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  • AIP Publishing  (20)
  • 1
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    Online Resource
    Control of the flow around a finite square cylinder with a flexible plate attached at the free end
    AIP Publishing ; 2022
    In:  Physics of Fluids Vol. 34, No. 2 ( 2022-02-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 34, No. 2 ( 2022-02-01)
    Abstract: A flexible plate vertically clamped at the free-end leading edge was used to modulate the aerodynamic forces on a wall-mounted finite square cylinder. The side width (d) of the cylinder was 40 mm and the aspect ratio (H/d) was 5. The flexible plate was made of low-density polyethylene, with a width of d and thickness of 0.04 mm. The length of the flexible plate ranged from d/8 to d. All measurements were carried out in a low-speed wind tunnel with the free-stream velocity (U∞) ranging from 4 to 20 m/s, corresponding to a Reynolds number ranging from 10 960 to 54 800. It was found that the flexible plate behaves distinctly depending on its length and has significant effects on the aerodynamic forces on the finite square cylinder. When U∞ is smaller than the critical velocity Ucr, which is closely related to the length of the plate, the plate statically deforms, having a negligible influence on the aerodynamic forces on the cylinder. When U∞ exceeds Ucr, the plate flaps periodically, resulting in a significant reduction in the aerodynamic forces. The maximum reduction in the mean drag, fluctuating drag, and fluctuating lateral force reaches approximately 5%, 25%, and 60%, respectively. The reduction in the aerodynamic forces is insensitive to both the plate length and flapping frequency. Flow visualization and particle image velocimetry results point out that the flapping plate induces large-scale vortices in the free-end shear flow, which suppress the formation of spanwise vortex shedding and make the upper part of the near wake symmetrical. The flapping configuration of the flexible plate and the corresponding pressure fluctuation on the free end were also addressed.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/5.0082181
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2022
    detail.hit.zdb_id: 1472743-2
    detail.hit.zdb_id: 241528-8
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  • 2
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    Transition of wake flows past two circular or square cylinders in tandem
    AIP Publishing ; 2021
    In:  Physics of Fluids Vol. 33, No. 8 ( 2021-08-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 33, No. 8 ( 2021-08-01)
    Abstract: This paper presents the dependence of flow transitions and flow regimes on Reynolds number Re (≤ 105) and scaled cylinder center-to-center spacing S* (≤ 10) for two tandem cylinders. Both circular and square cross-sectional geometries are considered, and a comparison of results is made between the two geometries. To do so, data available from the literature are collected, and supplementary numerical simulations are done to complement the literature data so as to provide clear-cut flow maps in the Re–S* domain. We identified the borders of Re- and S*-dependent flows, including steady, unsteady, transition, slender/extended body, alternating reattachment, bistable, and co-shedding flows. The flow interference between two tandem cylinders with S* & lt; S*cr is susceptible to stabilize the flow that requires a higher Re for the onset of vortex shedding than the isolated cylinder counterpart, where S*cr is the critical spacing for the onset of co-shedding flow. Besides, the flow interference generally postpones and advances the onset of secondary vortex shedding associated with the transition from two- to three-dimensional vortex shedding for S* & lt; S*cr and S* & gt; S*cr, respectively. Two flow maps are made for the two geometries, both yielding fundamental contributions toward a better understanding of the flow interference effect on various flow transitions. The flow maps determine the regions of a dearth of knowledge, which future studies may pay attention to. Finally, the drag inversion, hysteresis, and flow patterns in the proximity of the onset of vortex shedding for tandem square cylinders (1.5 ≤ S* ≤ 7) are discussed.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/5.0062978
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2021
    detail.hit.zdb_id: 1472743-2
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  • 3
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    Reynolds number effect on the wake of two staggered cylinders
    AIP Publishing ; 2009
    In:  Physics of Fluids Vol. 21, No. 12 ( 2009-12-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 21, No. 12 ( 2009-12-01)
    Abstract: This work aims to investigate, based on the measured/reported Strouhal number (St) and the flow structure, the Reynolds number (Re) effect on the wake of two identical cylinders with a diameter of d over P∗=P/d=1.2–4.0 and α=0°–90°, where P is the center-to-center spacing between the two cylinders and α is the angle of incident flow with respect to the line through the two cylinder centers. The Re range examined is from 1.5×103 to 2.0×104. Two hotwires were used to measure St simultaneously behind each of the two cylinders. The St-Re relationship is classified into four distinct types, i.e., types 1–4. Each is linked to distinct initial conditions, viz., interactions between the four shear layers around the cylinders. Type 1 occurs at small P∗, not exceeding 1.25. The two cylinders act like a single body, producing a single St across the wake throughout the range of Re examined. On the other hand, type 2 occurs at small α ( & lt;10°). Although single valued, the St in type 2 displays a sudden jump with increasing Re due to a switch in the shear layer, separated from the upstream cylinder, from overshooting to reattachment on the downstream cylinder (type 2A) or from reattachment to coshedding vortices (type 2B), depending on P∗. Type 3 is in the region of intermediate P∗ [(1.2–1.5)–2.2] and α (10°–75°). Two distinct St occur at low Re. The lower and the higher ranges of St are associated with the downstream and upstream cylinders, respectively. With increasing Re, the higher St collapses to the lower, which is attributed to a change in the inner shear layer, separated from the upstream cylinder, from squeezing through the gap between cylinders to reattachment on the downstream cylinder. Type 4 occurs at large P∗ ( & gt;1.2–2.2) and again exhibits at low Re two St, above and below the Strouhal number St0 in the wake of an isolated cylinder, both changing suddenly or progressively to St0 with increasing Re, which results from a change in the inner shear layer, separated from the upstream cylinder, from reattachment on the downstream cylinder to forming vortices between the cylinders. These types of Re-St relationships are also connected to the flow structure modes reported in literature. The dependence of the St and St-Re relationships on P∗ and α is provided, which may be used for the prediction of St in related problems.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/1.3275846
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2009
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  • 4
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    Drag reduction of circular cylinder using linear and sawtooth plasma actuators
    AIP Publishing ; 2021
    In:  Physics of Fluids Vol. 33, No. 12 ( 2021-12-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 33, No. 12 ( 2021-12-01)
    Abstract: The flow around a single circular cylinder modified by the Dielectric Barrier Discharge (DBD) plasma actuator is investigated experimentally at a Reynolds number Re = 5.25 × 103. The main purpose of the present study is to establish a comprehensive understanding of the effects of linear and sawtooth DBD plasma actuators (under steady and unsteady actuation modes, with different applied voltage Va, normalized modulation frequency F+, and duty cycle DC) on the time-averaged drag coefficient CD and flow characteristics of the cylinder. Extensive measurements are performed in the near wake of the cylinder using a load cell, hotwire anemometer, flow visualization, and particle image velocimetry techniques. The maximum drag reductions are 58% and 22.8% under the steady and unsteady actuation modes of the linear actuators, respectively, when a voltage of Va = 11 kV is applied. On the other hand, the maximum drag reductions achieved by the steady and unsteady actuation modes of the sawtooth actuators are 51.8% and 53.0%, respectively. The corresponding power efficiencies are 1.6%, 0.9%, 1.4%, and 2.1%, respectively. A detailed examination of the near wake dynamics unveils that the mechanisms behind the substantial drag reductions are quite different for the two different actuators, which account for the difference in drag reduction and efficiency.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/5.0077700
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2021
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  • 5
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    Near-wake structures of a finite square cylinder with a flapping film at its free end
    AIP Publishing ; 2023
    In:  Physics of Fluids Vol. 35, No. 9 ( 2023-09-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 35, No. 9 ( 2023-09-01)
    Abstract: As a follow-up study of Wang et al. [“Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)], this paper presents an experimental study of flow around a wall-mounted finite square cylinder with a vertically clamped flapping film at its free end. The width (d) of the square cylinder was 40 mm, and the aspect ratio (H/d) was 5, where the height H was 200 mm. The flexible film was made of low-density polyethylene, with a thickness of 0.04 mm and the width and length (l) each of d. Flow visualization and particle image velocimetry were conducted in the central lateral plane and several horizontal planes to reveal the 3D structure of the flapping induced vortex (FIV) and its effects on the cylinder near wake. All measurements were done in a low-speed wind tunnel at a flow speed of U∞ = 5 m/s with a Reynolds number of 13 700 based on U∞ and d. Previous study suggests that the flapping film reduces aerodynamic forces of the cylinder significantly and that the fluctuating lateral force is reduced by 60% [Wang et al., “Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)] . Vortices that shed from the trailing edge of the flapping film connect those from the side edges, forming n-shape FIVs downstream. FIVs induce more high-speed flow downwards into the wake, which suppresses the mean recirculation zone near the free end but enlarges it in the lower part of the wake. The two legs of n-shape FIVs are symmetrically arranged near the cylinder free end, whose effects diminish gradually as approaching the bottom wall, where alternating Karman vortex shedding still prevails.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/5.0153537
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2023
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  • 6
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    Dependence of square cylinder wake on Reynolds number
    AIP Publishing ; 2018
    In:  Physics of Fluids Vol. 30, No. 1 ( 2018-01-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 30, No. 1 ( 2018-01-01)
    Abstract: The wake of a square cylinder is investigated for Reynolds number Re & lt; 107. Two-dimensional (2D) laminar simulation and three-dimensional (3D) large-eddy simulation are conducted at Re ≤ 1.0 × 103, while experiments of hotwire, particle image velocimetry, and force measurements are carried out at a higher Re range of 1.0 × 103 & lt; Re & lt; 4.5 × 104. Furthermore, data covering a wide Re range, from 100 to 107, in the literature are comprehensively collected for discussion and comparison purposes. The dependence on Re of the recirculation bubble size or vortex formation length, wake width, shear-layer transition, time-mean drag force, and Strouhal number is discussed in detail, revealing five flow regimes, each having distinct variations of the above parameters. With increasing Re, while the streamwise recirculation size enlarges at Re & lt; 50 (steady flow regime), the vortex formation length reduces at 50 & lt; Re & lt; 1.6 × 102 (laminar flow regime), remains unchanged at 1.6 × 102 & lt; Re & lt; 2.2 × 102 (2D-to-3D transition flow regime), and decreases at 2.2 × 102 & lt; Re & lt; 1 × 103 (shear layer transition I regime), approaching asymptotically a constant at Re & gt; 1.0 × 103 (shear layer transition II regime). Meanwhile, the wake width decreases with Re in the laminar flow regime, grows in 2D-to-3D transition and shear layer transition I regimes, and levels off in the shear layer transition II regime. The narrowest wake width is identified in the 2D-to-3D transition flow regime, corresponding to a minimum time-mean drag force and a largest Strouhal number. With increasing Re, the shear-layer transition length rapidly declines in the shear layer transition I regime where the transition occurs downstream of the trailing corner of the cylinder. On the other hand, it slowly tapers off in the shear layer transition II regime where the transition takes place upstream of the trailing corner. An extensive comparison is made between the dependence on Re of a circular cylinder wake and a square cylinder wake, with their distinct natures highlighted.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/1.4996945
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2018
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  • 7
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    Numerical study of wake and aerodynamic forces on two tandem circular cylinders at Re = 103
    AIP Publishing ; 2019
    In:  Physics of Fluids Vol. 31, No. 4 ( 2019-04-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 31, No. 4 ( 2019-04-01)
    Abstract: Three-dimensional large eddy simulations were carried out to investigate the flow around two tandem circular cylinders at a subcritical Reynolds number of Re = 103. The cylinder center-to-center spacing ratio L/D is varied from 1.25 to 6, where D is the cylinder diameter. In order to enhance the understanding of flow physics around two circular cylinders, particular attention is devoted to fluctuating forces, shear-layer reattachment, flow separation, wake recirculation, Strouhal number (St), and phase lag (ϕ) between the fluctuating lift of the two cylinders. The flow structure around the cylinders is highly sensitive to L/D. A change in L/D thus leads to overshoot flow (L/D ≤ 1.25), reattachment flow (1.5 ≤ L/D ≤ 3.5), and coshedding flow (L/D ≥ 4). The boundaries are characterized by drastic changes in the flow structure and a discontinuous drop/rise in St and forces. The St drops at the boundary between overshoot and reattachment flow regimes and jumps at the boundary between reattachment and coshedding flow regimes, while fluctuating forces and ϕ both jump at both boundaries. The flow separation on the downstream cylinder is much delayed (122°–128°) in the reattachment flow regime compared to that on the single cylinder (95°) or upstream cylinder (92°–95.5°). The fluctuating pressure on the entire surface of either cylinder is low for the overshoot flow because the two cylinders are enclosed by the upstream-cylinder-generated shear layers having the longest wake recirculation. The ϕ is almost zero in the overshoot flow. With increasing L/D, ϕ linearly increases in the reattachment and coshedding regimes with different gradients, larger in the latter regime than in the former, by nearly twice.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/1.5087221
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
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  • 8
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    Control of flow and heat transfer over two inline square cylinders
    AIP Publishing ; 2019
    In:  Physics of Fluids Vol. 31, No. 12 ( 2019-12-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 31, No. 12 ( 2019-12-01)
    Abstract: Laminar flow around and heat transfer from two inline square cylinders under an active flow control (uniform blowing and suction) are numerically investigated at Reynolds numbers of 70–150, a Prandtl number of 0.71, and a cylinder-gap spacing (G) ratio of G/d = 1–5, where d is the cylinder side. A finite-volume code based on a collocated grid arrangement is employed in the two-dimensional numerical simulations. Uniform blowing and suction are applied to the upstream cylinder only (referred to as UFC) or applied to both cylinders (referred to as OFC). The purpose of using these two flow controls is to reduce time-mean and fluctuating forces and to suppress vortex shedding. The noncontrol case is referred to as the reference case where vortex shedding occurs from both cylinders for G/d ≥ 3 and from the downstream cylinder only for G/d & lt; 3. For UFC, vortex shedding from the upstream cylinder is suppressed for G/d = 1–5 examined. A drag reduction of more than 50% occurs for the upstream cylinder with G/d = 1–5, while the downstream cylinder has such a high drag reduction for G/d ≥ 3 only. In the case of OFC, vortex shedding from either cylinder is suppressed while the time-mean and fluctuating forces reduce for the entire G/d range. The maximum reduction in the total drag force (sum of both cylinders) is about 70%. The blowing hinders heat transfer from the cylinders while the suction enhances it.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/1.5128751
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
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  • 9
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    Flow structure around and heat transfer from cylinders modified from square to circular
    AIP Publishing ; 2019
    In:  Physics of Fluids Vol. 31, No. 8 ( 2019-08-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 31, No. 8 ( 2019-08-01)
    Abstract: This work aims at numerically investigating the influence of corner modification on the flow structure around and heat transfer from a square cylinder at a Reynolds number Re = 150 based on the cylinder width d and freestream velocity. The sharp corners of the square cylinder are rounded with r/d = 0 (square), 0.125, 0.25, 0.375, and 0.5 (circular), where r is the radius of the corner. The rounded corners have a profound effect on the flow structure from the perspective of flow separation, vortex strength, separation bubble, and wake bubble each playing a role in heat transfer from different surfaces of the cylinder. The boundary layer having a higher friction coefficient on the front and side surfaces leads to a higher local heat transfer. A shorter wake bubble renders a higher heat transfer from the rear surface. The increase in r/d from 0 to 0.5 leads to a 33% enhancement in the heat transfer from the cylinder. The enhancement largely results from a shrink in the wake bubble and an increase in vortex strength. The minimum time-mean drag and fluctuating forces are achieved at r/d = 0.25 and 0.125, respectively. The effect of r/d in various Reynolds averaged quantities is discussed.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/1.5109693
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
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  • 10
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    Flow map of foil undergoing combined fast and slow pitching
    AIP Publishing ; 2021
    In:  Physics of Fluids Vol. 33, No. 10 ( 2021-10-01)
    Details
    In: Physics of Fluids, AIP Publishing, Vol. 33, No. 10 ( 2021-10-01)
    Abstract: The drag-thrust transition and wake structures of a pitching foil undergoing combinations of fast and slow pitching are systematically investigated. The foil locomotion having combinations of fast and slow pitching is made by setting a variable s defined as the fraction of the pitching time required on the upper side of the wake centerline. On the other hand, time 1-s is required for the foil to pitch on the lower side of the wake centerline. Compared to the symmetric pitching (s = 0.5) case, the time-mean thrust rapidly increases and the drag-thrust boundary advances with increasing |s − 0.5|. The Kármán vortex street slants and produces thrust when |s − 0.5| is sufficiently large, which supersedes the previous thumb rule that only reverse Kármán vortex street can produce thrust. The faster forward stroke determines the slant direction of the vortex street. The detailed wake structures produced by the pitching foil are discussed, showing how the combined pitching affects vortex dynamics, drag-thrust transition, slant direction, and wake jet. This work provides a physical basis for understanding the hydrodynamics of native swimmers which may be useful to design bio-inspired underwater robots.
    Type of Medium: Online Resource
    ISSN: 1070-6631 , 1089-7666
    URL: Article
    DOI: 10.1063/5.0063992
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2021
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