Vortex Oscillations Around a Hemisphere-Cylinder Body with a High Fineness Ratio

Abstract

In our previous studies (Ma and Liu, Phy Fluids, 2014; Ma, Huang, and Liu, Chin J Aeronaut, 2014), the unsteady vortex flows around a pointed-ogive slender body were studied by means of numerical simulations and experiments, and low-frequency vortex oscillations were found over the forebody as angles of attack (AOAs) more than 65 o . In this investigation, the vortex unsteadiness around a hemisphere-cylinder body with a fineness ratio (the ratio of length L to diameter D of a body, L/D) of 24 at AOAs of 10 o to 80 o was studied using Large Eddy Simulation (LES) and Dynamic Mode Decomposition (DMD). The Reynolds number (Re) based on the cylinder diameter of the body is 22000. The results show that the vortex oscillations still exist over the hemisphere-nose slender body, but the vortex behaviors are much different from the ones over the pointed-nose bodies. The vortex oscillation exists over the forebody at the whole range of AOAs, and occurs even at the AOA of 10 o . The oscillation is characterized by alternate oscillations of a forebody leeward vortex pair up and down and in-phase swings from side to side. The vortex shedding can be found at the afterbody as AOAs more than 20 o , and the shedding region moves forwards gradually with AOAs increasing, and accordingly the region of vortex oscillations contracts and eventually only exists near the nose as AOAs sufficiently high. The vortex oscillation and shedding all induce fluctuating side forces along the body, but the ones from vortex oscillations are larger. The frequencies of vortex oscillations are similar to the ones of vortex shedding at the AOAs of 10 o -40 o with St=0.085-0.12, in which the flow fields over the afterbody are dominated by vortex shedding and forebody-vortex wakes together; while at AOAs of 50 o -80 o , the frequencies along the body are apparently divided into two regimes in which the vortex oscillations over the forebody have the frequencies of 0.053-0.064, and the vortex shedding over the afterbody has the frequencies of 0.16-0.2. Additionally, as AOAs beyond 40 o , the frequencies are basically proportional to the sine function of AOAs. The simulated frequencies agree well with previous experimental results obtained by hotwire velocity measurements (Hoang et al., Exp Fluids, 1999). The oscillatory global modes based on the sectional flow snapshots were obtained through DMD analysis. The results show that except the mean flows, the most energetic modes correspond to the vortex oscillation at the forebody and to the vortex shedding at the afterbody respectively, and the frequencies from DMD are identical to the ones of the side forces obtained by fast Fourier transform. In addition, both the time-averaged side forces and vorticity fields show that the mean flow fields for the vortex oscillations are symmetric, and no apparent asymmetry exists. Therefore, the vortex pair over the forebody oscillates around a symmetric mean flow field.

Vortex induced vibrations of wind turbine blades: Influence of the tip geometry

“…For an inclination angle of 40 ○ , this flow pattern was replaced by a set of vortical structures that were essentially steady. Similar shedding patterns were observed in the simulations of Ma and Yin (2018), which considered a hemisphere-cylinder body, and for delta wing geometries (Gursul, 2005;Luo and Ma, 2018), where the existence of leading edge vortices is well known. Jiang et al (2015) studied the flow around an inclined prolate spheroid at 45 ○ , representative of a submarine geometry, by means of numerical simulations.…”

Turbulent wake behind a concave curved cylinder

“…It has been extensively investigated (e.g. by Ramberg (1983), Snarski (2004), Zhao, Cheng & Zhou (2009) and Ma & Yin (2018), and the many other studies mentioned therein). The features of the inclined cylinder wake are highly dependent on the angle of attack.…”

“…As the angle of attack increases from (i.e. cylinder axis aligned with inflow) to , the inclined cylinder wake is usually classified into four types: attached flow, symmetric vortices, steady asymmetric vortices and unsteady vortex shedding (Zdravkovich 1997; Ma & Yin 2018). Steady asymmetric vortices that appear at medium high attack angles are most representative.…”

Stability of Low-Reynolds-Number Separated Flow Around an Airfoil Near a Wavy Ground

“…Another unique aspect of the present study concerns the introduction of three-dimensionality to this problem, through the use of global flow stability analysis. To develop effective control strategies to suppress flow separation and improve aerodynamic performance requires a sound understanding of the instability characteristics of the base flow [12][13][14][15][16][17][18]. Linear stability analysis has been used to successfully predict the primary stability of two-and three-dimensional flows [19].…”