Turbulent junction flow is commonly seen in various turbomachinery components, heat exchangers, submarine appendages, and wing-fuselage attachments, where the approach boundary layer separates and rolls up into a coherent system of vortices upstream of a bluff body. The highly unsteady behavior of this flow causes high pressure fluctuations on the wall, and if the fluid temperature is different than the wall temperature, also causes high heat transfer. One of the signature features of these flows is a bimodal distribution of velocity around the vortex system. In this paper, the flow physics as well as heat transfer of the turbulent junction flow are investigated using PIV and IR measurements respectively. Among the three objectives of this paper, the first one is to demonstrate the unique experimental setup that captures temporally resolved turbulent flow-field measurements. The second objective is to analyze the dynamics of primary vortex for various Reynolds numbers. The final objective is to investigate the effect of the unsteady junction flow on the endwall heat transfer.

Turbulent junction flow is a three-dimensional unsteady phenomenon occurring in the flow upstream of the leading edge of bodies attached to a surface, such as in turbine rotors and stators, heat exchangers, submarine appendages, and wing-fuselage attachments. One of the signature features of this type of flow is the presence of bimodal behavior in the probability density functions of velocity, but the bimodal phenomenon has not been observed in surface heat flux measurements. However, it is well-known that time-mean levels of heat flux are significant. In situations where the body experiences high freestream turbulence, mean heat flux is further increased, but the mechanisms of the enhancement are unclear. In this paper, a test section for simultaneous time-resolved heat flux and flowfield measurements in front of a common research wing is highlighted. Time-resolved unsteady heat flux is also reported for a range of Reynolds numbers at high freestream turbulence. Time-resolved heat flux measurements from the symmetry plane of the junction region are compared with measurements downstream of the airfoil to determine if there are correlated behaviors. Also, a comparison between the effects of baseline freestream turbulence and high freestream turbulence on junction heat transfer is presented. It is found that at the plane of symmetry, high freestream turbulence increases endwall heat transfer at low Reynolds number and has negligible influence on endwall heat transfer at high Reynolds number.

The horseshoe vortex system is a common flow feature in many natural and industrial flows occurring near the junction of a blunt obstacle with the endwall surface. In industrial settings, such as in high temperature gas turbine engines, the dynamic behavior of the horseshoe vortex has been shown to contribute significantly to the pressure loading and heat transfer behavior on surfaces near the leading edge of the obstacle. Fundamental studies of the horseshoe vortex have characterized its time mean and dynamic behavior at low freestream turbulence conditions, and studies using industry relevant geometries, such as cylindrical pin fin arrays common to cooling applications, have captured dynamic behavior of the vortex at high freestream turbulence. The isolated effect of high freestream turbulence on the dynamic behavior of the vortex, independent of upstream wake effects found in pin fin arrays and other industry geometries, however, is not well understood. This study seeks use high-speed time resolved stereo particle image velocimetry (SPIV) measurements of the horseshoe vortex system taken at varied freestream turbulence levels in front of a single Rood wing obstacle to better understand the isolated effect of freestream turbulence on the vortex position and vortex breakdown dynamics.

Turbine vanes and blades are generally manufactured as single or double airfoil sections that must each be installed onto a turbine disk. Between each section, a gap at the endwalls through the blade passage is present, through which high pressure coolant is leaked. Furthermore, sections can become misaligned due to thermal expansion or centrifugal forces. Flow and heat transfer around the gap is complicated due to the interaction of the mainstream and the leakage flow. An experimental and computational study was undertaken to determine the physics of the leakage flow interaction for a realistic turbine blade endwall, and assess whether steady RANS CFD, commonly used for non-axisymmetric endwall design, can be used to accurately model this interaction. Computational models were compared against experimental observations of endwall heat transfer on a contoured endwall with a midpassage gap. Endwall heat transfer coefficients were determined experimentally by using infrared thermography to capture spatially-resolved surface temperatures on a uniform heat flux surface (heater) attached to the endwall. Predictions and measurements both indicated an increase in endwall heat transfer with increasing gap leakage flow, although the distribution of heat transfer coefficients along the gap was not well captured by CFD. A misalignment of the blade endwall causing a forward-facing step for the near-endwall flow resulted in a large highly turbulent recirculation region downstream of the step and high local heat transfer that was overpredicted by CFD. Conversely, a backward-facing step reduced turbulence and local heat transfer. The misprediction of local heat transfer around the gap is thought to be caused by unsteady interaction of the passage secondary flow and gap leakage flow, which cannot be well-captured by a steady RANS approach.

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