Corresponding author : Cheng See Yuan Email : chengutem.edu.my
EFFECT OF SURFACE ROUGHNESS ON DRAG OF LOGGERHEAD CARAPACE
Alan Chan Kah Poh
1
, Cheng See Yuan
1
, Ahmad Kamal Mat Yamin
1
, Ainil Jesita Jalaluddin
1
, Iskandar Shah Ishak
2
, Shuhaimi Mansor
2 1
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka
Hang Tuah Jaya, 75450 Ayer Keroh, Melaka
2
Faculty of Mechanical Engineering, Universiti Teknologi Malaysia,
81310 UTM Skudai, Johor
ABSTRACT
The present investigation primarily studies the effect of surface roughness on the drag coefficient, Cd of a Loggerhead sea turtle carapace using a subsonic wind tunnel. The
pressure coefficient, Cp distribution across the Loggerhead carapace was also investigated and is compared to the Cp trend of an airfoil in order to deduce the
aerodynamics features of the Loggerhead carapace. One-to-five-scaled models are created based on the dimensions of a real Loggerhead turtle with simplification. Four
roughness scales were employed to capture the Cd trend at increasing Reynolds numbers, Re. As expected, the Cd levelled off with Re for all four models investigated. However, the
Re where constant Cd began varies with relative roughness of the carapace models. Good correlation between the Cd and relative roughness is obtained. In addition, the
wind tunnel results are able to capture the Cp trend of the carapace models and compared to Cp values of an airfoil. Results reveal that the upper surface of the
Loggerhead carapace is streamlined but with restrictions of angle of attack.
Keywords :
drag, roughness, carapace, wind tunnel
1.0 INTRODUCTION
In the shores of Shark Bay, Western Australia, a relatively undisturbed foraging ground, forms an excellent feeding ground for sea turtles and hosts a rich marine ecosystem
Heithaus et al. 2005. A research led by Dr. Mike Heithaus over a span of ten years has revealed the fact that green sea turtles Chelonia mydas are less likely to be attacked by
tiger sharksGaleocerdo cuvier when compared to Loggerheads Caretta caretta, sometimes as much as five times Heithaus et al. 2002. Although both are of the same
family of Cheloniidae, the green and Loggerhead sea turtle are as different as tanks and flying saucers.
The cause to why Loggerhead sea turtles are at higher risks of being attacked by tiger sharks at Shark Bay, Western Australia are comprised of many factors, one of which is
the Loggerhead’s habit of not cleaning its shell thus allowing the build-up of roughness over time. However, to the author’s knowledge, there has yet to be any comprehensive
study on how drag upon the shell is influenced by the roughness build-up on loggerhead carapace. In the context of this project, the main objective is to study the effects of
roughness built up on the shell of loggerhead sea turtles in relation to drag.
The investigation had focused on the Loggerhead sea turtle that dwells within the Caribbean Seas near Curacao, Netherlands Antilles. A simplified model was created
based on the dimensions of the real Loggerhead sea turtle. Verification of the designed
Corresponding author : Cheng See Yuan Email : chengutem.edu.my
model was reflected based on the values of blockage ratio. Following this, the surface roughness of the models was defined and analysis based on wind tunnel testing results
was done to examine static drag in relationship with the surface roughness. 2.0
WIND TUNNEL MODEL
2.1 Model Dimensions The 1:5 scaled, simplified wind tunnel models of the loggerhead turtle carapace were
created based on the actual turtle carapace dimensions as presented in the study by Epperly et al., and the dimensioning conventions used by Dr. Wyneken 2001.
Accordingly, the Standard Carapace Length SCL and Standard Carapace Width SCW of 0.92 m and 0.63 m, respectively, were adopted as the length to width ratio of the scaled
down carapace models of the present study.
This model scale has also taken into consideration the blockage ratio and dynamics similarity of wind tunnel test, which entails information of the actual swimming speed of
the loggerhead turtle. Hence, the mean swimming speed of 0.5721 ms presented by Nagelkerken et al. 2003 is adopted. As for the temperature and density of the sea water,
the values are taken at 26.7 ºC and 1027 kgm
3
, respectively. The design of the model is first created using a commercial CAD program, Solidworks 2006, as shown in Figure 1. It
is then fabricated using thermoplastic via Rapid Prototyping with the final model as shown in Figure 2.
Figure 1: Simplified wind tunnel model of loggerhead carapace.
Figure 2: Model product of Rapid Prototyping
Corresponding author : Cheng See Yuan Email : chengutem.edu.my
The blockage ratio of less than 0.05 based on the requirement of aeronautics study is attained. The frontal area used for the calculation of blockage ratio is estimated using
method introduced by Scott Thor 2007a. 2.2 Surface Roughness Definition
In the present study, the relative roughness of a finished surface is defined as follow:
Relative roughness = L
1 Where
refers to the mean roughness height of fifteen tabulated points determined with a scope, and
L
is the chord length of the model. Data was retrieved from a test-slate with the respective roughness to be tabulated. In total, four roughness models were used
as summarized in shown in Table 1. For convenience in the discussion, the models are designated as Models A, B, C and D respectively.
Table 1: Relative roughness data Specimen
Relative Roughness
A Smooth
B 0.430
C 0.456
D 0.556
Figure 3: Pressure-tapping numbering
Corresponding author : Cheng See Yuan Email : chengutem.edu.my
A model with eleven pressure tappings was created as shown in Figure 3, using eleven polyurethane tubes. The relative distance, xL of the tapping points from the anterior tip is
shown in Table 2. Table 2: Relative distance of tapping points
Tapping no.
Relative Distance from anterior tip, xL
1 0.00
2 0.04
3 0.17
4 0.28
5 0.36
6 0.47
7 0.53
8 0.63
9 0.73
10 0.81
11 1.00
3.0 WIND TUNNEL TESTING
