Structure of turbulent wedges created by isolated surface roughness

Experimental Investigation of Isolated Roughness Induced Transition in a Swept Wing Boundary Layer

“…Surface roughness is a parameter that plays a fundamental role in the laminar-to-turbulent transition process of twoor three-dimensional boundary layers (BL). Specifically, previous investigations [1][2][3][4][5] observed that a sufficiently large isolated roughness element applied on the surface of a wing, leads to transition via the formation and spreading of a turbulent wedge. Numerous works [e.g.…”

“…The downstream development of these structures results in a flow configuration dominated by the alternation of low-and high-speed streaks, which destabilize the edges of the turbulent flow region, enhancing its spanwise spreading [e.g. 5,9]. The spectral analysis of the collected hot wire data identifies an oscillatory behaviour of the horseshoe legs [3], dominated by frequency peaks that correspond to those of the roughness cylinder shedding.…”

Remote surface damage detection on rotor blades of operating wind turbines by means of infrared thermography

“…Emerging damages range from initial surface defects to structural threats. In avoidance of rotor blade breakdown, a variety of re-mote damage detection methods have been developed, some of which are widely used in practice (Ciang et al, 2008;Márquez et al, 2012;Li et al, 2015). Most methods are based on measuring a global effect by local sensors (e.g., acoustic emission), thus lacking in detectability of small damages and their exact localization.…”

“…In general, single surface roughness elements generating so-called turbulent wedges in the BL were the subject of several basic studies. Thermographic flow visualization based on chemical surface treatment for the purpose of defect identification lacks in practicability or spatial resolution (Zhong et al, 2003;Kuester and White, 2016). Other studies investigated roughness elements that significantly exceed typical dimensions encountered on rotor blades (Rudolph et al, 2009).…”

“…In the field of wind turbine maintenance, IRT is usually not considered a suitable inspection method on site up to now. According to some studies, the application of IRT requires thermal excitation to enhance thermal contrast, which is very laborious (Ciang et al, 2008;Márquez et al, 2012;Li et al, 2015). However, the latest development of infrared cameras yielded compact cameras with a thermal resolution of better than 15 mK (Carlomagno and Cardone, 2010;Rogalski, 2011).…”