A thin PSP coating may slightly modify the overall shape of a model and produces local surface roughness and topological patterns. These unwanted changes in model geometry may alter flows over a model and affect the integrated aerodynamic forces (Engler et al. 1991; Sellers 1998a). Hence, this paint intrusiveness to flow should be considered as an error source in PSP measurements. The effects of a paint coating on pressure and skin friction are directly associated with locally changed flow structures and propagation of the induced perturbations in flow; these local effects may collectively alter the integrated aerodynamic forces. When a local paint thickness variation is much smaller than the boundary layer displacement thickness, a thin coating does not alter the inviscid outer flow. Instead of directly altering the outer flow, a rough coating may indirectly result in a local pressure change by thickening the boundary layer; coating roughness may reduce the momentum of the boundary layer to cause early flow separation at certain positions. Therefore, the effective aerodynamic shape of a model is changed and as a result the pressure distribution on the model is modified; this effect is mostly appreciable near the trailing edge due to the substantial development of the boundary layer on the surface. Vanhoutte et al. (2000) observed an increment in the trailing edge pressure coefficient relative to the unpainted model, which was consistent with an increase in the boundary layer thickness at the trailing edge. For certain models such as high-lift models, a coating may change the gap between the main wing and slat or flap when the gap is small; thus, the pressure distribution on the model is locally influenced. In addition, a coating may influence laminar separation bubbles near the leading edge at low Reynolds numbers and high angles-of-attack. The perturbations induced by a rough coating near the leading edge may enhance mixing that entrains the high-momentum fluid from the outer flow into the separated region. The perturbations could be amplified by several hydrodynamic instability mechanisms such as the Kelvin-Helmholtz instability in the shear layer between the outer flow and separated region and the cross-flow instability near the attachment line on a swept wing. Consequently, the coating causes the laminar separation bubbles to be suppressed. Vanhoutte et al. (2000) reported this effect that led to a reduction in drag.
Schairer et al. (1998a, 2002) observed that a rough coating on the slats slightly decreased the stall angle of a high-lift wing. Also, they found that the empirical criteria for ‘hydraulic smoothness’ and ‘admissible roughness’ based on 2D data by Schichting (1979) were not sufficient to provide a satisfactory explanation for their observation. Indeed, in 3D complex flows on the high-lift model, the effect of the coating on the cross-flow instability and its interactions with the boundary layer and other shear layers such wakes and jets are not well understood. Schairer et al. (1998a, 2002) and Mebarki et al. (1999) found that a rough coating moved a shock wave upstream and the pressure distribution was shifted near the shock location. This change might be caused by an interaction between the shock and the incoming boundary layer affected by the coating. In an attached flow at high Reynolds numbers, a rough coating increases skin friction by triggering premature laminar-turbulent transition and increasing the turbulent intensity in a turbulent boundary layer (Mebarki et al. 1999; Vanhoutte et al. 2000). An increase in drag due to a rough coating was observed in airfoil tests in high subsonic flows (Vanhoutte et al. 2000). In fact, premature transition by coating roughness has been often observed in TSP transition detection experiments (see Chapter 10). Amer et al. (2001, 2003) reported that a very smooth coating on the upper surface of a delta wing model at Mach 0.2 and a semi-span arrow-wing model at Mach 2.4 did not significantly change the drag coefficients of these models. Generally speaking, the effect of a coating on aerodynamic forces highly depends on flows over a specific model configuration; there is no universal conclusion on this effect.