Do riblets exhibit fully rough behaviour?
Davide Gatti, Lars H. von Deyn, Pourya Forooghi, Bettina Frohnapfel
Abstract
Abstract Trapezoidal riblets are experimentally investigated in both their drag-reducing and drag-increasing regimes in an air turbulent channel flow facility. The nondimensional riblet spacing $$s^+$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>s</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> and height $$h^+$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>h</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> in viscous units are varied by changing the bulk Reynolds number in the range $$5 \times 10^3<{\mathrm{Re}}_b<8.5 \times 10^4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>5</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>3</mml:mn></mml:msup><mml:mo><</mml:mo><mml:msub><mml:mi>Re</mml:mi><mml:mi>b</mml:mi></mml:msub><mml:mo><</mml:mo><mml:mn>8.5</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>4</mml:mn></mml:msup></mml:mrow></mml:math> . The change in skin-friction drag is accurately measured by comparing the pressure drop across the test section with smooth and ribbed walls at given values of $${\mathrm{Re}}_\mathrm{b}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Re</mml:mi><mml:mi>b</mml:mi></mml:msub></mml:math> . The results show that riblets in the drag-increasing regime behave like typical “ k ”-type roughness in the fully rough regime only up to $$h^+ \sim 30$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>h</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>∼</mml:mo><mml:mn>30</mml:mn></mml:mrow></mml:math> , at odds with present understanding. Further increase in $$h^+$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>h</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> yields a constant relative change of wall shear stress and thus a clear deviation from the fully rough behaviour. The present results are accessible online at the following digital object identifier: 10.5445/IR/1000089978. Graphic abstract