READ Me file for the Dataset for The instantaneous structure of secondary flows in turbulent boundary layers DOI: https://doi.org/10.5258/SOTON/D0721 These files contain the data used in the publication: Vanderwel C., Stroh A., Kriegseis J., Frohnapfel B. and Ganapathisubramani B., "The instantaneous structure of secondary flows in turbulent boundary layers", Journal of Fluid Mechanics, In Press, 2018. (dx.doi.org/xx.xxxx/jfm.2018.xxx) Abstract: Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is not very sensitive to the Reynolds number range investigated, and direct numerical simulations and experiments produce similar results in the mean flow as well as the dispersive and turbulent stress distributions. The numerical methods capture time-resolved features of the instantaneous flow and provide insight into the near-wall flow structures, that were previously obscured in the experimental measurements. Proper orthogonal decomposition was shown to capture the essence of the secondary flows in relatively few modes and is useful as a filter to analyse the instantaneous flow patterns. The secondary flows are found to create extended regions of high Reynolds stress away from the wall that comprise predominantly sweeps similar to what one would expect to see near the wall and which are comparable in magnitude to the near-wall stress. Analysis of the instantaneous flow patterns reveals that the secondary flows are the result of a non-homogeneous distribution of mid-size vortices. Experimental Details: Experimental dataset is previously published under http://dx.doi.org/10.5258/SOTON/D0720 with a corresponding publication under https://doi.org/10.1017/jfm.2015.292 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 a ratio of S/W = 6. 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. Numerical Details: The carried out DNS is based on a pseudo-spectral solver for incompressible boundary layer flows developed at KTH/Stockholm. The Navier-Stokes equations are numerically integrated using the velocity-vorticity formulation by a spectral method with Fourier decomposition in the horizontal directions and Chebyshev discretization in the wall-normal direction. For temporal advancement, the convection and viscous terms are discretized using the 3rd order Runge-Kutta and Crank-Nicolson methods, respectively. The simulation domain represents an open turbulent channel flow with periodic boundary conditions applied in streamwise and spanwise directions, while the wall-normal extension of the domain is bounded by no-slip boundary conditions at the lower domain wall (y = 0) and symmetry boundary conditions (v = 0, ∂u/∂y = ∂w/∂y = 0) at the upper boundary (y = δ). The flow is driven by a prescribed constant pressure gradient (CPG) which results in a bulk Reynolds number of Re_b = U δ/ν = 7161. The resulting friction Reynolds number for the present case is given by Re_τ = 495. Simulation configuration - Grid nodes: Nx x Ny x Nz = 768 × 301 × 384 - Domain size: Lx x Ly x Lz = 8δ × δ × 4δ - Resolution: ∆x+=5.2, ∆y_min+=0.014, ∆y_max+=2.6, ∆z+ = 5.2 The surface structure is introduced through an immersed boundary method (IBM) based on the method proposed by Goldstein et al. (1993) and is essentially a proportional controller which imposes zero velocity in the solid region of the numerical domain. The Lego bricks are placed on the lower domain wall in such a way that the surface height H of the large raised surface elements is given by H/δ = 8.6% which corresponds to the experimental set-up. Data Files: The data is distributed into separate folders corresponding to figures from the manuscript. In every folder filenames mark the origin of the data ("*_experiment.mat", "*_simulation.mat"). Each file contains MATLAB data consisting of the plotted quantities and corresponding coordinates. The data is non-dimensionalized as shown in the manuscript figures utilizing free stream velocity U∞ and boundary layer thickness δ. Additionally, a single 3-dimensional instantaneous snapshot is saved in VTK-format under "3d_snapshot_simulation.vtr", which can be viewed with VTK-compatible visualization software (e.g. ParaView). The file contains three velocity components and the lamba2-criterion used for the figure 10 in manuscript. A time series of 3000 3-dimensional velocity fields is available upon request (alexander.stroh@kit.edu). Reference: Please provide a reference to the article above when using this data. Please direct questions regarding experimental setup/data to Christina Vanderwel at c.m.vanderwel@soton.ac.uk Please direct questions regarding numerical setup/data to Alexander Stroh Licence: Creative Commons: Attribution 4.0 International (CC BY 4.0) https://creativecommons.org/licenses/by/4.0/ December, 2018 Updated with minor changes and licence added, January 2019