Journal Of Applied Fluid Mechanics
Volume:1 Issue: 2, May-Jun 2008

Cold dark matter (CDM) cosmology based on the Jeans 1902 criterion for gravitational instability gives predictions about the early universe contrary to fluid mechanics and observations. Jeans neglected viscosity, diffusivity, and turbulence: factors that determine gravitational structure formation and contradict small structures (CDM halos) forming from non-baryonic dark matter particle candidates. From hydro-gravitational-dynamics (HGD) cosmology, viscous-gravitational fragmentation produced supercluster (10^46 kg), cluster, and galaxy-mass (10^42 kg) clouds in the primordial plasma with the large fossil density turbulence (3 ×10 ^ -17 kg m ^ -3) of the first fragmentation at 10^12 s, and a protogalaxy linear morphology reflecting maximum stretching on vortex lines of the plasma turbulence at plasma-gas transition at 10^13 s. Gas protogalaxies fragmented into proto-globular-star-cluster mass (10 ^36 kg) clumps of protoplanet gas clouds that are now frozen as earth-mass (10^ 24-25 kg) Jovian planets of the baryonic dark matter, about 30,000,000 rogue planets per star. Observations contradict the prediction of CDM hierarchical clustering cosmology that massive Population III first stars at 10^16 s existed but support the HGD prediction of gentle formation of small first stars in globular-star-clusters soon after 10^13 s.

Flow separation is mostly an undesirable phenomenon and boundary layer control is an important technique for flow separation problems on airfoils and in diffusers. Longitudinal (streamwise) vortices are produced by the interaction between jets and a freestream. This technique is known as the vortex generator jet method of separation, or stall control. The vortex generator jet method is an active control technique that provides a time-varying control action to optimize performance under a wide range of flow conditions because the strength of longitudinal vortices can be adjusted by varying the jet speed. In the present study, an active separation control system using vortex generator jets with rectangular orifices has been developed. The active separation control system can be practically applied to the flow separation control of a two-dimensional diffuser. It was confirmed that the proposed active separation control system could adaptively suppress flow separation for the flow fields caused by some changes in freestream velocity and the divergence angle of the diffuser.

We extend the modified log-wake law, which was developed for turbulent pipe and boundary layer flows, to turbulent flows in open-channels. Like those in pipes and boundary layers, turbulent velocity profiles in open-channels can be approximated with three components: (1) the law of the wall that results from the constant bed shear stress; (2) the law of the wake that reflects the effects of gravity, secondary currents and bed roughness; and (3) the cubic correction near the maximum velocity. The modified log-wake law compares very well with experimental data from Coleman, Lyn, Kironoto and Graf and Sarma et al. It also replicates the measured velocity profiles of the Mississippi River. In particular, it can well model the velocity dip phenomenon in open-channels where the conventional log-wake law fails.

A numerical investigation is carried out for laminar sinusoidal pulsating flow through a modeled arterial stenosis with a trapezoidal profile up to peak Reynolds number of 1000. Finite element based numerical technique is used to solve the fluid flow governing equations where the fluid is assumed to be viscous, incompressible and Newtonian. The effects of pulsation, stenosis severity, Reynolds number and Womersley number on the flow behavior are studied. The dynamic nature of pulsating flow disturbs the radial velocity distribution and thus generates recirculation zone in the post-stenotic region. The peak wall shear stress develops for 65% stenosis (by area) is 3, 2.2, and 1.3 times higher than that for 30%, 45%, and 55% stenosis, respectively. Peak wall shear stress and wall vorticity appear to intense at the throat of the stenosis. It is also observed that the peak wall vorticity seems to increase with the increase of stenosis size and Reynolds number. However, the peak values of instantaneous wall vorticity are not greatly affected by the variation of Womersley number.

The dispersion phenomenon has resulted from the various water flow magnitude and direction in porous media. The dissolved tracer tends to spread due to dispersion and then travel time of tracer through the porous media increases. In unsaturated porous media, dispersion coefficient varies with non-linear Darcy’s velocity and the water content. These effects observed in both of the laboratory scale sand and soil columns (20 cm). The unsaturated infiltration column and tracer tests have been used to interpret the relationships between Darcy’s velocity and the water content together with the dispersion coefficient. However, the dispersivity coefficient cannot be measured directly, it has to determine from advection-dispersion equation (ADE), which can be used to model the tracer transport in unsaturated porous media. The model was used to describe the non-linear functions of water contents and dispersivities for both porous media. The simulations have been verified that the dispersion of tracer through soil is higher than sand column and also travel time of tracer through soil is longer than sand column. Even though, soil has very low degree of pore velocity, the high dispersivity is observed in the simulations. The water content and tracer concentration profiles reveal that the increase of dispersivity induces the increase of flow path distance and the decrease of pore velocity. The maximum dispersivity was observed when the water content of porous media is relatively low; this leads the maximum of spreading of tracer.

The deposition behavior of fine sediment is an important phenomenon, and yet unclear to engineers concerned about reservoir sedimentation. An elliptic relaxation turbulence model (V2-f model) has been used to simulate the motion of turbid density currents laden with fine solid particles. During the last few years, the V2-f turbulence model has become increasingly popular due to its ability to account for near-wall damping without use of damping functions. V2-f model has also proved to be superior to other RANS (Reynolds-Averaged Navier-Stokes) methods in many fluid flows where complex flow features are present. This current becomes turbulent at low Reynolds number (order 1000). The k-e model, which was standardized for high Reynolds number and isotropic turbulence flow, cannot simulate the anisotropy and non-homogenous behavior near the wall. In this study, the turbidity current with uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. The model has been validated by available experimental data sets. Moreover, results have been compared with the standard k-e turbulent model. The deposition of particles and the effects of their fall velocity on concentration distribution, Richardson number, and deposition rate are also investigated. The results show that the coarse particles settle rapidly and make the deposition rate higher.

The phase distribution of water-oil flows was studied experimental in dispersed liquid-liquid in horizontal tubes. Under most conditions the pattern was oil continuous in water dispersed or water continuous in oil dispersed flow continuously and there is entrainment in the form of drops of phase into the other. The investigations were carried out through the cross-sectional phase distribution in the flow of mixtures of water and a kerosene such as EXXSOL-D80 in a horizontal 25.4 mm bore stainless steel section. The phase fraction distribution was determined using a traversing beam gamma densitometer, with the beam being traversed in three directions (00, 450 and 900 of the vertical line passing through the axis of the tube). Measurements were made at three positions spaced along the 9.7 m test section length (1.0 m, 5.85 m and 7.72 m along the horizontal tube). The measurements were done in the Two-phase Oil Water Experimental Rig facility. This facility allows the two fluids to be fed to the test section before they separated and returned once more to the test line. The flow developed naturally from an initial stratified flow in which the oil and water were introduced separately at the top and the bottom of the test section respectively. It was found that the liquids were fully inter-dispersed by the end of the test section. The results were also used to define the flow patterns in water-oil liquid-liquid flow system. The phase fraction distribution was shown to be homogeneously mixed near to the outlet of the test section.

The design, development and optimization of an internal combustion engine require the application of a modern sophisticated analysis tool. In addition to experimental work, numerical calculations are now necessary to provide an insight into the complex in-cylinder process. The combustion process and its emission characteristics in a compressed natural gas direct injection engine were analyzed and investigated. The numerical studies were performed on a single cylinder of a 1.6-liter engine running at wide open throttle. The grid generation was established through an embedded algorithm for moving mesh and boundary in order to provide a more accurate transient condition. The combustion process was modelled with the eddy break-up model of Magnussen for unpremixed or diffusion reaction with three global reaction scheme. The computational fluid dynamics (CFD) simulations at two baseline conditions are carried out to examine the fluid flow, air-fuel mixing formation, combustion process, carbon monoxide emission distribution as well as NO emission formation occurred inside engine cylinder. The CFD results were compared with the experimental data and showed a very good agreement for two baseline conditions. A set of parametric studies were carried out by varying the timings of start of injection (SOI) and start of ignition (SI). The examined engine performance is in-cylinder pressure, while the considered emissions to be minimised are CO and NO levels. In order to study the effect of injection timing, the SOI timing was varied from 120º -140º with fixed ignition timing at 19º bTDC. On the other hand, SI timing was positioned from 15º-23º bTDC with fixed SOI timing for studying its influences. The CFD results indicated that slightly retarded SOI and SI timing can be chosen to reduce CO and NO levels while increasing engine performance.