Numerical study of detailed flow affecting a direct measuring skin-friction gauge

The detailed flow characteristics of a direct measuring skin-friction gauge are examined. This type of device uses a small, movable head mounted flush to a wall, such that the head is assumed to be exposed to the same shear stress from the flow as the surrounding wall. The resulting deflection can be monitored by instruments such as strain gauges mounted on a flexure below the head. The goal of this study was to develop an understanding of the effects that the geometric design and installation parameters of the device have on the surrounding flow and the ability of the gauge to reflect the undisturbed shear stress value. Factors influencing the performance of a skin-friction gauge include the presence of the necessary gaps in the wall around the floating head, improper installation/misalignment issues with respect to the wall, and extraneous forces arising from pressure gradients. All of these effects are studied here computationally with a three-dimensional, Navier-Stokes code based on the finite element technique. It was found that smaller gap size does indeed produce the highest quality of measurement output because a larger gap results in increased exposure of the device to the external flow. Results presented here indicate that misalignment is the most significant factor that must be controlled and that it can induce an error of more than a factor of two in the measurement for the conditions studied. Finally, the configuration studied here exhibits errors of 17% or less under both strong favorable and adverse pressure gradient situations, despite the complex force fields placed on the head in these situations.