DSpace Academic/Research Unit: Mechanical & Manufacturing Eng

Fast numerical simulation of vortex shedding in tube arrays using a discrete vortex method

Wed, 01 Jan 2003 00:00:00 GMT

Title: Fast numerical simulation of vortex shedding in tube arrays using a discrete vortex method Author: MESKELL, CRAIG; Sweeney, Conor Abstract: Vortex shedding may occur in tube arrays, resulting in strong excitation forces at discrete frequencies. In the past the Strouhal numbers governing vortex shedding in these systems were determined primarily by experiment. This paper presents a computationally inexpensive method of numerical simulation for the unsteady flow through a rigid normal triangular tube array which determines both the frequency of vortex shedding and the instantaneous flow structure. The technique used is based on a discrete vortex method similar to the cloud-in-cell approach which has been applied to flow problems for small numbers of cylinders. However, in the current implementation the flow velocity calculation is carried out on an unstructured grid using a finite element discretization. Thus, the complex geometry associated with a tube array can be easily accommodated. The method, referred to as the “Cloud-in-element” method, is validated for the standard case of flow over a single cylinder and then applied to flow through a normal triangular array with a pitch-to-diameter of 1.6. The Reynolds number is 2200. The Stouhal number obtained from the numerical simulation is 1.27, which is within 6% of the value available in the literature. Qualitatively, the vortex shedding pattern obtained is in agreement with published flow visualization. Description: PUBLISHED

A Decrement Method for Quantifying Nonlinear and Linear Damping in Multidegree of Freedom Systems

Sat, 01 Jan 2011 00:00:00 GMT

Title: A Decrement Method for Quantifying Nonlinear and Linear Damping in Multidegree of Freedom Systems Author: MESKELL, CRAIG Abstract: A method is presented which can estimate the linear and nonlinear damping parameters in a lightly damped multidegree of freedom system which allows the system to be decomposed into a set of single degree of freedom nonlinear systems. Only a single response measurement from a free decay test is required as input ensuring that the magnitude of the damping parameters is not compromised by phase distortion between measurements. The response is band-pass filtered in the time domain, around each of the natural frequencies. While this provides a free response measurement for each mode, it introduces a restriction as the natural frequencies must be distinct and separated. The instantaneous energy of each trace is used to describe the long-term evolution of the mode. This is achieved by using only the peak amplitudes in each period, and so the stiffness and inertial forces are effectively ignored, and only the damping forces are considered. Thus, the method is not unlike the familiar decrement method, which can estimate the viscous damping in linear systems. The method is developed for a weakly nonlinear, lightly damped two-degree-offreedom system, with both linear and Coulomb damping. Simulated response data is used to demonstrate the accuracy of the technique. Description: PUBLISHED

Expansion in the presence of FGF-2 enhances the functional development of cartilaginous tissues engineered using infrapatellar fat pad derived MSCs

Sun, 01 Jan 2012 00:00:00 GMT

Title: Expansion in the presence of FGF-2 enhances the functional development of cartilaginous tissues engineered using infrapatellar fat pad derived MSCs Author: BUCKLEY, CONOR TIMOTHY; KELLY, DANIEL JOHN Abstract: MSCs from non-cartilaginous knee joint tissues such as the infrapatellar fat pad (IFP) and synovium possess significant chondrogenic potential and provide a readily available and clinically feasible source of chondroprogenitor cells. Fibroblast growth factor-2 (FGF-2) has been shown to be a potent mitotic stimulator during ex vivo expansion of MSCs, as well as regulating their subsequent differentiation potential. The objective of this study was to investigate the longer term effects of FGF-2 expansion on the functional development of cartilaginous tissues engineered using MSCs derived from the IFP. IFP MSCs were isolated and expanded to passage 2 in a standard media formulation with or without FGF-2 (5ng/ml) supplementation. Expanded cells were encapsulated in agarose hydrogels, maintained in chondrogenic media for 42 days and analysed to determine their mechanical properties and biochemical composition. Culture media, collected at each feed, was also analysed for biochemical constituents. MSCs expanded in the presence of FGF-2 proliferated more rapidly, with higher cell yields and lower population doubling times. FGF-2 expanded MSCs generated the most mechanically functional tissue. Matrix accumulation was dramatically higher after 21 days for FGF-2 expanded MSCs, but decreased between day 21 and 42. By day 42, FGF-2 expanded MSCs had still accumulated ∼1.4 fold higher sGAG and ∼1.7 fold higher collagen compared to control groups. The total amount of sGAG synthesised (retained in hydrogels and released into the media) was ∼2.4 fold higher for FGF-2 expanded MSCs, with only ∼25% of the total amount generated being retained within the constructs. Further studies are required to investigate whether IFP derived MSCs have a diminished capacity to synthesise other matrix components important in the aggregation, assembly and retention of proteoglycans. In conclusion, expanding MSCs in the presence of FGF-2 rapidly accelerates chondrogenesis in 3D agarose cultures resulting in superior mechanical functionality. Description: PUBLISHED

Porous poly(vinyl alcohol)/sepiolite bone scaffolds: preparation, structure and mechanical properties

Sun, 01 Jan 2012 00:00:00 GMT

Title: Porous poly(vinyl alcohol)/sepiolite bone scaffolds: preparation, structure and mechanical properties Author: CHEN, BIQIONG; Killeen, David; Frydrych, Martin Abstract: Porous poly(vinyl alcohol) (PVA)/sepiolite nanocomposite scaffolds containing 0-10 wt.% sepiolite were prepared by freeze-drying and thermally crosslinked with poly(arylic acid). The microstructure of the obtained scaffolds was characterised by scanning electron microscopy and micro computer tomography, which showed a ribbon and ladder like interconnected structure. The incorporation of sepiolite increased the mean pore size and porosity of the PVA scaffold as well as the degree of anisotropy due to its fibrous structure. The tensile strength, modulus and energy at break of the PVA solid material that constructed the scaffold were found to improve with additions of sepiolite by up to 104%, 331% and 22% for 6 wt.% clay. Such enhancements were attributed to the strong interactions between the PVA and sepiolite, the good dispersion of sepiolite nanofibres in the matrix and the intrinsic properties of the nanofibres. However, the tensile properties of the PVA scaffold deteriorated in the presence of sepiolite because of the higher porosity, pore size and degree of anisotropy. The PVA/sepiolite nanocomposite scaffold containing 6 wt.% sepiolite was characterized by an interconnected structure, a porosity of 89.5% and a mean pore size of 79 μm and exhibited a tensile strength of 0.44 MPa and modulus of 14.9 MPa, which demonstrates potential for this type of materials to be further developed as bone scaffolds. Description: IN_PRESS

Measurement of grinding temperatures using a foil/workpiece thermocouple

Sun, 01 Jan 2012 00:00:00 GMT

Title: Measurement of grinding temperatures using a foil/workpiece thermocouple Author: TORRANCE, ARCHIBALD ANDREW Abstract: The high temperature generated in abrasive processes is the main factor responsible for thermal damage to a ground surface. It can be predicted through the thermal balance of the heat fluxes in the process. Such predictions can be experimentally verified using a foil/workpiece thermocouple. To estimate the thermal behaviour of such a sensor, it was dynamically calibrated with a laser beam to measure its frequency response. It was found that the response of the sensor has a time constant dependant on the thermal load and cannot be modelled by a simple first order function. In the calibration conditions used, the sensor is fast enough to measure the surface temperature with a time constant less than 100 μs. A high frequency acquisition system allows the signal to be measured at the local grit scale so that the activity of grits and the contact stability between the foil and the workpiece during grinding can be studied. By using the peak temperature and the local cooling after a peak, suitably processed, a local background temperature can be defined. It is shown that this background temperature can be evaluated more accurately by matching the global cooling curve to a finite element solution. The temperatures obtained from the local minima of the local diffusive cooling curve agree better with measured results than a temperature obtained by low pass filtering, which can overestimate the background temperature and so the partition ratio. Description: IN_PRESS

Torsion of monofilament and polyfilament sutures under tension decreases suture strength and increases risk of suture fracture

Sun, 01 Jan 2012 00:00:00 GMT

Title: Torsion of monofilament and polyfilament sutures under tension decreases suture strength and increases risk of suture fracture Author: SIMMS, CIARAN KNUT Abstract: Background: A continuous running suture is the preferential method for abdominal closure. In this technique the suture is secured with an initial knot and successive tissue bites are taken. At each tissue bite, the needle is rotated through the tissue; in doing so, the suture can twist around the knot which acts as an anchor. Objective: To determine the effect of axial torsional forces on sutures used in abdominal closure. Methods: The effect of axial twisting on polydioxanone (PDS*II), polyglactin (Vicryl), polypropylene (Prolene) and nylon (Ethilion) sutures was investigated using a uniaxial testing device. Results: The maximum tensile force withstood for untwisted sutures was determined: polydioxanone failed at a tensile force of 116.4±0.84 N, polyglactin failed at 113.9±2.4 N, polypropylene failed at 71.1±1.5 N and nylon failed at 61.8±0.5 N. Twisting decreased the maximum tensile force of all sutures; one complete twist per 10 mm (i.e. 15 twists) decreased the tensile strength of polydioxanone by 21%, polyglactin by 23%, polypropylene by 16% and nylon by 13%, p<0.001. Excessive twisting caused a non linear decrease in suture strength, with one twist per 75 mm (i.e. 20 twists) of polydioxanone decreasing strength by 39%, P<0.001. Conclusion: The effect of excessive twisting on the mechanical properties of sutures is a previously unrecognised phenomenon. Surgeons should be aware that this can result in a decrease in suture strength and reduce the elasticity of the material, and therefore need to adapt their practice to reduce the torsional force placed on sutures. Description: IN_PRESS

Experimental Investigations of Coherence Based Noise Source Identification Techniques for Turbomachinery Applications - Classic and Novel Techniques

Sat, 01 Jan 2011 00:00:00 GMT

Title: Experimental Investigations of Coherence Based Noise Source Identification Techniques for Turbomachinery Applications - Classic and Novel Techniques Author: Bennett, Gareth; Davis, Ian Abstract: Coherence based noise source identi cation techniques utilise the ordinary linear coherence function between two signals to identify the contribution of one or more sources to the noise measured at a location of interest. Such techniques can assess the relative contributions of broadband noise sources, which are the dominant noise sources in turboshaft engines, to the sound radiated at a far-field location. All such techniques require the placement of dynamic pressure sensors close to these sources. Experimental studies undertaken at the Engine Acoustics Branch of the German Aerospace Agency, DLR, as part of an EU FP7 project, TEENI, have provided data for a range of test scenarios. The test rig has been designed to represent the key acoustic elements of a real turboshaft engine. Each noise source identification technique were applied in a two noise source region scenario. The aim of these techniques was to identify the contribution of one or both of these noise sources to the noise measured at a sensor of interest. Test set-ups where each source is present in isolation were also investigated to provide benchmark auto-spectra. These benchmarks are compared to the resulting spectra from several coherence based techniques in order to assess each technique's efficacy in identifying the relative contribution of one or both source region(s) to the noise measured by a receiver of interest. A modal CSA technique is also proposed and tested, based on the ve-microphone conditional spectral analysis technique, which uses decomposed acoustic modes at the duct inlet to identify the contribution of both of the source region(s) to the amplitudes of the modes propagating at the inlet end of the duct. The novel technique was tested for broadband noise source identification, by investigating how accurately it could identify the contributions of two broadband noise source regions to the modal content at the duct inlet. Description: PUBLISHED

Spatial Noise Source Identification of Tonal Noise in Turbomachinery Using the Coherence Function on a Modal Basis

Sat, 01 Jan 2011 00:00:00 GMT

Title: Spatial Noise Source Identification of Tonal Noise in Turbomachinery Using the Coherence Function on a Modal Basis Author: Davis, Ian; Bennett, Gareth Abstract: In modern helicopter engines, the core noise generated by the combustor and turbine stages dominates the core sound radiated from the engine exhaust. Identi cation of the specific contributions of each rotor-stator stage to the core noise is impeded by the lack of available space and the complex duct geometry within the engine. Modal decomposition is an advanced technique which can be used to analyse detailed information on the modal content of the core noise radiating from the engine. This paper advances previous work whereby a technique of identifying noise source location in ducts using the coherence function on a modal basis was developed. A proof of concept was developed using analytical data, with a full radial modal decomposition performed at two axial locations upstream and downstream in the duct. For the case of a real helicopter engine however, there is not enough space to perform a full modal decomposition upstream of the turbine. In this paper, the aforementioned technique is applied to experimental data in order to verify its usefulness for application inside a real turboshaft engine. The technique is also advanced to calculate the coherence between the modes found from a full modal decomposition close to the exit plane of the test duct in the flow (analogous to the engine exhaust) and a singlenpressure measurement upstream (analogous to a point inside the engine close to a noise source of interest). This better represents a realisable technique within the small confines of an engine, as only one pressure transducer is required in the region of each source location of interest. Experimental data measured within an FP7 European research project by one of the consortium partners (German Aerospace Centre (DLR)) was be used to validate this technique. Description: PUBLISHED

Aeroacoustic Noise Source Identification Using Irregularly Sampled LDV Measurements Coupled with Beamforming

Sat, 01 Jan 2011 00:00:00 GMT

Title: Aeroacoustic Noise Source Identification Using Irregularly Sampled LDV Measurements Coupled with Beamforming Author: BENNETT, GARETH; Garcia-Pedroche, Miguel Abstract: This paper develops a technique which relates dynamic pressure measurements taken outside the hydrodynamic eld of a low speed jet to single point, one component velocity measurements taken within the jet using a Laser Doppler Velocimetry (LDV) system. To improve the signal to noise ratio of the pressure measurement, a coherence based acoustic beamforming technique is applied and focussed on the LDV measurement volume in order to evaluate its contribution to the measured sound. As the pressure samples are acquired at regular time steps whereas the velocity at irregular time intervals dependent on the particle seeding, the calculation of the auto- and cross-spectra of the pressure and veloc- ity measurements require special treatment. The signal processing procedures required to calculate these are described and implemented in the paper. To validate the proposed tech- nique real experimental data was analysed. Laser Doppler Velocimetry measured velocity uctuations created by rods placed in a jet. Synchronously, a planar random array focused on the same plane as the LDV measurement, measured pressure uctuation outside the hydrodynamic eld. The same set of data was then analysed using conventional and co- herence based beamforming algorithms. The results from both techniques were compared. While conventional beamforming identi ed the contribution from the main sources within the ow, the coherence based method was able to single out the source related to the local measurement. The results shown demonstrated how coherence based beamforming, combined with LDV, can be a useful tool for aeroacoustic noise source investigations. Description: PUBLISHED

Cell-matrix interactions regulate mesenchymal stem cell response to hydrostatic pressure

Sun, 01 Jan 2012 00:00:00 GMT

Title: Cell-matrix interactions regulate mesenchymal stem cell response to hydrostatic pressure Author: THORPE, STEPHEN; BUCKLEY, CONOR TIMOTHY; KELLY, DANIEL JOHN; STEWARD, ANDREW Abstract: Both hydrostatic pressure (HP) and cell-matrix interactions have independently been shown to regulate the chondrogenic differentiation of mesenchymal stem cells (MSCs). The objective of this study was to test the hypothesis that the response of MSCs to hydrostatic pressure will depend on the biomaterial within which the cells are encapsulated. Bone marrow derived MSCs were seeded into either agarose or fibrin hydrogels and exposed to 10 MPa of cyclic HP (1 Hz, 4 h/day, 5 days/week for 3 weeks) in the presence of either 1 or 10 ng/ml TGF-β3. Agarose hydrogels were found to support a spherical cellular morphology, while MSCs seeded into fibrin hydrogels attached and spread, with clear stress fiber formation. Hydrogel contraction was also observed in MSC-fibrin constructs. While agarose hydrogels better supported chondrogenesis of MSCs, HP only enhanced sulphated glycosaminoglycans (sGAG) accumulation in fibrin hydrogels, which correlated with a reduction in fibrin contraction. HP also reduced alkaline phosphatase activity in the media for both agarose and fibrin constructs, suggesting that this stimulus plays a role in the maintenance of the chondrogenic phenotype. This study demonstrates that a complex relationship exists between cell-matrix interactions and hydrostatic pressure which plays a key role in regulating the chondrogenic differentiation of MSCs. Description: PUBLISHED