Carl Q. Howard

Power transmission from a vibrating body to a circular cylindrical shell through passive and active isolators

The transmission of harmonic vibratory power from a vibrating rigid body into a thin supporting cylindrical shell through multiple passive and active isolators is investigated theoretically. The model allows for the transmission of vertical and horizontal harmonic forces and moments about all three coordinate axes. Results show that over a frequency range from 0 to 200 Hz, the real power transmission into the supporting shell can be reduced substantially by employing in parallel with existing passive isolators, active isolators adjusted to provide appropriate control force amplitudes and phases.

Exhaust stack silencer design using finite element analysis

(Dated: November 2, 1999) Abstract Classical analytical models used for prediction of the performance of reactive silencers are limited to conditions where the dimensions of the duct and resonators are small compared to the wavelength of the sound. Finite Element Analysis does not suffer from such limitations and has therefore been used to analyze the design of a reactive silencer for the exhaust stack of a 980MW power station. To assist in the design process, resonators of various dimensions were analyzed using FEA which has led to the derivation of expressions for the resonance frequencies of slot-type rhomboid shaped resonators as a function of the geometry. An important design issue is the influence that adjacent resonators have on the overall performance of the system. It was found that when resonators of similar resonance frequency are in close proximity, they can interact and lead to a decrease in the overall performance compared to that of a single resonator. PACS numbers: 43.50 Gf

Quantification of the ultrasound induced sedimentation of Microcystis aeruginosa

It has been known for more than 40 years that vacuolate organisms can be induced to sediment with ultrasound. However, robust indicators are still needed to compare the efficacy of different treatments. A repeatable index is proposed that makes it possible to quantify the ultrasonic induced sedimentation. The procedure is used to monitor the long term sedimentation of Microcystis aeruginosa after sonication. Results reveal that the sedimentation process continues after gas vesicles have fully recovered, although at a slower rate.

Unifying model of carpal mechanics based on computationally derived isometric constraints and rules-based motion – the stable central column theory

This study was part of a larger project to develop a (kinetic) theory of carpal motion based on computationally derived isometric constraints. Three-dimensional models were created from computed tomography scans of the wrists of ten normal subjects and carpal spatial relationships at physiological motion extremes were assessed. Specific points on the surface of the various carpal bones and the radius that remained isometric through range of movement were identified. Analysis of the isometric constraints and intercarpal motion suggests that the carpus functions as a stable central column (lunate–capitate–hamate–trapezoid–trapezium) with a supporting lateral column (scaphoid), which behaves as a ‘two gear four bar linkage’. The triquetrum functions as an ulnar translation restraint, as well as controlling lunate flexion. The ‘trapezoid’-shaped trapezoid places the trapezium anterior to the transverse plane of the radius and ulna, and thus rotates the principal axis of the central column to correspond to that used in the ‘dart thrower’s motion’. This study presents a forward kinematic analysis of the carpus that provides the basis for the development of a unifying kinetic theory of wrist motion based on isometric constraints and rules-based motion.

Vibration Analysis of Waffle Floors

Abstract Two-way grillage or waffle floors are used extensively in semiconductor factories as they provide high impedance mounts for manufacturing equipment that is extremely vibration sensitive. This paper presents a mathematical model for the analysis of vibration at the center of a bay and the transmission of vibration along a waffle floor. The mathematical model is compared with finite element models and experimental results from several manufacturing buildings, and shows good agreement. Trends are shown for the displacement and resonance frequency of the floor as the thickness of the floor, size of the bays and the stiffness of the columns are varied.

Active vibration isolation experiments using translational and rotational power transmission as a cost function

Active vibration isolation experiments were conducted using a transducer that measures translational and rotational power transmission from a vibrating mass, through a single-axis active isolator and into a beam. The transducer is capable of measuring forces and moments along six axes and an accelerometer array measures its motion. By combining the measured force and velocity signals the translational and rotational power transmission was measured. Comparisons were made of the effectiveness of several cost functions for minimizing the vibration transmitted into the beam. The results show that active vibration isolation using power transmission as a cost function to be minimized is limited by the phase accuracy of the transducers. The best results were obtained from the minimization of the weighted sum of force and velocity.

Active control of energy density in a one-dimensional waveguide: A cautionary note (L)

Acoustic energy density has been shown to be a highly effective cost function for active noise control systems. Many researchers have used the sound field in a one-dimensional waveguide to trial their control strategies before moving onto more realistic three-dimensional sound fields. This letter aims to shed some light on the observations made in the early papers on one-dimensional energy density control and also shows that some of the analysis was incorrect and the conclusions reached may be flawed.

Molecular Dynamics Simulations of Sound Wave Propagation in a Gas and Thermo-Acoustic Effects on a Carbon Nanotube

Molecular dynamics (MD) simulations have been performed to study sound wave propagation in a simple monatomic gas (argon) and the thermo-acoustic effects on a single walled carbon nanotube (CNT). The objective of this study was to understand the acoustic behavior of CNTs in the presence of acoustic waves propagating in gaseous media. A plane sound wave was generated within a rectangular domain by oscillating a solid wall comprising Lennard-Jones (LJ) atoms with the same intermolecular potential as the gas molecules. A CNT was aligned parallel to the direction of the flow at the wall at the opposite end of the domain. Interatomic interactions in the CNT were modeled using the REBO potential. The behavior of the sound wave propagation in argon gas without the CNT was validated by comparison with a previous study. The simulation results show that the thermo-acoustic behavior of CNTs can be simulated accurately using MD and that large-scale MD can be performed in the ultrasonic frequency range. This investigation will contribute to an improved understanding of the acoustic absorption mechanism of these nanoscopic fibers.

Development of a transducer for active vibration isolation using translational and rotational power transmission as a cost function

When conducting active vibration control experiments it is often necessary to omit the measurements of the contribution of power transmission due to rotational moments because of the lack of suitable transducers. Here, a transducer is described which can be used to measure the translational and rotational power transmission from a source to a receiving structure. A description of the procedure used to calibrate the device is also included. The results from the calibration show that whilst the amplitude of the forces, moments, translational, and rotational displacements can be measured accurately, it is the phase accuracy of these measurements that limit the accuracy of measurements of power transmission.

A tool for the optimisation of vibro-acoustic systems using a parallel genetic algorithm and a distributed computing network

An asynchronous parallel genetic algorithm (GA) was used in conjunction with a distributed computing network to optimise the locations and parameters for combined Helmholtz resonators and tuned mass dampers, called passive vibro-acoustic devices (PVADs), placed on the walls of a composite cylinder model of the payload bay in a rocket used to launch satellites. The vibro-acoustic response was calculated using modal coupling theory with the modal response calculated using commercial finite element software. Binary and integer representations of the chromosomes in the genetic algorithm were used in the optimisation, and it was found that the integer representation converged twice as fast as the binary representation. Optimisations were conducted with a varying number of PVADs. The use of the distributed computing environment reduced the time taken to conduct the optimisation to 3 days compared to 75 days on a single desktop computer.