Roll-modes generated in turbulent boundary layers with passive surface modifications

Abstract

A unique class of directional surfaces arranged in a converging-diverging (herringbone) pattern are studied experimentally in a zero pressure gradient turbulent boundary layers. Hot-wire measurements using both single and cross-wire show that these small surfaces are able to generate large-scale counter rotating roll-modes/vortices within the turbulent boundary layer, resulting in dramatic spanwise variation in the boundary layer thickness δ (50% variation for the strongest case). The results reveal that above the converging region, the local mean velocity decreases while the turbulence intensity increases, resulting in locally thicker boundary layer. Over the diverging region, the opposite situation occurs, where the mean velocity increases and the turbulence intensity decreases, resulting in a locally thinner boundary layer. The strong perturbation effect from these surfaces to the overall flow dynamics seems unusual, considering that their peak-to-trough height is approximately only 1% of the boundary layer thickness. This study, also investigates the behavior of the large-scale counter-rotating roll-modes when the surface reverts from the herringbone pattern back to the smooth wall, to see how far they persist over the smooth wall. Our preliminary results show that the roll-modes above the smooth wall still persist even at 40δ downstream. The results of this study show that the herringbone surface roughness pattern can act as a novel method of generating counter rotating roll-modes (vortices) for flow control purposes in various engineering applications. Nomenclaturex Streamwise direction/distance, m y Spanwise direction/distance, m z Wall-normal direction/distance, m U Mean streamwise velocity, m/s V Mean spanwise velocity, m/s W Mean wall-normal velocity, m/s U ∞ Freestream velocity, m/s U τ Skin friction velocity, m/s U sa Spanwise averaged mean velocity, m/s δ Boundary layer thickness, m δ s Boundary layer thickness over smooth wall, m δ r Boundary layer thickness over rough surface, m δ sa Boundary layer thickness averaged over one spanwise wavelength, m δ rs Boundary layer thickness averaged over one spanwise wavelength of rough to smooth case, m Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 30, 2015 | http://arc.aiaa.org |

Vortical structures and development of laminar flow over convergent-divergent riblets

“…Inspired by this finding, a series of experimental studies on convergent-divergent riblets has been carried out in a flat plate turbulent boundary layer at the University of Melbourne. Similar results to those reported by Koeltzsch et al 21 were also obtained by Nugroho et al 23 Using a cross-wire, Nugroho et al 24 revealed the presence of a large-scale counter-rotating roll mode in the time-averaged flow field in cross-stream planes. Their finding was also confirmed and extended by Kevin et al 25 with their stereoscopic PIV measurements.…”

“…This observation is in agreement with the finding from previous experiments. [23][24][25] A comparison of the control effects between convergent-divergent riblets and synthetic jet is provided. In the active flow control experiments with synthetic jets, the upwash and downwash of fluid in the near-wall region are induced by the rotations of hairpin legs, 36,37 with an upwash leading to a decreased local skin friction and a downwash resulting in an increased one.…”

Numerical and experimental study of mechanisms responsible for turbulent secondary flows in boundary layer flows over spanwise heterogeneous roughness

“…(They also showed that streamwise velocity fluctuations were elevated within the analogous LMP, which is consistent with the findings of Mejia-Alvarez & and Barros & Christensen (2014), though for a profoundly different topography.) In fact, another very recent study by Nugroho et al (2014) showed that the spanwise heterogeneities exhibited counter-rotating mean flow vortices with the same rotational sense as observed by Barros & Christensen (2014). Such patterns are consistent also with those reported by Reynolds et al (2007) for flow over regular arrays of cubes wherein mean flow heterogeneity occurred at integer multiples of the periodic spanwise roughness spacing and for which enhanced turbulence intensity was noted coincident with regions of reduced streamwise momentum (LMPs).…”

“…Recent experimental and numerical work (Mejia-Alvarez & Christensen 2010, 2013Nugroho et al 2013Nugroho et al , 2014Willingham et al 2013;Barros & Christensen 2014) has shown that spanwise heterogeneous, streamwise-elongated roughness has a dramatic influence on dynamics of the turbulent boundary layer. Specifically, the presence of a secondary mean flow in the form of counter-rotating roll cells has been reported.…”

“…Some recent studies have shown that δ-scale mean flow heterogeneities exist in the spanwise-wall normal plane of rough-wall turbulent boundary layer flows (Reynolds et al 2007;Nugroho, Hutchins & Monty 2013;Willingham et al 2013; Barros & Christensen 2014;Nugroho et al 2014). For flow over a multiscale complex roughness (a gas turbine blade damaged by the accumulation of fuel deposits (Bons et al 2001)), which exhibited large-scale FIGURE 1.…”