Yangzi Huang

Bio: Yangzi Huang is an academic researcher from Syracuse University. The author has contributed to research in topics: Vortex & Lyapunov exponent. The author has an hindex of 3, co-authored 5 publications receiving 65 citations.

Detection and tracking of vortex phenomena using Lagrangian coherent structures

Abstract: The formation and shedding of vortices in two vortex-dominated flows around an actuated flat plate are studied to develop a better method of identifying and tracking coherent structures in unsteady flows. The work automatically processes data from the 2D simulation of a flat plate undergoing a $$45^{\circ }$$ pitch-up maneuver, and from experimental particle image velocimetry data in the wake of a continuously pitching trapezoidal panel. The Eulerian $$\varGamma _1$$ , $$\varGamma _2$$ , and Q functions, as well as the Lagrangian finite-time Lyapunov exponent are applied to identify both the centers and boundaries of the vortices. The multiple vortices forming and shedding from the plates are visualized well by these techniques. Tracking of identifiable features, such as the Lagrangian saddle points, is shown to have potential to identify the timing and location of vortex formation, shedding, and destruction more precisely than by only studying the vortex cores as identified by the Eulerian techniques.

Tracking coherent structures in massively-separated and turbulent flows

Abstract: Coherent vortex structures are tracked in simulations of massively-separated and turbulent flows using Lagrangian saddles found as intersections of positive and negative finite-time Lyapunov exponent ridges. This allows vortices to be tracked objectively in time and space in a variety of flows.

Eulerian and Lagrangian Methods for Detecting Vortex Formation and Shedding

Abstract: The visualization and tracking of the vortices shed from a pitching panel is studied to develop a better method of identifying and tracking coherent structures in unsteady flows. The research processes a large amount of data from the 2D simulation of a flat plate undergoing a 45 pitch-up maneuver. The Eulerian Γ1 and Q functions, as well as Lagrangian coherent structures (LCS) analysis are applied at the center and boundary of the vortices. The multiple vortices forming and shedding from the plate are visualized well by their vortex centers and boundaries. The saddle points and vortex centers are tracked, which could time the vortex separation and shedding more precisely. The research reveals the rich vortex information and dynamics details available by combining methods.

Definitions of vortex vector and vortex

Abstract: Although the vortex is ubiquitous in nature, its definition is somewhat ambiguous in the field of fluid dynamics. In this absence of a rigorous mathematical definition, considerable confusion appears to exist in visualizing and understanding the coherent vortical structures in turbulence. Cited in the previous studies, a vortex cannot be fully described by vorticity, and vorticity should be further decomposed into a rotational and a non-rotational part to represent the rotation and the shear, respectively. In this paper, we introduce several new concepts, including local fluid rotation at a point and the direction of the local fluid rotation axis. The direction and the strength of local fluid rotation are examined by investigating the kinematics of the fluid element in two- and three-dimensional flows. A new vector quantity, which is called the vortex vector in this paper, is defined to describe the local fluid rotation and it is the rotational part of the vorticity. This can be understood as that the direction of the vortex vector is equivalent to the direction of the local fluid rotation axis, and the magnitude of vortex vector is the strength of the location fluid rotation. With these new revelations, a vortex is defined as a connected region where the vortex vector is not zero. In addition, through direct numerical simulation (DNS) and large eddy simulation (LES) examples, it is demonstrated that the newly defined vortex vector can fully describe the complex vertical structures of turbulence.

A Definition of Vortex Vector and Vortex

Abstract: Vortex is ubiquitous in nature. However, there is not a consensus on the vortex definition in fluid dynamics. Lack of mathematical definition has caused considerable confusions in visualizing and understanding the coherent vortical structures in turbulence. According to previous study, it is realized that vortex is not the vorticity tube and vorticity should be decomposed into a rotational part which is the vortex vector and a non-rotational part which is the shear. In this paper, several new concepts such as fluid rotation of local point, the direction of fluid rotation axis and the strength of fluid rotation are proposed by investigating the kinematics of fluid element in the 2D and 3D flows. A definition of a new vector quantity called vortex vector is proposed to describe the local fluid rotation. The direction of the vortex vector is defined as the direction of local fluid rotation axis. The velocity components in the plane orthogonal to the vortex vector have zero derivatives along the vortex vector direction. The magnitude of the vortex vector is defined as the rotational part of vorticity in the direction of the vortex vector, which is the twice of the minimum angular velocity of fluid around the point among all azimuth in the plane perpendicular to vortex vector. According to the definition of the vortex vector, vortex is defined as a connected flow region where the magnitude of the vortex vector at each point is larger than zero. The new definition for the vortex vector and vortex follows three principles: 1. Local in quantity, 2. Galilean invariant, 3. Unique. The definitions are carefully checked by DNS and LES examples which clearly show that the new defined vortex vector and vortex can fully represent the complex structures of vortices in turbulence.

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

Abstract: The dynamic stall development on a pitching airfoil at Re = 106 was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterized by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analyzed for various sinusoidal motions revealing an increase in the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness, and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent.