Boundary-Layer Transition

Tani, Itirô

doi:10.1146/annurev.fl.01.010169.001125

Cited by 202 publications

(87 citation statements)

References 15 publications

(20 reference statements)

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Correlation methods can predict the transition induced in the wake of roughness elements (Tani 1969;Reshotko 2008). The prediction is based on roughness-based Reynolds number, Re k , and the ratio of the roughness height (k) to either boundary layer length scale or displacement thickness.…”

mentioning

confidence: 99%

Turbulence in a transient channel flow with a wall of pyramid roughness

Seddighi

Pokrajac

et al. 2015

J. Fluid Mech.

View full text Add to dashboard Cite

A direct numerical simulation investigation of a transient flow in a channel with a smooth top wall and a roughened bottom wall made of close-packed pyramids is presented. An initially stationary turbulent flow is accelerated rapidly to a new flow rate and the transient flow behaviour after the acceleration is studied. The equivalent roughness heights of the initial and final flows are k + s = 14.5 and 41.5, respectively. Immediately after the acceleration ends, the induced change behaves in a 'plug-flow' manner. Above the roughness crests, the additional velocity due to the perturbation flow is uniform; below the crest, it reduces approximately linearly to zero at the bottom of the roughness elements. The interaction of the perturbation flow with the rough wall is characterised by a series of events that resemble those observed in roughness-induced laminar-turbulent transitions. The process has two broad stages. In the first of these, large-scale vortices, comparable in extent to the roughness wavelength, develop around each roughness element and high-speed streaks form along the ridge lines of the elements. After a short time, each vortex splits into two, namely (i) a standing vortex in front of the element and (ii) a counter-rotating hairpin vortex behind it. The former is largely inactive, but the latter advects downstream with increasing strength, and later lifts away from the wall. These hairpin vortices wrap around strong low-speed streaks. The second stage of the overall process is the breakdown of the hairpin vortices into many smaller multi-scale vortices distributed randomly in space, leading eventually to a state of conventional turbulence. Shortly after the beginning of the first stage, the three components of the r.m.s of the velocity fluctuation all increase significantly in the near-wall region as a result of the vortical structures, and their spectra bear strong signatures of the surface topology. During the second stage, the overall turbulence energy in this region varies only slightly, but the spectrum evolves significantly, eventually approaching that of conventional turbulence. The direct effect of roughness on the flow is confined to a region up to approximately three element heights above the roughness crests. Turbulence in the core region does not begin to increase until after the transition near the wall is largely complete. The processes of transition over the smooth and rough walls of † Email addresses for correspondence: m.seddighi@ljmu.ac.uk, s.he@sheffield.ac.uk Turbulence in a transient channel flow with a rough wall 227 the channel are practically independent of each other. The flow over the smooth wall follows a laminar-turbulent transition and, as known from previous work, resembles a free-stream turbulence-induced boundary layer bypass transition.

show abstract

mentioning

confidence: 99%

Turbulence in a transient channel flow with a wall of pyramid roughness

Seddighi

Pokrajac

et al. 2015

J. Fluid Mech.

View full text Add to dashboard Cite

show abstract

“…Isolated 3D roughness such as insect roughness shows "more critical" behavior, with the transition front moving forward rapidly once a critical roughness Reynolds number is reached. Tani found that the critical roughness Reynolds number on isolated, cylindrical roughness is proportional to (k/d) 2/5 , where k is the roughness height and d is the roughness diameter [30]. Typical proportionality constants for this relationship lie between 600…”

Section: Ic Historical Airfoil Designmentioning

confidence: 99%

Roughness Sensitivity Comparisons of Wind Turbine Blade Sections

Wilcox

2017

View full text Add to dashboard Cite

One explanation for wind turbine power degradation is insect roughness. Historical studies on insect-induced power degradation have used simulation methods which are either unrepresentative of actual insect roughness or too costly or time-consuming to be applied to wide-scale testing. Furthermore, the role of airfoil geometry in determining the relations between insect impingement locations and roughness sensitivity has not been studied.To link the effects of airfoil geometry, insect impingement locations, and roughness sensitivity, a simulation code was written to determine representative insect collection patterns for different airfoil shapes. Insect collection pattern data was then used to simulate roughness on an NREL S814 airfoil that was tested in a wind tunnel at Reynolds numbers between 1.6 × 10 6 and 4.0 × 10 6 . Results are compared to previous tests of a NACA 63 3 -418 airfoil.Increasing roughness height and density results in decreased maximum lift, lift curve slope, and lift-to-drag ratio. Increasing roughness height, density, or Reynolds number results in earlier bypass transition, with critical roughness Reynolds numbers lying within the historical range. Increased roughness sensitivity on the 25% thick NREL S814 is observed compared to the 18% thick NACA 63 3 -418.Blade-element-momentum analysis was used to calculate annual energy production losses of 4.9% and 6.8% for a NACA 63 3 -418 turbine and an NREL S814 turbine, respectively, operating with 200 µm roughness. These compare well to historical field measurements. iii ACKNOWLEDGEMENTS

show abstract

“…Relatively well-established techniques include streamlining, which can reduce the adverse pressure gradient encountered or move back the position of minimum pressure, and suction (Atik et al 2005) to remove slower, near-wall particles and entrain faster ones; while the use of wall heat transfer (Chang 1970) and rotating cylinders (Modi 1997) has also been studied. Tripping the boundary layer to go turbulent through the use of roughness elements (whether active (Tani 1969) or passive (Pruessner & Smith 2015)) also remains popular when avoiding separation. Other ideas, for example from nature (Bushnell & Moore 1991;Fish & Lauder 2006), may also inspire new and novel techniques.…”

Section: Introductionmentioning

confidence: 99%

The impact of static and dynamic roughness elements on flow separation

2017

View full text Add to dashboard Cite

The use of static or dynamic roughness elements has been shown in the past to delay the separation of a laminar boundary layer from a solid surface. Here, we examine analytically the effect of such elements on the local and breakaway separation points, corresponding respectively to the position of zero skin friction and presence of a singularity in the roughness region, for flow over a hump embedded within the boundary layer. Two types of roughness elements are studied: the first is small and placed near the point of vanishing skin friction; the second is larger and extends downstream. The forced flow solution is found as a sum of Fourier modes, reflecting the fixed frequency forcing of the dynamic roughness. Solutions for both the static and dynamic roughness show that the presence of the roughness element is able to move the separation points downstream, given an appropriate choice of roughness frequency, height, position and width. This choice is found to be qualitatively similar to that observed for leading-edge separation. Furthermore, for a negative static roughness a small region of separated flow forms at high roughness depth, although there is a critical depth above which boundary-layer breakaway moves suddenly upstream.

show abstract

Boundary-Layer Transition

Cited by 202 publications

References 15 publications

Turbulence in a transient channel flow with a wall of pyramid roughness

Turbulence in a transient channel flow with a wall of pyramid roughness

Roughness Sensitivity Comparisons of Wind Turbine Blade Sections

The impact of static and dynamic roughness elements on flow separation

Contact Info

Product

Resources

About