Accurate Measurements of Wall Shear Stress on a Plate with Elliptic Leading Edge

Abstract

A micro-floating element wall shear stress sensor with backside connections has been developed for accurate measurements of wall shear stress under the turbulent boundary layer. The micro-sensor was designed and fabricated on a 10.16 cm SOI (Silicon on Insulator) wafer by MEMS (Micro-Electro-Mechanical System) processing technology. Then, it was calibrated by a wind tunnel setup over a range of 0 Pa to 65 Pa. The measurements of wall shear stress on a smooth plate were carried out in a 0.6 m × 0.6 m transonic wind tunnel. Flow speed ranges from 0.4 Ma to 0.8 Ma, with a corresponding Reynold number of 1.05 × 106~1.55 × 106 at the micro-sensor location. Wall shear stress measured by the micro-sensor has a range of about 34 Pa to 93 Pa, which is consistent with theoretical values. For comparisons, a Preston tube was also used to measure wall shear stress at the same time. The results show that wall shear stress obtained by three methods (the micro-sensor, a Preston tube, and theoretical results) are well agreed with each other.

Flexible smart sensing skin for “Fly-by-Feel” morphing aircraft

“…By now, plenty of research has proved that wall shear stress (WSS) measurement along the leading edge is a realistic way to detect the separation line/point in real-time. Basically, WSS measurement can be divided into two major schemes: hot-wire/film based shear-stress sensor [108,109] and floating-element based sensor [110][111][112]. The structure of Hotwire/film sensors is relatively simple, employing heated components on the sensor surface to indirectly measure the variation of shear stress, with fast response (around dozens of kilohertz) and high sensitivity (<1 Pa) [108,109].…”

“…Oppositely, floating-element based sensors directly sense the shear-stress force with the floating element. Most of the floating element structures apply moving components for shear stress measurement [110][111][112].…”

“…This sensor has a sensitivity of 0.65 Pa/nm. Ding et al [111] designed a capacitive sensor based on the micro-floating element to accurately measure WSS of the turbulent boundary layer range from 34 to 93 Pa. Additionally, high aspect-ratio micropillars were used to sense WSS using optical reflection or camera-based detection methods [112]. In Figure 6(a), a novel nanoparticle resistive-based multiaxial sensor was proposed, in which a structured PDMS diaphragm and mesa were mounted on a polyimide substrate.…”

Design and applications of MEMS flow sensors: A review

“…This is particularly important when the flow state transitions from laminar to turbulent, or when the flow is fully turbulent. This is because the magnitude of wall shear stress in the turbulent boundary layer is larger than that in the laminar boundary layer [89]. However, these devices typically measure wall shear stress in order of Pascals, which is insufficient for most aerodynamic applications where the wall shear stress can be of the order of tens of Pascal or even much more when the flow speed is increased [88].…”

Skin-Friction-Based Identification of the Critical Lines in a Transonic, High Reynolds Number Flow via Temperature-Sensitive Paint

“…Floating elements [ 31 ] provide an average measurement of the skin friction over a certain area, but clearly not the distribution of the skin-friction field. Localized skin-friction data can be obtained via micro-floating element sensors based on Micro-Electro-Mechanical Systems (MEMS) [ 27 , 32 ], via hot films [ 33 , 34 ] and via wall hot wires [ 35 ], but these single, surface-based sensing elements have to be arranged in arrays to obtain a skin-friction distribution, which still has limited spatial resolution. Moreover, the required complexity of the arrays increases for the determination of the direction of the skin-friction vector, and the application of all these surface-mounted sensors in high Reynolds number flows is particularly difficult because of their relatively large size compared to the small thickness of the boundary layer.…”