RECIRCULATING FLOW IN SHALLOW OPEN CHANNEL via THE RNG κ-ε MODEL

The steady recirculating flow behind a sudden expansion in a shallow channel is studied using the renormalization group (RNG) κ-ε model. The predicted recirculating flow and the reattachment length of the recirculating flow are well-supported by the reported experimental data. The computed effective viscosity, turbulence kinetic energy and its dissipation rate given by the RNG κ-ε model are smaller than that by the standard κ-ε model, while the reattachment length of the recirculating flow is larger. The RNG κ-ε medel overcomes the shortcoming of underpredicting the length of the recirculation zone by the standard κ-ε model.

EXPERIMENTAL STUDIES ON SWIRLING AND RECIRCULATING TWO-PHASE FLOW FIELD IN A COLD MODEL OF DUAL-INLET SUDDEN-EXPANSION COMBUSTOR

The axial and tangential velocities of gas and particle phases and particle concentration for turbulent swirling and recirculating gas-particle (simulating gas-droplet) flows in a cold model of a dual-inlet sudden-expansion combustor with partially tangential central tubes, proposed by the present authors, were measured by using a 2-D LDV system and a laser optic fiber system combined with a sampling probe. The results show that there are both gas and particle strongly reverse flows and swirling flows in the head part of the combustor. The velocity slip between gas and particle phases is remarkable. The particle concentration is higher near the wall and lower near the axis. There are two peaks in the concentration profiles near the inlet tubes. The above-obtained flow characteristics are favorable to ignition, flame stabilization and combustion. The results can also be used to validate the numerical modeling.

THE APPLICATION AND DEVELOPMENT OF TURBULENCE MODELS IN BUOYANT RECIRCULATING FLOWS

In this paper, a modified κ-ε turbulence model, a simplified algebraic stress model and a developed two-fluid model have been presented based on numerical modeling of turbulent buoyant recirculating flows. The calculated results by these models are in good agreement with experiments. However, the last model is much better for simulating gravity-stratified flows.

Impact of an artificial chute cutoff on the river morphology and flow structure in Sipaikou area of the Upper Yellow River

Artificial chute cutoff can fundamentally eliminate the threat of flood caused by the meandering river,but it significantly changes its morphodynamic characteristics.Channel adjustments after cutoff are rapid,which makes it difficult to study the interaction between river morphology and flow structure only through field measurement.However,numerical simulations can provide insights into the hydrodynamic characteristics after artificial chute cutoffs.In this study,both field measurement and numerical simulation are employed to investigate the flow structure and bed morphology caused by an artificial chute cutoff in Sipaikou area of the Upper Yellow River in 2018.The measured hydrological data provide boundary conditions and initial values for the numerical model.The field measurement results reveal that the concave bank of the study area is severely scoured up to 270 m after the artificial cutoff,and a 20 m deep scour hole and a 2.26 km long pool are formed at the entrance and near the left bank of the chute channel.The numerical simulation results of velocity at typical cross-sections are in good agreement with the measurement results.Flow separation and stagnation zones are observed near the right bank during the low flow conditions(discharge of at least 902 m^(3)/s),but this phenomenon is not seen during larger flow conditions(discharge exceeds 2000 m^(3)/s).Interestingly,flow recirculation zones are also found near the left and right banks of the scour hole.Further,a long flux belt is formed at the scour hole and the pool.Consequently,the impact of the bed topography on the hydrodynamic characteristics is relatively prominent when the discharge is small,while the impact on the river banks and river bed is more noticeable when the water discharge is large.In addition,high shear stress is observed near the left bank at the downstream of the studied area,which indicates that the left bank at the downstream is still being scoured.These results suggest that bank protection measures along the left bank are required to maintain the effectiveness of the artificial chute cutoff.

Transient behaviors of a flame over a Tsuji burner

The present study investigated numerically the physical mechanisms underlying the transient behaviors of the flame over a porous cylindrical burner. The numerical results showed that a cold flow structure at a fixed inflow velocity of Uin = 0.6 m/s in a wind tunnel could be observed in two co-existing recirculation flows. Flow variations occur repeatedly until t = 4.71 s, and then a vortex existed steadily behind the burner and no shading occurred. The ignition of flammable mixture led to a rapid rise in gas temperature and a sudden gas expansion. When it reached the stable envelope flame condition, Uin is adjusted to an assigned value. Two blow-off mechanisms were identified. It was also found in the study flame shapes with buoyancy effects agreed with the ones observed experimentally by Tsai. Furthermore, the lift-off flame would appear briefly between the envelopes and wake ones, and was stabilized as a wake flame.

Numerical Study of the Feed Flow and Its Influence on the Separation Efficiency of a Gas Centrifuge

In a Zippe-type 3-pole gas centrifuge, feed gas is introduced through a sonic nozzle into the rarefied region in the rotor. Introduction of the nonrotating feed gas will slow the whirl flow and introduce a secondary recirculating flow in the meridian plane. The effects of feed gas on the output of a gas centrifuge are investigated. The non-linear. axisymmetric N-S equations are used to calculate the secondary flow induced by the feed gas. Three types of numerical schemes. an implicit scheme similar to the Beam-Warming scheme. an implicit unfactorized scheme and an improved Newton-Raphson scheme are used. The Cohen separation theory with axial variation is used forcalculating the isotope concentration. Optimization of the output is achieved by automatic variation of the weighting factors for a number of linear flow solutions which can be superimposed. A Rome type centrifuge is analyzed as an example. Results show the recirculating flow caused by the feed gas. especially the acceleration loss. has an important effect on the output of a gas centrifuge.