Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator
Axial compressors in aero engines are prone to suffering a breakdown of orderly flow when operating at the peak of the pressure rise characteristic. The damaging potential of separated flows is why a safe distance has to be left between every possible operating point and an operating point at which stall occurs. During earlier investigations of stall inception mechanisms, a new type of prestall instability has been found. In this study, it could be demonstrated that the prestall instability characterised by discrete flow disturbances can be clearly assigned to the subject of “Rotating Instabilities”. Propagating disturbances are responsible for the rise in blade passing irregularity. If the mass flow is reduced successively, the level of irregularity increases until the prestall condition devolves into rotating stall. The primary objective of the current work is to highlight the basic physics behind these prestall disturbances by complementary experimental and numerical investigations. Before reaching the peak of the pressure rise characteristic flow, disturbances appear as small vortex tubes with one end attached to the casing and the other attached to the suction surface of the rotor blade. These vortex structures arise when the entire tip region is affected by blockage and at the same time the critical rotor incidence is not exceeded in this flow regime. Furthermore, a new stall indicator was developed by applying statistical methods to the unsteady pressure signal measured over the rotor blade tips, thus granting a better control of the safety margin.
Non-synchronous vibration in axial compressors: Lock-in mechanism and semi-analytical model
Abstract Non-Synchronous-Vibration (NSV) in high-speed turbomachinery compressors is an aeroelastic phenomenon which can have devastating consequences, including loss of rotor blades. Despite extensive research over the past two decades its underlying mechanisms are not yet understood. This paper aims to explain the physical mechanisms causing NSV in a modern transonic compressor rotor. Referring to previous experimental results and using validated computational fluid dynamics (CFD), a parametric study is performed in order to characterize the aerodynamic disturbance causing NSV, and to understand the lock-in mechanism between the fluid and the structure seen during NSV. The results show that the process is driven by aerodynamics in the tip region. Under highly throttled conditions, the tip leakage flow blocks the passage and causes the disturbance, which is characterised as a vorticity fluctuation, to propagate circumferentially in the leading edge plane. It is found that the propagation speed of the disturbance is determined by the mean flow conditions and only its phase is periodically modulated through interaction with oscillating blades. This is the mechanism facilitating lock-in. Based on these findings a semi-analytic model is developed and calibrated with the numerical results. The model is capable of simulating the lock-in process and correctly predicts unstable vibration modes.
Fascism and the right in Europe, 1919-1945
This new text places interwar European fascism squarely in its historical context and analyses its relationship with other right wing, authoritarian movements and regimes. Beginning with the ideological roots of fascism in pre-1914 Europe, Martin Blinkhorn turns to the problem-torn Europe of 1919 to 1939 in order to explain why fascism emerged and why, in some settings, it flourished while in others it did not. In doing so he considers not just the 'major' fascist movements and regimes of Italy and Germany but the entire range of fascist and authoritarian ideas, movements and regimes present in the Europe of 1919-1945.