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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Numerical Simulation of Flushing Deposits in Pipelines

Coverston, Joseph S 08 February 2019 (has links)
The purpose of this research is to reduce the amount of waste generated in Department of Energy nuclear cleanup efforts currently underway. Due to the highly radioactive nature of the waste, any fluid that contacts the waste must then be treated and processed as waste. To minimize the fluids contaminated during flushing, this research aims to provide a basis for the flushing of High Level Waste (HLW) pipelines. Edgar Plastic Kaolin (EPK) with solid particles of a nominal diameter of 1 micron was used as a simulacrum for HLW. An Eulerian-Eulerian simulation built in StarCCM+ software, with a k-ω turbulence model, and a drag coefficient to connect the solid EPK phase with the liquid phase, was used to simulate the flushing of pipelines. Velocities from 3 ft/s to 10 ft/s were investigated to find the highest volumetric efficiency, and it was determined that 10 ft/s was the optimal flushing velocity.
22

Measurements of jet velocity in unstratified and stratified fluids

Liu, Ying 05 1900 (has links)
No description available.
23

A more robust wall model for use with the two-equation turbulence model

Mallone, Kevin Charles January 1995 (has links)
The applicability of computational fluid dynamics (CFD) modelling schemes to turbulent wall-bounded flows is a matter of concern. In the near-wall region of bounded flows, the standard high Reynolds number k-e model is not valid and requires the use of empirical wall models to mimic the behaviour of this region. A theoretical study of the physics of prevalent wall modelling techniques showed that the velocity distribution took no account of the pressure gradient. To determine the effect of this shortcoming, a typical transient three-dimensional flow was analysed using current CFD methods and the results compared with experimental flow measurements. Consideration of these results showed that the 'traditional' wall model was unable to replicate observed flow features in the near-wall region: further analysis of the computational results confirmed that these poor flow predictions arose from the inability of the model to consider local pressure gradient effects. Consequently, a strong case was made for a more robust wall model for use in conjunction with the standard high Reynolds number k-e model. A number of boundary layer analyses were reviewed and Coles' law of the wake (1956) presented as a viable candidate for the development of a new wall modelling scheme. In theory, Coles' law (1956) provides a description of bounded flows under arbitrary pressure gradients up to the point of near-separation and may be extended to the study of reversed flows. A generic algorithm for Coles' law was prepared and used to study the fundamental test cases of U-bend and backward facing step flows. In a comparison between documented experimentation, 'conventional' CFD modelling and Coles' law models of these flows, the Coles' law model was shown to provide a viable alternative to 'traditional' schemes. Consequently, the Coles' law model of the near-wall region, being valid for pressure-driven flows, offers an extension to the range of flows for which the standard high Reynolds number k-e model may be used.
24

Bubbles in a gas fluidised bed

Mojtahedi, Wahab January 1983 (has links)
No description available.
25

The influence of near bed turbulent flow structures on scour hole development at pipe culvert outlets

Liriano, Sarah Louise January 1999 (has links)
This thesis presents the results of experimental measurements of scouring and turbulent velocity measurements in and around scour holes downstream of pipe culvert outlets. Centreline scour hole profiles have been measured at 4 flow rates and 4 tailwater depths resulting in up to 16 different experimental conditions. These results have enabled the maximum scour depth downstream of culvert outlets to be quantified in terms of flow rate and tailwater depth. An equation in terms of densimetric Froude number is presented to predict scour depth incorporating constants dependent on tailwater depth calculated from additional equations. The prediction of scour depth for different tailwater depths is of immediate benefit to design engineers. Additionally, measurementosf scourh ole profiles haveb een madeu sing four experimental facilities of different sizes. Froudian scaling protocols have been used to investigate the effect of model scale on scour hole development. Three experimental model facilities have been used at the University of Hertfordshire and a fourth prototype scale facility used at HR Wallingford. Model scale has been found to have an effect with small-scale models failing to accurately predict maximum scour depth. This is of particular significance as previous equations predicting scour depth downstream of outlets have been derived from studiesu sing small-scalem odel facilities. Velocity data was collected for three tailwater depths at one flow rate using a downward facing 3 component Acoustic Doppler Velocimeter. A method of bed fixing was used to enable velocity measurements to be made in scour holes at different stages of development without changes in bed form during the velocity data collection. From this data mean velocity vectors and contours have been plotted at different stages of scour hole development and turbulence intensities and Reynolds stresses have been determined for the centreline profiles. Further, using the quadrant analysis technique, the near-bed bursting events have been examined at different stages of development, which has shown that different events occur at different locations in the scour hole. The analysis of the turbulent flow structure in the scour hole has revealed that initial scour is a result of high velocities exceeding the critical velocity for sediment transport. As the scour hole develops the velocities close to the bed reduce and a gradual increase in scour depth takes place as a result of quadrant 4 events (sweeps) impacting on the bed in this region. Upstream of the dune the jet comes into contact with the bed and flow structures similar to those observed downstream of backward facing steps or dunes in open channel flow are noted. In particular it is suggested that hairpin vortices may be present in this region and lead to further scouring. The identification of flow structures in the scour hole may lead to the development of computer simulations of scouring downstream of pipe culvert outlets which in the long term could be used as a design tool.
26

The influence of different time varying antecedent flows on the stability of mixed grain size deposits

Saadi, Yusron January 2002 (has links)
The objective of this work was to examine the impact of unsteady flows on the erosion and movement of mixed grain size sediment. Time varying flows were examined as flowrates in natural rivers are rarely constant. There are very few reported studies on the movement of sediment in unsteady open channel flow and most of those used single sized sediment. River reach has its own sedimentological character and non-uniform beds exhibit very different behaviour from that of single sized material. Therefore it was thought important to examine the impact of time varying flow on the stability of water worked mixed grain size sediment beds. The thesis reports on a series of laboratory experiments in which a bimodal sediment bed was exposed to different flow hydrographs. The flow hydrographs consisted of constant flowrate with different duration and time varying flows with different rising and falling limb but had the same peak flowrate. Each experiment was followed by a stability test in which a standard "triangular shaped hydrograph" was used to assess the stability of each water worked deposit. The stability observation demonstrated that grain size fractions have different thresholds of motion when beds are formed by different antecedent flow patterns. The bed stability increased as the antecedent constant flow hydrograph progressed. The rising and falling limbs of the flowrate hydrographs were found to have a significant effect on the bed stabilisation process. It revealed that the shortest rising limb of flow hydrograph formed the weakest bed while the longest recession limb of flow hydro graph formed the most stable bed. It is believed that the short period of flowrate acceleration did not allow the coarse grains to stabilise with numerous exposed large grains spread on the bed. In a longer duration of recession limb of hydrograph, the coarse grains moved and eventually deposited over a length of time. As the flowrate declined the finer grains also rolled and then deposited forming a strong bond with the coarse grams. These experiments also provided important information on the flow structures and the changes in the bed topography as the tests progressed. There is strong evidence that only upward interactions (ejections) with high momentum magnitude were able to transport coarser grains. The lack of change in the distribution of downward looking-bed interactions (sweeps) in all tests indicated that these features are not important in determining transport. Changes in bed topography were also measured and characteristics of the distribution of bed surface elevation were linked to the observed changes in bed stability.
27

Computational fluid dynamic modelling of stirred reactors : power, baffle stresses, mixing times and semi-batch precipitation

Bujalski, Jakub Michal January 2003 (has links)
A commercial CFD (Computational Fluid Dynamics) code CFX (version 4.2 to 4.4) from AEA Technologyl'' has been used to compute the fluid flow, power number, Po, the stresses on baffles, mixing time and a precipitation reaction in a mixing vessel. The impellers investigated were Rushton turbine and 4 or 6 blade 45° pitch blade turbine. The impeller generated flow was modelled primarily using the sliding mesh technique, with additional modelling using Multiple Frames of Reference (MFR) for the mixing time simulations. The Po was estimated from three different methods i.e. specific energy dissipation rate, ET, summation, torque acting on the impeller surfaces, POp(primary power number), and the reaction torque acting on the vessel walls and baffles, POs (the secondary power number). The Po from the summation of ET, was underpredicted as compared with experimental values in all the simulations by over 50%. The investigation of the calculated power numbers for the vessels found that the closest and most consistent values of Po compared to experimental results were obtained from the torque acting on the impeller surfaces, POp. The value of POs was found to be greatly dependent on the sliding mesh simulation parameters and an improvement in the POsprediction could be obtained by using a small time step. A further investigation lead to the computation of the tangential forces and subsequently the axial pressure distribution on the baffles. The baffle pressure distribution depends on the impeller type and its clearance and was better predicted for greater impeller clearances and for the radial flow impellers. The mixing times simulations were performed using a computational method analogous to the experimental method of probe responses. The system was in the high transitional flow regime (Re=8800) and a low Reynolds k-e turbulence model was used in the development of the flow field. The simulations were compared with experimental results (based on decolorisation technique) and to three different mixing time correlations giving mixing times at three different levels of homogenisation (i.e. 90%, 95% and 99%). Worryingly, the simulation results were found to depend on the radial feed position even though the experimental results suggest that it does not. At certain radial position, the simulated mixing time responses accurately predicted the mixing times from the experiments and empirical correlations. CFD based flow visualisation showed that the feed position influenced where the majority of the tracer was initially distributed. The further the radial position was from the axis of the impeller, the more the bulk of the tracer moved towards the low velocity region near the vessel walls, leading to an overestimate of the mixing time. The sliding mesh and MFR simulations of the velocity fields were used for the computation of the mixing time. The results were similar in each case. The precipitation modelling was achieved through the coupling of the CFD hydrodynamics and user defined precipitation model. This approach was able to predict the performance of a semi-batch process involving the precipitation of BaS04 with 270 s addition time. The results (i.e. mean crystal size (d[4,3]) and the particle size distributions) were compared with experimental results for a double feed precipitation reaction for a number of feed configurations and concentration ratios. Overall reasonable trends and agreement have been obtained for the modelled Po, mixing time and baffle stresses. The precipitation model was less successful and was very dependant on the different crystal shape factors used in the simulation model. Further experimental work is required in order to define this parameter accurately, especially as experiments have shown that it varies during the addition time.
28

The development of a generalised finite element scheme for heat transfer and fluid flow analysis

Shemirani, Faramarz January 1991 (has links)
Colletotrichum gloeosporioides is the causal agent of anthracnose disease of mangoes. Infection occurs when humidity is high and rain-dispersed spores germinate and form an appressorium on immature mangoes. The infection then becomes quiescent until the fruit is harvested. On ripe fruit infection is visible as black, sunken lesions on the surface. At the pre-harvest stage, the disease is controlled with the application of a range of fungicides, and at the post-harvest stage by hot benomyl treatment. The extensive use of benomyl, both pre- and post-harvest, has resulted in the occurrence of isolates of C. gloeosporioides resistant to this fungicide. To devise an alternative strategy of disease control, the potential for biological control of anthracnose has been investigated. Potential microbial antagonists of C. gloeosporioides were isolated from blossom, leaves and fruit of mango, and screened using a series of assay techniques. In total 650 microorganisms, including bacteria, yeasts and filamentous fungi, were isolated and tested for their inhibition of growth of C. gloeosporioides on malt extract agar. Of these 650 isolates, 121 inhibited the fungus and were further tested on their ability to inhibit spore germination in vitro. Of these, 45 isolates, all bacteria and yeasts, were inoculated onto mangoes, which were artificially inoculated with C. gloeosporioides, and assessed for their potential to reduce the development of anthracnose lesions. A further selection was made, and 7 isolates were chosen to be used in a semi-commercial trial in the Philippines. This final screening procedure yielded two potential candidates for field trials, isolate 204 (identified as Bacillus cereus) and isolate 558 (identified as Pseudomonas fiuorescens). A field trial involving pre-harvest application of the biological control agent, was conducted using isolate 558. This isolate was chosen for this purpose since in in vitro experiments it significantly reduced germination of C. gloeosporioides spores. In the field trial 558 was applied in combination with nutrients and compared to treatments which had received no treatment or which had received conventional fungicide (benomyl) application. On spraying, high numbers of 558 were recorded on the leaf surface, but no reduction in post-harvest development of disease was observed. Failure of disease control was attributed to rapid death of the bacterium on the phylloplane. Inpost-harvest trials, isolates 204 and 558 were both tested in combination with different application methods, including the addition of sticker, peptone, fruit wax or a sucrose polyester. Application of 204 did not reduce disease development. Application of 558, however, did significantly reduce anthracnose development compared to the control fruit. No additional benefit was achieved by incorporating the bacteria in peptone, fruit wax or sucrose polyester. The mode of action of isolate 558 was investigated in detail. There was no evidence for parasitism taking place, or the production of volatile compounds, in the suppression of disease development. No antibiotic compounds were detected, but isolate 558 did produce a siderophore. A sharp increase in pH was also observed in culture media in which 558 was grown. Disease control may result from a combination of these two factors.particularly efficient in terms of storage requirements and computational speed. It also takes advantage of the nature of the system of equations to be solved. Several laminar benchmark exercises with and without heat transfer are performed. These include developing and fully developed isothermal duct flow, backward facing step flow, natural convection in square cavity and jet impingement with heat transfer. Results show that the adopted equal order velocity-pressure method can predict the benchmark solutions efficiently and accurately. Spurious pressure modes are also shown to be completely absent. In modelling turbulent flows, the k-c two equation eddy viscosity model is employed. The advection part of the k and e equations are discretised by the upwind technique developed in this research. Special treatment of the source terms eliminate the possibility of producing negative values of k or e during the iterative solution sequence, which can cause convergence difficulties. By combining the Law of the Wall and the Log Law of the Wall to determine shear stresses near solid regions, the need for an excessively fine mesh in these regions is avoided.
29

Instability problems in fluids

Griffith-Jones, Robert Glyn January 1980 (has links)
No description available.
30

Study of hydrodynamic force coefficients for a grooved squeeze film damper

Zhang, Jia Xin January 1994 (has links)
No description available.

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