Tham khảo tài liệu 'the mems handbook (1st ed) - m. gad el hak part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 30 FIGURE Typical mean velocity profile measured with a MEMS sensor and a conventional single hot-wire. Here U uT f uTy v where uT is the friction velocity. From Lofdahl L. et al. 1992 Exp. Fluids 12 391-393. 1992 Springer-Verlag. With permission. of this micro-velocity-sensor was tested in a two-dimensional flat-plate boundary layer at a Reynolds number based on distance from the leading edge of X 106. Figure compares the mean velocity profile in a turbulent boundary layer as measured with a conventional and MEMS-based sensor. The microsensor measured mean velocities with the same accuracy as a corresponding conventional hot-wire. Moreover it was also demonstrated that the microsensor had spatial and temporal resolutions that made it suited for turbulence measurements. Figure shows the streamwise turbulence intensity and the Reynolds stress as measured by a conventional X-wire and by two silicon microsensors. The MEMS-based sensors operated with good resolution even when the temperature of the heated part was reduced considerably. A clear drawback of this micro-hot-film is the proximity of the heated part of the sensor to the surface of the chip rendering the probe insensitive to changes in flow direction. This makes the silicon sensor unsuited for use in three-dimensional flows where the primary flow direction is not known a priori. Jiang et al. 1994 presented a MEMS-based velocity sensor with the hot-wire freestanding in space without any nearby structures so that cooling velocities can be determined in the same way as with a conventional hot-wire. Their sensor is shown in Figure and has a polysilicon hot-element that is greatly reduced in size typically about am thick 1 am wide and 10-160 am long. The dynamic performance and sensitivity of this sensor have been tested. A heating time of 2 as and a cooling time of 8 as for the 30-qm-long sensor in constant-current mode have been achieved. For constant-temperature operation a time
