Impact of Leading Edge Roughness in Cavitation Simulations around a Twisted Foil

The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness elements on the flow structures in the cavitation prediction. The results include detailed comparisons of cavitation pattern, vorticity distribution, and force predictions with the experimental measurements. It is noted that the presence of roughness generates very small cavitating vortical structures which interact with the main sheet cavity developing over the foil to later form a cloud cavity. Very similar to the experimental observation, these interactions create a streaky sheet cavity interface which cannot be captured in the smooth condition, influencing both the richness of structures in the detached cloudy cavitation as well as the extent and transport of vapour. It is further found to have a direct impact on the pressure distribution, especially in the mid-chord region where the shed cloud cavity collapses

Developing Computational Methods for Detailed Assessment of Cavitation on Marine Propellers

Cavitation often brings negative effects, such as performance degradation, noise, vibration, and material damage, to a marine propulsion systems, but for optimum performance, cavitation is almost inevitable. Therefore, it is necessary to improve the understanding of cavitation in order to maximize the performance without encountering severe problems. Experimental tests can only provide limited information about this complex phenomenon. This thesis deals with improving numerical simulations methodologies that can offer a more complete picture of the cavitation process, making it possible to investigate the flow in more detail with some confidence, thus enabling an improved design.Numerical simulations of non-cavitating and cavitating flows are conducted using OpenFOAM. The flow is modelled using Implicit Large Eddy Simulation and considering the two phases, i.e. vapour and liquid, as a homogeneous mixture through a volume fraction transport equation method along with the Schnerr-Sauer mass transfer model.To avoid manual calibration of the mass transfer model coefficients, which may significantly affect both the accuracy and stability of the numerical predictions, an approach is suggested and tested to compute the mass transfer rate based on the flow local time scale during the solution procedure. Moreover, the saturation pressure is modified in order to take into account the shear stress effects on the liquid rupturing.To test the proposed modifications, several test cases consisting of 2D and 3D hydrofoils and model scale propellers are simulated and the results are compared with experimental data. Integral quantities, local pressure data, and cavitation extent are studied for both the non-cavitating and the cavitating flows. Furthermore, the computational set-up is tested by varying domain size, mesh type and resolution, numerical schemes, and mass transfer model coefficients.The overall results compare well with the available experimental data, provided the mesh resolution is sufficient. The proposed mass transfer model modifications give a considerably improved prediction of pressure distribution and cavity extent. Some results yield overpredicted cavitation, indicating discrepancies between the modelling approach and model scale experimental techniques

Computational Modelling for Cavitation and Tip Vortex Flows

Cavitation often brings negative effects, such as performance degradation, noise, vibration, and material damage, to marine propulsion systems, but for optimum performance, cavitation is almost inevitable. Therefore, it is necessary to better understand cavitation in order to maximize the performance without encounter- ing severe problems. Experimental tests can only provide limited information about this complex phenomenon. This thesis deals with improving computational methodologies that can offer a more complete picture of the cavitation process, making it possible to investigate the flow in more details with a higher level of confidence, which eventually enables an improved design. The study describes cavitation behaviour in the early stage of the formation, i.e. cavitation inception and its interaction with tip vortex structures, as well as in the developed form, i.e. sheet and cloud cavitation. The analysis of the tip vortex flows is associated with the spatial mesh resolution, the sub-grid scale and the turbulence modelling, as well as the cavitation-vortex interaction. For inception prediction, different inception methods are investigated to char- acterize tip vortex flows around an elliptical foil and high skewed low noise pro- pellers. The adopted inception models cover different levels of complexity in- cluding wetted flow analysis, Eulerian cavitation simulations, and simplified La- grangian Rayleigh-Plesset bubble dynamics models. For simulations of developed sheet/cloud cavitating flows, a homogeneous two-phase mixture method is adopted along with the Schnerr-Sauer mass transfer model. A manual calibration of the mass transfer model coefficients may signifi- cantly affect both accuracy and stability of the numerical predictions. In order to avoid this issue, an approach is suggested and tested to compute the mass transfer rate based on the flow local time scale during the solution procedure. Comparison between high speed videos and numerical results clearly shows the capability of the developed method in predicting the cavitating structures. It is shown that in addition to the well-captured difference in e.g. the amount of cav- itation, the simulation is capable of correctly predicting the small though crucial differences in flow features and cavitation inception characteristics of different propellers designs. The strong dependency of the inception on the initial nuclei sizes are demonstrated, and it is shown that for weaker propeller tip vortices this dependency becomes even more significant

Comparative analysis of tip vortex flow using RANS and LES

The current study focuses on the numerical analysis of tip vortex flows, with the emphasis n the investigation of turbulence modelling effects on tip vortex prediction. The analysis includes comparison of RANS and LES methods at two different mesh resolutions. Implicit LES, ILES, modelling is employed here to mimic the turbulent viscosity. In RANS, the two equation k-ω SST model is adopted. In order to also address possible benefits of using streamline curvature variations in RANS, two curvature correction methods proposed for k-ω SST are tested, and compared. ILES results show very good agreement with the experimental observations. The predicted vortex in ILES is also stronger than RANS predictions. ILES has predicted accelerated vortex core axial velocity very well, while tested RANS models under predict the axial velocity. Adoption of curvature correction has not improved the tip vortex prediction, even though it has reduced the turbulent viscosity at the vortex core

Propeller tip vortex cavitation mitigation using roughness

This paper presents an investigation of roughness application on marine propellers in order to alter their tip vortex properties, and consequently mitigate tip vortex cavitation. SST kOmega model along with a curvature correction is employed to simulate the flow on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter. The roughness is modelled by using a rough wall function to increase the turbulent properties in roughed areas. In one case, roughness geometry is included as a part of the blade geometry, and the flow around them are resolved. To minimize the negative effects of the roughness on the propeller performance, the roughness area is optimized by simultaneous consideration of the tip vortex mitigation and performance degradation. For the considered operating condition, it is found that having roughness on the tip region of suction side can reduce the cavitation inception by 18 % while keeping the performance degradation in a reasonable range, less than 2%

Investigations of Tip Vortex Mitigation By Using Roughness

The application of artificial roughness to mitigate tip vortex cavitation inception is analyzed through numerical and experimental investigations carried out on an elliptical foil. Different roughness configurations and sizes are tested and effects on cavitation inception, drag, and lift, are studied. Implicit Large Eddy Simulation (ILES) is employed to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. Two different approaches including using a rough wall function and resolving the flow around roughness elements are evaluated. New experiments, performed in the cavitation tunnel at Kongsberg Hydrodynamic Research Centre, for the rough foil are presented.The vortical structures and vorticity magnitude distributions are employed to demonstrate how different roughness patterns and configurations contribute to the vortex roll-up and consequently on the tip vortex strength. It is found that the application of roughness on the leading edge, tip region and trailing edge of the suction side are acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The analysis of boundary layer characteristics shows a separation line caused by roughness is the reason for a more even distribution of vorticity over the tip compared to the smooth foil condition leading to a reduction in vortexstrength. For the optimum roughness pattern, both the numerical results and experimental measurements show a decrease in the tip vortex cavitation inception as large as 33 % compared to the smooth foil condition with a drag force increase observed to be less than 2 %

New Insights into Roughness Applications in Tip Vortex Cavitation Inception Mitigation

Tip vortex cavitation (TVC) is usually the first type of cavitation that appears on a propeller. Therefore, it is considered as the main cavitation characteristics to avoid in the design procedure of low-noise propellers, where their operating profiles demand very low radiated noise emissions. The current study includes both numerical and experimental analyses of blade surface roughness application in order to mitigate TVC inception. The investigation consists of applying roughness application on a classical benchmark, an elliptical foil, and on a propeller selected from a Kongs-berg research series of highly skewed propellers having a low effective tip load. The numerical simulations are performed on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter by using Implicit Large Eddy Simulation (ILES) for unsteady simulations, and RANS using the SST k − ω model with a curvature correction for steady simulations. Two approaches are considered to include roughness in the numerical simulations: using a rough wall function and resolving the flow around the roughness elements. To minimize the negative effects of the roughness on the propeller performance, the roughness area is optimized by simultaneous consideration of the tip vortex mitigation and performance degradation. Experimental measurements of the elliptical foil are conducted to support the CFD study at different operating conditions and with different roughness patterns while LDV and high-speed video recordings are used to collect the data. The tested conditions include both cavitating and inception of TV flows on the smooth and roughened foil to provide further insights on the usage of roughness. For the elliptical foil, it is found that the application of roughness can reduce the cavitation number for cavitation inception, σ i , by 35 % while keeping the performance degradation less than 1% compared to the smooth foil condition. The average reduction of the TVC inception number achieved by using roughness on the propeller is around 21% with a performance degradation of around 1.5% compared to the smooth propeller condition

Propeller tip vortex mitigation by roughness application

In this study, the application of surface roughness on model and full scale marine propellers in order to mitigate tip vortex cavitation is evaluated. To model the turbulence, SST k−ω model along with a curvature correction is employed to simulate the flow on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter according to our previous guidelines. The effect of roughness is modelled by modified wall functions. The analysis focuses on two types of vortices appearing on marine propellers: tip vortices developing in lower advance ratio numbers and leading edge tip vortices developing in higher advance ratio numbers. It is shown that as the origin and formation of these two types of vortices differ, different roughness patterns are needed to mitigate them with respect to performance degradation of propeller performance. Our findings clarify that the combination of having roughness on the blade tip and a limited area on the leading edge is the optimum roughness pattern where a reasonable balance between tip vortex cavitation mitigation and performance degradation can be achieved. This pattern in model scale condition leads to an average TVC mitigation of 37% with an average performance degradation of 1.8% while in full scale condition an average TVC mitigation of 22% and performance degradation of 1.4% are obtained

Propeller Tip Vortex Mitigation By Roughness Application

In this study, the application of surface roughness on model and full scale marine propellers in order to mitigate tip vortex cavitation is evaluated. To model the turbulence, SST kOmegamodel along with a curvature correction is employed to simulate the flow on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter according to our previous guidelines. The effect of roughness is modeled by modified wall functions. The analysis focuses on two types of vortices appearing on marine propellers: tip vortices developing in lower advance ratio numbers and leading-edge tip vortices developing in higher advance ratio numbers. It is shown that as the origin and formation of these two types of vortices differ, different roughness patterns are needed to mitigate them with respect to performance degradation of propeller performance. Our findings clarify that the combination of having roughness on the blade tip and a limited area on the leading edge is the optimum roughness pattern where a reasonable balance between tip vortex cavitation mitigation and performance degradation can be achieved. This pattern in model scale condition leads to an average TVC mitigation of 37% with an average performance degradation of 1.8% while in full scale condition an average TVC mitigation of 22% and performance degradation of 1.4% are obtained

Experimental Analysis of Tip Vortex Cavitation Mitigation By Controlled Surface Roughness

This study presents results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil. High-speed video recordings and Laser Doppler Velocimetry (LDV) measurements are employed to provide further details on the cavitation behaviour and tip vortex flow properties in different roughness pattern configurations. The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared to the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions. The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33 % in the optimized roughness pattern compared to the smooth foil condition where the drag force increase is observed to be around 2 %. During the tests, no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed. However, the drag force is found to be higher with a non-uniform roughness distribution