the
performance of their devices and mooring systems before deploying these systems at sea. The BEM
rotor model was written in C++ to create a back-end tool that is fed continuously updated data on the
OCT’s orientation and velocities as the simulation is running. The custom designed code was written
specifically so that it could operate within the OrcaFlex environment. An approach for numerically
modeling the entire OCT system is presented, which accounts for the additional degree of freedom
(rotor rotational velocity) that is not accounted for in the OrcaFlex equations of motion. The properties
of the numerically modeled OCT were then set to match those of a previously numerically modeled
Southeast National Marine Renewable Energy Center (SNMREC) OCT system and comparisons were
made. Evaluated conditions include: uniform axial and off axis currents, as well as axial and off axis wave fields. For comparison purposes these conditions were applied to a geodetically fixed rotor, showing nearly identical results for the steady conditions but varied, in most cases still acceptable accuracy, for the wave environment. Finally, this entire moored OCT system was evaluated in a dynamic environment to help quantify the expected behavioral response of SNMREC’s turbine under uniform current.

A mobility power flow approach is used to study the response of an infinitely-long cylindrical shell with an internal plate discontinuity. The shell is excited by either a ring radial force or by a plane acoustic wave. The junction between the shell and the internal plate is assumed to be radially pinned such that in-plane waves of the plate can be neglected. The junction forces are expressed in terms of the mobility functions of the plate and the shell. From knowledge of the junction forces and velocities, the power input, the power flow from the shell to the plate, the shell response and the radiated far-field scattered pressure are determined for the circumferential mode n = 0. The results show how the energy propagates from one structure to the other, and present a very clear picture of the characteristics of the scattering pattern from the junction forces.

Wind loads on a bridge may be classified into two types: the buffeting loads and the self-excited loads. The research reported in this thesis is concerned with experimental determination of the self-excited loads in the frequency domain, in particular, their non-dimensional coefficients, called flutter derivatives. The experiments were conducted in a water channel with water substituting for air. Five bridge-section models of different shapes were tested, each of which was driven to move harmonically by linkages, and the forces on the linkages were measured to determine the fluid loads. A thin-plate model, simulating an airfoil, was also tested and the results were compared with those obtained from the thin airfoil theory. The setup of the experiments and data acquisition, processing and analysis are presented herein.

In thick structures, vibrational power can propagate by both in-plane and out-of-plane waves. In performing measurements of power flow or structural intensity, it would be required that the components associated with the in-plane or out-of-plane waves be identified. Using a frequency wavenumber approach, the measured structural intensity can be decomposed into its different wave components. In this thesis, simulated structural intensity measurements are presented to demonstrate the use of this frequency wavenumber technique. The results obtained show the distribution of the structural intensity into the wave components. The implementation of this technique using a laser based instrument is discussed. The required characteristics of the instrument, the number of channels, the spacing between the channels, and the phase accuracy, are described. Also, a table to perform the scanning for the frequency wavenumber analysis is presented.

Structural intensity is propagated through a thick structure by both in-plane and out-of-plane (transverse) waves. These waves propagate at different phase speeds and therefore it is important to distinguish the components of the structural intensity associated with each wave type. To show the presence of these different wave components, experimental results are performed on a thick beam. Using a frequency-wavenumber analysis, the different waves and contributions to the structural intensity are identified. The significance of the contributions to the structural intensity are a function of both frequency and thickness of the structure. Using simulated measurements on a thick L-shaped plate, the relative importance between the in-plane and out-of-plane contributions to structural intensity as a function of frequency and thickness is demonstrated. It is shown that in-plane wave contributions increase in importance as frequency or thickness increases.