Transient Growth and Transition Induced by Random Distributed Roughness

Downs, Robert S.; White, Edward B.; Denissen, Nicholas

doi:10.2514/1.31696

Cited by 73 publications

(38 citation statements)

References 23 publications

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“…The largest-amplitude roughness modes (post-windowing) are shown in table 2. These wavelengths are provided not to suggest that receptivity for each wavelength is linear; both experiments (White et al 2005;Downs et al 2008) and computations (Tumin & Reshotko 2005;Kurz & Kloker 2014) have shown nonlinear receptivity to surface roughness, even for roughness with a zero spanwise mean. Instead, these wavelengths are provided in order to fully define the roughness surface and relate the predominant roughness wavelengths to the boundary layer thickness.…”

Section: Roughness Designmentioning

confidence: 99%

“…Transition occurred through the breakdown of these T-S waves rather than an instability initiated by the roughness. Downs, White & Denissen (2008) used rapid prototyping to create patches of 'random' roughness with a known streamwise and spanwise wavelength distribution. Unlike previous experiments with sand-grain roughness, the rapid-prototyped roughness had a zero mean value.…”

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confidence: 99%

“…Direct numerical simulation of the Downs et al (2008) experiment by Drews et al (2011) and Drews (2012) examined the flow downstream of distributed roughness patches. The DNS and experiment were in good agreement.…”

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confidence: 99%

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Roughness receptivity and shielding in a flat plate boundary layer

Kuester

White

2015

J. Fluid Mech.

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Surface roughness can affect boundary layer transition by acting as a receptivity mechanism for transient growth. While experiments have investigated transient growth of steady disturbances generated by discrete roughness elements, very few have studied distributed surface roughness. Some work predicts a 'shielding' effect, where smaller distributed roughness displaces the boundary layer away from the wall and lessens the impact of larger roughness peaks. This work describes an experiment specifically designed to study this effect. Three roughness configurations (a deterministic distributed roughness patch, a slanted rectangular prism, and the combination of the two) were manufactured using rapid prototyping and installed flush with the wall of a flat plate boundary layer. Naphthalene flow visualization and hotwire anemometry were used to characterize the boundary layer in the wakes of the different roughness configurations. Distributed roughness with roughness Reynolds numbers (Re kk ) between 113 and 182 initiated small-amplitude disturbances that underwent transient growth. The discrete roughness element created a pair of high-and low-speed steady streaks in the boundary layer at a sub-critical Reynolds number (Re kk = 151). At a higher Reynolds number (Re kk = 220), the discrete element created a turbulent wedge 15 boundary layer thicknesses downstream. When the distributed roughness was added around the discrete roughness, the discrete element's wake amplitude was decreased. For the higher Reynolds number, this provided a small but measurable transition delay. The distributed roughness redirects energy from longer spanwise wavelength modes to shorter spanwise wavelength modes. The presence of the distributed roughness also decreased the growth rate of secondary instabilities in the roughness wake. This work demonstrates that shielding can delay roughness-induced transition and lays the ground work for future studies of roughness-induced transition.

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Section: Roughness Designmentioning

confidence: 99%

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confidence: 99%

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Roughness receptivity and shielding in a flat plate boundary layer

Kuester

White

2015

J. Fluid Mech.

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“…In the few studies that have been carried out, the majority have used a spanwise array of roughness elements and the relative height of the roughness has been large. Recently, the experiments of Downs, White & Denissen (2008) and Kuester & White (2013) and the DNS simulations of Drews et al (2011) and Drews (2012) were carried out to study roughness-induced transition due to patches of random elements in a Blasius boundary layer. The results showed the occurrence of transient growth in the roughness-induced disturbances, as observed previously for cases of isolated roughness.…”

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

Seddighi

Pokrajac

et al. 2015

J. Fluid Mech.

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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.

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“…Transition relationships for distributed roughness are less well-defined due to difficulties in defining a "typical" surface. Experiments by Downs suggest that the mechanism for supercritical distributed roughness is similar to bypass observed for isolated roughness [31]. Downs was not able to fully analyze the transition mechanism for subcritical distributed roughness but suggests that it may be associated with TS wave growth.…”

Section: Ic Historical Airfoil Designmentioning

confidence: 97%

Roughness Sensitivity Comparisons of Wind Turbine Blade Sections

Wilcox

2017

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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

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Transient Growth and Transition Induced by Random Distributed Roughness

Cited by 73 publications

References 23 publications

Roughness receptivity and shielding in a flat plate boundary layer

Roughness receptivity and shielding in a flat plate boundary layer

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

Roughness Sensitivity Comparisons of Wind Turbine Blade Sections

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