Flow Unsteadiness and Stability Characteristics of Low-Re Flow Past an Inclined Triangular Cylinder

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Wei Zhang ◽  
Hui Yang ◽  
Hua-Shu Dou ◽  
Zuchao Zhu
Keyword(s):  
Growth Rate ◽  
Sensitivity Analysis ◽  
Unsteady Flow ◽  
Maximum Velocity ◽  
Flow Patterns ◽  
Wake Flow ◽  
Near Wake ◽  
Flow Unsteadiness ◽  
Flow Past ◽  
Far Wake

The present study investigates the two-dimensional flow past an inclined triangular cylinder at Re = 100. Numerical simulation is performed to explore the effect of cylinder inclination on the aerodynamic quantities, unsteady flow patterns, time-averaged flow characteristics, and flow unsteadiness. We also provide the first global linear stability analysis and sensitivity analysis on the targeted physical problem for the potential application of flow control. The objective of this work is to quantitatively identify the effect of cylinder inclination on the characteristic quantities and unsteady flow patterns, with emphasis on the flow unsteadiness and instability. Numerical results reveal that the flow unsteadiness is generally more pronounced for the base-facing-like cylinders (α → 60 deg) where separation occurs at the front corners. The inclined cylinder reduces the velocity deficiency in the near-wake, and the reduction in far-wake is the most notable for the α = 30 deg cylinder. The transverse distributions of several quantities are shifted toward the negative y-direction, such as the maximum velocity deficiency and maximum/minimum velocity fluctuation. Finally, the global stability and sensitivity analysis show that the spatial structures of perturbed velocities are quite similar for α ≤ 30 deg and the temporal growth rate of perturbation is sensitive to the near-wake flow, while for α ≥ 40 deg there are remarkable transverse expansion and streamwise elongation of the perturbed velocities, and the growth rate is sensitive to the far-wake flow.

Quantification of wake unsteadiness for low-Re flow across two staggered cylinders

2019 ◽  
Vol 233 (19-20) ◽  
pp. 6892-6909 ◽  
Author(s):  
Wei Zhang ◽  
Xiaojun Li ◽  
Zuchao Zhu
Keyword(s):  
Sensitivity Analysis ◽  
Shear Layer ◽  
Unstable Mode ◽  
Flow Patterns ◽  
Temporal Variations ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Flow Unsteadiness ◽  
Far Wake ◽  
Perturbation Growth

This work performs a numerical investigation on the two-dimensional flow across two circular cylinders in staggered arrangement at Re = 100. The seaparting distances between the centers of the cylinders are D/ d = 4–10 with Δ D/ d = 2 and T/ d = 0.0–2.0 with Δ T/ d = 0.5 in the streamwise and transverse directions, respectively, in which d is the cylinder diameter. Although the low- Re flow across staggered cylinders has been studied in a number of works, the authors mainly concerned about the identification and transition of various flow patterns. In this work, our objective is to quantitatively reveal the characteristics of flow unsteadiness as affected by the two separating distances. The flow unsteadiness is assessed from several aspects, including the spatial distributions and temporal variations of instantaneous flow patterns, fluctuating characteristic quantities, and fluctuating flow in the gap and in the near- and far-wake regions. To investigate the inherent instability of the flow, the global linear stability and sensitivity analysis is further carried out to demonstrate the unstable mode of perturbation growth and the critical flow patterns that destabilize the flow. The numerical results reveal that the wake flow between the two centerlines and beside the upstream cylinder is the most intensely perturbed. The flow around the downstream cylinder exhibits great fluctuation as perturbed by the destabilized shear layer of the upstream cylinder. The flow downstream of both cylinders shows multiple peak fluctuation of velocity because of the complex interactions between the destabilized shear layer and the wake vortices, resulting in the bidirectional transverse propagation of fluctuation. The stability analysis demonstrates that the unstable mode of perturbation growth is more significant in the far-wake region as the two cylinders are placed in proximity; the sensitivity analysis shows that the gap flow is crucial for the flow destabilization at small D, while the wake flow of cylinder- B is more significant for large D.

scholarly journals Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

2017 ◽  
Vol 7 (1) ◽  
pp. 20160090 ◽  
Author(s):  
Roi Gurka ◽  
Krishnamoorthy Krishnan ◽  
Hadar Ben-Gida ◽  
Adam J. Kirchhefer ◽  
Gregory A. Kopp ◽  
...  
Keyword(s):  
Unsteady Flow ◽  
Bird Species ◽  
Unsteady Aerodynamics ◽  
Flow Patterns ◽  
Flapping Flight ◽  
Wake Flow ◽  
Flapping Wings ◽  
Flow Structures ◽  
Near Wake ◽  
Time Resolved

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper ( Calidris mauri ), European starling ( Sturnus vulgaris ) and American robin ( Turdus migratorius ) using long-duration, time-resolved particle image velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named ‘double branch’). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag.

On the Intrinsic Mechanism of Wakes Interference Past Two Cylinders Towed Beneath a Free Surface

2013 ◽  
Author(s):  
Dominique R. Rajaona ◽  
Toky Ramanakoto
Keyword(s):  
Free Surface ◽  
Maximum Velocity ◽  
Ccd Camera ◽  
Cylinder Wake ◽  
Near Wake ◽  
Interference Phenomenon ◽  
Flow Past ◽  
Intrinsic Mechanism ◽  
Drag And Lift ◽  
Horizontal Cylinders

An experimental study of the flow past a set of two horizontal cylinders is presented. The cylinders are towed in a uniformly accelerated and decelerated motion in a visualization tank in order to enhance the vortex effects. The main cylinder (D = 0.04 m; L/D = 16) is placed in the flow past a front one (d = 0.002m; L/D = 16). They are towed beneath the free surface and the drag and lift forces are measured. The main cylinder wake pattern is visualized by an embarked CCD camera. The Reynolds number based on the maximum velocity is from 0 to 14000 and the Froude number based on the main cylinder immersion from 0.2 to 1.2 for an acceleration value of 0.15m.s-2. It is shown that the near wake is made of a combination of the main cylinder Von Karman vortices and those of the front cylinder. The interference phenomenon and the free surface effects are studied by varying the depth parameter and the two cylinders arrangements.

scholarly journals Two-scale dynamics of flow past a partial cross-stream array of tidal turbines

2013 ◽  
Vol 730 ◽  
pp. 220-244 ◽  
Author(s):  
Takafumi Nishino ◽  
Richard H. J. Willden
Keyword(s):  
Analytical Model ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Wake Region ◽  
Near Wake ◽  
New Model ◽  
Tidal Turbines ◽  
Type Design ◽  
Flow Past ◽  
Far Wake

AbstractThe characteristics of flow past a partial cross-stream array of (idealized) tidal turbines are investigated both analytically and computationally to understand the mechanisms that determine the limiting performance of partial tidal fences. A two-scale analytical partial tidal fence model reported earlier is further extended by better accounting for the effect of array-scale flow expansion on device-scale dynamics, so that the new model is applicable to short fences (consisting of a small number of devices) as well as to long fences. The new model explains theoretically general trends of the limiting performance of partial tidal fences. The new model is then compared to three-dimensional Reynolds-averaged Navier–Stokes (RANS) computations of flow past an array of various numbers (up to 40) of actuator disks. On the whole, the analytical model agrees well with the RANS computations, suggesting that the two-scale dynamics described in the analytical model predominantly determines the fence performance in the RANS computations as well. The comparison also suggests that the limiting performance of short partial fences depends on how much of device far-wake mixing takes place within the array near-wake region. This factor, however, depends on the structures of the wake and therefore on the type/design of devices to be arrayed.

scholarly journals Effect of Corner Radius in Stabilizing the Low-Re Flow Past a Cylinder

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Wei Zhang ◽  
Ravi Samtaney
Keyword(s):  
Sensitivity Analysis ◽  
Stability Analysis ◽  
New Physics ◽  
Flow Past ◽  
Corner Radius ◽  
Flow Past A Cylinder ◽  
The Stability ◽  
Variation Trends ◽  
Far Wake ◽  
Good Agreement

We perform global linear stability analysis on low-Re flow past an isolated cylinder with rounded corners. The objective of the present work is to investigate the effect of cylinder geometry (corner radius) on the stability characteristics of the flow. Our investigation sheds light on new physics that the flow can be stabilized by partially rounding the cylinder in the critical and weakly supercritical flow regimes. The flow is first stabilized and then gradually destabilized as the cylinder varies from square to circular geometry. The sensitivity analysis reveals that the variation of stability is attributed to the different spatial variation trends of the backflow velocity in the near- and far-wake regions for various cylinder geometries. The results from the stability analysis are also verified with those of the direct simulations, and very good agreement is achieved.

Immediate and Near Wake Flow Patterns Behind Slotted Disks

AIAA Journal ◽  
10.2514/2.564 ◽  
1998 ◽  
Vol 36 (9) ◽  
pp. 1626-1634 ◽  
Author(s):  
Hiroshi Higuchi ◽  
Jinzhong Zhang ◽  
Shojiro Furuya ◽  
Brian K. Muzas
Keyword(s):  
Flow Patterns ◽  
Wake Flow ◽  
Near Wake

Immediate and near wake flow patterns behind slotted disks

AIAA Journal ◽  
1998 ◽  
Vol 36 ◽  
pp. 1626-1634
Author(s):  
Hiroshi Higuchi ◽  
Jinzhong Zhang ◽  
Shojiro Furuya ◽  
Brian K. Muzas
Keyword(s):  
Flow Patterns ◽  
Wake Flow ◽  
Near Wake
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