Ph.D. Defense

Bichuan Mo

(Advisor: Prof. Vigor Yang)

"Flow Dynamics and Mixing of Jet in Crossflow with Cylindrical Cavity"

Thursday, November 21 

12:00 pm

Montgomery Knight Building 317

Abstract

Jet flow injection into cross flow (JICF) is a common fluid dynamics phenomenon that occurs in nature and many engineering applications, such as the plume from a volcano meeting with the crosswind and fuel injection in combustion devices. Flow dynamics and mixing characteristics are major concerns for a JCIF system, especially for engineering applications. This thesis explores the flow dynamics and mixing characteristics of the JICF with a cylindrical cavity surrounding the concentric injection orifice. The cavity is utilized to generate vortices to modulate the initial evolution of the jet and subsequent flow development. In this thesis, a baseline case without a cavity and four cases with different cavity geometries are studied. Special attention is given to the cavity flow structure, windward rolling vortices of the jet, their interaction with the crossflow, and jet mixing characteristics.

The theoretical formulation is based on the conservation equations of mass, momentum, energy, and species concentration in three dimensions. Turbulence closure is achieved using a large-eddy-simulation (LES) technique. A compressible-flow version of the static Smagorinsky model is employed. The numerical framework employs a density-based, finite-volume methodology. Temporal integration is achieved using a fourth-order Runge-Kutta method with explicit physical time stepping. Spatial discretization is performed using a second-order central difference scheme in a generalized coordinate. Fourth-order scalar artificial dissipation is implemented. A multi-block domain decomposition along with a message passing interface is applied to optimize computational efficiency.

The flow dynamics and mixing characteristics of an air jet issued from a cylindrical cavity in an air crossflow are numerically studied. The cavity, aligned concentrically with the jet, is located beneath the crossflow wall. The jet-to-crossflow velocity ratio is 4, and the Reynolds number for the jet flow is 1.39×104 based on its diameter and centerline velocity. The cavity significantly influences the early evolution of the jet and its interaction with the

crossflow. Complex vortical structures are observed. Notably, windward vortices on the jet surface increase in size, accompanied by a reduction in the Strouhal number. For a deep cavity, these vortices break down and result in small vortical tubes in the jet streamwise direction due to secondary instability. Also examined are leeward shear-layer vortices, hanging vortices, wake vortices, and the recirculating flow within the cavity. Their roles in the mixing between the jet fluid and the crossflow are identified. The cavity enhances mixing. The effect is significant in the near field but diminishes in the far field. By adjusting the cavity geometry, it is determined that the cavity depth exercises a more profound impact on jet evolution and mixing than the cavity radius. The most substantial influence occurs when a narrow and deep cavity is implemented. These findings may serve as guidelines for optimizing cavity design for effective modulation of jet behaviors.

Committee

· Prof. Vigor Yang - School of Aerospace Engineering (advisor)

· Prof. Joseph C. Oefelein - School of Aerospace Engineering

· Prof. Lakshmi N Sankar - School of Aerospace Engineering

· Prof. Wenting Sun - School of Aerospace Engineering

· Prof. Yingjie Liu - School of Mathematics