Volumetric velocimetry

Recognizing that most real flows are three-dimensional (3D) is as straightforward as recognizing they are time varying. Nevertheless, in fluid mechanics, we often assume we are in front of two-dimensional (2D) or even one-dimensional (1D) problems, just like we often assume them to be steady state. The roots of such simplifications are both conceptual and methodological: flows that are homogeneous in one or more spatiotemporal dimensions are easier to understand and theoretically more tractable, their governing equations are less expensive to integrate numerically, and (what is critical here) their velocity fields are much easier to characterize experimentally.