An Air-Bearing Floating-Element Force Balance for Friction Drag Measurement

It is extremely difficult, if not impossible, for existing force balances to capture very small skin-friction drag (SFD) in a perturbed turbulent boundary layer (TBL), which is characterized by the unpredictable, nonuniform distribution of static surface pressure. A novel force balance is proposed, which combines the level principle, as deployed in Cheng et al.'s (2020, “A High-Resolution Floating-Element Force Balance for Friction Drag Measurement,” Meas. Sci. Technol., 32, p. 035301) force balance, with a single-degree-of-freedom air bearing mechanism. This mechanism acts to eliminate disturbances, such as nonuniform static pressure on the wall associated with high Reynolds number TBL or a TBL under control. As a result, the developed balance may be used to accurately measure SFD in the order of 10⁻³ N in a TBL with or without control. This balance has been successfully applied to measure the drag reduction (DR) of a TBL manipulated using one array of streamwise microjets, at friction Reynolds number Re_τ = 3340 ∼ 5480.