Abstract:
This paper reviews the advantages and disadvantages of currently available wall-shear-stress measurement techniques for practical applications, such as over the surfaces of air and land vehicles. These techniques include the direct methods, such as the shear-stress balance and the MEMS balance, and the in-direct methods, such as Preston tube, vision-based methods, and hot-film sensors. This paper concludes that these techniques suffer problems of convenience or reliability, and are often too expansive for a large amount of implementation, none of them is capable of practical applications. The MEMS-based hot-film sensor is promising because it is low-cost, relatively convenient, and fast-response. However, major shortcomings in reliability are still to be solved, e.g. most of the Joule heat generated by the film transfer to the wall instead of the fluids, reducing the measurement sensitivity and creating unwanted sensitivities to other parameters. As a result of these unwanted sensitivities, the calibration results were found to drift with time. This well-recognized heat-loss problem excludes hot-film methods from practical applications, as well as quantitative laboratory investigations. The recent development of a dual-layer sandwich-structured hot-film technology effectively solves the heat-loss problems. In this method, two layers of thin metal films are stacked together with a thin insulation layer in-between. Two separate constant-temperature-anemometry systems keep both films at the same working temperature so that the lower film blocks the heat generated by the upper film, making them only transfer to fluids and reducing the heat loss to the substrate to as low as 5% of the total heat generation. The dual-film technique significantly improves the sensitivity and reliability of the hot-film method, making it promising for practical applications. In this paper, the details of this dual-layer hot-film sensor and the related calibration-free measurement technique were reviewed.