Sebastian Oberst

DEVELOPMENT OF COUPLED FE/BE MODELS TO INVESTIGATE THE STRUCTURAL AND ACOUSTIC RESPONSES OF A SUBMERGED VESSEL

An analytical model and a fully coupled finite element/boundary element model are developed for a simplified physical model of a submarine. The submerged body is modeled as a ring-stiffened cylindrical shell with finite rigid end closures, separated by bulkheads into a number of compartments and under axial excitation from the propeller-shafting system. Lumped masses are located at each end to maintain a condition of neutral buoyancy. Excitation of the hull axial modes from the propeller-shafting system causes both axial motion of the end closures and radial motion of the shell, resulting in a high level of radiated noise. In the low frequency range, only the axial modes in breathing motion are examined, which gives rise to an axisymmetric case, since these modes are efficient radiators. An expression for the structurally radiated sound pressure contributed by axial movement of the end plates and radial motion of the shell was obtained using the Helmholtz integral equation. In the computational model, the effects of the various influencing factors (ring stiffeners, bulkheads, realistic end closures, and fluid loading) on the free vibrational characteristics of the thin walled cylinder are examined. For both the analytical and computational models, the frequency responses, axial and radial responses of the cylinder, and the radiated sound pressure are compared.

Termites utilise clay to build structural supports and so increase foraging resources

Many termite species use clay to build foraging galleries and mound-nests. In some cases clay is placed within excavations of their wooden food, such as living trees or timber in buildings; however the purpose for this clay is unclear. We tested the hypotheses that termites can identify load bearing wood, and that they use clay to provide mechanical support of the load and thus allow them to eat the wood. In field and laboratory experiments, we show that the lower termite Coptotermes acinaciformis, the most basal species to build a mound-nest, can distinguish unloaded from loaded wood, and use clay differently when eating each type. The termites target unloaded wood preferentially, and use thin clay sheeting to camouflage themselves while eating the unloaded wood. The termites attack loaded wood secondarily, and build thick, load-bearing clay walls when they do. The termites add clay and build thicker walls as the load-bearing wood is consumed. The use of clay to support wood under load unlocks otherwise unavailable food resources. This behaviour may represent an evolutionary step from foraging behaviour to nest building in lower termites.

Instability analysis of friction oscillators with uncertainty in the friction law distribution

Despite substantial research efforts in the past two decades, the prediction of brake squeal propensity, as a significant noise, vibration and harshness (NVH) issue to automotive manufactures, is as difficult as ever. This is due to the complexity of the interacting mechanisms (e.g. stick-slip, sprag-slip, mode coupling and hammering effect) and the uncertain operating conditions (temperature, pressure). In particular, two major aspects in brake squeal have attracted significant attention recently: nonlinearity and uncertainty. The fugitiveness of brake squeal could be attributed to a number of factors including the difficulty in accurately modelling friction. In this paper, the influence of the uncertainty arising from the tribological aspect in brake squeal prediction is analysed. Three types of friction models, namely the Amonton-Coulomb model, the velocity-dependent model and the LuGre model, are randomly assigned to a group of interconnected oscillators which model the dynamics of a brake system. The complex eigenvalue analysis, as a standard stability analysis tool, and the friction work calculation are performed to investigate the probability for instability arising from the uncertainty in the friction models. The results are discussed with a view to apply this approach to the analysis of the squeal propensity for a full brake system.

Cryptic termites avoid predatory ants by eavesdropping on vibrational cues from their footsteps

Eavesdropping has evolved in many predator-prey relationships. Communication signals of social species may be particularly vulnerable to eavesdropping, such as pheromones produced by ants, which are predators of termites. Termites communicate mostly by way of substrate-borne vibrations, which suggest they may be able to eavesdrop, using two possible mechanisms: ant chemicals or ant vibrations. We observed termites foraging within millimetres of ants in the field, suggesting the evolution of specialised detection behaviours. We found the termite Coptotermes acinaciformis detected their major predator, the ant Iridomyrmex purpureus, through thin wood using only vibrational cues from walking, and not chemical signals. Comparison of 16 termite and ant species found the ants-walking signals were up to 100 times higher than those of termites. Eavesdropping on passive walking signals explains the predator detection and foraging behaviours in this ancient relationship, which may be applicable to many other predator-prey relationships.

The role of pad-modes and nonlinearity in instantaneous mode squeal

Disc brake squeal is a major source of customer dissatisfaction and related warranty costs for automobile manufacturers. Although mode coupling is recognized as a mechanism often found in squealing brakes, recent research results show that friction induced pad-mode instabilities could be the cause of instantaneous mode squeal reported in the literature. In this paper, the nonlinear characteristics of instantaneous mode squeal initiated by pad-mode instabilities are studied by analyzing phase space plots of vibrations and sound pressure for a numerical model of a pad-on-plate system as the friction coefficient increases. Results show that as the friction coefficient increases from 0.05 to 0.65, attractors of vibration in the phase space transits from limit cycle to quasi-periodic, showing signs of approaching chaotic behavior. It is shown here that the correlation of the sound pressure behavior in the phase-space with structural vibration is crucial to understanding the role of pad modes and nonlinearity i...

Novel method for pairing wood samples in choice tests

Choice tests are a standard method to determine preferences in bio-assays, e.g. for food types and food additives such as bait attractants and toxicants. Choice between food additives can be determined only when the food substrate is sufficiently homogeneous. This is difficult to achieve for wood eating organisms as wood is a highly variable biological material, even within a tree species due to the age of the tree (e.g. sapwood vs. heartwood), and components therein (sugar, starch, cellulose and lignin). The current practice to minimise variation is to use wood from the same tree, yet the variation can still be large and the quantity of wood from one tree may be insufficient. We used wood samples of identical volume from multiple sources, measured three physical properties (dry weight, moisture absorption and reflected light intensity), then ranked and clustered the samples using fuzzy c-means clustering. A reverse analysis of the clustered samples found a high correlation between their physical properties and their source of origin. This suggested approach allows a quantifiable, consistent, repeatable, simple and quick method to maximize control over similarity of wood used in choice tests.

Nonlinear interactions as trigger for chaotic vibrations in a simplified brake system

In automotive disc-brake squeal, much of the focus in the past two decades has been directed to the analysis of a brake systems vibration response in the frequency domain using the complex eigenvalue analysis (CEA) to predict the number of unstable vibration modes. Unfortunately, it is well known that not all predicted unstable vibration modes will lead to squeal and the magnitude of negative damping does not always indicate squeal propensity. In the complex eigenvalue approach, only linearised equilibrium is analysed and non-linear behaviour of the brake system is not taken into account. On the other hand, non-linear (transient) time-domain analysis simulates the dynamic behaviour closer to a real brake system, but is rarely applied as it is computationally expensive and frequency domain analyses are very popular in industry practice. In this paper, a simplified brake system in the form of an isotropic pad-on-disc system is considered. Specifically, the pad motion and a related instability are investiga...

Quantifying ant activity using vibration measurements.

Ant behaviour is of great interest due to their sociality. Ant behaviour is typically observed visually, however there are many circumstances where visual observation is not possible. It may be possible to assess ant behaviour using vibration signals produced by their physical movement. We demonstrate through a series of bioassays with different stimuli that the level of activity of meat ants (Iridomyrmex purpureus) can be quantified using vibrations, corresponding to observations with video. We found that ants exposed to physical shaking produced the highest average vibration amplitudes followed by ants with stones to drag, then ants with neighbours, illuminated ants and ants in darkness. In addition, we devised a novel method based on wavelet decomposition to separate the vibration signal owing to the initial ant behaviour from the substrate response, which will allow signals recorded from different substrates to be compared directly. Our results indicate the potential to use vibration signals to classify some ant behaviours in situations where visual observation could be difficult.

Detection of unstable periodic orbits in mineralising geological systems

Worldwide, mineral exploration is suffering from rising capital costs, due to the depletion of readily recoverable reserves and the need to discover and assess more inaccessible or geologically complex deposits. For gold exploration, this problem is particularly acute. We propose an innovative approach to mineral exploration and orebody characterisation, based on the analysis of geological core data as a spatial dynamical system, using the mathematical tools of dynamical system analysis. This approach is highly relevant for orogenic gold deposits, which-in contrast to systems formed at chemical equilibrium-exhibit many features of nonlinear dynamical systems, including episodic fluctuations on various length and time scales. Feedback relationships between thermo-chemical and deformation processes produce recurrent fluid temperatures and pressures and the deposition of vein-filling minerals such as pyrite and gold. We therefore relax the typical assumption of chemical equilibrium and analyse the underlying processes as aseismic, non-adiabatic, and inherent to a hydrothermal, nonlinear dynamical open-flow chemical reactor. These processes are approximated using the Gray-Scott model of reaction-diffusion as a complex toy system, which captures some of the features of the underlying mineralisation processes, including the spatiotemporal Turing patterns of unsteady chemical reactions. By use of this analysis, we demonstrate the capability of recurrence plots, recurrence power spectra, and recurrence time probabilities to detect underlying unstable periodic orbits as one sign of deterministic dynamics and their robustness for the analysis of data contaminated by noise. Recurrence plot based quantification is then applied to three mineral concentrations in the core data from the Sunrise Dam gold deposit in the Yilgarn region of Western Australia. Using a moving window, we reveal the episodic recurring low-dimensional dynamic structures and the period doubling route to instability with depth, embedded in and originating from higher-dimensional processes of the complex mineralisation system.