Session: 40-09 Fan and Compressor Vibrations

Paper Number: 129233

129233 - Numerical Analysis of Oscillating Blade in Linear Cascade at Transonic Flow 

In the current scenario, aircraft engines are moving towards increasing the thrust-to-weight (T/W) ratio by reducing the specific fuel consumption. Thrust can be accomplished by increasing the combustion chamber exit temperature, but this has been limited by high load and thermal stress acting on the turbine blades. So, instead of increasing the exit temperature, increase the efficiency of the engine components (compressor, turbine) with minimal weight. It can be obtained by very thin slender blades, which are primarily placed in the initial stages of the compressor and fan blades. Especially in the compressor blade, since it has an adverse pressure gradient over the airfoil, careful design of the airfoil and diffusion in the flow velocity should not be more than it can withstand. 

Most modern civil aircraft have been operated generally at cruising Mach 0.8, more than the critical Mach number. At some location over the suction surface in an airfoil, shock occurs. It changed with a sudden increase in the drag components, and flow separation occurred. When the aircraft operates at Mach numbers 0.7 to 0.95, the shock wave will oscillate from the leading edge to the trailing edge. It can be highly nonlinear in the slope of lift curve due to shock wave oscillation. At low subsonic speeds, the shock wave creates and oscillates at a very high angle of attack. But in the high subsonic Mach number, the shock wave will oscillate at even zero angle of attack, and the airfoil curvature is enough to accelerate the flow to create local shock at certain locations.

It should be a significant compromise between the aerodynamic and structural loads. When shock oscillates, it changes unsteady loads on the airfoil, which alters the flow phenomenon over time. The small perturbation also leads to blade flutter and LCO oscillations. Unlike a single airfoil, in the cascade, the flow phenomenon ultimately changes under the influence of the neighboring blade. In the flutter analysis, most of the blade stability is not greatly impacted by the bending mode vibration. The torsional mode is only predominantly prone to the instability of blade.

In this present computational study, it is reported that there are significant changes in the flow dynamics at the central blade flow passage by changing the inlet Mach number from 0.8, 0.85, stagger angle 45 to 50, and the chord from 46.3 mm in the linear cascade of 5 blades with two false blades at both ends of the wall to get the periodicity between the flow passages. The standard test configuration (STC-10) airfoil has been chosen for high subsonic flow conditions. The chamber angle is 10 degrees. The simulation was carried out on commercial software ANSYS 2022.

To simplify the problem, Classical method (fluid and structure equation uncoupled) and the influence coefficient method (one blade oscillation and measuring parameters in all blades) were adopted to carry out the simulation.

The unsteady simulation was carried out to determine the forced vibration with shock dynamics. In the linear cascade row of 5 blades, the central blade is only subject to torsional vibration at 500 Hz (corresponding reduced frequency of 0.5), with an amplitude of 2 deg and different phase angles.

 The oscillating blade setup in a linear cascade setup was simulated using computational work. The main goal of this simulation was to determine marginal flutter stability in two set of geometries. The CFD technique was developed for steady state analysis at M = 0.8, and initially it showed a good comparison between the original paper and current simulation data. Based on the prior knowledge, the unsteady pressure coefficient and phase lead were calculated using time-dependent analysis. A total of 16 cases were simulated, each with a different frequency and incidence angle.

To calculate the aerodynamic damping and flutter stability in oscillating blades, the energy method and work per cycle are used. The interaction between structural and aerodynamic modes has been neglected in adopting classical methods.

Presenting Author: Ragupathy S INDIAN INSTITUTE OF TECHNOLOGY KANPUR

Presenting Author Biography: I am Ragupathy, doing Phd at IIT kanpur, India. MY specialization is turbomachinery. Now i am working in oscillation compressor blade in transonic conditions.

Authors:

Ragupathy S INDIAN INSTITUTE OF TECHNOLOGY KANPUR
Abhijit Kushari INDIAN INSTITUTE OF TECHNOLOGY KANPUR
M C Keerthi INDIAN INSTITUTE OF TECHNOLOGY DHARWAD