Wake Distribution Research Articles

The Effect of the Fillets on Submarine Wake Field and Propeller Unsteady Bearing Force

The uniformity of the wake on the submarine propeller disk is the key factor affecting the vibration and noise performance of the propeller. To study the influence of the fillets on the uniformity of submarine wake field and propeller unsteady bearing force, the submarine resistance calculation and self-propulsion calculation were carried out using the Detached-Eddy Simulation (DES) method. The characteristics of the wake field and hydrodynamic performance of the original and improved plan were compared and analyzed in detail. The fillets improve the wake distribution at the propeller disk by reducing the horseshoe vortex, and the fillets mainly affect the flow field characteristics of the inner radius area. The fillets can effectively reduce the axial velocity pulsation and improve the quality of the wake field. The frequency–domain curve shows that the bearing force and torque pulsation value of the improved plan are smaller than that in the original one, indicating that the fillets are conducive to controlling the vibration and noise performance of the submarine propeller. The results show that the fillets can effectively improve the flow field quality in the wake fields and reduce the propeller unsteady bearing force.

선미 부가물 수정에 따른 프로펠러 캐비테이션 성능 향상 연구

In order to improve the propeller cavitation performance composed of Cavitation Inception Speed (CIS), cavitation extent and pressure fluctuation, it needs to improve the wake distribution that flows into the propeller. The warship propeller cavitation is strongly influenced by the wake created at the V-strut of various appendages. The inflow characteristics of the V-strut were investigated using Computational Fluid Dynamics (CFD) and the twisted angles of the V-strut were aligned with upstream flow. The resistance and self-propulsion tests for the model ship with the existing and modified V-struts were conducted in Towing Tank (TT), and wake distribution, CIS, cavitation observation and pressure fluctuation tests were conducted in Large Cavitation Tunnel (LCT). The propeller behind the modified V-strut showed better cavitation characteristics than that behind the existing V-strut. Another model test was conducted to investigate rudder cavitation performance by the change of the V-strut. The rudder cavitation characteristics were not improved by the change of the operating conditions. On the basis of the present study, it is thought that the stern appendages for better propeller cavitation performance would be developed.

Numerical Study of Flow Downstream a Step with a Cylinder Part 2: Effect of a Cylinder on the Flow over the Step

In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the available literature data, in Part 2 the effect of the cylinder was investigated. In the laminar regime, different Reynolds numbers were considered. In the turbulent regime, the effects on the flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step were comparatively studied. When the cylinder was positioned below the step edge mid-plane, flow over the step was not altered by a cylinder. However, in other locations of a cylinder, the added cylinder modified the structure of flow, increasing the skin friction coefficient in the recirculation zone. Furthermore, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point and remained stable in the flow recovery process. Moreover, the presence of the step significantly influenced the dynamics of the vortex generation and shedding leading to an asymmetric wake distribution.

Wake-adapted ducted propeller for full-scale generic underwater vehicle: parametric study on blade skew for unsteady propeller thrust

A parametric study of blade skew for unsteady propeller thrust in the wake of an underwater vehicle (BB2) is presented. Unsteady thrust forces transmit via the propeller shaft and produce undesirable noise and vibration, particularly at frequencies corresponding to multiples of the blade number. The effectiveness of blade skew in reducing noise and vibration has been reported in previous studies, though not for underwater vehicles. The amplitudes of the dominant harmonics of the unsteady propeller thrust are determined using a Fourier series. The balanced skew reduces the dominant fifth harmonic more significantly than the biased skew at identical total-skew angles. The total-skew angle strongly influences the unsteady propeller thrust magnitude for balanced skew designs, whereas the effect of the maximum forward-skew angle is negligible. Significant fluctuations in one-blade loads are observed when the blade passes through the downstream wake of the vehicle's sail and casing. Increasing the blade total-skew reduces the blade load fluctuation when the blade approaches the top dead centre position, but has limited influence on the fluctuations when the blade moves away from this position. The nominal wake distribution at the propeller plane is analysed, and the fifth harmonic is identified as having the largest amplitude. This suggests that an increase in the number of blades would reduce the unsteady propeller thrust of the full-scale BB2 underwater vehicle.

Research on the flow characteristics of the bulk carrier wake field based on particle image velocimetry

The wake field has three-dimensional flow separation characteristics, affecting various ship performances. The research on the characteristics of the wake field can provide theoretical guidance for the formation of new ship design methods and provide an understanding of flow mechanisms for improving ship performance. To explore the flow mechanism of the wake field, particle image velocimetry was used to carry out the detailed flow measurement of bulk carriers. First, the experimental uncertainty and convergence are analyzed. Then, the spatial distribution characteristics of the time-averaged field, instantaneous field, and turbulent flow statistics are discussed in detail, and a criterion for discriminating turbulent anisotropy is proposed. The results show that the vortex structure significantly affects flow characteristics, and the axial velocity contours present a U-form distribution with prominent “hook-like” features. Compared with the time-averaged velocity field, the instantaneous velocity field is chaotic and has multiple additional vortex structures, and the velocity contours and streamlines have prominent non-smooth characteristics. The wake field has an apparent double vortex structure, and the aggregation of many small vortices forms the bilge vortex. The instantaneous rotation characteristics of vortices in the wake field are highly time-dependent and fluctuate with time. The turbulent kinetic energy, the root mean square of fluctuation velocity, and the Reynolds stress have a U-form distribution. The U-form region is concentrated in the area with a large gradient. The wake distribution is in a state of turbulent anisotropy, and the kinetic energy change layer and low kinetic energy region have a low turbulent anisotropy.

Experimental Study on the Influence of a Two-Dimensional Cosine Hill on Wind Turbine Wake

The accurate prediction of wind energy distribution on the terrain has been of great significance for wind farm selection. Therefore, this paper evaluated the influence of a two-dimensional hill with a relatively large slope (i.e., 0.83) on wind turbine wake. Firstly, the wakes on flat terrain and wind characteristics around a single hill were investigated by using a hot-wire anemometer. Subsequently, wake distributions combined with the hill were measured when the turbine was located 2D in front and on the hilltop. The results of the hill showed that a flow separation was formed within 6D of the leeward side (where D is the rotor diameter). The turbulence intensity increased initially and then decreased as height increased, leading to a high turbulence region 2.28 times the hill height, according to the experiment of a wind turbine combined with the hill. In conclusion, wake recovery was promoted on the windward side and 4D behind the hilltop. As the longitudinal coordinate increased, the intensity of the turbulence changed to a single peak, and the peak value was more than twice as high as on flat terrain. Based on this, it may be possible to optimize the design of wind turbines for better performance.

Large eddy simulation for the effects of ground roughness and atmospheric stratification on the wake characteristics of wind turbines mounted on complex terrains

The wake characteristics of wind turbine mounted on complex terrains are of great significance in wind energy utilization. The ground roughness and atmospheric stratification are two key factors for this issue and their coupling effect has not been fully studied before. In this research, the wake distributions of wind turbine mounted on two typical complex terrains and a real complex terrain are investigated using large eddy simulations and actuator disk model with rotation. The ground roughness and atmospheric stratification are simulated by adding source term in the governing equations of fluid dynamics to generated temperature gradient and vegetation canopy. It is found that the effects of ground roughness and atmospheric stratification on the wake of wind turbine are superimposed or counteracted depending on the shape of complex terrain. The presence of atmospheric stratification reduces the power of wind turbine drastically by more than 40% and the rough ground causes a small decrease in the power of wind turbine on typical complex terrains. For the multiple wind turbines on a real terrain with rough ground, the influence of atmospheric stratification on the wake characteristics and power production of each wind turbine strongly affected by the micrositing of wind turbines. The usual practice in industry of multiplying the oncoming wind profile by the turbine power curve overestimates the power production of wind turbines on complex terrains with atmospheric stratification by up to 670% compared with the numerical results.

Hydrodynamic Forces and Wake Distribution of Various Ship Shapes Calculated Using a Reynolds Stress Model

The Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics (CFD) calculation using a two-equation turbulence model, such as the k–omega shear-stress transport (SST) model, is a mainstream method with sufficient accuracy for the estimation of integral hydrodynamic forces and moment at both the model-scale and full scale. This paper confirmed that the Reynolds stress model (RSM) has sufficient estimation accuracy of viscous resistance and wake distribution at the hull design stage. Herein, the ability of RSMs to estimate the viscous resistance and wake distribution of a JBC ship is evaluated. Specifically, the verification and validation (V&V) method is employed to indicate the numerical and model uncertainties of each turbulence model used to estimate the viscous resistance. The RSMs showed higher numerical uncertainty than the k–omega SST. However, the uncertainty of the experimental measurements is generally smaller than the numerical uncertainty. Moreover, the linear pressure–strain (LPS) and the linear pressure–strain two-layer (LPST) models show less comparison error of the viscous resistance than the k–omega SST. Furthermore, the LPST and k–omega SST models are applied to twenty ships with various full and fine hull forms to calculate the viscous resistance and compare it with the experimental results. The viscous resistance of the LPST model showed a small difference when employed in experimental fluid dynamics (EFD) and CFD calculations. Using the LPST model, the viscous resistance can be estimated with high accuracy in our setting. For industrial use, this study could provide an important insight into the designing of various types of vessels.

Quantification of 3D spatiotemporal inhomogeneity for wake characteristics with validations from field measurement and wind tunnel test

To further understand the spatiotemporal inhomogeneity characteristics of wake profile caused by the inflow change of upstream wind turbine and reduce the influence of wake expansion on the power output and fatigue load of downstream wind turbine, a three-dimensional full wake model (3DJGF model) and a three-dimensional temporal wake model (3DJGF-T model) are proposed and verified in this paper. Assuming that the downstream wake distribution of wind turbine is double-Gaussian distribution, a new 3DJGF model is derived based on the law of conservation of mass and considering the wind shear effect. Based on the 3DJGF model, the time variable is introduced to calculate the delay time, and the 3DJGF-T model of the whole flow field is obtained. The accuracy of the proposed 3DJGF model and 3DJGF-T model is verified by wind field measurement and wind tunnel test, and the relative error of the 3DJGF-T model is analyzed: under the time-varying characteristics of different incoming wind speeds, the error of the newly proposed 3DJGF-T model is basically within 4%. The results show that the 3DJGF-T model can effectively describe the spatiotemporal inhomogeneity distribution and expansion characteristics of wake, which can provide a reference for the dynamic load analysis of downstream wind turbine considering the dynamic adjustment process of wind turbine control strategy, further improve the wake model and optimize the control strategy of wind turbine.

A novel entrainment wind farm flow model for power prediction

ABSTRACT The wind farm flow (WFF) models, which are enabled to predict the power output of downstream, located turbines within a wind farm. A WFF model consists of two main model components: a single wake model and a superposition model. Two WFF models, the Root Sum Square (RSS) superposition model incorporated into the single wake models (e.g. Bastankah and Porté-Agel wake (BPA) model and Jensen wake model), have been extensively applied in engineering. But the WFF models above-mentioned tend to overestimate the power deficit of the whole wind farm. Hence, a novel entrainment wind farm flow (NEWFF) model is proposed in this paper, which is a combination of a modified linear entrainment wake (MLEW) model and the RSS superposition model. Different from the previous linear entrainment wake (LEW) model, the MLEW model developed in this paper considers the impacts of terrain roughness variables on the wake distribution of downstream wind turbines. The MLEW model significantly improves the accuracy of the wake simulation over the advanced BPAW model and JW model, as well as the LEW model, as shown in two cases from the TNO wind tunnel. Finally, several cases of Horns Rev offshore wind farm and Lillgrund offshore wind farm are utilized to validate the NEWFF model. Compared with the latest advanced Zong and Agel superposition wake (ZASW) model and BPA wind farm flow (BPAWFF) model, it has been demonstrated that the prediction results obtained with the NEWFF model exhibit the best agreements with measured power data under full and partial wake conditions.

The hydrodynamic performance analysis of a submarine with new pump-jet propulsor

A new type of pump-jet propulsor (PJP) was designed and installed on the DARPA Suboff. The installation position of this new PJP is different from the traditional PJP, which gives more possibilities to improve the comprehensive performance of submarine. The duct of PJP has been automatically modeled based on pumped rotor, stator and hull shape. The rotor of PJP has been modeled as an actuator disc based on body force method coupled with the experimental open water data. The hydrodynamic performance analysis was applied on the self-propulsion point, and the self-propulsion characteristics were estimated by the thrust identity method. Moreover, the pressure and velocity distribution for different duct geometric parameters have been compared and discussed. The results indicate that the size of the duct has an impact on the generation of negative pressure zone and the wake distribution. When the geometric parameters of duct are equal to r = 0.75R, L = 3.5D and H = 0.8D, the overall performance is superior. The new pump jet destroys the circumferential correlation of the duct vortex and accelerates the dissipation of the vortex.

Analysis of The Effect of Pitch Angle on Propeller Modification by Considering Wake Distribution on Propeller Performance

The propeller is an important component of the ship’s propulsion system, and its design related to the safety and ship’s cost. The propeller is operated in an uneven flow field behind the ship so it has an important influence on the cavity, vibration and hydrodynamic performance. Therefore, optimization calculations are needed to get most economical in addition to meeting the provisions of the Energy Efficiency Design Index. Optimization software through polynomial regression values can be used for selecting propeller. However, when the propeller is installed on the hull, the Velocity Advance (Va) on the propeller will change from the distribution at the time of the open water test because of the shape of the hull. This reduces the performance of propeller. So it is necessary to calculate the engine propeller hull matching to get a match on each propulsion component. Blade element theory concept can be used in designing the propeller for the engine hull matching, with the hypothesis that if the pitch angle is greater than Va, cavitation will occur. This condition is not allowed and if the pitch angle is too small for Va, the thrust cannot be maximized against flow, then it must adjust to the Velocity Advance (Va) distribution on the ship’s hull, because increasing the thrust value will also increasing propeller’s relative rotative efficiency (ηR) and open water efficiency (ηO) and hull efficiency (ηH). This situation makes a proportional increase in the value of the propulsive coefficient. The simulations are carried out on the size of the model with a dual domain (single-hull) by turning the propeller while the ship is stationary and seeing the convergent speed generated by the ship. The simulation data that will be taken are propeller thrust and torque because these values are considered sufficient to compare the simulation results. It’s getting the thrust foil value that making model S_T, S_eta and S_KomL S_T model is done by looking at the highest thrust, S_eta model with highest efficiency method and on S_KomL with highest thrust and efficiency by looking at the streamlined model, the thrust and efficiency values are equal to or higher of the MP687. It is showed that open water efficiency value in the two modified models has decreased, namely S_T model of 0.341, S_KomL of 0.345 and S_eta of 0.301 with original MP687 model of 0.371. While the hull efficiency of S_T model has decreased to 1,449 and S_KomL model increased by 1,591 and S_eta model increased by 3.8 but in MP687 model was 1,526, and the relative rotate efficiency of S_T model increased to 1,096, S_KomL model also increased by 1,099 and also S_eta model of 1.08 which originally on the MP687 1,055 model. This will result in the Propulsive Coefficient (PC) value in S_T model of 0.543 with decreasing, S_KomL 0.604 and S_eta 1.25 with an increase from the original MP687 0.597

풍력터빈의 후류 우회를 위한 요각도 분석

The wake deficit from wind turbines is a major factor in reducing the performance of wind farms. Numerous studies have been performed on yaw angle control for the wind turbine rotor to mitigate the wake effect. It is necessary to establish a physical understanding of wake deflection and deformation at various yaw angles. In the present study, the turbine wake development at various yaw angles(-30° ~ 30°) and inflow wind speeds(7 ~ 9m/s) were analyzed via Large-Eddy Simulation coupled with Actuator Disk Model. Simulations were performed under uniform inflow conditions with the NREL 5MW wind turbine model placed at the center. The center, width, and shape of wake deficit profiles were calculated based on the velocity field and the vorticity of the wake. It shows that as the wake center deflection did not increase linearly with the yaw angle, and the slope of the wake deflection varied from the starting point to the end. In addition, the asymmetry of the wake distributions was analyzed based on the vorticity field which evolved into a kidney-shaped wake profile. Lastly, it was found that the wake deficit distribution held a self-similarity profile for the given range of uniform inflows.

STUDY OF FROUDE AND HUGHES METHODS BY NUMERICAL TOWING TANK

The resistance of a cargo ship is calculated by numerical towing tank. RANSE multi-phase parallel solver with K-Z SSTturbulent model and VOF formulation is applied. Computational results from double model (without free surface) areused to obtain 1+k in Hughes’ method and those with free surface are analyzed by both Froude and Hughes’ approachesto investigate model and full scale correlation. ITTC recommended uncertainty study is carried out to evaluate numericalerror due to grid density. The computed wave elevation, wake distribution and resistance components by fine, mediumand coarse meshes are cross-compared and validated against experiment data where applicable. It is found that gridresolution has most effect on wave pattern. The predicted friction and viscous-pressure resistance coefficients arerelatively grid independent from present numerical simulation.

CFD Simulation for Estimating Efficiency of PBCF Installed on a 176K Bulk Carrier under Both POW and Self-Propulsion Conditions

In this paper, the efficiency of Propeller Boss Cap Fins (PBCF) installed at the bulk carrier was estimated under both Propeller Open Water (POW) and self-propulsion conditions. For this estimation, virtual model-basin tests (resistance, POW, and self-propulsion tests) were conducted through Computational Fluid Dynamics (CFDs) simulation. In the resistance test, the total resistance and the wake distribution according to ship speed were investigated. In the POW test, changes of thrust, torque coefficient, and open water efficiency on the propeller according to PBCF installation were investigated. Finally, the International Towing Tank Conference (ITTC) 1978 method was used to predict the effect of PBCF installation on self-propulsive coefficient and brake horsepower. For analyzing incompressible viscous flow field, the Reynolds-Averaged Navier–Stokes (RANS) equation with SST k-ω turbulence model was calculated using Star-CCM+ 11.06.010-R8. All simulation results were validated by comparing the results of model tests conducted at the Korea Research Institute of Ships and Ocean Engineering (KRISO). Consequently, for the self-propulsion test with the PBCF, a 1.5% reduction of brake horsepower was estimated in the simulation and a 0.5% reduction of the brake horsepower was estimated in the experiment.

Cooperative multiagent optimization method for wind farm power delivery maximization

Reducing wake losses and improving the overall power output of wind farms have become a research focus in attempts to optimize wind farm power generation. A cooperative multiagent optimization method (CMAOM) for wind farm power delivery maximization has been proposed in this paper. In the CMAOM, a wind farm wake distribution calculation model, based on the Jensen wake model, was constructed, and each turbine was then assigned as an agent; the CMAOM was used to reduce wake losses and improve the overall wind farm power output. The agent, multiagent objective function and grid environment were defined in this study using wind turbine characteristics, and the CMAOM, including the neighborhood competition operator, mutation operator, and self-learning operator, were calibrated using wind turbine aerodynamic correlation characteristics. The Danish Horns Rev wind farm was selected as a case study, and the CMAOM and particle swarm optimization (PSO) algorithm were used to conduct analyses there. The results showed that the CMAOM proposed in this paper was more effective than the PSO algorithm and that the wind farm overall power output was increased by 7.51% for a 270° incoming wind direction and an incoming wind speed of 8.5 m/s.

A novel three-dimensional wake model based on anisotropic Gaussian distribution for wind turbine wakes

The development of a more advanced three-dimensional wake model for wind power generation is presented based on a multivariate Gaussian distribution. The newly-presented model is closer to reality as it truly depends on two independent dimensions (namely horizontal and vertical directions) rather than the radius of a circle. For this reason, the general expression of wake expansion rate in each dimension is specifically developed. In addition, by taking into account the inflow wind shear effect, this current model is able to accurately capture the asymmetric distribution of the vertical wake profile. Four cases including experimental data from wind tunnels and field observations as well as high-fidelity numerical simulation are used to validate the present model. Compared with conventional models, this new model is capable of predicting the wake distribution of a single wind turbine reasonably well. The proposed model is highly simple with a low computational cost. Before applying this model, no additional numerical calculation or trial calculation is required. Wake velocity at any given spatial position can be calculated in an accurate and fast manner. Because of its accuracy, universality and low cost, the present three-dimensional wake model is able to make contributions to farm-level applications such as layout optimization and control strategies and therefore benefit the power output of wind farms.

On the effect of velvet structures on trailing edge noise: experimental investigation and theoretical analysis

This study is inspired by the velvety structures on an owl's upper wing surface. Anechoic wind tunnel experiments were conducted to study the effect of the velvety structures on trailing edge noise as well as the boundary layer flow of a flat plate model. The tests were conducted in The Hong Kong University of Science and Technology low-speed wind tunnel, ultra-quiet noise injection test and evaluation device (UNITED). It was found that the trailing edge noise spectra are significantly modified by the velvety structures. In general, the velvety structures increase the low-frequency noise below a cross-over Strouhal number $St_c$ but reduce the spectral level at higher frequencies. The velvety surface also changes the boundary layer characteristics in terms of the boundary layer thickness, non-dimensional velocity distribution and turbulence distribution. Vortex shedding is suppressed by the velvety coating despite the blunt trailing edge. An analytic model is proposed for the trailing edge noise of a flat plate, including the effect of finite trailing edge thickness and velvety structures on the flat plate surface. The model uses the near wake distribution of the mean and fluctuating velocities in the streamwise direction as the input. The predictions, which require no empirical corrections, match well with the experiments for both the baseline and velvet-coated configurations. With a detailed non-dimensional analysis, this study proposes a potential aeroacoustic function of velvet structures, i.e. noise control through manipulation of boundary layer statistics.

Statistical Method of Bubble Distribution in Ship Wake Based on Digital Image

A large number of bubbles are formed in the wake of a ship when it is sailing. Statistical analysis of the wake bubbles can infer the important information of the ship such as draft, ship width and speed, which is of great significance in the military field. The experimental device is used to simulate the actual wake bubbles in this paper, and the detection method combining the sobel operator and the regionprop function is used to make effective statistics on the wake distribution. Experiments show that this paper can perform distribution statistics of wake bubbles in different situations and obtain better statistical results.

Evaluation of Aerodynamic Characteristics of Mega-Yacht Superstructures by CFD Simulations

Air wake distribution around the superstructure of a mega-yacht is a key concern for the designer because of various reasons such as comfort expectations in recreational deck areas, self-noise generation, air pollution and temperature gradients due to exhaust interactions, and safety of helicopter operations such as landing/take off and hovering. The Reynolds-averaged Navier-Stokes (RANS) technique in computational fluid dynamics (CFD) is frequently used in studies on mega-yacht hydrodynamics and aerodynamics with satisfactory results. In this article, a case study is presented for the utilization of CFD in a mega-yacht's superstructure design. The flow field in recreational open areas has been analyzed for the increase in velocity due to the existence of the superstructure. A reduction in self-noise of the mast structure has been aimed by reducing flow separation and vorticity. Time-dependent velocity data obtained with scale-resolving simulations are presented for the evaluation of helicopter landings. The capabilities and limitations of the RANS technique are discussed along with recent developments in modeling approaches.