Hydrodynamic Design

The application of marine bonded hoses has increased in recent times, due to the need for more flexible conduits and flexible applications in the offshore industry. These marine structures include Catenary Anchor Leg Moorings (CALM) buoys and ocean monitoring buoys. Their attachments include floating hoses, submarine hoses and submarine cables. However, the structural performance challenges of a CALM buoy system from its hydrodynamics water waves and other global loadings, have led to the need for this investigation. In this study, a detailed presentation on the motion characterisation of the CALM buoy hose system is presented. The CALM buoy is a structure with six degrees of freedom (6DoF). A well-detailed experimental presentation on the CALM buoy hose model conducted in Lancaster University Wave Tank is presented using three novel techniques, which are: a digital image captured using Imetrum systems, using an Akaso 4K underwater camera, using wave gauges arranged in a unique pattern and using underwater Bluetooth sensors. The buoy model was also found to respond uniquely for each motion investigated under water waves. The results showed that the higher the profile, the higher the response of the buoy. Thus, this study confirms the existence of flow patterns of the CALM buoy while floating on the water body. Full article

Underwater transportation is an essential approach for scientific exploration, maritime construction and military operations. Determining the hydrodynamic coefficients for a complex underwater transportation system comprising multiple vehicles is challenging. Here, the suitability of a quick and less costly semi-empirical approach to obtain the hydrodynamic coefficients for a complex transportation system comprising two Unmanned Underwater Vehicles (UUVs) is investigated, where the interaction effects between UUVs are assumed to be negligible. The drag results were verified by Computational Fluid Dynamics (CFD) analysis at the steady state. The semi-empirical results agree with CFD in heave and sway; however, they were overpredicted in surge due to ignoring the wake effects. Furthermore, experiments were performed for the validation of the time-domain motion simulations with semi-empirical and CFD results. The simulations which were performed with the CFD drags were close to the experiments. The semi-empirical approach could be relied on once a correction parameter is included to account for the interactive effect between multiple UUVs. Overall, this work makes a contribution by deriving a semi-empirical approach for the dynamic and controlling system of dual UUVs, with CFD and experiments applied to ascertain its accuracy and potential improvement. Full article

This paper presents CFD (Computational Fluid Dynamics) simulations of the performance of a planing hull in a calm-water condition, aiming to evaluate similarities and differences between results of different CFD models. The key differences between these models are the ways they use to compute the turbulent flow and simulate the motion of the vessel. The planing motion of a vessel on water leads to a strong turbulent fluid flow motion, and the movement of the vessel from its initial position can be relatively significant, which makes the simulation of the problem challenging. Two different frameworks including k-ε and DES (Detached Eddy Simulation) methods are employed to model the turbulence behavior of the fluid motion of the air–water flow around the boat. Vertical motions of the rigid solid body in the fluid domain, which eventually converge to steady linear and angular displacements, are numerically modeled by using two approaches, including morphing and overset techniques. All simulations are performed with a similar mesh structure which allows us to evaluate the differences between results of the applied mesh motions in terms of computation of turbulent air–water flow around the vessel. Through quantitative comparisons, the morphing technique has been seen to result in smaller errors in the prediction of the running trim angle at high speeds. Numerical observations suggest that a DES model can modify the accuracy of the morphing mesh simulations in the prediction of the trim angle, especially at high-speeds. The DES model has been seen to increase the accuracy of the model in the computation of the resistance of the vessel in a high-speed operation, as well. This better level of accuracy in the prediction of resistance is a result of the calculation of the turbulent eddies emerging in the water flow in the downstream zone, which are not captured when a k-ε framework is employed. The morphing approach itself can also increase the accuracy of the resistance prediction. The overset method, however, overpredicts the resistance force. This overprediction is caused by the larger vorticity, computed in the direction of the waves, generated under the bow of the vessel. Furthermore, the overset technique is observed to result in larger hydrodynamic pressure on the stagnation line, which is linked to the greater trim angle, predicted by this approach. The DES model is seen to result in extra-damping of the second and third crests of transom waves as it calculates the stronger eddies in the wake of the boat. Overall, a combination of the morphing and DES models is recommended to be used for CFD modeling of a planing hull at high-speeds. This combined CFD model might be relatively slower in terms of computational time, but it provides a greater level of accuracy in the performance prediction, and can predict the energy damping, developed in the surrounding water. Finally, the results of the present paper demonstrate that a better level of accuracy in the performance prediction of the vessel might also be achieved when an overset mesh motion is used. This can be attained in future by modifying the mesh structure in such a way that vorticity is not overpredicted and the generated eddies, emerging when a DES model is employed, are captured properly. Full article

In this study we investigated the performance of NACA 0012 hydrofoils aligned in tandem using parametric method and Neural Networks. We use the 2D viscous numerical model (STAR-CCM+) to simulate the hydrofoil system. To validate the numerical model, we modeled a single NACA 0012 configuration and compared it to experimental results. Results are found in concordance with the published experimental results. Then two NACA 0012 hydrofoils in tandem configuration were studied in relation to 788 combinations of the following parameters: spacing between two hydrofoils, angle of attack (AOA) of upstream hydrofoil and AOA of downstream hydrofoil. The effects exerted by these three parameters on the hydrodynamic coefficients Lift coefficient ($C_L$), Drag Coefficient ($C_D$) and Lift-Drag Ratio (LDR), are consistent with the behavior of the system. To establish a control system for the hydrofoil craft, a timely analysis of the hydrodynamic system is needed due to the computational resource constraints, analysis of a large combination and time consuming of the three parameters established. To provide a broader and faster way to predict the hydrodynamic performance of two hydrofoils in tandem configuration, an optimal artificial neural network (ANN) was trained using the large combination of three parameters generated from the numerical simulations. Regression analysis of the output of ANN was performed, and the results are consistent with numerical simulation with a correlation coefficient greater than $99.99\%$. The optimized spacing of $6.6c$ are suggested where the system has the lowest $C_D$ while obtaining the highest $C_L$ and LDR. The formula of the ANN was then presented, providing a reliable predicting method of hydrofoils in tandem configuration. Full article

The hydroelastic behaviour of a river-sea-going ship hull is analysed experimentally and numerically. A segmented ship model connected by a steel backbone is tested in regular waves, and its high-frequency vibrations such as springing and whipping responses are identified. The hydroelastic response of the ship is numerically calculated using a hydroelastic time domain method based on strip theory, which is extended to include an improved model of the slamming load. The slamming forces in the bow section are determined using the Modified Longvinovich Model (MLM) instead of the Von Karman model. The vertical motions and wave-induced loads are calculated and compared with the experimental results. The response amplitude operators of the vertical loads and the high-order harmonics are analysed under different speeds, showing good agreement with the experiments. The slamming loads on the bow section of a river-to-sea ship are predicted utilizing the MLM model and compared with the Arbitrary Lagrangian Eulerian algorithm by LS-DYNA and with the measured results. Full article

To estimate the response of wave energy converters to different sea environments accurately is an ongoing challenge for researchers and industry, considering that there has to be a balance between guaranteeing their integrity whilst extracting the wave energy efficiently. For oscillating wave surge converters, the incident wave field is changed due to the pitching motion of the flap structure. A key component influencing this motion response is the Power Take-Off system used. Based on OpenFOAM, this paper includes the Power Take-off to establish a realistic model to simulate the operation of a three-dimensional oscillating wave surge converter by solving Reynolds Averaged Navier-Stokes equations. It examines the relationship between incident waves and the perturbed fluid field near the flap, which is of great importance when performing in arrays as neighbouring devices may influence each other. Furthermore, it investigates the influence of different control strategy systems (active and passive) in the energy extracted from regular waves related to the performance of the device. This system is estimated for each wave frequency considered and the results show the efficiency of the energy extracted from the waves is related to high amplitude pitching motions of the device in short periods of time. Full article

A mathematical model for the problem of wave diffraction by a floating fixed truncated vertical cylinder is formulated based on Boussinesq equations (BEs). Using Bessel functions in the velocity potentials, the mathematical problem is solved for second-order wave amplitudes by applying a perturbation technique and matching conditions. On the other hand, computational fluid dynamics (CFD) simulation results of normalized free surface elevations and wave heights are compared against experimental fluid data (EFD) and numerical data available in the literature. In order to check the fidelity and accuracy of the Boussinesq model (BM), the results of the second-order super-harmonic wave amplitude around the vertical cylinder are compared with CFD results. The comparison shows a good level of agreement between Boussinesq, CFD, EFD, and numerical data. In addition, wave forces and moments acting on the cylinder and the pressure distribution around the vertical cylinder are analyzed from CFD simulations. Based on analytical solutions, the effects of radius, wave number, water depth, and depth parameters at specific elevations on the second-order sub-harmonic wave amplitudes are analyzed. Full article