Thurmelec (France)

Surface acoustic wave (SAW) sensors represent a prominent technologi-cal avenue for quantifying mechanical stress-induced deformations. Nonetheless, the integration of these microelectromechanical systems (MEMS) sensors remains a complex challenge. Consequently, a common approach within a multitude of research involves the utilization of an adhesive formulation. However, this mode of integration continues to exhibit deficiencies in terms of measurement repeatability, particularly when confronted with harsh environments characterized by pronounced mechanical and thermal stresses. In this context, the utilization of backside reflow soldering for SAW sensor integration stands as a notably dependable alternative to epoxy bonding. In this letter, we present the results of our feasibility study concerning the reflow soldering of a SAW sensor onto a binocular copper load cell to measure static strains and a ferromagnetic steel sheet to measure dynamic strains resulting from magnetostriction. To use reflow soldering, a nickel/copper electrodeposition on the ferromagnetic steel sheet was carried out, demonstrating the potential to apply this technique to various materials. The results have indicated that reflow soldering enhances the transmission of static and dynamic mechanical stresses to the SAW sensor, exhibiting a coefficient threefold greater than that achieved through simple adhesive bonding. Furthermore, this integration technique demonstrated the long-term stability of the measurement.

A microscale low power high temperature gradient calorimetric (HTGC) sensor measuring both mean and fluctuating bidirectional wall shear stress is presented. The micromachined sensor is composed of three free-standing $$3\,\upmu \text {m}\times 1$$ mm micro-wires mechanically supported using perpendicular micro-bridges. The static and dynamic characterisations were performed in a turbulent boundary layer wind tunnel on a flat plate configuration, and compared to the one obtained with a conventional hot-film probe. The results demonstrated that the calorimetric sensor behaves similarly to the hot-film in constant temperature anemometry with nonetheless lower power consumption and better spatial resolution and temporal response. Additionally, its calorimetric measurement detected the direction of the wall shear stress component orthogonal to the wires, corresponding to the shear stress sign in 2D flows. The calibrated HTGC micro-sensor was then used for unsteady flow separation detection downstream a 2D square rib for $$Re_h= 2.56\times 10^4$$ . The calorimetric micro-sensor enabled self-correlated measurements and consequently successfully achieved the detection of flow separation and the reattachment point around $$x/h=10.7$$ .

This paper reports a calorimetric micro-sensor designed for aerodynamic applications. Measuring both the amplitude and the sign of the wall shear stress at small length-scale and high frequencies, the micro-sensor is particularly suited for flow separation detection and flow control. The micro-sensor was calibrated in static and dynamic in a turbulent boundary layer wind tunnel. Several micro-sensors were embedded in various configurations for measuring the shear stress and detecting flow separation. Specially, one was embedded inside an actuator slot for in situ measurements and twelve, associated with miniaturized electronics, were implemented on a flap model for active flow control experiments.

This paper presents an efficient and high-sensitive micro-sensor designed for wall shear stress measurement. The main technical application targeted is flow separation detection for closed-loop active flow control. The sensor is a temperature-resistance transducer operating on heat transfer. The device is micro-structured with three substrate-free wires presenting a high aspect ratio, and periodic perpendicular micro-bridges ensuring mechanical toughness and thermal insulation. This design achieves a homogeneous temperature distribution along the wires. Welded on a flexible printed circuit, the sensor is wall-mounted on a wind tunnel. The experiments, conducted in both attached and separated flow configurations, demonstrate the sensor sensitivity to wall shear stress up to 2.4 Pa and the ability of the 3-wires based design to perform flow direction sensing for back-flow detection.