Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
shown for the response amplitude of the angle of attack (left panel) show about 20% deviation in the measured range of the angle of attack compared with the demand range of the angle of attack which again was expected based on the frequency response presented in Figure 12. As was explained before, mechanical operation
angle. This error is relatively more absolute error of about ±0.5 degree caused by the mechanical linkage of the model.
The observed phase lag between measured angle of attack and demand angle of attack increases
this discrepancy is due to of the T-Foil actuator which
introduced a dead-space in the stepper-motor gearbox. This slack causes an error when an angle is demanded for the stepper-motor and its response is not exactly equal to the demand
significant for the low demand T-Foil incidence (α) range of ±5º as the ratio of response deflection to the demand deflection decreases with decreasing range of the demand control angle range. The results shown for the unsteady lift coefficients (right panel) with a ±5º range show that the Theodorsen theory somewhat over predicts the magnitude of unsteady lift at the all frequencies in this case. However the quasi-static calculation is relatively close to the experimental magnitude of unsteady lift at all frequencies. imperfections
These outcomes are all related to in the mechanism becoming relatively larger for small movements.
In general, it can be said that there is an acceptable agreement between experimental data and theoretical data. Thus, it is acceptable to use the Theodorsen theory in combination with the quasi-static calculation to predict the dynamic lift coefficients in numerical simulations of motion control systems as the basis for evaluating appropriate control algorithms.
6. CONCLUSIONS 7. Under steady conditions the effect of low Reynolds
number on lift performance is not very significant and the results obtained here show that the model scale T- Foil performs adequately to act as a control surface on the bow of an INCAT Tasmania 2.5m catamaran model. Similar results were found at
different water diminished due to the flow
velocities and it is evident that the T-Foil performance is not
effect of low operating
Reynolds Number. The lift curve slope of the T-foil was found to be 2.45 per radian, this being 61% of the value for an ideal foil of the same aspect ratio with elliptic load distribution.
Under unsteady conditions, the magnitude of the
measured angle of attack as a ratio to the demand angle of attack decreases as the frequency increases. This ratio is close to unity for the range of frequencies up to 4 Hz for which model testing will be conducted. This outcome can be explained on the basis of the mechanical operation of the stepper motor used to drive the T-foil where there is a dead-space in the stepper-motor gearbox as well as slack in the connections between the motor and T-foil. As a consequence, when an angle is demanded for the stepper-motor, its output angle is not exactly equal to the demand angle. This error is relatively more significant for the lower demand angle range and is due to an
The support of INCAT Tasmania, Revolution Design, the University of Tasmania and the Australian Research Council is gratefully acknowledged.
8. 1.
REFERENCES
http://www.incat.com.au/#.
2. DAVIS M. R. and HOLLOWAY D. S., "Motion and
catamarans in oblique seas," International shipbuilding progress, vol. 50, pp. 333-370, 2003.
3. HOLLOWAY D. and DAVIS M., "Ship motion computations using a high Froude number time domain strip theory," Journal of ship research, vol. 50, pp. 15-30, 2006.
4.
JACOBI G., THOMAS G., DAVIS M., HOLLOWAY D., DAVIDSON G. and ROBERTS T., "Full-scale motions of a large high-speed catamaran: The influence of wave environment, speed and ride control system," International Journal of Maritime Engineering, vol. 154, pp. A143-A155, 2012.
5. JACOBI G., THOMAS G., DAVIS M. R., and DAVIDSON G., "An insight into the slamming
passenger discomfort on high speed ACKNOWLEDGEMENTS
The general conclusion of this investigation is that the unsteady performance of the low Reynolds number model scale T-foil with a relatively low aspect ratio is adequate for application to scale model towing tank tests. It is therefore anticipated that tank testing of a complete 2.5m catamaran model fitted with a model RCS system will lead to the identification of the best motion control algorithms for reducing ship motions and thus contribute significantly to improvement of passenger comfort and reduction of structural loads.
with
increasing frequency reaching about 30º at 4 Hz. The phase lag increases approximately in proportion to frequency and thus appears to be caused by time delay and slew rate limitations in the stepper motor actuation system.
It was found that that there is a generally moderately good agreement
between the temporal variation of
experimentally measured lift coefficients and theoretical lift coefficients derived from a combination of the static lift curve slope and the Theodorsen theory for unsteady lift. This leads to the conclusion that it is acceptable to use the Theodorsen theory for the effect of frequency in combination
with quasi-static predictions at low
frequency to predict the dynamic lift coefficients during model testing to evaluate control algorithms.
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©2015: The Royals Institution of Naval Architects
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