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The floating-element (FE) principle, introduced nearly a century ago, remains one of the most versatile direct wall shear stress measurement methods. Yet, its intrinsic sources of systematic error, associated with the flow-exposed gap, off-axis load sensitivity, and calibration, have thus far limited its widespread application. In combination with the lack of standard designs and testing procedures, measurement reliability still hinges heavily on individual judgement and expertise. This paper presents a framework to curb these limitations, whereby the design and operation of a FE balance are leveraged by an analytical model that attempts to capture the behaviour and predict the relative contribution of the systematic sources of error. The design is based on a parallel-shift linkage and a zero-displacement force-feedback system. The FE has a surface area of 200 × 200 mm, and measurement sensitivity is adjustable depending on the surface condition and the Reynolds number. It is thus suitable for application in a wide range of low-speed, boundary-layer wind tunnels, small or large scale. Measurements of the skin friction coefficient over a smooth wall show a remarkable agreement with oil-film interferometry, especially for Re θ > 1.3 × 10 4 . The discrepancy relative to the empirical Coles–Fernholz relation ( κ = 0.39 and C = 4.352 ) is within 0.5 %, and the level of uncertainty is below 1 % for a confidence interval of 95 %. [ABSTRACT FROM AUTHOR]

This paper presents a general method to extract a surface pressure field from a skin friction field in complex flows as an inverse problem, focusing on its application to global luminescent oil-film (GLOF) skin friction measurements. The main technical aspects of this method are described, including the basic equation relating surface pressure to skin friction, variational method, numerical algorithm, and approximate method with the constant boundary enstrophy flux (BEF). The proposed method is evaluated through simulations in the Falkner–Skan flow and the flow over a 70°-delta wing to investigate the effects of the Lagrange multiplier, downsampling rate, noise level, and the value of a constant source term in the approximate method. Further, the approximate method is applied to the skin friction fields obtained by GLOF measurements in the flow over a 65°-delta wing and the square junction flow to obtain the normalized surface pressure fields. The proposed method provides a useful tool to obtain the high-resolution fields of both surface pressure and skin friction by GLOF measurements in complex flows (particularly at low speeds). The normalized surface pressure field extracted from the skin friction field in the square junction flow, where skin friction lines are superposed. [ABSTRACT FROM AUTHOR]

The movement of a thin oil film on the surface of fast-rotating disks or propeller blades is analyzed in the present work with the aim to find a means for skin-friction measurements. A new skin-friction measurement technique is proposed, the rotational-flow global interferometry skin-friction method, utilizing the relationship between the oil-flow deflection angles, the induced centrifugal forces, and the skin friction. Finally, some results from initial wind-tunnel experiments using the proposed technique are presented. [ABSTRACT FROM AUTHOR]