Browsing by Author "Sun, Rui"

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    Large Eddy simulation of Trailing Edge Acoustic Emissions of an Airfoil
    Wu, Jinlong; Devenport, William J.; Paterson, Eric G.; Sun, Rui; Xiao, Heng (Virginia Tech, 2015-06)
    The present investigation of trailing edge acoustic emission of an airfoil concerns the effects of the broadband noise generated by the interaction of turbulent boundary layer and airfoil trailing edge, and the tonal noise generated by the vortex shedding of trailing edge bluntness. Large eddy simulation (LES) is performed on an NACA0012 airfoil with blunt trailing edge at a Reynolds number Rec = 400; 000 based on the airfoil chord length for three different configurations with different angles of attack. In order to reproduce and compare with the result from experiment in the literature, numerical tripping is tested and chosen to control the boundary layer development to guarantee a similar boundary layer thickness near the airfoil trailing edge. The near wall region inside the boundary layer is directly resolved by LES simulation with Van Driest damping, in order to obtain the instantaneous data in that region. With these instantaneous data from aerodynamic simulation, the acoustic predication is conducted by the Curle's analogy, which is suitable for stationary surface in free ow. To validate the numerical solutions, both ow simulation and acoustic integration results are compared to experimental data and simulation results available in the literature, and good agreement is achieved. The aerodynamic results show that the similar boundary layer development of experimental result can be reproduced by simulation with a suitable choice of numerical tripping, and the similar instantaneous behavior of ow inside the boundary layer is therefore guaranteed, which is vital for the acoustic prediction. The aeroacoustic results show that the acoustic prediction changes with the lift and drag force provided by the airfoil. Basically speaking, it's a result that the unsteady force around the surface is closely related to the mean force provided by an airfoil, which means that the noise control of a given airfoil is coupled with the optimization of its aerodynamic performance. As for the approximation made in the implemetation of Curle's analogy, it is shown in the aeroacoustic results that the airfoil can be treated as a compact point only if low frequency acoustic emission is of interest, and such kind of approximation can cause obvious problem if very high frequency acoustic emission is concerned.
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    A Novel Rough Wall Boundary Condition for LES of high Reynolds Number Flows
    Xiao, Heng; Liu, Yu; Sun, Rui; Devenport, William J. (Virginia Tech, 2015-06)
    The interactions between rough surfaces and fluid flows play an important role in turbulence simulation. The understanding of roughness elements at the wall (i.e., buildings and terrain features) to aerodynamics flow is crucial in wind energy from farm identification and assessment to turbine blade design. In this work, we propose a novel rough-wall boundary condition for LES to simulate flows over rough surfaces at high Reynolds numbers. The proposed rough-wall boundary condition consists of two parts: (1) smooth-wall modeling for high Reynolds number flow; (2) wall-modeling for roughness surface. To reduce the computational costs for high Reynolds number flow, a wall-modeling mesh is applied at the bottom of the boundary layer following (Kawai and Larsson 2012). In this procedure, the wall-modeling mesh will obtain velocity from LES mesh, solve for the shear stress according an equilibrium equation of boundary layer, and provides the calculated wall shear stress back to LES mesh. To verify the smooth wall-modeling LES part, the simulation of high Reynolds number flow in a channel is performed. The Reynolds number of the verification case is Re__8=u_c 8/u,,3.01x 10^5 and the thickness of the wall model is h_wm=0.18. The comparison of normalized streamwise velocity between the experiment, wall-modeling LES and pure LES are shown in the figure below. It is noted that the LES mesh of the modeling LES and pure LES are the same, but the wall-modeling LES will update the shear stress at the wall via wall modeling. Therefore, the wall-model LES results are the combination of the results of the wall-modeling part below h_wm and the LES part above h_wm. From the figure, it is can be seen that the wall modeling improves the results of LES when using relatively coarse grid at the boundary. Another part of the present model is the simulation of the influence of roughness elements. In the presented rough wall boundary condition, the flow around the roughness element, at the inner region of turbulent boundary layer, is not fully resolved. Instead, a one-layer roughness mesh is used to resolve the geometry of roughness elements. On the roughness mesh, the roughness geometry is adequately represented via the surface elevation. By projecting the instantaneous pressure onto the roughness surface, the instantaneous roughness shear stress is obtained. Since the smooth-wall and roughness shear stress are obtained, the total wall shear stress is obtained by adding the two parts. Then, the so obtained total wall shear stress is used to correct the flow at the near wall region. The LES mesh size, lix^+, liy^+ and liz^+ (in streamwise, wall-normal, and spanwise directions, respectively) in the present simulations can be as large as 50 to 4000, which is favorable for high-Reynolds number flow simulations in applications of wind turbines. Moreover, the presented wall model can solve roughness elements having size of K^+ ranging from 100 to several hundred wall units, which can be used to estimate the influence of roughness elements at different sizes. According to the results from the simulations, the presented rough wall-modeling boundary condition can perform high fidelity simulation for turbulent flow at higher Reynolds number by using a relatively low computational cost. The velocity profiles and Reynolds stress agree favorably with experimental data and numerical results in the literature. Therefore, the merits of the proposed rough-wall model are demonstrated.
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    Particle-Resolving Simulations of Dune Migration: Novel Algorithms and Physical Insights
    Sun, Rui (Virginia Tech, 2017-06-26)
    Sediment transport is ubiquitous in aquatic environments, and the study of sediment transport is important for both engineering and environmental reasons. However, the understanding and prediction of sediment transport are hindered by its complex dynamics and regimes. In this dissertation, the open-source solver SediFoam is developed for high-fidelity particle-resolving simulations of various sediment transport problems based on open-source solvers OpenFOAM and LAMMPS. OpenFOAM is a CFD toolbox that can perform three-dimensional flow simulations on unstructured mesh; LAMMPS is a massively parallel DEM solver for molecular dynamics. To enable the particle-resolving simulation of sediment transport on an arbitrary mesh, a diffusion-based algorithm is used in SediFoam to obtain the averaged Eulerian fields from discrete particle data. The parallel interface is also implemented for the communication of the two open-source solvers. Extensive numerical simulations are performed to validate the capability of SediFoam in the modeling of sediment transport problems. The predictions of various sediment transport regimes, including `flat bed in motion', `small dune' and `vortex dune', are in good agreement of with the experimental results and those obtained by using interface resolved simulations. The capability of the solver in the simulation of sediment transport in the oscillatory boundary layer is also demonstrated. Moreover, this well-validated high-fidelity simulation tool has been used to probe the physics of particle dynamics in self-generated bedforms in various hydraulic conditions. The results obtained by using SediFoam not only bridge the gaps in the experimental results but also help improve the engineering practice in the understanding of sediment transport. By using the particle-resolving simulation results and the insights generated therein, the closure terms in the two-fluid models or hydro-morphodynamic models can be improved, which can contribute to the numerical modeling of sediment transport in engineering scales.
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    Wide Area System Islanding Detection, Classification, and State Evaluation Algorithm
    Sun, Rui (Virginia Tech, 2013-03-12)
    An islanded power system indicates a geographical and logical detach between a portion
    of a power system and the major grid, and often accompanies with the loss of system
    observability. A power system islanding contingency could be one of the most severe
    consequences of wide-area system failures. It might result in enormous losses to both the power utilities and the consumers. Even those relatively small and stable islanding events may largely disturb the consumers\' normal operation in the island. On the other hand, the power consumption in the U.S. has been largely increasing since 1970s with the respect to the bloom of global economy and mass manufacturing, and the daily increased requirements from the modern customers. Along with the extreme weather and natural disaster factors, the century old U.S. power grid is under severely tests for potential islanding disturbances. After 1980s, the invention of synchronized phasor measurement units (PMU) has broadened the horizon for system monitoring, control and protection. Its real time feature and reliable measurements has made possible many online system schemes. The recent revolution of computers and electronic devices enables the implementation of complex methods (such as data mining methods) requiring large databases in power system analysis. The proposed method presented in this dissertation is primarily focused on two studies: one power system islanding contingency detection, identification, classification and state evaluation algorithm using a decision tree algorithm and topology approach, and its application in Dominion Virginia power system; and one optimal PMU placement strategy using a binary integral programming algorithm with the consideration of system islanding and redundancy issues.