  Visual anemometry (VA) leverages observations of fluid-structure interactions
to infer incident flow characteristics. Recent work has demonstrated the
concept of VA using both data-driven and physical modelling approaches applied
to natural vegetation. These methods have not yet achieved generalization
across plant species and require site-specific calibration. We conducted a
laboratory study in an open circuit wind tunnel using overhead imagery of three
vegetation species to assess the utility of vegetation displacement fields for
wind speed inference. Both the wind and vegetation speeds exhibited a
two-parameter Weibull distribution. The relationship between the scale factor
(one of the two parameters) of the wind and vegetation was found to be well
described by a sigmoid function, indicating three regions of distinct
structural response to the wind loading at low, intermediate, and high wind
speeds. Within an intermediate range of wind speeds, the wind and vegetation
scale factors are nearly linearly proportional, thereby facilitating VA.
Importantly, the wind and vegetation scale factors were found to be
uncorrelated in low and high wind regimes, revealing a fundamental constraint
on VA using structure response data. We discuss the physical basis for these
regimes and present additional parametric relationships that can be exploited
in the intermediate wind regime to potentially generalize inference of the wind
speed and direction.
