The dynamic character of the hemisphere-cylinder wake

Hoang, Nguyen Quang; Rediniotis, Othon K.; Telionis, Demetri P.

doi:10.1007/s003480050305

Cited by 10 publications

(8 citation statements)

References 10 publications

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“…This finding is interesting, because it seems to imply that a type of new global mode might exist over hemisphere cylinders. However, the oscillatory frequencies obtained by Le Clainche et al 29,30 are different from the ones in previous experiment 25 due to the much lower Reynolds numbers. The studies of Le Clainche et al 29,30 also revealed 3D spatial structures on the separation bubble, horn vortices and leeward vortices over a hemisphere cylinder.…”

Section: Recent Years Direct Numerical Simulation (Dns) and Stabilitcontrasting

confidence: 67%

“…Therefore, if the asymmetric vortices exist over a hemisphere cylinder, the underlying mechanisms will be probably similar to the case for the prolate spheroid. Flow unsteadiness over hemisphere cylinders has also been studied in experiments and numerical simulations [25][26][27][28][29][30] , but not so many. Hoang et al 25 measured flow fluctuations over a hemisphere cylinder at various AOAs using hot wires, in which the Reynolds number based on cylinder diameter was 22000.…”

Section: Introductionmentioning

confidence: 99%

“…Flow unsteadiness over hemisphere cylinders has also been studied in experiments and numerical simulations [25][26][27][28][29][30] , but not so many. Hoang et al 25 measured flow fluctuations over a hemisphere cylinder at various AOAs using hot wires, in which the Reynolds number based on cylinder diameter was 22000. The study found a type of velocity fluctuations with the frequencies lower than Kármán vortex shedding, and they suggested that the low frequency fluctuations probably come from vortex heaving, but no information on flow patterns was provided.…”

Section: Introductionmentioning

confidence: 99%

“…The mean drag coefficient (CD) on the body was selected as a global quantity to show the grid convergence, as conducted in previous studies 29,42 . Meanwhile, since the present investigation was focused on the vortex oscillation, the non-dimensional frequencies (St) of the total side-force on the body were also calculated in the grid refinement study and compared with published experimental data 25 . The GCI is defined as GCI j+1, j (%) =3|(fj+1-fj)/fj (r p -1)|×100 where the f is any integral or field quantities on coarse (j+1) and fine (j) grids, r is the refinement ratio from fine to coarse mesh sizes, and p is the observed order of accuracy for numerical methods used.…”

mentioning

confidence: 99%

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

Yin

2018

AIAA Journal

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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.

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Section: Recent Years Direct Numerical Simulation (Dns) and Stabilitcontrasting

confidence: 67%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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

Yin

2018

AIAA Journal

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“…They believed that the vortices near the fore-body were nearly stationary with no significant unsteadiness. Hoang et al 19 measured the unsteady flow fields around a hemisphere-cylinder body using hotwires and detected a type of unsteady phenomenon where the frequencies of fluctuations are lower than the ones of Karman vortex shedding, but the sources of fluctuations were not clear, and they conjectured that the low frequency fluctuations could be caused by forebody vortex heaving. Ma et al 20,21 also studied the difference of unsteadiness at the fore-body and after-body of a slender body at extreme high AOAs.…”

Section: Introductionmentioning

confidence: 98%

Low-frequency unsteadiness of vortex wakes over slender bodies at high angle of attack

Liu

2014

Chinese Journal of Aeronautics

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A type of flow unsteadiness with low frequencies and large amplitude was investigated experimentally for vortex wakes around an ogive-tangent cylinder. The experiments were carried out at angles of attack of 60-80°and subcritical Reynolds numbers of 0.6-1.8 · 10 5 . The reduced frequencies of the unsteadiness are between 0.038 and 0.072, much less than the frequency of Karman vortex shedding. The unsteady flow induces large fluctuations of sectional side forces. The results of pressure measurements and particle image velocimetry indicate that the flow unsteadiness comes from periodic oscillation of the vortex wakes over the slender body. The time-averaged vortex patterns over the slender body are asymmetric, whose orientation is dependent on azimuthal locations of tip perturbations. Therefore, the vortex oscillation is a type of unsteady oscillation around a time-averaged asymmetric vortex structure. ª 2014 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.

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