Near-Wall Lagrangian Particle Tracking Measurements of Flow over Checkerboard Roughness by means of 4-Pulse Shake-the-Box
T. Rich(1), C. Schmidt(1), J. Schmitt(1), P. Sujar-Garrido(1,2), B. Frohnapfel(1), J. Kriegseis(1)
1: Institute of Fluids Mechanics, Karlsruhe Institute of Technology, Germany
2: Department of Engineering Mechanics, KTH Royal Institute of Technology, Sweden
DOI:
Four-pulse Lagrangian Particle Tracking (LPT) by means of the Shake-the-Box (STB) approach was used to examine the flow over three surfaces within a zero pressure gradient turbulent boundary layer (TBL). The three surfaces examined were a smooth wall reference case, a uniform sandpaper wall, and a checkerboard pattern consisting of 50% coverage of each surface. At low Reynolds numbers in turbulent channel flow, the checkerboard surface showed surprisingly low drag compared to the average drag of the uniform (smooth and rough) reference cases. The corresponding flow field suggests that the flow perceives the checkerboard surface as a homogeneous boundary condition, which is not the case at higher Reynolds number. The present investigation aims to identify whether a similar phenomenon is present in TBLs. The volumetric nature of STB measurements is used to examine the form that heterogeneity takes in the velocity and acceleration fields above the checkerboard. Regular repeating patterns are found in both streamwise velocity and acceleration fields, that correspond to the checkerboard roughness below. Quantitative magnitudes of accelerations are unexpectedly high and this is examined statistically using the context of smooth wall data to examine a possible bias error.
