Comparison of different plasma actuation strategies for aeroelastic control on a linear compressor cascade

Abstract

The objective of this paper is to investigate the effects of Dielectric Barrier Discharge (DBD) plasma actuators on the aeroelastic control of a subsonic compressor cascade, for unsteady load mitigation and enhancement in the cascade aeroelastic response. Numerical simulations of the blades oscillating in traveling wave mode are performed with the commercial solver Ansys FLUENT®. Results are then validated through comparison with experimental data to assess the stability and convergence of the reference grid. Aeroelastic control is achieved by means of two AC-DBD plasma actuators installed on the trailing edge of the compressor blades, one on the suction side and one on the pressure side. Alleviation of the blade load is realized by operating alternately the actuators through a cosinusoidal function. Moreover, the effect of the actuation force phase on the blade load control is evaluated. Based on the actuation phase, results show that it is possible to manipulate effectively the unsteady lift and moment coefficients, stabilizing the compressor performance, alleviating fatigue phenomena and enlarging the flutter limits of the cascade. Plasma actuators confirm to be a very promising technology for active flow control in turbomachines and aeronautical applications.

Introduction

Aeroelastic flutter phenomenon represents a severe issue for the structural integrity of many aerospace applications [1]. Such critical conditions must be strictly avoided as may affect wings, control surfaces and even the engine operation. In this latter, aeroelastic problems generally affect the fan, the compressor first stages and even the rear stages of the low-pressure turbine. To minimize the structural weight of the engine, blades are made increasingly slender, longer and more flexible and, hence, more likely susceptible to flutter when subjected to unsteady dynamic loads. In high loading conditions, the blades can experience large deformations caused e.g. by bending and twisting motions. In this case, these torsional loads modify the angle of attack of the blade, that can develop a lift force in phase with the bending mode. Vibrations are then violently amplified and can no longer be compensated by the structural damping [2], [3]. This results in the immediate loss of the blade – referred to as flutter – or in high cycle fatigue failure (HCF). Beside the instability associated to flexo-torsional couplings, diverging oscillations or limit cycles can be also associated to one single degree of freedom, such as the blade torsion [1], [2], [3], [4].

Among several technologies able to manipulate the flow over a surface [4], plasma actuator (PA) are very promising for aeronautical applications. This device modifies the momentum of the flow by inducing a local speed perturbation, which leads to a change in the velocity profile over the blades surface, possibly preventing or delaying boundary layer separation [5], [6]. Plasma actuators are particularly attractive for aviation gas turbine engines. Advantages are numerous and include fast responses, lightweight, absence of moving parts, low power requirement and easy installation [7]. Furthermore, they are suitable for a high temperature environment (i.e. turbine blades). Over the last years, there have been successful applications of active flow control in turbomachinery, principally aiming at improving the stability range and the efficiency of turbines and compressors. In Ref. [8] an experimental investigation of active flow control is carried out to reduce the unsteady interaction between the stator-rotor blades of a turbofan. In Ref. [9] dielectric barrier discharge plasma actuators are employed to control tip leakage flow of a turbine rotor. In Ref. [10] pulsed-DC waveform powered plasma actuator are used to delay stall inception in an axial fan. In Ref. [11] different high-voltage (HV) excitation waveforms of a single dielectric barrier discharge (SDBD) plasma actuator are optimized to control flow separation and aerodynamic losses in low-pressure turbines (LPTs). In Ref. [12] experimental investigations are conducted to control flow separation on a LPT rotor blade through a micro-fabricated SDBD plasma actuator. In Ref. [13] investigations on the effect of multiple suction side plasma actuators to control tip leakage flow are performed.

In parallel to this, recent works have dealt with the employments PA devices for controlling the aeroelastic response of turbomachines components, such as aircraft engine blades. These studies also explore the enlargement of the flutter stability boundaries and unsteady load alleviation on isolated oscillating airfoils. Glaz et al. [14] evaluate the effects on the aeroelastic stability of using nanosecond pulsed dielectric barrier discharge plasma actuator in post-stall regime. De Giorgi et al. [15] carry out numerical investigations of multi-DBD plasma micro-actuators for load mitigation and stability control on an isolated oscillating airfoil. In regards to flutter control of a compressor cascade, Motta et al. [16], [17] optimize the steady and unsteady airloads acting on a subsonic compressor cascade employing DBD plasma actuators as virtual control surfaces. In Ref. [18] a numerical assessment on the capability of PAs to control the aeroelastic response of a compressor cascade in transonic regime is performed.

In this regard, the aim of the present paper is to assess the feasibility of using dielectric barrier discharge plasma actuators (DBD-PAs) through a proper modulation of the input AC voltage signal as a novel approach for lift enhancement, mitigation of the torsional oscillations and aeroelastic control of vibrating airfoils operating in (or close to) unstable conditions.

Based on the application and power supply – Nanosecond Pulsed (NP) or Alternate Current (AC) – this type of technology is able to induce different flow fields in the actuator neighboring region. For low-Reynolds numbers, plasma actuators powered by AC voltage have been found to be particularly effective [19]. Both numerical and experimental investigations have widely applied AC power supply for DBD plasma actuation to boundary-layer control, leading-edge control and trailing edge flow separation control, as reported in Refs. [20], [21], [22]. Current investigation will therefore focus only on the performance provided by AC driven DBD plasma actuators, hereinafter referred to as AC-DBD actuators.

The purpose of this study is the evaluation of the unsteady aerodynamic performance of trailing edge plasma actuators on a compressor cascade. Actuators are triggered alternately by proper phase shifting of the actuation law. The actuators are placed on the suction and pressure sides of the central blades. For the first time to the authors' knowledge, optimization of the aerolasic performance of two micro-actuators has been performed through a proper application of the phase shift in the cosinusoidal actuation law.

Fluid dynamics of turbomachinery internal flows is one of the most complex phenomena encountered in practice, because of the limited flow space in the passage of a blade row and the strong interactions that generate on the surfaces of the blades. Moreover, the effect of the motion of the adjacent blades has a considerable influence on the aeroelastic response and flutter characteristics of the cascade [1]. In this context, a preliminary numerical assessment is carried out to demonstrate the active vibration control capability of dielectric barrier discharge plasma actuators in a compressor cascade. Here, flow disturbances that already per se influence the stability of the oscillating airfoils will interact with the plasma-induced flow field. The cascade oscillates according to a traveling wave mode, with blades oscillating at the same frequency but with different phase. This motion is meant to be representative of the pressure waves encountered on turbomachinery annuli [5] and is extensively used for both experimental [1] and numerical [6] aeroelastic assessments. The phase difference of two adjacent blades is referred to as interblade phase angle (IBPA) and – together with the reduced frequency – is of fundamental importance in affecting the cascade stability, see e.g. [1].

Considering the lack of experiments in actual aviation gas turbine engines in ground test facilities, potential applications of AC-DBD plasma actuators for flow control in aero engine blades will be explored and discussed. Though AC-DBD actuators are very well suited for landing and takeoff manoeuvres, the most critical phases of a typical aircraft mission, they have to be tested in all flight conditions in order to operate as flow control devices. Therefore, the numerical study presented here could be extended to future practical applications in aeronautical field.

Starting from the work of Motta et al. [16], where a uniform force per unit volume is applied over a rectangular plasma region, the modeling approach of the AC-DBD-PAs employed in this study is based on the formulation proposed by Shyy [23]. In reality, the active discharge region does not distribute evenly above the dielectric layer, but rather concentrates towards the exposed electrode where the electric field is stronger, forming a plasma region similar to a triangular zone. With Shyy's approach the lines of the electric field, as well as the body force generated by plasma discharge, can be considered as linearized. In addition to this, Shyy's model [5] has been chosen because of its numerous advantages, such as wide applicability, smooth implementation in the solver, fast response and very low computational cost for unsteady CFD applications. Moreover, despite its simplicity, Shyy's approximation is able to fully mimic the AC-DBD plasma actuator on the surrounding airflow. The effects of the alternate actuation on the compressor cascade is illustrated and discussed. As anticipated, a phase-controlled actuation is fundamental to obtain a significant load reduction on the lift and moment coefficient, consistently with Ref. [16]. Realistic operating conditions of the compressor cascade are performed.

The outline of this work is as follows. Section 2 addresses the AC-DBD plasma actuator model and relative characterization. Section 3 describes the geometric domain and settings for the CFD analysis. Mesh convergence studies of the reference grid are realized through comparisons to experimental data in steady and unsteady RANS simulations. Control of aeroelastic response of the cascade in terms of load mitigation, aerodynamic damping and hysteresis loops of the aerodynamic coefficients are discussed in Section 4. Final remarks are presented in Section 5.