No description available.

No description available.

No description available.

In this thesis, the tonal noise produced by flow over perforated plates at oblique angles of incidence is studied experimentally. A two-dimensional model of a perforated plate is used, where the circular holes of a typical perforated plate are replaced by a series of long rectangular Aluminum slats with an adjustable gap width between them. The slats are 3.175 mm thick and the gap width between them is set to 3.175 mm, 6.35 mm, and 12.7 mm. This simplified model is mounted at the exit of an open-loop wind tunnel and tested at angles of incidence of 0° to 40° and flow velocities of 0 to 30 m/s. An angle of 0° is defined as flow parallel to the plate. The acoustic response is studied using microphone measurements, and flow visualization is done using particle image velocimetry. The effect of the angle of incidence, flow velocity, gap width, and streamwise position are investigated. The flow visualization reveals that tonal noise is produced by the periodic shedding and impingement of vortices at the trailing edge of the gaps. Vortices form in the unstable free shear layer originating at the leading edge of the gap and impinge on the downstream side of the gap. At the downstream corner, these vortices separate into vortex pairs, consisting of one positively rotating and one negatively rotating vortex. These vortices are shed periodically, leading to the production of tonal noise at the shedding frequency. The effect of the angle of incidence is investigated by changing the angle of the plate with respect to the flow. For a given gap width, tones are produced only for a specific range of angles. Depending on the plate geometry, this range of angles is typically around 5° to 30°. Within this range of angles, the free shear layer impinges on the downstream side of the gap. For angles which are too small or too large, the free shear layer misses this downstream side and tones are not produced. For a larger gap width, tones are produced at smaller angles of incidence. Similarly, for a given plate geometry, there is a preferred range of flow velocities at which tonal noise is produced. The velocity at which the free shear layer is the most unstable at the tone frequency produces the strongest vortices and the loudest tones. The optimal velocity is lower for larger gap widths. Finally, it is found that the magnitude of the produced tones increases in the streamwise direction over repeated gaps along the length of the plate. This is due to the local flow conditions changing in the streamwise direction, only reaching the optimal conditions after a certain length of the plate. / Thesis / Master of Applied Science (MASc)

Recent progresses by the aircraft industry in the development of a quieter supersonic transport have opened the possibility of overland supersonic flights, which are currently banned by aviation authorities in most countries. For the ban to be lifted, the sonic booms the aircraft generate at supersonic speed must be acceptable from a human-perception point of view, in particular inside buildings. The problem of the transmission of sonic booms inside buildings can be divided in several aspects such as the external pressure loading, structure vibration, and interior acoustic response. Past investigations on this problem have tackled all these aspects but were limited to simple structures and often did not account for the coupled fluid-structure interaction. A more comprehensive work that includes all the effects of sonic booms to ultimately predict the noise exposure inside realistic building structures, e.g. residential houses, has never been reported. Thus far, these effects could only be investigated experimentally, e.g. flight tests. In this research, a numerical model and a computer code are developed within the above context to predict the vibro-acoustic response of simplified building structures exposed to sonic booms, at low frequency. The model is applicable to structures with multiple rectangular cavities, isolated or interconnected with openings. The response of the fluid-structure system, including their fully coupled interaction, is computed in the time domain using a modal-decomposition approach for both the structural and acoustic systems. In the dynamic equations, the structural displacement is expressed in terms of summations over the "in vacuo" normal modes of vibration. The interior pressure is expressed in terms of summations over the acoustic modes of the rooms with perfectly reflecting surfaces (hard walls). This approach is simple to implement and computationally efficient at low frequency, when the modal density is relatively low. The numerical model is designed specifically for this application and includes several novel formulations. Firstly, a new shell finite-element is derived to model the structural components typically used in building construction that have orthotropic characteristics such as plaster-wood walls, floors, and siding panels. The constitutive matrix for these types of components is formulated using simple analytical expressions based on the orthotropic constants of an equivalent orthotropic plate. This approach is computationally efficient since there is no need to model all the individual subcomponents of the assembly (studs, sheathing, etc.) and their interconnections. Secondly, a dedicated finite-element module is developed that implements the new shell element for orthotropic components as well as a conventional shell element for isotropic components, e.g. window panels and doors. The finite element module computes the "in vacuo" structural modes of vibration. The modes and external pressure distribution are then used to compute modal loads. This dedicated finite-element module has the main advantage of overcoming the need, and subsequent complications, for using a large commercial finite-element program. Lastly, a novel formulation is developed for the fully coupled fluid-structure model to handle room openings and compute the acoustic response of interconnected rooms. The formulation is based on the Helmholtz resonator approach and is applicable to the very low frequency-range, when the acoustic wavelength is much larger than the opening dimensions. Experimental validation of the numerical model and computer code is presented for three test cases of increasing complexity. The first test structure consists of a single plaster-wood wall backed by a rigid rectangular enclosure. The structure is excited by sonic booms generated with a speaker. The second test structure is a single room made of plaster-wood walls with two double-panel windows and a door. The third test structure consists of the first room to which a second room with a large window assembly was added. Several door configurations of the structure are tested to validate the formulation for room openings. This latter case is the most realistic one as it involves the interaction of several structural components with several interior cavities. For the last two test cases, sonic booms with realistic durations and amplitudes were generated using an explosive technique. Numerical predictions are compared to the experimental data for the three test cases and show a good overall agreement. Finally, results from a parametric study are presented for the case of the single wall backed by a rigid enclosure. The effects of sonic-boom shape, e.g. rise time and duration, and effects of the structure geometry on the fluid-structure response to sonic booms are investigated. / Ph. D.

Foreign object ingestion (FOI) into jet engines is a recurring scenario during the operation life of aircraft. Objects can range from as small as a pebble on the tarmac to the size of a large bird. Among the potential ingestion scenarios, damage caused by smaller objects may be considered to be negligible. Alternatively, larger objects can initiate progressive damage, potentially leading to catastrophic failure, compromising the integrity of the structure, and endangering the safety of passengers. Considering the dramatic increase in air traffic, FOI represents a crucial safety hazard, and must be better understood to minimize possible damage and structural failure. The main purpose of this study is to develop a unique methodology to assess the response and dynamic damage progression of an advanced, high-bypass propulsion system in the event of an FOI during operation. Using a finite element framework, a unique modeling methodology has been proposed in order to characterize the FOI response of the system. In order to demonstrate versatility of the computational analysis, the impact characteristics of two most common foreign object materials, bird and ice, were investigated. These materials were then defined in finite element domain, verified computationally, and then validated against the existing physical experiments. In addition to the mechanics of the two FOI materials, other material definitions, used to characterize the structures of the high-bypass propulsion system, were also explored. Both composite materials and rate dependent definitions for metal alloys were investigated to represent the damage mechanics in the event of an FOI. Subsequently, damage sequence of high-bypass propulsion systems subject to FOI was developed and assessed, using a uniquely devised Fluid-Structure Interaction (FSI) technique. Using advanced finite element formulation, this approach enabled the accurate simulation of the comprehensive damage progression of the propulsion systems by including aerodynamic interaction. Through this strategy, fluid mechanics was combined with structural mechanics in order to simulate the mutual interaction between both continua, allowing the interpretation of both the additional damage caused by the fluid flow and disrupted aerodynamics induced by the dynamic deformation of the fan blade. Subsequently, this multidisciplinary-multiphysics computational approach, in the framework of the comprehensive analysis methodology introduced, enabled the effective determination of details on the overall progressive impact damage, not traditionally available to propulsion designers. / PHD

The point absorber, one of the most popular types of ocean wave energy converter (WEC), usually consists of a rigid body buoy that can be efficiently modeled using existing WEC simulation tools. However, new wave energy technologies have looked to utilize flexible buoy structures to decrease costs, improve power generation, and increase portability. In addition to replacing rigid body designs, the combination of multiple renewable energy sources is another area that shows promising potential for increasing WEC power generation. With these concepts in mind, this work considers a new WEC design that features a flexible-inflatable buoy, an ocean current harvesting turbine, and a buoy shape that has been optimized for simultaneous wave and current energy harvesting. For this device, conventional modeling techniques cannot be used due to the highly nonlinear hydrodynamic interactions that result between the flexible buoy and the ocean waves. As a result, a Fluid-Structure Interaction (FSI) model must be used to determine how the flexibility of the buoy will influence the device's power generation. Currently, high-fidelity FSI modeling approaches are computationally expensive and unsuitable for early design decisions. As a result, this thesis utilizes a mid-fidelity method, the generalized modes modeling approach, to accurately and efficiently model the FSI of a WEC's flexible buoy. The resulting flexible buoy model was then compared to a rigid design to determine the performance differences between a rigid and flexible buoy, with a complex, optimized shape. / Master of Science / The ocean is a vast potential energy resource with a variety of different sources of renewable energy. Of these sources, ocean waves and ocean currents are two potentially massive power reserves present in many coastal areas. To capture energy from these sources, this work discusses the development of a device that can harvest energy from ocean waves and ocean currents simultaneously. In addition to harvesting energy from multiple sources, this device also features a flexible-inflatable buoy, with a shape that has been optimized for this unique application. However, since this device utilizes flexible materials, traditional modeling techniques used for rigid body designs would not be applicable. As a result, this work looks to model the interaction between the flexible buoy and the ocean waves to accurately predict the power generation of this device's wave energy converter.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / Aortic valve replacements are frequently performed during heart surgery. However, since this is quite a stressful procedure, many patients are turned down for medical reasons. Stented valves, designed and manufactured for percutaneous insertion, eliminate many of the risks involved in open-heart surgery, thus providing a solution to patients not deemed strong enough for open-chest aortic valve replacements. The aortic valve is a complex structure, and therefore numerical simulation is necessary to obtain flow and stress data to support the design of a prosthetic heart valve in the absence of viable physical measuring methods. To aid in the design of a prosthetic heart valve, various finite element valve models were created, and the fluid structure interaction (FSI) between the valves and the blood was simulated using commercial finite element software. The effect of the geometry of the leaflets on the haemodynamic behaviour over the cardiac cycle was investigated. It was found that leaflet dimensions should be chosen judiciously, because of their considerable effect on the stress distribution and performance of the valve. A simple leaflet geometry optimisation was done for a 20 mm and 26 mm valve, respectively, by means of existing geometry relationships found in the literature. Simulations were done to obtain the maximum leaflet attachment forces that can be used by a stent designer for fatigue loading, or to investigate the structural strength of the stent. These simulations were numerically validated. The effect of leaflet thickness and stiffness on resistance to opening, stress distribution and strain were investigated. Results showed that leaflet thickness has a greater effect on the performance of the valve than leaflet stiffness, and thereby validated the results of similar tests contained in the literature. After simulating over-, as well as under-dilation of a stented valve, it was found that problems associated with over-dilation can be minimised to a certain extent by increasing the coaptation1 region of the leaflets. A simple pulse duplicator was designed based on a four-element Windkessel model. The pulse duplicator was used to study the performance of the prototype valves by means of high-speed photography, the results of which were fed into one of the numerical finite element models and compared to real valve performance. Some of the prototype valves showed efficiencies of 88%.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Bioprosthetic heart valves (BHVs) for transcatheter aortic valve implantation (TAVI) have been rapidly developing over the last decade since the first valve replacement using the TAVI technique. TAVI is a minimally invasive valve replacement procedure offering lifesaving treatment to patients who are denied open heart surgery. The biomedical engineering research group at Stellenbosch University designed a 19 mm balloon expandable BHV for TAVI in 2007/8 for testing in animal trials. In the current study the valve was enlarged to 23 mm and 26 mm diameters. A finite element analysis was performed to aid in the design of the stents. New stencils were designed and manufactured for the leaflets using Thubrikar‟s equations as a guide. The 23 mm valve was manufactured and successfully implanted into two sheep. Fluid structure interaction (FSI) simulations constitute a large portion of this thesis and are being recognized as an important tool in the design of BHVs. Furthermore, they provide insight into the interaction of the blood with the valve, the leaflet dynamics and valve hemodynamic performance. The complex material properties, pulsating flow, large deformations and coupling of the fluid and the physical structure make this one of the most complicated and difficult research areas within the body. The FSI simulations, of the current valve design, were performed using a commercial programme called MSC.Dytran. A validation study was performed using data collected from a cardiac pulse duplicator. The FSI model was validated using leaflet dynamics visualisation and transvalvular pressure gradient comparison. Further comparison studies were performed to determine the material model to be used and the effect of leaflet free edge length and valve diameter on valve performance. The results from the validation study correlated well, considering the limitations that were experienced. However, further research is required to achieve a thorough validation. The comparative studies indicated that the linear isotropic material model was the most stable material model which could be used to simulate the leaflet behaviour. The free edge length of the leaflet affects the leaflet dynamics but does not greatly hinder its performance. The hemodynamic performance of the valve improves with an increase in diameter and the leaflet dynamics perform well considering the increased surface area and length. Many limitations in the software prevented more accurate material models and flow initiation to be implemented. These limitations significantly restricted the research and confidence in the results. Further investigation regarding the implementation of FSI simulations of a heart valve using the commercial software is recommended. / AFRIKAANSE OPSOMMING: Bio-prostetiese hartkleppe (Bioprosthetic Heart Valves - BHVs) wat gebruik word vir transkateter aortaklep-inplantings (Transcatheter Aortic Valve Implantation - TAVI) het geweldig vinnige ontwikkeling getoon in die afgelope tien jaar sedert die eerste klepvervanging wat van die TAVI prosedure gebruik gemaak het. TAVI is ʼn minimaal indringende klepvervangingsprosedure wat lewensreddende behandeling bied aan pasiënte wat ope-hart sjirurgie geweier word. Die Biomediese Ingenieurswese Navorsingsgroep (BERG) by Stellenbosch Universiteit het in 2007/8 ʼn 19 mm ballon-uitsetbare BHV vir TAVI ontwerp vir eksperimente met diere, en hierdie tesis volg op die vorige projekte. In die huidige studie is die klep vergroot na 23 mm en 26 mm in deursnee. ʼn Eindige element analise is gedoen om by te dra tot die ontwerp van die rekspalke vir die klep. Nuwe stensils is ontwerp en vervaardig vir die klepsuile, deur gebruik te maak van Thubrikar se vergelykings. Die 23 mm klep is vervaardig en suksesvol in twee skape ingeplant. Vloeistruktuur interaksie (Fluid Structure Interaction (FSI)) simulasies vorm ‟n groot deel van die tesis en word gesien as ʼn noodsaaklike hulpmiddel in die ontwerp van BHVs. Die simulasies verskaf ook insig in die interaksie van die bloed met die klep, die klepsuil-dinamika en die klep se hemodinamiese werkverrigting. Die komplekse materiaal eienskappe, polsende vloei, grootskaalse vervorming, die verbinding van die vloeistof en die fisiese struktuur maak van hierdie een van die mees gekompliseerde voorwerpe om te simuleer. Die FSI simulasies van die huidige ontwerp, is uitgevoer deur van kommersiële sagteware, MSC.Dytran, gebruik te maak. ʼn Geldigheidstudie wat data gebruik het vanaf die hartklop-nabootser, is uitgevoer. Die FSI model word geverifieer deur klepsuil dinamika visualisering en ʼn vergelyking van die drukgradiënt gebruik te maak. Verdere vergelykende studies is uitgevoer om te bepaal watter materiaal model om te gebruik, asook die uitwerking van die klepsuil-vrye rand en klepdeursnee op die klep se werkverrigting. Die resultate van die studie korreleer goed, in ag genome die beperkings wat ervaar is. Verdere navorsing is egter nodig vir ʼn volledige geldigheidstudie. Vergelykende studies het getoon dat die liniêre isotropiese materiaalmodel die meer stabiele materiaalmodel is wat kan gebruik word om klepsuilgedrag te simuleer. Die vrye-rand lengte van die klepsuil affekteer die dinamika van die klepsuil, maar belemmer nie die werkverrigting grootliks nie. Die hemodinamiese werkverrigting van die klep verbeter met die toename in deursnee en die klepsuil-dinamika vertoon goed in ag genome die verhoogde oppervlak area en lengte. Die vele beperkings in die sagteware het die implementering van meer akkurate materiaalmodelle verhoed. Hierdie beperkings het ʼn verminderde vertroue in die resultate tot gevolg gehad. Verdere ondersoek rakende die implementering van die FSI simulasies van ʼn hartklep deur kommersieel beskikbare sagteware te gebruik, word aanbevel.

The response of submerged structures subject to loading by underwater blast waves is governed by complex interactions between the moving or deforming structure and the surrounding fluid and these phenomena need to be thoroughly understood in order to design structural components against underwater blast. This thesis has addressed the response of simple structural systems to blast loading in shallow or deep water environment. Analytical models have been developed to examine the one-dimensional response of both water-backed and air-backed submerged rigid plates, supported by linear springs and loaded by underwater shock waves. Cavitation phenomena as well as the effect of initial static fluid pressure are explicitly included in the models and their predictions were found in excellent agreement with detailed FE simulations. Then, a novel experimental apparatus has been developed, to reproduce controlled blast loading in initially pressurised liquids. It consists of a transparent water shock tube and allows observing the structural response as well as the propagation of cavitation fronts initiated by fluid-structure interaction in a blast event. This experimental technique was then employed to explore the one-dimensional response of monolithic plates, sandwich panels and double-walled structures subject to loading by underwater shock waves. The performed experiments provide great visual insight into the cavitation process and the experimental measurements were found to be in good agreement with analytical predictions and dynamic FE results. Finally, underwater blast loading of circular elastic plates has been investigated by theoretically modelling the main phenomena of dynamic plate deformation and fluid-structure interaction. In addition, underwater shock experiments have been performed on circular composite plates and the obtained measurements were found in good correlation with the corresponding analytical predictions. The validated analytical models were then used to determine the optimal designs of circular elastic plates which maximise the resistance to underwater blast.