Non-axisymmetric Flow Fields Research Articles

Emergence of lobed wakes during the sedimentation of spheres in viscoelastic fluids

The motion of rigid particles in complex fluids is ubiquitous in natural and industrial processes. The most popular toy model for understanding the physics of such systems is the settling of a solid sphere in a viscoelastic fluid. There is general agreement that an elastic wake develops downstream of the sphere, causing the breakage of fore-and-aft symmetry, while the flow remains axisymmetric, independent of fluid viscoelasticity and flow conditions. Using a continuum mechanics model, we reveal that axisymmetry holds only for weak viscoelastic flows. Beyond a critical value of the settling velocity, steady, non-axisymmetric disturbances develop peripherally of the rear pole of the sphere, giving rise to a four-lobed fingering instability. The transition from axisymmetric to non-axisymmetric flow fields is characterized by a regular bifurcation and depends solely on the interplay between shear and extensional properties of the viscoelastic fluid under different flow regimes. At higher settling velocities, each lobe tip is split into two new lobes, resembling fractal fingering in interfacial flows. For the first time, we capture an elastic fingering instability under steady-state conditions, and provide the missing information for understanding and predicting such instabilities in the response of viscoelastic fluids and soft media.

Non-Axisymmetric Flows and Rotordynamic Forces in an Eccentric Shrouded Centrifugal Compressor—Part 2: Analysis

AbstractAn integrated analytical model to predict non-axisymmetric flow fields and rotordynamic forces in a shrouded centrifugal compressor has been newly developed and validated. The model is composed of coupled, conservation law-based, bulk-flow submodels, and the model takes into account the flow coupling among the blades, labyrinth seals, and shroud cavity. Thus, the model predicts the entire flow field in the shrouded compressor when given compressor geometry, operating conditions, and eccentricity. When compared against the experimental data from part 1, the new model accurately predicts the evolution of the pressure perturbations along the shroud and labyrinth seal cavities as well as the corresponding rotordynamic stiffness coefficients. For the test compressor, the cross-coupled stiffness rotordynamic excitation is positive; the contribution of the shroud is the highest; the contribution of the seals is less than but on the same order of magnitude as that of the shroud; and contribution of impeller blades is insignificant. The new model also enables insight into the physical mechanism for pressure perturbation development. The labyrinth seal pressure distribution becomes non-axisymmetric to satisfy mass conservation in the seal cavity, and this non-axisymmetry, in turn, serves as the influential boundary condition for the pressure distribution in the shroud cavity. Therefore, for accurate flow and rotordynamic force predictions, it is important to model the flow coupling among the components (e.g., impeller, shroud, labyrinth seal, etc.), which determines the non-axisymmetric boundary conditions for the components.

Non-Axisymmetric Flow Fields in Axisymmetric CVD Reactor Setups Revisited: Influence on the Film's Non-Uniformity

Non-Axisymmetric Flow Fields in Axisymmetric CVD Reactor Setups Revisited: Influence on the Film's Non-Uniformity

Modeling the slag behavior in three dimensional CFD simulation of a vertically-oriented oxy-coal combustor

Slagging and fouling are important phenomena associated with ash handling and discharge in coal combustion and gasification. A slag model has been developed for modeling the ash/slag fate including char/ash deposition, char wall burning, molten slag flow, as well as solid slag layer formation on the reactor wall. The submodels are implemented as user defined functions in a three-dimensional computational fluid dynamics (CFD) code, and applied for the simulation of a pilot scale coal slagging combustion facility. The results on the two-dimensional cylindrical reactor wall show uneven char/ash deposition distribution due to the non-axisymmetric flowfield and char trajectories in the reactor. Molten ash properties such as the temperature of critical viscosity (Tcv) and slag viscosity are critical to the slag layer buildup and solid slag layer formation, and eventually affect the ash partition between slag and fly ash. The proportion of ash captured on the cylindrical wall decreases from 44.1% to 23.5% when Tcv increases from 1580°K to 1780°K. Since the reactor is configured with refractory-brick wall, the slag layer has limited impact on the heat transfer through the wall due to its small thickness.

Non-axisymmetric flow field in an axial impulse turbine

Phenomena such as hard landings or geometric flaws can cause non-axisymmetric tip clearance in turbines. Such geometric imperfections induce flow distortions which can, in turn, cause self-excited vibration of the rotor, or rotordynamic instability. Flow field perturbations in a single-stage, unshrouded impulse turbine caused by non-axisymmetric tip clearance have been investigated experimentally. Steady velocity and pressure data have been acquired at the design point with and without static turbine casing offset. Perturbations in tangential velocity and casing wall pressure have been obtained, and rotordynamic forces along and perpendicular to the axis of offset have been inferred. Compared to an unshrouded 20% reaction turbine, the forces due to tangential force asymmetry are much smaller, but the forces due to pressure asymmetry are comparable.

Transport phenomena in vertical reactors for metalorganic vapor phase epitaxy: I. Effects of heat transfer characteristics, reactor geometry, and operating conditions

Effects of operating conditions, reactor geometry, and heat transfer characteristics on flow patterns and growth rate uniformity in vertical, axisymmetric reactors for metalorganic vapor phase epitaxy (MOVPE) are described. Finite element solutions of two- and three-dimensional models of transport phenomena in vertical MOVPE reactors identify regions of inlet flow rates, susceptor rotations, and pressures where flow recirculations due to natural convection are minimized and good deposition rate uniformity is achieved. Particular attention is placed on understanding the influence of reactor geometry and heat transfer characteristics on flow fields and film thickness variations. The numerical computations demonstrate that modifications in the orientation of the reactor, the inlet size and its distance from the susceptor as well as the shape of the reactor enclosure can be used effectively to optimize reactor performance. Baffles can also be used advantageously to improve uniformity in existing reactor enclosures but at the expense of more complex flow patterns. The simulations underscore the importance of including accurate heat transfer treatments in MOVPE models by illustrating that different flow fields result from the commonly used thermal boundary conditions and a detailed heat transfer model. The model predictions are shown to be in good agreement with experimentally observed flow fields, wall temperatures and growth rates for GaAs. Nonlinear transport phenomena lead to the existence of multiple steady flows for the same operating parameters as well as the breaking of axisymmetry and development of fully three-dimensional flow patterns. An example of a non-axisymmetric flow field is given and the assumption of axisymmetry in models of vertical reactors is discussed.

Poloidal Magnetic Fields in Galactic Central Regions

We discuss the origin of the observed strong poloidal fields Bz in the central regions of galaxies which have gaseous rings. In the context of galactic disk dynamo models only weak poloidal fields but strong toroidal fields result (Bϕ > Bz). Therefore we tie the strength of the poloidal fields to the central activity and apply known and tested ideas rigorously. A battery process on galactic scales is discussed which ensures the existence of a large scale magnetic field in the inner galactic region. The frozen-in field may be amplified by compression and turbulent stretching; the resulting field is poloidal. The central activity provides a nonaxisymmetric flow field which, just as the α – ω dynamo produces mostly Bϕ > Bz, can produce Bz ≥ Bϕ. This model explains the structure and strength of the magnetic field in star-burst galaxies like M82 (Lesch et al., 1989).

A Method for Wind-Tunnel Investigation of Sonic Booms

A theoretical study for a new method for wind-tunnel investigation of sonic boom problem based on large models is presented. Based on the well-known quasi-linear approach, a method for extrapolation of three-dimensional wind-tunnel data obtained in the very near field of an aircraft configuration is obtained. Several numerical examples for axisymmetric and nonaxisymmetric flowfields are given. Comparisons of the present results with those based on the current sonic boom theory show that the three-dimensional near-field effects are important.