Turbulent pipe flow response to wall changes targeting specific azimuthal modes

We present an experimental study on the response of turbulent pipe flow at Reτ = 3486 to rapid changes in pipe shape that are designed to manipulate the Large Scale and Very Large Scale Motions in wall-bounded turbulence. Stereo PIV measurements were taken 5 pipe diameters downstream of 3D printed pipe inserts designed to target specific azimuthal Fourier mode numbers m = 3 and 15. Direct manipulation of the flow momentum (vortex generators) and secondary flows induced by Reynolds stresses (sinusoidally varying wall shape) are considered. The designs successfully modified the mean flow to contain structures mimicking the desired Fourier mode shapes. The energy was added directly to the targeted modes, and secondary peaks in energy existed in non-targeted modes indicating nonlinear interactions. At the same time, other modes showed relatively significant losses of energy, indicating that the pipe inserts were gathering energy into predefined flow structure.