Interaction Phenomena of High Frequency Pulsed Blowing in LP Turbine-Like Boundary Layers at High Speed Test Conditions

Abstract

The use of fluidic oscillators for active flow control applications is a proven and efficient concept. For the well-known highly loaded LP turbine profile T161, the total pressure losses could already reduced by 40% at low Reynolds numbers, were usually flow separation occurs. For further improvements of the active flow control concept, it is essential to understand the driving flow phenomena responsible for the loss reduction mechanism, which are discussed in this paper. The results presented are based on experimental investigations on a flat plate with pressure gradient, imposed with an aerodynamically highly loaded low pressure turbine suction side flow and equipped with active flow control. The analogy to the suction side of the T161 is shown and validated against former cascade measurements. Based on the T161 equivalent operating point of Re = 70,000 and a theoretical out flow Mach number of Ma2,th = 0.6, the focus is set on the interaction of the boundary layer flow with high frequency actuation. The chosen actuator, a high frequency coupled fluidic oscillator, is designed to independently adjust mass flow and frequency. The flat plate is equipped with an array of high frequency actuators to control the flow separation. For this study one oscillator operating point at 6.7kHz is presented and the influence on transition and loss reduction compared to the non-actuated case is discussed. This oscillator operating point was found to be most efficient and the steady and unsteady mixing behavior of the high frequency actuator impact and the low pressure turbine like suction side boundary layer flow is investigated in much detail. Depending on the measurement technique, the isentropic Mach number distribution, frequency spectra, standard deviation, skewness and kurtosis are evaluated. The most important results are on the one hand, that the chosen concept is more efficient compared to former studies in means of mass flow investment, which is mainly based on the chosen oscillator outlet position and frequency. On the other hand, in a transonic flow the mixing and interaction of the high frequency pulses and the boundary layer flow require about 10% of the surface length to even establish and about to 30% to be completed. These results of the mixing behavior between actuator and boundary layer for compressible flow conditions help to attain a fundamental understanding for future designs of active flow control concepts.

High Frequency Boundary Layer Actuation by Fluidic Oscillators at High Speed Test Conditions

“…To ensure comparability with former results, the experimental testbed was validated against the T161 cascade measurements. Bettrich et al [15] proved very good agreement between the flat plate with pressure gradient and the T161 suction side flow. The operating point was chosen at Reynolds number Re = 70, 000, theoretical outflow Mach number of Ma 2,th = 0.6, and an inflow free stream turbulence level of T u ≈ 4%.…”

“…5. The differences near the leading edge are typical for any flat plate setup, as discussed in [15]. Altogether, the design goals of the experimental testbed for the planned investigations is fully achieved.…”

“…The adjustable diffuser is used to fine-tune the flat plate boundary layer characteristic. More details and aspects of the design process can be found in [15].…”

“…More details on the Preston probe head shape and size, the procedure to determine the boundary layer condition (laminar, turbulent or separated), and the determination of the transition point are given in [15,22]. Furthermore, the total pressure loss coefficient calculated from the Preston probe data is used to compare different actuator operating points.…”

“…Since the high speed investigations at ISA showed many promising results [2,9,13,14] for AFC applied on the T161, this paper focuses on a T161-like suction side flow of a flat plate with pressure gradient. The massive flow separation is controlled with high frequency periodic excitation [15], induced by a newly developed coupled fluidic oscillator [16].…”