29,816 research outputs found
Boundary layer development and flow-induced noise of airfoils:The effect of high inflow turbulence on trailing-edge noise generation
Human factors regarding wind turbines have come into play in recent years, primarily due to the aerodynamic noise. The primary noise source of a wind turbine is trailing-edge noise. However, when the inflow is turbulent, which is a predominant condition for wind turbines, leading-edge noise becomes an additional source of noise. The effect of the inflow turbulence on trailing-edge noise generation has yet to be studied. Therefore, this thesis first studies trailing-edge noise generation for uniform inflow conditions by analyzing the boundary layer development. Later, the analyses are extended for the turbulent inflow condition. The results show the strong influence of the inflow turbulence on the boundary layer development and trailing-edge noise. Finally, this thesis presents a methodology to predict wind turbine noise. The results found in this thesis contribute to a better understanding and evaluation of noise generation with and without inflow turbulence for airfoils, which is relevant for wind turbines and other applications. Accounting for the influence of inflow turbulence on trailing-edge noise is pivotal for accurately assessing aerodynamic noise and for designing effective noise mitigation strategies for a particular frequency range
A New Recycling Method to Generate Turbulent Inflow Profiles
The accuracy of the scale-resolving simulations for practical geometries strongly depends on the inflow boundary conditions. Imposing experimentally observed turbulent inflow profiles for the numerical simulations is a major challenge. Existing methods available in the literature assume self-similar behavior, which is not true for most of the experiments. In the present work, we formulate the turbulent inflow profile generation technique as an optimization problem. An adjoint technique is exploited to evaluate the sensitivities of multiple input parameters for the present problem. The present formulation is then tested to generate a laminar boundary layer profile, turbulent boundary layer profile, and turbulent jet profile
An improved turbulent boundary layer inflow condition, applied to the simulation of jets in cross-flow
The jet acting perpendicular to a cross-flow boundary layer is a commonly studied complex turbulent flow. Our research was motivated by their potential application in separation delay devices, where jets can be used to produce streamwise vortices in a similar manner to conventional solid vortex generating vanes.
This thesis addresses two problems; firstly the generation of inflow conditions for the simulation of a spatially developing turbulent boundary layer, and secondly the simulation of low velocity ratio jets interacting with the boundary layer. Our approach involved refining a popular turbulent inflow generation technique, validating the accuracy of our improved method against well established direct numerical simulation data. This turbulent boundary layer was used to simulate a low velocity ratio perpendicular jet test-case, which was validated against experimental data. Finally, a pitched and skewed jet model was investigated.
Our modifications to the turbulent boundary layer inflow generation method were successful, addressing problems described by various authors regarding the stability and accuracy of the technique. Secondly we have found excellent agreement in our perpendicular jet in cross flow test-case, and have produced what we believe to be the first documented unsteady numerical simulation of the flow field behind a low velocity ratio pitched and skewed jet
Large-eddy simulation of turbulent wall-pressure fluctuations using the finite element method
In the present dissertation, turbulent wall-pressure fluctuations are characterized. To capture the turbulent characteristics of the flow, large-eddy simulation is used to resolve the large scale motions of the flow directly. A wall-adapting local eddy-viscosity model is selected to account for the effect of small scale motions. The streamwise/upwind Petrov-Galerkin method is chosen to discretize the computational domain and a second-order backward difference formula is applied for the time integration. Maintaining turbulent flow throughout the simulation domain to properly characterize turbulence is critical in investigating wall-pressure fluctuations. In order to reduce the size of the simulation domain an inflow generation method, a variant of the recycling and rescaling method, is used. In this method, the turbulent velocity profile from a specific plane within the computational domain is recycled and rescaled propriately, and re-introduced at the inlet of the domain at every time step iteration. In the proposed method, the mean velocity profile is fixed at the inlet while the velocity fluctuations are recycled and rescaled to obtain the desired turbulent characteristics. This method is simple and effective and maintains the turbulent flow throughout the simulation domain. The non-reflecting boundary conditions with a sponge layer are applied at the top and exit of the computational domain to remove unwanted reflections from the boundary. In order to examine the present inflow generation method and the ability to capture the wall-pressure fluctuations, numerical results are verified on a flat plate with a zero pressure gradient. The mean velocity profile, the RMS velocity fluctuations, and the friction velocity over time are investigated to show the effectiveness of the present inflow turbulent generation method. Computed wall-pressure fluctuations are evaluated using the time-averaged statistics and the spectra, to show that they are characterized well using the present method
Assessing the sensitivity of turbine cascade flow to inflow disturbances using direct numerical simulation
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