Spatial Resolution Limitations of Particle Image Velocimetry in High Reynolds Number Turbulent Boundary Layers

Abstract Particle image velocimetry (PIV) has become a standard technique in turbulence experiments for its non-intrusive ability to simultaneously measure multiple points in a flow field. However, inherent volume averaging of the interrogation windows as well as fixed limitations of camera spatial resolution and related hardware set the minimum and maximum scales of structures that can be resolved with this measurement technique. Resolution limitations present themselves as an attenuation of the measured turbulence statistics, seen particularly in the Reynolds wall-normal stress components. This experimental study investigates the impact of hardware-level resolution limitations on the resolved turbulence statistics of PIV measurements made of both rough and smooth wall turbulent boundary layers influenced by external pressure gradients. Velocity fields are measured with PIV using two lens magnifications; one with a baseline lens and one with the addition of a teleconverter lens to view the same flow-field with double magnification (i.e. a finer spatial resolution). A comparison of the turbulence statistics between the rough and smooth wall, components of velocity, and pressure gradient magnitudes are analyzed. The smooth wall turbulence data consistently show a much higher sensitivity to the spatial resolution, particularly for the wall-normal component, due to the smaller eddy length-scales. A fixed camera resolution limits the measured range of scales which affects comparative magnitudes of turbulence for flows with different local Reynolds numbers. The rough wall data measurements show a variable sensitivity to the camera resolution depending on local Reynolds number and streamwise flow history. A statistical convergence study based on the anistropy of turbulence scales is necessary in experimental design for the study of turbulent boundary layers.