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Aerodynamic Characteristics of the Portuguese Caravel

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Aerodynamic Characteristics of the Portuguese Caravel Aerodynamic Characteristics of the Portuguese Caravel Nuno Jorge Jesus da Silveira 16/05/11 Abstract: The Portuguese Caravel was extensively used as the main vessel on sea exploration during the XV and XVI centuries, but technical and operational information have been lost. In order to re-acquire knowledge, wind tunnel tests were conducted for a model of a Portuguese Caravel. In the tests, the model faced different wind directions, the sails being adjusted for each direction and data collected. For a given wind speed, the navigation speed in different directions was estimated by the equilibrium between the aerodynamic and hydrodynamic forces acting on the hull. The hydrodynamic resistance coefficient of the hull was estimated using an empirical formula. The estimated navigation speeds were compared to an historic maritime record. The present results suggest that the Portuguese Caravel had a uniform performance across the navigation directions tested and that the estimated navigation speeds are in agreement with the historic record information. Keywords: Portuguese Caravel, Aerodynamics, Experimental, Navigation, Speed, Wind Tunnel 1. Introduction This study‟s purpose is to determine the Portuguese Caravel operability by evaluating The Portuguese Caravel, namely the so its sailing speed as a function of the called “discovery model”, was extensively navigation angle (between the wind and the used in the XV and XVI centuries. Its ability sailing course) using the equilibrium between to sail windward and out manoeuvre other the aerodynamic and hydrodynamic forces vessels in case of danger permitted the action on the caravel. The aerodynamic force Portuguese to gain the edge against other is measured in wind tunnel tests performed nations in terms of sea exploration and for a representative model whereas the mercantile expansion. hydrodynamic force is estimated from Due to its importance in the territorial and empirical correlations based on the ship´s economic expansions, technical and geometry and weight. operational information regarding it was Due to the lack of knowledge on the sails considered classified and so passed only by behaviour a set of different relative oral transmission through generations of ship sail-course sailed positions were tested. For builders resulting in actual scarce each of these positions, the sail form and its information, the most available being the one aerodynamic angle of attack were varied in supported by artistic means (paintings and order to obtain the best performance. literature). Understanding how these ships behave sailing becomes a natural, but challenging, 2. Vessel’s Brief Description task following an intense historical research Historical references say that the Portuguese on construction techniques. Caravel could have two or three latin (triangular) sails, although two masts were common. It could sail windward, was fast for its time and highly manoeuvrable. Fig. 3 - Portuguese Caravel Bartolomeu Dias. 3. Sailing forces geometry Fig. 1 - Artistic drawing of a three sail Portuguese Caravel Sailing forces are obtained according to Fig. 4, where β is the apparent wind angle, λ There is no absolute knowledge on what the the yaw angle and the difference between Caravel‟s real shape was, but even so, two both, β-λ, is the navigation angle. different Caravels real scale representations were made, Figs. 2 and 3, with divergent results. Fig. 4 – Sailing forces and angles [1] The yaw angle is originated by the aero-hydrodynamic equilibrium, but during aerodynamic essays λ was considered 0°. 4. Basic concepts Fig. 2 – Spanish Caravel Niña III. As all sailing vessels, the navigation force is due to the aerodynamic force generated in the sails. As such, aerodynamics comprehension was required in order to velocity. However the main issue of the know the ship‟s behaviour and a brief insight difference in Reynolds number (laminar or is presented here. turbulent flow) may be overcome using a transition wire placed at the sail‟s leading 4.1. Dimension Analysis and Similarity edge, according to Gibbings criteria [3] (5). The aerodynamic force is dependent on the air properties, body geometry and air-body (5) orientation (1) Fortunately, this effect is by the sail (1) supporting beam – the so called antenna - and, adapting the transition wire‟s criteria to and by conducting a dimension analysis [2], the model‟s antenna dimension, it was the resulting aerodynamic force coefficient is possible to establish that the minimal wind defined as (2). speed in the wind tunnel tests to force transition to turbulent flow should be 1.76 m/s or Re = 52300, considering average sail‟s (2) chord as the characteristic length. (3) CF depends also on the wind-sails relative position, angle of attack α and sails camber θ. Therefore, CF is dependent on the Reynolds Sails are flexible wings but once they take a number (3), angle of attack and camber, Fig. stable form, they resemble thin wings and 5. aerodynamics studies can be conducted with identical procedures to those applied to thin wings. Fig. 5 – Angle of attack and aerodynamic force. [1] Fig. 6 – C and C variation with α and θ. In order to obtain representative coefficients, L D similarity laws have to be respected and tests From those studies, coefficients of Re should be equal to the real one. The aerodynamic force decompositions, CL and equality of Reynolds number is impossible to CD increase at first with α and then CL achieve because it would require a wind decrease while CD keeps increasing due to velocity larger than wind tunnel top wind the occurrence of flow separation, Fig. 6 With θ increase, CL and CD results with α are (8) anticipated (results translation movement in α‟s axis, Fig. 6) (9) 5. Wind Tunnel Facilities and Model The aerodynamic tests were performed at a the LNEC‟s open circuit 9 m long wind tunnel that has a 3.1 x 2.0 m2 cross section. The air flow is established by a set of six 1.1 kW fans providing velocities up to 18 m/s. The flow velocity [2] (6) was determined from the dynamic pressure acquired by a Pitot tube connected to a Betz type manometer, with atmospheric pressure and temperature Fig. 7 – Balance, aerodynamic and correction. course-sailed referential. (6) The aerodynamic forces generated in the model were measured by a previously calibrated balance made of a deformable column, equipped with strain gauges, rigidly fixed to the model and a base by top and bottom rigid plates under the wind tunnel. The model used for testing was adapted from Fig. 8 - Model in the wind tunnel. an already existent one, assembled by Dr The model adaptation consisted mainly on Amaral Xavier [4] to [12], at a scale of aprox. the reinforcement at the main mast base and 1:40 and showing only the dry part of the hull, on the elimination of the gap between the hull Fig. 7 to 9. bottom and the wind tunnel floor. Extra Due to the balance assembling and fixation lashing points for sail‟s “loose” end were also geometry the measured force had to be provided outside the hull and a compensation translated from the balance reference axis weight to balance the mass centre position. (YY-ZZ) to both aerodynamic referential (DD- LL) and course sailed referential (SS-HH), as seen in Fig. 7, according to equations (7) to (9). (7) Fig. 9 - Applied adaptations 6. Wind Tunnel Tests Fig. 10 - Trimmed sail. Latin sails in the XV/XVI centuries did not use a boom at the sail´s bottom chord (as Once the mainsail‟s position is determined, nowadays ships). Therefore, they require a the secondary sail was hoisted while the way to fix the sail‟s “loose” end. This fact main was kept untouched. Except sail trim, makes the identification of the best sail all secondary sail procedures were the same position troublesome. To overcome this as those applied to the mainsail. difficulty, the tests were divided in two In phase 2, the navigation angle was varied phases: as well as the sails‟ angle of attack in relation to the reference values due to the sails-hull 1. For a given navigation angle, the sails‟ air interference. A total of five angles of best position (angle of attack) was attack were tried out for each navigation determined via the highest measured angle and for each angle of attack nine sailing force; velocities were tested. 2. Based upon that “reference” sails‟ The required test parameters were navigation position, the aerodynamic forces were angles, angles of attack, velocities, forces recorded varying the navigation angle. and sail chords. In phase 1) reaching the best angle of attack requires the sail to “cross” the ship‟s deck 7. Results taking the antenna leading edge to stay 7.1. Forces Coefficients Determination outboard, Fig. 10, and also requiring trimming the sail. From phase 1. results it was possible to Actually such sail position is often seen in establish the range of valid Re to ensure paintings and illustrations, as well as in similarity conditions (elimination of Reynolds present ships still using that kind of sails. number influence) – Re > 53200, Fig. 11 and 12. The force coefficients CL, CD, CFs were then evaluated by linear regression applied to the pairs force versus dynamic pressure, Fig. 12. Tab. 1 – Errors from used equipment. 7.3. Navigation Velocity Estimation For each navigation angle the highest sailing force coefficient, CFs, was taken out of five tested sail positions, taking into account the evaluated uncertainties. The aero-hydrodynamic equilibrium is Fig. 11 – Line mark (black) from Gibbings criteria. needed to estimate the ship‟s speed (11) and 4 y = 0,9537x + 0,1373 CFhydro and had to be evaluated. 3 R² = 0,999 F (N) F 2 L (11) 1 y = 0,4631x + 0,1243 D R² = 0,997 0 0 1 2 3 4 The hydrodynamic force coefficient had to be (Pt- Pe).A (N) empirically estimated by (12) to (15) [13], due to the lack of model„s full hull geometry and Fig.
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