Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

Several effects of nearly isotropic free-stream turbulence in transitionally rough turbulent boundary layers are studied using data obtained from laser Doppler anemometry measurements. The free-stream turbulence is generated with the use of an active grid, resulting in free-stream turbulence levels of up to 6.2 %. The rough surface is characterized by a roughness parameter k + ≈ 53, and measurements are performed at Reynolds numbers of up to Re θ = 11 300. It is confirmed that the free-stream turbulence significantly alters the mean velocity deficit profiles in the outer region of the boundary layer. Consequently, the previously observed ability of the Zagarola & Smits (J. Fluid Mech., vol. 373, 1998, p. 33) velocity scale U ∞ δ * /δ to collapse results from both smooth and rough surface boundary layers, no longer applies in this boundary layer subjected to high free-stream turbulence. In inner variables, the wake region is significantly reduced with increasing free-stream turbulence, leading to decreased mean velocity gradient and production of Reynolds stress components. The effects of free-stream turbulence are clearly identifiable and significant augmentation of the streamwise Reynolds stress profiles throughout the entire boundary layer are observed, all the way down to the inner region. In contrast, the Reynolds wall-normal and shear stress profiles increase due to free-stream turbulence only in the outer part of the boundary layer due to the blocking effect of the wall. As a consequence, there is a significant portion of the boundary layer in which the addition of nearly isotropic turbulence in the free-stream, results in significant increases in anisotropy of the turbulence. To quantify which turbulence length scales contribute to this trend, second-order structure functions are examined at various distances from the wall. Results show that the anisotropy created by adding nearly isotropic turbulence in the free-stream resides mostly in the larger scales of the flow. Furthermore, by analysing the streamwise Reynolds stress equation, it can be predicted that it is the wall-normal gradient of u 2 v term that is responsible for the increase in u 2 profiles throughout the boundary layer (i.e. an efficient turbulent transport of turbulence away from the wall). Furthermore, a noticeable difference between the triple correlations for smooth and rough surfaces exists in the inner region, but no significant differences are seen due to free-stream turbulence. In addition, the boundary layer parameters δ * /δ 95 , H and c f are also evaluated from the experimental data. The flow parameters δ * /δ 95 and H are found to increase due to roughness, but decrease due to free-stream turbulence, which has significance for flow control, particularly in delaying separation. Increases

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“…Radmsky & Thole 2002;Roberts & Yaras 2005). These studies were carried out for conditions replicating those found on turbine blades.…”

Section: Introductionmentioning

confidence: 99%

Effects of free-stream turbulence on rough surface turbulent boundary layers

et al. 2009

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“…The reference length L = 1220 mm is taken as the distance between the leading edge of the test plate and the outflow boundary of the computational domain. This length is consistent with the dimensions of the test surface in experimental studies of transition phenomena undertaken by the authors' research group at Carleton University (e.g., Yaras, 2001;Yaras, 2002;Roberts and Yaras, 2005a).…”

Section: Computational Domain and Boundary Conditionssupporting

confidence: 83%

“…A spatially-uniform, time-invariant inlet velocity and a static pressure that remains fixed in an area-averaged sense were specified for the inflow and outflow boundaries, respectively. The length of the test surface is consistent with the dimensions of the test surface in experimental studies of transition phenomena undertaken by the authors' research group at Carleton University (Yaras, 2001;Yaras, 2002;Roberts and Yaras, 2005a).…”

Section: Computational Domain and Boundary Conditionssupporting

confidence: 77%

“…Transitional separation bubbles are frequently observed on the suction surface of highly-loaded airfoils, deteriorating the performance of the airfoil through reduced lift and increased drag (Mayle, 1991;Hodson and Howell, 2005). Numerous studies have been conducted to understand the effects of flow Reynolds number, streamwise pressure gradient, surface roughness, periodic free-stream unsteadiness, and vortical and acoustical disturbances on separation-bubble transition (Roberts and Yaras, 2005a;Malkiel and Mayle, 1996;Yang and Voke, 2001;Volino and Bohl, 2004).…”

Section: Introductionmentioning

confidence: 99%

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Numerical Investigations of Instability and Transition in Attached and Separated Shear Layers

Brinkerhoff¹

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This integrated thesis documents a series of five complementary numerical investigations aimed at understanding the flow instability and laminar-to-turbulent transition process in attached and separated shear layers. Direct numerical simulation was used in these studies to accurately resolve the spatial and temporal scales of the simulated flows. The first two investigations simulate the flow in an idealized computational domain that approximates the conditions on the suction surface of a low-pressure turbine (LPT) blade. The first study investigates the interaction of the Tollmien-Schlichting instability in the attached-flow region with the Kelvin-Helmholtz instability in the separated-flow region. The interaction produces packets of coherent vortices that facilitate transition to a fully-turbulent boundary layer.The effect of swept-blade conditions on the transition process is investigated in the second numerical study by sweeping the simulation domain by 45 • . The resulting three-dimensional pressure field creates the potential for a crossflow-induced instability mode, but its effect on the transition process is minimal and transition follows the same mechanisms observed in the unswept configuration.The characteristics of the coherent vortices that facilitate transition to turbulence in the first study are further investigated through a series of fundamental studies of an isolated turbulent spot that is triggered by a transverse jet. The first of these fundamental studies demonstrates the sensitivity of the formation and structure of the coherent vortices to the level of free-stream acceleration. A subsequent fundamental study identifies the regeneration process by which the turbulent spot grows laterally through the formation of packets of hairpin vortices along the spanwise edges of the spot and longitudinally by the increase in the spatial scale of the hairpin vortices. The final fundamental study investigates the effects of elevated free-stream turbulence levels and LPT-realistic pressure distributions on the transition process. Under elevated turbulence conditions, bypass transition occurs as streamwise streaks i form in the laminar boundary layer and roll-up into hairpin-shaped vortices via a Kelvin-Helmholtz instability mode that is accelerated by the free-stream turbulence. The wave-packet regeneration model identified under low free-stream turbulence is promoted by elevated freestream turbulence, increasing the growth-rate of turbulent spots under elevated turbulence conditions.ii

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“…30,31 Examples for the application of specifically adapted surfaces structures can also be found in nature. Sharks possess riblet structures on their skin which are assumed to damp the spanwise and the wall-normal velocity fluctuations and, thus, the momentum transport, reducing the turbulent drag.…”

Section: Transition Control (Velvet)mentioning

confidence: 99%

Owl Inspired Silent Flight

Bachmann¹,

Winzen²

2014

Handbook of Biomimetics and Bioinspiration

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Urbanization and the constant upgrade of airports and wind power plants have led to the need of innovative wing-designs and applications to reduce noise of aircraft and rotors. How efficiently noise can be reduced during flight is shown by owls. Owls evolved unique wing geometries, surface-and edge-features to reduce flight noise to a minimum. Wings of owls are huge in comparison to the body mass. This in combination with a specific profiling leads to a wing geometry that produces much lift even at low flight-speeds. A velvety surface texture reduces friction noise of feathers and influences the boundary layer of the wing to delay separation. Serrations at the leading edge and fringes at the trailing edge of each remex influence both lift control and noise production by reducing turbulent eddies. Further, fringes merge neighboring feathers at the spread wing to create a smooth surface. Finally, the dense and porous plumage which covers the whole body acts as an acoustic absorber that dampens noise at the point of production. The complete knowledge of the physical mechanisms underlying silent flight of owls could lead to a future wing design and to applications that reduce noise pollution for humans and nature.

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Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

Cited by 62 publications

References 35 publications

Effects of free-stream turbulence on rough surface turbulent boundary layers

Effects of free-stream turbulence on rough surface turbulent boundary layers

Numerical Investigations of Instability and Transition in Attached and Separated Shear Layers

Owl Inspired Silent Flight

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