Bifurcation analysis of two-dimensional laminar flow through sudden expansion channel

In this work, bifurcation characteristics of unsteady, viscous, Newtonian laminar flow in two-dimensional sudden expansion and sudden contraction-expansion channels have been studied for different values of expansion ratio. The governing equations have been solved using finite volume method and FLUENT software has been employed to visualize the simulation results. Three different mesh studies have been performed to calculate critical Reynolds number (Recr) for different types of bifurcation phenomena. It is found that Recr decreases with the increase in expansion ratio (ER).

A STUDY ON SUDDEN EXPANSION HYDRODYNAMIC PHENOMENA OCCURRING IN CYLINDRICAL PIPES

Introduction: This paper studies the frequency with which hydrodynamic parameters change in the sudden expansion section of axisymmetric pressure flow, based on the boundary layer equations. Methods: The suggested method reveals the regularity of changes in the hydrodynamic parameters of the flow in the transitional area, making it possible to obtain a velocity profile in any cross-section under common initial and boundary conditions. Based on the general solutions, we studied the hydrodynamic processes occurring in the transitional area of the effective sudden cross-section expansion within the axisymmetric pressure movement, in the following cases: a) when the velocity is constant at any point of the inlet face; b) when the velocity is distributed along the inlet face according to the parabolic law. Our calculations were carried out for different values of the expansion factor. Results: Based on the results of the computer-aided experimental study, we obtained velocity diagrams along the length of the transitional area with constant and parabolic velocity distributions for fluid inflowing into the expanded section. We also determined the patterns of pressure distribution along the length of the relevant section.

HYDRODYNAMICAL INSTABILITY OF NEWTONIAN FLOW BEFORE AN AXISYMMETRIC SUDDEN CONTRACTION

The sizes of the vortex region before the axisymmetric sudden contraction of the circular pipe at the Newtonian flow have been investigated. Area ratios 0.250 and 0.500 were considered. The sizes of the vortex region have the extreme dependence with a maximum at the transition of the laminar flow into a turbulent flow one. When the Reynolds number at the laminar flow increase, these sizes also increase, and they decrease at the turbulent flow. In both cases, the sizes of the vortex region are proportional to the Reynolds number. A transition region between laminar flow and turbulent flow lies in the range of the Reynolds number from 3000 to 5300 and 750…1300, determined by the diameter of a bigger pipe of sudden expansion and a step height correspondingly

The Phenomenon of Hysteresis in a Swirling Stream

The article considers a swirling current with a sudden expansion. As a result of an experimental study, it was found that two flow modes can exist under these conditions. In the first case, there is an intense twist at the outlet of the vortex device, a zone of return currents and precessional oscillations. In the second case, there is no zone of return currents and precession. At the same time, the gas flow through the device increases abruptly

Magma pressure discharge induces very long period seismicity

Volcano seismicity is one of the key parameters to understand magma dynamics of erupting volcanoes. However, the physical process at the origin of the resulting complex and broadband seismic signals remains unclear. Syn-eruptive very long period (VLP) seismic signals have been explained in terms of the sudden expansion of gas pockets rising in the liquid melt. Their origin is linked to a magma dynamics which triggers the explosive process occurring before the explosive onset. We provide evidence based on acoustic, thermal, and ground deformation data to demonstrate that VLP signals at Stromboli are generated at the top of the magma column mainly after the explosion onset. We show that VLP amplitude and duration scale with the eruptive flux which induces a decompression of 103–104 Pa involving the uppermost ~ 250 m of the feeding conduit. The seismic VLP source represents the final stage of a ~ 200 s long charge and discharge mechanism the magma column has to release excess gas accumulated at the base of a denser and degassed magma mush. The position of the VLP seismic source coincides with the centroid of the shallow mush plug and tracks elevation changes of the magma free surface.

Effect of Asymmetry of Channels on Flows in Parallel Plates with a Sudden Expansion

In order to quantitatively grasp the influence of asymmetry of a channel, flow in an eccentric sudden expansion channel, in which the channel centers are different on the upstream and downstream sides, was calculated to be less than the Reynolds number of 400, where the expansion rate was 2. The asymmetry of a channel is expressed by an eccentricity S, where a symmetric expansion channel is S = 0 and a channel with one side step is S = 1. Both flows firstly reattached on the wall located on the short and long side of a sudden expansion and were observed in the range of S ≤ 0.2, although only the former was seen in the range of S > 0.2. The critical Reynolds number of the multiple solutions increases parabolically to S. At least two separation vortices occur, and the third separation vortex is generated in both solutions above the critical Reynolds number of the third vortex. The length of an entrance region increases linearly to the Reynolds number and slightly with the increase in S. The pressure drop coefficient is proportional to the power of the Reynolds number and increases with S.

Experimental study of the local structure of a bubble flow in a flat channel with sudden expansion

The hydrodynamic structure of the flow in a flat channel with sudden expansion was studied at constant flow rates of liquid and gas in the vertical flow at Re = 6600 and gas content β = 0.03. The measurements were carried out using the PLIF method; and with this view, fluorescent particles for PIV studies and the dye Rhodamine G were added to distilled water. An optical threshold filter was installed on the lens of the video camera. When processing images to obtain data on the local gas content, only bubbles falling into the plane of the laser beam were considered (the boundary glows, casting a shadow).

Effect of Cavities in Suddenly Expanded Flow at Supersonic Mach Number

The contribution from the base drag due to the sub-atmospheric pressure is significant. It can be more than two-thirds of the net drag. There is a need to increase the base pressure and hence decrease the base drag. This research examines the effect of Mach Number on base pressure. To accomplish this objective, it controls the efficacy in an enlarged duct computed by the numerical approach using Computational Fluid Dynamics (CFD) Analysis. This experiment was carried out by considering the expansion level and the aspect cavity ratio. The computational fluid dynamics method is used to model supersonic motion with the sudden expansion, and a convergent-divergent nozzle is used. The Mach number is 1.74 for the present study, and the area ratio is 2.56. The L/D ratio varied from 2, 4, 6, 8, and 10, and the simulated nozzle pressure ratio ranged from 3 to 11. The two-dimensional planar design used commercial software from ANSYS. The airflow from a Mach 1.74 convergent-divergent axi-symmetric nozzle expanded suddenly into circular ducts of diameters 17 and 24.5 mm with and without annular rectangular cavities. The diameter of the duct is taken D=17mm and D=24.5mm. The C-D nozzle was developed and modeled in the present study: K-ε standard wall function turbulence model was used with the commercial computational fluid dynamics (CFD) and validated. The result indicates that the base pressure is impacted by the expansion level, the enlarged duct size, and the passage’s area ratio.