Browsing by Author "Ahn, Seungki"

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    An experimental study of flow over a 6 to 1 prolate spheroid at incidence
    Ahn, Seungki (Virginia Tech, 1992-10-05)
    In two-dimensional flow, the point of flow separation from the surface coincides with the point at which the skin friction vanishes. However, in three-dimensional flow, the situation is much more complex and the flow separation is rarely associated with the vanishing of the wall shear stress except in a few special cases. Though the effects of cross-plane separation are substantial and have been recognized for some time, the phenomenon of flow separation over three-dimensional bodies is still far from being completely understood. The flow is so complex that no completely satisfactory analytical tools are available at the moment. In an attempt to logically identify the various effects and parametric dependence while simultaneously minimizing configuration dependent issue, the flow over a 6 to 1 prolate spheroid, which is a generic three-dimensional body, is investigated. For the identification of the general flow pattern and better understanding of the flow field, surface-oil-flow visualization tests and force and moment tests were performed. The angle of attack effect and Reynolds number effect on the separation location are studied with natural transition. Forces and moments tests, surface pressure distribution measurements as well as the surface presure fluctuations, and mini-tuft flow visualization tests were made to document the flow characteristics on the surface of the body with an artificial boundary layer trip. It was found that there exists a critical Reynolds number at which the flow characteristics of the afterbody changes. This critical Reynolds number was also confirmed by the force and moment tests. Above this Reynolds number, as the Reynolds number increases, the separation lines do not change their circumferential location but stretch to the upstream of the body. For the low supercritical Reynolds number range, the angle of attack effect on the location of the primary separation is not as prominent as in the higher Reynolds number range where the cross-flow component effect becomes dominant. Surface pressure fluctuation data and surface pressure spectra were measured and documented for the first time for this type of three-dimensional flow. For the extension of the study to unsteady transient motion effects, a new Dynamic-Plunge-Pitch-Roll (DyPPiR) model mount was designed and developed to generate required transient motions. The measurements carried out during this study are to be used as reference data to identify the unsteady transient effect of the flow field undergoing unsteady transient maneuvers.
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    Some unsteady features of turbulent boundary layers
    Ahn, Seungki (Virginia Polytechnic Institute and State University, 1986)
    For steady free-stream, zero and favorable pressure gradient turbulent boundary layers, the unsteadiness in the form of turbulent fluctuations was investigated. Phase ensemble-averaged flow characteristics of a large amplitude periodic unsteady turbulent boundary layer was also investigated at a reduced frequency k = 0.61 based on the length of the converging and diverging test section with amplitude to mean velocity ratio of 0.8. In steady flow cases, both zero and favorable pressure gradient flows show good two-dimensional flow characteristics and mean flow characteristics are compared with other researchers’ data. Measured power spectral data show good agreement with those of Klebanoff, Ueda and Hinze, Perry, Lim and Henbest for the zero pressure gradient flows and Jones and Launder for the favorable pressure gradient flow. The power spectral data measured in the turbulent wall region of the zero pressure gradient flow closely follow the model equation proposed by Perry, Lim and Henbest. Convective wave speed also show good agreement with those of Favre, Gaviglio and Dumas and Sternberg within the experimental uncertainties. In the inner region of the boundary layer where y+ < 40, convective wave speed is higher than local mean velocity at all eddy scales as observed by Kline, Reynolds, Schraub and Runstadler. In the unsteady flow case, in the absence of flow reversal, the flow behaves in a quasi-steady manner and can be described by the steady flow structure as in the case of moderate amplitude flows. The Ludwieg·Tillmann skin friction equation and the Perry-Schofield universal velocity defect law hold at these phases. Except the laminariscent velocity profile observed during the acceleration phases, the large amplitude unsteady flow shows basically the same flow characteristics as the moderate amplitude flows.