These files contain the data used in the publication: 

  Vanderwel C. and Ganapathisubramani B., “Effects of spanwise spacing on large-scale secondary
  flows in rough-wall turbulent boundary layers”, Journal of Fluid Mechanics, Vol. 774, R2, 2015.
  (dx.doi.org/10.1017/jfm.2015.292)



Abstract:

Large-scale secondary flows can sometimes appear in turbulent boundary layers
formed over rough surfaces, creating low- and high-momentum pathways along the
surface (Barros & Christensen, J. Fluid Mech., vol. 748, 2014, R1). We investigate
experimentally the dependence of these secondary flows on surface/flow conditions by
measuring the flows over streamwise strips of roughness with systematically varied
spanwise spacing. We find that the large-scale secondary flows are accentuated when
the spacing of the roughness elements is roughly proportional to the boundary layer
thickness, and do not appear for cases with finer spacing. Cases with coarser
spacing also generate delta-scale secondary flows with tertiary flows in the spaces in
between. These results show that the ratio of the spanwise length scale of roughness
heterogeneity to the boundary layer thickness is a critical parameter for the occurrence
of these secondary motions in turbulent boundary layers over rough walls.



Experimental Details:

All the details of the experiments are documented in the above article. 
Experiments were performed in the University of Southampton’s suction wind
tunnel using Lego bricks to form the roughness elements. The wind
tunnel has a working section 4.5 m long with a 0.9 m x 0.6 m cross-section. In this
study, strips of Lego bricks having a width of W=16 mm and height of H=9.6 mm
were aligned with the flow direction and extended over the full 4.5 m length of the
wind tunnel test section. These dimensions do not include the array
of ‘bumps’ used to connect the pieces together, which have a diameter of 4.8 mm
and a height of 1.7 mm and which covered the floor of the wind tunnel uniformly.
The centre-to-centre spacing, S, of the elevated strips was varied to test five cases
with ratios of S/W = 2, 3, 6, 8 and 12. 

Measurements of the velocity field that developed over the surface were acquired 
using stereo particle image velocimetry (PIV) in a cross-section normal to the flow 
direction at a position 4 m downstream of the leading edge. We apply the convention 
that x, y, z are the streamwise, wall-normal and spanwise directions, and U, V, W are 
the corresponding velocities in those directions. 

In all of the test cases, the free stream velocity was set to U1 D 15 m s??1.
For each case, a total of 1500 image pairs were acquired with an image pair
separation time of 15 ms at a rate of 2 Hz, which was slow enough such that
each measurement could be considered independent. Vectors were determined with
LaVision’s DaVis 8.2.2 software, using window sizes of 32 pixel x 32 pixel with
50% overlap, resulting in a resolution of one vector per 0.9 mm.



Data Files:

Instantaneous Stereo-PIV maps are separated into five folders representing the five cases 
investigated in the article:

  Spacing (S/W) = 2, 3, 6, 8, 12

Each file contains MATLAB data consisting of U,V, and W velocity maps, and the X, Y coordinates.
Use the included sample matlab script "DisplayInstImages.m" to read and display the data in MATLAB.



Reference: 

Please provide a reference to the article above when using this data.
Please direct questions to Christina Vanderwel at c.m.vanderwel@soton.ac.uk