Unsteady Computational and Experimental Fluid Dynamics Investigations of Aerodynamic Loads of Large Optical Telescopes

This book is intended to serve as a reference text for advanced scientists and research engineers to solve a variety of fluid flow problems using computational fluid dynamics (CFD). Each chapter arises from a collection of research papers and discussions contributed by the practiced experts in the field of fluid mechanics. This material has encompassed a wide range of CFD applications concerning computational scheme, turbulence modeling and its simulation, multiphase flow modeling, unsteady-flow computation, and industrial applications of CFD. The importance of applying computational fluid dynamics (CFD) for large optical telescope flow analyses in the early design phase was emphasized and some critical challenges for accurate flow field prediction were drawn. Also, a thorough literature review was performed specifically on the role of CFD that can play towards accurate prediction of pressure loads on telescopes structure. Some recent CFD and experimental investigations, on a scaled model of a very large optical telescope housed within a spherical enclosure, were performed and led to the following remarks. In general, when the enclosure opening was facing into the wind, both the CFD and measured data revealed that the flow was highly unsteady inside and outside the enclosure, causing unsteady wind loads on the enclosure and the telescope structure. Outside the enclosure, owing to the formation of a boundary layer over the floor, a distinct unsteady horseshoe vortex was formed. A strong shear layer was noted to evolve across the enclosure opening. The shear layer was decidedly unstable, and tended to roll up into a series of small vortices that interacted with the aft edge of the enclosure opening, resulting in large pressure fluctuations on the primary mirror surface. The mean pressure inside the enclosure was roughly uniform. The flow was separated on the back of the enclosure, starting from the aft edge of the enclosure opening towards the floor. A spectral analysis of the pressure signal on the primary mirror surface showed the existence of at least three principal oscillatory modes. Owing to the elevated Mach number used for most of the simulations, the first CFD mode frequency was understandably underestimated. However, when the actual wind tunnel Mach number was used in the simulations, the spectral analysis showed excellent agreement between the CFD and measured data, demonstrating the relevance of simulating appropriately the acoustic waves generated by the interaction of the shear layer vortices with the enclosure opening edge. Good comparisons were also obtained between the CFD predictions and the measurements for the mean pressure coefficients and their standard deviations around and inside the enclosure surfaces and on the primary mirror surface. For the purposes of this study, the level of agreement obtained with the experimentally observed phenomena justifies the assumption of a hydraulically smooth enclosure surface and fully turbulent flow. Despite the good agreement between CFD and experimental results for the flow behavior inside the enclosure, the flow physics around the enclosure external surface was not properly simulated by assuming fully turbulent flows, as the infrared measurements showed a large laminar run on the front region of the enclosure, followed by transition, fully turbulent flows, and finally separated flows. Furthermore, from the smoke visualization and infrared measurements, it appeared that the flow around the enclosure was strongly affected by the tunnel floor, causing an unsteady horseshoe vortex to form, and by the enclosure opening, which disrupted the transition and separation locations expected on sphere flows in the supercritical regime. These have the effect of delaying the transition location and triggering earlier flow separation. For bodies with a high sensitivity to the boundary layer state, such as a spherical enclosure, there is a need to simulate transition, even in the high Reynolds number case. Future work involving high Reynolds number tests and CFD simulations using hybrid and zonal DNS, LES and URANS simulations is required to address properly these flow phenomena As an example of the evaluation of interfacial flows, two methodologies were proposed for the evaluation of the GE phenomena. One is the CFD-based prediction methodology and the other is the high-precision numerical simulation of interfacial flows. In the development of the CFD-based prediction methodology, the vortical flow model was firstly constructed based on the Burgers theory. Then, the accuracy of the CFD results, which are obtained on relatively coarse computational mesh without considering interfacial deformations for the reduction of the computational costs, was discussed to determine the occurrence indicators of the two types of the GE phenomena, i.e. the elongated gas core type and the bubble pinch-off type. In this study, the gas core length was selected as the indicator of the elongated gas core type with considering the three times allowance. On the other hand, the downward velocity gradient was determined empirically as the indicator of the bubble pinch-off type. Finally, the developed CFD-based prediction methodology was applied to the evaluation of the GE phenomena in an experiment using 1/1.8 scale partial model of the upper plenum in reactor vessel of a large-scale FR. As a result, the GE occurrence observed in the 1/1.8 scale partial model experiment was evaluated correctly by the CFD-based prediction methodology. Therefore, it was confirmed that the CFD-based prediction methodology can evaluate the GE phenomena properly with relatively low computational costs. In the development of the high-precision numerical simulation algorithms, the highprecision volume-of-fluid algorithm, i.e. the PLIC algorithm, was employed as the interfacetracking algorithm. Then, to satisfy the requirement for accurate geometrical modeling of complicated spatial configurations, an unstructured mesh scheme was employed, so that the PLIC algorithm was newly developed on unstructured meshes. Namely, the algorithms for the calculation of the unit vector normal to an interface, reconstruction of an interface, volume fraction transport through cell-faces and surface tension were newly developed for high accurate simulations on unstructured meshes. In addition, to establish the volume conservation property violated by the excess or too little transport of the volume fraction, the volume conservative algorithm was developed by introducing the physics-basis correction algorithm. Physics-basis considerations were also conducted for mechanical balances at gas-liquid interfaces. By defining momentum and velocity independently at gasliquid interfaces, the physically appropriate formulation of momentum transport was derived, which can eliminate unphysical behaviors near the gas-liquid interfaces caused by conventional formulations. Furthermore, the improvement was necessary to satisfy the mechanically appropriate balances between pressure and surface tension at gas-liquid interfaces, so that the physically appropriate formulation was also derived for the pressure gradient calculation at gas-liquid interfaces. As the verification of the developed PLIC