No description available.

No description available.

No description available.

Bottom of the mountainous rivers is generally composed of natural gravels. Flow depth in such rivers is generally shallow, with the ratio of water depth to size of bed materials (known as relative submergence) rarely higher than 20. In this type of flow, gravels intrusion induces significant spatial variation of the flow characteristics near bed region, which is known as roughness layer. The simultaneous effects of natural gravels and water surface cause formation of complicated flow structure which is to some extent different from the flow with high relative submergence (flow with relative submergence higher than 40). Despite abundance of studies in shallow flows, there are only a limited number of studies concerning spatial organization of near bed flow field for such type of flow, with also contradictory results. The spatial organization of near bed turbulent flow characteristics is also important for transport of fine sediment. Transport of fine sediments is generally correlated to the asymmetry of vertical velocity. Asymmetry of vertical velocity also arises from a quasi-cyclic process of upward motion of low-velocity fluid parcels (ejection) and downward motion of high-velocity parcels (sweep), together known as bursting process. Spatial organization of bursting process and asymmetry of vertical velocity in near bed and respect to bed topography has not been inscribed properly. In heterogeneous flows, the use of spatially averaged turbulent transport equations, known as Double Averaged Navier-Stikes equations (DANS), is common. In DANS equations viscous drag, form drag and correlation of spatial fluctuation of time averaged velocities (known as form induced stresses) are explicitly expressed. Despite prevailing usage of DANS equations in study of gravel bed flow, examination of vertical velocity has not been performed appropriately by applying double averaging method. Also, the role of form induced stresses in vertical momentum flux has not been highlighted. In present thesis, Stereoscopic Particle Image Velocimetry at near bed horizontal layer and Digital Particle Image Velocimetry in vertical planes are employed together with laser scanning of bed elevations to study flow field and turbulence structure over a coarse immobile gravel bed in submergence conditions ranges from 5 to 10. Spatial organization of flow characteristics at the near bed region is analyzed respect to bed topography. This analysis is also composed of spatial distribution of bursting process and vertical momentum flux. Moreover, vertical profiles of double averaged turbulent flow characteristics and form induced stresses with different relative submergences are compared. Results show that near bed flow field is characterized by a strip structure induced by secondary currents. Such structure tends to be disrupted by the effect of gravel protrusions. To better analyze the interaction between the flow field and gravel bed protrusions, cross-correlations of different velocity components and bed elevations in a horizontal layer just above gravel crests are computed. These results show that upward and downward flows occur not randomly on the bed, but in correspondence to upstream and downstream side of gravels. Also, turbulent momentum flux is directed downward in the downstream side of gravel crests and it is directed upward in upstream side of gravel crests. This is due to prevalence of ejection and sweep events respectively in upstream and downstream sides of gravel crests. These results are in agreement with formation of separation and reattachment zones around gravel crests. Moreover, spatial distribution of sweep and ejection events are organized in streamwise elongated strips with high and low values which are consistent with presence of secondary currents cells. Results obtained by double averaging method show that relative submergence affects the normalwise double averaged turbulence intensity profiles all along the flow depth, while only a weak effect, limited to the near bed region, is noticed on streamwise double averaged turbulence intensity profiles. Logarithmic law parameterization of double averaged velocity profiles shows that parameters change considerably with relative submergence and, in some cases, no clear log-law region was found. These results challenge application of log-law in such type of flow. Analysis of the vertical velocity shows that far from the bed, vertical turbulence momentum flux is upward, while below gravel crests it is downward. This behavior is resulted by prevalence of ejection events far from the bed and sweep events below gravel crests. Results show that vertical momentum flux resulted by form induced component is not significant, except below gravel crests which are upward in to the water column. A limited number of qualitative observations in the real case of fine sediments presence in the matrix of rough bed is in agreement with the results of turbulent flow characteristics. Sand ribbons are clearly formed due to secondary currents. Also, fine materials are mostly deposited and eroded respectively in downstream and upstream sides of gravel crests. The results of present study show that in general some regions actively participate in transport, while the other regions do not participate in the transport. From this basis, Rouse criterion has been developed by considering spatial variation of vertical momentum flux.

"This dissertation is devoted to the computational investigation of supersonic gas flows in rectangular nanochannels with scales between 100 nm and 1000 nm, using an unstructured three-dimensional Direct Simulation Monte Carlo (U3DSMC) methodology. This dissertation also contributes to the computational mathematics background of the U3DSMC method with validations and verifications at the micronscale and nanoscale, as well as with the investigation of the statistical fluctuations and errors associated with U3DSMC simulations at the nanoscale. The U3DSMC code is validated by comparisons with previous two dimensional DSMC simulations of flows in micron-scale rectangular channels. The simulation involves the supersonic flow of nitrogen into a microchannel with height of 1.2 m and width of 6 m. The free stream conditions correspond to a pressure of 72,450 Pa, Mach number , Knudsen number and mean free path nm. The U3DSMC centerline temperature, heat flux to the wall, and mean velocity as a function of the transverse direction are in very good agreement with previous 2D results. Statistical fluctuations and errors in U3DSMC have added significance in nanoscale domains because the number of real particles can be very small inside a computational cell. The effect of the number of samples, the number of computational particles in a Delaunay cell, and the Mach number on the fractional errors of density, velocity and temperature are investigated for uniform and pressure-driven nanoscale flows. The uniform nanoflow is implemented by applying a and free stream boundary condition with m-3, K, nm in a domain that requires resolution of a characteristic length scale nm. The pressure-driven flows consider a nanochannel of 500 nm height, 100 nm width and 4 m length. Subsonic boundary conditions are applied with inlet pressure 101,325 Pa and outlet pressure of 10132.5 Pa. The analysis shows that U3DSMC simulations at nanoscales featuring 10-30 particles per Delaunay cell result in statistical errors that are consistent with theoretical estimates. The rarefied flow of nitrogen with speed ratio of 2, 5, and 10, pressure of 10.132 kPa into rectangular nanochannels with height of 100, 500 and 1000 nm is investigated using U3DSMC. The investigation considers rarefaction effects with =0.481, 0.962, 4.81, geometric effects with nanochannel aspect ratios of (L/H) from AR=1, 10, 100 and back-pressure effects with imposed pressures from 0 to 200 kPa. The computational domain features a buffer region upstream of the inlet and the nanochannel walls are assumed to be diffusively reflecting at the free stream temperature of 273 K. The analysis is based on the phase space distributions as well as macroscopic flow variables sampled in cells along the centerline. The phase space distributions show the formation of a disturbance region ahead of the inlet due to slow particles backstreaming through the inlet and the formation of a density enhancement with its maximum inside the nanochannel. The velocity phase-space distributions show a low-speed particle population generated inside the nanochannel due to wall collisions which is superimposed with the free stream high-speed population. The mean velocity decreases, while the number density increases in the buffer region. The translational temperature increases in the buffer region and reaches its maximum near the inlet. For AR=10 and 100 nanochannels the gas reaches near equilibrium with the wall temperature. The heat transfer rate is largest near the inlet region where non-equilibrium effects are dominant. For =0.481, 0.962, 4.81, vacuum back pressure, and AR=1, the nanoflow is supersonic throughout the nanochannel, while for AR=10 and 100, the nanoflow is subsonic at the inlet and becomes sonic at the outlet. For =0.962, AR=1, and imposed back pressure of 120 kPa and 200 kPa, the nanoflow becomes subsonic at the outlet. For =0.962 and AR=10, the outlet pressure nearly matches the imposed back pressure with the nanoflow becoming sonic at 40 kPa and subsonic at 100 kPa. Heat transfer rates at the inlet and mass flow rates at the outlet are in good agreement with those obtained from theoretical free-molecular models. The flows in these nanochannels share qualitative characteristics found in microchannels ad well as continuum compressible flows in channels with friction and heat loss. The rarefied flow of nitrogen with speed ratio of 2, 5, 10, at an atmospheric pressure of 101.32 kPa into rectangular nanochannels with height of 100 and 500 nm is investigated using U3DSMC. The investigation considers rarefaction effects with =0.0962 and 4.81, geometric effects with nanochannel aspect ratios of (L/H) of AR=1 and 10 and vacuum back-pressure. Phase plots and sample-averaged macroscopic parameters are used in the analysis. Under vacuum back pressure the centerline velocity decreases in the buffer region from its free stream value. For 0.481, 0.0962 and AR=1 the Mach number is supersonic at the inlet and remains supersonic throughout the nanochannel. For 0.481, 0.0962 and AR=10, the flow becomes subsonic at the inlet and shows a sharp increase in pressure. The Mach number, subsequently, increases and reaches the sonic point at the outlet. For 0.481, 0.0962 and AR=1 the translational temperature reaches a maximum near the inlet and decreases monotonically up to the outlet. For 0.481, 0.0962 and AR=10, the translational temperature reaches a maximum near the inlet and then decreases to come in near equilibration with the wall temperature of 273 K. "

This thesis is concerned with the numerical simulation of high Reynolds number, three-dimensional, incompressible flows in open domains. Many problems treated in Computational Fluid Dynamics (CFD) occur in free space: e.g., external aerodynamics past vehicles, bluff bodies or aircraft; shear flows such as shear layers or jets. In observing all these flows, we can remark that they are often unsteady, appear chaotic with the presence of a large range of eddies, and are mainly dominated by convection. For years, it was shown that Lagrangian Vortex Element Methods (VEM) are particularly well appropriate for simulating such flows. In VEM, two approaches are classically used for solving the Poisson equation. The first one is the Biot-Savart approach where the Poisson equation is solved using the Green's function approach. The unbounded domain is thus implicitly taken into account. In that case, Parallel Fast Multipole (PFM) solvers are usually used. The second approach is the Vortex-In-Cell (VIC) method where the Poisson equation is solved on a grid using fast grid solvers. This requires to impose boundary conditions or to assume periodicity. An important difference is that fast grid solvers are much faster than fast multipole solvers. We here combine these two approaches by taking the advantages of each one and, eventually, we obtain an efficient VIC-PFM method to solve incompressible flows in open domain. The major interest of this combination is its computational efficiency: compared to the PFM solver used alone, the VIC-PFM combination is 15 to 20 times faster. The second major advantage is the possibility to run Large Eddy Simulations (LES) at high Reynolds number. Indeed, as a part of the operations are done in an Eulerian way (i.e. on the VIC grid), all the existing subgrid scale (SGS) models used in classical Eulerian codes, including the recent "multiscale" models, can be easily implemented.

No description available.

This thesis is concerned with the numerical simulation of high Reynolds number, three-dimensional, incompressible flows in open domains. Many problems treated in Computational Fluid Dynamics (CFD) occur in free space: e.g., external aerodynamics past vehicles, bluff bodies or aircraft; shear flows such as shear layers or jets. In observing all these flows, we can remark that they are often unsteady, appear chaotic with the presence of a large range of eddies, and are mainly dominated by convection. For years, it was shown that Lagrangian Vortex Element Methods (VEM) are particularly well appropriate for simulating such flows. In VEM, two approaches are classically used for solving the Poisson equation. The first one is the Biot-Savart approach where the Poisson equation is solved using the Green's function approach. The unbounded domain is thus implicitly taken into account. In that case, Parallel Fast Multipole (PFM) solvers are usually used. The second approach is the Vortex-In-Cell (VIC) method where the Poisson equation is solved on a grid using fast grid solvers. This requires to impose boundary conditions or to assume periodicity. An important difference is that fast grid solvers are much faster than fast multipole solvers. We here combine these two approaches by taking the advantages of each one and, eventually, we obtain an efficient VIC-PFM method to solve incompressible flows in open domain. The major interest of this combination is its computational efficiency: compared to the PFM solver used alone, the VIC-PFM combination is 15 to 20 times faster. The second major advantage is the possibility to run Large Eddy Simulations (LES) at high Reynolds number. Indeed, as a part of the operations are done in an Eulerian way (i.e. on the VIC grid), all the existing subgrid scale (SGS) models used in classical Eulerian codes, including the recent "multiscale" models, can be easily implemented.

Ohlson(1995) and Feltham and Ohlson (1995) explain the importance of the financial reports through clean surplus relation and build up the relationship of market value, book value and earnings. The main structure of Ohlson model is the balance sheet and the income statement; however, the model doesn¡¦t contain the information of cash flows. The purpose of this paper is to find the relationship of accruals, cash flows, and market value. The results of this paper show that to divide net income into accruals and cash flows is good at forecasting abnormal earnings and valuing market value. To divide accruals into separate accruals also is helpful to forecast abnormal earnings and value market value. The cash flows and the accruals are different at forecasting and valuing. Key words¡GAccruals, Cash Flows, Market Value

A novel gas channel experiment was constructed to study the development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing helium-air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, short transients and was statistically steady. The facility was designed to be capable of large Atwood number studies of ABtB ~ 0.75. We describe work to measure the self similar evolution of mixing at density differences corresponding to 0.035 < ABtB < 0.25. Diagnostics include a constant temperature hot-wire anemometer, and high resolution digital image analysis. The hot-wire probe gives velocity, density and velocity-density statistics of the mixing layer. Two different multi-position single-wire techniques were used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the mixing as it develops to a self-similar regime in this flow. These measurements are to our knowledge, the first use of hot-wire anemometry in the Rayleigh-Taylor community. Since the measurement involved extensive calibration of the probes in a binary gas mixture of air and helium, a new convective heat transfer correlation was formulated to account for variable-density low Reynolds number flows past a heated cylinder. In addition to the hot-wire measurements, a digital image analysis procedure was used to characterize various properties of the flow and also to validate the hot-wire measurements. A test of statistical convergence was performed and the study revealed that the statistical convergence was a direct consequence of the number of different large three-dimensional structures that were averaged over the duration of the run.