Maria Grazia De Giorgi

Investigation of a Micro Dielectric Barrier Discharge Plasma Actuator for Regional Aircraft Active Flow Control

This paper reports a multitechnique investigation of a micro dielectric barrier discharge plasma actuator (DBDPA) as a promising system to control separated flows. The device was manufactured through a photolithographic technique and its performances and capabilities were compared with the ones of conventional macro DBDPAs. Alternate current operation under sinusoidal voltage excitation was studied in the absence of external flow by means of many experimental techniques like discharge imaging, flow visualizations, particle image velocimetry, infrared thermography, and electrical characterization. The influence of the operating parameters was investigated. The main results underlined that an increase in the voltage amplitude or frequency brought to a rise in the maximum induced velocity, electrical power dissipation, and actuator surface temperature. Moreover, it was assessed that the small heating of the micro DBDPA did not affect the actuated flow. A jet velocity up to 1.36 m/s was obtained at a 9.01 W/m electrical power dissipation per unit electrode length. The device realized by microelectronic fabrication technology allowed reaching a flow velocity magnitude comparable with the one of conventional macro DBDPAs, with a reduction in applied voltage, power dissipation, and actuator size. Furthermore, the induced wall jet was more confined in the area in proximity of the device, because of the limited plasma discharge extension.

Experimental and Numerical Investigations of Cavitating Flows

Cavitating flows have been investigated using both numerical and experimental methods. Three different cavitation models, based on mechanical or thermal equilibrium hypotesis, have been implemented in a general-purpose CFD code. As an external flows example, the behavior of a NACA 0015 hydrofoil was investigated. The cavitating flow field over the hydrofoil was predicted, and the results compared with experimental data, reported in literature. The general characteristics of the cavitating flow were well predicted. Especially, the cavity length was calculated with reasonable accuracy. Further, the cavitation models were applied to flows through an orifice, and the computed results were compared with the experimental observations, obtained with a CCD camera. The cavitation originates at the inlet of the flow constriction area. It grows intensively and transforms into a dense cloud. Shedding is observed in this stage. As flow rate was increased, it was observed that the cloud travels downstream of the hole oscillating about the exit position and it is connected to the hole inlet through a sheet having a complex turbulent structures.

Cavitation Regime Detection by LS-SVM and ANN With Wavelet Decomposition Based on Pressure Sensor Signals

A cavitating two-phase flow of water in a pipe with area shrinkage was experimentally investigated, acquiring at high sampling rate pressure signals and images of the cavitating flow field. The time series of the pressure fluctuations was analyzed in terms of power spectral density and related to the cavitation regimes. Furthermore, the fluctuations of the pressure measurements were also decomposed using the wavelet transform to analyze the frequency distribution of the signals energy with respect to the flow behavior. The energy content at each frequency band of the acquire signals is well related to cavitation flow-field behavior. Moreover, the artificial neural network and the least squares support vector machine (LS-SVM) were implemented to identify the cavitation regime, using, as inputs, the power spectral density distributions of the pressure fluctuations, and some features of the decomposed signals, as the wavelet energy for each decomposition level and wavelet entropy. Results indicate the most accurate model to be used in the cavitation regime identification, underlining the enhanced capability of LS-SVM trained with the input data set based on the wavelet decomposition features.

Aircraft Distributed Flow Turbulence Sensor Network with Embedded Flow Control Actuators

Several active and passive flow control systems are studied to improve the performances of fluid machineries and to increase aerodynamic efficiency of propulsion systems. Among all the well-known active flow control devices, the dielectric barrier discharge plasma actuator (PA) is in full expansion and of great interest in the scientific community. A PA modifies the following behaviour of a fluid by providing an electronically controllable disturbance that brings to drag reduction, flow separation control, enhanced mixing, and noise suppression. PA is potentially easy to construct, has no moving parts and has low power requirements. This leads to its possible applications for separation control in low pressure turbine blade and compressor cascade, tip clearance flow control and compressor stability range extension. The present work reports the design and fabrication of cheap Kapton-based flow turbulence capacitive sensors able to be embedded into aircraft wing profiles and airfoil structures for critical turbulence conditions detection and early-detected separated flows control. The embedded system will provide a Kapton-foil based pressure detection and linear/synthetic jet plasma actuators working in feedback, for prevention and active reduction of separated flow for regional aircraft applications.

Experimental and Numerical Analysis of a Micro Plasma Actuator for Active Flow Control in Turbomachinery

Nowadays several active flow control systems, particularly dielectric barrier discharge plasma actuators, appear to be effective for the control of flow stream separation and to improve performance of turbomachinery. However these applications require high actuation strength, higher than the one generated by conventional macro plasma actuators. Research is actually improving the design of plasma actuator in order to enhance the flow control capability and reduce the power consumption.In this contest, this work concerns the implementation of a micro plasma actuator for the active control in a compressor cascade. For this aim, firstly the micro actuator was developed and an experimental characterization of the flow induced by the device was done. The induced flow field was studied by means of Particle Image Velocimetry and Laser Doppler Velocimetry. The dissipated power was also evaluated. Experimental results were used to validate a multi-physics numerical model for the prediction of the body forces induced by the plasma actuator.Finally, the obtained body force field was used for modeling the separation control by means of the micro plasma actuator in a highly-loaded subsonic compressor stator.Copyright

Active Flow Control Techniques on a Stator Compressor Cascade: A Comparison Between Synthetic Jet and Plasma Actuators

In this work a CFD analysis is applied to study the suppression of the boundary layer separation into a highly-loaded subsonic compressor stator cascade, by different active flow control techniques. Active flow control techniques have the potential to delay separation and to increase the pressure ratio. In particular three different techniques have been applied: the actuation by steady jet, by zero net mass flux Synthetic Jet (SJA) and by plasma actuator.Several works have investigated the use of synthetic jet and plasma actuators on the airfoil, but only few studies have compared the effect of these devices.Concerning the synthetic jet actuator, a suction/blowing type boundary condition is used, imposing a prescribed sinusoidal velocity depending on velocity amplitude, jet frequency and jet angle of ejection with respect to the wall.Concerning the plasma actuation, the effect is modeled into numerical flow solvers by adding the paraelectric force that represents the plasma force into the momentum equation. The plasma, generated by Dielectric Barrier Discharge, acts as a momentum source to the boundary layer allowing it to remain attached throughout a larger portion of the airfoil. The time-averaged body force component, acting on the fluid, depends on the frequency and on the applied voltage, the charge density, the electrical field and the dimensional properties of the actuator, like width of the electrodes and gap between the electrodes. Using this numerical model, the effect of plasma actuators to suppress the flow separation over the blade has been investigated, increasing the turbo-machinery performance too.Finally, the comparison between the different actuation devices shows that, reducing the secondary flow structures, each actuation technique beneficially affects the performance of the stator compressor cascade, even if in the steady jet the costs are relevant.© 2012 ASME

Frequency Analysis and Predictive Identification of Flame Stability by Image Processing

Monitoring and characterization of combustion flames by digital image processing is an active research topic. This study experimentally investigates the feasibility of high speed visualization techniques for combustion instability monitoring in a swirl liquid-fueled lean combustor for different air/fuel ratios. Instability, in fact, is an unpleasant aspect in the combustive system that negatively impacts on combustion efficiency. This work investigates methods for extracting significant parameters using the geometrical and luminous data of the flame images; some flame features are related to the combustion regimes. The stability of the flame is identified using spectral and wavelet-based analysis of the pixel intensities of the flame images. In particular the most flame unstable regions were identified by analyzing the two dimensional maps of different physical quantities. The impact of the fuel/air ratio on the stability of the flame is investigated also by a Monochromator/Photomultiplier system (PMT). The results support the potential of the methods described for flame monitoring. NOMENCLATURE CCD

Assessment of the Combustion Behavior of a Pilot-Scale Gas Turbine Burner Using Image Processing

Experimental investigations were performed on a non-premixed liquid fuel-lean burner. The present work aims to the development of a methodology for the recognition of flame instability regimes in industrial and aeronautical burners. Instability, in fact, is an unpleasant aspect of combustive system that negatively impacts on combustion efficiency. The online monitoring of the occurrence of instability conditions, permits to adjust combustion parameters (as fuel or air mass flow, temperature, pressure, etc.) and to stabilize again the flame.High speed visualization systems are promising methods for on-line combustion monitoring.In this study two high speed visualization systems in the visible range and in the infrared spectral region were applied to characterize combustion efficiency and flame stability.Different processing techniques were used to extract representative data from flame images.Wavelet Decomposition and Spectral analysis of pixel intensities of flame images were used for feature extraction. Finally a statistical analysis was performed to identify the most unstable regions of the flame by the pixel intensity variance.Copyright

Modeling Nucleation Phenomena in Cavitating Flow

The aim of this investigation is to implement a cavitating flow model taking into account the nucleation for both water and cryogenic fluids, in particular hydrogen. In this paper we re-examine previously developed cavitation models, used for water liquid, including thermal effect for simulations of cryogenic fluids and transport equation for the bubble number. Thermal effects substantially impact the cavitation dynamics of cryogenic fluids. Different numerical cavitation models, based on different physics of mass exchange between liquid and vapor phases, and taking into account the nucleation, were used to simulate the flowfield in internal flow. The transport equation for bubble number is solved in conjunction with the mass and momentum conservation. Performances of the reported cavitation models are tested, comparing the predicted pressure with the experimental data, with non-cryogenic and cryogenic fluids. For non-cryogenic the phase change during rapid depressurization of water have been studied and the role of vapor bubbles nucleation and growth and the effect on the system fluid dynamics were modeled. For cryogenic fluid the cavitation phenomena of liquid hydrogen flowing in a Venturi has been investigated and the results have been compared with experimental data.

Spray and Combustion Modeling in High Pressure Cryogenic Jet Flames

The aim of the present work is the investigation of the combustion phenomenon in liquid-propellant rocket engines. The combustion of liquid oxygen and gaseous methane in a shear coaxial injector under supercritical pressure was analyzed. To realize an efficient numerical description of the phenomena, it is important to treat the LOx jet in a manner which takes into account its real behavior. In the present work different kinetics, combustion models and thermodynamics approaches were used in association with the description of the jet as a discrete phase. For all the approaches used, a comparison with experimental data from literature was performed.Copyright