A Nonlinear Method for Predicting Unsteady Sheet Cavitation on Marine Propellers

1983 ◽  
Vol 27 (01) ◽  
pp. 56-74
Author(s):  
Frederick Stern ◽  
William S. Vorus
Keyword(s):  
Complete Solution ◽  
Near Field ◽  
Fluid Velocity ◽  
Far Field ◽  
Static Potential ◽  
Nonlinear Method ◽  
Slender Body Theory ◽  
Marine Propellers ◽  
Sheet Cavitation ◽  
Dynamic Potential

A method is presented which provides a basis for predicting the nonlinear dynamic behavior of unsteady propeller sheet cavitation. The method separates the fluid velocity potential boundary-value problem into two parts, static and dynamic, which are solved sequentially in a forward time stepping procedure. The static potential problem is for the cavity fixed instantaneously relative to the propeller and the propeller translating through the nonuniform wake field. This problem can be solved by standard methods. The dynamic potential represents the instantaneous reaction of the cavity to the static potential field and thus predicts the cavity's deformation and motion relative to the blade. A solution is obtained for the dynamic potential by using the concepts of slender-body theory to define near-and far-field potentials which are matched to form the complete solution. In the far field, the cavity is represented by a three-dimensional spanwise line distribution of sources. In the near field, the cavity is approximated at each cross section as a semi-ellipse with unknown axes a(t), b(t), and position l(t) along the chord of the foil section. Conditions are derived that determine (a, b, l) by minimizing the square error in satisfying the dynamic boundary condition. These conditions yield the equations of motion of the cavity in the form of three coupled nonlinear second-order ordinary differential equations with time as the independent variable. The theory is presented for the general foil and not specifically for propellers. However, the method incorporates features in its formulation which facilitate its application to marine propellers. The method is demonstrated by using the steady noncavitating potential for the two-dimensional half-body as an approximation to the static potential. Both fixed and unsteady cavities are calculated. The unsteady cavities are calculated by varying the hydrostatic pressure in the half-body pressure field sinusoidally.

A computational investigation of noise spectrum due to cavitating and non-cavitating propellers

2019 ◽  
Vol 234 (2) ◽  
pp. 374-387
Author(s):  
Savas Sezen ◽  
Sakir Bal
Keyword(s):  
Near Field ◽  
Noise Spectrum ◽  
Research Field ◽  
Hydrodynamic Performance ◽  
Navier Stokes ◽  
Marine Propellers ◽  
Sheet Cavitation ◽  
Blade Number ◽  
Numerical Studies ◽  
Propeller Blades

In this article, noise spectrum of marine propellers is investigated in uniform flow under non-cavitating and cavitating conditions. New results are presented for this research field. Hydrodynamic performance of both non-cavitating and cavitating marine propellers is first analyzed by viscous and potential based flow solvers. In viscous solver, sheet cavitation on propeller blades is simulated with Schnerr–Sauer cavitation model based on Rayleigh Plesset equation using volume of fluid approach. Numerical hydrodynamic results based on viscous solver is compared with potential solver and then validated with experimental data of benchmark David Taylor Model Basin 4119 model propeller. Later, noise spectrum of model propellers is predicted by a hybrid method which combines Reynolds-averaged Navier Stokes and Ffowcs Williams Hawkings equations. Computed noise spectrum is compared with other numerical studies in the literature for the selected model propeller. In addition, hydrodynamic and hydroacoustic pressures are compared in near field to show reliability of numerical solution. Effects of blade number on hydrodynamic performance and noise spectrum are also investigated. Numerical results indicated that as blade number increases, propeller noise level decreases for different loading conditions due to decreased blade loading (circulation) per blade. However, propeller efficiency increases as blade number decreases.

Determination of Far-Field Pattern of Rigid Scatterers Using Independent Finite Element Method and Eigenfunction Expansion, Part 2: Nonaxisymmetric Scattering

1996 ◽  
Vol 118 (4) ◽  
pp. 583-590 ◽  
Author(s):  
C. Prabavathi ◽  
C. P. Vendhan
Keyword(s):  
Eigenfunction Expansion ◽  
Numerical Technique ◽  
Complete Solution ◽  
Near Field ◽  
Field Potential ◽  
Prolate Spheroid ◽  
Field Pattern ◽  
Far Field ◽  
Outer Domain ◽  
Contour Plots

The near- and far-field steady state scattered potentials around a rigid arbitrary obstacle subjected to plane incident wave are computed by FE and eigenfunction expansion approach. The near-field is bounded by a truncation surface which is spherical. The nonaxisymmetric damper equation developed by Bayliss et al. (SIAM J. Appl. Math. Vol. 42, pp. 430–451, 1982) is employed on this surface. The computed FE near-field potential on the truncation boundary is employed as a Dirichlet boundary condition to obtain the expansion coefficients which eventually help in obtaining the complete solution in the entire outer domain including the far-field. The numerical technique is applied to problems of rigid scattering of beam-on- and oblique-incident waves by rigid prolate spheroid and hemispherically capped cylinder and contour plots of far-field scattered potential are presented.

scholarly journals Mesh Properties for RANS Simulations of Airfoil-Shaped Profiles: A Case Study of Rudder Hydrodynamics

2021 ◽  
Vol 9 (10) ◽  
pp. 1062
Author(s):  
Suli Lu ◽  
Jialun Liu ◽  
Robert Hekkenberg
Keyword(s):  
Near Field ◽  
Reynolds Numbers ◽  
Far Field ◽  
Computational Domain ◽  
Ansys Fluent ◽  
Marine Propellers ◽  
Computational Fluid Dynamics Cfd ◽  
Naca 0012 ◽  
Mesh Properties

A good mesh is a prerequisite for achieving reliable results from Computational Fluid Dynamics (CFD) calculations. Mesh properties include mesh types, computational domain sizes, and node distributions. However, in literature, we found no clear consensus about what these properties should be. In this article, we performed a case study on ship rudders to determine what the suitable mesh properties are for airfoil-shaped profiles. A classic NACA 0012 profile is chosen as an example, and commercial packages ANSYS ICEM are applied for meshing with an ANSYS Fluent solver. With a strategy in consideration of relationships among different mesh properties, a comprehensive parametric investigation is conducted to study the impacts of these properties on the accuracy of rudder hydrodynamic coefficients obtained by CFD methods. The step-by-step study outputs recommended Reynolds numbers, domain sizes, and near- and far-field node distributions for mesh types with distinct topology structures, i.e., C-mesh, O-mesh, H-mesh, and Hybrid-mesh. Specifically, the study shows that a critical Reynolds number is needed for the perspective of efficiency, while a domain extending 60 times of the chord length enables the boundary effects to be negligible. As for node distributions, the near-field nodes should be treated carefully, compared with those in the far-field. After that, corresponding mesh properties for different calculation objectives are illustrated in detail based on the characteristics of mesh types mentioned above. With the proposed strategy for mesh refinements, impacts of different mesh properties on rudder hydrodynamics are clarified and recommended settings are applicable for other airfoil-shaped profiles such as wind turbines and marine propellers.

Magnetic Tag Antenna for UHF Near-field and Far-Field RFID Applications

2015 ◽  
Vol 5 (2) ◽  
pp. 119
Author(s):  
Mondher Dhaouadi ◽  
M. Mabrouk ◽  
T. Vuong ◽  
A. Ghazel
Keyword(s):  
Near Field ◽  
Far Field ◽  
Rfid Applications

Far field modeling of the Mamala Bay outfalls

1998 ◽  
Vol 38 (10) ◽  
pp. 323-330
Author(s):  
Philip J. W. Roberts
Keyword(s):  
Field Model ◽  
Near Field ◽  
Far Field ◽  
Field Modeling ◽  
Visitation Frequency ◽  
Statistical Variation ◽  
Long Time ◽  
Harmonic Average ◽  
Ocean Outfall ◽  
Acoustic Doppler Current Profilers

The results of far field modeling of the wastefield formed by the Sand Island, Honolulu, ocean outfall are presented. A far field model, FRFIELD, was coupled to a near field model, NRFIELD. The input data for the models were long time series of oceanographic observations over the whole water column including currents measured by Acoustic Doppler Current Profilers and density stratification measured by thermistor strings. Thousands of simulations were made to predict the statistical variation of wastefield properties around the diffuser. It was shown that the visitation frequency of the wastefield decreases rapidly with distance from the diffuser. The spatial variation of minimum and harmonic average dilutions was also predicted. Average dilution increases rapidly with distance. It is concluded that any impact of the discharge will be confined to a relatively small area around the diffuser and beach impacts are not likely to be significant.

Direction of Arrival Estimation Under Near-Field Interference Using Matrix Filter

2015 ◽  
Vol 23 (04) ◽  
pp. 1540007 ◽  
Author(s):  
Guolong Liang ◽  
Wenbin Zhao ◽  
Zhan Fan
Keyword(s):  
Near Field ◽  
Direction Of Arrival ◽  
Doa Estimation ◽  
Direction Of Arrival Estimation ◽  
Far Field ◽  
Data Simulation ◽  
Estimation Algorithms ◽  
Simulation Results

Direction of arrival (DOA) estimation is of great interest due to its wide applications in sonar, radar and many other areas. However, the near-field interference is always presented in the received data, which may result in degradation of DOA estimation. An approach which can suppress the near-field interference and preserve the far-field signal desired by using a spatial matrix filter is proposed in this paper and some typical DOA estimation algorithms are adjusted to match the filtered data. Simulation results show that the approach can improve capability of DOA estimation under near-field inference efficiently.

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