Showing papers on "Starting vortex published in 2021"

Simulation of flow induced by single-dielectric-barrier-discharge plasma actuator using a high-order flux-reconstruction scheme

Abstract: The starting vortex flow generated by a single-dielectric-barrier-discharge (SDBD) plasma actuator in still air is investigated numerically by direct numerical simulation (DNS) using a 4th order Navier–Stokes solver and Suzen's model for the plasma actuator. The Navier–Stokes equations and the equations for the electric field and the charge density distribution in the plasma body force model are solved by the same high-order method based on the flux reconstruction algorithm on arbitrary mixed grids, allowing easy extension to configurations of complex geometry. The computational method and its high orders of accuracy are verified for the problem of the translation of an isentropic vortex and validated for the vortex shedding problem behind a circular cylinder. The plasma body force model is then implemented in the Navier–Stokes code to perform highly resolved DNS of the starting vortex generated by an SDBD plasma actuator. The computed vortex pattern and its trajectory with time are compared with available experimental data. The computational results confirm similarity laws on the motion of the vortex.

Numerical Investigation of Inter-Rotor Spacing Effects on Wake Dynamics of Coaxial Rotors

Abstract: In this Paper, the wake dynamics of coaxial rotors are studied using the high-wake-resolution method. The high-wake-resolution method is a novel combination of the truncated vortex tube model for i...

Evaluating unsteady fluid dynamic forces in viscous flows from the vorticity field

Abstract: The vortex force map (VFM) method is a promising low-cost technique for predicting unsteady aerodynamic load from the vorticity field. Developed under potential flow assumption and applied to high-resolution computational fluid dynamics (CFD) data, the original VFM method needs to be further explored for viscous flows, especially at low Reynolds numbers and with incomplete knowledge of the flow data. In this paper, the VFM method is extended to general viscous flows, with both forces contributed from vorticity in the flowfield and on the body surface considered. We find that viscosity contributes to not only the skin-friction force but also a pressure force term. We also show that the extended vortex pressure force maps for general flow problems without the potential flow assumption lead to vanishing contributions from the far-field. The extended VFM method is applied to CFD flowfields for the starting flow of a cylinder and a NACA0012 airfoil, and it is used to analyze the force contribution effect of a given vortex. Moreover, acceptable accuracy of the method on truncated domains and under coarse sampling of typical particle image velocimetry measurement size is demonstrated.

On the starting vortex generated by a translating and rotating flat plate

Abstract: We consider the trailing-edge vortex produced in an inviscid fluid by the start-up motion of a two-dimensional flat plate. A general starting motion is studied that includes the initial angle-of-attack of the plate (which may be zero), individual time power laws for plate translational and rotational speeds and the pivot position for plate rotation. A vortex-sheet representation for a start-up separated flow at the trailing edge is developed whose time-wise evolution is described by a Birkhoff–Rott equation coupled to an appropriate Kutta condition. This description includes convection by the outer flow, rotation and vortex-image self-induction. It admits a power-law similarity solution for the (small-time) primitive vortex, leading to an equation set where each term carries its own time-wise power-law factor. A set of four general plate motions is defined. Dominant-balance analysis of this set leads to discovery of three distinct start-up vortex-structure types that form the basis for all vortex motion. The properties of each type are developed in detail for some special cases. Numerical and analytical solutions are described and transition between solution types is discussed. Singular and degenerate vortex behaviour is discovered which may be due to the absence of fluid viscosity. An interesting case is start-up motion with zero initial angle of attack coupled to power-law plate rotation for which time-series examples are given that can be compared to high Reynolds number viscous flows.

Vortex formation and frequency tuning of periodically-excited jet diffusion flames

Abstract: This paper presents an experimental study on the vortex formation and frequency tuning of jet diffusion flames under periodic excitations. The state-of-art laser diagnostic techniques were applied to provide temporally-resolved measurements for flame and flow structures. The results show that the fame surface deformation is synchronized with the convection of the inner vortex rings (IVRs), demonstrating the crucial role of IVRs in affecting the flame dynamics. The quantitative study on vortex formation and evolution is intended to understand two mechanisms: how the IVR forms, and how it is tuned to the forcing frequency of the perturbed fuel. The qualitative agreement between the predicted circulation growth and experimental data verifies the relevance of the classical starting vortex jet model in addressing the current problem, indicating the vortex growth is mainly dictated by the shear layer of the upstream fuel. The vortex tuning is closely related to the detachment of the main IVR, which is attributed to a secondary “razor vortex” when the shearing between the fuel jet and the coflow switches direction.