2,809 research outputs found
Numerical modelling of seepage beneath skirted foundations subjected to vertical uplift
No full textThis study reports the results of small strain finite element analyses undertaken to determine the rate of displacement of skirted foundations subjected to uplift loading, due to combined swelling and seepage. Compatibility of foundation movement with cumulative seepage of water into the skirt compartment is modelled using a layer of soft poroelastic material immediately below the foundation top plate. Performance of the model is first assessed for a range of stiffness values for the soft layer. The model is then used to investigate the rate of displacement for skirt depths ranging from 0.1 to 1 times the foundation diameter. The results are compared with available theoretical solutions and experimental results, and expressions are provided for calculating the equivalent seepage lengths and resulting uplift velocities. The effect on the seepage behaviour of the presence of a gap down the external skirt-soil interface is also discussed.</p
Large-deformation numerical modeling of short-term compression and uplift capacity of offshore shallow foundations
No full textLarge-deformation finite-element analysis has been used to model the undrained response of skirted shallow foundations in uplift and compression. Large-deformation effects involve changes in embedment ratio and operative local soil shear strength with increasing foundation displacement-either in tension or compression. Centrifuge model testing has shown that these changes in geometry affect the mobilized bearing capacity and the kinematic mechanisms governing failure in undrained uplift and compression. Small-strain finite-element analysis cannot by definition capture the effects of changing foundation embedment ratio and variation in local soil strength with foundation displacement. In this paper, load-displacement relationships, ultimate capacities, and kinematic mechanisms governing failure from largedeformation finite-element analyses are compared with centrifuge model test results for circular skirted foundations with a range of embedment between 10 and 50% of the foundation diameter. The results show that the large-deformation finite-element method can replicate the loaddisplacement response of the foundations over large displacements, pre-and postyield, and also capture differences in the soil deformation patterns in uplift and compression. The findings from this study increase confidence in using advanced numerical methods for determining shallow skirted foundation behavior, particularly for load paths involving uplift.</p
Experimental investigation of reverse end bearing of offshore shallow foundations
No full textShallow skirted foundations can mobilize uplift resistance from end bearing in the short to medium term. However, uncertainty exists over the magnitude of reverse end bearing resistance compared with resistance in compression, and how this might be affected by a gap between the external face of the foundation skirt and the adjacent soil. The study presented in this paper explores this problem through centrifuge model tests, investigating the effect of skirt embedment ratio on (i) the magnitude of reverse end bearing capacity compared with compression capacity, (ii) the uplift displacement associated with spontaneous loss of suction during uplift, and (iii) the existence of a vertical gap along the external skirt-soil interface. The results show that (i) peak uplift capacity equivalent to compression capacity can be mobilized for a fully sealed foundation with an intact skirt-soil interface, (ii) suction required for reverse end bearing can be maintained through considerable foundation displacement, even for a low skirt embedment ratio, and (iii) the presence of a vertical gap along the external skirt-soil interface causes abrupt loss of suction beneath the top plate after minimal foundation displacement, with subsequent uplift capacity being markedly reduced.</p
Turbulent drag reduction by hydrophobic surfaces with shear-dependent slip length
No full textThe stabilisation of a parabolic equilibrium profile in a three-dimensional (3D) turbulent channel flow for an incompressible fluid is addressed with the objective of achieving drag reduction. The formulation of this problem stems from Balogh’s work [1] where Lyapunov stability analysis was used to devise prototype feedback laws and prove global stability of the solutions. This treatment only considers the controller as a mathematical artefact, but it can actually be linked to physical control strategies modelling hydrophobic surfaces and porous media. In the former, only linear slip velocity boundary conditions (BC) were considered [8]. However, experiments [2] have suggested that the slip length may be shear-dependent. Motivated by these, the effect on drag reduction of a shear-dependent slip length surface is examined in the present study using Direct Numerical Simulations (DNS) at Re τ0 = u τ0 δ/ν ≃ 180. δ is the channel half height, u τ0 the wall-shear velocity for regular no-slip walls channel and ν the kinematic viscosity. The theoretical analysis in [5], is extended to this new model. The proposed formulation shows that the skin-friction coefficient can be reduced by tuning the parameters in the shear-dependent slip length model. The results, which verified by DNS simulations, show that by taking a slip length value based on a constant slip model [8] and combining it within a shear-dependent model, up to 50% drag reduction can be obtained. The effect of control is further assessed by formulating the Fukagata identity [4] with general boundaries; the weighted Reynolds shear-stress for each quadrant shows an enhanced reduction in the sweep/ejection events compared to the constant slip model
Performing content-based retrieval of humans using gait biometrics
No full textIn order to analyse surveillance video, we need to efficiently explore large datasets containing videos of walking humans. At surveillance-image resolution, the human walk (their gait) can be determined automatically, and more readily than other features such as the face. Effective analysis of such data relies on retrieval of video data which has been enriched using semantic annotations. A manual annotation process is time-consuming and prone to error due to subject bias. We explore the content-based retrieval of videos containing walking subjects, using semantic queries. We evaluate current biometric research using gait, unique in its effectiveness at recognising people at a distance. We introduce a set of semantic traits discernible by humans at a distance, outlining their psychological validity. Working under the premise that similarity of the chosen gait signature implies similarity of certain semantic traits we perform a set of semantic retrieval experiments using popular latent semantic analysis techniques from the information retrieval community
Hydrodynamical turbulence by fractal fourier decimation
No full textWe present a systematic numerical investigation of high-resolution 3D isotropic and homogeneous turbulence resolved on a decimated set of Fourier modes. Fractal decimation acts to decrease the effective dimensionality of the flow by allowing triadic interactions only in a set of Fourier modes N(k) proportional to k^DF for large k. While keeping the symmetries of the original 3D Navier-Stokes equations unchanged, a dramatic change in small-scale statistics is detected at decreasing the fractal dimension DF . Already at fractal dimension DF = 2.8, a global self-similar behaviour is observed in the inertial range of scales, the consequence of such transition are the restoration of the scaling symmetry and vorticity distribution that becomes close to Gaussian. We relate the results to the different roles of local vs non-local interactions in the energy transfer range
Metallers mobilitet i mark
No full textI rapporten beskrivs nuvarande kunskapsläge när det gäller metallers uppträdande i mark. Riskbedömningar för metaller i förorenad mark diskuteras. De flesta metaller binds i viss utsträckning i marken, oftast genom olika ytreaktioner med markens organiska material eller med järn- och aluminiumoxider, och ibland även genom utfällningsreaktioner. I vilken omfattning detta sker beror av faktorer som t.ex. pH, redoxförhållanden, löst organiskt material (DOC) i markvattnet, samt förekomst av konkurrerande joner. Det är också viktigt att beakta vilka former metallerna förekommer som i markvattnet. Komplexbildning med t.ex. DOC minskar i regel toxiciteten. För att bättre ta hänsyn till markkemiska förhållanden i riskbedömningen bör geokemiska modeller komma till ökad användning. Metaller lösta i vattnet följer med när vattnet strömmar och kan transporteras till grund- och ytvatten eller tas upp av växter. Hur snabbt detta går beror, förutom kemiska och biologiska processer, även på en rad olika jordegenskaper. Till exempel innehåller vissa jordar s.k. makroporer vilka ger upphov till preferentiellt flöde. Detta innebär att en del av metallerna snabbare kan transporteras genom marken, förbi jordpartiklar där metallerna annars skulle bindas. De olika modeller som används för att beskriva påverkan av markförorening på grundvatten kan delas in i två grupper: stationära modeller antar att källtermen är konstant och tar enbart hänsyn till den utspädning som sker i grundvattnet. Exempel på sådana modeller är den svenska riktvärdesmodellen, JAGG och RBCA. I de två senare modellerna finns möjlighet att ta hänsyn till nedbrytning av organiska ämnen. Tidsberoende modeller antar en avklingande källterm och en tidsberoende transport i grundvattnet, exempelvis RISC och TAC-modellen. För beräkning av transport i grundvattnet tas hänsyn till flödes- och fastläggningsmekanismer som advektion, dispersion och linjär sorption. Med hjälp av laktester kan man uppskatta den andel av metallföroreningen som är löslig i vatten. Denna information kan sedan användas för att beräkna adsorptionsparametrar (Kd-värden, m.m.) för spridningsmodeller. Laktester kan ibland ge missvisande resultat eftersom de orsakar utspädning av provet, bl.a. blir DOCkoncentrationerna alltför låga. Dessutom bör de inte användas för sulfidjordar. Geokemiska modeller kan användas för att förbättra tolkningen av laktester för att få till stånd realistiska uppskattningar av Kd-värden. Det finns ett antal spridningsmodeller som kan användas för att bedöma risken för spridning till grundvatten och ytvatten. De är kraftfulla verktyg som dock ännu inte använts i någon större utsträckning i Sverige. Detta beror på flera orsaker, bl.a. de hydrogeologiska förhållandena i Sverige, behovet av anpassningen av de modeller som är allmänt tillgängliga, svårigheter att ta fram bra dataunderlag samt en svag tradition i att använda modeller och förstå modellresultat. Ett par angelägna forsknings- och utvecklingsinsatser är att ta fram bättre uppskattningar av metalladsorption i svensk mark, och att anpassa existerande spridningsmodeller för användning i riskbedömningar.
Predicting adherence to antiretroviral therapy and retention to HIV care : effects of baseline biopsychosocial status and neuropsychological functioning
No full textThese drugs have demonstrated efficacy in improving immune function and reducing HIV-related morbidity and mortality, and while a cure is not available, patients on treatment may live longer, healthier lives. However, early optimism has been tempered by the growing recognition that meticulous adherence is a prerequisite for optimal clinical response and prevention of drug resistance
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