Physical analysis of tonal component in flap side-edge noise generation: on the role of cross-flow wake

This study aims to numerically investigate the flow dynamics at the flap side edge of a swept high-lift wing and its relation to the far-field noise thanks to an original scale-resolving approach coupling lattice Boltzmann method with Zonal Detached Eddy Simulation. The reason for this investigation stems from previous experimental work where it is shown that the flap noise radiated by the present configuration exhibits a quasi-tonal component of unknown origin around 6.6 kHz (Strouhal number upper S t Subscript c Sub Subscript italic flap Baseline equals 8 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi>S</mml:mi> <mml:mi>t</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>c</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="italic" class="MJX-tex-mathit">flap</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>8</mml:mn> </mml:math> ${St}_{c_{\textit{flap}}} = 8$ based on flap chord c Subscript italic flap <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mi>c</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="italic" class="MJX-tex-mathit">flap</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> $c_{\textit{flap}}$ ), similar to what has been reported in other experimental studies. In light of the simulation analysis, the flow can be divided into three phases: shear-layer roll-up, vortex merging and cross-flow wake. The first two phases are commonly described in the literature, but the third is rarely mentioned. Precisely, it is shown that the source of the tonal feature is located specifically within the cross-flow wake phase which is associated with the existence of a focal point and a near-wall reversed-flow field. The cross-flow wake induces what is called side trailing-edge noise responsible for the tonal component in the present flap side-edge spectrum. Further discussion of the mechanism as regards flap geometry or vortex breakdown suggests that it may be generalised to certain other configurations that have presented a focal point.