Resumo em Inglês:

This paper is concerned with trajectories to transfer a spacecraft between the Lagrangian points of the Sun-Earth system and the primaries. The Lagrangian points have important applications in astronautics, since they are equilibrium points of the equation of motion and very good candidates to locate a satellite or a space station. The planar circular restricted three-body problem in two dimensions is used as the model for the Sun-Earth system, and Lamaître regularization is used to avoid singularities during the numerical integration required to solve the Lambert's three-body problem. The results show families of transfer orbits, parameterized by the transfer time.

Resumo em Inglês:

The conventional grinding methods in some cases are not very efficient because the arising of thermal damages in the pieces is very common. Optimization methods of cutting fluid application in the grinding zone are essential to prevent thermal problems from interaction of the wheel grains with the workpiece surface. The optimization can happen through the correct selection of the cut parameters and development of devices that eliminate air layer effects generated around the grinding wheel. This article will collaborate with the development of an experimentation methodology which allows evaluating, comparatively, the performance of the deflectors in the cutting region to minimize the air layer effect of the high speed of the grinding wheel. The air layers make the cutting fluid jet to dissipate in the machine. An optimized nozzle was used in order to compare the results with the conventional method (without baffles or deflectors) of cutting fluid application. The results showed the high eficciency of the deflectors or baffles in the finish results.

Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

The present work aims to determine mathematical equations that describe the behavior of the components of the volumetric error in Three Coordinates Measuring Machines (3CMM). A general methodology using techniques of multiple regressions is presented. Such methodology, applied to a moving bridge type 3CMM, has made it possible to obtain, in a simple way, three regression equations from data collected through a direct calibration procedure, more specifically, the space grid method. The proposed model was statistically and experimentally evaluated. The statistic evaluation covers the calculation of the correlation coefficient of the samples, a residue analysis and the hypothesis verification. The experimental evaluation was made through the comparison of results expected by the model with the results obtained from the measurements of a ball bar. From these results, it is possible to verify that the model is adequate and that it is good in predicting volumetric errors in the machine.

Resumo em Inglês:

An overview is given of high-resolution numerical simulation results for gravity currents in the lock exchange configuration. Results are provided for Boussinesq and non-Boussinesq flows, and for both horizontal and sloping bottom geometries. Furthermore, currents driven by fluid density differences are discussed along with those driven by differential particle loading.

Resumo em Inglês:

Direct numerical simulations of compressible turbulent channel flow including passive scalar transport have been performed at five Mach numbers, M, ranging from 0.3 to 3.5 and Reynolds numbers, Re, ranging from 181 to 1030. The Prandtl and Schmidt numbers are 0.7 and 1.0, respectively, in all cases. The passive scalar is added to the flow through one channel wall and removed through the other, leading to an S-shaped mean scalar profile with non-zero gradient in the channel centre. The paper describes the set of compressible flow equations, which is integrated using high-order numerical schemes in space and time. Statistical equations are presented for fully developed flow, including budgets for the Reynolds stresses, the turbulent scalar fluxes and the scalar variance. Results are presented for second order moments and the terms in the mentioned balance equations. Outer scalings are found suitable to collapse incompressible and compressible data. The reduction in the near-wall pressure-strain and pressure-scalar gradient correlations due to compressibility is explained using a Green-function-based analysis of the fluctuating pressure field.

Resumo em Inglês:

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.

Resumo em Inglês:

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<FONT FACE=Symbol>»</FONT>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<Re<333,000), the actuator enables a reattachment of the flow and reveals an important sensitivity of the middle post stall regime to periodic excitation.

Resumo em Inglês:

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.

Resumo em Inglês:

Turbulence generated by a pair of oscillating grids in a water tank is investigated using the digital particle image velocimetry technique. Vertical and horizontal components of fluctuating velocities are measured. Experimental profiles of the turbulent kinetic energy k, and the energy dissipation rate, xi, obtained from the fluctuating velocities, are presented and compared with theoretical predictions. Other properties of interest, such as the length scale and the turbulent viscosity, obtained in the present study, are also reported. The turbulence generated with the pair of grids may be classified as "nearly isotropic" and theoretically the flow may be analyzed using the k-epsilon model. The experimental data agree well with the theoretical predictions. Also data of different sources found in literature, for pairs of vibrating grids, are compared with the present experimental results and theoretical predictions, showing good agreement. The studies conducted at the Laboratory of Environmental Hydraulics from the Department of Hydraulics and Sanitary Engineering, EESC/USP, permits to affirm that "double oscillating grids" turbulence can be viewed as nearly isotropic.

Resumo em Inglês:

The main objectives of this work are the formulation, implementation and applications of a numerical algorithm to simulate turbulent, incompressible, isothermal flows. The transient three-dimensional flow is analyzed using an explicit Taylor-Galerkin scheme and the finite element method with hexahedrical eight-node element. Turbulence is simulated using Large Eddy Simulation. For sub-grid scales two models were implemented, the classical Smagorinsky's model and the dynamic eddy viscosity model. For the second filtration, which is necessary in the dynamic model, a new method was developed based on independent finite elements that involve each node in the original mesh. The implemented scheme is efficient and good results with low additional computational cost were obtained. Results for two classical problems, the driven cavity and the backward facing step are presented. Comments about the model applicability for flows with high Reynolds numbers are also presented.

Resumo em Inglês:

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.

Resumo em Inglês:

The transition to turbulence in a stably stratified flow is a problem of considerable interest in fluid dynamics with applications in both geophysical sciences and engineering. This transition is controlled by competition between the vertical shear of the base flow and the buoyancy forces due to the density stratification. The present work investigates numerically the effect of stable stratification on the development of a Kelvin-Helmholtz (KH) instability and the formation of streamwise vortices, which are developed after the saturation of the primary billows of KH. The Direct Numerical Simulation (DNS) technique was used to solve the complete Navier-Stokes equations, in the Boussinesq approximation. Numerical tests were done with different Richardson numbers and forced initial conditions for velocity fluctuations. The results showed that high stratification inhibits the pairing process, reduces the buoyancy flux, weakens the vertical motions, decreases the thickness of the mixing layer and affects the formation of streamwise vortices. The three-dimensional results demonstrated that the streamwise vortices are clearly formed in non-stratified cases. In the stratified cases, on the other hand, the streamwise vortices are weakened, due to the streamwise density gradient, which decrease the levels of vorticity in the billows of KH, while increase in the braid zone.