12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

THE EFFECT OF LARGE EDDY BREAK-UP DEVICE ON ROUGH WALLTURBULENT BOUNDARY LAYER

Shubham Kumbhar
Lyazid Djenidi
Department of Mechanical Engineering Department of Mechanical Engineering
University of Newcastle
University of Newcastle
University Dr, Callaghan NSW 2308
University Dr, Callaghan NSW 2308
shubhambharat.kumbhar@uon.edu.au
lyazid.djenidi@newcastle.edu.au

Farzin Ghanadi
Md Kamruzzaman
Department of Mechanical Engineering Department of Mechanical Engineering
University of NewcastleUniversity of Newcastle
University Dr, Callaghan NSW 2308
University Dr, Callaghan NSW 2308
farzin.ghanadi@newcastle.edu.au
md.kamruzzaman@uon.edu.au

ABSTRACTet al., 2021). It can be said that a fair amount of work has been
dedicated to exploring the application of these devices on asmooth wall. Comparatively much less attention was given tothe LEBU effects on the rough wall. Bandyopadhyay (1986)
found that the outer layer devices are less effective for dragreduction in rough walls as compared to the smooth wall. But
no literature is found which depicts the interest to learn thedetailed effect of these so-called ‘drag enhancing’ devices (ofcourse, when looked at from a drag reduction perspective) on
the structure of rough wall turbulent boundary layer. The primary aim of the present study is to fill the critical knowledge
gap in the area of wall turbulence, by investigating how a fully
developed turbulent boundary layer responds to LEBU placedin the rough wall turbulent boundary layer. The results are also
compared with no LEBU results.

The effect of large eddy break up (LEBU) device on therough wall turbulent boundary layer up to Reθ ≈ 12000 is investigated using hot-wire anemometry. The pressure measurements (around a circular roughness element) are carried out
to determine skin-friction coefficient (c f ) and friction velocity(Uτ ). The LEBU is flat plate, installed at wall normal distanceof 0.8δ (local boundary layer thickness) from the wall. TheLEBU effect is observed immediate downstream of the device,where it creates a region of recirculating wake. At 43.7LB (LBis the LEBU chord length), maximum LEBU effect is reflectedin the reduction in turbulence intensity. The boundary layer recovers around 50.4LB .

INTRODUCTION
The large eddies present in the outer layer of the turbulent boundary layer can be manipulated using the Large-Eddy
Break-Up (LEBU) or Outer Layer Devices (OLD). These devices are known to reduce friction and induce total net dragon a smooth wall (Sahlin et al., 1988). There is no study tounderstand how these devices disturb the rough wall turbulent
boundary layer. The earlier outer layer manipulation studies
by Yajnik & Acharya (1978) and Hefner et al. (1980) expressing the capability of the thin plate device to reduce the friction drag provided a breeding ground for the number of LEBU
studies worldwide. Corke et al. (1981, 1982) reported a reduction in the turbulence intensity up to 100δ (boundary layerthickness) and serious net drag reduction (up to 20%) with tandem LEBU arrangement. In the follow up study by Anders &Watson (1985), the net drag reduction up to 7% was recorded.
The LEBU popularity dropped significantly after the towing
tank experiments carried out by Sahlin et al. (1988) proving
the incapability of the LEBU to reduce net drag owing to thedevice drag added by LEBU itself.
The LEBU devices studies continued even after several
failed attempts of using LEBU for net drag reduction. In recentyears the advancement of computer resources has brought a
new wave of numerical studies focussing more on understanding the LEBU effects (Kim et al., 2017; Chin et al., 2017; ChanEXPERIMENTAL SETUP
The open-return blower type wind tunnel at the universityof Newcastle is used for these experiments which was previously used by Kamruzzaman et al. (2015) to investigate thedrag of turbulent boundary layer with transverse circular rods.
The 5.4 m long test section is 0.15 m wide and 0.9 m highcross-section. The zero-pressure gradient is maintained in theentire working section of the wind tunnel by adjusting the one
side wall to compensate for the growth of the boundary layer.
The boundary layer is tripped at the contraction by a 4 mmdiameter rod followed by 170 mm long strip of No. 40 grit
sandpaper. Antonia et al. (1990) confirmed the behaviour of
the turbulent boundary layer (tripped by using a rod and sandpaper) to be similar as the fully developed turbulent boundarylayer. The rough wall consisted of the copper-coated circularrods placed equidistant over the entire width of the working
section (see figure 1). The diameter of the circular rods is 1.6mm, and the ratio is set to be λ /k = 16. According to Leonardi
et al. (2003) for λ /k = 16 the viscous drag is very small, andthe form drag can be closely approximated to the total drag.
The virtual origin (d0 ) is determined by using the method introduced by Jackson (1981) who measured d0 from base ofroughness with the distance to the centroid of the moment of1

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
present study, the LEBU is placed at the wall-normal distanceof 0.8δ because this location is proven to be effective to disturb the boundary layer in various studies for example (Corkeet al., 1982; Anders & Watson, 1985). The present study isconducted at free stream velocity of U∞ = 8 m/s and 12 m/s,
while the momentum thickness Reynolds numbers Reθ ranging from 7004 to 11985 with Uτ = 0.48 at 8 m/s and Uτ = 0.75
at 12 m/s.

the forces acting on rod roughness. This technique was usedby (Kamruzzaman et al., 2015; Talluru et al., 2016; Djenidi
et al., 2017). In this case, d0 /k = 0.48, d0 is determined byestimating the centroid of forces acting on roughness elementsfrom the pressure distribution around the roughness element.

RESULTS AND DISCUSSION
Pressure Measurements
Figure 2(a) & (b) describes the pressure distribution
around a roughness element for U∞ = 8 m/s and 12 m/s atdifferent x/LB (LB is the LEBU chord length). The measurements were taken at two different regions, close to the tunnel
inlet and further away (fully developed region). These distributions are used to calculate the (c f ) as follows.

Figure 1. A schematic of the experimental setup. The origin
of x is taken at the leading edge of the rough wall.

cf =Hot-wire anemometry is used for streamwise velocity
measurements. The Dantec 55P15 single hot-wire probe, has
spacing of 1.5 mm with 2.5 µm Wollaston Pt wire solderedbetween two prongs. In house built constant temperature
anemometer (CTA) circuit is used to operate hot-wire at an
overheat ratio of 1.5. The signal from the CTA is amplified,offset and low- pass filtered at cut-off frequency ( f c) of 12000
Hz with sampling frequency set at 2 f c. The positioning of the
hot-wire probe close to the wall is accomplished using Celestron digital microscope (resolution = 1 micron) mounted onfine threaded traversing system. The dial indicator with thedivisions of 0.001 m is set at zero when the microscope is focussed at the wall, the displacement is measured as the microscope focus is shifted from wall to probe. The Mitutoyo heightgauge (resolution = 0.01 mm) is used for 32 points measurements (spaced logarithmically) between 0.5 mm to 89.4 mm.
The temperature fluctuation is measured for the entire duration of the experiment using BAT-10 thermocouple (resolution= 0.1○ C). At the beginning and end of every experiment thehot-wire is calibrated against the pitot tube (fixed) immersedin the undisturbed free stream flow. To counter the hot-wirevoltage drift the linear interpolation is used between the preand post calibration data.
√The friction velocity (Uτ = τw /ρ, where, τw is the wall
shear stress and ρ is the density of the fluid) is obtained bymeasuring the pressure distribution around the rod and defect chart method (both methods give consistent values of Uτ
within 2%). The pressure measurement is carried out at x =2.54 m (origin of x is taken at the leading edge of the roughwall). At this location the circular rod is replaced by noncoated hollow steel cylinder. A hole of 0.32 mm diameteris drilled through the wall of cylinder, it is connected to theFurnace Control FC 0332 micromanometer (Range: 0 − 50Pa,Accuracy: 0.0125Pa, 0.025% full scale output). The cylinder
is rotated by 10○ (with minimum step angle resolution of 1.8)
from 0○ - 360○ with more than 60 seconds measurements ateach rotation. The 600 mm long LEBU is made of the Aluminium with 1 mm of thickness and 60 mm chord length tooccupy the tunnel’s entire span width. The ‘pin-and-strut’ configuration is used to hold the position of LEBU at x = 2.49m (fully developed region). This LEBU mechanism is designed to perform effective wall-normal distance study as well
as check the effects of the device at different locations. For the2FD2 kρU∞where, pressure drag (FD ) is,2π 12 Ps kcosα dαFD = ∫0(a)(b)
Figure 2. Distribution of the normalised static pressurearound a rod at different free stream velocities. (a) U∞ = 8m/s (b) U∞ = 12 m/s. Symbols, ○: 17.4LB , ◻: 20.6LB , +:40.8LB , ×: 43.0LB , △: 45.8LB , ◇: 48.2LB .

212th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
Figure 4(a) & (b) shows the mean velocity profiles for
U∞ = 8 m/s and 12 m/s respectively. It appears that LEBU
disturbs the boundary layer significantly. The inner layercomparison of LEBU and No LEBU profiles show significant deviation from each other. Immediately downstream ofthe LEBU, 42.4LB there is very strong region of recirculating
wake (y/δ ∼ 0.8). Also, the inner layer is affected the most byLEBU (y/δ ≤ 0.04) at 43.7LB .
Figure 4(c) & (d) presents the mean velocity profiles overrough wall (with and without LEBU) in the velocity defectform ((U∞ −U)/Uτ vs y/δ ). These profiles show a good collapse reflecting that seen in figure (a) & (b). The wake dueto the LEBU device is also observed clearly in velocity defect form around y/δ ∼ 0.8. The turbulence intensity profiles
are presented in the Figure 4(e) & (f). At 43.7LB the turbulence intensity is reduced. For Reθ ≥ 11072 this gap seems tohave closed by small amount which means for higher Reθ , theLEBU effect is reduced. The LEBU effect seems to have vanished from the outer layer at 50.4LB . It would be interesting
to compare these results with smooth wall to understand therecovery of boundary layer on different surfaces.

The streamwise variation of c f for U∞ = 8 m/s and 12 m/s
are shown in the figure 3. We found that c f is constant in therange of 40 to 50 for U∞ = 8 m/s. For U∞ = 12 m/s the valueof c f is increasing with x/LB . The change in magnitude of c f
for different velocities reflects a Reynolds number effect.

CONCLUSIONSThe hot-wire anemometry measurements are carried out
on the rough wall turbulent boundary layer subjected to theLEBU device. The pressure distribution around a roughness
element is measured to determine c f and Uτ . The c f variation for different velocities reflects the Reynolds number effect. The mean velocity profiles at seven streamwise locations are presented. The inner layer comparison of LEBU
and No LEBU profiles show significant deviation from eachother. Immediately downstream of the LEBU, 42.4LB there isvery strong region of recirculating wake (y/δ ∼ 0.8). The innerlayer is affected the most by LEBU (y/ 0.04) at 43.7LB. This
device also acts to reduce the turbulence intensity (at 43.7LB ).
The boundary layer recovers around 50.4LB as the LEBU effect (wake) from the outer layer disappears.

Figure 3. Variation in C f with x/LB for λ /k = 16. Symbols◻: LEBU, U∞ = 12 m/s, ○: LEBU, U∞ = 8 m/s.

Mean Velocity and Velocity DefectThe mean velocity profiles at seven streamwise locations
are presented in the figure 4 . As suggested by Kamruzzaman
et al. (2015) Uτ and δ is used as scaling parameters due totheir better scalability for rough wall. The No LEBU roughwall data is used for the purpose of comparison.

Table 1. Boundary layer properties at different streamwise
locations (Hot-wire measurements). Here, NLB: No LEBU.

Acknowledgments
The Australian Research Council (ARC) is gratefully acknowledged for the financial support of this work.

δ∗(m)

x(m)

Symbol U∞
δ(m/s) (m)NLB
42.4LB
43.7LB
45.0LB
46.4LB
47.7LB
49.0LB
50.4LB

◻
○+∗×◇▽

888888880.083 0.021 0.012 2873 6548
0.084 0.022 0.013 2641 7286
0.083 0.021 0.013 2663 7277
0.085 0.021 0.013 2678 7004
0.086 0.023 0.013 2756 7527
0.087 0.024 0.014 2845 7615
0.087 0.024 0.014 2839 7757
0.087 0.023 0.013 2827 7410

NLB
42.4LB
43.7LB
45.0LB
46.4LB
47.7LB
49.0LB
50.4LB◻
○+∗×◇▽

12121212121212
120.083 0.020 0.012 4151 9710
0.086 0.022 0.013 4049 11072
0.084 0.021 0.013 3968 10625
0.087 0.023 0.013 4398 11985
0.087 0.025 0.014 4304 11886
0.087 0.024 0.014 4358 11663
0.087 0.024 0.014 4360 11769
0.086 0.023 0.013 4240 11025

θ(m)

Reτ

Reθ
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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

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Figure 4. Comparison of a mean velocity, velocity defect, and turbulence intensity profiles between the 8 m/s and 12 m/s data (seetable 1 for symbols).

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