Session: 11-08-02: Multiphase Flows and Applications II

Paper Number: 165609

Suppression of Vortex Shedding From a Hemisphere by the Addition of a Porous Coating 

Vortex shedding commonly occurs for fluid flow around a bluff body.  Above a critical Reynolds number, due to the curvature of the body, the high shear boundary layer separates from the surface of the body; at a higher Reynolds number, the separated flow becomes unstable to the shedding of vortices (von Kármán vortex street). These vortices are periodically shed (Strouhal frequency) from the object into the wake.

Vortex shedding may be suppressed (via a mechanism not fully understood) by coating the bluff body with a solid porous shell. The porous layer alters the boundary layer near the surface of the bluff body, allowing for a more gradual transition from zero flow at the body surface to the free stream flow; this smoothing of the flow profile may be sufficient to stabilize fluctuations in the boundary layer.

The current study aims to understand the role of the porous layer in suppressing vortex shedding for a hemisphere mounted on a substrate. The domain is a rectangular duct with a cross section of 44mm×11mm; the hemisphere has a radius of 2.2 mm. The simulations are conducted at (hemisphere) Reynolds number 10000. The inlet velocity profile is fixed to be that for a fully developed empty duct flow. The outlet boundary condition requires the axial derivative of all flow quantities to vanish.  An implicit unsteady scheme with a time step of 1×10^-5 s was used. The Mentor’s SST k-ω turbulence model was employed. Vortex shedding is observed from the solid hemisphere with a measured Strouhal number of 0.1946, which is reasonable.

The porous layer is simulated by introducing Lagrangian multiphase solid particles with very high density, arranged in the form of hemispherical shells around the solid dome. Initial simulations were conducted with particle diameter d = 50 microns and interparticle distance of 60 microns; this results in a porosity of 41.3%; the porosity is controlled by changing the particle diameter.  The thickness of the coating is varied by successively adding shell layers.  The simulations are conducted under zero gravitational acceleration; thus, the Lagrangian particles do not settle, but their infinite inertia precludes their being set in motion by the fluid.

Q-criterion iso-surface plots are employed to visualize the three-dimensional structure and evolution of the vortex shedding. Temporal variation of vorticity and pressure, at the apex of the hemisphere, are monitored to quantify any damping of the vortex shedding.

Adding the first (1-layer) shell drastically reduces the vorticity and pressure oscillations at the apex of the hemisphere, although some long-time minor oscillations persist; this is also observed as activity in the Q-criterion plot. Upon increasing the shell thickness (2-layer shell), the vortex shedding completely damps out following a few oscillations; further increase in shell thickness (multi-layer shell) merely increases the damping of these oscillations.  Similarly, the Q-criterion plot becomes less active for these thicker shells. Additional results with different values of porosity will be presented. This study can help in the development of solutions to suppress vortex shedding in engineering applications.

Presenting Author: Satyasreet Jena University of Cincinnati

Presenting Author Biography: Satyasreet Jena is a doctoral student in the Department of Mechanical Engineering at the University of Cincinnati. Their current work investigates Multiphase flows with a focus on particle aerosolization in highly turbulent flows.

Authors:

Satyasreet Jena University of Cincinnati
Leonid A. Turkevich National Institute for Occupational Safety and Health
Milind A. Jog University of Cincinnati
Urmila Ghia University of Cincinnati