THE 22nd CHESAPEAKE SAILING YACHT SYMPOSIUM ANNAPOLIS, MARYLAND, MARCH 2016 Modal Analysis of Pressures on a Full-Scale Spinnaker Julien Deparday1, Naval Academy Research Institute, Brest, France Patrick Bot, Naval Academy Research Institute, Brest, France Fréderic Hauville, Naval Academy Research Institute, Brest, France Benoit Augier, Naval Academy Research Institute, Brest, France Marc Rabaud, Laboratoire FAST, Univ.Paris-Sud, CNRS, Université Paris-Saclay, F-91405, Orsay, France Dario Motta, University of Auckland, New-Zealand David Le Pelley, University of Auckland, New-Zealand NOTATION AWA Apparent Wind Angle AWS Apparent Wind Speed IQR Inter Quartile Range (Q3-Q1) ABSTRACT Q1 First Quartile Q3 Third Quartile POD Proper Orthogonal Decomposition Load While sailing offwind, the trimmer typically adjusts the CF Load Coefficient ( 1 2 ) 2 ρ S(AWS) downwind sail "on the verge of luffing", letting occasion- Pleeward−Pwindward ∆CP Differential pressure coefficient ( 1 ρ(AWS)2 ) ally the luff of the sail flapping. Due to the unsteadiness 2 ρ Density of air (1:25 kg/m3) of the spinnaker itself, maintaining the luff on the verge of S Sail Area of the asymmetrical Spinnaker (65 m2) luffing needs continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aero- INTRODUCTION dynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultaneously measured time- In research and development in sail aerodynamics, full-scale resolved pressures on the spinnaker, aerodynamic loads, testing, wind tunnel testing and numerical simulation have boat and wind data. Significant spatio-temporal patterns always been complementary. Numerical simulation allows are detected in the pressure distribution. In this paper we to efficiently investigate different designs without the cost present averages and main fluctuations of pressure distribu- of creating sails (Chapin et al., 2011, Durand et al., 2014, tions and of load coefficients for different apparent wind an- Ranzenbach et al., 2013, Viola et al., 2015). Nowadays, gles as well as a refined analysis of pressure fluctuations, us- advanced computational resources have enhanced numeri- ing the Proper Orthogonal Decomposition (POD) method. cal simulation and have allowed to couple fluid and struc- POD shows that pressure fluctuations due to luffing of the tural solvers to create Fluid-Structure Interaction simula- spinnaker can be well represented by only one proper mode tions (Chapin et al., 2011, Durand et al., 2014, Ranzenbach related to a unique spatial pressure pattern and a dynamic et al., 2013, Augier et al., 2014, Lombardi et al., 2012, Ren- behavior evolving with the Apparent Wind Angles. The zsch and Graf, 2010, Trimarchi et al., 2013). However wind time evolution of this proper mode is highly correlated with tunnel testing and full-scale testing are required for compar- load fluctuations. ison and validation (Hansen et al., 2002, Renzsch and Graf, Moreover, POD can be employed to filter the measured pres- 2013, Viola and Flay, 2011). Wind tunnel testing has the sures more efficiently than basic filters. The reconstruction advantage to be in a controlled environment where a bal- using the first few modes allows to restrict to the most en- ance can be used to measure the forces created by the sails ergetic part of the signal and remove insignificant variations on the boat frame (Campbell, 2014a, Flay, 1996, Graf and and noises. This might be helpful for comparison with other Müller, 2009, Zasso et al., 2005). Those results can easily measurements and numerical simulations. be used to create a Velocity Prediction Program (Campbell, 2014b, Le Pelley and Richards, 2011). Nevertheless with wind tunnel testing, some rules of similitude are violated as [email protected] the Reynolds number, or the ratio of fabric weight to wind pressure or the ratio of membrane stress to wind pressure. EXPERIMENTAL SETUP Full-Scale testing does not have those issues, and permits to determine yacht performance in real sailing conditions. An instrumented J/80 class sailing yacht, an 8 meter one- Those experiments need complex set-up in a harsh envi- design cruiser racer was used during those experiments. A ronment but actual aerodynamic loads can be assessed in tri-radial asymmetrical spinnaker with a surface of about a variety of ways. Sail boat dynamometers (Herman, 1989, 65 m2 with a 12 meter long rounded luff was hoisted as well Hochkirch and Brandt, 1999, Masuyama, 2014) measured as a mainsail of 17 m2. Boat and wind data, loads on the forces from upwind sails transmitted to the boat frame. Fos- standing rigging and on the sails were recorded. Moreover sati et al. (2015) created a sail boat dynamometer with the pressure taps, developed by the Yacht Research Unit from possibility of measuring aerodynamic forces of downwind the University of Auckland were stuck on the spinnaker to sails. Augier et al. (2012) carried out experiments where acquire the dynamic pressure distribution. They were syn- loads on the rigging lines and sails were measured. They chronized with the other data thanks to an acquisition soft- contributed to a better comprehension on interaction be- ware, RTMaps developed by Intempora which received ev- tween the wind, the rigging and the sails. Le Pelley et al. ery signal at their own rate and timestamped them “on the (2015) measured the forces and the directions on the three flow”. A resampling was applied during the post process- corners of spinnakers. Le Pelley et al. (2012), Lozej et al. ing to obtain synchronous data for easier analysis. Figure 1 (2012), Motta et al. (2014), Viola and Flay (2010) measured shows the arrangement of all the sensors set onboard. pressures on sails for upwind and downwind sails. This setup for downwind navigation is a further develop- However downwind sails are more complex to study than ment of the experimental system described in Augier et al. upwind sails mainly due to their non-developable 3D shape (2012) which was used for measurements in upwind navi- with highly cambered sections and massively detached flow gation. around a thin and very flexible membrane. Due to the dynamic behavior of this unsteady fluid-structure interac- 3D anemometer tion, the pressures on the sail vary quickly. Even in sta- ble conditions, offwind sails have an inherent unsteadi- ness. One key feature of spinnaker unsteadiness comes from the flapping at the leading edge, also called luffing. We have previously investigated pressure evolution during luff- ing (Deparday et al., 2014, Motta et al., 2015). In Depar- Pressure sensors day et al. (2014), we showed an example where flapping of spinnaker creates pressure peaks at the leading edge in- creasing the aerodynamic force dynamically by 50%. Due to the non-stationarity of the environment while sailing, spatio-temporal pressure data are complex to analyze and therefore to simulate. However significant and different spatio-temporal patterns can be spotted (Motta et al., 2015) and might be produced by different physical causes (Fluid- Structure Interaction, wind variations, boat motions, etc.). Load sensors In this paper we present an approach to decompose complex pressure evolutions into simpler modes. It would then allow GPS z easier analysis and comparison with simulations. Compass This paper presents results of full-scale experiments of an x instrumented J/80 class yacht in offwind conditions were loch loads, pressures on the spinnaker, boat and wind data were motion sensor measured. After presenting the experimental apparatus, av- erage and fluctuations of pressures and loads are presented. The next section is the use of the Proper Orthogonal De- Figure 1: General arrangement of the experimental set-up composition (POD) method on pressures to create a simpler on the J/80. 16 load sensors (green discs), 44 pressure taps model of the complex variations of pressure distribution in (red circles), and wind and boat sensors (blue squares). time. We show then that the method also helps to highlight the correlation between the main evolution of pressures and the variations of loads. Loads The standing rigging (shrouds, forestay and backstay) is fit- ted with custom-made turnbuckles and shackles equipped with strain gages. The running rigging (the corners of the mainsail and of the spinnaker -head, tack and clew-) is is no need for a measure of a reference pressure, a compli- equipped with instrumented shackles too. For the stand- cated task in full-scale experiments. Those sensors are stuck ing rigging and the mainsail the sensors are connected to on one side -at the intrados when sailing on portside tack- a load acquisition system Spider8 from HBM. Voltages are and are positioned facing 2mm-diameter holes on the sail to received from all strain gages and amplified. They are then measure the pressure jump across the sail without signifi- converted in digital data at a rate of 25 Hz. Thereafter cant air leak. Punctured light sail cloth patches are applied they are transferred to the real-time acquisition software, on the pressure taps to profile the sensors. This custom built RTMaps. Due to the high displacements of the spinnaker pressure system was designed by the Yacht Research Unit at -in the order of magnitude of 1 to 5 meters for the spinnaker the University of Auckland. These pressure transducers are used in those experiments-, the instrumented shackles on the connected by wires to the receiver inside the boat and thus three corners of the spinnaker communicate wirelessly to the are synchronized with the other data. acquisition system. The clew sensor is connected via a wire The pressure sensors have a sampling frequency of approxi- running along the foot of the sail to a small box located near matively 10 Hz with a maximum range of ±1 kPa and a res- the tack point of the spinnaker (see Figure 2).