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A FUNDAMENTAL INVESTIGATION OF THE EFFECT OF FREESTREAM TURBULENCE PARAMETERS ON THE TIME-MEAN AND DYNAMIC BEHAVIOR OF JUNCTION FLOW
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
March 30, 2019
Stephen P Lynch, Dissertation Advisor
Stephen P Lynch, Committee Chair
Karen Ann Thole, Committee Member
Rhett William Jefferies, Committee Member
Xiaofeng Liu, Outside Member
Turbulence Length Scale
Junction flow is a phenomenon that is common to both natural and industrial processes, occurring where an incoming flow above a wall meets an obstacle protruding from the wall, such as occurs at wing-body junctions on aircraft or turbine blade/vane endwalls in a gas turbine. This obstacle produces a region of adverse pressure which causes the incoming boundary layer to lift off of the wall, curl down upon itself, and form a region of backflow along the endwall in front of the obstacle. This backflow sustains the presence of a horseshoe vortex system – a coherent vortex feature centered near the endwall in front of the obstacle leading edge, with legs that wrap around the obstacle following the mainstream flow. In turbulent flows, the horseshoe vortex system is known to be complex in structure and highly unsteady in regards to the position of its primary vortex core. Due in part to its unsteadiness, the presence of the horseshoe vortex system within the junction significantly increases endwall heat transfer in front of the obstacle. It can also produce significant dynamic pressure loads on obstacle surfaces near the junction. Understanding and potentially controlling the dynamic behavior of the horseshoe vortex in an applied setting is currently difficult due to the need for a fundamental understanding of how freestream parameters such as Reynolds number and freestream turbulence levels affect flow in the junction. To address this need, a study was constructed to determine the effect of freestream Reynolds number, turbulence intensity, and integral length scale conditions on the dynamic and time-mean unsteadiness of the horseshoe vortex system and the surrounding junction flow field in front of a symmetric wing. Time-resolved measurements of the junction flow field were made using stereo particle image velocimetry (SPIV). These were analyzed to quantify the unsteadiness of the junction region and the horseshoe vortex system under a wide range of freestream turbulence levels (2%, 10%, and 17%) and body-thickness Reynolds number (7000, 25000, 80000) conditions relevant to industrial applications. This unsteadiness is dominated by oscillatory motions of the horseshoe vortex between backflow and zero-flow modes. Dynamic analysis of these motions and the temporal unsteadiness of the horseshoe vortex under varying Reynolds number and turbulence intensity conditions are highly unique contributions of this work. Freestream Reynolds number significantly influences the structure of the horseshoe vortex system and other vorticity features along the junction endwall. At low Reynolds number, the horseshoe vortex has a coherent dynamic structure and moves along the endwall in a coherent manner driven by large hairpin structures in the incoming boundary layer. This motion produces an elliptical distribution of time-mean turbulent kinetic energy along the junction endwall. At high Reynolds numbers, however, the incoming boundary layer is significantly more turbulent, causing the dynamic structure of the vortex and its motions along the endwall to be more chaotic. This is due to disruptive interactions with locally generated hairpin vortices and other small scale vorticity features in the incoming lower boundary layer, which occur frequently at high Reynolds number. These features destabilizing the vortex in backflow mode and increase local flow field unsteadiness. The result is a greater degree of randomness in vortex core position and a modest increase in time-mean turbulent kinetic energy in the core region at high Reynolds number, particularly beneath the time-mean core location. The presence of moderate to high freestream turbulence intensity significantly increases turbulent kinetic energy in the junction under low turbulent Reynolds number conditions, but is less effective as Reynolds number increases. Much of this augmentation occurs in the horseshoe vortex region and along the wing leading edge. This effect is caused by the impingement of freestream and upper boundary layer turbulence features along the wing leading edge and the subsequent transport of this fluctuating momentum along the wing leading edge to the junction corner region. At low Reynolds number, features are frequently entrained within the horseshoe vortex or endwall backflow, causing high fluctuating momentum in the region along the endwall. The presence of turbulence features in the junction can also initiate and energize sweeping vortex oscillatory motions at low Reynolds number. These mechanisms are less effective at high Reynolds numbers due to smaller scale of freestream turbulent eddy penetration which occurs at high Reynolds number in the junction, and the lack of large hairpin structures in the incoming boundary layer. At high freestream turbulence intensities, large integral length scales also played a role in augmenting the effect of turbulence intensity. Appreciable augmentation of the unsteadiness in the vortex core region and leading edge regions of the junction is observed for large integral length scales at high turbulence intensity under low and moderate turbulent Reynolds number conditions in comparison to the unsteadiness observed at similar turbulence intensity but smaller length scales. This is due to the increased rate of impingement of large turbulence features along the wing leading edge that occurs at length scale magnitudes above a certain threshold. Altogether, this work provides a detailed fundamental understanding of the effects of Reynolds number and freestream turbulence on the unsteadiness of flow in the junction, and a detailed explanation of the mechanisms that drive these effects to occur. The dynamic motion patterns of the horseshoe vortex are highly sensitive to turbulence in the incoming boundary layer, which varies in nature at varying Reynolds number. High levels of freestream turbulence can lead to a significant increase in unsteadiness of flow near the junction surfaces at low Reynolds number due in part to energized motions of the horseshoe vortex system. This understanding directly supports efforts to predict or control junction heat transfer and pressure loading effects in applications that include high freestream turbulence conditions. It is clear that any attempt to model the unsteady behavior of the junction flow field under moderate to high freestream turbulence conditions must account for Reynolds number, turbulence intensity, and at high turbulence levels, integral length scale, as influential driving parameters of unsteadiness in the junction.
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