Date of Award
Master of Science in Ocean Engineering
Understanding the flow field around objects (i.e. external flow) is very important for various engineering and scientific application at all scales. The overall objective of this study is to better understand the flow field and turbulence in a wave-current flume for internal and external flow studies using particle image velocimetry and to investigate the capabilities and limitations of the Lagrangian smoothed particle hydrodynamics (SPH) method to simulate external flows using the collected experimental data in the wave-current flume. Laboratory studies are useful for observation of physical phenomena in a controlled environment in addition to their being less expensive and more accessible than field testing. Numerical methods are additionally advantageous because they are less expensive than experimental and field work, they can be more easily applied at various scales, and they require less time and preparation than observational studies.
Smoothed Particle Hydrodynamics (SPH) is a popular Lagrangian numerical method to simulate the flow and wave fields around coastal and marine structures in a complex ocean environment. Currently, SPH models usually incorporate simple turbulence models to simulate real viscous flows as a default. At first, an experimental study was conducted to determine the effects of surface roughness on wake turbulence. A bare aluminum cylinder of diameter 0.0508 m (2") and an identical aluminum cylinder treated with an antifouling marine paint were set up individually across the channel of a recirculating flume in which a sheared flow was present. Using particle image velocimetry (PIV), the wake across approximately 5.5 diameter-lengths trailing the cylinder and with various Reynolds numbers was measured and the turbulence parameters and wake widths estimated. Further, a 2-D model of the cylinder system was developed in an SPH model to closely examine how the turbulence setting and parameterization can affect the hydrodynamic fields in the wake of the cylinder and how the model parameters can be tuned to refine the turbulent eddy representation. We focused on the turbulence modeling in DualSPHysics which is a SPH-based open-source code that can implement a Large Eddy Simulation (LES) method based on the Smagorinsky eddy viscosity model (Smagorinsky, 1963). The performance of the SPH model in simulating the turbulence was examined using the PIV data. In particular, the limitations of SPH regarding high Reynolds number simulations were explored. Results can be applied in SPH in the simulation of the flow field around marine and coastal structures.
The wave-current flume exhibited a sheared velocity with turbulent fluctuations, the latter of which were reduced by introducing a flow conditioner into the channel. Treating an aluminum cylinder with an antifouling marine paint was found to increase turbulent mixing in the near-field wake which also reduced the downstream shear compared to the unpainted case. This work may be advanced by testing the impact of additional roughnesses on wake turbulence and a sheared velocity. A larger shear may also be introduced to test the extent of the impact different surface roughnesses may have.
For the numerical modeling, a benchmark study was first performed using SPH at a Reynolds number of 180 like that of several literature studies to ensure that similar results could be obtained. The benchmark case showed fair agreement with the literature, so higher Reynolds numbers that matched some of the experimental cases were modeled. The numerical simulation of the experimental case, which involved a much higher Reynolds number, could model the general flow field; however, it underestimated the observed data largely because the simulation included a uniform velocity profile upstream of the cylinder, unlike the sheared velocity in the experiments, and no background turbulence, unlike the ambient turbulence present in the experiments. However, the model also had trouble with the higher Reynolds number as higher turbulence was introduced. The ViscoBound factor could be tuned to improve the output results, but further work is needed to assess this factor's impact. This model is also unable to implement a variable particle resolution, which is a key component of capturing boundary layer physics. While SPH is a powerful method for free-surface flows, more work needs to be done in developing its capabilities of modeling turbulent flows and incorporating a variable resolution.
Deeb, Sandra, "Measuring and Modeling Turbulence Using Particle Image Velocimetry and Smoothed Particle Hydrodynamics; Applications to External Flows" (2022). Open Access Master's Theses. Paper 2237.