Anti-singing Edge Effect on Propeller Open Characteristics

An operating propeller is the main source of vibration on the ship hull, especially on the stern. In this article, an improved numerical scheme is presented to predict the pressure fluctuations on the ship hull due to the unsteady sheet cavitation of a propeller. The calculated results are compared with the published results and experimental data carried out in the hydrodynamics and cavitation tunnel of Hamburgische Schiffbau-Versuchsanstalt to verify the improvement. The present method is based on a two-cycle iterating scheme which satisfies the boundary integral equation in time domain. The hull pressure fluctuations calculated by the first-cycle iterations are treated as the initial values for the second-cycle iterations. The solid angles of the elements will deviate from the standard value, .5, as the dramatic variation in geometry appears, and accumulate numerical errors in the calculating process. During the second-cycle iterations, a filter based on the solid angles on hull elements is proposed to minimize the iterating error. A container ship is treated as the computing sample in this study, and evidence is offered regarding the 16% improvement achieved by the present method. 1. Introduction A propeller operating in a nonuniform wake field in the ocean is one of the prime sources of vibration and noise affecting the ship. The prediction of hull pressure fluctuations caused by propellers is an important consideration for many vessels and worthy of being researched. Hull pressure fluctuation caused by the marine propeller has been widely investigated since the 1980s. Huse and Guoqiang (1982) developed a semi-empirical prediction method, in which the cavitation volume on the propeller blade surface can be estimated using experimental data. Breslin et al. (1982) proposed a model based on the potential flow theory, which simulates the ship hull by means of a simple panel method and the propeller by means of a lifting surface vortex lattice method. Kinns and Bloor (2004) used an acoustic boundary element (BE) model to simplify the problem of hull vibration excitation due to propeller sources and dipoles. The convergence of results for a cruise liner model was demonstrated with different element distributions. Kehr and Kao (2004) derived the incident blade rate pressure induced by unsteady sheet cavitation (monopole) and unsteady forces (dipole) of an operating propeller, for calculating the hull pressure fluctuations. Brouwer (2005) applied a stationary set of rings of monopole and dipole sources in the frequency domain to solve the propeller-induced noise and vibrations. Lee et al. (2006) integrated computational fluid dynamics (CFD) and the finite difference method for the computation of propeller-induced hull vibration. It was recommended that the phase difference of the propeller-induced pressure should be considered for preventing overprediction. Kao and Kehr (2006) developed a time domain iteration method to calculate the hull pressure fluctuations induced by the operating propeller. The phase difference is included in the iterating scheme, and the computation is robustly convergent. Seol and Moon (2009) derived the governing equation of pressure fluctuation induced by sheet cavitation according to the Ffowcs Williams approach. In the study by van Wijngaarden (2011), a potential flow boundary element method (BEM) with measured hull pressure data as input was applied to solve the hull pressure fluctuations, and the model experiments were also carried out. It was concluded that the computed hull pressure fluctuations due to non-cavitating propellers is reasonably accurate compared with the experimental data. Kehr and Kao (2011) calculated the pressure fluctuations on ship hull due to propeller sheet cavitation by using the method presented in Kao and Kehr (2006). The numerical result is higher than the experimental data provided by the hydrodynamics and cavitation tunnel (HYKAT) of HSVA (Wiemer 1999) by approximately 27%. The pressure fluctuations on ship stern induced by cavitating propellers were solved by Kanemaru and Ando (2011) with a surface panel method. The maximum amplitude appears in the first blade frequency and overestimates the experimental data by about 30%. Kim et al. (2012) predicted the hull pressure fluctuations induced by marine propeller sheet cavitation, and the Doppler effect was considered at the same time. Wei and Wang (2013) employed CFD to simulate the propulsion of a submarine. The finite element method and BEM were then combined to solve the submarine's structure and acoustic responses under the propeller excitations. The matched-field inversion technique applied in Lee et al. (2014) uses the fluctuating hull pressure field measured by receivers and the acoustic field calculated by BEM to define the sheet cavitation noise, source strengths, source positions, and number of sources. The equivalent source model for the propeller can result in more accurately extrapolated hull pressure distribution. In Wei et al. (2016), the unsteady forces of a submarine propeller were predicted by CFD, and the hull pressure fluctuations due to the non-cavitation propeller were calculated by the method proposed in Kao and Kehr (2006).

Edge noise is generated if turbulence interacts with solid edges. Reduction of trailing edge noise of airfoils can be achieved by replacing the solid material at the trailing edge by inlays of porous permeable material. The acoustic benefit of approximately 6 dB of such treatment is known from experiments. Enroute to numerically optimized porous properties, this paper presents a first principle based Computational Aeroacoustics (CAA) method for predicting the acoustic effect of a porous NACA0012 trailing edge. In a hybrid two-step CFD/CAA procedure the turbulence statistics from a solution of the Volume Averaged Navier-Stokes (VANS) equations is used as a basis for the prediction of turbulent-boundary-layer trailing-edge noise (TBL-TEN). For the acoustic part of the calculation, the Acoustic Perturbation Equations (APE) are solved in the flow field. Inside the porous regions, a different set of governing equations, referred to as Linear Perturbation Equations (LPE) will be solved. The LPE represent a modified form of the Linearized Euler Equations (LEE) with the APE vorticity source term shifted to the right-hand side. The new set of equations is derived by volume averaging the Navier-Stokes equations and decomposing the flow variables into a time-averaged mean part and a fluctuating part and isolating the vorticity source term to the right-hand side of the momentum equation. The LPE are verified by an analytical solution. The simulation results of a NACA0012 airfoil geometry with and without porous trailing edge treatment are compared to wind tunnel measurements. The noise reduction effect of such a trailing edge treatment is successfully demonstrated.

In this paper, the effects of porous trailing edge on airfoil aerodynamic and aeroacoustic characteristics are investigated. In order to reduce the airfoil trailing edge noise and in the mean while to minimize the negative influence on aerodynamic performance, a new filling method is proposed such that porous medium is only used in the suction side half of the trailing edge which is more sensitive to the noise generation. The Large Eddy Simulation (LES) for flow and Ffocws-Williams and Hawkings (FW-H) methods for acoustics are used. At a Reynolds number of 2.63×105 and various angles of attack, the NACA0018 airfoil profile with 20% chords porous trailing edge is studied under two porous configurations, namely a fully-porous and a suction-side-porous trailing edge types. The results show that the flowfield of the porous airfoils are also influenced due to the presence of the porous medium. The noise radiation from the newly proposed porous airfoil achieves a noise reduction of 4.3dB at an angle of attack α=0°, and 4.07dB at an angle of attack α=2°. On the other hand, the loss of lift force is about 0.8%-1.5% in the angle of attack range of 0~10°.

Circulation control technology has proven aerodynamic benefits, however, along with an increase in the coefficient of lift, parasitic drag also increases. This paper explores two drag reduction techniques investigated in order to decrease the negative effects caused by the geometric requirements of a circulation control airfoil. The first method tests the viability of inducing a constant, low exit velocity jet, similar to a “leak” from the upper and lower plenums of the blowing jets, in an attempt to find a threshold where a specific rate of leak can alleviate the adverse effects of the circular trailing edge. A second drag mitigation technique included altering the shape of the trailing edge; attached to the trailing edge of a conventional circulation control surface is a sharp trailing edge, with an inner curved channel offset. By modifying the distance between the two geometries, an ideal separation distance is identified. The aerodynamic qualities of the two test models are therein compared to conventionally shaped airfoils. The results indicate that two techniques wherein have the ability to reduce the drag, proven by empirical experimentation.

Detailed information on the flow field structure is often important in numerous industrial applications. Although commercial computational fluid dynamics packages are often capable of providing the required data, they are costly and not universally available. This study was motivated by the operation of an open-pool nuclear research reactor where low radiation levels can be maintained by the installation of a stable purified hot water layer in the upper part of the pool. Maintaining a stable stratification requires a detailed description of the structure of the velocity field. Due to the inherent complications and restrictions of performing accurate measurements in a pool of a real-size operating reactor, either smaller-scale models or oversimplified fluid dynamics computational schemes are routinely used. These methods cannot be validated, and therefore do not necessarily capture the large-scale behavior correctly. We present an alternative approach to evaluate the velocity components in the pool that is based on the potential flow theory. The model results are validated by measurements using particle image velocimetry. The presented potential theory allows for the quick and easy assessment of the global properties of the fluid velocity distribution within the pool, and in particular, close to its surface. The suggested computational models are flexible and allow for easily varying the spatial dimensions of the flow field. The technique thus can be upscaled, and enables the validation of numerical computations in various fluid mechanical installations where the flow field cannot be resolved.

The unsteady Kutta condition is discussed in the light of some recent experimental measurements made near the trailing edge of a long flat plate and a 10C4 airfoil. The hierachy of disagreement from the theoretically predicted zero trailing edge loading caused by viscous instabilities is found to be acoustically correlated vortex shedding, natural vortex shedding, Tollmien-Schlichting waves, and, by implication, turbulent boundary-layer eddies. The region of significant chordwise disagreement scales with the wake perturbation wavelength of the corresponding instability. Coordinating the vorticity of the turbulent boundary layer shed from the profile airfoil with a transverse acoustic resonance produced a distinct disagreement of the Kutta condition at high reduced frequency parameters ( = wc/U). In this case and for vortex shedding, the extrapolated loading coefficient at the trailing edge increased with the nondimensional acoustic amplitude. I. Introduction T^HE Kutta condition as applied in unsteady JL potential airfoil analyses is essentially an extension of steady theory. Kutta ! postulated that a value of circulation should be chosen in his steady potential model to avoid a velocity singularity at the sharp trailing edge of an airfoil. This condition can be established if the trailing edge is also the rear stagnation point. The resulting modeled flow pattern agrees with that observed in steady flow and also predicts the lift and its chordwise distribution well at low angles of attack. The theoretical consequences of this hypothesis are that the lift loading or chordwise vorticity jump approaches zero at the trailing edge. An alternative statement is that the surface velocities on either side of the airfoil approach a common value at the rear stagnation point. For rounded trailing edges, the position of the rear stagnation point is indeterminate , as there is no velocity singularity to be avoided and so fix its location. In this case and for the situation of real flows with viscosity, Taylor2 proposed the condition of zero net vorticity discharge to establish the steady lift value. Preston3 explained the deviation of the lift of an airfoil at low angles of incidence from the potential theory value as due to the profile alteration from the boundary-layer growth. His calculations incorporated Taylor's vorticity discharge condition. Various approximate steady lift calculation methods for the rounded trailing edge geometry have been proposed by Gostello 4 and others. These extend the upper and lower lift distributions, at a selected chordwise position, to the trailing edge to give zero loading and thereby remove the stagnation point indeterminacy. In the unsteady case there are all the previous theoretical difficulties and, in addition, the unsteady effects on the viscous boundary layer and the shed vorticity. The latter complicates the airfoil response, making it a function of the airfoil's vorticity history. However, same theoretical assumption for the Kutta condition, of no unsteady loading at the

Some numerical studies were carried out on micrometer-sized rising bubble bouncing near a rigid boundary. Taking surface tension into consideration, the bubble motion could be solved using the potential flow theory. A correction should be made in Bernoulli equation because the pressure gradient was caused by the viscosity between the bubble and the wall. The numerical result agree well with the experimental data. Based on the fundamental phenomenon, we have studied the influence of characteristic parameter on bubble bouncing behavior, and the balanced shape due to the action of the wall. With the increase of the rising distance of the bubble, the distance of the bubble bouncing downward and the period of bouncing would increase. However, they would not change obviously when the rising distance is large enough. Surface tension has great effect on the dynamic behavior of the bubble. The bouncing period decreases when surface tension becomes large, but the bouncing distance will have an increase before it gradually decreases. Finally, the balanced shape of the bubble due to the wall effect can be mainly controlled by buoyance parameter and the Weber number.

The ability of the avoidance to movable obstacle is inefficient in robot path planning using traditional artificial potential field. An improved potential field model based on potential flow theory is proposed to achieve high efficient mobile robot path planning. The Zhukovsky transformation is used to optimize the configuration and function of the potential model to satisfy the reliability of avoiding mobile obstacle, and a further investigation of the vortex is applied to prevent the local minima. The application of the improved potential field model to multi-obstacle potential superposition is discussed to verify the effectiveness of the algorithm. The simulation results indicate that the improved method is able to avoid mobile obstacles and prevent local minima efficiently.

In this study, non-cavitating and cavitating flow around the benchmark DTMB 4119 model propeller are solved using both viscous and potential based solvers. Cavitating and non-cavitating propeller radiated noises are then predicted by using a hybrid method in which RANS (Reynolds-averaged Navier-Stokes) and FWH (Ffowcs Williams Hawkings) equations are solved together in open water conditions. Sheet cavitation on the propeller blades is modelled by using a VOF (Volume of Fiuld) method equipped with Schnerr-Sauer cavitation model. Nevertheless, tip vortex cavitation noise is estimated by using two different semi-empirical techniques, namely Tip Vortex Index (TVI, based on potential flow theory) and Tip Vortex Contribution (TVC). As the reference distance between noise source and receiver is not defined in open water case for TVI technique, one of the outputs of this study is to propose a reference distance for TVI technique by coupling two semi-empirical techniques and ITTC distance normalization. At the defined distance, the starting point of the tip vortex cavitation is determined for different advance ratios and cavitation numbers using potential flow solver. Also, it is examined that whether the hybrid method and potential flow solver give the same noise results at the inception point of tip vortex cavitation. Results show that TVI method based on potential flow theory is reliable and can practically be used to replace the hybrid method (RANS with FWH approach) when tip vortex cavitation starts.

Potential and viscous hybrid calculation method for ship motion prediction considering the change of ship motion attitude

The use of pin fin arrays in channels is one of the best choices to enhance overall heat transfer in gas turbine trailing edge blade cooling. Furthermore, in this particular application, the use of cross-pins in the trailing edge section of a turbine blade is a good way for supplying structural integrity to the blade itself. In this paper, results of several 3D RANS calculations performed in channels with cross-pins disposition such as in a typical trailing edge of a gas turbine blade are shown. Numerical calculations were compared with experimental results obtained on the same geometries using a transient Thermochromic Liquid Crystals (TLC) based technique. Goals of this comparison are both the evaluation of the accuracy of CFD packages with standard two equation turbulence models in heat transfer problems with complex geometries and the analysis of flow details to complete and support experimental activity. Two computational domains have been considered: they both consist in a wedge shaped channel with a stream-wise normal pin fin or pedestal arrays. The aim of the numerical analysis is the evaluation of convective Heat Transfer Coefficient (HTC) on the planar bottom surface of the wedge-shaped duct: this surface is commonly named “endwall” surface. Detailed analysis of the flow field points out the coexistence of an horse-shoe vortex, a stagnant wake behind the pin and a mean flow acceleration due to convergent shape of the channel. Calculations reveal the presence of a weak jet-like flow field toward endwall surfaces caused by the strong recirculation behind each pin.

In article considers the peculiarities of designing the parameters of mass explosions in the conditions of development of mineral deposits on mountain slopes. In these conditions, the width of the working platforms, the height of the ledges and, accordingly, the length of the well charges is limited by the slope. Particular attention is paid to the initial stage of formation of the mechanical effect of the explosion in the marginal areas of the elongated charge. The specifics of the choice of technological parameters of a mass explosion in mountain conditions are studied, where due to a significant difference in height within one block the height of the ledge can vary within 5 ... 10 m, and the charge length can reach 2… 7 meters. Due to the reduction of the length of the downhole charge, the participation of its end parts becomes more significant, where up to 35 ... 40% are dissipative energy losses due to the change of the symmetry of the force field. The studies take into account the peculiarities of the manifestation and development of edge effects at the ends of the charge. Regularities of stress field formation taking into account boundary phenomena have been studied by analytical calculations based on the replacement of a linear charge by a system of concentrated charges. Based on the compiled algorithm, the parameters of the force field for different variants of instantaneous and delayed explosion of a system of concentrated charges are calculated. Calculations of boundary phenomena in the explosion of a well charge of limited size show that under the condition of instantaneous charge initiation simultaneously on both ends of the charge, the degeneration of the axial symmetry of the force field occurs from the ends of the charge to a depth of 50 radii. The results of the calculations indicate that in real conditions of detonation of short downhole charges, their mechanical effect is largely formed by the weakened end sections of the linear charge. Analysis of the results of calculations allows us to recommend rational parameters of the system of downhole charges, which minimize energy losses

Nowadays, deep-waters are the new frontier of the latest huge oil discovers. The most common technology for oil exploration in deep waters is based on the FPSO concept. Therefore, a key element for a successful exploration of deepwater oil fields is the accurate prediction of the unit motions in order to minimize the stop production due to excessive motions and maximum accelerations to which the process plant equipment and supports are subjected. The excessive motions of a FPSO can be caused not only due to rough weather but also due to mild weather at beam sea conditions. Beam sea conditions are faced by FPSOs in a spreading mooring configuration or even in single point mooring configuration but moored in regions where the swell sea is important, for instance, in Campos Basin offshore Brazil. In such cases, the accurate evaluation of roll motions is of prime importance. Moreover, in order to reach valuable results, a critical step is the proper evaluation of the different damping effects in particular viscous damping induced by the bilge keel, or those of risers. The state of the art assumption is that the roll damping is composed of two parts: the linear one and the quadratic one. The former accounts for the potential wave radiation and partly to some frictional effects, while the latter accounts for drag forces, flow separation, and other non-linear effects. In practice, the linear part can usually be neglected and the only remaining damping is the quadratic part. These fact strengths the role played by the non-potential damping in a reliable evaluation of the roll motion amplitude. In this paper, we will show the implementation of a practical procedure for the evaluation of the roll motions including the non-potential damping of a FPSO moored in a spreading configuration at Campos Basin and its validation through the comparison of the numerical results to the model tests. Introduction Compared to the other modes of motion, the evaluation of the roll motion appears to be much more complicated and the corresponding results more uncertain. The reasons for that are mainly related to the evaluation of the roll damping coefficient, which is usually dominated by the non-potential sources of damping. The usual procedure for calculation of the ship motions is the use of the so-called diffraction-radiation software based on the potential flow theory. The damping calculated by this kind of codes is linear and related to the wave making. As far as the linear theory is concerned the horizontal modes of motion (surge, sway, yaw) are usually out of resonance in the common sea conditions (periods of 5 to 25 seconds). In those cases the role of damping is not very important so that the potential flow theory evaluate correctly the (linear) amplitudes of these motions. The only modes of motion that may enter in the (linear) resonance are vertical modes (heave, pitch and roll) and for these motions the role of the damping is essential.

Unmanned Sea Surface Vehicle (USSV) motion simulation in time domain is an important component of USSV design and operation. This capability is needed for hull design, operator training, controller synthesis and testing. Many applications such as simulators for operator training require real-time performance. Traditional approaches based on strip theory, although fast, abstract hulls into slender bodies, making the simulation results insensitive to changes in geometry. Computational fluid dynamics (CFD) based approaches are accurate but very slow. Potential flow based approaches are sensitive to hull geometry and take lesser amount of time than CFD based approaches. The motion simulation using potential flow theory involves following four main operations: (1) computation of dynamic pressure head due to fluid flow around the hull under the ocean wave, (2) computation of wet surface, (3) computing surface integral of dynamic pressure head over wet surface, and (4) solving the rigid body dynamics equation. First three operations depend upon the boat geometry complexity and need to be performed at each time step, making the simulation run very slow. In this paper, we investigate the problem of model simplification for real-time simulation of USSV model in time domain using potential flow theory, with arbitrary geometry under ocean waves with inviscid and irrotational flow. Using clustering based simplification scheme and parallel computing we obtained real time simulation performance.

Hydrodynamic response for flexible connectors of mobile offshore base at rough sea states