Numerical and in vitro experimental study of arterial deformation and buckling under hypertension and atherosclerotic conditions

Cardiovascular diseases remain the major cause of mortality worldwide. Pathologies of the vasculature such as atherosclerosis are often related to biochemical and genetic factors as well as mechanical effects that strongly change the function and shape of arteries. The present work is part of a general research project which aims to better understand the mechanical mechanisms responsible for atherosclerotic plaque formation and rupture. The chosen approach is to use numerical fluidstructure interaction (FSI) methods to study the relative influence of hemodynamic parameters on the structural stresses generated on plaques. To this aim, a numerical study of a simplified straight vessel exposed to lumen pressure was investigated under quiescent and steady flow conditions. As the internal pressure or the steady velocity increases, the vessel buckles lead-ing to a non-linear large deformation behaviour. The results have been validated using theoretical predictions for the buckling thresholds. Further studies on idealised cardiovascular conditions such as stenosis (i.e., lumen constriction) or aneurysm like (i.e., arterial wall expansion) formation have also been performed

Experimental and Numerical Investigation of a Fully Confined Impingement Round Jet

The heat transfer characteristics of a fully confined impingement jet are experimentally and numerically evaluated. Full surface heat transfer coefficient distributions are obtained for the target and impingement plate of the model using the transient liquid crystal technique and a commercial CFD solver. The confined box consists of a single round jet impinging over a flat surface at relatively low jet-to-target plate distances, varied between 0.5 and 1.5 jet diameters. The impingement geometry is blocked from the three sides, and therefore, the air of the jet is forced to exit the model in a single direction resulting in a fully confined configuration. Experiments were carried out over a range of Reynolds varying between 16,500 and 41,800. The experimental data is compared to the numerical simulations aiming to quantify the degree of accuracy to which the heat transfer rates can be predicted

Determination of Vibro-Acoustic Properties of Brass Instruments

This research focusses on the influence of material and manufacturing techniques into the sound of brass instruments. It includes in a first step time-resolved pressure fluctuation measurements for a variety of brass instruments and with different players, varying pitches, volume levels, etc. In a second step, experimental and numerical investigations were performed on straight tubes made from different materials in order to quantify the coupling between the artificially excited air column an the surrounding wall material

Acoustic absorption properties of perforated gypsum foams

International audienceEfficient absorbers are important to create a pleasant acoustic environment, or to meet certain technical specifications. Standard solutions make use of open-pore foams or thin resonating plates. The first option requires a thick layer to be efficient at low frequencies, reducing the available space. Plate absorbers, on the other hand, can be thin constructions, but they have a very narrow absorption band. Wideband high absorption levels at low frequencies can be achieved by careful design of the porosity and pore connectivity of a foam. If the matrix material is sufficiently rigid, the acoustic absorption can be predicted in two steps. First, a finite element model of a representative volume element (RVE) yields a set of non-acoustic parameters: flow resistivity, thermal and viscous length, tortuosity, and thermal permeability. In the second step, these values lead to equivalent homogenized fluid properties, as formalized in the Johnson-Champoux-Allard- Lafarge (JCAL) model. Gypsum foams are rigid, closed-cell materials with a narrow, controllable, pore size and wall thickness distribution. The inherent acoustic absorption is weak due to the closed cells. We investigated how a rectangular array of sub-millimeter perforations of the foam results in a high acoustic absorption coefficient. JCAL model predictions based on a Kelvin cell RVE are validated by impedance tube measurements for a variety of perforation diameters. The results show that a suitable combination of pore geometry and perforation pattern leads to a perfect absorption peak below 1 kHz for foam layers as thin as several centimeters. An optimal design of the perforated foams, e.g. using random perforation patterns or graded pore sizes, can lead to the engineering of a desired absorption curve

Analysis and rebuilding of experiments on a heated carbon graphite model in the X2 expansion tube

Numerical simulations have been performed to study radiative and ablative processes experienced by a space vehicle entering a planetary atmosphere. The simulations are compared with spectroscopic measurements of a model tested at hypersonic entry conditions in the X2 expansion tube at the University of Queensland. The tests used a hemicylin-drical graphite model to reproduce the ejection of carbon-based species into the flow due to surface reactions of an ablative material. Measurements were performed for flow over a cold (room temperature) model, and over an electrically pre-heated model, to study the influence of the model temperature on the recorded spectra. The experiments were rebuilt using numerical and physical analysis, the hot wall case. The flow configuration is fully three-dimensional, hence simulations are performed accordingly

Reproducibility of sound-absorbing periodic porous materials using additive manufacturing technologies: Round robin study

The purpose of this work is to check if additive manufacturing technologies are suitable for reproducing porous samples designed for sound absorption. The work is an inter-laboratory test, in which the production of samples and their acoustic measurements are carried out independently by different laboratories, sharing only the same geometry codes describing agreed periodic cellular designs. Different additive manufacturing technologies and equipment are used to make samples. Although most of the results obtained from measurements performed on samples with the same cellular design are very close, it is shown that some discrepancies are due to shape and surface imperfections, or microporosity, induced by the manufacturing process. The proposed periodic cellular designs can be easily reproduced and are suitable for further benchmarking of additive manufacturing techniques for rapid prototyping of acoustic materials and metamaterials