Dynamics of Wake Behind a Low Aspect Ratio Pitching Plate

2011 ◽  
Author(s):  
A. K. De
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Shedding ◽  
Separation Bubble ◽  
Time Integration ◽  
Low Aspect Ratio ◽  
Unsteady Wake ◽  
Lower Reynolds Number ◽  
Vortex Streets ◽  
Predictor Corrector

Vectorized numerical simulations of unsteady wake behind a low-aspect ratio (double amplitude to span ratio 0.3) sinusoidally pitching plate (Strouhal number 0.4 and 0.5) are carried out for the Reynolds number, Re = 150,300. A non-staggered finite volume based predictor-corrector type of algorithm employing 2nd-order time integration and spatial schemes is used. The first sign of instability appears on the plate promoting separation which becomes more vigorous with the increase in Reynolds number. At low Reynolds number, the separation bubble originating near the plate diffuses in the near wake region and the whole wake oscillates with the plate. Fully developed vortex shedding from the rear edge of the plate is observed at a higher Reynolds number. The shed vortices are slowly convected and eventually diffuses completely at a large downstream distance. At a higher forced frequency the wake does not alter significantly at a lower Reynolds number. However, vigorous vortex shedding leading to two layers of vortex streets from both the sides of the plate is observed at a higher Reynolds number.

Secondary vortex, laminar separation bubble and vortex shedding in flow past a low aspect ratio circular cylinder

2021 ◽  
Vol 930 ◽  
Author(s):  
Gaurav Chopra ◽  
Sanjay Mittal
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Separation Bubble ◽  
Critical Regime ◽  
Secondary Vortex ◽  
Laminar Separation Bubble ◽  
Symmetric Mode ◽  
Supercritical Regime ◽  
Laminar Separation ◽  
Low Aspect Ratio

Large eddy simulation of flow past a circular cylinder of low aspect ratio ( $AR=1$ and $3$ ), spanning subcritical, critical and supercritical regimes, is carried out for $2\times 10^3 \le Re \le 4\times 10^5$ . The end walls restrict three-dimensionality of the flow. The critical $Re$ for the onset of the critical regime is significantly lower for small aspect ratio cylinders. The evolution of secondary vortex (SV), laminar separation bubble (LSB) and the related transition of boundary layer with $Re$ is investigated. The plateau in the surface pressure due to LSB is modified by the presence of SV. Proper orthogonal decomposition of surface pressure reveals that although the vortex shedding mode is most dominant throughout the $Re$ regime studied, significant energy of the flow lies in a symmetric mode that corresponds to expansion–contraction of the vortex formation region and is responsible for bursts of weak vortex shedding. A triple decomposition of the time signals comprising of contributions from shear layer vortices, von Kármán vortex shedding and low frequency modulation due to the symmetric mode of flow is proposed. A moving average, with appropriate size of window, is utilized to estimate the component due to vortex shedding. It is used to assess the variation, with $Re$ , of strength of vortex shedding as well as its coherence along the span. Weakening of vortex shedding in the high subcritical and critical regime is followed by its rejuvenation in the supercritical regime. Its spanwise correlation is high in the subcritical regime, decreases in the critical regime and improves again in the supercritical regime.

Vortex Shedding From a Bluff Body Adjacent to a Plane Sliding Wall

1991 ◽  
Vol 113 (3) ◽  
pp. 384-398 ◽  
Author(s):  
M. P. Arnal ◽  
D. J. Goering ◽  
J. A. C. Humphrey
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Solid Wall ◽  
Reynolds Numbers ◽  
Careful Examination ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Flow Past ◽  
Lower Reynolds Number

The characteristics of the flow around a bluff body of square cross-section in contact with a solid-wall boundary are investigated numerically using a finite difference procedure. Previous studies (Taneda, 1965; Kamemoto et al., 1984) have shown qualitatively the strong influence of solid-wall boundaries on the vortex-shedding process and the formation of the vortex street downstream. In the present study three cases are investigated which correspond to flow past a square rib in a freestream, flow past a rib on a fixed wall and flow past a rib on a sliding wall. Values of the Reynolds number studied ranged from 100 to 2000, where the Reynolds number is based on the rib height, H, and bulk stream velocity, Ub. Comparisons between the sliding-wall and fixed-wall cases show that the sliding wall has a significant destabilizing effect on the recirculation region behind the rib. Results show the onset of unsteadiness at a lower Reynolds number for the sliding-wall case (50 ≤ Recrit ≤100) than for the fixed-wall case (Recrit≥100). A careful examination of the vortex-shedding process reveals similarities between the sliding-wall case and both the freestream and fixed-wall cases. At moderate Reynolds numbers (Re≥250) the sliding-wall results show that the rib periodically sheds vortices of alternating circulation in much the same manner as the rib in a freestream; as in, for example, Davis and Moore [1982]. The vortices are distributed asymmetrically downstream of the rib and are not of equal strength as in the freestream case. However, the sliding-wall case shows no tendency to develop cycle-to-cycle variations at higher Reynolds numbers, as observed in the freestream and fixed-wall cases. Thus, while the moving wall causes the flow past the rib to become unsteady at a lower Reynolds number than in the fixed-wall case, it also acts to stabilize or “lock-in” the vortex-shedding frequency. This is attributed to the additional source of positive vorticity immediately downstream of the rib on the sliding wall.

Flow past finite cylinders of constant curvature

2018 ◽  
Vol 837 ◽  
pp. 896-915 ◽  
Author(s):  
Jessica K. Shang ◽  
H. A. Stone ◽  
A. J. Smits
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Shedding ◽  
Angle Of Incidence ◽  
Number Range ◽  
Independence Principle ◽  
Laminar Wake ◽  
The Difference ◽  
Visualization Experiments ◽  
Local Angle

Wake visualization experiments were conducted on a finite curved cylinder whose plane of curvature is aligned with the free stream. The stagnation face of the cylinder is oriented concave or convex to the flow at $230\leqslant Re_{D}\leqslant 916$, where $Re_{D}$ is the cylinder Reynolds number and the curvature is constant and ranges from a straight cylinder to a quarter-ring. While the magnitude of the local angle of incidence to the flow is the same for both orientations, the contrast in their wakes demonstrates a violation of a common approximation known as the ‘independence principle’ for curved cylinders. Vortex shedding always occurred for the convex-oriented cylinder for the Reynolds-number range investigated, along most of the cylinder span, at a constant vortex shedding angle. In contrast, a concave-oriented cylinder could exhibit multiple concurrent wake regimes along its span: two shedding regimes (oblique, normal) and two non-shedding regimes. The occurrence of these wake regimes depended on the curvature, aspect ratio and Reynolds number. In some cases, vortex shedding was entirely suppressed, particularly at higher curvatures. In the laminar wake regime, increasing the curvature or decreasing the aspect ratio restricts vortex shedding to smaller regions along the span of the cylinder. Furthermore, the local angle of incidence where vortex shedding occurs is self-similar across cylinders of the same aspect ratio and varying curvature. After the wake transitions to turbulence, the vortex shedding extends along most of the cylinder span. The difference in the wakes between the concave and convex orientations is attributed to the spanwise flow induced by the finite end conditions, which reduces the generation of spanwise vorticity and increases the incidence of non-shedding and obliquely shedding wakes for the concave cylinder.

Flow Pattern on the Surface of Airfoil With Lower Reynolds Number

2009 ◽  
Author(s):  
Xu Sun ◽  
Jia-Zhong Zhang
Keyword(s):  
Reynolds Number ◽  
Flow Pattern ◽  
Stokes Equations ◽  
Separation Bubble ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Driven Cavity ◽  
Naca0012 Airfoil ◽  
Lower Reynolds Number ◽  
Characteristic Based Split

In this paper, aerodynamic performance of the NACA0012 airfoil in the incompressible flow with a lower Reynolds number (Re) is investigated numerically from the viewpoints of flow pattern and nonlinear dynamics. First, the characteristic-based split (CBS) finite element method is introduced for the approximation of the incompressible Navier-Stokes equations, and then the lid-driven cavity flow and flow around a circular cylinder are calculated for varification. Then, at Re = 1000, flow fields around the NACA0012 airfoil at a series of angles of attack are simulated. With the increase of the attack angle, great change of the flow pattern appears, and the flow structures such as trailing edge vortex, separation bubble and shedding vortex are observed. Moreover, it is found that the separation bubble plays an important role in the deterioration of the flow stability at higher attack angles, and the vortex shedding can be taken as the result of a Hopf bifurcation while the bifurcation parameter is the angle of attack.

CFD Analysis of Low Aspect Ratio Rectangular Wings in Very Low Reynolds Number

2018 ◽  
Vol 2018.55 (0) ◽  
pp. C021
Author(s):  
Takaya MIWA ◽  
Yuta NATSUME ◽  
Daisuke SASAKI ◽  
Masato OKAMOTO ◽  
Koji SHIMOYAMA
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Low Reynolds Number ◽  
Cfd Analysis ◽  
Low Aspect Ratio ◽  
Low Reynolds
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