Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec FLUID STRUCTURE INTERACTION SIMULATION OF SPINNAKERS – GETTING


CLOSER TO REALITY (DOI No: 10.3940/rina.ijsct.2011.b2.115)


H F Renzsch, Technical University Delft, The Nethreland K U Graf, University of Applied Sciences Kiel, Germany SUMMARY


This paper describes the current implementation of FlexSail, a Fluid-Structure-Interaction program for the simulation of the behaviour of spinnakers. A short outline of general membrane theory is given, the major focus lies on the impact of wrinkling and validation. A numerical modelling of anisotropic wrinkling is presented, its impact on flying shape and flow forces investigated. A newly implemented solver is described. A series of test cases comparing the numerical results to data from the Yacht Research Unit Kiel Twisted Flow Wind Tunnel is presented and discussed.


NOMENCLATURE a


a


A ^ ¯ ˜


 in orientation of  AWA


xx, yy, xy 11, 22, 12


AWS 


1, 2, 1, 2 E


G H


 mi





PDyn Ri


Air 


m t


Vi xi


y+ 1.


displacements of the sail under wind load. Thus fluid structure interaction is needed.


scalar vector matrix


material coordinate system element coordinate system wrinkled coordinate system element axes material axes


apparent wind angle [deg] apparent wind speed [m s-1] strain


principal strains, stresses [N m-1] Young’s modulus [N m-1] shear modulus [N m-1] Hessian matrix rotation angle


virtual mass of node i Poisson number


dynamic pressure [N m-2]


total force on node i (residual) density of air [kg m-3] stress [N m-1]


material stress [N m-1] at time t


velocity of node i displacement of node i dimensionless wall distance INTRODUCTION


Typically, new spinnaker designs are evaluated by wind tunnel testing. Due to the problems associated with the simulation of the partially separated flow usually found around spinnakers, numerical


investigation of a new design is currently still a niche application.


Simulations of the flow around sails, in particular spinnakers not only have to cope with the problem of flow separation, they also have to account for the large


The historical development of Fluid-Structure-Interaction codes for the simulations of upwind sails lead to a split between structural and flow code that made sense for applications where the structural code


majority of the computational power. Usually the two codes where


structural code triggers the flow code, both codes in turn calculating until convergence individually.


These structural codes have been successfully coupled with RANSE codes in the past, yet, due to both codes calculating until convergence on each iteration, computation costs for the simulation were extremely high and practical solutions were limited to steady state.


FlexSail is a Fluid-Structure-Interaction program specifically designed to include a RANSE solver as flow code and still run in an efficient manner. To this end a different


coupling paradigm, suited to the global instationarity


computational costs of RANSE simulations, is used. This method is able to simulate a quasi-steady state as well as globally instationary behaviour of spinnakers, capable to solve


or stability problems


associated with downwind sail operation. 2. FLEXSAIL – BASIC IDEA


Like any other Fluid-Structure-Interaction program FlexSail iterates the flow and structural solver to find equilibrium in both solutions and a converged state in the coupling of


both. Flow is computed using the


commercial flow solver AnsysCFX 12.0, a program for the simulation of viscous flow by solving the steady or unsteady RANSE equations. The structural behaviour is simulated by a purpose-written membrane finite element code, capable of simulating large displacements and highly non-linear behaviour. RANSE solver.


It


needed the either coupled in batch mode or the


high


is embedded in the


What sets FlexSail apart from other FSI solution methods is the coupling paradigm. The basic idea is to run the flow simulation in an unsteady mode. That means that


©2011: The Royal Institution of Naval Architects B-71


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