Influence of wall roughness on Influence of wall roughness on near wall turbulence structurenear wall turbulence structure

bybyHaigermoser C.*Haigermoser C.*, Vesely L.*, La Polla M., , Vesely L.*, La Polla M.,

Onorato M., Onorato M., Politecnico di TorinoPolitecnico di Torino

XIV A.I.VE.LA. National Meeting XIV A.I.VE.LA. National Meeting Roma, 6-7 novembre 2006Roma, 6-7 novembre 2006

Influence of wall roughness on Influence of wall roughness on near wall turbulence structurenear wall turbulence structure

bybyHaigermoser C.*Haigermoser C.*, Vesely L.*, La Polla M., , Vesely L.*, La Polla M.,

Onorato M., Onorato M., Politecnico di TorinoPolitecnico di Torino

XIV A.I.VE.LA. National Meeting XIV A.I.VE.LA. National Meeting Roma, 6-7 novembre 2006Roma, 6-7 novembre 2006

* Marie Curie EST fellow

Roma, 6-7 November 2006Roma, 6-7 November 2006XIV A.I.VE.LA. National Meeting XIV A.I.VE.LA. National Meeting

OutlineOutlineOutlineOutline

1. Introduction2. Experimental Setup3. Results

1. General2. Vertical Plane PIV Measurements3. Horizontal Plane PIV Measurements

4. Summary and Conclusion

1. Introduction2. Experimental Setup3. Results

1. General2. Vertical Plane PIV Measurements3. Horizontal Plane PIV Measurements

4. Summary and Conclusion

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1. Introduction1. Introduction1. Introduction1. 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

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2. Experimental Setup2. Experimental SetupPIV SetupPIV Setup

2. Experimental Setup2. Experimental SetupPIV SetupPIV 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 CameraCCD Camera

LaserLaser

System HubSystem Hub Flow Manager (PC)Flow Manager (PC)

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2. Experimental Setup2. Experimental SetupFlow SetupFlow Setup

2. Experimental Setup2. Experimental SetupFlow SetupFlow Setup

Closed loop open-flow water tunnel with 350 x 500 x 1800 mm3 test section

Mean freestream velocity 0.3 m/s (Reθ ≈ 1900) Silicon Carbide Particles with 2 μm nominal

diameter PIV image size:

Closed loop open-flow water tunnel with 350 x 500 x 1800 mm3 test section

Mean freestream velocity 0.3 m/s (Reθ ≈ 1900) Silicon Carbide Particles with 2 μm nominal

diameter PIV image size:

0 5 10 15 20 25 30 35 40 45mm

0

5

10

15

20

25

30

35

mm

51mm

41mm60

0 V

isco

us

unit

s

800 Viscous units

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2. Experimental Setup2. Experimental SetupMeasurement SetupMeasurement Setup

2. Experimental Setup2. Experimental SetupMeasurement SetupMeasurement 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

xy

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2. Experimental Setup2. Experimental SetupRoughnessRoughness

2. Experimental Setup2. Experimental SetupRoughnessRoughness

2 different rough surfaces tested 2 different rough surfaces tested

FlowDirection

5

6 1

.7

Top View Side View

3D roughness

3.5

1.3

Side ViewTop View

FlowDirection

2D roughness

Roma, 6-7 November 2006Roma, 6-7 November 2006XIV A.I.VE.LA. National Meeting XIV A.I.VE.LA. National Meeting

4. Results4. ResultsGeneralGeneral

4. Results4. ResultsGeneralGeneral

0

5

10

15

20

25

1 10 100 1000 10000

y+

U+

Smooth2D Roughness3D Roughness

Mean velocity profiles:

Mean velocity profiles:

Generic results: Generic results:

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4. Results4. ResultsTurbulenceTurbulence4. Results4. Results

TurbulenceTurbulence

0

1

2

3

4

5

6

7

8

9

10

1 10 100 1000y+

U'2

+ ,

V'2+

Smooth Wall (DeGraff) V'̂ 2+ 2D Roughness U'̂ 2+ Smooth (DeGraff) U'̂ 2+ 2D Roughness V'̂ 2+

Smooth U'̂ 2+ Smooth V'̂ 2+ 3D Roughness U'̂ 2+ 3D Roughness v'̂ 2+

•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.

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4. Results4. ResultsVertical PlaneVertical Plane

4. Results4. ResultsVertical PlaneVertical Plane

2D Roughne

ss

2D Roughne

ss

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 Roughne

ss

3D Roughne

ss

FLOWFLOW

Smooth wallSmooth wall Wall

Wall

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4. Results4. ResultsVertical PlaneVertical Plane

4. Results4. ResultsVertical PlaneVertical Plane

Results of Spatial Correlation with streamwise fluctuating velocity, yref=0.2δ:

Results of Spatial Correlation with streamwise fluctuating velocity, yref=0.2δ:

3D Roughness3D Roughness 2D Roughness2D Roughness

Increased inclination of vortex heads in a vortex package especially for 3D roughnessIncreased inclination of vortex heads in a vortex package especially for 3D roughness

+

+

+

+

∆y+

∆x+

Smooth wallSmooth wall

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4. Results4. ResultsVertical PlaneVertical Plane

4. Results4. ResultsVertical PlaneVertical Plane

Probability of a measurement point to be involved in a clockwise swirling motion Λci < 0 :

Probability of a measurement point to be involved in a clockwise swirling motion Λci < 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

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4. Results4. ResultsVertical PlaneVertical Plane

4. Results4. ResultsVertical PlaneVertical Plane

Positive vortices – 2 explications:Positive vortices – 2 explications:

Positive vortices induced by negative vortices:

Positive vortices are a part of hairpin-like vortices:

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4. Results4. ResultsVertical PlaneVertical Plane

4. Results4. ResultsVertical PlaneVertical Plane

Correlations Λrci\ Λp

ci, yref=0.2δ:Correlations Λrci\ Λp

ci, yref=0.2δ:

• Results of a study by Natrajan, Wu and Christensen for smooth wall (TAM report 2006): Θ1= 65º, Θ2= 230º, d1=135, d2=103.5.

• Similar results for 3D roughness as for smooth wall• Increased Θ2 for 2D roughness

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4. Results4. ResultsHorizontal PlaneHorizontal Plane

4. Results4. ResultsHorizontal PlaneHorizontal Plane

3D Roughness (y+

= 50)3D Roughness (y+

= 50)2D Roughness (y+

= 50)2D Roughness (y+

= 50)

FLOW FLOW

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5. Summary and Conclusion5. Summary and Conclusion5. Summary and Conclusion5. Summary and Conclusion

Skin friction with 2D roughness higher than Skin friction with 2D roughness higher than with 3D roughness in spite of smaller with 3D roughness in spite of smaller roughness heightroughness height

Essentially the same flow structure Essentially the same flow structure characteristics observed like for smooth wallcharacteristics observed like for smooth wall

Higher inclination of vortex packets Higher inclination of vortex packets Position of negative vortices with respect to Position of negative vortices with respect to

positive vortices similar positive vortices similar Increased low speed streak spacing with Increased low speed streak spacing with

roughnessroughness Vortex strength higher especially for the 2D Vortex strength higher especially for the 2D

casecase Pyramids create “attached” vorticesPyramids create “attached” vortices

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ReferencesReferencesReferencesReferences

[1] Jimenez J. Turbulent flow over rough walls. Annu. Rev. Fluid Mech. 36, pp. 173-196, 2004.

[2] Tachie, Bergstrom, Balachandar. [2] Tachie, Bergstrom, Balachandar. Rough wall boundary layers in Rough wall boundary layers in shallow open channel flowshallow open channel flow. J. of Fluids Engineering, Vol. 122, 2000.. J. of Fluids Engineering, Vol. 122, 2000.

[3] Natrajan, Wu, Christensen. [3] Natrajan, Wu, Christensen. Spatial signatures of retrograde Spatial signatures of retrograde spanwise vortices in wall turbulence.spanwise vortices in wall turbulence. TAM report 2006. TAM report 2006.