The design of transonic compressors increasingly focuses on higher blade loading, sparking interest in shock oscillation mechanisms in highly loaded transonic fans operating at cruise altitude. At such conditions, low chord Reynolds numbers (1.4 Mio.) may sustain a laminar boundary layer on the suction side of the blade up to the shock-wave/boundary-layer interaction (SBLI). The resulting interaction with large separation (pre-shock Mach number of 1.6) cause shock oscillations and structural excitation. In this study, we demonstrate that a canonical research configuration enables the experimental investigation of a specific shock oscillation mechanism relevant to transonic fans at altitude, providing a basis for validation. Using Large Eddy Simulations and experimental data, we show that the oscillation mechanism depends on the conditions at the SBLI rather than the geometry. The oscillation arises from the growth and self-suppression of the upstream laminar section of the separation bubble. Periodic collapse of this laminar section generates turbulence that entrains the separation bubble, influencing the dynamics of the reflected shock. The reflected shock movement resembles the cascade passage shock behavior, driven by blockage variations from the separation bubble. Additionally, we examine the numerical requirements to resolve this mechanism. These findings provide insights to advance compressor designs and hypersonic applications featuring similar mechanisms.

Reynolds numbers at cruise altitude can be such that a laminar boundary layer persists on the suction side of a transonic fan blade up to the shock-wave/boundary-layer interaction (SBLI). In a transitional SBLI which exhibits sufficiently large shock-induced separation, a shock oscillation mechanism characterized by growth and natural suppression of the upstream laminar section of the separation bubble occurs. To validate the shock oscillation mechanism observed in large eddy simulations (LES), the shock oscillation mechanism is studied experimentally using high-speed Schlieren and spark-light shadowgraphy. A characteristic length based on the distance of laminar separation shock travel is proposed. Strouhal numbers from LES and the experiment collapse at around 0.075. A strong dependency of the oscillation mechanism on free-stream turbulence and boundary-layer state is shown. Dominant oscillation frequencies are an order of magnitude lower for the turbulent interaction as opposed to the laminar case. For the laminar case, dynamic mode decomposition showed a strong relationship of the laminar separation shock with the separation bubble and reflected shock movement. The turbulent interaction shows a significantly lower reflected shock travel distance. The findings experimentally confirm that stabilization of the shock is achieved by tripping the boundary layer.

At cruise altitudes, the Reynolds number may become sufficiently low to allow a laminar boundary layer to persist on the suction side of a transonic fan blade up to the shock-wave/boundary-layer interaction (SBLI). In such a transitional SBLI with sufficiently large shock-induced separation, a shock oscillation mechanism occurs, with the source at the upstream growth (until a critical length) and natural suppression (through shear layer instabilities) of the separation bubble. The oscillation cycle is characterized by a temporarily vanishing upstream laminar part of the separation bubble. The suppression of this laminar part is accompanied by downstream advection of turbulence and subsequent entrainment into the bulk separation bubble, affecting the reflected shock movement. In order to study the spatial behavior of the mechanism, particle image velocimetry of a highly separated transitional oblique SBLI at Mach 2.3 is conducted in the high speed aerodynamics laboratory of Delft University of Technology. Statistical quantities, including root mean square velocity fluctuations, phase averages, and spatial modes from proper orthogonal decomposition, are investigated. The entrainment strength varies depending on the phase of the oscillation, and the turbulent shear layer is not fully developed. The main growth and shrinking mode of the separation bubble were extracted, which affects the slip line region size and shock position. Secondary modes that affect the shear layer undulation and upstream effects were also extracted. The study provides quantitative analyses of an important shock oscillation type, with the focus on capturing the separation bubble size variation and upstream effects.