Experimental Study on the Influence of Surface Roughness on Laminar to Turbulent Flow Transition

Models for how surface roughness influences laminar to turbulent flow have not been proven with experimental results in the moderate roughness zone which makes these models less useful for predicting results in engineering. Even though classical stability theory considers only smooth surfaces, in reality, many systems have roughness that can impact the way transitions happen, affecting facts such as drag, heat flow and energy efficiency. It explored, through experiments, how the roughness of the surface (with Ra from 0.5 to 25 µm) impacts the critical Reynolds number for turbulence to occur. One way we studied transition instabilities was with a hot-wire wind tunnel which we used to complement precise measurements provided by stylus profilometry. ANOVA, Pearson correlation and linear regression were applied to numerically study the relationship between roughness and global and local flow conditions. It was found that the smoother the surface, the faster the flow over it; velocity decreased by about 43% between the smoothest and the roughest surfaces (0.5 µm to 25 µm roughness). This relationship was further confirmed by the derived regression model (Flow Velocity = 0.1452 – 0.0026×Ra, R² = 0.96) which proved that each increased μm of roughness reduces velocity by 0.0026 m/s. There were differences among roughness classes as ANOVA indicated (F = 583.2, p < 0.001), other than those between Ra = 6.0 and 8.0 µm which suggested a possible saturation effect. This research shows empirically that a moderate degree of roughness helps the transition from laminar to turbulent flow, improves theoretical models and is useful for improving surfaces in aerodynamics, piping systems and microfluidics. By relating roughness measurements to transition details, the study closes a major knowledge gap and makes fluid flow control in engineering design more predictable.