An Experimental Study of Boundary Layer Characteristics in Flat Plate Transition Region at Different Surface Roughness

The study concentrates on investigating the effect of surface roughness of painted and non-painted flat plate surfaces on boundary layer characteristics in the laminar to turbulent transition region. The flat plate is made of a PVC three-dimensional printed surface of 45 cm length × 20 cm width × 0.5 cm thickness dimensions. Three types of the above PVC flat plate are examined. The first one is unpainted (5.725 µm arithmetic mean roughness Ra), the second one is water-based painted (3.03 µm Ra), and the third one is oil-based painted (0.35 µm Ra). Static pressure measurements of the three surfaces using a Pitot tube near the plate surface at 35, 40, and 45 cm axial locations from the leading edge to calculate the wall velocity within the transition region. The tests are carried out in a test section of a low-speed wind tunnel. The effect of Reynolds no. variation at 25, 27, 29 m s-1 tunnel velocity is examined. Transition laminar results at maximum tunnel velocity show a 10% velocity increase, 5.12% Boundary Layer Thickness (BLT) decrease, 5.15% Skin Drag Coefficient (SDC) decrease, and 14.48% Average Drag Coefficient (ADC) decrease of water-based painted under non-painted surfaces. In addition, a 10.4% velocity increase, a 5.34% BLT decrease, a 5.3% SDC decrease, and a 15.34% ADC decrease of oil-based painted under water-based painted surfaces can be noticed. The transition turbulent results at maximum tunnel velocity show a 10% velocity increase, a 55% BLT decrease, a 2.24% SDC decrease, and a 11.85 ADC decrease of water-based painted under non-painted surfaces. In addition, a 10.4% velocity increase, 2.56% BLT decrease, 2.06% SDC decrease, and 12.36% ADC decrease of oil-based painted under water-based painted surfaces can be noticed. Laminar and turbulent comparison of ADC results in transition region at maximum Reynolds number show a 60.34% rise of non-painted surface, a 61.51% rise in water-based painted surface, and a 62.82% rise in oil-based painted surface. The painting surface roughness is passive, cheap, and has excellent control on the separation of laminar side and much larger of turbulent side in the transition region.