Showing papers in "Journal of The Brazilian Society of Mechanical Sciences and Engineering in 2006"

Inverse and optimization problems in heat transfer

Abstract: This paper presents basic concepts of inverse and optimization problems. Deterministic and stochastic minimization techniques in finite and infinite dimensional spaces are revised; advantages and disadvantages of each of them are discussed and a hybrid technique is introduced. Applications of the techniques discussed for inverse and optimization problems in heat transfer are presented. Keywords : Inverse problems, optimization, heat transfer

Parametric optimization of wire electrical discharge machining (WEDM) process using taguchi method

Abstract: Wire electrical discharge machining (WEDM) is a specialized thermal machining process capable of accurately machining parts of hard materials with complex shapes. Parts having sharp edges that pose difficulties to be machined by the main stream machining processes can be easily machined by WEDM process. Technology of the WEDM process is based on the conventional EDM sparking phenomenon utilizing the widely accepted non-contact technique of material removal with a difference that spark is generated at wire and work piece gap. Since the introduction of the process, WEDM has evolved as a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highest degree of dimensional accuracy and surface finish. This paper outlines the development of a model and its application to optimize WEDM machining parameters. Experiments are conducted to test the model and satisfactory results are obtained. The methodology described here is expected to be highly beneficial to manufacturing industries, and also other areas such as aerospace, automobile and tool making industries.

Multimodal vibration damping through piezoelectric patches and optimal resonant shunt circuits

Abstract: Piezoelectric elements connected to shunt circuits and bonded to a mechanical structure form a dissipation device that can be designed to add damping to the mechanical system. Due to the piezoelectric effect, part of the vibration energy is transformed into electrical energy that can be conveniently dissipated. Therefore, by using appropriate electrical circuits, it is possible to dissipate strain energy and, as a consequence, vibration is suppressed through the added passive damping. From the electrical point of view, the piezoelectric element behaves like a capacitor in series with a controlled voltage source and the shunt circuit, commonly formed by an RL network, is tuned to dissipate the electrical energy, more efficiently in a given frequency band. It is important to know that large inductances are frequently required, leading to the necessity of using synthetic inductors (obtained from operational amplifiers). From the mechanical point of view, the vibration energy can be attenuated in a single mode, or in multiple modes, according to the design of the damping device and the frequency band of interest. This work is devoted to the study of passive damping systems for single modes or multiple modes, based on piezoelectric patches and resonant shunt circuits. The present contribution discusses the modeling of piezoelectric patches coupled to shunt circuits, where the basics of resonant shunt circuits (series and parallel topologies) are presented. Following, the devices used in passive control (piezoelectric patch and synthetic inductors) are analyzed from the electrical and experimental viewpoints. The modeling of multi-degree-of-freedom mechanical systems, including the effects of the passive damping devices is revisited, and, then a design methodology for the multi-modal case is defined. Also, it is briefly reviewed the optimization method used for design purposes, namely the LifeCycle Model. Finally, experimental results are reported, illustrating the success of using the methodology presented in passive damping applications applied to mechanical and mechatronic systems.

Some observations on wear and damages in cemented carbide tools

Abstract: Cutting tools are subjected to extremely unfavorable conditions during machining operations. High cutting temperatures, compressive and shearing stresses, chemical attacks, variable cyclic thermal and mechanical loads are some adverse conditions that wear and damage these tools. Therefore, it is crucial to understand the process of tool wear and damage and how the cutting parameters affect it in order to underpin decisions regarding the most favorable conditions to address the problem. This article treats on some forms and mechanism of wear and damage that cemented carbides can undergo during machining. Special attention was given to damages caused during interrupted cutting (e.g., milling), such as fracture, chipping and thermal fatigue. Experimental details and results of the latter phenomenon, which was studied under different cutting conditions, are discussed and confronted with literature. Keywords : Cemented carbide tools, milling, tool damage, thermal fatigue

Biodegradable fibrous thermal insulation

Abstract: This study investigates the potential of naturally occurring biodegradable fibers for use as building thermal insulation. The use of biodegradable building thermal insulation would alleviate the environmental problems presently associated with the disposal of currently used man-made non-biodegradable thermal insulations. The apparent thermal conductivity(l) for biodegradable coconut and sugarcane fiber were investigated in accordance with ASTM C 518 over the density ranges 40 kg/m3 to 90 kg/m3 and 70 kg/m3 to 120 kg/m3 for the test temperature ranges 13.2oC to 21.8oC and 18oC to 32oC, respectively. The experimental data were used to determine empirical equations for l variation with density and temperature for both coconut and sugarcane fiber. Comparison of l at 24oC for coconut and sugarcane fiber were made with seven different conventional insulation from published data. The results indicated that the minimum l for both coconut and sugarcane fiber are within the range normally associated with building thermal insulation. The l variation with density and mean temperature for both coconut and sugarcane fiber were consistent with the behavior of loose-fill thermal insulation.

Vorticity shedding and acoustic resonance in tube bundles

Abstract: This paper describes the vorticity shedding excitation in tube bundles and its relation to the acoustic resonance mechanism. These phenomena are investigated by means of velocity and pressure measurements, as well as with the aid of extensive visualization of the unsteady flow structure at the presence and absence of acoustic resonance. Vorticity shedding excitation is shown to be generated by either jet, wake, or shear layer instabilities. The tube layout pattern (in-line or staggered), the spacing ratio, and Reynolds number determine which instability mechanism will prevail, and thereby the relevant Strouhal number for design against vorticity shedding and acoustic resonance excitations. Strouhal number design charts for vortex shedding in tube bundles are presented for a wide range of tube patterns and spacing ratios. Regarding the acoustic resonance mechanism, it is shown that the natural vorticity shedding, which prevails before the onset of resonance, is not always the source exciting acoustic resonance. This is especially the case for in-line tube bundles. Therefore, separate "acoustic" Strouhal number charts must be used when appropriate to design against acoustic resonances. To this end, the most recently developed charts of acoustic Strouhal numbers are provided.

Cascade controlled pneumatic positioning system with LuGre model based friction compensation

Abstract: This paper proposes a cascade controller with friction compensation based on the LuGre model. This control is applied to a pneumatic positioning system. The cascade methodology consists of dividing the pneumatic positioning system model into two subsystems: a mechanical subsystem and a pneumatic subsystem. This division allows the introduction of friction compensation at force level in the pneumatic positioning system. Using Lyapunov´s direct method, the convergence of the tracking errors is shown under the assumption that the system parameters are known. Experimental results illustrate the main characteristics of the proposed controller. Keywords : Servopneumatics, robotics, cascade control, friction compensation

Wear behavior of uncoated carbide inserts under dry, wet and cryogenic cooling conditions in turning C-60 steel

Abstract: Environmental pollution, inconveniences and health hazards due to conventional application of cutting fluids essentially required for cooling and lubrication have been a great concern of the industries and the modern societies. Further they are also ineffective in controlling the high cutting temperature and rapid tool wear. One of the possible and potential techniques to overcome such problem is application of cryogenic cooling particularly by liquid nitrogen specially where the cutting temperature is a major constraint in achieving high productivity and job quality. The present work deals with experimental investigation in the role of cryogenic cooling by liquid nitrogen jets on tool wear, dimensional deviation and surface finish in turning of C-60 steel at industrial speed-feed combination by uncoated carbide inserts (SNMG and SNMM) of different geometric configurations. The results have been compared with dry and wet machining. The results of the present work indicate substantial reduction in tool wear, which enhanced the tool life, dimensional accuracy and surface finish. This may be mainly attributed to reduction in cutting zone temperature and favourable change in the chip-tool interaction. Further it was evident that machining with soluble oil cooling failed to provide any significant improvement in tool life, rather surface finish deteriorated. Furthermore, it provides environment friendliness and improves the machinability characteristics.

Heat transfer coefficient in a shallow fluidized bed heat exchanger with a continuous flow of solid particles

Abstract: This work shows the experimental study of a continuous gas-solid fluidized bed with an immersed tube where cold water is heated by fluidized solid particles presenting inlet temperature from 450 to 700°C. Experiments were carried out in order to verify the influence of solid particle flow rate and distance between baffles immersed in a shallow fluidized bed. The solid material was 254µm diameter silica sand particles, fluidized by air in a 0.90m long and 0.15m wide heat exchanger. The measurements were taken at steady state conditions for solid mass flow rate from 10 to 100 kg/h, in a heat exchanger with the presence of 5 or 8 baffles. Bed temperature measurements along the length of the heat exchanger were experimentally obtained and heat balances for differential control volumes of the heat exchanger were made in order to obtain the axial profile of the bed-to-tube heat transfer coefficient. The results showed that heat transfer coefficient increases with the solid particle mass flow rate and with the presence of baffles, suggesting that these are important factors to be considered in the design of such heat exchanger.

Development of an omnidirectional vision system

Abstract: Omnidirectional vision systems can provide images with a 360° of field of view. For this reason, they can be useful to robotic applications such as navigation, teleoperation and visual servoing. An effective way to construct this type of vision system is by combining lenses and mirrors together resulting on a system that does not require the movement of the camera to the direction of attention of the robot. A typical construction is by mounting a convex mirror in front of a camera aligning the center of the mirror with the optical axis of the camera. The most common convex mirror shapes used are conic, parabolic, hyperbolic and spherical. In this work we present two types of mirror that were constructed: a spherical mirror, used in the initial tests, and a hyperbolic, used for actual robot tasks. The hyperbolic mirror was manufactured using an ultra-precision CNC machine. Additionally, a software was developed to create panoramic and perspective images from the image acquired by the system. This work shows the development of an omnidirectional vision system, presenting the formulation used to determine a suitable mirror shape, the mechanical solutions used to build a fully operational system, and the results of the developed algorithm.

A specialized genetic algorithm for the electrical impedance tomography of two-phase flows

Abstract: The main objective of this work is to contribute to the development of a new tomographic reconstruction method well suited for processing signals obtained from electrical or other soft sensing field probes. The adopted approach consists in formulating the reconstruction problem in terms of an error function, which assesses the difference between a prospective and the actual internal contrast distribution (3D image), and searching for its minimum with the help of a specialized genetic algorithm (GA). Numerical simulations have been performed to demonstrate the feasibility of the proposed reconstruction method, as well as to emphasize the relation between the ill-posed nature of the problem and the topology of the minimization hyper-surface, and the importance of considering this relation when designing the numerical solution procedure. Results show that convergence is greatly enhanced when a priori information is introduced in the error function. Keywords : Electrical impedance tomography, genetic algorithm, inverse problem, optimization, multiphase-flow

Carbon dioxide evaporation in a single microchannel

Abstract: Despite its importance for designing evaporators and condensers, a review of the literature shows that heat transfer data during phase change of carbon dioxide is very limited, mainly for microchannel flows In order to give a contribution on this subject, an experimental study of CO2 evaporation inside a 08 mm-hydraulic diameter microchannel was performed in this work The average heat transfer coefficient along the microchannel was measured and visualization of the flow patterns was conducted A total of 67 tests were performed at saturation temperature of 233°C for a heat flux of 1800 W/(m2°C) Vapor qualities ranged from 0005 to 088 and mass flux ranged from 58 to 235 kg/(m2s) An average heat transfer coefficient of 9700 W/(m2°C) with a standard deviation of 35% was obtained Nucleate boiling was found to characterize the flow regime for the test conditions The dryout of the flow, characterized by the sudden reduction in the heat transfer coefficient, was identified at vapor qualities around 085 Flow visualization results showed three flow patterns For low vapor qualities (up to about 025), plug flow was predominant, while slug flow occurred at moderated vapor qualities (from about 025 to 050) Annular flow was the flow pattern for higher vapor qualities

Structural optimization for statics, dynamics and beyond

Abstract: Structural optimization has matured to the point that it can be routinely applied to a wide range of real design tasks. The purpose here is threefold. First, the general optimization task will be defined. Second, the state of the art in structural optimization will be reviewed. Finally, examples will be presented to demonstrate the level of sophistication possible in applying this technology. It is concluded that, while much research always remains, optimization technology has matured to the point where it can and should be used routinely for engineering design.

Automatic system for thermal damage detection in manufacturing process with internet monitoring

Abstract: This work involved the development of a smart system dedicated to surface burning detection in the grinding process through constant monitoring of the process by acoustic emission and electrical power signals. A program in Visual Basic® for Windows® was developed, which collects the signals through an analog-digital converter and further processes them using burning detection algorithms already known. Three other parameters are proposed here and a comparative study carried out. When burning occurs, the newly developed software program sends a control signal warning the operator or interrupting the process, and delivers process information via the Internet. Parallel to this, the user can also interfere in the process via Internet, changing parameters and/or monitoring the grinding process. The findings of a comparative study of the various parameters are also discussed here.

The Lagrange equations for systems with mass varying explicitly with position: some applications to offshore engineering

Abstract: The usual Lagrange equations of motion cannot be directly applied to systems with mass varying explicitly with position. In this particular context, a naive application, without any special consideration on non-conservative generalized forces, leads to equations of motions which lack (or exceed) terms of the form 1/2(¶m/¶q.2), where q is a generalized coordinate. This paper intends to discuss the issue a little further, by treating some applications in offshore engineering under the analytic mechanics point of view.

In-process grinding monitoring through acoustic emission

Abstract: This work aims to investigate the efficiency of digital signal processing tools of acoustic emission signals in order to detect thermal damages in grinding processes. To accomplish such a goal, an experimental work was carried out for 15 runs in a surface grinding machine operating with an aluminum oxide grinding wheel and ABNT 1045 Steel as work material. The acoustic emission signals were acquired from a fixed sensor placed on the workpiece holder. A high sampling rate data acquisition system working at 2.5 MHz was used to collect the raw acoustic emission instead of the root mean square value usually employed. Many statistical analyses have shown to be effective to detect burn, such as the root mean square (RMS), correlation of the AE, constant false alarm rate (CFAR), ratio of power (ROP) and mean-value deviance (MVD). However, the CFAR, ROP, Kurtosis and correlation of the AE have been presented more sensitive than the RMS. Keywords : Grinding, burn detection, acoustic emission, electrical power, monitoring

Influence of loading, contamination and additive on the wear of a metallic pair under rotating and reciprocating lubricated sliding

Abstract: This paper deals with an experimental study of wear and friction responses from lubricated sliding. Tests were carried out using a tribometer having devices for both continuous and reciprocating motion. The tested specimens were pins of AISI 52100 steel and counter-faces of AISI 8640 steel. The lubricant was paraffin mineral oil, VI 100. The presence of additives and contamination in the lubricant was investigated under two mechanical loading levels, determined by the velocity/load relation. Wear was evaluated in terms of morphology of the worn surfaces and by dimensional analysis of worn area of the pins. It was possible to obtain a ranking of influences on wear of mechanical loading, mechanical motion, oil additive and contamination presence in oil. Keywords : Wear, paraffin oil, abrasive contaminant, rotating motion, reciprocating motion, mechanical loading, mixed lubrication

Applied nonlinear dynamics of non-smooth mechanical systems

Abstract: This paper introduces practically important concept of local non-smoothness where any dynamical system can be considered as smooth in a finite size subspace of global hyperspaceW. Global solution is generated by matching local solutions obtained by standard methods. If the dynamical system is linear in all subspaces then an implicit global analytical solution can be given, as the times when non-smoothness occurs have to be determined first. This leads to the necessity of solving a set of nonlinear algebraic equations. To illustrate the non-smooth dynamical systems and the methodology of solving them, three mechanical engineering problems have been studied. Firstly the vibro-impact system in a form of moling device was modelled and analysed to understand how the progression rates can be maximised. Periodic trajectories can be reconstructed as they go through three linear subspaces (no contact, contact with progression and contact without progression). In the second application frictional chatter occurring during metal cutting has been examined via numerical simulation method. The analysis has shown that the bifurcation analysis can be very useful to make an appropriate choice of the system parameters to avoid chatter. The last problem comes from rotordynamics, where nonlinear interaction between the rotor and the snubber ring are studied. The results obtained from the developed mathematical model confronted with the experiment have shown a good degree of correlation.

Virtual kinematic chains to solve the underwater vehicle-manipulator systems redundancy

Abstract: This paper addresses the kinematics of the Underwater Vehicle-Manipulator Systems (UVMSs). Due the adittional degrees of freedom (dofs) provided by the vehicle, such systems are kinematically redundant, i.e. they possess more dofs than those required to execute a given task, and need to be solved using some redundancy technique. We present an approach based on introducing kinematic constraints. The approach uses the screw representation of movement and is based on the so-called Davies method used to solve the kinematics of closed kinematic chains. We describe the vehicle-manipulator system as an open-loop chain and present a virtual kinematic chain concept that allows closing this open chain. Applying the Davies method to this resulting closed chain, the UVMS direct kinematic is solved. The inverse kinematics is obtained using the same approach, by introducing extra constraints derived from energy savings requirements. The proposed approach is compared to another redundancy resolution method to illustrate the ability of the proposed strategy.

Transient stability of empty and fluid-filled cylindrical shells

Abstract: In the present work a qualitatively accurate low dimensional model is used to study the non-linear dynamic behavior of shallow cylindrical shells under axial loading. The dynamic version of the Donnell non-linear shallow shell equations are discretized by the Galerkin method. The shell is considered to be initially at rest, in a position corresponding to a pre-buckling configuration. Then, a harmonic excitation is applied and conditions to escape from this configuration are sought. By defining steady state and transient stability boundaries, frequency regimes of instability may be identified such that they may be avoided in design. Initially a steady state analysis is performed; resonance response curves in the forcing plane are presented and the main instabilities are identified. Finally, the global transient response of the system is investigated in order to quantify the degree of safety of the shell in the presence of small perturbations. Since the initial conditions, or even the shell parameters, may vary widely, and indeed are often unknown, attention is given to all possible transient motions. As parameters are varied, transient basins of attraction can undergo quantitative and qualitative changes; hence a stability analysis which only considers the steady-state and neglects this global transient behavior, may be seriously non-conservative. Keywords : Cylindrical shells, fluid-structure interaction, parametric instability, nonlinear vibrations

Foam flow of oil-refrigerant R12 mixture in a small diameter tube

Abstract: This work presents an experimental investigation of the ester oil ISO VG10-refrigerant R134a mixture flow with foam formation through a straight horizontal 3.22 mm ID diameter, 6.0 m length tube. An experimental apparatus was designed to permit the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns of the two-phase flow. Tests were performed at different mass flow rates, several refrigerant mass fractions at the inlet of the flow, and inlet mixture temperatures around 28 and 39 °C. A liquid mixture flow with constant temperature and pressure gradient could be seen at the inlet of the tube. As the flow proceeded towards the exit of the tube the pressure drop produced a reduction of the refrigerant solubility in the oil yielding to formation of the first bubbles. Initially, small and few bubbles could be noticed and the flow behaved as a conventional two-phase flow. Eventually, the bubble population increased and foam flow was observed at the exit of the flow. Due to the great formation of bubbles, both the temperature and pressure gradient of the mixture were greatly reduced in this region of the flow.

Vehicle modeling by subsystems

Abstract: Computer simulations have become very popular in the automotive industry. In order to achieve a good conformity with field test, sophisticated vehicle models are needed. A real vehicle incorporates many complex dynamic systems, such as the drive train, the steering system and the wheel/axle suspension. On closer inspection some force elements such as shock absorbers and hydro-mounts turn out to be dynamic systems too. Modern vehicle models consist of different subsystems. Then, each subsystem may be modeled differently and can be tested independently. If some subsystems are available as a set of nested models of different complexity it will be even possible to generate overall vehicle models which are well tailored to particular applications. But, the numerical solution of coupled subsystems is not straight forward. This paper shows that the overall vehicle model can be solved very effectively by suitable interfaces and an implicit integration algorithm. The presented concept is realized in the product ve-DYNA, applied worldwide by automotive companies and suppliers.

Experimental Study of the Bistable Flow in Tube Arrays

Abstract: Flow through circular cylinder arrays are commonly found in several engineering application as offshore structures, heat exchangers, transmission lines and chimneys, therefore the understanding of the several phenomena that occur due the interaction between flowing fluid and these structures is very important. This work analyzes experimentally the presence of phenomenon called bistable or biased flow, which is able to cause alternations on the flow modes in cylinder arrays. For the experimental work, two cylinders are placed side-by-side, or forming a tube bank with in line arrangement. The first arrangement corresponds to the case where the phenomenon is easily detected, and in line tube bank, the phenomenon has been also found. The experiments were performed in a wind channel using hot wire anemometry additionally to the classical statistic and spectral tools, wavelet transforms are used. The bistable flow can be an important generator of dynamic instabilities, since it alternates the lift and drag coefficients, thus alternating the structure dynamic response.

Identification of flutter parameters for a wing model

Abstract: A flexible mounting system has been developed for flutter tests with rigid wings in wind tunnel. The two-degree-of-freedom flutter obtained with this experimental system can be described as the combination of structural bending and torsion vibration modes. Active control schemes for flutter suppression, using a trailing edge flap as actuator, can be tested using this experimental setup. Previously to the development of the control scheme, dynamic and aeroelastic characteristics of the system must be investigated. Experimental modal analysis is performed and modes shape and frequencies are determined. Then, wind tunnel tests are performed to characterize the flutter phenomenon, determining critical flutter speed and frequency. Frequency response functions are also obtained for the range of velocities below the critical one showing the evolution of pitch and plunge modes and the coupling tendency with increasing velocity. Pitch and plunge data obtained in the time domain during these tests are used to evaluate the ability of the Extended Eigensystem Realization Algorithm to identify flutter parameter with increasing velocity. The results of the identification process are demonstrated in terms of the evolution of frequency and damping of the modes involved in flutter. Keywords : Identification, flutter, EERA, aeroelasticity

Adaptive techniques applied to offshore dynamic positioning systems

Abstract: Dynamic positioning systems (DPS) comprise the deployment of active propulsion to maintain the position and heading of a vessel. Several sensors are used to measure the actual position of the floating body, while a control algorithm is responsible for the calculation of forces to be delivered by each propeller, in order to counteract all environmental forces, such as wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous amount of power to be attenuated, possibly causing damage to the propeller system. That is the reason why a filtering algorithm is to be put in place to separate high-frequency components from the low-frequency ones, which are, then, fed into the control loop. Usual commercial systems apply Kalman filtering technique to perform such task, due to the smaller phase-lag introduced in the control loop compared to conventional low-pass filters. The Kalman filter draws on a model of the system to be controlled, which, in turn, depends on an unknown parameter, related to the wave frequency. Adaptive filtering is called upon with a view to perform an on-line estimation of such parameter. Most control algorithms, however, rely on fixed gains, thus making it possible for a noticeable performance degradation to take place in some situations, as those associated to mass variation during a loading operation. This paper presents the application of model-reference adaptive control (MRAC) technique to DPS's, cascaded with the commonly used adaptive Kalman filter. The model of a dynamically-positioned shuttle tanker exposed to waves and current is employed to highlight the advantages of the adaptive controller compared to commonplace fixed-gain controllers.

Film thickness and wave velocity measurement using reflected laser intensity

Abstract: The objective of this work is to develop a film thickness and velocity measurement technique using laser intensity measurement in liquid film flow. This technique was developed for annular flow studies, but it has been scarcely used due to the equipment's complexity, as compared with other techniques. The laser technique uses the reflection of the laser beam in water interface and the attenuation of its intensity to determine the interface position. Thus, the relation between intensity and thickness must be obtained by calibration. A theoretical model was proposed for the optical phenomena present in the film thickness measurement and its results were compared with experimental results using a planar mirror. A controlled experiment was conducted using one-dimensional waves in a short channel where the wave frequencies were varied by changing the vibrator's frequency and the film thickness was modified by changing the liquid volume. The reference film thickness was obtained by the analysis of photographic data taken through the transparent channel walls, allowing the comparison to the results from the laser technique. The experimental results for the film thickness presented a good agreement with the reference thickness from photographic data. The interface wave propagation velocities were measured with good accuracy, showing good agreement with the theoretical data.

Intelligent systems for welding process automation

Abstract: This paper presents and evaluates the concept and implementation of two distinct multi-sensor systems for the automated manufacturing based on parallel hardware. In the most sophisticated implementation, 12 processors had been integrated in a parallel multi-sensor system. Some specialized nodes implement an Artificial Neural Network, used to improve photogrammetry-based computer vision, and Fuzzy Logic supervision of the sensor fusion. Trough the implementation of distributed and intelligent processing units, it was shown that parallel architectures can provide significant advantages compared to conventional bus-based systems. The paper concludes with the comparison of the main aspects of the transputer and the DSP-based implementation of sensor guided robots.

Flow control with EHD actuators in middle post stall regime

Abstract: We analyze the modifications of the flow around a NACA 0015 airfoil when the flow is perturbed with an electrohydrodynamic (EHD) actuator. The device used consists of two bare electrodes flush mounted on the surface of the model operated in a discharge regime characterized by the formation of a plasma sheet contouring the body in the interelectrode space. In this study, we analyze the influence on the aerodynamic performance of the airfoil in the middle post stall regime (angle of attack»20o). The analysis is undertaken with measurements of the surface pressure distribution and of the flow fields with Particle Image Velocimetry technique. The experiments indicate that at moderate Reynolds numbers (150,000

Active flutter suppression in a 2-D airfoil using linear matrix inequalities techniques

Abstract: Department of Mechanical Design State University of Campinas - UNICAMP Cidade Universitaria, Rua Mendeleiev s/n, 13083-970 Campinas, SP

Large eddy simulation applied to reciprocating compressors

Abstract: This paper considers the application of large-eddy simulation (LES) to predict the performance of hermetic reciprocating compressors utilized in refrigeration. In such devices the pressure difference between the suction/discharge chamber and the cylinder, established by the piston motion, is responsible for the valve opening. Once the valves are open, the flow dictates the pressure distribution on the valve reed surface and, consequently, the resultant force that will govern the valve dynamics and its displacement from the seat. The methodology developed herein applies LES, combined with the Smagorinsky sub-grid model, to account for the compressible turbulent flow through the discharge valve. A one-degree of freedom model is adopted for the valve dynamics, and a finite volume methodology to solve the flow field throughout the discharge valve. For the remainder of the compression cycle, an integral formulation is employed, with effective flow and force areas being used to evaluate the dynamics and mass flow rate for the suction valve. Numerical results demonstrate that the methodology is capable of predicting important flow features in the discharge process, at a reasonable computational cost.