Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 DISCUSSION
FLUID STRUCTURE INTERACTION SIMULATION OF SPINNAKERS – GETTING CLOSER TO REALITY
H F Renzsch, Technical University Delft, The Netherlands and K U Graf, University of Applied Sciences Kiel, Germany (Vol 153 Part B2, 2011)
COMMENT
Dr Ignazio Maria Viola Lecturer in Naval Architecture, Newcastle University
I think that having a FSI code based on Reynolds Averaged Navier-Stokes equations, instead of Potential Flow methods, is very important. Your effort is leading to new significant breakthroughs. Moreover, your paper offers very interesting solutions to deal with wrinkling, which is certainly an issue for spinnakers.
I would suggest few points which you might like to discuss further.
(1) I wonder if you would like to provide some references of previous works in order to offer a wider background to your work.
(2) It is not clear to me how the spinnaker is trimmed in the CFD simulations. You stated that it is trimmed accordingly to the setting recorded during the experiments but,
later, differences
between the numerical and the experimental flying shapes are presented. Are the length of the sheet and the position of the boom imposed, while the direction of the sheet is computed?
(3) The proposed FSI code has certainly great potentiality. However, before using it as a design tool, I think that its numerical uncertainty should be
estimated, for instance with a (4)
verification and validation analysis as proposed in ref [15] by Stern, Wilson, Shao, “Quantitative V&V of CFD simulations and certification of CFD codes”, Int. J. Numer. Meth. Fluids 2006; 50:1335–1355. Figure
22 shows numerical/experimental
differences in the forces larger than 20%. For instance, while at 112.5º the numerical and experimental drive forces are in good agreement, at 90º the drive force is over- estimated by roughly 20%. Similarly, while at 120º the numerical and experimental side forces are in good agreement, at 105º the side force is under-estimated by roughly 20%. Until further
developments are made, I think that this
disagreement does not allow concluding that the FSI code can be used as a design tool.
(5) Figure 17 shows the pressure coefficient, Cp, on a sail section. Near the leading edge, on the windward side, Cp is more than 1.2, while the maximum Cp shall be Cp=1 at the stagnation point. A dynamic pressure over-estimation was also reported
by other authors in sail
aerodynamics (Richards and Lasher, “Wind tunnel and CFD modelling of pressures on downwind sails”, In ref [16]: BBAA VI
Int.
Colloquium on: Bluff Bodies Aerod. & Appl. Milano, Italy, July 20-24, 2008) and I wonder if you would like to comment on this.
AUTHORS’ RESPONSE
(1) The idea for FlexSail was borne out of the attempt to supplement the tests conducted on spinnakers in a twisted flow wind-tunnel by full-scale simulations. The need for
viscous flow simulation arose out of the
partially separated flow observed around spinnakers. The structural simulation has been significantly inspired by the work of Peter Heppel and Prof. Vinicius Arcaro.
The development references 17-21.
(2) During the measurements the position of spinnaker pole and sheet lead as well as sheet length are recorded. The pole’s end, the lead and the halyard “sheave” (actually a knot) are takes as fixed points, the direction and elongation of the sheet are computed.
(3) We have to admit that an uncertainty analysis has not been carried out as yet. The principal focus until now has been on the development of the code.
(4) As indicated within the text, some effect on the forces might be due to the omission of the mast in the simulations. A typical source of such differences can be difficulties in the prediction of separation points. Figure 9 of the paper indicates that grid insensitivity has not yet been reached.
After finding some errors in the windtunnel’s velocity measurements, the
simulation were reassessed. The following
shows the results of this reassessment of the calculated respectively
results of measurement and diagram
measured forces based on calculated
respectively measured velocities at the same location upwind of the masthead. While still not perfect, the results show a marked improvement of agreement of the driving force areas (Ax), the side force areas (Ay) still require quite some improvement.
©2014: Royal Institution of Naval Architects B-107 of FlexSail can be tracked by
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