The effects of tunnel blockage and aspect ratio on the mean flow past a circular cylinder with Reynolds numbers between 10 4 and 10 5

West, G. S.; Apelt, C. J.

doi:10.1017/s0022112082000202

Cited by 364 publications

(149 citation statements)

References 3 publications

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“…The total geometric blockage of the two cylinders was 4.2%, less than the level ͑6%͒ that may affect the flow appreciably. 17 The length-todiameter ratio of the cylinder was 48. Figure 1͑a͒ shows the arrangement of the cylinders and the definitions of P and ␣.…”

Section: Methodsmentioning

confidence: 99%

Reynolds number effect on the wake of two staggered cylinders

et al. 2009

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This work aims to investigate, based on the measured/reported Strouhal number ͑St͒ and the flow structure, the Reynolds number ͑Re͒ effect on the wake of two identical cylinders with a diameter of d over P ‫ء‬ = P / d = 1.2-4.0 and ␣ = 0°-90°, where P is the center-to-center spacing between the two cylinders and ␣ is the angle of incident flow with respect to the line through the two cylinder centers. The Re range examined is from 1.5ϫ 10 3 to 2.0ϫ 10 4 . 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 ‫ء‬ , 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 ␣ ͑Ͻ10°͒. 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 2A͒ or from reattachment to coshedding vortices ͑type 2B͒, depending on P ‫ء‬ . Type 3 is in the region of intermediate P ‫ء‬ ͓͑1.2-1.5͒-2.2͔ and ␣ ͑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 ‫ء‬ ͑Ͼ1.2-2.2͒ and again exhibits at low Re two St, above and below the Strouhal number St 0 in the wake of an isolated cylinder, both changing suddenly or progressively to St 0 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 ‫ء‬ and ␣ is provided, which may be used for the prediction of St in related problems.

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Section: Methodsmentioning

confidence: 99%

Reynolds number effect on the wake of two staggered cylinders

et al. 2009

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“…Since the forces are measured for the full length of the cylinder and the flow velocity near the tunnel wall is smaller than in the free stream, the present measurement of C D is 0.9 %, 2.7 %, 3.6 % and 7.2 % below that measured by Nebres & Battill (1993), Ting et al (1998), Khalak & Williamson (1996) Gartshore (1987), respectively. Based on their measurements, West & Apelt (1982) suggested that the blockage had virtually no effect on C D if it was less than 6 % and could have a very small effect if it was between 6 % and 7.5 %. Therefore, the present blockage (7.5 %) is expected to have a negligible effect on C D .…”

Section: Fluid Forcesmentioning

confidence: 99%

Turbulent wake of an inclined cylinder with water running

Alam

Zhou

2007

J. Fluid Mech.

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This paper presents the results of an experimental study of fluid dynamics around an inclined circular cylinder with and without water running over its surface, covering water rivulet formation, fluid forces on the cylinder, near wake and their interrelationships. The cylinder inclination angle (α) with respect to incident flow was between 55 • and 80 • . It has been found that water running over the cylinder surface may behave quite differently, depending on the Reynolds number (Re), and subsequently impact greatly upon the fluid dynamics around the cylinder. As such, five flow categories are classified. Category A: one water rivulet was observed, irrespective of α, at the leading stagnation point at a small Re. Category B: the rivulet splits into two, symmetrically arranged about the leading stagnation line, once Re exceeds a critical value that depends on α. The two rivulets may further switch back to one, and vice versa. Category C: two symmetrical straight rivulets occur. Category D: the two rivulets shift towards the flow separation line with increasing Re and oscillate circumferentially. The oscillation reaches significant amplitude when the rivulets occur at about 70 • from the leading stagnation point. This increased amplitude is coupled with a rapid climb in the mean and fluctuating drag and lift, by a factor of near 5 for the fluctuating lift at α = 80 • . Meanwhile, the flow structure exhibits a marked variation. For example, Strouhal number and vortex formation length decrease, along with an increase in spanwise vorticity concentration, velocity deficit, and coherence between vortex shedding and fluctuating lift. All these observations point to the occurrence of a 'lock-in' phenomenon, i.e. the rivulet oscillation synchronizing with flow separation. A rivulet-vortex-induced instability is proposed to be responsible for the well reported rain-wind-induced vibration associated with the stay cables of cablestayed bridges. Category E: the two rivulets shift further downstream just beyond the separation line; the shear layers behind the rivulets become highly turbulent, resulting in weakened vortex shedding, flow fluctuating velocities and fluctuating fluid forces. Based on the equilibrium of water rivulet weight, aerodynamic pressure and friction force between fluid and surface, an analysis is developed to predict the rivulet position on the cylinder, which agrees well with measurements.

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“…All the measurements have been taken at the same value of free stream velocity equal to The noise level of the rms values of velocity has been evaluated according to the procedure provided in Romano et al [13] to be equal to 0.5 %. table 1) compared to the usually reported values (Okajima [14] for instance), however this can be linked to the blockage effect reported by West and Apelt [15]. A simple blockage ratio correction applied to the velocity value Table 1: Parameters of the shedding of the medium and the small bar (see figure 1a), AR stands for aspect ratio of a bar cross section and δ for bar thickness small bar medium bar…”

Section: Methodsmentioning

confidence: 90%

The triple decomposition of a fluctuating velocity field in a multiscale flow

2015

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A new method for the triple decomposition of a multiscale flow, which is based on the novel optimal mode decomposition (OMD) technique, is presented. OMD provides low order linear dynamics, that fits a given data set in an optimal way and is used to distinguish between a coherent (periodic) part of a flow and a stochastic fluctuation. The method needs no external phase indication since this information, separate for coherent structures associated with each length scale introduced into the flow, appears as the output. The proposed technique is compared against two traditional methods of the triple decomposition, i.e. bin averaging and proper orthogonal decomposition (POD). This is done with particle image velocimetry (PIV) data documenting the near wake of a multiscale bar array. It is shown that both traditional methods are unable to provide a reliable estimation for the coherent fluctuation while the proposed technique performs very well. The crucial result is that the coherence peaks are not observed within the spectral properties of the stochastic fluctuation derived with the proposed method however these properties remain unaltered at the residual frequencies. This proves the method's capability 1 of making a distinction between both types of fluctuations. The sensitivity to some prescribed parameters is checked revealing the technique's robustness. Additionally, an example of the method application for analysis of a multiscale flow is given, i.e. the phase conditioned transverse integral length is investigated in the near wake region of the multiscale object array.

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The effects of tunnel blockage and aspect ratio on the mean flow past a circular cylinder with Reynolds numbers between 10 4 and 10 5

Cited by 364 publications

References 3 publications

Reynolds number effect on the wake of two staggered cylinders

Reynolds number effect on the wake of two staggered cylinders

Turbulent wake of an inclined cylinder with water running

The triple decomposition of a fluctuating velocity field in a multiscale flow

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