IMPACT OF HIGH FREESTREAM TURBULENCE ON LPT ENDWALL FLOW–PART II: ENDWALL FLOW DYNAMICS

Abstract The flow field surrounding low-pressure turbine blades is associated with significant losses in the endwall region. The exact location and behavior of the endwall structures is tied to the incoming boundary layer state; therefore, the incoming freestream turbulence is expected to impact dynamic characteristics of these structures and the associated losses. As discussed in Part I, the incoming turbulence intensity was varied between <1% and 7% at three different Reynolds numbers within a linear cascade of high-lift high-work low-pressure turbine blades. The focus of this paper will be on the effect elevated freestream turbulence has on the unsteady dynamics of the endwall flow structures. Three planes of high-speed stereoscopic particle image velocimetry were collected within the passage of the turbine blades capturing cross-sectional slices of key endwall vortices. From a detailed flow analysis completed with SPIV measurements, it was established that elevating the turbulence levels had a direct impact on the locations and time-dependent characteristics of the endwall topology. Differences in temporal characteristics were observed in all planes with the introduction of elevated turbulence; however, the bimodal behavior documented by other researchers near the leading edge was still observed. The horseshoe vortex was found to intermittently lose coherence at the leading edge and in the passage. The frequency of the loss of coherence events as well as the vortex position was altered as the freestream turbulence was elevated.