Investigation on the performance of a ducted propeller in oblique flow Qin Zhang Rajeev K. Jaiman∗ Research Fellow Assistant Professor Keppel-NUS Corporate Laboratory Keppel-NUS Corporate Laboratory Department of Mechanical Engineering Department of Mechanical Engineering National University of Singapore National University of Singapore Singapore 119077 Singapore 119077 Email: [email protected] Email: [email protected] Peifeng Ma Research Engineer Keppel Offshore and Marine Technology Centre 628130,Singapore Email: [email protected] Jing Liu Research Engineer Keppel Offshore and Marine Technology Centre 628130,Singapore Email: [email protected] In this study, the ducted propeller has been numerically in- Nomenclature vestigated under oblique flow, which is crucial and challeng- ui The velocity component. ing for the design and safe operation of thruster driven vessel t The time. and dynamic positioning (DP) system. A Reynolds-Averaged r The density of fluid. Navier-Stokes (RANS) model has been first evaluated in the p The pressure. quasi-steady investigation on a single ducted propeller oper- p¥ The static pressure in the free stream. ating in open water condition, and then a hybrid RANS/LES n The kinematic viscosity of fluid. model is adapted for the transient sliding mesh computa- q The angular coordinate. tions. A representative test geometry considered here is a J The advance coefficient. marine model thruster which is discretized with structured JBP The advance coefficient at Bollard Pull condition. hexahedral cells, and the gap between the blade tip and noz- U¥ The inlet current flow. zle is carefully meshed to capture the flow dynamics. The n The rotation rate of the propeller, corresponding to the computational results are assessed by a systematic grid con- RPM (revolutions per minute). vergence study and compared with the available experimen- D The propeller diameter. tal data. As a part of novel contribution, multiple incidence b The inflow angle. angles from 15◦ to 60◦ have been analyzed with varying ad- Tp The thrust from propeller component. vance coefficients. The main emphasis has been placed on T arXiv:1801.03211v1 [physics.flu-dyn] 10 Jan 2018 n The thrust from nozzle component. the hydrodynamic loads that act on the propeller blades and Tsp The thrust from non-ducted propeller. nozzle as well as their variation with different configurations. Qp The ducted propeller torque. The results reveal that while the nozzle absorbs much effort Qsp The non-ducted propeller torque. from the oblique flow, the imbalance between blades at dif- t The ratio of propeller thrust to total thrust. ferent positions is still noticeable. Such unbalance flow dy- K The thrust coefficient based on propeller component. namics on the blades and the nozzle has a direct implication TP K The thrust coefficient based on nozzle component. on the variation of thrust and torque of a marine thruster. TN KQP The torque coefficient based on propeller component. h The ducted propeller efficiency. Utip The velocity of the blade tip. ∗ [email protected] Cp The pressure coefficient. 1 Introduction for industrial purpose with commercial software [22,23,13]. In the recent years, there has been a growing trend of re- To investigate the wake flow of ducted propeller and re- search on ducted propellers (i.e., thrusters) due to their vast duce the computational resource requirement, several hybrid industrial applications such as in the DP systems of semi- models have been developed and explored. One of the ex- submersible and marine vessels. To achieve high accuracy in amples is the modeling of combined vortex-lattice method operation and safe stationkeeping, the ducted propeller per- (MPUF-3A) with a RANS solver in a commercial code for formances need to be analyzed in a detailed manner under the unsteady flow analysis to predict the effective wake of varying conditions, which is crucial and challenging for the thrusters [24] and thruster-hull interaction [25]. A coupled success of a DP controlled platform or vessel. For example, hybrid approach whereby the actuator disk model for the pro- during DP operation, thrusters underneath the vessel coun- peller blade and the RANS model for the ducted propeller teract wave and current forces from different directions. To wake flow field was explored in [26, 27]. The MRF steady- maintain the predefined position and heading, the ducted pro- state simulation was compared with the sliding mesh tran- peller is likely to work in the off-design conditions. Unbal- sient simulation in the case of a single ducted propeller with anced hydrodynamic forces on blades and nozzle can lead to a rudder by [28], which showed one percent difference in variation of thrust and torque. The understanding of ducted prediction of forces. In the study of [29], the MRF method propeller related hydrodynamic can pave a way for improv- in OpenFOAM was evaluated with a marine propeller us- ing the strategy for DP algorithms, which contributes to a ing simpleFOAM steady state solver, the results agreed very safer and optimal marine operations. Furthermore, the relia- well with the experimental data. It can be noted that as bility and efficiency of the thruster design directly transform compared to transient sliding-interface technique, the MRF into a lower energy cost and a higher performance. method generally requires much lesser computing resource In literature, ducted propeller has been extensively stud- and provides a reasonable prediction related to the hydro- ied through physical experiments. In [1], the authors inves- dynamic force. The investigation of an open propeller and tigated the in-tubed propellers for aeronautical applications. a ducted propeller has also been extended to the off-design The authors [2] systematically conducted a series of phys- condition. A detail analysis of performances for a marine ical experiments to analyze the characteristics of DP ship propeller operating in oblique flow was conducted from 10◦ thrusters. A principal guideline for the selection of thrusters to 50◦ by [30,31]. In [26], the wake of an azimuthing thruster for dynamic positioning vessels has been proposed by [3]. (7◦) was investigated physically and numerically. The hy- In [4], the authors summarized the existing experimental drodynamics of tilted thrusters (7◦) was investigated by us- data, and proposed several empirical formulas for the mo- ing MRF steady state simulation in [23]. There is a lack mentum loss during the thruster-thruster interaction. Com- of systematic investigation dealing with the influence of the pared to the ducted propeller loading study, the investigation off-design condition on a single ducted propeller, which mo- of wake flow of ducted propeller is a relatively unexplored tivates the present study. area. In [5], Nienhuis investigated the effects of thruster in- This numerical study first investigates the single ducted teraction and measured velocities in the wake flow with LDV propeller under a wide range of advance coefficients and in- (Laser Doppler Velocimetry). PIV (Particle Image Velocime- flow angles via quasi-steady MRF-RANS model simulation try) measurement of wake flow has been conducted by [6] at and transient sliding mesh hybrid LES/RANS model simula- the downstream location up to 1.5 thruster propeller diame- tion. The results of ducted propeller force are compared with ter. The most recent physical experiment about thruster wake the experimental and the recent numerical data. The pro- flow was conducted by MARIN for the configurations such peller blades and nozzle is mainly investigated through the as single thruster in open water conditions, thruster under a force and pressure, the flow field in front and back of ducted plate, thruster under a barge [7], semi-submersible and DP propeller is also revealed. These investigations can be also drill ship [8, 9]. used in the structural analyses for thruster-driven vessel and Apart from the physical experimental studies, the ap- DP system to determine the stress and vibrations that occur plication of computational fluid dynamics (CFD) to analysis in the unit under different operating conditions. and design of marine propellers has been considered in the last 20 years [10]. Multiple models and methods adopted in open propeller investigation have also been extended to 2 Numerical Modeling ducted propeller-related interaction effects. For instant, vor- The ducted propeller flow dynamics is governed by the tex lattice lifting-line theory is used to model an asymmetric- Navier-Stokes equations of incompressible flow posed on a ducted propeller with no gap between the duct and the pro- moving frame. For the sake of completeness, we briefly peller [11]. The Multiple Reference Frame (MRF) method present the RANS model and the underlying numerical de- as a quasi-steady method is generally used to calculate the tails. ducted propeller performance such as thrust, torque and ef- ficiency [12, 13, 14, 15, 16, 4, 17, 18, 19]. Recently, the scale effect on the open water characteristics of ducted propellers 2.1 Governing equations was investigated with the MRF method via a commercial The complex flow fields around the blades of the solver [20, 21]. The transient sliding-interface simulation propeller is obtained by solving the three-dimensional of full blade geometry with a moving mesh was conducted Reynolds-averaged Navier-Stokes equations, which can be written in the Cartesian coordinate form as: 2 ¶ui ¶ui 1 ¶p ¶ ui ¶ 0 0 + u j = − + n 2 + (−uiu j) (1) ¶t ¶x j r ¶xi ¶xi ¶x j ¶u i = 0 (2) ¶xi (a) Front view. (b) Side view. 0 0 where ui is the velocity component, ui and u j are the fluctu- ating velocities, t is the time, r is the density of fluid, p is Fig. 1. Geometry of the ducted propeller.The blade position accord- ing to angular coordinate is labeled in the left figure. the pressure, n is the kinematic viscosity of fluid, xi are the coordinates.