Boundary Layer Dynamics and Structure, Time-resolved PIV Study

Turbulent boundary layer is characterized not only by vertical profile of mean velocity, but also by more complex variables as shear stress tensor, intermittency factor, integral length scale, etc. From aerodynamical point of view, it is desired to find one universal vertical profile for given variable independent on the surface roughness and the Reynolds number. In atmospheric sciences, however, there are several different scaling procedures, which don't lead to the same results.

Proposed paper presents time-resolved PIV measurement of wind-tunnel boundary layers. This unique technique allows us to analyze spatial and temporal behavior of the flow in high resolution and thus look at the classical problem of boundary layer scaling by new eyes. We measured the flow field of the whole depth of the boundary layers above 3 different types of surface roughness (slightly rough, rough, and moderately rough) and for 4 different inlet velocity (i.e. Reynolds number in the range from 5000 to 41000). We used two scaling approaches. The first is based on boundary layer depth delta and the corresponding velocity at this height. The second is based on the constant reference height H=100 m in the full scale and the velocity at this height.

The minor Reynolds number dependence was found in the profiles and will be discussed. The vertical profile of the intermittency factor reveals several vertical subranges (the inertial subrange among others). Flow structure is slightly more coherent (reveals longer integral length scales) in these subranges and the boundaries between these regions produce smaller but stronger shear driven vortices.