Direct Numerical Simulations of Trailing-Edge Noise Generated by Turbulent Boundary-Layers

AbstractThe role of radiative energy transfer in turbulent boundary layers is carefully analysed, focusing on the effect on temperature fluctuations and turbulent heat flux. The study is based on direct numerical simulations (DNS) of channel flows with hot and cold walls coupled to a Monte-Carlo method to compute the field of radiative power. In the conditions studied, the structure of the boundary layers is strongly modified by radiation. Temperature fluctuations and turbulent heat flux are reduced, and new radiative terms appear in their respective balance equations. It is shown that they counteract turbulence production terms. These effects are analysed under different conditions of Reynolds number and wall temperature. It is shown that collapsing of wall-scaled profiles is not efficient when radiation is considered. This drawback is corrected by the introduction of a radiation-based scaling. Finally, the significant impact of radiation on turbulent heat transfer is studied in terms of the turbulent Prandtl number. A model for this quantity, based on the new proposed scaling, is developed and validated.

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The influence of harmonic wall motion on transitional boundary layers

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.591 ◽

2014 ◽

Vol 760 ◽

pp. 63-94 ◽

Cited By ~ 13

Author(s):

M. J. Philipp Hack ◽

Tamer A. Zaki

Keyword(s):

Numerical Simulations ◽

Boundary Layers ◽

Wall Motion ◽

Direct Numerical Simulations ◽

Bypass Transition ◽

Power Requirement ◽

Optimal Values ◽

Wall Oscillation ◽

Transition Delay ◽

Secondary Instabilities

AbstractThe influence of harmonic spanwise wall motion on bypass transition in boundary layers is investigated using direct numerical simulations. It is shown that the appropriate choice of the forcing parameters can achieve a substantial stabilization of the laminar flow regime. However, an increase of the forcing amplitude or period beyond their optimal values diminishes the stabilizing effect, and leads to breakdown upstream of the unforced case. For the optimal wall-oscillation parameters, the reduction in propulsion power substantially outweighs the power requirement of the forcing. The mechanism of transition delay is examined in detail. Analysis of the pre-transitional streaks shows that the wall oscillation substantially reduces their average amplitude, and eliminates the most energetic streaks. As a result, the secondary instabilities that precede breakdown to turbulence are substantially weakened – an effect demonstrated by linear stability analyses of flow fields from direct numerical simulations. The outcome is transition delay owing to a significant reduction in the frequency of occurrence of turbulent spots and a downstream shift in their average inception location. Finally, it is shown that the efficiency of the forcing can be further improved by replacing the sinusoidal time dependence of the wall oscillation with a square wave.

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