A numerical study of scale effects on performance of a tractor type podded propeller

International Journal of Naval Architecture and Ocean Engineering.
2014.
Jun,
6(2):
380-391

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

- Published : June 30, 2014

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INTRODUCTION

A podded propeller is consisted of propeller and pod housing which is usually composed with pod, strut and fin. In case of a tractor-type podded propeller, since its slipstream is disturbed by the pod housing located behind the propeller, the performance of the propeller blades is different from that of a conventional propeller. Also the force acting on the pod housing has different characteristics compared to the drag in uniform flow because of flow acceleration, pressure change and swirl flow, which are induced by rotating propeller blades. Therefore, in order to design a podded propeller with a good propulsive performance, it is necessary to understand not only how the interaction between propeller and pod housing has an effect on propulsive performance, but also which components of the performance are influenced by propeller loading and Reynolds number. However, the information mentioned above is rarely found, especially regarding the full scale propulsive performance, because there exist difficulties in obtaining the full scale performance of a podded propeller from model test due to the limitation of experimental facility and the uncertainty of full scale extrapolation method. Currently, there are various methods for estimating the full scale performance of a podded propeller(
ITTC, 2005
;
2008
). It is mostly accepted that the ITTC correction method can be applied for the propulsive performance of propeller with the consideration of the interaction. Meanwhile, in case of a full scale drag of the pod housing, various methods considering acceleration of propeller slipstream and Reynolds number have been suggested (
Holtrop, 2001
;
Sasaki et al., 2004
). Also, the method for utilizing the drag ratio of model scale to full scale using CFD has been proposed (
Lobatchev and Chicherine, 2001
;
Chichern et al., 2004
). Considering the difficulty of a full scale experiment, this can be measured as a reasonable option. The applications of CFD analysis to podded propeller continue to grow and the studies on the drag of pod housing (
Sanchez-Caja and Pylkkanen, 2004
;
Deniset et al., 2006
) and the interaction of podded propeller (
Kim and Kim, 2002
;
Ohashi and Hino, 2004
;
Sanches-Caja and Pylkkanen, 2006
;
Gaggero et al., 2010
;
Zhang and Wang, 2006
) have been performed.
Despite the studies mentioned above, the CFD analysis of podded propeller performance in both model and full scale did not yet improve the method of estimating the full scale performance of podded propellers from model tests nor provide any significant result useful to design better propeller and pod housing. Especially, the study regarding the pod housing drag, which changes according to propeller loading and Reynolds number and the change of propeller performance induced by the effect of pod housing, is lacking despite its high importance.
In this study, the CFD analysis for a tractor type podded propeller is carried out. The performance of the podded propeller is analyzed from model to full scale to investigate its scale effects. The pod housing drag is scrutinized specifically to find out its variation with propeller loading and Reynolds number. Also, in order to study how the propulsive performance of propeller blades and the drag of pod housing are influenced by the interaction between the propeller and the pod housing, the calculations for both propeller without pod housing and pod housing without propeller are performed.
DESCRIPTION OF GEOMETRY AND CONDITIONS

- Geometry

The tractor type podded propeller consists of propeller, pod, strut and fin. The principal dimensions of the podded propeller are given in
Table. 1
. The features of pod housing include the cross-sectional area, in which the pod is 21% of propeller disk area, and the relatively thick strut.
Principal dimensions of the podded propeller.

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- Calculation conditions

In order to investigate the effect of Reynolds number (Rn), numerical analysis is conducted for 6 Reynolds numbers from model scale to full scale. The Reynolds number is defined by the length of the podded propeller and inflow velocity (
Calculation conditions.

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NUMERICAL METHOD

For the 3-dimensional incompressible steady state turbulent flow, the governing equations are continuity and momentum equation (RANS equation). Through a process of discretization based on the finite volume method, the algebraic equations are solved. A commercial code, Fluent (V13), is used for the computations. The convection and diffusion terms of momentum equation are discretized by QUICK and 2nd order central-difference scheme, respectively. Turbulence model is the realizable k- ε model which is popularly used for numerical analysis of ship flows, with standard wall function. SIMPLEC algorithm is used for the velocity-pressure coupling and MRF (Moving Reference Frame) scheme is adopted for propeller rotating.
Fig. 4
shows the computational domain which is defined by the boundaries as follows: the inlet and the external boundaries are located at 2.5L from the hub nose and pod side, respectively, with velocity inlet condition; the outlet boundary is located at 4L from pod tail with pressure outlet condition. As a result, the 3 dimensional structured spatial grids of O-H type around the podded propeller are generated. Also, a multi-block grid for using MRF scheme is generated and then the grids are divided into two blocks of fixed block and rotating block for rotating propeller.
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RESULTS AND DISCUSSION

- Performance in model scale

The numerical results of model scale are compared with experiments which are carried out in the large cavitation tunnel (SCAT) of Samsung Ship Model Basin (SSBM). The inflow speed of the test section is 5
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- Scale effect and interaction in propulsive performance

In order to investigate into the scale effect, the calculations are carried out for podded propeller from model scale to full scale Reynolds numbers. The inflow velocity is varied among 3
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- Scale effect and interaction in pod housing drag characteristics

The interaction will also act in pod housing as much as the podded propeller blade, thus, the drag variation of pod housing has been investigated.
Fig. 11(a)
shows the drag of pod housing in larger scale than in
Fig. 7(b)
. The
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CONCLUSIONS

In this paper, the CFD analysis for the tractor-type podded propeller is carried out for 6 Reynolds numbers from model scale to full scale. From the results, the scale effect by Reynolds number and interaction between podded propeller blade and pod housing by propeller loading are investigated. Firstly, it is confirmed that the numerical results of model scale agree well with the experimental results of a large cavitation tunnel and the usage of CFD analysis for estimating the performance of podded propeller is reliable at least for model scale Reynolds number.
It is found that the increments of the thrust and the torque of podded propeller blade by the interaction with pod housing are caused by the change of the slipstream of podded propeller which is altered by the pod housing. And those are mainly influenced by advance coefficient, i.e. propeller loading, which is affected relatively little by the change of Reynolds number.
On the other hand, it is confirmed that the drag of pod housing with propeller in operation is different from that of pod housing without propeller due to the acceleration and swirl of propeller slipstream which is altered by propeller loading as well as the pressure recovery and friction according to Reynolds number, which suggests that the pod housing drag under the condition of propeller in operation is the key factor of the scale effect on the performance between model and full scale podded propellers. The so called ‘drag ratio’, which is the ratio of pod housing drag to total thrust of podded propeller, increases as the advance coefficient increases due to accelerated flow in the slipstream of the podded propeller. However, the increasing rate of the drag ratio reduces continuously as the Reynolds number increases from model to full scale progressively. The pressure recovery in trailing edge of the strut and the pod appears faster in full scale than in model scale, and the friction coefficient is smaller in full scale than model scale. However, in spite of the fast pressure recovery near the pod tail end in full scale, the pressure component of the drag of pod is larger in full scale than in model scale due to the pressure distribution in the region of the junction of the pod tail which is affected by the strut.
From this study, it is confirmed that the scale effect by Reynolds number and the interaction between propeller and pod housing is quite important, and when estimating the full scale performance of podded propeller blades and pod housing for design, the consideration of scale effect and interaction is necessary. Also, this information with respect to scale effect can be used to suggest a full scale extrapolation method about the pod housing drag and, moreover, an extrapolation method for full scale performance of pod housing drag has been suggested and the method is applied for the pod housing drag ratio of model scale to full scale using CFD results of this study. The detail information can be found in
Park et al. (2013)
.
Acknowledgements

This research was sponsored by research grant PNS1860 under the administration of the ministry of knowledge economy, Korean government.

Choi J.K.
,
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A study of using wall function for numerical analysis of high reynolds number turbulent flow
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Citing 'A numerical study of scale effects on performance of a tractor type podded propeller
'

@article{ E1JSE6_2014_v6n2_380}
,title={A numerical study of scale effects on performance of a tractor type podded propeller}
,volume={2}
, url={http://dx.doi.org/10.2478/IJNAOE-2013-0186}, DOI={10.2478/IJNAOE-2013-0186}
, number= {2}
, journal={International Journal of Naval Architecture and Ocean Engineering}
, publisher={The Society of Naval Architects of Korea}
, author={Choi, Jung-Kyu
and
Park, Hyoung-Gil
and
Kim, Hyoung-Tae}
, year={2014}
, month={Jun}