Indian Journal of Geo-Marine Sciences Vol. 41(2), April 2012, pp. 124-133
Numerical Study on Hydrodynamic Performance of Shallow Underwater Glider Platform Ting M. C.*, M. Abdul Mujeebu, M. Z. Abdullah & M. R. Arshad School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia [*E-mail:
[email protected]] Received 20July 2010; Revised 07 July 2011 Underwater glider is the new generation of autonomous underwater glider which has been used for ocean observation and wide range of marine environment monitoring. It glides through seawater horizontally and vertically by carried additional wings. This paper presents the investigation of hydrodynamic characteristics of the shallow underwater glider by using Computational fluid dynamics (CFD) code, FLUENT 6.3.26. A 3-dimensional Spalart-Allmaras turbulence model is used to estimate hydrodynamic performance of the glider. The effect of angle of attack (varied from -8º to +8º) at various Reynolds numbers and the flow behavior of underwater glider at different flow conditions are studied. The characteristics such as lift and drag forces, lift and drag coefficients and pressure distribution over fixed wings are evaluated in a flow field with Reynolds number ranging from 105 to 106. Lift and drag coefficients are observed to rise as function of angle of attack. Wake formation occurs at the tail and the junction of main wing and fuselage body, and the nose is exerted to highest pressure force compared with the other parts. [Keywords: Underwater glider; Computational fluid dynamics; Hydrodynamic performance; Spalart-Allmaras; Lift; Drag.]
Introduction Underwater vehicles play an important role in ocean monitoring, experimentation and other underwater researches. These vehicles are subdivided into three main categories such as manned submarine vehicles (MSV), remotely operated vehicles (ROV) and autonomous underwater vehicles (AUV). Though MSV is effective in underwater exploration it has limitations in terms of high cost, and high operating time and manpower. But ROV has fewer constraints on pilot training and safety issues compared to MSV. It is a tethered vehicle which is controlled from surface by remote operators and is extremely maneuverable. The third type of vehicles, AUV, requires no manpower during the operation. It contains its own power and is able to run autonomously to accomplish the pre-defined missions. Some of the principles and design of AUV has been studied and developed by many previous researchers1-7. Underwater glider (UG) is new generation of AUV which glide through ocean both horizontally and vertically by altering its weight in the water. It carries wings to propel and control the pitch attitude. Unique features of the UG are ability to
achieve long endurances per deployment and long term measurement missions which last for weeks or months. This glider uses buoyancy as its main means of propulsion, so no need of power driven propulsion. It has significantly low operating cost, low power consumption and quiet operation8-10. The three models of UG currently being developed and demonstrated commercially, are Spray11 by Scripps Institution of Oceanography, Seaglider12 devised by Woods Hole Oceanography, University of Washington, and Slocum13 from the Webb Research Corporation. The UG can be used for both coastal and deep ocean environments depending on the design and types of missions. Spray, Seaglider and Slocum thermal gliders are used for deep-ocean observations with maximum pressure ranges from 100 dbar to 1500 dbar, and Slocum Electric UG is designed for shallow coastal operation purpose with operating depth of 4-200 m14. In general, UG is an electronic based platform which collects data by using integrated scientific sensors such as conductivity, temperature and depth sensors (CTD). It executes the preprogrammed mission every time when submerged and updates when surfaced, through communication with the control centre10. Recent developments in CFD
TING et al.: HYDRODYNAMIC PERFORMANCE OF SHALLOW UNDERWATER GLIDER PLATFORM
could offer the cost-effective and good prediction of problems such as the study on hydrodynamic characteristics. Use of simulation codes such as SC/Tetra and Matlab, to investigate UG characteristics and Ansys™ Fluent to study on AUV hull were reported by few researchers15-18. As part of the ongoing research on underwater vehicles, at University Sains Malaysia (USM), a prototype of UG was developed named as “USM underwater glider” for shallow underwater applications. In the present study, a detailed numerical analysis is carried out, to observe the hydrodynamic performance of the said prototype and the limitations of its design. The finite volume based CFD software FLUENT 6.1.3 is used for the 3D simulations, by exploiting the Spalart-Allmaras turbulence model. For both main wing and stabilizer wing sections, the airfoil model NACA 0012 is used. The effect of angle of attack (AOA) at various Reynolds numbers and the flow behavior of UG at different flow conditions are studied. USM Shallow Underwater Glider USM underwater glider has been designed to operate in seawater, which is not more than 30 m deep19. It is still under development and will be tested for its workability in the practical real environment. Table 1 shows the general specification of the underwater glider. Total length of the vehicle is 1.3 meters with a diameter of 0.17 meters. Its weight does not exceed 30 kg including all payloads, sensors and other add-on equipments. The shape of the vehicle is cylindrical (as shown in Fig. 1) which offers better resistance on pressure and gives larger payload compared to the streamline shape. The main wings are designed to be adjustable in order to facilitate various experimental arrangements. National Advisory Committee for Aeronautics (NACA) 0012 airfoil profile has been
chosen for the wing design. Total wing span is 1 m with no sweep angle design. Numerical modeling CFD analysis is performed to investigate the hydrodynamic coefficients, flow behavior and pressure distribution on the surface of the underwater glider. It helps to examine the performance and limitation of the underwater glider design. The low speed model analysis is performed at different Reynolds numbers and angles of attack. Flow is considered three dimensional steady state and incompressible. The governing equations
Continuity equation: … (1) Spalart-Allmaras one equation model is given by equation (2):
… (2) In case of the steady state,
Table 1—Specifications of USM underwater glider Dimensions
Operation Depth Operation Time Main Power Sensors
0.17 m (Diameter) 1.3 m (Length) 1.0 m (Wing Span) 30 m maximum More than 2 hours Lithium-Ion Eco-Sounder Transducer IMU 5 Degrees of Freedom Gyrocompass Depth Sensor Distance Sensor
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Fig. 1—3D model of underwater glider.
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And turbulent eddy viscosity is computed from equation (3):
… (3) Where
… (4) And is the density, ν=µ/ρ is molecular kinematic viscosity, and µ is the molecular dynamic viscosity. ,
,
and
are constants.
… (5)
Equations (6) and (7) are used to estimate the lift and drag forces (FL and FD) respectively FL= ρV2ACL
… (6)
FD= ρV2ACD
… (7)
the outer shape of the underwater glider design is modified and simplified for easy meshing and saving the computational cost. The underwater glider is splitted into two symmetrical parts in GAMBIT (Figs 3 and 4). Forces acting on the left and right side of the glider are same since the shape of the glider is lateral symmetry. Therefore only half of the underwater glider is modeled and considered in the flow simulation, as shown in Fig. 4. An external brick volume is modeled, which is used as fluid flow. Brick volume is subtracted by the glider volume, to obtain the required flow domain to study the effect of external flow on the UG model. Size of the flow domain is further increased in order to reduce the effect of wall boundary layer. Sizing function is applied to the flow domain in such way that mesh is finer at the glider surface and coarser away from the glider.
where, A is the reference area chosen for calculating lift and drag force. In this study, wing area is considered to be the reference area, which is 0.04 m2. CL,CD, and V are the lift and drag coefficients, and the velocity of fluid respectively. The simulation model
3D geometric modeling of the shallow underwater glider is made using Solidworks. Fig. 2 shows the exploded view of the conceptual design of the shallow underwater glider. For the purpose of CFD analysis,
Fig. 2—Exploded view of the model [
Fig. 3—Symmetry view of the model
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Boundary Conditions
The boundary conditions considered are, for inletvelocity inlet that ranges from 0.3 m/s to 0.5 m/s, for outlet-pressure outlet condition, and for UG surface and other walls of flow domain is wall with no slip condition. Symmetry plane uses symmetry boundary. Validation and grid independency test
In order to verify the simulation method and to ensure the accuracy of the simulation results, previous study by Arima et al.16 who focus on UG with independently controllable main wings has been used. They used different design and size of the glider. The model has been made according to their glider for the validation purposes. Fig. 5 shows the view of the UG which is constructed using Solidworks. Unstructured mesh is generated using GAMBIT, consisting of about 1 million cells, as shown in Fig. 6. SCRYU/Tetra for Windows of Software Cradle Co. Ltd was used by Arima et al. for CFD analysis; in this project, FLUENT 6.3.26 is used. Lift and drag coefficient is calculated with wing area as reference area. The Spalart-Allmaras turbulent model is utilized with SIMPLE scheme for the Pressure-Velocity Coupling. Three different discretization schemes namely, First-order upwind, second-order upwind and third-order MUSL (Monotone Upstream-Centered
Schemes for Conservation Laws) are used. The results are compared in order to determine which scheme is more appropriate in the simulations. The first-order and third-order MUSL scheme are tested to obtain the better scheme for the simulation. Fig. 7 indicates that the simulation results obtained from MUSL scheme agree well with the publication results. The thirdorder MUSL scheme is found to be suitable for arbitrary meshes and better accuracy compared to the other schemes. The different between simulation and publication results does not exceed 7% for lift-to-drag ratio. Therefore, this scheme has been selected for simulation on USM underwater glider. A few cases have been run to determine the effect of mesh density on the lift coefficient for USM underwater glider. Table 2 shows the simulation results of the lift coefficient, CL, for the UG at the angle of attack AOA= -2º. Seven cases of different mesh sizes are checked, and the difference lift coefficient is studied. The deviation between case 2 and case 3 are very small (less than 1%) compared with other cases. Therefore, case 3 is adopted for the simulation. Results and Discussion Lift and drag force distributions
Figures 8 and 9 respectively show the distributions of lift and drag forces on the UG. Lift force is mainly generated by the main wing and the fuselage body. For example at +4º, main wing contributes 65% and fuselage body, 26%, of the total lift force acting on the UG, while the tail part contributes only 9%. For the drag force distribution, fuselage body and main wing contribute highest percentages compared with tail wing, tail cone and stabilizer. Since Reynolds number is inversely proportional to the viscosity of
Fig. 4—Half of UG model in GAMBIT
Fig. 5—Boundary Conditions of UG Model
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Fig. 6—Model in Solidworks
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the fluid passing over the body, viscous force is less significant compared to the pressure force, which is negligible; drag force in this case is dominant by the pressure force. The drag force is more significant when AOA increases. Since pressure force increases as function of frontal area of the body, it makes sense when AOA increases pressure force follows to raise as total area against flow direction also increases. Pressure force is dominant for fuselage body since it is cylindrical in shape and has largest frontal area. Effects of AOA on lift and drag coefficients
Figures 10 and 11 show the effects of AOA on the lift and drag coefficients respectively. As the UG changes its position from AOA = 0º to ±8º, lift coefficient increases linearly, as expected. Due to the low cruising speed of the glider, which is varied between 0.3 m/s and 0.5 m/s, lift coefficient for increased velocity slightly rises with difference less than 2%. Drag coefficient ranges from 0.1 to 0.23, and drag force continues to increase as AOA rises. From Fig. 11, drag coefficient at AOA= -8º is higher compared with the other positions. This is due to the wake formed at the main wing and rear zone of the UG. The lift coefficient variations are small for the speed ranges of 0.3 m/s to 0.5 m/s which are less than 2%. This small variation is due to low velocity which is below 1 m/s. In addition, the velocity increases also relatively small, only 0.1 m/s. For lower angle of attacks, the variation in lift coefficient is also small. The drag coefficient shows slightly higher percentage different compared with the lift coefficient. For higher velocity, the difference in drag coefficients become smaller as observed in Fig. 12. This is due to low velocity with small velocity increment. Flow visualization Stream lines
Flow visualization method helps to interpret the hydrodynamic behavior. Figures 12 and 13 show the stream lines for AOA +8º and – 8º respectively. Wake is formed at upper surface of airfoil for +8º and at lower surface of airfoil for –8º. This shows that wake region is more chaotic for –8º condition, and this leads to higher drag force to act on the UG. Fig. 7—(a) The meshed model; (b) Drag coefficient vs angle of attack; (c) Lift coefficient vs angle of attack; & (d) L/D vs angle of attack, comparison of results from three discretization schemes with the result of Arima et al.17.
Velocity profile
Figures 14 and 15 show the velocity profiles for UG at symmetry plane for both zero and +80 of AOA. As the fluid passes through the glider surface, velocity
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Fig. 8—Lift force distribution on UG.
Fig. 9—Drag force distribution on UG.
Fig. 10—CL vs AOA for various cruising speeds
Fig. 11—CD vs AOA for various cruising speeds
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Fig. 12—Percentage Difference for CL and CD with Variation of Speed for Different AOA
Fig. 12a—Stream line at -8º AOA
magnitude changes along the surface. Area around the nose of the glider exhibits low velocity since the pressure magnitude is largest at this part; and zero velocity is observed on all the surface of the glider. Velocity of the fluid increases along the curved surface of the nose of the glider; this phenomenon formed a region with higher velocity at the upper region of the glider. As AOA increases, the flow decelerates along the upper surface of the glider. Therefore a low velocity region can be observed on the upper surface of the glider which is oriented at +80. Wake is formed at the tail part of the glider. The wake is weak due to low cruising speeds which are between 0.3 m/s and 0.5 m/s. As the AOA rises, the wake region also continues to expand. Fluid stream detaches itself from the glider at the rear region of the glider. Figures 16 and 17 show the detailed views of the wake formed at the tail part of the glider at 0 and +80 of AOA. Reverse flow phenomenon is observed at the tail cone section and rear region of the tail wing, in both the cases. The tail cone has curved downhill side and it has a narrow section. As the fluid flow passes
Fig. 13—Stream line at +8º AOA
Fig. 14—Velocity Profile at 8 degree AOA (symmetry plane)
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Fig. 15—Velocity Profile at 0 degree of AOA (symmetry plane)
Fig. 17—Velocity Contour at Tail Part of Glider at 0 Degree of AOA.
Fig. 16—Velocity Contour at Tail Part of Glider at 8 Degree of AOA.
through the body fuselage which is straight and moves to the curved tail cone region, reverse flow occurs at the bottom of the tail cone; the velocity is relatively lower than the fluid velocity at other regions. Similar phenomenon is observed at the rear part of the tail wing as in Fig. 18. Reverse flow occurs due to the sharp edge without streamlining and the fluid stream detached from the glider surface. Due to low fluid velocity, pressure is comparatively higher at the tail part. Figures 19 and 20 show the velocity contours for the fluid flow through the fixed main wing at +80 of AOA. Flow stream climbed along the fixed wing smoothly without forming wake except for the region where main wing is attached to the fuselage body. Leading edge of the airfoil shows the highest velocity
Fig. 18—Flow behavior at tail part
relative to the trailing edge. Circulation of flow occurs due to mixing flow between flow pass through curved surface of the fuselage body and the fixed main wing. The velocity of this region is relatively low compared to upstream flow velocity. The wake region keeps growing until it regains its velocity and the velocity profile becomes flat again behind the fixed main wing. The wake region expands depending on the
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Fig. 22—Pressure Distribution at zero degree AOA Fig. 19—Flow behavior at the fixed main wing.
Fig. 23—Pressure distribution at +6º AOA Fig. 20—Vector of the flow at location A
Fig. 21—Pressure Distribution at + 8º AOA
effect of AOA; higher the AOA, larger the wake region be formed. Static pressure distribution
Figures 21 and 22 show the static pressure distribution on the UG. At any orientation, the nose of the UG has the highest magnitude of static pressure. Large frontal area which is normal to the flow direction is subjected to higher pressure impact compared to other parts, and it contributes to higher drag force. Drag force is a function of wall shear and pressure force. Pressure drag coefficient is dominant compared with viscous drag coefficient in this case. Viscous drag coefficient is a function of viscosity. The effect of viscosity is less as it is inversely proportional to Reynolds number which is high. As UG changes its orientation from zero AOA to positive
Fig. 24—Bottom view of pressure distribution at +6º of AOA
and negative AOA, fluid separates from the body and generates low pressure region at the back especially behind the main wings, and at the tail part. Pressure difference between front and rear part of UG makes pressure drag dominant in this case. For lift force, it is considered to depend on the pressure distribution on the surface of the lifting body. As in Figs 23 and 24, as AOA increases positively, pressure distribution on the bottom surface is higher than upper surface of main wings and fuselage body, and positive lift force is generated which enables the UG to move upward. The wing span is taken to be the total distance between the tips of main wings, which include the width of the UG body.
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Conclusion Three-dimensional numerical analysis has been performed on the USM shallow underwater glider model with the goal to study the hydrodynamic behavior at different flow conditions. A simple Spalart-Allmaras turbulent model has succeeded in producing 3D flow behavior of the underwater glider at different angles of attack and cruising speeds. Lift and drag coefficients are observed to rise as function of angle of attack. Wake formation occurs at the tail part and also at the junction part between main wing and fuselage body. The highest pressure force is exerted on the nose of the UG compared to the other parts. The drag force can be reduced further by modifying the tail design, joining part of main wing and fuselage body, and the nose of the UG. Due to lack of previous results on similar study, the present findings could not be validated properly. Therefore as future work, adequate experiments on the UG may be performed to substantiate the proposed numerical analysis. References 1
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