No / Computational Fluid Dynamics (CFD) represents a useful tool to study natural currents in the rivers and estuaries with erosive materials; therefore, it is always in the keen interest for scientists to further study and advance it, especially when numerical model has the advantages compared to actual laboratory experiment in terms of cost, time, and restrictions on conditions of the physical models and field collections. The present study deals with the hydro-morphological investigation and numerical modeling of a group of vertically stationed parallel groynes using FLOW-3D commercial software. To validate the results of the FLOW-3D simulation, it has been compared to the experiments from literature. Besides, a SSIIM 2.0 software has also been employed to compare with some of the FLOW-3D results. It was found that the accuracy of the FLOW-3D model influenced by the approach Froude number and the critical velocity ratio (Uavg/Ucr). Even though it underestimated the measured scour depth (due to complex and intense vortices, which reduce the accuracy of the numerical models), but general results from the model have reproduced the measured data well.

There has been a great deal of interest in investigating numerous unique types of electrostatic and electromagnetic waves and instabilities in dusty plasmas. Dusty plasmas are characterized by the presence of micrometer or submicrometer size dust grains immersed in a partially or fully ionized plasma. In this study, a two-dimensional numerical model is presented to study waves and instabilities in dusty plasmas. Fundamental differences exist between dusty plasmas and electron-ion plasmas because of dust charging processes. Therefore, a primary goal of this study is to consider the unique effects of dust charging on collective effects in dusty plasmas. The background plasma electrons and ions here are treated as two interpenerating fluids whose densities vary by dust charging. The dust is treated with a Particle-In-Cell PIC model in which the dust charge varies with time according to the standard dust charging model. Fourier spectral methods with a predictor-corrector time advance are used to temporally evolve the background plasma electron and ion equations. The dust charge fluctuation mode and the damping of lower hybrid oscillations due to dust charging, as well as plasma instabilities associated with dust expansion into a magnetized background plasma are investigated using our numerical model. Also, an ion acoustic streaming instability in unmagnetized dusty plasmas due to dust charging is investigated. The numerical simulation results show good agreement with theoretical predictions and provide further insight into dust charging effects on wave modes and instabilities in dusty plasmas. / Ph. D.

Yes / Numerical simulations are carried out to investigate the spreading of two-dimensional plane turbulent aerated and nonaerated jets in a tank filled with finite water depth. A multiphase model is applied to simulate the problem under investigation. The governing equations, their numerical scheme and the boundary conditions are presented. Aerated and non-aerated turbulent jets are simulated for a range of the jet velocity and width at exit, the initial air content at exit and the water depth in tank. The simulated results show that a self-similar Gaussian velocity distribution exists from the distance downstream being larger than five jet slot width for both the aerated and nonaerated jets. Good agreement between the simulated velocity profiles and available laboratory experiments is obtained. The simulated slope of the jet velocity decay along the jet centreline is in good agreement with the experimental measurements. The effect of air content on pressure distribution and the maximum impinging hydrodynamic pressure at the tank bottom is discussed. / the Open Funding from the State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (SKHL1302)

The utilization of ammonia as a fuel is a pragmatic approach to pave the way towards a low-carbon economy. Ammonia compromises almost 18 % of hydrogen by mass and accepted as one of the hydrogen combustion enablers with existing infrastructure for transportation and storage. From an environmental and sustainability standpoint, ammonia combustion is an attractive energy source with zero carbon dioxide emissions. However, from a practical point of view, the direct combustion of ammonia is not feasible due to the low reactive nature of ammonia. Due to the low combustion intensity, and the higher nitrogen oxide emission, ammonia was not fully investigated and there is still a lack of fundamental knowledge of ammonia combustion. In this thesis, the computational study of ammonia premixed flame characteristics under various hydrogen addition ratios and moderate or intense low oxygen dilution (MILD) conditions were investigated. Particularly, the heat release characteristics and dominant reaction pathways were analyzed. The analysis revealed that the peak of heat release for ammonia flame occurs near burned gas, which raises a question regarding the physics of this. Further analysis identified the dominant reaction pathways and the intermediate species (NH2 and OH), which are mainly produced in the downstream and back diffused to the leading edge and produce some heat in the low-temperature zone. To overcome low reactivity and poor combustion performance of pure ammonia mixture, the onboard ammonia decomposition to hydrogen and nitrogen followed by blending ammonia with hydrogen is a feasible approach to improve ammonia combustion intensity. With increasing hydrogen amount in the mixture, the enhancement of heat release occurs due to both transport and chemical effect of hydrogen. Another approach to mitigate the low reactive nature of ammonia may be eliminated by applying the promising combustion concept known as MILD combustion. The heat release characteristics and flame marker of ammonia turbulent premixed MILD combustion were investigated. The high fidelity numerical simulation was performed to answer fundamental questions of ammonia turbulent premixed combustion characteristics.

An immersed boundary spectral method is used to simulate laminar boundary layer flow over a periodic array of cylindrical surface roughness elements. Direct comparisons are made with experiments by using a roughness-based Reynolds number Re[subscript k] of 216 and a diameter to spanwise spacing ratio d/[lamda] of 1/3. Near-field differences between three similar studies are presented and addressed. The shear layer developed over the roughness element produces the downstream velocity deficit region while splitting of the vortex sheet shed the trailing edge forms its lateral modes. Additional geometrical configurations are simulated for comparisons with experimental results and future analysis by linear stability theory. Total disturbance energy E[subscript rms] is fairly consistent with experimental results while spanwise energy components vary significantly. Physical relaxation of the disturbance wake is found to remain a prominent issue for this simulation technique. / text

<p>This research presents the results of numerical study of heat transfer and pressure drop in a heat exchanger that is designed with different shape pin fins. The heat exchanger used for this research consists of a rectangular duct fitted with different shape pin fins, and is heated from the lower plate. The pin shape and the compact heat exchanger (CHE) configuration were numerically studied to maximize the heat transfer and minimize the pressure drop across the heat exchanger. A three dimensional finite volume based numerical model using FLUENT© was used to analyze the heat transfer characteristics of various pin fin heat exchangers. The simulation applied to estimate the heat transfer coefficient and pressure drop for a wide range of Reynolds numbers with different pin fins. Circular pin configuration variations included changes in pin spacing, axial pitch and pin height ratio. Rectangular and drop-shaped pin variations also included changes in length and aspect ratio. Correlations for Nusselt number and friction factor were developed. The optimum drop shaped pin array was shown to match the heat transfer rates obtained by the optimum circular pin configuration while incurring less than one third the specific fluid friction power losses. The data and conclusions of this study can be applied to the optimization of different heat exchangers which are used in industry, especially oil cooler in power transformers which are currently working with low cooling efficiency. It can also be used in the design of electronic components, turbine blade cooling or in other high heat flux dissipation applications requiring a low-profile, high area-density based micro-heat exchanger design. This study also shows that numerical models backed with experimental analysis can reduce both the time and money required to create and evaluate engineering concepts, especially those that deal with fluid flow and heat transfer. In the following chapters, first the problems which are encountered by power transformer suppliers are described. Then pin fin technology is studied with more details as a novel solution to the oil cooling problem. Some studies on behavior of power transformer coolers are also conducted to make their problems more clear. Available experimental data in the Iran Transfo company have been used for validation of these studies. They are presented as separated papers at the end of thesis. Finally the results of pin fin studies are presented and horizontal continuous casting (HCC) is explained as a manufacturing method for pin fins production. A separate paper which is based on experimental study on HCC is also included at the end of thesis.</p> / <p>Forskningen presenterad är ett resultat av en numerisk studie av värmeöverföring och tryckfall i en värmeväxlare designad med olika former av Kylflänsar. Värmeväxlaren består av ett rektangulärt kanal utrustat med olika former av Kylflänsar och är uppvärmd underifrån. Kylflänsar forma och den kompakta värmeväxlarens utformning är studerade numeriskt för att maximera värmeöverföringen och minimera tryckfallet över värmeväxlaren.En tredimensionell finit volym baserad på en numerisk modell i FLUENT© användes för att analysera värmeöverföringsegenskaper för olika Kylflänsar konfigurationer. Genom simuleringar uppskattades värmegenomgångstalet och tryckfallet för olika Reynolds tal och Kylflänsar konfigureringar. Cirkulära Kylflänsar konfigurationer inkluderar variation av avstånd mellan Kylflänsar, och förhållandet mellan axiellt avstånd och höjd. Rektangulära och droppformade Kylflänsar inkluderade även variation för längd och aspekt förhållande. Korrelation mellan Nusselts tal och friktionsfaktor utvecklades. Optimal matris för hur droppformade Kylflänsar placerades visades överensstämma med optimal överföring för cirkulära Kylflänsar men bara med en tredjedel av friktionsförlusterna för fluiden. Data och slutsatser från studien kan användas inom för optimering av värmeväxlare använda i industrin, speciellt oljekylda högspänningstransformatorer som har låg effektivitet i kylningen. Resultaten kan även användas inom design av elektronikkomponenter, kylning av turbinblad eller andra komponenter med högt värmeflöde där låg profil, och stor ytdensitet behövs. Studien visar att kombinationen av numeriska modeller som valideras genom experiment kan reducera både tid och kostnad vid utveckling och utvärdering av ingenjörsverktyg, speciellt inom fluidmekanik och värmeöverföring. I följande kapitel beskrivs först problem som identifierats av tillverkare av högspänningstransformatorer. Kylflänsar studeras i detalj som en ny lösning till de identifierade problemen med oljekylning. Några studier har genomförts för att ytterligare belysa problemen kring högspänningstransformatorers kylning. Tillgängliga data från Iran Transfo company har använts för validering av resultat från studierna. Studierna presenteras som separata artiklar i slutet av avhandlingen. Avslutningsvis presenteras resultat från studierna av Kylflänsar och en horisontell kontinuerlig gjutprocess (HCC) för tillverkning av Kylflänsar. HCC-studien presenteras som en separat artikel inkluderad sist i avhandlingen.</p>

A series of local and bench scale laboratory experiments and bench and field scale numerical simulations were conducted to develop a better understanding of the interrelationship between nonwetting phase (NWP) source zones and downgradient aqueous phase concentrations in saturated porous media contaminated by immiscible organic liquids. Specific emphasis was placed on the factors governing the rate of NWP source zone evolution and the factors governing the rate of mass transfer from the NWP to the aqueous phase. Hysteretic NWP relative permeability-saturation (krN-SW) relationships were measured at the local scale for six sands to examine the relationship between krN-SW functions and porous media type. Parameterization of the measured constitutive relationships revealed a strong correlation between mean grain diameter and the maximum value of NWP relative permeability. The measured krN-SW relationships, were validated through a bench scale experiment involving the infiltration, redistribution, and immobilisation of NWP in an initially water saturated heterogeneous porous medium. This match of simulation to experiment represents the first validation of a multiphase flow model for transient, fixed volume NWP releases. Multiphase flow simulations of the bench scale experiment were only able to reproduce the experimental observations, in both time and space, when the measured krN-SW relationships were employed. Two-dimensional field scale simulations of a fixed volume NWP release into a heterogeneous aquifer demonstrate the influence of spatially variable krN-S relationships correlated to porous media type. Both the volume of the NWP invaded porous media, and the length of time during which NWP is migrating, will be under predicted if variable (correlated) kr,N is not accounted for in the numerical model iv formulation. This under prediction is exacerbated as the mean intrinsic permeability of the release location decreases. A new, thermodynamically-based interfacial area (IFA) model was developed for use in the single-boundary layer expression of mass transfer as an alternative to existing empirical correlation expressions. The IFA model considers consistency and continuity of constitutive relationships, energy losses, effective specific interfacial area for mass transfer, and dissolution of residual NWP. A bench scale experiment involving the release and dissolution of a transient NWP source zone in heterogeneous porous media was conducted to evaluate the appropriateness of the developed IFA model when utilised to predict NWP dissolution rates. Comparison of measured downgradient dissolved phase concentrations and source zone NWP saturations in time and space with those from numerical simulations of the experiment reveal that the proposed IFA model is superior to both a local equilibrium assumption and existing empirical correlation expressions. This represents the first mass transfer model validated for the dissolution of a complex NWP source zone. Twodimensional simulations at the field scale of multiphase flow and dissolution suggest that employing existing mass transfer expressions instead of the IFA model lead to incorrect predictions of the life spans of NWP source zones, downgradient dissolved phase concentrations, and the rate of mass flux through a downgradient boundary. The practical implication of this research is that accurate numerical predictions of the evolution of a transient NWP source in porous media require consideration of krN-S relationships and NWP / aqueous phase IFA, as these factors dictate the rates of the key subsurface contaminant processes of migration and dissolution, respectively.

Flows over hydraulically smooth walls are predominant in turbulence studies whereas real surfaces in engineering applications are often rough. This is important because turbulent flows close to the two types of surface can exhibit large differences. Unfortunately, neither experimental studies nor theoretical studies based on conventional computational fluid dynamics (CFD) can give sufficiently accurate, detailed information about unsteady turbulent flow behaviour close to solid surfaces, even for smooth wall cases. In this thesis, therefore, use is made of a state of the art computational method “Direct Numerical Simulation (DNS)” to investigate the unsteady flows. An “in-house” DNS computer code is developed for the study reported in this thesis. Spatial discretization in the code is achieved using a second order, finite difference method. The semi-implicit (Runge-Kutta & Crank-Nicholson) time advancement is incorporated into the fractional-step method. A Fast Fourier Transform solver is used for solving the Poisson equation. An efficient immersed Boundary Method (IBM) is used for treating the roughness. The code is parallelized using a Message Passing Interface (MPI) and it is adopted for use on a distributed-memory computer cluster at University of Aberdeen as well as for use at the UK’s national high-performance computing service, HECToR. As one of the first DNS of accelerating/decelerating flows over smooth and rough walls, the study has produced detailed new information on turbulence behaviours which can be used for turbulence model development and validations. The detailed data have enabled better understanding of the flow physics to be developed. The results revealed strong non-equilibrium and anisotropic behaviours of turbulence dynamics in such flows. The preliminary results on the rough wall flow show the response of turbulence in the core and wall regions, and the relationship between the axial and the other components are significantly different from those in smooth wall flows.

The dispersion and mixing of passive scalars released from two concentrated sources into open-channel and homogeneous isotropic turbulent flows are studied using direct numerical simulation (DNS). The simulations are conducted using two fully-parallelized in-house codes developed using the FORTRAN 90/95 programming language. A comparative study has been conducted to investigate the effects of the source separation distance, Reynolds number, relative length scales of the plume and turbulent flow, and source elevation on the dispersion and mixing of two plumes. For both flow configurations, four distinct stages in the downwind development of the cross correlation between the fluctuating concentration fields have been identified which feature zero, destructive and constructive interferences and a complete mixing state. Differences between the exceedance probability of concentrations for the single and total plumes are highlighted and analyzed, and the effects of destructive and constructive interference on the exceedance probabilities for the total plume are used to explain these differences. It is found that the relationship between the third- and fourth-order concentration moments and the second-order concentration moment can be well predicted using a clipped-gamma model. This leads to an interesting conclusion that all the higher-order (third-order and above) moments of the total concentration can be inferred from a knowledge of only the first- and second-order concentration moments of each single plume and of the cross correlation coefficient. From a spectral analysis, it is observed that there exists a range of `leading scales' at which the rate of turbulent mixing of the two plumes becomes the most efficient and the coherency spectrum of the plumes approaches the asymptotic value of unity quicker than at any other scales. / October 2016

Abstract Turbulence and turbulent mixing play a key role in a wide range of engineering and scientific applications. One of the major difficulties when simulating these phenomena comes from their multi-scale nature. The range of scales that can be explicitly simulated is still too limited, even with current supercomputers, to study accurately most geophysical or engineering flows. A solution is to model the small scales in order to reduce the computational cost. Unfortunately, current models are of variable accuracy, in particular due to many constraining and sometimes empirical hypotheses. In order to improve the models used in turbulence simulations, and, more generally, to progress in our understanding of turbulence and turbulent mixing, it is crucial to study the behavior of the small-scale motions of turbulent flows. Rather than a traditional statistical approach, a physics-based description of the dynamics of the flow at these scales can lead to a better appreciation of the role of the small-scale motions in turbulence as well as drastically improve their prediction, particularly in regions of strong and fast variations that, currently, most models fail to capture. First, the relation between external forcing and the dissipation rate of turbulent kinetic energy has been studied. Theory represents the energetic equilibrium of turbulence as a cascade of energy from the larger scales to the smaller ones at a constant dissipation rate. A number of turbulence models rely on some sort of prediction of this dissipation rate, mostly based on (semi-)empirical considerations. Analyses of the Navier-Stokes equations show that for a given forcing on the flow, the dissipation rate could be precisely estimated at high enough Reynolds number. The case of a forcing on two low wavenumbers is presented in this dissertation. The low Reynolds number simulations were made for forces of increasing amplitude on the highest of the two wavenumbers. The results confirm the existence of a regime for which the dissipation rate could be precisely estimated and show a dependence on the dominant wavenumber, predicted by the analytical derivations. Second, the small-scale dynamics of a concentration of passive tracers, or passive scalars, is investigated. When advected by a turbulent flow, the passive scalar field for which the scalar diffusivity is smaller than the kinematic viscosity can create scales even smaller than those of the flow itself. The small-scale dynamics is analyzed with regards to the local topology of the flow, which is determined by the second and third invariant of the velocity gradient tensor. The phase-plane defined by these invariants allows an identification of the streamline patterns in the neighborhood of the location where they are computed. Probability density functions and conditioned statistics based on the local flow structure show that large scalar dissipation occurs in biaxial extensional regions located near vortices. Large scalar dissipation fluctuations pose a great challenge for traditional numerical simulations. Their scales, which could be several orders of magnitude smaller than the smallest velocity scales, may cause numerical errors that can significantly affect the accuracy of the solution. The study presented in this dissertation establishes the foundation for a new modeling strategy based on the flow topology and the combination of Eulerian and Lagrangian transport method.