Application of a BEM time stepping algorithm in understanding complex unsteady propulsion hydrodynamic phenomena

We solve the problem of unsteady potential flow around a system of arbitrarily moving rigid or flexible, lifting or non-lifting bodies, in an infinite fluid free of distributed vorticity. For the solution we use a time stepping algorithm and a potential based formulation of the corresponding free BVP. Nonlinear free shear layer dynamics are included in our modeling. This is a major innovation in treating complex unsteady propulsion problems since no simplifying assumptions (like that of a helicoidal wake) are used regarding the wake model. Bilinear quadrilateral elements are used to describe body and shear layer geometry at each time t. Three types of Kutta conditions can be alternatively applied for the determination of the shed vorticity from lifting bodies. The theoretical and numerical aspects of the method are presented followed by a number of applications, elucidating the qualitative and quantitative physical characteristics of a number of complex unsteady propulsion problems.     

Numerical flow analysis of single-stage ducted marine propulsor

The present work has solved 3D incompressible RANS equations on a rotating, non-orthogonal multi-blocked grid to efficiently analyze a ducted marine propulsor with rotor–stator interaction. To handle the interface boundary between a rotor and a stator, the sliding multi-block technique using the cubic spline interpolation and the bilinear interpolation was applied. To validate the present code, the flow of a single stage turbine flow was simulated. Time averaged pressure coefficients were compared with experiments and good agreements were obtained. After the code validation, the flowfield around a single-stage ducted marine propulsor having a single stage of rotor and stator was successfully simulated and the hydrodynamic performance coefficients were computed.     

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.     

A three-dimensional wake impingement model and applications on tandem oscillating foils

Potential flow based vortex numerical methods have been widely used in aerodynamics and hydrodynamics. In these methods, vortices shed from lifting bodies are traced by using vortex filaments or dipole panels. When the wake elements encounter a downstream body, such as a rudder behind a propeller or a stator behind a rotor, a treatment is necessary to divert the wake elements to pass by the body. This treatment is vital to make wake simulations realistic and to satisfy the non-penetration condition during wake body interaction. It also helps to avoid pure numerical disturbances such as when a vortex filament or an edge of a dipole panel passes through the collection point of a body element; this is a singularity for induced velocity and it will introduce a large numerical disturbance. This necessary treatment for three-dimensional problems with geometrical complexity has not been found to date. In this study, a wake impingement model was developed to divert wake elements to slip over the body surface, model the vortex/body interaction, and predict forces on fluctuating components. The model was also tested on configurations of oscillating foils in tandem with an existing panel method code. Simulation results with the wake impingement model are shown to be in closer agreement with limited published experimental data than those without the model. With the established wake impingement model, force fluctuations on the after body due to the wake vortex impingement were investigated based on a series of simulations. The series varied several parameters including distance between two foils, oscillating frequency, span, rear foil pitch angle, swap angle and vertical position.     

RANS Simulation of Podded Propulsor Performances in Straight Forward Motion

The Computational Fluid Dynamics (CFD) approach is adopted to study the steady and unsteady performances of the podded propulsor by the Fluent software package. While the interactions of the propeller blades with the pod and strut are time-dependent by nature, the mixing plane model is employed firstly to predict the steady performance, where the interactions are time-averaged. Numerical experiments are carried out with systematically varied mesh sizes to investigate the dependence of the predicted force values on the mesh sizes. Furthermore, the sliding mesh model is employed to simulate the unsteady interactions between the blades, pod and strut. Based on the numerical results, the characteristics of unsteady hydrodynamic forces are discussed, and the applicability of the mixing plane model is investigated for puller-type podded propulsor.     

吊舱推进器与船体间相互影响研究进展

吊舱推进器是近年来发展起来的一种新型船舶推进装置.针对吊舱推进器与船体之间影响的成因、干扰形式和研究方法进行了阐述,总结了吊舱推进器与船体之间相互影响的理论研究和模型试验方面取得的成果.最后提出该领域的主要研究方向,作为进一步研究的参考.     

A novel maneuverable propeller for improving maneuverability and propulsive performance of underwater vehicles

The existing propulsor that can perform both propulsion and maneuvering along axis of rotation is propeller/rotor for a helicopter. Helicopter propellers when maneuvering increase or decrease their blades’ pitch cyclically to create imbalanced thrust and hence maneuvering force/torque. A “maneuverable propeller” was developed and its performance on both maneuvering and propulsion is assessed. The “maneuverable propeller” is an alternative of the existing helicopter rotors. The novelty of this propulsor is that the imbalanced thrust force/torque is created by cyclically increasing or decreasing the angular speed of their blades relatively to the hubs/shafts, to provide the desired maneuvering torque. This maneuverable propeller is hence defined as the Cyclic Blade Variable Rotational Speed Propeller (CBVRP). One of the best advantages is that the maneuvering torque created by the “maneuverable propeller” is much higher, about 5 times of the shaft torque of the same propeller at thrust only mode. The “maneuverable propeller” has wide applications for both surface ships and underwater vehicles that require high maneuverability for cruising inside the narrow passage.     

Effect of waves on cavitation and pressure pulses of a tanker with twin podded propulsion

There is increasing interest in optimizing ships for the actual operating condition rather than just for calm water. In order to optimize the propeller designs for operations in waves, it is essential to study how the propeller performance is affected by operation in waves. The effect of various factors that influence the propeller is quantified in this paper using a 8000 dwt chemical tanker equipped with twin-podded propulsion as a case vessel. Propeller performance in waves in terms of cavitation, pressure pulses, and efficiency is compared with the performance in calm water. The influence of wake variation, ship motions, RPM fluctuations and speed loss is studied. Substantial increase in cavitation and pressure pulses due to wake variation in the presence of waves is found. It is found that the effect of other factors is relatively small and easier to take into account as compared to wake variation. Therefore, considering the wake variation at least in the critical wave condition (where the wavelength is close to ship length) in addition to calm water wake is recommended in order to ensure that the optimized propeller performs well both in calm water and in waves.     

A general equation for performance analysis of multi-component propulsor with propeller

An integral panel method (IPM) that treats the different components of multi-component propulsors as a whole is presented for efficient propulsor performance analysis. The IPM requires consider only one blade of the propeller in the performance analysis, which significantly reduces the number of computation grid. The control equations of the IPM are derived in detail for podded propulsors, contra-rotating propellers and hybrid contra-rotating shaft pod propulsors, and based on these derivations, a general control equation for multi-component propulsors with propeller is derived. Comparison between numerical results and experimental data show that the IPM provides good accuracy for the performance analysis of multi-component propulsors with propeller. In addition, the error sources of IPM are discussed, and the reasonableness of these errors is evaluated.