271,057 research outputs found
Structure of unsteady stably stratified turbulence with mean shear.
No full textThe statistics of unsteady turbulence with uniform stratification N (Brunt–Väisälä frequency) and shear α(=dU1/dx3) are analysed over the entire time range (00 and \it Ri>0.25 respectively, oscillatory momentum and positive and negative density fluxes develop. Above a critical value of \it Ri\scriptsize\it crit(∼0.3), their average values are persistently countergradient. This structural change in the turbulence is the primary mechanism whereby stable stratification reduces the fluxes and the production of variances. It is quite universal and differs from the energy and stability mechanisms of Richardson (1926) and Taylor (1931). The long-time asymptotics of the energy ratio ER(=\it PE/VKE) of the potential energy to the vertical kinetic energy generally decreases with \it Ri(≥0.25), reaching the smallest value of 3/2 when there is no shear (\it Ri→∞). For strong mean shear (\it Ri<0.25), RDT significantly overestimates ER since (as in unstratified shear flow) it underestimates the vertical kinetic energy VKE. The RDT results show that the asymptotic values of the energy ratio ER and the normalized vertical density flux are independent of the initial value of ER, in agreement with DNS. This independence of the initial condition occurs because the ratios of the contributions from the initial values PE0 and KE0 are the same for PE and VKE and can be explained by the linear processes. Stable stratification generates buoyancy oscillations in the direction of the energy propagation of the internal gravity wave and suppresses the generation of turbulence by mean shear. Because the shear distorts the wavenumber fluctuations, the low-wavenumber spectrum of the vertical kinetic energy has the general form E33(k)∝(αtk)−1, where (LXαt)−1≪k≪L−1X (LX: integral scale). The viscous decay is controlled by the shear, so that the components of larger streamwise wavenumber k1 decay faster. Then, combined with the spectrum distortion by the shear, the energy and the flux are increasingly dominated by the small-k1 components as time elapses. They oscillate at the buoyancy period π/N because even in a shear flow the components as k1→0 are weakly affected by the shear. The effects of stratification N and shear α at small scales are to reduce both VKE and PE. Even for the same \it Ri, larger N and α reduce the high-wavenumber components of VKE and PE. This supports the applicability of the linear assumption for large N and α. At large scales, the stratification and shear effects oppose each other, i.e. both VKE and PE decrease due to the stratification but they increase due to the shear. We conclude that certain of these unsteady results can be applied directly to estimate the properties of sheared turbulence in a statistically steady state, but others can only be applied qualitatively
Analysis of an alternative topology for steel-concrete-steel sandwich beams incorporating inclined shear connectors
No full textThis thesis presents a new concept in steel-concrete-steel sandwich construction in which a bi-directional corrugated-strip core is proposed as an alternative inclined shear connector. The focus is on the feasibility study of fabrication techniques and the theoretical study of the structural responses of both unfilled and concrete-filled steel-concrete-steel sandwich beams under static flexural loading using numerical and analytical methods. Two possible fabrication techniques to create the proposed bi-directional corrugated strip core are presented. The unfilled sandwich beam is studied using a finite element method and three analytical methods referred to as the modified stiffness matrix, the braced frame analogy, and the discrete beam methods The finite element method is used to investigate the stiffness and strength behaviour of the unfilled sandwich beam.The modified stiffness matrix method provides good correlation with the finite element method. The other two analytical methods are less accurate. The assessment of the effect of geometrical parameters defining the bi-directional corrugated-strip core is carried out. The responses of the strength and stiffness, especially the transverse shear stiffness, are examined and discussed. The optimum configuration of the core is found at the angle of the inclined part of the corrugation is about 45°. The concrete-filled sandwich beam is studied using the finite element method. The finite element method is used to investigate the transverse shear strength and the crack development of a four-point loaded concrete-filled sandwich beam. The assessment of the effect of geometrical parameters defining the inclined shear connectors is carried out. The responses of the concrete-filled sandwich beam are examined and discussed. The optimum advantage of the transverse shear strength of the concrete-filled sandwich beam is also found when the inclined shear connectors align at an angle 45º. It is found that creating the proposed core with a 45° pattern provides a great advantage in transverse shear stiffness and strength in both the unfilled and concrete-filled sandwich beams
Reversible Rail Shear Apparatus Applied to the Study of Woven Laminate Shear Behavior
No full textThe multitude of in-plane shear tests existing in the literature seems to demonstrate the complexity of developing a test adapted to all experimental works. In a general framework of investigation of translaminar cracks in thin laminates, a test able to reproduce a pure in-plane shear loading was required. The laminate studied is notably employed as helicopter blade skin, and cyclic torsion induced by aerodynamic load involves cyclic in-plane shear. This particular application established some specifications for the test needed to carry out this study. To comply with them, an original technological solution has been developed from a three-rail shear test apparatus. This paper describes the resulting “reversible rail shear test” solution and its application to the study of in-plane shear behavior of a thin glass-epoxy laminate. The results concern plain and notched coupons under quasi-static loading, and crack growth tests under cyclic loading
Evaluation of bed shear stress under turbid flows through measures of flow deceleration
No full textIn previous work it has been observed that the direct application of the law-of-the-wall in flows of high concentrations of cohesive sediments, results in overestimation of the bed shear stress. In this experimental work the effect of various turbidity levels on the drag coefficient and shear stress over a smooth bed is investigated. For this purpose, turbid flows were simulated in a laboratory annular flume (the Lab Carousel), using kaolinite suspensions of different concentrations, varying from 0 to 60 g/l. Flow velocity measurements were taken in the turbulent boundary layer of hydraulically smooth flows and the values of bed drag coefficient and shear stress were calculated using the method of Flow Deceleration. The results obtained after the processing of the velocity time-series and the application of the Flow Deceleration method indicate that there is no evidence of shear stress modification for suspended sediment concentrations below 3 g/l. However, at higher concentrations the flow in the Lab Carousel exhibited different boundary layer characteristics. Therefore, the bed drag coefficient showed a significant response to the increase of the suspended clay concentration by decreasing up to ~ 50% from the clear water value over the range 3-60 g/l under a flow speed range of 0.8 m/s. The equivalent decrease in bed shear stress was calculated as ~ 40%. These results are in accordance with earlier findings of various researchers, thus confirming the validity of the Flow Deceleration method, used in accurate estimations of the bed drag coefficient and shear stress within the Lab Carousel
Shear transfer between precast prestressed bridge beams and in-situ concrete crosshead in continuous structures
No full textA detailed investigation was made to study the shear transfer between precast prestressed beams and in-situ concrete in a relatively new method of construction of continuous bridge decks where the ends of precast beams are
connected to an integral in-situ crosshead away from the supports.
Two series of tests were carried out. In the first series 1/3 scale models of the M. o. T, C&CA M-8 sections were used, and these were modified in the second series to study the effect of the beam's top flanges within the connection.
One of the most important mechanisms of shear transfer proved to be the top flanges of the precast beam.
For the precast beams with top flanges (first series), and with a 300mm beam embedment length, it was discovered that:
a) The shear force is transferred from a small length at the end of the beam.
b) The in-situ concrete nibs (concrete surrounding the web) can take this shear force without stirrups.
c) There is no need either to project all the bars from the precast into the in-situ concrete or to prestress the connection transversely as a means of improving shear transfer.
d) It was possible to transfer the whole shear force at the connection with a reduced embedment length of 100mm with nib stirrups.
For the precast beam without top flanges, the transfer of the shear force at the connection required other improving details. In this respect transverse prestressing and web shear connectors were utilized effectively. The effect of
projecting bars was also examined.
In the general behaviour of composite continuous beams subjected to shear a detailed comparison was made between different Code predictions for the web cracking shear and web crushing strength. A mathematical model is also
proposed to predict the stirrup stress according to shear span, effective depth and stirrup ratio when failure is controlled by web crushing. Stirrup stress measurement in the vicinity of continuous support made it possible to predict the enhanced shear strength and a design method is proposed for the continuous beams. A comparion is also made between different Code predictions in this respect.
To obtain more information about the strength of web shear connectors used in the secod series, a separate dowel shear specimen was designed. Different interface conditions including bond, dowel bar size and strength and the effect of shrinkage were examined. A design method is proposed together with a comparison with different Code predictions
Settling of particles in homogeneous shear turbulence
No full textThe settling of (inertial) particles is studied in homogeneous shear turbulence. A drift velocity perpendicular to gravity is measured due to the interplay between the homogeneous shear turbulence and gravity acting on the particles. We introduce a model to predict and understand this phenomenon
Development of a direct simple shear apparatus for peat
No full textThis paper discusses the design and development of a new direct simple shear (DSS)
apparatus for testing peat soils. The apparatus has been designed to test peat at low
effective stresses, representative of its in-situ condition and allow the deformation of
the specimen to be monitored. This device uses Particle Image Velocimetry (PIV)
image analysis techniques to monitor the side of the peat specimen and provide an
insight into the behaviour of peat during shearing. A set of comparative tests on
remoulded clay have been conducted with another widely used DSS apparatus and has
shown both to yield similar undrained strength ratios (su/σ'vc) for a range of stress
levels. Application of the apparatus to peat soils is demonstrated by a set of tests on a
high water content blanket bog peat. Analysis of these tests using the PIV technique
reveals the complex shear strain and volumetric strain behaviour of peat undergoing
shearing. Identification of partial slippage of a specimen is also demonstrated through
these analyses.Not applicabl
Shear strengthening of wall panels through jacketing with cement mortar reinforced by GFRP grids
No full textThis paper gives the results of a series of shear tests carried out on historic wall panels reinforced with an innovative technique by means of jacketing with GFRP (Glass Fiber Reinforced Plastics) mesh inserted into an inorganic matrix. Tests were carried out in situ on panels cut from three different historic buildings in Italy: two in double-leaf rough hewn rubble stone masonry in Umbria and L'Aquila and another with solid brick masonry in Emilia. Two widely-known test methods: the diagonal compression test and the shear-compression test with existing confinement stress. The test results enabled the determination of the shear strength of the masonry before and after the application of the reinforcement. The panels strengthened with the GFRP exhibited a significant improvement in lateral load-carrying capacity of up to 1060% when compared to the control panels. A numerical study assessed the global behavior and the stress evolution in the unreinforced and strengthened panels using a finite element code
Continuity development between precast beams using prestressed slabs, and its effect on flexure and shear
No full textDevelopment of continuity between precast prestressed bridge girders by post-tensioning the insitu top slab In the regions of hogging moment is a relatively new technique which forms the basis for this research study. Compared
to the more conventional method of using reinforcing steel in the slab over the interior supports, prestressed slabs will ensure a crack free more durable bridge deck, and will therefore reduce the maintenance costs.
The effect that such a slab has on flexural and shear behaviour of the bridge deck has been studied both analytically and experimentally by considering
composite beams based on M-8 standard precast beam section. Comparison of the design of bridge decks with a prestressed slab and a reinforced concrete slab indicated that a partially prestressed slab with a prestress considering up to 50% of the live load will ensure the slab remains crack free under total service load. Although secondary effects and the two stage construction of such a slab tend to
increase the prestress requirement for the slab, the same two effects considerably reduce the positive midspan moments, resulting in a decrease in the prestress
required in the precast beams (and thus a possible increase in the span range) for given standard precast beam sections.
The experimental investigation consisted of testing eleven 1/3-scale M-8 continuous composite beams in two series, Series-A and Series-B. Series A, in which three beams were tested as double cantilevers was planned to study the
effects of prestressed slab on overall flexural behaviour. A considerable improvement in crack control under service loads and a higher ratio of measured to calculated ultimate moment capacity was obtained in beams with a prestressed
slab. The continuity developed using insitu prestressed slabs was very effective at all levels of loading. Recommendations have been made for the flexural design of
continuous bridge decks with this type of prestressed slabs.
In Series B, effect of prestressed slabs on shear strength at the continuity connection has been studied. A considerable increase in web shear cracking load was obtained for beams with prestressed slabs, resulting in a
decrease In the amount of shear reinforcement required for such beams. The different methods of predicting web shear cracking strength and web crushing strength according to current design codes were compared with experimental
values, and based on the results, recommendations for the design for vertical shear of composite beams subjected to hogging moments have been made
Wall-shear stress measurements of turbulent flow over ribbed surfaces using the micro-pillar shear stress sensor MPS3
No full textThe drag reduction effect of a semi-circular riblet-structured surface in a turbulent boundary layer is experimentally investigated using the micro-pillar shear stress sensor MPS3. The MPS3 sensor is a novel tool for the quantitative measurement of the wall-shear stress distribution and possesses a high spatial and temporal resolution. The effectiveness and mechanisms of a ribbed surface in skin friction reduction are to be examined in comparison with the flow case of a flat surface
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