Measurements of wall shear stress of a turbulent boundary layer using a micro-shear-stress imaging chip

Abstract

Measurements of wall shear-stress streaks of a turbulent boundary layer in the channel ow were carried out using a MEMS-based micro-shear-stress imaging chip, which contains about 100 sensors. The chip is designed and fabricated by surface micromachining technology. One arrray of 25 micro-shear-stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous spanwise distribution of the surface shear stress. The statistics of high shear-stress streaks were established. Based on the measurement, the physical quantities associated with the high shear-stress streaks, such as their length, width and peak shear-stress level, were obtained. We found out that a high correlation exists between the peak shear-stress level and front-end shear-stress slope of a high shear-stress streak. This important property is currently being applied to the design of a real-time ow control logic.

Electrokinetic Microactuator Arrays for Control of Vehicles

“…Thus effective shear stress sensor design must minimize the heat loss to the substrate. For this reason, the proposed sensor design improves the frequency response and sensitivity of thermal sensors by using a vacuum cavity underneath the thermal sensor, as has been reported by Breuer (2000), Huang et al (1996), and Kimura et al (1999). Since the thin film element is then separated from the substrate by a vacuum cavity and a membrane, the heat will still be conducted from the film to the membrane and then from the membrane to the substrate.…”

“…Several approaches for developing practical wall shear stress sensors based on these two approaches can be found in the literature (e.g., Huang et al 1996Huang et al , 1999Kimura et al 1999;Liu et al 1999;Gad-el-Hak 1989Löfdahl & Gad-el-Hak 1999;Padmanabhan et al 1995;Shajii et al 1992;Schmidt et al 1988;Breuer 2000). Of particular relevance for the present study, Schmidt et al (1988) fabricated a microscale floating element sensor with a sensing area of 500 × 500 µm 2 using MEMS techniques, and Huang et al (1996) developed a microscale thermal sensor to measure surface shear stresses, but noted that the resulting sensitivity of their sensors was less than desirable.…”

“…Arrays of thermal sensors would appear to be more appropriate for this purpose due to their inherently higher robustness, as noted above, and the fact that their inherent simplicity more readily permits fabrication in dense arrays collocated among actuators and processing electronics. Related to this, Kimura et al (1999) have successfully used MEMS processes to fabricate a microscale wall shear stress sensor array composed of 100 thermal sensors, and used this array under laboratory conditions to measure instantaneous spanwise distributions of surface shear stress. Their sensors were spaced 300 µm apart, and the sensitive area -defined by the membrane where the polysilicon hot wire was located -measured 200 × 200 µm 2 .…”

MEMS Shear Stress Sensors: Promise and Progress

“…Several research groups have presented shear-stress spectra and/or statistical moments measured using vacuum cavity sensors compensated by constant temperature feedback circuits. 26,31,46 Clearly, the uncertainty of such data is unknown. …”

Wall shear stress hot film sensor for use in gases

“…However its measurement is complex and is generally limited to experimental models. The measurement of wall shear stresses is relevant when studying the drag of blunt or aerodynamic objects, the pressure drop through conduits, the convective heat transfer of surfaces exposed to flows, etc., but it can also be used to detect the passage of fluid-dynamic structures such as hairpin vortices, vortex shearing behind obstacles or large scale coherent structures [1] [2] [3] [4] [5]. Having a good temporal resolution in this kind of devices is of great interest, one reason for this is that the fluctuations of wall shear are relevant in the understanding of the flow and its effect on walls.…”