Incipient motion criterion in sediment transport is very important, as it defines the flow condition that initiates sediment motion, and is also frequently employed in models to predict the sediment transport at higher flow conditions as well. In turbulent flows, even a reasonably accurate definition of incipient motion condition becomes very difficult due to the random nature of the turbulent process, which is responsible for sediment motion under incipient conditions. This work investigates two aspects, both of which apply to incipient sediment transport conditions. The first one deals with the role of turbulence in initiating sediment motion. The second part deals with the nature of sediment-fluid interaction for more general and complex flows where the number of sediment particles that form the rough surface is varied. The first part of this work that investigates the role of turbulence in initiating sediment motion, uses a video camera to simultaneously monitor and record the sediment (glass ball) motion and corresponding fluid velocity events measured by a three-component laser Doppler Velocimeter (LDV). The results of the single ball experiment revealed that the number of LDV flow measurements increase dramatically (more than four folds) just prior to the ball motion. The fluid mean velocity and its root-mean-square (rms) values also are significantly higher than the values that correspond to the flow conditions that yield no ball motion. The second part of the work, investigation of the fluid-sediment interaction, includes five tests with varying number of sediment particles. In order to understand the nature and extent of fluid-solid interaction, velocity profile measurements using the 3-D laser system were carried out at three locations for each of these five cases. Plots of mean velocities, rms quantities located the universal layer at about 1.5 ball diameters above the porous bed. However, at higher sediment particle concentrations, this distance reduced and the beginning of the universal layer approached the top of the porous bed. / Ph. D.

Geothermobarometric, micro- and macro-structural data indicate that peak metamorphic pressure and temperature of the Greater Himalayan Sequence (GHS) of the Garhwal Region of India increase dramatically across the Main Central Thrust (MCT). Metamorphic pressure and temperature increase from ~5 kbar and ~550 ºC in the Lesser Himalayan Crystalline Sequence (LHCS) in the footwall to ~14 kbar and ~850 ºC at ~3 km above the MCT in the hanging wall (GHS). Pressures decrease slightly upsection to ~8 kbar and temperatures remain nearly constant at ~850 ºC to the structurally overlying South Tibetan Detachment (STD). The LHCS exhibits a high temperature-depth gradient (30 ºC/km) whereas the lower GHS has a much lower temperature-depth gradient (18 ºC/km) that increases to ~28 ºC/km near the STD. The pressure-temperature pattern is consistent with conduction of heat from the hotter (initially deeper) GHS into the colder (initially shallower) footwall of LHCS and conductive cooling of the hotter hanging wall of GHS along the STD. Numerical "channel flow" models predict a pressure-temperature pattern for the exhumation of the GHS similar to what is observed in the Garhwal Region of India. However, observed pressures (~10-14 kbar) are higher than predicted in the models (~10-12). The higher pressure of the GHS is likely due to the greater exhumation from displacement along the Munsiari Thrust (MT). In other words, the GHS in the Eastern Garhwal region provides a deeper view of the channel material than elsewhere in the range. The temperature-depth ratios of the Eastern Garhwal region also exhibit a very different pattern of conductive heating and cooling of the LHCS and GHS respectively, than elsewhere in the range. Ductile features within the GHS exhibit sheath fold geometries, indicative of high degrees of ductile flow. Overprinting the ductile structure are two populations of extensional conjugate fractures oriented both parallel and perpendicular to the orogen. These fractures crosscut major tectonic boundaries in the region such as the MCT and STD, and are found throughout the LHCS, GHS, and Tethyan Sedimentary Sequences (TSS). The crosscutting of these brittle structures across the major tectonic boundaries in the area indicate that the various tectonolithic sequences were exhumed during widespread extensional deformation as one coherent block.

Neko is a portable framework for high-order spectral element flow simulations[1]. Originally from Nek5000, Neko not only inherited its numerical stability and well-performance, but also added the benefits of dynamic memory and support for many current accelerator backends. As a newly developed code, Neko requires comprehensive validation, and certain functions are yet to be implemented. To this end, the present work performed simulations in Nek5000 and Neko and compared the results. A straight channel with friction Reynolds number Reτ = 590 is used as the test case. Some functions are implemented in the current post-processing framework, and the statistics from different solvers are compared. It is shown that the results from tested solvers are similar, although two methods for discretization, i.e., PnPn−2 and PnPn, are used. Besides, a convergence analysis and a resolution study are done to further compare and validate the code.

Yes / There has been little study on the microscopic bubble entrainment and diffusion process on the high-speed self-aerated flows although the problem under investigation is theoretically important and has important engineering application. This study presents an experimental investigation on visual processes of free surface air entrainment and single bubble diffusion in supercritical open channel flows. The typical surface deformation, single air bubble rising and penetration are recorded using a high-speed camera system. Results show that for a single bubble formation process, surface entrapment development and bubble entrainment through a deformation evolution underneath the free surface are the two main features. The shape variation of local surface deformation with time follows an identical power law for different bubble size generations. The entrained bubble size depends on both size scale and shape of entrapped free surface. As the single bubble moves downstream, its longitudinal velocity is approximately the same as that of water flow surrounded it, while its vertical velocity for rising and penetration increases with the increase of the water flow velocity. An empirical-linear relationship for the bubble rising and penetration velocity with water flow velocity is obtained. This study demonstrates that the microscopic bubble movement can improve the self-aeration prediction in the open channel flow and advance the knowledge of our understanding of the macroscopic and microscopic air–water properties in hydraulic engineering. / National Natural Science Foundation of China (Grant number 51609162), Sichuan Science and Technology Program (Grant number 2019JDTD0007) and the Open funding of the State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University (Project No: Skhl1809).

Yes / Manning’s roughness coefficient (n) has been widely used to estimate flood discharges and flow depths in natural channels. Therefore, although extensive guidelines are available, the selection of the appropriate n value is of great importance to hydraulic engineers and hydrologists. Generally, the largest source of error in post-flood estimates is caused by the estimation of n values, particularly when there has been minimal field verification of flow resistance. This emphasizes the need to improve methods for evaluating the roughness coefficients. Trinidad and Tobago currently does not have any set method or standardised procedure that they use to determine the n value. Therefore, the objective of this study was to develop a soft computing model in the calculation of the roughness coefficient values using low flow discharge measurements for a stream. This study presents Gene-Expression Programming (GEP), as an improved approach to compute Manning’s Roughness Coefficient. The GEP model was found to be accurate, producing a coefficient of determination (R2) of 0.94 and Root Mean Square Error (RSME) of 0.0024.

<p>We study the effect of minute perturbations by using blowing jets at upstream and bio-inspired micro denticles on turbulence large-scale motions which are observed to be crucial in controlling heat transfer, noise and drag reduction. This work is divided into two phases. In first phase, we studied the effect of blowing perturbations at upstream on large-scale motions and associated co?herent vortical structures which are crucial in enhancing heat transfer by promoting mixing. The second phase is focused on impact of flow dynamics in preventing the biofouling using micro bioinspired structures and the importance of flow regime in designing the antifouling coating us?ing bioinspired structures is demonstrated, and subsequently, separation bubble dynamics and its characterization is carried out for a transonic channel imposed with pressure gradient to further expand our thesis outcomes to utilize micro bioinspired structures in aerospace applications, noise reduction, and to delay separation.</p> <p><br></p> <p>Extensive studies were focused on the importance of large-scale motions (LSM) and their con?tribution to TKE and turbulence mixing. Although there are studies focusing on the λ2 coherent vortical structures and large-scale motions separately, there are no studies addressing the control?ling using upstream perturbations on the large-scale motions and their associated λ2 vortices. In the first phase of our studies, we used the DNS data of channel flow for Reτ = 394 generated using in-house code. In these simulations, we created blowing perturbations using spanwise jets of low blowing ratio, 0.2, placed at upstream. The spatial large-scale motions are extracted using a a novel 3D adaptive Gaussian filtering technique developed based on Lee and Sung [1] for turbulent pipe flows. POD is used to extract the energetic large-scale motions and coherent vortical structures are extracted using λ2-criterion for its efficiency in educing coherent structures in cross flow jets. The results show that the upstream perturbations enhance streamwise heat flux via energetic LSM and also create a secondary peak of scalar production in the log-layer showing that the perturbations alter LSMs to enhance the heat transfer. Filtered large-scale field from Gaussian filtering technique have an integral length scale greater than 2h (where h is channel half-height) are used to obtain λ2 vortices. The resulted λ2 vortices are of ring-type and have higher signature of temperature than their counterpart. The pre-multiplied spectra shows that the upstream perturbations can excite the large-scale wave-numbers which are in the same order as the jet diameter and spacing between them. Simulations show the presence of secondary peak in the log-layer and increased turbulence production which are eminent of large-scales. Furthermore, our results suggest that jet spacing and diameter are crucial in exciting large-scale field to control turbulent flows.</p> <p><br></p> <p>Evans, Hamed, Gorumlu, et al. [2] modeled the denticles present on Mako shark skin into a diverging micro-pillars. They conducted experimental studies in a water tunnel using these on the back of airfoil exposed to an adverse pressure gradient flow. They observed that presence of these pillars reduced the re-circulation bubble (form drag) by 50%. They proposed a blowing and suction type mechanism by which the micro pillars interact with the boundary layer. However, the details of underlying interfacial mechanism is not completely understood. The unique impact of flow conditions on anti-biofouling and the corresponding mechanisms for the first time is illustrated. We employed commercially available bioinspired structures as micro-diverging pillars making it feasible to apply in real life. We demonstrated the underlying mechanism by which bio?inspired structures are responsible for anti-biofouling. To study the pressure gradient effects on the separation under transonic conditions, we performed direct numerical simulations (DNS) in a non?equilibrium flow created by a sinsuoidal contraction and also, we quantified the separation length,</p> <p>detachment, and attachment points of separation bubble imposed with various pressure gradients and their variation in the transonic and subsonic regimes. We noticed that the resultant shear at the attachement led to the enhancement of coherent structures which are extended into the outer layer under transonic flow which is quite different than the subsonic flow.</p>

Turbulent structures are known to be important physical processes in gravel-bed rivers. A number of limitations exist that prohibit the advancement and prediction of turbulence structures for optimization of civil infrastructure, biological habitats and sediment transport in gravel-bed rivers. This includes measurement limitations that prohibit characterization of size and strength of turbulent structures in the riverine environment for different case studies as well as traditional numerical modeling limitations that prohibit modeling and prediction of turbulent structure for heterogeneous beds under high Reynolds number flows using the Navier-Stokes equations. While these limitations exist, researchers have developed various theories for the structure of turbulence in boundary layer flows including large eddies in gravel-bed rivers. While these theories have varied in details and applicable conditions, a common hypothesis has been a structural organization in the fluid which links eddies formed at the wall to coherent turbulent structures such as large eddies which may be observed vertically across the entire flow depth in an open channel. Recently physics has also seen the advancement of topological fluid mechanical ideas concerned with the study of vortex structures, braids, links and knots in velocity vector fields. In the present study the structural organization hypothesis is investigated with topological fluid mechanics and experimental results which are used to derive a vortex model for gravel-bed flows. Velocity field measurements in gravel-bed flow conditions in the laboratory were used to characterize temporal and spatial structures which may be attributed to vortex motions and reconnection phenomena. Turbulent velocity time series data were measured with ADV and decomposed using statistical decompositions to measure turbulent length scales. PIV was used to measure spatial velocity vector fields which were decomposed with filtering techniques for flow visualization. Under the specific conditions of a turbulent burst the fluid domain is organized as a braided flow of vortices connected by prime knot patterns of thin-cored flux tubes embedded on an abstract vortex surface itself having topology of a Klein bottle. This model explains observed streamline patterns in the vicinity of a strong turbulent burst in a gravel-bed river as a coherent structure in the turbulent velocity field.

In recent years, jet noise has been an active area of research due to an increase in the use of aircraft in both commercial and military applications. To meet the noise standards laid out by government agencies, novel nozzle design concepts are being developed with an aim to attenuate the noise levels. To reduce the high costs incurred by experiments, simulation techniques such as large eddy simulation (LES) in combination with a surface integral acoustic method have received much attention for investigating various nozzle concepts. LES is utilized to predict the unsteady flow in the nearfield, whereas the surface integral acoustic method is used for the computation of noise in the farfield. However, Reynolds numbers at which nozzles operate in the real world are very high making wall-resolved LES simulations prohibitively expensive. To make LES simulations affordable, wall-models are being used to model the flow in the near wall region. Using a highly scalable, sixth-order finite-difference-based, in-house LES code, both wall-resolved and wall-modeled simulations of jets through the baseline short metal chevron (SMC000) nozzle were carried out earlier using an implicit LES (ILES) approach. However, differences exist in noise levels between the two simulations. Understanding the cause and reducing the differences between the two methodologies, while at the same time improving the fidelity of the wall-modeled LES is the main aim of the present work. Three new wall-models are implemented in the in-house LES code. A generalized equilibrium wall-model (GEWM) is implemented along with two wall-models that can account for non-equilibrium effects. First, a series of preliminary SMC000 wall-modeled LES simulations were performed and analyzed using the GEWM. The effect of turbulent length scales and velocity fluctuations specified at the inflow, wall-model formulation, and wall-normal grid refinement are analyzed. The adjustment of the fluctuations levels at the inflow proves to be useful in producing flowfields similar to that of the wall-resolved simulation. The newly implemented wall-models are validated for non-canonical problems such as an accelerating boundary layer developing over a flat plate and flow through a converging-diverging channel. It is noticed that the Reynolds number should be high enough for the non-equilibrium wall-models to be effective. At low Reynolds numbers, both equilibrium and non-equilibrium models produce similar wall shear-stresses. However, the wall shear stress boundary conditions supplied by the wall-models do not affect the mean velocity, turbulent kinetic energy, and Reynolds shear stress. Since all the wall-models produce similar results, and the GEWM is the most economical among the implemented wall-models, it is used in performing two wall-modeled LES SMC000 nozzle simulations for noise predictions. The inflow velocity and density fluctuations are varied between the simulations. The first SMC000 simulation uses similar inflow conditions as the previous wall-resolved SMC000 simulation. The second wall-modeled simulation was carried out by reducing the density and velocity fluctuations added to the mean flow at the inlet by 65%. The flowfield and acoustics agree reasonably well in comparison with the wall-resolved LES and similar experiments. Lowering of the velocity and density fluctuations in the wall-model LES improves the agreement of the far-field noise predictions with the wall-resolved LES at most observer locations. However, the preliminary SMC000 simulations performed using a higher Reynolds number and Mach number than that of the previous case show that the approach of adjusting the velocity and density fluctuations added to the mean flow have minimal impact on the developing flowfield which in turn affects the farfield noise. Thus, unless a more effective wall-modeling method is developed, possibly employing an explicit SGS model, the postdictive process of using a wall-model while adjusting the velocity and density fluctuations, seems to be an affordable tool for testing various nozzle designs, subject to the Reynolds number and Mach number being used.

Thesis (M.Sc. (Applied Mathematics)) --University of Limpopo, 2007 / The thermodynamic irreversibility in any fluid flow process can be quantified through entropy analysis. The first law of thermodynamics is simply an expression of the conservation of energy principle. The second law of thermodynamics states that all real processes are irreversible. Entropy generation is a measure of the account of irreversibility associated with the real processes. As entropy generation takes place, the quality of energy (i.e. exergy) decreases. In order to preserve the quality of energy in a fluid flow process or at least to reduce the entropy generation, it is important to study the distribution of the entropy generation within the fluid volume. In this dissertation, the inherent irreversibility in the flow of a variable viscosity fluid in both a closed channel and an open channel is investigated. The channel is assumed to be narrow, so that the lubrication approximation may be applied and the fluid viscosity is assumed to vary linearly with temperature. Both the lower and the upper surfaces of the channel are maintained at different temperature. The simplified form of governing equations is obtained and solved analytically using a perturbation technique. Expressions for fluid velocity and temperature are derived which essentially expedite to obtain expressions for volumetric entropy generation numbers, irreversibility distribution ratio and the Bejan number in the flow field. In chapter 1, a historic background of the study is highlighted. Both closed and open channels problem are investigated in chapters 2 and 3. In chapter 4, generally discussion on the overall results obtained from the investigation is displayed together with possible areas of future research work.

The depth-averaged Saint-Venant equations, which are used for most computational flow models, are adequate in simulating open channel flows with insignificant curvatures of streamlines. However, these equations are insufficient when applied to flow problems where the effects of non-hydrostatic pressure distribution are predominant. This study provides a comprehensive examination of the feasibility of a simple one-dimensional Boussinesq-type model equation for such types of flow problems. This equation, which allows for curvature of the free surface and a non-hydrostatic pressure distribution, is derived using the momentum principle together with the assumption of a constant centrifugal term at a vertical section. Besides, two Boussinesq-type model equations that incorporate different degrees of corrections for the effects of the curvature of the streamline are investigated in this work. One model, the weakly curved flow equation model, is the simplified version of the flow model based on a constant centrifugal term for flow situations that involve weak streamline curvature and slope, and the other, the Boussinesq-type momentum equation linear model is developed based on the assumption of a linear variation of centrifugal term with depth.