Abstract
In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder - coated with a rough, hydrophobic material - is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction present in the working fluid. For small volume fractions (<![CDATA[$\unicode[STIX]{x1D6FC}), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air , the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at play: on the one hand, the roughness invokes an extension of the log layer - resulting in an increase in drag - and, on the other hand, there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.
| Original language | English |
|---|---|
| Article number | A61 |
| Journal | Journal of fluid mechanics |
| Volume | 883 |
| Early online date | 25 Nov 2019 |
| DOIs | |
| Publication status | Published - 25 Jan 2020 |
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Keywords
- UT-Hybrid-D
- drag reduction
- Taylor-Couette flow
- coating
Cite this
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Bubbly drag reduction using a hydrophobic inner cylinder in Taylor-Couette turbulence. / Bullee, Pim A.; Verschoof, Ruben A.; Bakhuis, Dennis; Huisman, Sander G.; Sun, Chao; Lammertink, Rob G.H.; Lohse, Detlef.
In: Journal of fluid mechanics, Vol. 883, A61, 25.01.2020.Research output: Contribution to journal › Article › Academic › peer-review
TY - JOUR
T1 - Bubbly drag reduction using a hydrophobic inner cylinder in Taylor-Couette turbulence
AU - Bullee, Pim A.
AU - Verschoof, Ruben A.
AU - Bakhuis, Dennis
AU - Huisman, Sander G.
AU - Sun, Chao
AU - Lammertink, Rob G.H.
AU - Lohse, Detlef
N1 - CUP deal
PY - 2020/1/25
Y1 - 2020/1/25
N2 - In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder - coated with a rough, hydrophobic material - is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction present in the working fluid. For small volume fractions (<![CDATA[$\unicode[STIX]{x1D6FC}), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air , the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at play: on the one hand, the roughness invokes an extension of the log layer - resulting in an increase in drag - and, on the other hand, there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.
AB - In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder - coated with a rough, hydrophobic material - is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction present in the working fluid. For small volume fractions (<![CDATA[$\unicode[STIX]{x1D6FC}), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air , the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at play: on the one hand, the roughness invokes an extension of the log layer - resulting in an increase in drag - and, on the other hand, there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.
KW - UT-Hybrid-D
KW - drag reduction
KW - Taylor-Couette flow
KW - coating
UR - http://www.scopus.com/inward/record.url?scp=85075783477&partnerID=8YFLogxK
U2 - 10.1017/jfm.2019.894
DO - 10.1017/jfm.2019.894
M3 - Article
AN - SCOPUS:85075783477
VL - 883
JO - Journal of fluid mechanics
JF - Journal of fluid mechanics
SN - 0022-1120
M1 - A61
ER -