DMR effect on drag reduction of a streamlined body measured by Magnetic Suspension and Balance System

TL;DR

This study experimentally validates that distributed micro-roughness coatings can significantly reduce aerodynamic drag on streamlined bodies by modifying boundary layer behavior, with reductions up to 43.6%.

Contribution

First experimental validation of DMR's drag reduction effects using magnetic suspension measurements, confirming boundary layer friction reduction as the primary mechanism.

Findings

Up to 43.6% drag reduction observed in transitional flow.

Drag reduction mainly due to skin friction decrease, not flow separation.

Flow patterns remain similar regardless of surface condition, indicating boundary layer modification.

Abstract

This study experimentally investigates the aerodynamic drag reduction capabilities of distributed micro-roughness (DMR) coatings on a streamlined model, utilising the 1-m magnetic suspension and balance system (MSBS) at Tohoku University. Previous direct numerical simulations (DNS) indicated that DMR can mitigate turbulent-energy growth by suppressing Tollmien--Schlichting (TS) waves and influencing the breakdown of streamwise vortices. The present work provides the first experimental validation of these effects using an interference-free MSBS, which is essential for accurate measurement in the laminar and transitional regimes. A streamlined model was tested with two rows of artificial tripping tape to induce transition; the DMR height was approximately 1% of the local boundary layer thickness, significantly smaller than typical roughness elements. Direct aerodynamic drag measurements using the MSBS revealed a substantial reduction of up to 43.6% within the transitional flow regime. Crucially, integrated analysis using wall-resolved large-eddy simulations (LES) and dynamic oil-flow visualisation confirmed that this benefit does not mainly originate from the suppression of flow separation. The LES drag decomposition established that the total pressure-drag budget is subordinate to skin friction, a finding complemented by oil-flow observations, which revealed qualitatively similar flow patterns regardless of the surface condition. Consequently, the observed drag reduction is primarily ascribed to friction drag reduction achieved through the modification of the boundary layer state. These findings provide compelling experimental evidence for the efficacy of DMR and offer valuable insights for optimising surface designs for passive flow control.