Podded Propulsor Research Articles

A Numerical Study of the Hydrodynamic Noise of Podded Propulsors Based on Proper Orthogonal Decomposition

Podded propulsors have become a focal point of research in the field of marine propulsion in recent years due to their high efficiency, low noise, and excellent maneuverability. To investigate the acoustic characteristics induced by the flow field of podded propulsors, a high-precision unsteady numerical simulation was conducted using the Delayed Detached Eddy Simulation (DDES) coupled with Ffowcs Williams–Hawkings (FW-H) equations. Multiple spatial acoustic receiving arrays were employed, and analysis methods including Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) were utilized to determine the spatial distribution of the acoustic field of the podded propulsor. The results show that the blade passing frequency and the shaft frequency consistently dominate as the primary characteristic frequencies. On the plane of the propeller disk, the distribution of sound pressure levels is uniform without distinct directivity. Across the space curved surface, approximately the first ten POD modes encompass 99.8% of the total energy, and their spatial distribution characteristics of sound pressure are closely related to the pod structure. Additionally, these modes exhibit characteristic frequencies such as the blade passing frequency and shaft frequency. The spatial distribution of sound pressure at a single frequency on the spatial surface corresponds well with the results obtained from the POD analysis.

EXPERIMENTAL INVESTIGATION OF HYBRID CONTRA-ROTATING SHAFT POD PROPULSOR IN THE CAVITATION TUNNEL

In order to analyze the interactions among various components of hybrid contra-rotating shaft pod (HCRSP) propulsor and effects of test devices, the model test of HCRSP propulsor was conducted in a cavitation tunnel. In the present study, the interactions among fore propeller, aft propeller, and the pod of HCRSP propulsor are analyzed with the influence of a long-axis dynamometer. It was found that the long-axis dynamometer shows great influence on the pod propulsor, which is made up of a pod and an aft propeller, but little influence on the fore propeller. Especially, under the condition of using a long-axis dynamometer or not, the difference of thrust coefficient and torque coefficient of the aft propeller with a high advance coefficient reached 10%. Also, the fore propeller could accelerate the flow of the pod propulsor. In detail, the pod and aft propeller show blocking and suction effects, respectively, and these impacts could cancel each other out within a certain range.

Numerical Study on the Unsteady Flow Field Characteristics of a Podded Propulsor Based on DDES Method

The podded propulsor has gradually become an important propulsion device for high technology ships in recent years because of its characteristics of high maneuverability, high efficiency, low noise, and vibration. The performance of podded propulsor is closely related to its flow field. To study the unsteady flow field characteristics of podded propulsor, the DDES (delayed detached eddy simulation) method was used to carry out high-precision transient numerical simulations. Results showed that the pod has a significant influence on the unsteady flow field. The rotor–stator interaction between the propeller and pod can be observed, leading to the periodic fluctuation of thrust on the propeller. On the surface of pod, pressure distribution changes with time, leading to the difference of local lateral force. In the spatial region affected by the propeller wake flow, pressure distribution presents a spiral characteristic, both in the region far away from the pod, and in the region of the wake flow of strut and fin. The vortex structures of podded propulsor are complex since the interference of the pod. In addition to the tip, root and hub vortex, strut and fin vortices also occur. The vortices generated by the effect of mutual inductance between vortices are also discussed.

Novel Multi-Physics Computational Simulation of a 10 kW Permanent Magnet Motor for Podded Propulsion

This paper presents a 10 kW, 12-slot 10-pole surface-mounted permanent magnet synchronous motor (SPMSM) design with fractional-slot concentrated winding for a podded propulsion system. Its load is a propeller that is proportional to the square of the rotational speed and the fifth power of the propeller diameter. Taking this into account, three SPMSMs with rated rotational speeds of 600, 1200, and 1800 rpm with the same rated output power of 10 kW were analyzed. These were designed under the same conditions (i.e., torque per rotor volume, air-gap length, current density, power factor, fill-factor, and supply voltage). Based on the SPMSMs designed by electromagnetic analysis, the housing of a podded propulsor for each SPMSM was modeled for mechanical analysis, including such parameters as forced vibration, radiated noise, and modal acoustics analysis in air and water. From the modal acoustics analysis, it is confirmed that the natural frequencies of a structure in water are lower than those in air because of the added mass effect of water.

Ice load prediction of a podded propulsor under the dynamic azimuthing condition

In the process of cutting an ice layer by dynamic azimuthing of a podded propulsor, the unsteady and high pulsation characteristics of the ice load may compromise the safety and structural strength of podded propulsors. In this study, the failure criterion of simulated materials for the ice-breaking problem and the ice load calculation method were derived based on the peridynamics and the idea of panel division. The contact judgment method between the propeller and ice under the dynamic azimuthing condition of the podded propulsor was proposed. A numerical calculation model of the milling action of the ice–podded propulsor was established, and a numerical simulation of the ice–propeller milling process under the dynamic azimuthing conditions of the podded propulsor was realized. Moreover, numerical simulations under different ice layer moving velocities, propeller rotational speeds, podded propulsor azimuthing rates, and azimuth angles were carried out, and the ice layer breaking phenomenon and the variation characteristics of the propeller ice load under the dynamic azimuthing condition of the podded propulsor were obtained. The research results can guide technical support for the load evaluation, structural safety analysis, mechanism research of load components, and the selection of polar navigation steering strategies for ice-class podded propulsors.

Analysis of Vibration Characteristics of Podded Propulsor Shafting Based on Analytical Method

Podded propulsors are widely used in warships and cruise ships, which have a higher requirement of vibrational and acoustic design. Therefore, studying vibration characteristics and the transmission mechanism of podded propulsor shafting is significant for reducing vibration and ensuring the safe operation of ships. This paper establishes a model of podded propulsor shafting by analytical method. The shafting is simplified to a heterogeneous variable cross-section beam, while bearings are seen as springs. The podded propulsor shafting has one radial-thrust hybrid bearing and one radial bearing. The excitations from the propeller and cabin are considered. The influences of bearing stiffness, bearing location, and excitation on vibration characteristics of shafting are analyzed. The main conclusions are as follows: Based on the analysis of the area that resonance frequency is sensitive to the change of bearing stiffness, the resonance frequencies of the shafting can be adjusted to the proper range. The large span between hybrid bearing and radial bearing leads to low stiffness of shafting and low resonances frequencies. Under radial excitations, the low vibration always occurs at the hybrid bearing, motor shafting, or propeller end of shafting. This research provides theoretical support for the design and optimization of vibration reduction of podded propulsor shafting.

METHODOLOGIES TO PREDICT HYDRODYNAMIC CHARACTERISTICS OF PUSHER AND PULLER PODDED PROPULSORS IN OBLIQUE FLOWS

This paper presents the outcome of a numerical simulation based research program to evaluate the propulsive characteristics of puller and pusher podded propulsors in a straight course and at static azimuthing conditions while operating in open water. Methodologies to predict the propeller thrust and torque, and pod forces and moments in three dimensions using a Reynolds-Averaged Navier Stokes (RANS) solver at multiple azimuthing conditions and pod configurations are presented. To obtain insight into the reliability and accuracy of the results, grid and time step dependency studies are conducted for a podded propulsor in straight-ahead condition. The simulation techniques and results are first validated against measurements of a bare propeller and a podded propulsor in straight ahead condition for multiple loading scenarios and in both puller and pusher configurations. Next, simulations were carried out to model the podded propulsors in the two configurations at multiple loading conditions and at various azimuthing angles from +30° to –30° in 15° increments. The majority of the simulations are carried out using both steady state and unsteady state conditions, primarily to evaluate the effect of setup conditions on the computation time and prediction accuracy. The predicted performance characteristics of the pod unit using the unsteady RANS method were within 1% to 5% of the corresponding experimental measurements for all the loading conditions, azimuthing angles and pod configurations studied. The non-linear behaviour of the performance coefficients of the pod unit are well captured at various loading and azimuthing conditions in the predicted results. This study demonstrates that the RANS solver, with proper meshing arrangement, boundary conditions and setup techniques can predict the performance characteristics of the podded propulsor in multiple azimuthing angles, pod configurations and in the various loading conditions with a same level of accuracy as experimental results. Additionally, the velocity and pressure distributions on and around the pod-strut- propeller bodies are discussed as derived from the RANS predictions.

STUDY ON PROPULSIVE PERFORMANCE OF TWO GEOMETRICALLY SIMILAR PODDED PROPULSORS

This paper presents the outcome of a research to evaluate the effect of size on the propulsive performance of podded propulsors in cavitating and non-cavitating open water conditions. Two cases are examined, namely: propeller-only case and pod-unit case. In the propeller-only case, a commercial propeller dynamometer is used to measure the thrust and torque of two propellers of different size at the four quadrants of propellers with varied shaft and flow speeds. Also, both propellers are tested at different tunnel pressure to study and compare the behaviour under similar cavitation conditions. In the pod-unit case, two geometrically similar but different sized pod-units are tested using two separate custom-made pod dynamometer systems in two towing tank facilities in straight-ahead and static azimuthing conditions. The study showed that the performance characteristics stabilize at lower Reynolds Number for the smaller propeller than the larger propeller. The propulsive performance of the two propellers was comparable in the four-quadrant experiments. Also, the experiments at the cavitating conditions showed that the cavitation characteristics of the two propellers were consistent at corresponding operating conditions. The experiment results of the two pod-units were also comparable for forces and moments in the three coordinate directions in the straight-ahead and static azimuthing conditions. A brief discussion on the uncertainty assessments for each of the measurements is also presented.

Study on power matching of podded propulsor for polar ships

Objective To enhance the pertinence and applicability of polar ships with podded propulsion in design, the power matching problem should be researched. Method Based on the classification rules of ice classes abroad and the results of ship investigation, the seaworthiness range of ships with different polar classes in the four seasons of the Arctic could be confirmed. Based on the Lindqvist formula, the ice resistance of three typical pod propulsion ships in the polar region was calculated, and then the required power of the pod propulsion ship could be obtained. Based on the investigation of the mature pod thruster products in the market, a preliminary selection method of pod thruster for polar ships was established. Results Combined with the current single machine power and arrangement types of pod thrusters, the power matching results and preliminary suggestions for the selection of various ship types under different ice levels were obtained. Conclusion The research results can provide reference for thruster selection in the design stage.

Electric Podded Propulsor Propeller with Control Algorithm of Boat Based on Data Processing from Sensors

Electric Podded Propulsor Propeller with Control Algorithm of Boat Based on Data Processing from Sensors

Simulation of turbulent effective wakes for propellers in off-design conditions by a correction factor approach

This paper presents a procedure for the estimation of propeller effective wakes in oblique flows. It shows how a recently developed method for controlling coupling errors can be applied to analyze propellers operating in off-design conditions. The approach allows the use of fast potential flow methods for the representation of the propeller in the context of viscous flow solvers and works accurately for a wide range of advance numbers and incidence angles with a minimum computational cost. The new method makes it possible to disclose flow phenomena on the effective wake that were hidden in conventional approaches of effective wake simulation. Different application cases are analyzed, such as a propeller-shaft configuration in inclined flow, a pod propulsor in an oblique inflow, and a ship hull advancing at a yaw angle. A dipole-like distortion on the effective wake is unmasked for a uniform flow incident to a propeller mounted on an inclined shaft. The flow component perpendicular to the axis is found to be responsible for the distortion. The effect of the direction of propeller rotation on the effective wake is illustrated for a single-shaft ship moving at a yaw angle. In particular, keel vortices are either attracted to or repelled from the propeller disk depending on the sign of the yaw angle or alternatively on that of the propeller rotation.

Numerical simulation on the hydrodynamic performance of an azimuthing pushing podded propulsor in reverse flow and rotation

In this study, the hydrodynamic performance of a reverse podded propulsor under azimuthing conditions is evaluated. The shear stress transfer (SST) k-omega turbulence model and sliding mesh technique are adopted for this evaluation, and the commercially available software, STAR-CCM+ is used for the numerical simulation. An extensive convergence study was performed with respect to time step and grid size to verify the numerical simulation. The hydrodynamic performance is analyzed under different advance velocities as the propulsor sinusoidally azimuths from 130° to 230°. The forces and moments generated by the propulsor in steady oblique flow are compared with those under azimuthing condition. It is found that the forces and axial moments of the propulsor in oblique flow generally varies in between the envelopes of the transient ones under an azimuthing state, while the vertical and horizontal moments are observed too strongly deviate from one another.

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice-podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice-podded propulsor interaction.

Equivalent standard manoeuvres for pod-driven ships

Procedures for carrying out the manoeuvring tests presented in International Maritime Organisation (IMO) resolution MSC.137(76) are based on the capabilities of ships with conventional steering-propulsion systems. Therefore they do not correspond to ships with other steering-propulsion devices, like pod drives. IMO is aware of this shortcoming and for ships with non-conventional steering and propulsion system may permit the use of comparative steering angles to the rudder angles specified by the resolution. To elaborate these comparative angles for pod-driven gas carrier, experiments with free-sailing manned model have been carried out. For comparison two sister ships have been taken into consideration, i.e. twin-pod and twin-propeller twin-rudder configurations. For both ship versions, manoeuvres according to standard IMO procedures have been carried out. For twin-pod configuration, additionally manoeuvres with various pod propulsor settings have been performed to find the comparative, i.e. rudder-equivalent pod propulsor angles. Results show an obvious fact, that for twin-pod ship version similar results of standard manoeuvre parameters, as for twin-propeller twin-rudder version, are obtained for other than IMO recommended 35° steering angle. It is shown that, for turning circle parameters, a single comparative or rudder-equivalent angle does not exist. Various turning circle parameters are obtained for different pod propulsor angles.

Hydrodynamic Interactions between Bracket and Propeller of Podded Propulsor Based on Particle Image Velocimetry Test

In this study, particle image velocimetry was used to measure the fine flow-field characteristics of an L-type podded propulsor in various working conditions. The flow-field details at different cross-sections between the propeller and the inclined bracket were compared and analyzed, allowing for more intuitive comparison of the flow-field characteristics of L-type podded propulsors. The interference mechanisms among the propeller, pod, and bracket of the L-type podded propulsors at different advance coefficients, deflection angles, and deflection directions were investigated in depth. The results of this study can serve as reference material and provide technical support for the design and practical shipbuilding application of L-type podded propulsors. Therefore, the results have theoretical significance and practical engineering value.

Prediction of Self-Propulsion Performance of Ship Model with Double L-Type Podded Propulsors and Conversion Method for Full-Scale Ship

In this study, the self-propulsion performance of a ship model with double-L-type podded propulsors was predicted. Additionally, a conversion method for the performance of a full-scale ship was established based on the correction method published by the International Towing Tank Conference (ITTC) for the scaling effect of a single podded propeller and research reports on pod tests conducted by different ship research institutes. The thrust deduction and wake fraction of the ship were also analyzed. Furthermore, the self-propulsion performance of a full-scale ship with double L-type pods was determined, the full- and model-scale ships compared in terms of their flow fields and pressure charts, and the influence of the scaling effect analyzed. In addition, the calculation results were compared with the conversion results of a full-scale ship, and the reliability of the method adopted for the performance estimation of a full-scale ship with double podded propulsors was verified. The findings reported herein can provide statistics and technical support for the design of L-type podded propulsors and their application in full-scale ships, which are of theoretical significance and practical value in the engineering domain.

NUMERICAL ESTIMATION ABOUT PERFORMANCE OF CPP AND PODDED PROPULSOR ON MANY OFF-DESIGN CONDITIONS USING RANS APPROACH

On the off-design condition of CPP or podded propulsor, the angle of attack against the propeller blade or the flow interaction between propeller and pod housing is quite different from the propeller design condition. The flow around them is very complex. Hence there is the difficulty for numerical estimating. In order to verify the applicability to wide range of the off-design condition, RANS simulations were executed. The calculation range for CPP was complete two quadrant condition each at several pitch setting. Regarding podded propulsor, pull type and push one having the common propeller and housing were targeted and the range of pod steering angle were set to all 360 degrees. Hydrodynamic performance as these calculated results were compared with published experimental results including the rotational moment of changing CPP’s pitch angle or steering the pod housing. Then, it was found that there were good agreements except for the particular kind of the operating conditions.

Analysis of Hydrodynamic Performance of L-Type Podded Propulsion with Oblique Flow Angle

In this study, the Reynolds-averaged Navier–Stokes (RANS) method and a model experimental test in a towing tank are used to investigate the unsteady hydrodynamic performance of L-type podded propulsion under different oblique flow angles and advance coefficients. The results show that the load of the operative propeller increases with oblique flow angle and the bracket adds resistance to the pod due to the impact of water flow, leading to a reduced propeller thrust coefficient with increased oblique flow angle. Under a high advance coefficient, the speed of increase of the pressure effect is higher than that of the viscosity effect, and the propeller efficiency increases with the oblique flow angle. The nonuniformity of the inflow results in varying degrees of asymmetry in the horizontal and vertical distributions of the propeller blade pressure. Under high oblique flow angle, relatively strong interference effects are seen between venting vortexes and the cabin after blades, leading to a disorderly venting vortex system after the blade. The numerical simulation results are in good agreement with the experimental values. The study findings provide a foundation for further research on L-type podded propulsors.

A coupled potential–viscous flow approach for the prediction of propeller effective wakes in oblique flow

This paper presents a method for the estimation of propeller effective wakes in oblique flows. It extends to inclined flows an approach based on correction factors previously developed for the estimation of effective wakes in straight flow. The approach converts propeller-induced velocities approximately predicted via potential flow theory into viscous-induced velocities on the basis of a viscous flow RANS analysis. The correction factors are a function of both the radial and angular positions on the propeller disk. They are calculated for a reference advance number and work accurately in a neighboring continuous region of advance numbers. This procedure allows controlling one of the errors present in the calculation of effective wakes, namely the error derived from coupling a potential flow method for the representation of the propeller with a RANS solver. Consequently, it permits calculating the effective wake more precisely in off-design conditions, reducing the CPU time, and therefore, enlarging its range of applicability to situations like those resulting from ship maneuvering. The approach is tested for a podded propulsor unit in oblique flow.

Modeling Techniques of Puller Podded Propulsor in Extreme Conditions

This article presents the outcome of a numerical simulation-based research to evaluate the propulsive characteristics of a puller podded propulsor in extreme azimuthing and at multiple initial loading conditions. A Reynolds-Averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD) code is used to model and predict the propulsive characteristics of the podded propulsor at extreme oblique flow conditions in which highly unsteady, fully 3D, and separated flows exist. The forces and moments of the podded propulsor in the three coordinate directions as well as the thrust and torque of the propeller in static azimuthing conditions in the range of -180°–180° at advance coefficients of 0.20 and 0.8 are predicted and compared with a corresponding dataset obtained in a previous model testing campaign. The predictions of the performance characteristics of the pod unit were within 1–5% of the corresponding measurements, which also captured the nonlinear trends for all the loading conditions, and azimuthing angles. In addition, the velocity and pressure distributions around the podded propulsor as derived from the RANS predictions reveal the highly separated and 3D flow and complex interaction between the propeller and pod-strut bodies, which was more evident in the extreme azimuthing and moderate to light propeller loading conditions. This study demonstrates that the RANS-based CFD code, with proper meshing arrangement, boundary conditions, and setup techniques, can predict the performance characteristics of the podded propulsor in extreme azimuthing and in various loaded conditions with the same level of accuracy of the measurements. 1. Introduction Podded propulsors have the potential to become a popular main propulsion system because they allow more flexibility in the design of the internal arrangement of a ship, potentially reduce noise and vibration, and increase maneuverability (Pakaste et al. 1999; Carlton 2002). The propeller, pod, and strut are the three main components of a podded propulsor. There are multiple propeller-pod-strut configurations that are available commercially, namely the puller type, pusher type, duel-end type, ducted type, and contrarotating type configurations. One of the popular configurations of the podded systems is the puller type, in which the propeller is located upstream of the pod and strut (Islam 2009).