Influence of wall roughness on near wall turbulence structure by Haigermoser C.*, Vesely L.*, La Polla M., Onorato M., Politecnico di Torino XIV A.I.VE.LA. National Meeting Roma, 6-7 novembre 2006 Influence of wall roughness on near wall turbulence structure by Haigermoser C.*, Vesely L.*, La Polla M., Onorato M., Politecnico di Torino XIV A.I.VE.LA. National Meeting Roma, 6-7 novembre 2006 * Marie Curie EST fellow

XIV A.I.VE.LA. National Meeting Roma, 6-7 November 2006 OutlineOutline 1. 1.Introduction 2. 2.Experimental Setup 3. 3.Results 1. 1.General 2. 2.Vertical Plane PIV Measurements 3. 3.Horizontal Plane PIV Measurements 4. 4.Summary and Conclusion 1. 1.Introduction 2. 2.Experimental Setup 3. 3.Results 1. 1.General 2. 2.Vertical Plane PIV Measurements 3. 3.Horizontal Plane PIV Measurements 4. 4.Summary and Conclusion

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Introduction   Turbulent boundary layer measurements on rough walls using PIV   2 different rough walls tested   Aim: Influence of roughness on coherent structures with reference to a smooth wall turbulent boundary layer   Turbulent boundary layer measurements on rough walls using PIV   2 different rough walls tested   Aim: Influence of roughness on coherent structures with reference to a smooth wall turbulent boundary layer

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Experimental Setup PIV Setup   Double pulsed Nd: YAG, Q–switched laser, with 200 mJ of energy per pulse (5-6 ns pulse duration)   1280 x 1024 pixel CCD Camera   DANTEC system hub   Software: DANTEC Flow Manager   Double pulsed Nd: YAG, Q–switched laser, with 200 mJ of energy per pulse (5-6 ns pulse duration)   1280 x 1024 pixel CCD Camera   DANTEC system hub   Software: DANTEC Flow Manager CCD Camera Laser System Hub Flow Manager (PC)

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Experimental Setup Flow Setup   Closed loop open-flow water tunnel with 350 x 500 x 1800 mm 3 test section   Mean freestream velocity 0.3 m/s (Re θ ≈ 1900)   Silicon Carbide Particles with 2 μm nominal diameter   PIV image size:   Closed loop open-flow water tunnel with 350 x 500 x 1800 mm 3 test section   Mean freestream velocity 0.3 m/s (Re θ ≈ 1900)   Silicon Carbide Particles with 2 μm nominal diameter   PIV image size: 51mm 41mm 600 Viscous units 800 Viscous units

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Experimental Setup Measurement Setup Laser Sheet   Avoiding laser deflection (image deflection) due to surface waves by covering the water surface with Plexiglas   Time between two images was 1 ms   Avoiding laser deflection (image deflection) due to surface waves by covering the water surface with Plexiglas   Time between two images was 1 ms  1000 recordings to ensure data convergence  Measurements in vertical and horizontal plane  1000 recordings to ensure data convergence  Measurements in vertical and horizontal plane z FLOW Sand Paper Roughness Camera Plexiglas x y

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Experimental Setup Roughness   2 different rough surfaces tested Flow Direction Top View Side View 3D roughness Side ViewTop View Flow Direction 2D roughness

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results General  Mean velocity profiles:  Generic results:

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Turbulence Increased turbulence at y + >100 due to increased turbulence in the tunnel ([1] Tachie, Bergstrom, Balachandar, 2000). 2D roughness shows reduced turbulence level of u- velocity close to the wall ([2] Jimenez, 2004). Due to form drag.

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Vertical Plane 2D Roughness  Strong shear layer separates low and high momentum zones  Vortex heads situated in shear layer  Vortex heads belong to a package  Strong shear layer separates low and high momentum zones  Vortex heads situated in shear layer  Vortex heads belong to a package FLOW 3D Roughness FLOW Smooth wall Wall

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Vertical Plane Results of Spatial Correlation with streamwise fluctuating velocity, y ref =0.2δ: 3D Roughness 2D Roughness Increased inclination of vortex heads in a vortex package especially for 3D roughness ∆y+ ∆x+ Smooth wall

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Vertical Plane Probability of a measurement point to be involved in a clockwise swirling motion Λ ci < 0 : Probability of a measurement point to be involved in a clockwise swirling motion Λ ci < 0 :  Comparable values of Pω for y + > 100  Higher peak values for rough walls  Peak value for 3D roughness shifted farer away from the wall  Comparable values of Pω for y + > 100  Higher peak values for rough walls  Peak value for 3D roughness shifted farer away from the wall  Smooth wall swirling motions appear to be weaker  2D roughness produces strongest clockwise vortices  Smooth wall swirling motions appear to be weaker  2D roughness produces strongest clockwise vortices

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Vertical Plane Positive vortices – 2 explications: Positive vortices induced by negative vortices: Positive vortices are a part of hairpin-like vortices:

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Vertical Plane Correlations Λ r ci \ Λ p ci, y ref =0.2 δ: Results of a study by Natrajan, Wu and Christensen for smooth wall (TAM report 2006): Θ 1 = 65º, Θ 2 = 230º, d 1 =135, d 2 = Similar results for 3D roughness as for smooth wall Increased Θ 2 for 2D roughness

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Results Horizontal Plane 3D Roughness ( y+ = 50 ) 2D Roughness ( y+ = 50 ) FLOW

XIV A.I.VE.LA. National Meeting Roma, 6-7 November Summary and Conclusion Skin friction with 2D roughness higher than with 3D roughness in spite of smaller roughness height Skin friction with 2D roughness higher than with 3D roughness in spite of smaller roughness height Essentially the same flow structure characteristics observed like for smooth wall Essentially the same flow structure characteristics observed like for smooth wall Higher inclination of vortex packets Higher inclination of vortex packets Position of negative vortices with respect to positive vortices similar Position of negative vortices with respect to positive vortices similar Increased low speed streak spacing with roughness Increased low speed streak spacing with roughness Vortex strength higher especially for the 2D case Vortex strength higher especially for the 2D case Pyramids create “attached” vortices Pyramids create “attached” vortices

XIV A.I.VE.LA. National Meeting Roma, 6-7 November 2006 ReferencesReferences [1] Jimenez J. Turbulent flow over rough walls. Annu. Rev. Fluid Mech. 36, pp , [2] Tachie, Bergstrom, Balachandar. Rough wall boundary layers in shallow open channel flow. J. of Fluids Engineering, Vol. 122, [2] Tachie, Bergstrom, Balachandar. Rough wall boundary layers in shallow open channel flow. J. of Fluids Engineering, Vol. 122, [3] Natrajan, Wu, Christensen. Spatial signatures of retrograde spanwise vortices in wall turbulence. TAM report [3] Natrajan, Wu, Christensen. Spatial signatures of retrograde spanwise vortices in wall turbulence. TAM report 2006.