Unsteady flow around an ogive cylinder

Abstract

The time-dependent flow around ah ogive cylinder undergoing large-amplitude, harmonic pitching motion was investigated using flow visualization techniques. The slender body of revolution was towed in an 18 m water channel at Reynolds numbers up to 1.2xl0 5 . Fluorescent dyes were either introduced uniformly from the body's porous surface or placed as horizontal sheets in the slightly stratified tank prior to a run. The dyes were excited with a sheet of laser light projected in the desired plane to mark the flow in the separation region around the body, the flow in the wake, and the potential flow away from the body. The separation process and the geometry of the leeward vortices were studied under different reduced frequencies and Reynolds numbers. The unsteady separation phenomenon is found to be significantly different from the separation on a body in steady flight. Two distinct separation regions, the forebody and aftbody vortex pairs, evolve in the unsteady case. NomenclatureD = maximum body diameter / = pitching frequency H = height of separation region K -reduced frequency, irfD/UL =body length R D = Reynolds number, U^D/v t = time, s £/«, = towing speed x,y,z = Cartesian coordinates fixed with the body oi(t) = time-dependent angle between the body centerline and the towing direction di(t) =rate of change of angle of attack <XA(*) = time-dependent angle between the body centerline and the relative velocity vector (aerodynamic angle of attack) ce 0 =mean pitching angle #00 = amplitude of harmonic motion j8 = angle between the plane of symmetry of the separation vortex and the vertical axis v = kinematic viscosity

The lateral force hysteretic characteristic of a pitching slender body without and with synthetic jet control

“…Their series experimental results showed that during the downward pitching, the separated zone moved downstream, and vice versa, and the size and symmetric degree of the leeward vortices had great change with the change of the pitching parameters. 8 Montividas et al introduced the concept of the effective angle of attack that was changed with different reduced frequencies, and found the hysteresis loop of the separation point with the change of the angle of attack, and obtained effective control that the separation was symmetric using winglet devices control during the pitching motion of the slender body. 9 Ericsson pointed out that to model the hysteretic that occurs in this condition, time must be included to account for flow transmission and observed that the development of the flow field for pitching motion depended on the local flow field and the flow field at the tip.…”

Aerodynamic loads on bluff bodies under wind gusts

“…Concerning the external pressure study, some interesting works related to unsteady aerodynamic loads have been found such as Gad-el-Hak et al (1986) who reported the results of time-dependent flow around cylinders undergoing large-amplitude and harmonic motions. Raza et al (1986) presented the numerical and physical analysis of the pressure and velocity fields in the near wake of a circular cylinder.…”

“…Concerning the external pressure loads on bluff bodies, some interesting works related to unsteady aerodynamic loads have been found such asGad-el-Hak & Chih-Ming Ho (1986) that have reported the results of time-dependent flow around cylinders undergoing large-amplitude and harmonic motions. Their main work is focused on the geometry and separation process of the leeward vortices at different reduced flow frequencies and Reynolds numbers.…”