The Limitations of Using “Ra” to Describe Surface Roughness

2016 ◽  
Vol 138 (10) ◽  
Author(s):  
Martin N. Goodhand ◽  
Karl Walton ◽  
Liam Blunt ◽  
Hang W. Lung ◽  
Robert J. Miller ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Skin Friction ◽  
Rough Surfaces ◽  
Leading Edge ◽  
Amplitude Parameter ◽  
Roughness Amplitude ◽  
The Relationship ◽  
Roughness Size

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centerline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely, the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ. To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested. The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.

The Limitations of “Ra” to Describe Surface Roughness

2015 ◽  
Author(s):  
Martin N. Goodhand ◽  
Karl Walton ◽  
Liam Blunt ◽  
Hang W. Lung ◽  
Robert J. Miller ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Skin Friction ◽  
Rough Surfaces ◽  
Leading Edge ◽  
Amplitude Parameter ◽  
Roughness Amplitude ◽  
The Relationship ◽  
Roughness Size

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centreline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ. To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case, was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested. The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.

A Critical Assessment of Reynolds Analogy for Turbine Flows

2005 ◽  
Vol 127 (5) ◽  
pp. 472-485 ◽  
Author(s):  
J. Bons
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Skin Friction ◽  
Adverse Pressure Gradient ◽  
Design Parameters ◽  
Freestream Turbulence ◽  
Reynolds Analogy ◽  
Pressure Drag

The application of Reynolds analogy 2St/cf≅1 for turbine flows is critically evaluated using experimental data collected in a low-speed wind tunnel. Independent measurements of St and cf over a wide variety of test conditions permit assessments of the variation of the Reynolds analogy factor (i.e., 2St/cf) with Reynolds number, freestream pressure gradient, surface roughness, and freestream turbulence. While the factor is fairly independent of Reynolds number, it increases with positive (adverse) pressure gradient and decreases with negative (favorable) pressure gradient. This variation can be traced directly to the governing equations for momentum and energy which dictate a more direct influence of pressure gradient on wall shear than on energy (heat) transfer. Surface roughness introduces a large pressure drag component to the net skin friction measurement without a corresponding mechanism for a comparable increase in heat transfer. Accordingly, the Reynolds analogy factor decreases dramatically with surface roughness (by as much as 50% as roughness elements become more prominent). Freestream turbulence has the opposite effect of increasing heat transfer more than skin friction, thus the Reynolds analogy factor increases with turbulence level (by up to 35% at a level of 11% freestream turbulence). Physical mechanisms responsible for the observed variations are offered in each case. Finally, synergies resulting from the combinations of pressure gradient and freestream turbulence with surface roughness are evaluated. With this added insight, the Reynolds analogy remains a useful tool for qualitative assessments of complex turbine flows where both heat load management and aerodynamic efficiency are critical design parameters.

Separate and Combined Effects of Surface Roughness and Turbulence Intensity on Vane Heat Transfer

1997 ◽  
Author(s):  
Ronald S. Bunker
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Turbulence Intensity ◽  
Pressure Ratio ◽  
Development Program ◽  
Leading Edge ◽  
Thin Foil ◽  
Combined Effects ◽  
Average Roughness

A research and development program has been undertaken to ascertain the effects of surface roughness levels on vane external heat transfer, with varied conditions of inlet freestream turbulence intensity and vane Reynolds number. A transonic linear vane cascade was constructed to operate at a nominal overall pressure ratio of 1.86. Airfoil heat transfer distributions are measured using a thin-walled stainless steel airfoil having imbedded thermocouples. The methodology incorporates a thin-foil surface heater to provide a known heat flux condition, with room temperature mainstream air at approximately 5 atm pressure. Heat transfer is characterized for uniform surface average roughness levels of 0.4, 1.85, and 4.5 micrometers, with inlet turbulence intensity levels from 4 to 13%. Airfoil Reynolds number based on axial chord length and exit velocity ranges from 2.2 to 4.8 · 106. Results show consistent individual and combined effects of Reynolds number, turbulence, and surface roughness changes. Higher roughness levels tend to dominate turbulence effects in most regions except the leading edge.

scholarly journals Simulation of synthetic gecko arrays shearing on rough surfaces

2014 ◽  
Vol 11 (95) ◽  
pp. 20140021 ◽  
Author(s):  
Andrew G. Gillies ◽  
Ronald S. Fearing
Keyword(s):  
Surface Roughness ◽  
Rough Surfaces ◽  
Adhesive System ◽  
Single Fibre ◽  
Order Of Magnitude ◽  
Synthetic Adhesives ◽  
Contact Models ◽  
Nanoscale Features ◽  
The Relationship

To better understand the role of surface roughness and tip geometry in the adhesion of gecko synthetic adhesives, a model is developed that attempts to uncover the relationship between surface feature size and the adhesive terminal feature shape. This model is the first to predict the adhesive behaviour of a plurality of hairs acting in shear on simulated rough surfaces using analytically derived contact models. The models showed that the nanoscale geometry of the tip shape alters the macroscale adhesion of the array of fibres by nearly an order of magnitude, and that on sinusoidal surfaces with amplitudes much larger than the nanoscale features, spatula-shaped features can increase adhesive forces by 2.5 times on smooth surfaces and 10 times on rough surfaces. Interestingly, the summation of the fibres acting in concert shows behaviour much more complex that what could be predicted with the pull-off model of a single fibre. Both the Johnson–Kendall–Roberts and Kendall peel models can explain the experimentally observed frictional adhesion effect previously described in the literature. Similar to experimental results recently reported on the macroscale features of the gecko adhesive system, adhesion drops dramatically when surface roughness exceeds the size and spacing of the adhesive fibrillar features.

Effect of Surface Roughness Location and Reynolds Number on Compressor Cascade Performance

2010 ◽  
Author(s):  
Seung Chul Back ◽  
Garth V. Hobson ◽  
Seung Jin Song ◽  
Knox T. Millsaps
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Leading Edge ◽  
Suction Side ◽  
Pressure Side ◽  
Compressor Cascade ◽  
Suction Surface ◽  
Trailing Edge ◽  
Blade Loading ◽  
Linear Compressor Cascade

An experimental investigation has been conducted to characterize the influence of surface roughness location and Reynolds number on compressor cascade performance. Flow field surveys have been conducted in a low-speed, linear compressor cascade. Pressure, velocity, and flow angles have been measured via a 5-hole probe, pitot probe, and pressure taps on the blades. In addition to the entirely smooth and entirely rough blade cases, blades with roughness covering the leading edge; pressure side; and 5%, 20%, 35%, 50%, and 100% of suction side from the leading edge have been studied. All of the tests have been done for Reynolds number ranging from 300,000 to 640,000.Cascade performance (i.e. blade loading, loss, and deviation) is more sensitive to roughness on the suction side than pressure side. Roughness near the trailing edge of suction side increases loss more than that near the leading edge. When the suction side roughness is located closer to the trailing edge, the deviation and loss increase more rapidly with Reynolds number. For a given roughness location, there exists a Reynolds number at which loss begins to visibly increase. Finally, increasing the area of rough suction surface from the leading edge reduces the Reynolds number at which the loss coefficient begins to increase.

Aeroelastic Dynamics of a NACA 0012 Airfoil in the Transitional Reynolds Number Regime

2006 ◽  
Author(s):  
Dominique Poirel ◽  
Yael Harris ◽  
Aze´mi Benaissa
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Degrees Of Freedom ◽  
Initial Conditions ◽  
Reynolds Numbers ◽  
Flow Structures ◽  
Hot Wire ◽  
Initial Work ◽  
Naca 0012 ◽  
The Relationship

The work discussed herein is a focused extension of a series of studies that were carried out at the Aeroelasticity Laboratory of the Royal Military College of Canada in recent years. Initial work revealed the presence of self-excited oscillations over certain ranges of airspeed when a NACA 0012 airfoil was immersed in the laboratory’s wind tunnel and allowed to oscillate freely in both pitch and heave. The range of airspeeds tested corresponded to Reynolds numbers in the low-to-moderate regime. While the aeroelastic apparatus is capable of two-degrees-of-freedom motion, the present work concerns only the motion of the airfoil when it is constrained to rotate in pure pitch. A parametric investigation is presently being undertaken to more fully comprehend the airfoil’s pitch behaviour, specifically the amplitude and frequency of its oscillations which are observed in the following range of chord based Reynolds numbers: 5.0 × 104 ≤ Rec ≤ 1.2 × 105. This paper focuses on the effect of the stiffness of the springs used in the apparatus. Other parameters such as surface roughness, turbulence intensity, temperature and initial conditions are also briefly discussed. In conjunction with the pitch oscillation measurements, preliminary results reveal vortices to be present in the wake. In an attempt to determine the frequency and character of these flow structures, as well as to understand the relationship between the airfoil motion and wake dynamics, hot-wire anemometry measurements have been performed.

Predicting Skin Friction for Turbulent Flow Over Randomly-Rough Surfaces Using the Discrete-Element Method: Part II — Skin Friction Validation

2003 ◽  
Author(s):  
Stephen T. McClain ◽  
B. Keith Hodge ◽  
Jeffrey P. Bons
Keyword(s):  
Surface Roughness ◽  
Wind Tunnel ◽  
Discrete Element Method ◽  
Turbulent Flow ◽  
Skin Friction ◽  
Rough Surfaces ◽  
Roughness Element ◽  
Discrete Element ◽  
Randomly Rough Surface ◽  
Element Method

The discrete-element method for predicting skin friction for turbulent flow over rough surfaces considers the drag on the surface to be the sum of the skin friction on the flat part of the surface and the drag on the individual roughness elements that protrude into the boundary layer. The discrete-element method has been widely used and validated for roughness composed of sparse, ordered, and deterministic elements. This paper extends the validation of the discrete-element to include real (random and closely packed) surface roughness. To analyze flow over a randomly-rough surface using the discrete-element method, the roughness element blockage fraction and the roughness element cross-section area distributions as a function of height must be determined from surface profilometer measurements. The technique developed for determining these distributions was described in Part 1. This paper, Part 2, describes the modifications that were made to the discrete-element roughness method to extend the validation to real surface roughness. These modifications include accounting for the deviation of the roughness element cross sections from circular configurations and the determination of the location of the computational “surface,” that differs from the physical surface. Two randomly-rough surfaces, two analog surfaces were generated using a three-dimensional printer for wind-tunnel testing. The analog surfaces were created by replacing each random roughness element from the original randomly-rough surface with an elliptical roughness element with the equivalent plan area and eccentricity. The results of the wind tunnel skin friction measurements and the discrete-element method predictions for each of the six surfaces are presented and discussed. For each randomly-rough and analog surface studied, the discrete-element method predictions are within 7% of the experimentally measured skin friction coefficients.

Effects of Reynolds Number and Surface Roughness Magnitude and Location on Compressor Cascade Performance

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Seung Chul Back ◽  
Garth V. Hobson ◽  
Seung Jin Song ◽  
Knox T. Millsaps
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Leading Edge ◽  
Side Surface ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Pressure Side ◽  
Compressor Cascade ◽  
Suction Surface ◽  
Blade Loading

An experimental investigation has been conducted to characterize the influence of Reynolds number and surface roughness magnitude and location on compressor cascade performance. Flow field surveys have been conducted in a low-speed, linear compressor cascade. Pressure, velocity, and loss have been measured via a five-hole probe, pitot probe, and pressure taps on the blades. Four different roughness magnitudes, Ra values of 0.38 μm (polished), 1.70 μm (baseline), 2.03 μm (rough 1), and 2.89 μm (rough 2), have been tested. Furthermore, various roughness locations have been examined. In addition to the as manufactured (baseline) and entirely rough blade cases, blades with roughness covering the leading edge, pressure side, and 5%, 20%, 35%, 50%, and 100% of suction side from the leading edge have been studied. All of the tests have been carried out for Reynolds numbers ranging from 300,000 to 640,000. For Reynolds numbers under 500,000, the tested roughnesses do not significantly degrade compressor blade loading or loss. However, loss and blade loading become sensitive to roughness at Reynolds numbers above 550,000. Cascade performance is more sensitive to roughness on the suction side than pressure side. Furthermore, roughness on the aft 2/3 of suction side surface has a greater influence on loss. For a given roughness location, there exists a Reynolds number at which loss begins to significantly increase. Finally, increasing the roughness area on the suction surface from the leading edge reduces the Reynolds number at which the loss begins to increase.

Alternate Scales for Turbulent Boundary Layer on Transitional Rough Walls: Universal Log Laws

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Noor Afzal
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Velocity Profiles ◽  
Pressure Gradients ◽  
Wall Roughness ◽  
Extensive Experimental Data

The present work deals with four new alternate transitional surface roughness scales for description of the turbulent boundary layer. The nondimensional roughness scale ϕ is associated with the transitional roughness wall inner variable ζ=Z+∕ϕ, the roughness friction Reynolds number Rϕ=Rτ∕ϕ, and the roughness Reynolds number Reϕ=Re∕ϕ. The two layer theory for turbulent boundary layers in the variables, mentioned above, is presented by method of matched asymptotic expansions for large Reynolds numbers. The matching in the overlap region is carried out by the Izakson–Millikan–Kolmogorov hypothesis, which gives the velocity profiles and skin friction universal log laws, explicitly independent of surface roughness, having the same constants as the smooth wall case. In these alternate variables, just above the wall roughness level, the mean velocity and Reynolds stresses are universal and do not depend on surface roughness. The extensive experimental data provide very good support to our universal relations. There is no universality of scalings in traditional variables and different expressions are needed for inflectional type roughness, monotonic Colebrook–Moody roughness, k-type roughness, d-type roughness, etc. In traditional variables, the velocity profile and skin friction predictions for the inflectional roughness, k-type roughness, and d-type roughness are supported well by the extensive experimental data. The pressure gradient effect from the matching conditions in the overlap region leads to the universal composite laws, which for weaker pressure gradients yields log laws and for strong adverse pressure gradients provides the half-power laws for universal velocity profiles and in traditional variables the additive terms in the two situations depend on the wall roughness.

A new measure of roughness for defining the aerodynamic performance of sports balls

2007 ◽  
Vol 221 (7) ◽  
pp. 789-806 ◽  
Author(s):  
S J Haake ◽  
S R Goodwill ◽  
M J Carre
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Skin Friction ◽  
Aerodynamic Performance ◽  
Complete Characterization ◽  
Surface Asperity ◽  
Statistical Measures ◽  
Tennis Balls ◽  
Rate Decay

A new analysis is presented of the major findings in sports ball aerodynamics over the last 20 years, leading to a new method for defining surface roughness and its effects on the aerodynamic performance of sports balls. It was shown that the performance of balls in soccer, tennis, and golf are characterized by the position of the separation points on the surface of the ball, and that these are directly influenced by the roughness of the surface at a given Reynolds number and spin rate. The traditional measure of roughness k/D (the ratio of surface asperity dimension to diameter) was unable to predict the transition from laminar to turbulent flow for different sports balls. However, statistical measures of roughness commonly used in tribology were found to correlate well with the Reynolds number at transition and the minimum Cd after transition. It was concluded that this new measure and a further one of dimension should allow the complete characterization of the aerodynamic performance of sports balls. The effects of surface roughness on spin rate decay were also considered, and it was found that tennis balls had spin decay over six times that of golf balls and was due to the increased skin friction of the nap.

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