Turbulence

A turbulent water current induced by winds, through a friction force at the sea surface and subjected to the Coriolis force in shallow water was studied. A Large Eddy Simulation model developed by Zikanov et al. is used to solve the Navier-Stokes equations. To define the bottom boundary condition, a drag coefficient parameter, based on the ideas of Csanady, is used to evaluate the shear stress at the bottom. To find a suitable bottom boundary condition for this LES simulation, several cases were considered with change in drag coefficient property. The effect of variation in the depth of the water column was also considered. Variation in surface deflection of the current, variation of the mass flux and distribution of eddy viscosity with depth of the water column are determined. The cases are compared with the case of a deep water column. Numerical results are also compared with field observations.

The purpose of this research is to study the modification of a turbulent flow as it passes through a cascade of flat plates. The results will then be compared with experimental results obtained in a companion experimental study being conducted at Virginia Tech. In a typical marine propulsor turbulent flow passes through a set of inlet guide vanes (IGVs) and then interacts with the propeller blades: this process creates unwanted vibration and sound. The purpose of this research is to determine if the arrangement of the IGVs can be used to reduce the propulsor noise generation. In this study the incoming flow to the propeller is modeled as homogeneous turbulence and the IGVs are represented by a cascade of flat plates. We will consider the equations, which describe the blade response to an incoming harmonic gust, and we will represent the turbulent flow using a modal description.

Application of a small autonomous underwater vehicle (AUV) is described as a platform for measurement of oceanic turbulence in coastal waters during cold atmospheric fronts. The turbulence package, mounted on the AUV, allows horizontal profiling and measurement of small-scale fluctuations of velocity and temperature and other characteristics of the flow in the ocean mixed layer. The turbulence measurements were made in conjunction with current profile measurements, conductivity, temperature, and depth measurements, providing the background conditions. The navigation and tracking data from the ship and the underwater vehicle are also presented. The primary focus of this research was to collect and analyze data from the ocean in order to resolve the turbulent velocity fluctuations and the dissipation rates of turbulent kinetic energy. The aim of this thesis is to explain the approach for measurement and analysis of ocean data. It includes the manufacture of the measurement probes, the preparation of the electronic system, the coding of the acquisition software and use of several algorithms for detecting the presence of turbulence and mixing. Two observational oceanographic experiments are described as a basis for illustrating the techniques and methods in data acquisition and analysis of the oceanographic and turbulent quantities.

The motion stability of long-span bridges under turbulent wind is studied. A new stochastic theory, developed on the basis of a new wind turbulence model, is applied to experimentally measured bridge deck models to determine the stochastic stability boundaries. The new turbulence model has a finite mean-square value and a versatile spectral shape, and is capable of closely matching a target spectrum, such as the Dryden or the von Karman spectrum, by changing the parameters of the model. The bridge motion is represented as a linear system of single degree of freedom in torsion. A bridge is generally subject to two types of wind loads: the buffeting loads and the self-excited loads. Only the self-excited loads are considered in the investigation, since the buffeting loads, which appear as inhomogeneous terms in the differential equation of motion, do not affect the motion stability of a linear system. In the absence of turbulence, the onset of flutter instability occurs at a critical wind velocity at which a pair of complex-conjugate eigenvalues of the combined structural-fluid system becomes purely imaginary. The corresponding eigenvectors describe the interaction between the structure and the surrounding fluid. Upon the introduction of turbulence, the composition of the structural and fluid components is changed. Since the turbulence portion of the flow fluctuates randomly in time, a new state of balance between the energy inflow from fluid to structure, and the energy outflow from structure to fluid, can only be reached in the statistical sense, or equivalently, in the sense of long-time average under the ergodicity assumption. It is the random deviation from the deterministic flutter mode that renders either the stabilizing or destabilizing effect possible. The asymptotic sample stability boundary of the motion is obtained. The aerodynamic constants for the theoretical analysis are measured experimentally in a forced vibration test conducted in a water channel, with water substituting for air as the working fluid. For a particular bridge deck model, the computed stability boundary shows that the presence of turbulence in the wind flow can be either stabilizing or destabilizing depending on the peak frequency and band-width of the turbulence spectrum.

Boundary layers are regions where turbulence develops easily. In the case where the flow occurs on a surface showing a certain degree of roughness, turbulence eddies will interact with the roughness elements and will produce an acoustic field. This thesis aims at predicting this type of noise with the help of the Computational Fluid Dynamics (CFD) simulation of a wall jet using the Reynolds Average Navier-Stokes (RANS) equations. A frequency spectrum is reconstructed using a representation of the turbulence with uncorrelated sheets of vorticity. Both aerodynamic and acoustic results are compared to experimental measurements of the flow. The CFD simulation of the flow returns consistent results but would benefit from a refinement of the grid. The surface pressure spectrum presents a slope in the high frequencies close to the experimental spectrum. The far field noise spectrum has a 5dB difference to the experiments.

Simulating the exact chaotic turbulent flow field about any geometry is a dilemma between accuracy and computational resources, which has been continuously studied for just over a hundred years. This thesis is a complete walk-through of the entire process utilized to approximate the flow ingested by a Sevik-type rotor based on solutions to the Reynolds Averaged Navier-Stokes equations (RANS). The Multiple Reference Frame fluid model is utilized by the code of ANSYS-FLUENT and results are validated by experimental wake data. Three open rotor configurations are studied including a uniform inflow and the rotor near a plate with and without a thick boundary layer. Furthermore, observations are made to determine the variation in velocity profiles of the ingested turbulent flow due to varying flow conditions.

Ji and Wang (2010) propose that the dominant source of sound from a forward facing step is the stream wise dipole on the face of the step and that sources acting normal to the flow are negligible. Sound radiation normal to flow of forward facing steps has been measured in wind tunnel experiments previously by Farabee and Casarella (1986, 1991) and Catlett (2010). A method for evaluating sound radiation from surface roughness proposed in Glegg and Devenport (2009) has been adapted and applied to flow over a forward facing step which addresses the sound normal to the flow that was previously unaccounted for. Far-field radiation predictions based on this method have been compared with wind tunnel measurements and show good agreement. A second method which evaluates the forcing from a vortex convected past surface roughness using RANS calculations and potential flow information is also evaluated.