9 research outputs found

    A Survey of Factors Affecting Blunt Leading-Edge Separation for Swept and Semi-Slender Wings

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    A survey is presented of factors affecting blunt leading-edge separation for swept and semi-slender wings. This class of separation often results in the onset and progression of separation-induced vortical flow over a slender or semi-slender wing. The term semi-slender is used to distinguish wings with moderate sweeps and aspect ratios from the more traditional highly-swept, low-aspect-ratio slender wing. Emphasis is divided between a selection of results obtained through literature survey a section of results from some recent research projects primarily being coordinated through NATO s Research and Technology Organization (RTO). An aircraft context to these studies is included

    Numerical investigation of shallow-water effects on hydrokinetic turbine wake recovery

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    Thrust, power and intermediate wake predictions obtained using resolved rotating blade with sliding mesh simulations for a hydrokinetic turbine (HKT) are assessed using the open-source flow solver OpenFOAM. Single- and two-phase URANS and DES computations are performed for three-blade, 0.5m diameter (D) turbine mounted on a stanchion that intersects the free surface with a tip-speed ratio λ = 6.15. The thrust and power predictions compare within 5% of the experimental data. Results show that the thrust predictions are dominated by the pressure distribution on the blades, whereas the shear stress plays a significant role in the power predictions. The turbine performance showed unsteadiness with amplitudes around 3% of the mean, due to the disruption of the flow each time a blade passed in front of the stanchion. The wake recovery is primarily due to the growth of shear layers (originating from the blade tips) towards the turbine axis, which are primarily caused by the cross-plane turbulent velocity. The shear layer growth is enhanced by the turbulence produced by the stanchion. Predictions of the mean wake profile compared within 10% of the experimental data, which is significant improvement over previous Fluent predictions that showed large errors of 22%. The improved predictions in OpenFOAM is attributed to better turbulence predictions. Two-phase results show that the interaction between the wake and free-surface is initiated by the interaction of stanchion with the free-surface. The free-surface creates a blockage effect that accelerates the flow in the upper bypass region and enhances the wake recovery

    Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library

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    In this paper we employ two implementations of the fictitious domain (FD) method to simulate water-entry and water-exit problems and demonstrate their ability to simulate practical marine engineering problems. In FD methods, the fluid momentum equation is extended within the solid domain using an additional body force that constrains the structure velocity to be that of a rigid body. Using this formulation, a single set of equations is solved over the entire computational domain. The constraint force is calculated in two distinct ways: one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method and another using a fully-Eulerian approach of the Brinkman penalization (BP) method. Both FSI strategies use the same multiphase flow algorithm that solves the discrete incompressible Navier-Stokes system in conservative form. A consistent transport scheme is employed to advect mass and momentum in the domain, which ensures numerical stability of high density ratio multiphase flows involved in practical marine engineering applications. Example cases of a free falling wedge (straight and inclined) and cylinder are simulated, and the numerical results are compared against benchmark cases in literature.Comment: The current paper builds on arXiv:1901.07892 and re-explains some parts of it for the reader's convenienc

    Unstructured grid technologies for hydrodynamic applications with bodies in relative motion and mesh deformation

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    Unstructured grid technologies for hydrodynamic applications with bodies in relative motion and mesh deformation are presented. A parallel universal mesh deformation scheme is developed to manage deforming surface and volume grids for both aerodynamic and hydrodynamic applications. The approach is universal and independent of grid type. Also, it requires minimal inter-processor communication and is thus perfectly suitable to a parallel platform. The original scheme of Allen (2006) has difficulty deforming volume grids in regions near concave geometry features and for abrupt grid resolution changes. Several modifications are proposed to overcome these problems. Grid quality can be improved significantly by adding a smoothing algorithm and additional surface mesh connectivity. The mesh deformation scheme is demonstrated and validated by solutions of several synthetic jet test cases from a NASA Langley Workshop. Application to the free surface flow over the S175 container ship undergoing two-node harmonic bending is also demonstrated. The resulting viscous mesh shows good quality throughout the harmonic deformation with large scale vortex shedding occurring at the bow and stern. Overset grids technologies are adopted to simulate the flow past multiple bodies in relative motion. A generalized library DiRTlib and a grid assembly code SUGGAR both developed by Noack (2005) are used to facilitate integration of overset grids method into Tenasi flow solver. Both static and dynamic cases are tested. First, for verification, simulation of an oscillating cylinder using overset grids is compared with a baseline configuration using a single grid in rigid motion (i.e., no relative motion between the cylinder and the farfield), and the solutions agree with each other very well. Then interaction between two oscillating cylinders with same amplitude and frequency but 180-degree phase difference is studied. It is found that significant low pressure is generated between these two cylinders when passing each other at close separation distance. Finally, as the primary motivation for developing simulation capability for modeling the dynamics of interacting platforms, a Subo passing beneath a container ship is simulated, and satisfactory results are obtained

    A review on the turbulence modelling strategy for ship hydrodynamic simulations

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    Ship operations are accompanied by turbulent regimes that play a significant role in the hydrodynamic characteristics. With the ongoing development of computational technologies, it is now feasible to numerically simulate turbulent ship flows with a high degree of detail. Turbulent simulations, however, tend to be computationally expensive and require a trade off between computational costs and fidelity. Whilst a range of turbulence modelling strategies is available in Computational Fluid Dynamics, there is a lack of up-to-date recommendations on their suitability for different ship-flow simulation scenarios. Addressing this gap, the present work reviews the state-of-the-art of turbulence modelling for ship hydrodynamic applications. As a result, this paper introduces the most known turbulence modelling approaches used in the field, followed by a thorough discussion of their applicabilities and limitations. Furthermore, this paper provides recommendations for the selection of turbulence modelling strategies versus various ship simulation scenarios, such as resistance prediction, ship flow modelling, self-propulsion, and cavitation analyses. It is expected that the present paper will provide decision-making support by helping CFD users minimise the time spent on trial and error, as well as providing valuable insights to promote the advancement of turbulence modelling

    Large -eddy simulation of ship wakes

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    The objective of the present study is to improve our understanding of turbulent wake flows. Large-Eddy Simulation (LES) technique is applied for this purpose. A readily available code was used with necessary modifications. Three dimensional incompressible Navier-Stokes equations are solved in non-orthogonal curvilinear coordinates. Finite-volume approach is implemented on a non-staggered grid. The core of the numerical scheme is a fractional step method. The overall accuracy of the method is second order in both space and time.;The LES approach has been validated for four cases: channel flow, flow past a square body, a shear layer flow, and open channel flow. Different subgrid-scale models and numerical schemes have been tested for these benchmarks. Comparisons between the simulations and experiments show the capability of this LES method.;An efficient and accurate Random Flow Generation (RFG) approach has been improved to provide turbulent initial and inflow conditions for developing wake flow calculations. The RFG method can handle anisotropy and inhomogeneity, and it satisfies the instantaneous continuity equations. This approach has been verified by reproducing a turbulent channel flow based on a Direct Numerical Simulation (DNS). Perfect agreements have been obtained.;A typical one equation sub-grid scale model has been selected and modified to include the backscatter of energy by applying the RFG algorithm. Meanwhile, an attempt was made to implement the one equation sub-grid scale model dynamically. Comparisons have been made between the experiments and the simulation results using different one equation sub-grid scale models.;The RFG approach along with the LES technique has been applied to the wake behind a flat plate. Effects of grid resolution and SGS models on the turbulent flow field have been investigated. Good results are obtained as compared to experiments.;Wake flows behind a Naval ship model (5415) have been studied in details by applying the combined LES-RFG method. The calculation domain starts from a plane behind the ship model. Because of the lack of experimental data, Reynolds Averaged Navier-Stokes (RANS) calculations are used to provide the RFG procedure with the information on the inflow boundary. The ship wake flow with an impose surface wave has been studied. The result shows that the wave surface has a significant influence on the turbulent kinetic energy distribution. Finally, part of the ship hull is included in the simulation of the ship wake with a static wave surface (Fr = 0.28) to investigate the difference between RANS and LES results in the near wake of a ship model. The overall quality of the LES calculations is found to be very good. In particular the large coherent structures with significant vorticity concentration and gradients could be captured in the wake of a ship model which is not possible to accomplished by RANS. These structures have significant importance in the transplant of entrained air bubbles

    Validation of hydrokinetic turbulent wake predictions and analysis of wake recovery mechanism

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    Turbulent simulations are performed for a three-bladed, horizontal-axis hydrokinetic turbine for two different tip-speed-ratios using a resolved rotating blade model with a sliding interface. The thrust, power and intermediate wake predictions are validated by comparison with experimental data, and the results are analyzed to understand the wake recovery mechanism. The thrust and power predictions compare within 4.3% of the experimental data for both tip-speed ratios. The wake predictions improve with grid resolution, and MILES, which includes resolved anisotropic turbulence, performs better than both URANS and IDDES models. Nonetheless, the errors in the intermediate wake predictions are large ~27%.These errors are due to under prediction of cross-plane turbulent fluctuations, which delays the shear layer growth and wake recovery, and are caused by numerical dissipation of resolved turbulence at the interface between the rotating and stationary domains. The far-wake deficit shows self-similarity with a Gaussian profile, and wake recovery is independent of the tip-speed ratio. Future work should focus on investigating improved interface boundary conditions, and procuring additional experimental data for validation
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