Investigation of wall-bounded turbulence over sparsely distributed roughness M. Placidi, B. Ganapathisubramani and M. Tan Faculty of Engineering and the Environment Fluid Structure Interactions (FSI) Research Group Acknowledgement: This project is supported by Lloyd Registered Educational Trust FSI Away Day 2012 Background Sparse distribution of surface roughness is found in abundance in natural environments and in a variety of engineering applications. Barnacle growth on ship’s hull (Figures 1 and 2) and Atmospheric Boundary Layer (Figure 3) are classical examples. References: Jimenez, J Turbulent flows over rough walls. Annual Review of Fluid Mechanics 36 (1), 173–196. Townsend, A. A The structure of turbulent shear flow; Krogstad, P. A. & Antonia, R Surface roughness effects in turbulent boundary layers. Experiments in Fluids 27, 450–460, Ganapathisubramani, B. & Shultz, M. P Turbulent boundary layer structure over sparsely distributed roughness pp. 1–6. Volino, R. J., Schultz, M. P. & Flack, K. A Turbulence structure in rough and smooth wall boundary layers. Experiments in Fluids 592, 1–31. Grimmond, C. S. B. & Oke, T. R Aerodynamic properties of urban areas derived, from analysis of surface form. Journal of Applied Meteorology 38 (9), 1262– Future Work Analyzing the effect of urban-like roughness on the turbulence structure; Considering the problem of step change in roughness; Studying the effect of roughness on pollutant dispersion. Experimental Facilities and Method 3x2 Wind tunnel 0.9 x 1.35 x 4.5 m long test section Velocity range = 0.1 – 25 m/s Slightly favorable pressure gradient PIV “friendly” Lego® Elements cylindrical and rectangular 2x2 bricks Staggered arrays Aims and Objectives Systematic study of the effect of Frontal and Plan solidity on the structure of the turbulence; Evaluate the importance and the effects of additional parameters on the turbulence structure; Investigate the validity of Townsend’s similarity Hypothesis in the sparse regime. Regular roughness can be characterized using two parameters: 1.Frontal Solidity λ F =A F /A T (which is the ratio of the total projected frontal area of the roughness per unit wall parallel area); 2.Plan Solidity λ P =A P /A T (which is the ratio of the total plan area of the roughness per unit wall parallel area). Roughness Characterization Jimenez (2004) pointed out that while boundary layers over surfaces with a dense distribution of roughness elements have been the object of numerous studies, the effects of sparse roughness has been poorly investigated (Figure 4). Figure 1: Accumulation of marine growth (barnacles) on the ship’s hull. ©2009 Igor Sterzhantov. Figure 2: Maintenance of ship hull in situ. ShipHullPerformance.org. Figure 3: Urban boundary layers and turbulence in the urban canopy. Princeton University. Figure 4: Equivalent sand roughness for various surfaces versus the frontal solidity. Jimenez (2004). Figure 5: Frontal and Plan Solidity. Grimmond and Oke (1998). Figure 6: Experimental set up Experimental Technique Particle Image Velocimetry (PIV) PIV is based on the measurement of the displacement of small tracer particles, that are released into the fluid; The tracer particles are illuminated within a thin light sheet generated from a pulsed light source; the light scattered by the particles is recorded onto two subsequent image frames by a CCD camera; Cross-correlating the particle-image patterns in small interrogation windows, between the first and second image frame, allows the evaluation of the displacement hence, the local fluid velocity. Figure 7: Schematic of a typical PIV measurements system. Scarano 2010 Townsend’s Similarity Hypothesis Townsend (1976) stated that the turbulence structure is una ff ected by the surface condition, at su ffi cient distance from the wall. Similarity only holds if the Reynolds Number is su ffi ciently high and if the mean height of the roughness elements is small, compared to the boundary layer thickness. Our understanding of rough walls heavily relies on this similarity. Krogstad & Antonia 1999, Volino et al and Ganapathisubramani & Schultz 2011 have shown evidence of lack of similarity for rough walls characterized by sparse roughness. Figure 8: Classical PIV image. Streamwise (x,y) plane. Colorbar shows mean streamwise velocity U.