tailieunhanh - The MEMS Handbook MEMS Applications (2nd Ed) - M. Gad el Hak Episode 1 Part 8

Tham khảo tài liệu 'the mems handbook mems applications (2nd ed) - m. gad el hak episode 1 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Sensors and Actuators for Turbulent Flows 6-31 average of Equation over a large number of cycles and assuming that the turbulent fluctuations are small a linearized expression for the periodic wall-shear stress is obtained. To summarize the classical expression Equation gives a relation between the wall-shear stress and the heat transfer from the wall. This expression assumes steady laminar zero-pressure-gradient flow and is not valid in a turbulent environment. Menendez and Ramaprian 1985 have derived an extended version of Equation valid for a periodically fluctuating freestream velocity Equation . However the latter relation contains some assumptions that are questionable for turbulent flows. For instance in the thermal boundary layer it is assumed that the temperature distribution is self-similar and that the local thickness varies linearly. These assumptions are relevant for a streamwise velocity oscillation and a weak fluctuation but certainly not in a turbulent flow which is strongly unstable in all directions. Calibration Several methods and formulas are in use for calibrating hot-film shear probes operated in the constanttemperature mode and the choice of method depends on flow conditions and sensor used. In this subsubsection two static calibration methods are discussed. Both are based on the theoretical analysis leading to the relation between rate of heat transfer and wall-shear stress as discussed in the last subsubsection. The challenge is of course to be able to use the shear-stress sensor in a turbulent environment. If a laminar flow facility is used to calibrate the wall-shear-stress sensor then Equation can be rewritten more conveniently in time-averaged form T w A- B where Tw is the desired mean wall-shear stress e2 is the square of the mean output voltage and A and B are calibration constants. The term B represents the heat loss to the substrate in a quiescent surrounding and this procedure is similar to a .