Effect of abrasion and biofouling on aerodynamic performanceof industrial surface coatings

F. Gökhan Ergin
Dantec Dynamics, Skovlunde, Denmark, 2740
gokhan.ergin@dantecdynamics.com

performance via two major mechanisms; fouling anderosion. Surface fouling is caused by external material(e.g. dust/oil, ice, biofouling such as insects, barnacles andalgae) building up on the surface, changing the roughnesslevels, hence reducing performance. Surface erosion (wearand corrosion) is often due to weathering including UVdegradation, abrasion and friction cause surface erosion of
the coating, leading to loss of material over time, hence aloss of performance associated with loss of active
component and change in surface morphology.
Nanomaterial-based paints are interesting as they canboth achieve desired roughness levels, and are able tomodify the surface energy of the underlying
material. Controlling the transition location in boundary
layers offers considerable advantages in numerous
industrial applications.
Using nanomaterial-based
coatings, the flow-surface interaction can be engineered todelay the transition of flow to turbulence, and hence
reduce flow forces, and save costs for structures such as
aerofoils, winglets, wings and wind turbine blades.
The NATURAL project is aimed to develop methods
that allow the evaluation of surfaces at the nanoscale andcorrelate the measured surface structure with aerodynamic
performance. The testing technique involves thedevelopment of an understanding of the relationshipbetween physical properties of fluid flow at the surface,surface roughness morphology and surface energy (Workpackages 4 and 5). This measurement method
provides, for the first time, a technique that will allow
these novel coatings to be evaluated in a manner thatallows nanometer-scale characteristics to be correlatedwith their effects on the flow behaviour and aerodynamic
efficiency.
To achieve the objective of NATURAL Work
Packages 4 and 5, the “yellow” wind tunnel in the theDepartment of Mechanical Engineering at the Technical
University of Denmark (DTU) was modified to makeboundary-layer profile measurements over a variety ofsurface samples. Once these modifications were made, 11different fibreglass samples were fabricated, coated andsubjected to typical wear patterns. Six of these plates aretested in pristine condition, (without abrasion or
biofouling), two were biofouled in a special
environmental chamber, and three were subject toabrasion using sandblasting. One of the pristine plates(FG-P) was not coated. This plate was used achieve the

ABSTRACT
Effect of abrasion and biofouling on aerodynamics
performance of industrial surface coatings is investigatedexperimentally. The test section of the DTU closed-loop
wind tunnel is refurbished to perform automated boundary
layer stability experiments. The modifications satisfy themeasurement requirements imposed by NATURAL EU
project work packages 4 and 5. The design features
include a sliding test section front window, a computercontrolled three-axis traverse, angle of attack adjustment,
zero-pressure-gradient alignment, quick replacement of
test plates and repeatable alignment between plates.
Boundary layer profile measurements were performed onpristine, abraded and biofouled flat plates using a constanttemperature anemometry system. The anemometers were
controlled using state-of-the-art electronics, a computercontrolled hotwire calibrator and LabVIEW. Both laminarand turbulent boundary layer profiles were measured.

INTRODUCTION
The aerodynamic performance of a vehicle is closely
linked with its surface roughness. For external surfaces,roughness can lead to increased skin friction drag andreduced lift force. For instance, erosion of aircraft surfaces
during routine flight has been claimed to increase fuelconsumption and CO2 emissions by almost 5%. Similarly,
fouling of ship hulls by marine organisms is estimated toincrease fuel consumption by up to 40% [1]. Besides
increasing drag, lift can also be compromised leading tosimilar performance losses. For example for wind turbines
operating in cold climates ice accretion can cause a loss of
20-50% of electricity production; whereas for those
operating in warmer climates, fouling of the blade leadingedge by insects can reduce power production from windturbines by up to 50% [2]A variety of surface coatings have been considered toprevent the accumulation of surface roughness.
Conventional paints and procedures have not beensatisfactory in reducing these effects. New coatings withnanometer-sized powder may be more successful and havesignificant potential performance improvements. To be
successful in the field, any such coating must have good
durability. In time under environmental exposure,unwanted changes in the nano-structure often reduces1baseline flow condition: a zero-pressure-gradient (ZPG)
laminar boundary layer flow over the entire length. Thisway the flow field over remaining five pristine nanocoatings can be compared to the baseline flow condition.
Similarly, the flow field over abraded and biofouled
samples could be compared their pristine counterparts.
The overview of samples and a short description is given
in Table 1. All plates have dimensions 725mm x 294mm x10mm and a 25mm-long elliptical leading edge (Fig. 2).
Only the FG-P plate has ten Ø3mm pressure taps for ZPG
alignment.

Automated positioning
According to Ref. [3] and Ref. [4], as of 2008 there
are 14 out of 383 wind tunnels that are equipped with anautomated sensor positioning system. With the automatedpositioning system described here, the DTU Yellow windtunnel is the 15th known (other wind tunnels may havebeen equipped with such a system since 2008) wind tunnel
with this capability.
The automated positioning of sensors was achieved
using a custom-built three-axis traversing mechanism. The
final design is shown in Fig. 1a and the actual mechanism
mounted on the DTU wind tunnel is shown in Fig. 1b. The
shorter y-axis lead screw is attached on the longer x-axis
lead screw in the horizontal plane. The x-axis lead screwis attached on two vertical lead screws of the z-axis
traverse mechanism. The z-axis lead screws are driven bya single stepper motor and the power distribution isachieved by the drive shaft connecting two gearboxes, thecyan colored parts in Fig. 1a. The minimum step size ineach direction is 6.25µm with a positioning accuracy of
1µm. The hotwire support arm is attached on the y-axis of
the traverse accessing the test section through a
streamwise slit cut in the front window. The streamwise
slit on the sliding front window is sealed with during eachmeasurement in order to avoid flow through the slit (Fig.
1b).

Table 1. Overview of the pristine (P), abraded (A) andbiofouled (B) samples.
Sample
FG-P
K-P
2-P
2I-P
2O-P
2R-P
2-B
2I-B
2-A
2O-A
2R-A

Description
Fiber Glass – Pristine
Coating 1 – Pristine
Coating 2 – Pristine
Coating 2 + I nanoparticles – Pristine
Coating 2 + O nanoparticles – Pristine
Coating 2 + R nanoparticles – Pristine
Coating 2 + Biofouled
Coating 2 + I nanoparticles – Biofouled
Coating 2 – Abraded
Coating 2 + O nanoparticles – Abraded
Coating 2 + R nanoparticles – Abraded

TEST SECTION DESIGN
The wind tunnel facility used during the experiments
is the “yellow” wind tunnel belonging to the Departmentof Mechanical Engineering at the Technical University ofDenmark (DTU). The facility is a closed-loop subsonicfacility erected in 1976, with a nominal maximum flow
speed of 60 m/s and a measured background turbulence
intensity level of 0.15% at 20m/s. The contraction ratio is12.94:1, the nominal test section dimensions are 0.3 m ×
0.3 m × 0.75m, with an increasing cross sectional area
along the streamwise direction to compensate fordeveloping boundary layers. Although the wind tunnel isequipped with a cooling system for the drive fan, thetemperature of the flow cannot be controlled during theexperiments. As a result, the air temperature in the tunnelrises from approximately 22°C to 26°C during the courseof the experiments. Two interchangeable test sections areavailable which enables a quick change to a different
experimental setup. One of these test sections was
modified to meet the boundary layer measurement
requirements imposed by the NATURAL EU project work
packages 4 and 5. These requirements include (i)automated measurement of laminar, transitional andturbulent boundary layer profiles, (ii) angle of attack
adjustment from outside the test section, (iii) zeropressure-gradient alignment during tunnel operation, (iv)quick replacement of test plates and (v) maintainingalignment between plates, (vi) minimum flow disturbanceand vibrations due to probe support equipment (vii) realtime normalisation of boundary layer velocity using thefreestream velocity.

ab
Figure 1. Computer-controlled
three-axis
traversing
system (a) the design schematic (b) the actual unit.
Sealing the slit during experiments implies thatmeasurements can be taken in cross-sectional y-z planes
perpendicular to the flow direction. The flat plate
schematic, assumed coordinate system and themeasurement planes are shown in Fig. 2. Traverse
movements in the y-direction is simply through the2

opening between duct tape and through the streamwise
slit. However, in order to achieve traversing movements inthe z-direction requires a sliding front window coupledwith the z-axis lead screws. The sliding front window isplaced in a vertical rail on the test section and is pulled up
and down using a pulleys and belts (Fig. 1b) The negative
pressure inside the test section helps seal the window on
the test section frame.

closer the bottom and 5 closer to the top of the plate (Fig.
4a). Pressure readings along the plate are taken in order tosatisfy the ZPG requirement (Fig. 4b).

a
Pressure Variation-P [Pa]250

Figure 2. Flat plate schematic, assumed coordinate systemand the measurement plane.
Angle of attack adjustment
The flat plates with elliptical leading edges aremounted vertically in the test section in order to minimize
dust collection over time (Fig. 3a). Using special
removable fixtures and guide rails, the plates are mounted
on two freely rotating carousels, one on the bottom, andone on the top wall of the test section. The angularorientation of the plate with respect to the incoming flow
can be changed by rotating a micrometer screw on the topof the test section (Fig. 3b). The angle indicator provides acourse scale and the micrometer provides a finer scale for
precise angle of attack adjustments. Zero angle of attack
was used during the experiments performed in this work.

aPtop
Pbottom

125

0
0

b

125

250

375

500

625

750

X [mm]

Figure 4. (a) Back of the alignment plate during ZPG
alignment, (b) pressure variation along the plate.
Quick plate replacement and repeatablealignment
It is essential to install a new test plates in areasonable amount of time and without changing thealignment. For repeatable alignment, the plates aremounted on two guide rails using a total of sevensetscrews, fixed on the bottom and top carousels (visible
in Fig. 3a and 4a). Once the ZPG alignment is performed,
the guide rail positions are locked using four setscrews,
one at each corner of the plate. One of these setscrews(front top corner) is visible in Fig. 3b. For quick
replacement, the plates are mounted on the rails using
special removable fixtures (visible in Fig. 3a).
Minimizing flow disturbance and flow induced
vibrations
The hotwire support arm profile is streamlined tominimize the flow disturbances (Fig 5a). The leading edge
is machined in a bluff semi circular shape and the trailingedge is tapered to resemble an aerofoil shape. The hotwire
cables were placed in a groove cut along the trailing edge.
The probes could be inserted / replaced by retracting the
hotwire support arm fully out through the slit, i.e. without
having to remove the plate in the test section (Fig 5a). In
order to minimize flow induced vibrations the hotwire
probe support lengths were kept short (Fig 5b). The
boundary layer probe support has a 5deg angle withrespect to the flow direction in order to keep safety amargin between the flat surface and the tip of the supportarm.

b

Figure 3. (a) Test section view as seen by the flow, (b)angle of attack adjustment for the plate carousel and theangle indicator.
Zero pressure gradient alignment
One of the pristine flat plates (FG-P) is used for thezero pressure gradient (ZPG) alignment. This plate was
not coated and therefore has a smooth flat surface finish
that can produce a laminar boundary layer profile over theentire length for a tunnel speed of 20m/s. The alignment
plate is tapped with 10 holes for pressure readings, 53

suggested by Benjamin and Roberts [5]. Voltages werecorrected according to T  Tx 
Vc  Vm 1  c
 Tw  Tx 

0.55

(1)

where Vc is the corrected voltage, Vm is the measured
voltage, Tc is the calibration temperature, Tw is the wire
temperature and Tx is the elevated ambient temperatureduring measurements.
Experiments were conducted on 4 measurement
planes for each plate; 125mm, 250mm, 375mm and
500mm from the plate leading edge. Boundary layer
velocity was normalised with the freestream velocity inorder to minimize the effect of freestream speed
fluctuations. The boundary layer type was identified foreach plane before the actual experiment by measuring aboundary layer profile on each edge and in the middle.
The measurement height was set to either η=8 (laminar)and η=16 (turbulent) depending on the boundary layer
type. Corrections of arbitrary wall normal coordinates
were performed according to Ref. [6]. Wall position ateach spanwise location is estimated by fitting a line to thevelocity profile between data points where the velocity is
30% of the freestream velocity, Ufs, to 18% of that value
then extrapolating to zero velocity. Then a quadratic fit isperformed on the wall positions vs. spanwise coordinate
and this information is used for correcting the wall-normal
positions of the acquired data. The difference between
Ref. [6] and the current study is the number of points usedin the quadratic fit. In Ref. [6], because of the presence ofthe roughness array, the wall location estimates weremade at three spanwise positions between neighbouring
roughness elements, and at two spanwise positions at eachend of the spanwise range (Fig. 2 in Ref [6]). Here, due tothe presence of distributed roughness, all spanwise
positions were used for the quadratic fit. This is useful in
two aspects: first the number of data points is increased
for a better wall location estimate, and second thisapproach provides an unambiguous definition of aneffective wall surface in the presence of distributedroughness. Finally the data files are saved in .txt format
compatible with Tecplot. As an example, a data fileheader and first few lines of data is included in Appendix
A1. The overview of measurement range and resolution
for the experiments are provided in Table 2.

ab
Figure 5. Boundary layer and freestream hotwire probes(a) during insertion into probe support outside the test
section, (b) during measurements in the boundary layer.
MEASUREMENT SYSTEM
The measurement system is a constant-temperature
anemometer (CTA) system manufactured by Dantec
Dynamics (DD, Skovlunde Denmark). The system
consists of a StreamLine Pro Frame and Controller withthree CTA velocity channels, StreamLine Pro Automatic
Calibrator with Nozzle I, a Pitch/Yaw – Roll (PYR)
manipulator to perform velocity calibrations, StreamWare
Pro software and a custom-developed LabVIEW program.
The system controller is a Dell Optiplex 990 PC with anIntel Core i7-2600, 3.4GHz CPU and 8GB Ram, runningon a 64-bit Windows 7 Enterprise SP1 operating system.
The measured voltages were transferred to the computer
via a National Instruments 4-channel simultaneous
sampling differential USB A/D converter. The 55P15
boundary layer probe and the 55P11 freestream probe arepositioned at multiple grid points using a computercontrolled 3-axis traversing system. Both probes aremounted on the traversing mechanism using the hotwire
support arm.
The 1:20 bridge configuration is selected for highsensitivity on the CTA electronics. The probe was placedin the vertical free jet with a flat, low-turbulent velocity
profile during the velocity calibration. Nozzle I was used
to cover a velocity range of 1-60m/s. StreamWare Pro wasused for the velocity calibration process, and Labview
programs were used for measuring laminar and turbulent
velocity profiles. The linearization of voltages was
performed using a 5th order polynomial. Since thetemperature in the test section may rise as much as 5°C
during the course of the measurements, temperaturecorrection was performed using a correction procedure

Table 2. Measurement range and resolution
SampleSpanwise range, z
Wall-normal range
and resolution [mm]
η and resolution
FG-P–25 to +25, 1mm
0 to 8, 0.1
K-P–25 to +25, 1mm
0 to 8, 0.1
2-P–25 to +25, 1mm
0 to 8, 0.1
2I-P–25 to +50, 5mm
0 to 16, 0.1
2O-P–25 to +25, 1mm
0 to 8, 0.1
2R-P–25 to +25, 1mm
0 to 8, 0.1
2-B–50 to +50, 2.5mm
0 to 8, 0.1
2I-B–50 to +50, 2.5mm
0 to 16, 0.1
2-A–50 to +50, 2.5mm
0 to 16, 0.1
2O-A–50 to +50, 2.5mm
0 to 16, 0.1
2R-A–50 to +50, 2.5mm
0 to 16, 0.1

4boundary layer profiles. The abrasion via sandblastingcertainly increased the roughness level for smootherpristine plates, but not enough to reach the criticalroughness levels to trigger turbulence. The sandblasted
surfaces were found to have a homogenous and subcriticalroughness levels. This raises an interesting question: “Canabrasion via sandblasting also reduce roughness levels ofsupercritically rough surfaces in their pristine condition(such as the 2I plate). Unfortunately the 2I plate was nottested in the abraded state, and this question will be thefocus of future examinations.

RESULTSThe results are presented as time-averaged steady flow
field and the unsteady fluctuations. The steady flow fieldis represented by iso-velocity lines, with 10% incrementsof the normalized boundary layer velocity; i.e. 0.1Ufs,
0.2Ufs, 0.3Ufs, 0.1Ufs … 0.9Ufs. The unsteady flow
field is represented by colored contour maps with thecorresponding contour palette in each (sub)figure.
The following comparisons are made:1. Comparison of flow field among 6 Pristine plates
2. Comparison of flow field among 3 Abraded plates
3. Comparison of flow field between two Biofouled
plates
4. Comparison of flow field on 2I coating in Pristineand Biofouled state
5. Comparison of flow field on 2O coating in Pristineand Abraded state
6. Comparison of flow field on 2R coating in Pristineand Abraded state
7. Comparison of flow field on Plate 2 in Pristine,
Abraded and Biofouled state

Third, a comparison between each biofouled plate ismade to assess the performance after aging. The biofouled
plate 2 produced a laminar flow whereas the biofouled 2I
plate produced a turbulent flow in most of the domain(Fig. 8). The spreading angle for this turbulent spot iscalculated as ~8° w.r.t. the x-axis in the x-z plane.

First, a comparison among 6 pristine plates is made inorder to assess the performance in pristine state: All
pristine plates except 2I produced a laminar boundary
layer with minimal turbulence intensity throughout thedomain. Only the 2I plate produced a turbulent boundary
layer in one partition of the measurement region (Fig. 7).
The streamwise spreading of the turbulent spot is also
clear: The turbulent spot appears on the right edge of themeasurement domain at x=250mm, then spreads leftwards
covering approximately half of the z-domain at x=375mm,
and covering approximately three quarters of the z-domain
at x=500mm. This indicates that the 2I plate has
sufficiently large localized roughness levels already in thepristine state to trigger turbulent flow in the boundary
layer. The spread angle for this turbulent spot is calculated
as ~9° with respect to the x-axis in the x-z plane.

Figure 8. Flow field measured on biofouled 2I plate
Since the 2I-coated plate produces turbulence in theboundary layer both in its pristine state and in the
biofouled state, a comparison of flow fields can reveal
qualitative effects of biofouling (Fig. 9, left: pristine,
right: biofouled). Upon comparison of the flow fields, thefirst observation is that the turbulence is observed alreadyat x=125mm, and the flow fluctuations cover half of themeasurement domain. This means the turbulent spots forthe biofouled plate appear much closer to the leadingedge, and this could be because of the combined effect ofsupercritical roughness levels due to the coating andbiofouling. The similarity in the spreading angles (~8° and~9°) is also visible in Fig. 9.

Figure 7. Flow field over the pristine 2I plate.
Second, a comparison among the 3 abraded plates ismade in order to assess the performance in the abradedstate: Interestingly, all abraded plates produced laminarab
Figure 9. Comparison of flow field over 2I coating in
Pristine (left) and Biofouled (right) state5The 2O and 2R coatings were tested in their pristine
and abraded states. In both tests, both plates produced
laminar boundary layer profiles regardless of theircondition. These two coatings equally provide very goodoverall performances in terms of flow disturbances in theboundary layer. These two plates were not tested in the
biofouled state.

REFERENCES[1] D. Kane “Hull and Propeller Performance
Monitoring: Fuel Conversion and Emissions Reduction”,
in Climate Change and Ships: Increasing Energy
Efficiency, Proceedings, SNAME, February 16-17, 2010.
[2] N. Dalili, A. Edrisy, R. Carriveau. “A review ofsurface engineering issues critical to wind turbine
performance” Renewable and Sustainable Energy
Reviews 13 (2009) 428–438[3] M. Goodrich, J. Gorham, N. Ivey, S. Kim, M.
Lewis, C. Minkus. “Wind Tunnels of the Eastern
Hemisphere” A Report Prepared by the Federal Research
Division, Library of Congress for the Aeronautics
Research Mission Directorate, National Aeronautics andSpace Administration, August 2008[4] M. Goodrich, J. Gorham. “Wind Tunnels of theWestern Hemisphere” A Report Prepared by the Federal
Research Division, Library of Congress for the
Aeronautics Research Mission Directorate, National
Aeronautics and Space Administration, June 2008[5] SF Benjamin, CA Roberts “Measuring flow
velocity at elevated temperature with a hot wire
anemometer calibrated in cold flow” Int. Journ. of Heat &Mass Transfer 45 (2002) 703 – 706[6] EB White, FG Ergin “Using laminar-flow velocity
profiles to locate the wall behind roughness elements”
Experiments in Fluids 36 (2004) 805–812

Finally the plate with coating 2 was tested in all threestates; i.e. in pristine, abraded and biofouled states. Plate 2
produced laminar boundary layer profiles in all states andtherefore coating 2 provides the best overall performance,by producing the least amount of flow disturbances in theboundary layer.
CONCLUSIONS AND FUTURE WORK
Since the objective of the NATURAL project is to
identify coating(s) which can keep their initial surface
structure in their aged (biofouled and/or abraded) state, aperformance criterion can be defined by its aerodynamic
performance: If the coating can produce a laminarboundary layer in its pristine state and can keep this typeof flow in its aged state, the coating is successful. In
contrast, if a coating cannot produce a laminar boundary
layer profile in its pristine condition or keep it in its aged
condition, then the coating is not successful. Based on thisargument, three main conclusions can be reached: Coating 2 (which does not contain nanoparticles) isthe most successful since it is able to produce alaminar boundary layer and keep this flow condition
in its aged state (both biofouled and abraded).
 Coating 2O and 2R are successful since they are able
to produce a laminar boundary layer and keep thisflow condition in their abraded state. Since thesepanels were not biofouled no definitive conclusion
can be made on their success in the biofouled state.
 Coating 2I is unsuccessful as it produces a turbulent
boundary layer in its pristine and aged state. Since
this panel was not abraded no definitive conclusion
can be made on its success in the abraded state.

APPENDIX A1. A SAMPLE DATA FILE
TITLE = "FG-P"
VARIABLES = "eta" "z [mm]" "x [mm]" "Ubl/Ufs""u'bl [m/s]"
ZONE T="Natural Data" I=81 J=51 K=4 f=point
0
-25
125
0
0
0.1 -25125
0.0382 0.0093
0.2 -25125
0.0764 0.0186
0.3 -25125
0.1146 0.0278
0.4 -25125
0.1528 0.0371
0.5 -25125
0.1896 0.0376
0.6 -251250.2252 0.0437
0.7 -25125
0.2621 0.0554
0.8 -25
125
0.299
0.056
0.9 -25125
0.3356 0.062
1
-25
125
0.3734 0.0736…

Future investigations will be focused on three subjects(i) measurement of multiple velocity components in theboundary layer and (ii) how the distributed surface
roughness of an individual coating is linked to the flow
field it produces. (iii) Although panel 2I was not abraded,since it produces a turbulent boundary layer already in its
pristine stage due to high roughness amplitudes, there is achance that that the abrasion could have produced a better
result (such as a delayed transition location, at least) byreducing the surface roughness amplitude.
ACKNOWLEDGMENTS
The research leading to these results has received
funding from the European Union Seventh Framework
Program (FP7 2007-2013) under grant agreement number
310397. The author thanks Allan Kruse Christensen formaking the engineering drawings and for his efforts
during manufacturing and installation, and Prof. Edward
B. White for reviewing the manuscript and Prof. Knud
Erik Meyer for providing access to the wind tunnel.

6