A fluid dynamic structure includes a surface and a microstructure mounted on the surface. The microstructure is defined by a plurality of peaks and valleys (i.e., riblets) and includes a transition region in which each peak and valley changes from a first profile to a second profile along a chord length of the structure. The peaks and/or valleys may have a variable dimension along the length of the microstructure. The peaks and valleys can be non-linear along the chord length and have a smooth transition from the first profile to the second profile. The different profiles of the microstructure are optimized to reduce noise generated by turbulent fluid flow across the structure.