A visual study of turbulent spots

Perry, A. E.; Lim, T. T.; Teh, E. W.

doi:10.1017/s0022112081002966

Cited by 137 publications

(97 citation statements)

References 9 publications

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“…We note that the observations made by Carstensen, Sumer, and Fredsøe 1 were actually analogous to those made for steady boundary layers (e.g. Gad-el-Hak, Blackwelder, and Riley; 4 Perry, Lim, and Teh; 5 Chambers and Thomas; 6 Browand and Plocher; 7 Seifert and Wygnanski; 8 and Matsubara and Alfredsson 9 ).…”

Section: Introductionsupporting

confidence: 75%

“…The results of this study confirmed the experimental findings of Carstensen, Sumer, and Fredsøe. 1 In the numerical simulations the turbulent spots appeared at about the end of the accelerating phases at the Reynolds numbers investigated (Re = 3.0 × 10 5 and Re = 4.5 × 10 5 ) and caused spikes in the wall shear stress and the velocity fields. Previously, direct numerical simulations (see, for example, Akhavan, Kamm, and Shapiro; 11 Vittori and Verzicco; 12 and Costamagna, Vittori, and Blondeaux 13 ) have also successfully been utilised to capture transition to turbulence over hydraulic smooth beds, in the form of spikes in the wall shear stress signal (or in the near wall velocity signal) prior to the near wall flow reversal in oscillatory boundary-layer flows, in line with experimental observations (Hino, Sawamoto, and Takasu; 14 and Jensen, Sumer, and Fredsøe 15 ).…”

Section: Introductionmentioning

confidence: 92%

“…The turbulent spots, cf. Figure 1, form locally in time and space in each half-cycle of the motion for Re > 1.5 × 10 5 . At the critical Reynolds number (Re cr = 1.5 × 10 5 ) the turbulent spots first appear in the deceleration stage prior to the near-bed flow reversal.…”

Section: Introductionmentioning

confidence: 99%

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A note on turbulent spots over a rough bed in wave boundary layers

2012

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This study is a continuation of the investigation of turbulent spots in wave boundary layers over a smooth wall reported by Carstensen et al. [J. Fluid Mech. 646, 169–206 (2010)10.1017/S0022112009992825]. The present paper summarises the results of an experimental investigation of turbulent spots in wave boundary layers over a rough wall. Two kinds of experiments were made: flow visualization tests and velocity profile measurements. The experiments show that turbulent spots as observed in the smooth-wall wave boundary layers are present irrespective of wall roughness conditions. As in the smooth-wall case the turbulent spots over the rough wall appear locally in time and space and grow in size as time progresses.

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Section: Introductionsupporting

confidence: 75%

Section: Introductionmentioning

confidence: 92%

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A note on turbulent spots over a rough bed in wave boundary layers

2012

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“…Hairpin structures can be seen near the leading edge of the spot, reminiscent of the conceptual picture of Perry et al 23 Quasistreamwise vortices are evident at the lateral extremities of the spot ͑see Sec. V for an explanation of their origin͒.…”

Section: B Structure Of a Spotmentioning

confidence: 62%

Strong interaction of a turbulent spot with a shock-induced separation bubble

Krishnan

Sandham

2007

Physics of Fluids

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Direct numerical simulations have been conducted to study the passage of a turbulent spot through a shock-induced separation bubble. Localized blowing is used to trip the boundary layer well upstream of the shock impingement, leading to mature turbulent spots at impingement, with a length comparable to the length of the separation zone. Interactions are simulated at free stream Mach numbers of two and four, for isothermal ͑hot͒ wall boundary conditions. The core of the spot is seen to tunnel through the separation bubble, leading to a transient reattachment of the flow. Recovery times are long due to the influence of the calmed region behind the spot. The propagation speed of the trailing interface of the spot decreases during the interaction and a substantial increase in the lateral spreading of the spot was observed. A conceptual model based on the growth of the lateral shear layer near the wingtips of the spot is used to explain the change in lateral growth rate.

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“…In their model, an initial disturbance to a laminar boundary layer causes three-dimensional undulations in initially straight vortex bundles aligned in the spanwise direction. Fluctuating velocities induced by the wavy lines of vorticity cause the rapid growth of the turbulent spot in both the spanwise and streamwise directions, resulting in the patterns illustrated in Perry et al (1981). It is probable that there is a direct connection between the observed growth pattern of turbulent spots and the orientation observed in the turbulent boundary layer.…”

Section: Discussionmentioning

confidence: 96%

Three-dimensional structure of a low-Reynolds-number turbulent boundary layer

2004

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A low-Reynolds-number zero-pressure-gradient incompressible turbulent boundary layer was investigated using a volumetric imaging technique. The Reynolds number based on momentum thickness was 700. The flow was tagged with a passive scalar from two spanwise dye slots to distinguish between fluid motions originating in the inner and outer portions of the boundary layer. The resulting volumetric scalar field was interrogated using a laser sheet scanner developed for this study. Twoand three-dimensional time-dependent visualizations of a 50 volume time series are presented (equivalent to 17δ in length). In the outer portion of the boundary layer, scalar structures were observed to lie along lines in the (x, z)-plane, inclined to the streamwise (x-)direction in the range ±50 • . The ejection of brightly dyed fluid packets from the near-wall region was observed to be spatially organized, and related to the passage of the large-scale scalar structures. IntroductionHere, we present the short-term dynamics of a low-Reynolds-number turbulent boundary layer using three-dimensional visualizations of the scalar field. The interpretations are based on inspection of individual events in a data set comprising a 50 volume time series, equivalent to 17δ in length.It is a well-established observation that a turbulent boundary layer displays structure, in that 'coherent motions' or 'organized motions' are recognized to be an important component of the velocity and pressure fields. Such coherent structures were described by Robinson (1991) as 'a three-dimensional region of the flow over which at least one fundamental flow variable (velocity component, density, temperature, etc.) exhibits significant correlation with itself or with another flow variable over a range of space and/or time that is significantly larger than the smallest local scales of the flow.' Different features appear in the near-wall and outer-layer regions, and we have a broad understanding of their general features. In contrast, the fundamental nature of the motions, their distinguishing characteristics, and the nature of their interactions, are not at all settled. According to an early review by Cantwell (1981), the structure

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A visual study of turbulent spots

Cited by 137 publications

References 9 publications

A note on turbulent spots over a rough bed in wave boundary layers

A note on turbulent spots over a rough bed in wave boundary layers

Strong interaction of a turbulent spot with a shock-induced separation bubble

Three-dimensional structure of a low-Reynolds-number turbulent boundary layer

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