Abstract
Three methods have been used to experimentally determine the roughness function (ΔU +) for several rough surfaces. These include the rotating disk, the towed plate, and the velocity profile methods. The first two are indirect methods in as much as they rely on measurements of overall torque or resistance and boundary layer similarity laws to obtain ΔU +, whereas the velocity profile method provides a direct measurement of ΔU +. The present results indicate good agreement between the towed plate and the velocity profile methods for all of the surfaces tested. Tests for the rotating disk were carried out at much higher unit Reynolds numbers. Using this method, the results for sandpaper rough surfaces agree within their uncertainty with a Nikuradse-type roughness function in the fully rough regime, while a spray painted surface agrees with a Colebrook-type roughness function.
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Abbreviations
- B :
-
smooth wall log-law intercept, =5.0
- C F :
-
overall frictional resistance coefficient, \( = {{\left( {F_{{\text{D}}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {F_{{\text{D}}} } \right)}} {{\left( {\tfrac{1} {2}\rho U_{{\text{e}}} ^{2} S} \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {\tfrac{1} {2}\rho U_{{\text{e}}} ^{2} S} \right)}} \)
- C f :
-
skin-friction coefficient, \( = {{\left( {\tau _{{\text{o}}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {\tau _{{\text{o}}} } \right)}} {{\left( {\tfrac{1} {2}\rho U_{{\text{e}}} ^{2} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {\tfrac{1} {2}\rho U_{{\text{e}}} ^{2} } \right)}} \)
- C m :
-
torque coefficient, =(2M)/(ρR 5(φω)2)
- F D :
-
drag force
- k :
-
arbitrary measure of roughness height
- K :
-
acceleration parameter, \( = \frac{\nu } {{U^{2}_{{\text{e}}} }}\frac{{{\text{d}}U_{{\text{e}}} }} {{{\text{d}}x}} \)
- L :
-
plate length
- M :
-
torque
- N :
-
number of samples or replicates
- R :
-
disk radius
- R a :
-
centerline average roughness height, \( = \frac{1} {N}{\sum\limits_{i = 1}^N {{\left| {y_{i} } \right|}} } \)
- R q :
-
root mean square roughness height, \( = {\sqrt {\frac{1} {N}{\sum\limits_{i = 1}^N {y^{2}_{i} } }} } \)
- R t :
-
maximum peak to through height, =y max−y min
- R z :
-
ten point roughness height, \( = {\sum\limits_{i = 1}^5 {{\left( {y_{{\max i}} - y_{{\min i}} } \right)}} } \)
- Re L :
-
Reynolds number based on plate length, =U e L/ν
- Re R :
-
Reynolds number based on disk radius, =φωR 2/ν
- Re θ :
-
momentum thickness Reynolds number, =U e θ/ν
- S :
-
wetted surface area
- U :
-
mean velocity in the x direction
- U e :
-
freestream velocity relative to surface
- U τ :
-
friction velocity, \( = {\sqrt {{\tau _{{\text{o}}} } \mathord{\left/ {\vphantom {{\tau _{{\text{o}}} } \rho }} \right. \kern-\nulldelimiterspace} \rho } } \)
- u′:
-
streamwise fluctuating velocity
- ν′:
-
wall-normal fluctuating velocity
- x :
-
streamwise distance from plate leading edge
- y :
-
normal distance from the boundary
- δ :
-
boundary layer thickness
- ΔU + :
-
roughness function
- ΔU +′:
-
roughness function slope, =d(ΔU +)/d(lnk +)
- ε :
-
wall datum offset
- φ :
-
swirl factor, \( = {{\left( {\frac{1} {{{\sqrt {C_{{\text{m}}} } }}}} \right)}_{\infty } } \mathord{\left/ {\vphantom {{{\left( {\frac{1} {{{\sqrt {C_{{\text{m}}} } }}}} \right)}_{\infty } } {{\left( {\frac{1} {{{\sqrt {C_{{\text{m}}} } }}}} \right)}_{{{\text{en}}}} }}} \right. \kern-\nulldelimiterspace} {{\left( {\frac{1} {{{\sqrt {C_{{\text{m}}} } }}}} \right)}_{{{\text{en}}}} } \)
- κ :
-
von Kármán constant, =0.41
- ν :
-
kinematic viscosity of the fluid
- Π :
-
wake parameter
- θ :
-
momentum thickness, \( = {\int\limits_0^\delta {{\left( {1 - \frac{U} {{U_{{\text{e}}} }}} \right)}^{2} {\text{d}}y} } \)
- ρ :
-
density of the fluid
- τ o :
-
wall shear stress
- ω :
-
angular velocity
- +:
-
inner variable (normalized with U τ or U τ /ν)
- en:
-
disk rotating in enclosed tank
- min:
-
minimum value
- max:
-
maximum value
- R:
-
rough surface
- S:
-
smooth surface
- ∞:
-
disk rotating in unconfined fluid
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Acknowledgements
MPS would like to thank the Office of Naval Research for financial support of this research under the direction of Dr. Steve McElvaney. The authors would also like to acknowledge the ongoing financial support of the disk drag measurements from Mr. William Stoffel (NSWC-Carderock, Code 981, Shipboard R&D Office) and Dr. Alan Roberts (CNO, N420, Energy Plans & Policy). Thanks, as well, to Dr. Eric Holm, Ms. Elizabeth Haslbeck, and Ms. Jean Montemarano from NSWC-Carderock (Code 641) for allowing us to use the rotating disk facility and for providing technical guidance. Many thanks go to Mr. Steve Enzinger, Mr. Don Bunker, and the USNA Hydromechanics Laboratory and Technical Support Division staff for assisting with the project. We are also indebted to Prof. Michelle Koul for helping with the laser profilometry.
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Schultz, M.P., Myers, A. Comparison of three roughness function determination methods. Exp Fluids 35, 372–379 (2003). https://doi.org/10.1007/s00348-003-0686-x
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DOI: https://doi.org/10.1007/s00348-003-0686-x