University of Beira Interior
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On The Stratification Of Density At Supercritical Liquid-Like Conditions
No full textEfficiency optimization of power systems such as diesel and liquid propelled rocket engines leads to mixture behavior to enter transcritical and supercritical regimes, where distinct coupling mechanisms from those at subcritical conditions govern the thermophysical processes.
In the present manuscript, we numerically evaluate the effect of pseudo-boiling at supercritical liquid- and gas-like conditions (reminiscent to liquid and gas phases) and compare the results against experimental nitrogen injection data, which serves as a surrogate for the oxygen-hydrogen propellant combination encountered in most liquid rocket engines. Thus, we can evaluate supercritical fluid behavior using nitrogen without including combustion and finite-rate kinetics into the computational model.
We can show the occurrence of density stratification at both liquid- and gas-like conditions, directly connected to the event of the pseudo-phase change inside the injector, meaning that in certain situations, we can have the same fluid behavior for both pseudo-phases. However, density stratification at liquid-like conditions has not been demonstrated in the literature. Therefore, it is an argument in favor of the importance of injector heat transfer in the computational modeling of such flows.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Grid Generation with Boundary Point Distribution Control using Heterogeneous Parallel Architectures
No full textA parallel numerical method for 2D orthogonal grid generation with boundary point distribution is presented. The method is based on the solution of a system of partial differential equations. The parallel machine uses Intel PC’s and Alpha workstation platforms already available in a local area network. The UNIX operating system is used together with PVM for message passing control. The control of the boundary point distribution (Dirichlet type of boundary conditions) is used to optimize the interprocessor communication and the grid quality. Reductions of the total turnaround time by a factor of 1.6 for the case of 3 processors, while maintaining the quality characteristics of the final grid, were obtained. The parallel method allows also the specification o the distance of the first grid node to the boundary, which may be most interesting to viscous flow calculations in Computational Fluid Dynamics. The present techniques and algorithms are successfully applied to different geometry’s, including C-grids around airfoils.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
The First Aerial Raid From Portugal to Macau
No full textIn 1920, Brito Pais and Sarmento de Beires tried an unsuccessful flight attempt from Amadora, Lisbon to Madeira Island aboard the airplane Breguet XIV A2, named "Cavaleiro Negro". Despite the scarce means of navigation they reach Madeira, which failed to land due to dense fog. On the way back after 8 hours of flight time, they alight at the Atlantic Ocean for lack of fuel and were rescued at about 500 km from Lisbon. In 1922, Gago Coutinho and Sacadura Cabral conducted the First Aerial Cross of the South Atlantic, flying from Lisbon to Rio de Janeiro. The Portuguese Aeronautics rejoiced auspicious days that time, with its aviation pioneers trying consecutively to reach more distant places along intercontinental flights. In 1923 Gago Coutinho and Sacadura Cabral were contemplating to perform an Around the World Flight. However, Sacadura died in 1924, while piloting an airplane acquired for the circumnavigation voyage. Also in 1924 the pilots Brito Pais and Sarmento de Beires idealized the conducting of a trip to Macau as an aspiration for a future Portuguese Around the World Flight attempt. On 7 April 1924, those pilots departed from Vila Nova de Milfontes in a Breguet XVI Bn2 airplane, beginning a trip to Macau. On 7 May an engine failure forced them to crash in India. On 30 May they managed to continue the Voyage in a De Havilland DH9 aircraft before being forced to end their attempt in 20 June in flying over Macau. A typhoon hindered their efforts to land and the airplane was crash landed in Chinese Territory about 800 meters from the Hong Kong Border. On 25 June 1924, they were shuttled back to Macau by boat. Brito Pais, Sarmento de Beires and Manuel Gouveia returned to Portugal, via North America, visiting several Portugueses nuclei at China, Japan, United States and England. They went to Portugal on 9 September, after having flown a total of 16760 kilometers in 117: 41h facing often extremely adverse atmospheric conditions, sandstorms and inaccurate navigation maps.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
The History of Aerospace/Aerospace/Aeronautics Engineering in Portugal
No full textIn the present study the origins of the aeronautics engineering education in Portugal are investigated, as well as the relations between the different initiatives that occurred in the beginning of the twentieth century. This paper aims at reporting the most relevant initiatives in Portugal and the actors involved. From 1963 to 1974 the Portuguese Air Force in together with a Portuguese University taught the first Aeronautics Engineering Military Degree at Portugal. In early 1990s two Portuguese Universities and the Portuguese Air Force started their Aerospace/Aeronautics Engineering Degrees teaching. In the 21st Century the Aerospace/Aeronautics Engineering courses at Portugal are connected with the most advanced Engineering Schools in Europe providing skills and competences integrating several disciplines.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
A Design Study of a Low-Cost Heterogeneous Parallel Computer
No full textA design study of a low-cost heterogeneous parallel computer based on already existing hardware is presented. The UNIX operating system is used together with PVM for message passing control. The parallel machine is constructed using existing Intel PC's and Alpha workstation platforms. The new techniques and algorithms are successfully applied to the grid generation problem based on the solution of partial differential equations with control of the boundary point distribution. Two different programming models were used. One uses decomposition of the domain into as many parts as there are processors, and revealed high reductions on the total turnaround time, although load balancing was a major problem. The other programming model has the possibility of changing the position of the points of the interface between each sub-grid, and allows reductions of the total turnaround time up to 1.6 times for the case of 3 processors, while the quality characteristics of the final grid are maintained. The communication times were reduced up to 7.5% of the total turnaround times, but the recursive nature of the numerical iterative process of the grid generation method causes a substantial increase in the number of iterations, reducing the parallel machine performance.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Liquid film dynamic on the spray impingement modeling
No full textThe present paper addresses a liquid film submodel included into a computational model that aims at reproducing the spray impingement phenomena. This numerical extension incorporates the spread of the liquid film over the neighboring nodes due to the dynamic motion induced by the film inertia and also the exchange of mass between the liquid layer and the incident and splashing particles. Moreover, a dimensionless film thickness parameter is embedded in the submodel by mean of an experimentally deduced correlation that can be fitted and updated to specified conditions. In order to realize how the model behaves with different influencing parameters, a thorough investigation is performed: the results that are obtained with and without the inclusion of the liquid film submodel are compared against the experimental data for two crossflow velocities. The integration of the computational extension with the spread/splash transition criterion is also evaluated by considering two types of expressions: one that includes the effect of the film thickness and one that does not. The results show that the latter option combined with the submodel does not distinctly enhance the simulation results, contrary to what happens with the transition criterion that considers the film thickness as an influencing parameter. In this case, the model with the computational extension reveals better prediction results, which indicates the necessity of considering it for spray impingement simulations along with a splash threshold that depends on the liquid layer.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Numerical Study of Mixing of Co-Axial Jets
No full textThis paper presents a numerical study of the effect of the turbulence intensity in the turbulent mixing of coaxial confined jets. The objective of this work is to study the effect of the initial levels of turbulence of each coaxial jet in the process of turbulent mixing for diameter ratios less than 2, which is the case of very low bypass turbofan engines. The computational method was tested using the available experimental data, and revealed good capabilities for the simulation of this kind of flows. The effect of the velocity ratio of the two streams is presented for velocity ratios of the two streams of 0.3, 1.5, and 6, and turbulence intensity ratios between 10-5 and 10 5. The results revealed that for relatively high turbulence intensities of the outer jet a vortex is always present for velocity ratios of 6 or larger, and this is independent of the turbulence intensity ratio. When the turbulence intensity of the inner jet is kept constant and the ratio of turbulence intensities is increased, the recirculation zone decreases in size, its center moves further upstream until it disappears completely. The existence of a favorable pressure gradient as a similar effect, revealing that this is an additional parameter to be taken into consideration together with the velocity ratio and turbulence intensity ratio as far as the recirculation zone is concerned.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Simulations of a Plunging Airfoil Undergoing Unequal Ascending and Descending Velocities at Low Reynolds Numbers
No full textDynamic stall is an effect frequently seen in nature and rotary wings where flow separation induces strong force oscillations. In order to simulate such phenomena, reduced-order models (ROMs) such as the LESP-modulated discrete-vortex method (LDVM) are often used whose applicability at low Reynolds number is discussed here. CFD computations with the SST were conducted to obtain the critical leading-edge suction parameter (LESP) by analyzing the skin friction coefficient distribution for the Reynolds number tested of 1,500. Results explore both symmetrical and asymmetrical plunging of a NACA0012 airfoil following a triangular velocity profile, indicating a good agreement between experimental, CFD, and LDVM computations, which makes the latter, a very efficient and adequate method to study wake configurations of oscillating airfoils at low Reynolds numbers.Grant co-sponsored
by Santander-UBI BID/FE/2019. The authors also acknowledge the financial support of the Brazilian agencies: the
National Council for Scientific and Technological Development – CNPq (grants #131346/2020-2 and #306824/2019-1)
and the São Paulo State Research Agency – FAPESP (grants #2017/02926-9 and #2020/00326-7).info:eu-repo/semantics/publishedVersio
Numerical Study of the Mixing Co-Axial Jets
No full textThis paper presents a numerical study of the effect of the turbulence intensity in the turbulent mixing of coaxial confined jets. The objective of this work is to study the effect of the initial levels of turbulence of each coaxial jet in the process of turbulent mixing for diameter ratios less than 2, which is the case of very low bypass turbofan engines. The computational method was tested using the available experimental data, and revealed good capabilities for the simulation of this kind of flows. The effect of the velocity ratio of the two streams is presented for velocity ratios of the two streams of 0.3, 1.5, and 6, and turbulence intensity ratios between 10 -5 and 10 5. The results revealed that for relatively high turbulence intensities of the outer jet a vortex is always present for velocity ratios of 6 or larger, and this is independent of the turbulence intensity ratio. When the turbulence intensity of the inner jet is kept constant and the ratio of turbulence intensities is increased, the recirculation zone decreases in size, its center moves further upstream until it disappears completely. The existence of a favorable pressure gradient as a similar effect, revealing that this is an additional parameter to be taken into consideration together with the velocity ratio and turbulence intensity ratio as far as the recirculation zone is concerned.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Experimental Study of a Ground Vortex: The Effect of the Crossflow Velocity
No full textThe study of the complex flowfield produced by a single jet impinging on a wall through a crossflow has been carried out over the past several decades with many different motivations and applications. This type of flow is most relevant to the successful design and operation of vertical and/or short take-off and landing aircrafts. The increased momentum of the wall jet is consistent with the movement of the separation point in the X direction, but the region of higher mean vertical velocity spreads in the wall direction. Nevertheless, their results did not exhibit any bimodal LDV histograms of discrete frequencies that could be associated with any instabilities or oscillations. To avoid the influence of the impinging region, a plane wall jet is produced independently. The wall jet collides with the boundary layer produced using a conventional wind tunnel giving rise to a highly curved region, which can be studied for different velocity ratios between the wall jet and crossflow.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio