J. Sousa

Experimental and numerical investigation of flow oscillations in a rectangular cavity

The unsteady, incompressible, moderate Reynolds number flow past a rectangular cavity is experimentally and numerically investigated. Laser-Doppler anemometry, flow visualization and unsteady numerical simulation using fully second-order accuracy in time and space, were the tools employed to meet this purpose. Large-amplitude organized oscillations are reported to occur in the investigated geometry due to fluid-dynamic instability. Detailed flow visualization and unsteady predictions clearly show that the instability process involves a complex coupling of shear layer and recirculating flowfield dynamics. The paper also demonstrates the accuracy of the present calculations.

Influence of impingement edge geometry on cavity flow oscillations

and back portion of the airfoil is shown in Figs. 2b and 2c, respectively. The smooth transition of the grid from one type to another and the decency of the grid at sharp corners and between close surfaces are clearly shown, which indicate excellent compatibility and communication between the two grid strategies as controlled by a common background grid. The grid was generated using a Silicon Graphics IRIS 4D/210 VOX workstation. The codes performance is about 360 triangles/s on that workstation and 2350 triangles/s on a Cray Y-MP. The total turnaround time required for generating a grid around a complex geometry, including the setup time, is about 1 h. To examine the fidelity of the grids generated with the present method, a turbulent viscous-flow computation was performed on the grid shown in Fig. 2. The flow solution was obtained with an available node-based, upwind flow solver8 using the BaldwinEarth turbulence model at a Mach number of 0.2, an angle of attack of 16.2 deg, and a Reynolds number of 9 X 106. Comparisons for surface pressure distributions and velocity profiles are shown in Figs. 3a and 3b, respectively. Excellent agreement with experimental data confirms the viability of the generated grid. Concluding Remarks A new method of unstructured viscous grid generation has been introduced. The approach is conceptually simple but powerful, and capable of producing high-quality viscous unstructured grids for complex configurations with ease. Being based on a totally unstructured grid strategy, the method is fully automatic and flexible and, thus, alleviates the difficulties stemming from the structural limitations of the existing structured or semi-unstruct ured techniques. Because of an efficient grid-marching strategy and a simple front-detection algorithm, the method is highly efficient and operational on small workstations. The method is also self-sufficient for insertion of grid points in the boundary layer and beyond. The method has been applied to two-dimensional problems with satisfactory results. The basic elements of the technique, however, have been primarily designed for its subsequent extension for generating three-dimensional highly stretched tetrahedral grids which is currently in progress. The full benefit of the method will be realized for generation of three-dimensi onal viscous grids where the complexity of the problem becomes excessive.

Spray Characteristics of Angled Liquid Injection into Subsonic Crossflows

The spray characteristics of angled liquid injection into subsonic crossflows were experimentally investigated. The experiments were conducted using water as the test liquid and included variations of the nozzle injection angle, air velocity of the crossflow, and liquid flow rate. An initial assessment of the spray characteristics performed with the aid of a visualization technique revealed that the overall jet penetration, breakup length, and atomization quality are significantly affected by the nozzle injection angle and, to a lesser extent, by the liquid-to-air momentum flux ratio. Subsequently, detailed droplet size and droplet velocities were obtained for various sprays by the use of phase Doppler anemometry. The data reveal that there is an evident longitudinal droplet velocity/diameter correlation for all droplets, with small droplets being faster than larger droplets. Furthermore, the droplet mean diameters diminish on increasing the nozzle injection angle but are affected only marginally by the liquid-to-air momentum flux ratio.

Confined vortex breakdown generated by a rotating cone

Confined vortex breakdown generated by a rotating cone within a closed cylindrical container has been studied both by numerical simulation and by experimental techniques. A comprehensive investigation of the various flow regimes has been carried out by flow visualization. From laser–Doppler measurements of the entire flow field (three velocity components) detailed maps of the time-averaged flow structures for single and double breakdown have been constructed. Three-dimensional time-dependent simulations of steady and unsteady breakdown have been performed. Steady numerical and experimental flow fields obtained at Reynolds number 2200 for a gap ratio of 2 show notable agreement. At critical Reynolds numbers of approximately 3095, for a gap ratio of 2, and 2435, for a gap ratio of 3, the flow was observed becoming unsteady. The periodic behaviour exhibited by the unsteady flow suggested the occurrence of a supercritical Hopf bifurcation. This conjecture was confirmed by the evolution of the oscillation amplitude as a function of criticality, measured for a gap ratio of 3. The dynamical behaviour of unsteady vortex breakdown structures is depicted by numerical simulation of two distinct oscillatory regimes, at Reynolds numbers 2700 and 3100. A thorough analysis of the numerical results has shown that whereas the former regime is characterized by the steady oscillation of closely axisymmetric breakdowns, the latter displays precession of breakdown structures about the central axis. Additionally, it was observed that the mode bringing about the Hopf bifurcation is non-axisymmetric, with azimuthal periodicity of π/2 radians. From examination of measured velocity power spectra at higher Reynolds numbers, a transition scenario was also educed. In the present case, the Ruelle–Takens–Newhouse theorem has been shown to apply.

Engineering aspects of the tokamak ISTTOK

AbstractThe first Portuguese magnetic confinement experiment, the tokamak ISTTOK, has been in operation since 1993. This tokamak device is described and the main technological features, as well as the novel techniques of its diagnostics and control and data acquisition system, are reported. A synopsis of the experimental activity is also presented.

Low-Reynolds-Number Effects in Passive Stall Control Using Sinusoidal Leading Edges

The objective of the presentwork is to investigate the application of a sinusoidal leading edge to the design ofmicro air vehicles. Wind-tunnel tests of wings with low aspect ratios of 1 and 1.5, rectangular planforms, and five distinct leading edges [four sinusoidal leading edges and one baseline (straight) leading edge for each aspect ratio] have been conducted. The Reynolds numbers of 70,000 and 140,000 have been analyzed. For the higher Reynolds number, a proper combination of amplitude andwavelength can lead to a substantial increase in lift for angles of attack greater than the baseline stall angle. Maximum lift coefficient gains of the order of 45% were achieved by combining both large amplitude and large wavelength. At the lower Reynolds number, the benefits can be extended to low angles of attack, leading to a dramatic increase in the range of operation. The results depend strongly on the aspect ratio.

A high performance reconfigurable hardware platform for digital pulse processing

This paper presents a new reconfigurable hardware platform which uses both digital signal processors (DSP) and field programmable gate arrays (FPGA) to attain high resolution and real-time processing in nuclear spectrometry experiments. The module was designed in order to provide a high digital pulse processing (DPP) yield with the capacity of being reconfigurable according to the experimental conditions and the desired data output. This allows unprecedented real time processing capabilities implemented in the FPGA such as pulse pileup correction through adaptive filtering and effective signal-to-noise ratio (SNR) control and optimization. The module uses a Virtex-II Pro FPGA (Xilinx) and a DSP from the TMS320C64xx family (Texas Instruments) and is implemented on a PCI board to be used in a host workstation. Special emphasis is given to the scalability of the module along with the potential capacity of being used as a portable stand-alone instrument. This reconfigurable hardware platform allows us to simultaneously benefit from the advantages of the hardware based digital spectrometers, namely, their high throughput, and of the flexibility of the software based configurations. Besides, this same platform can be used as a general purpose high-speed data acquisition and processing unit.

Experimental Investigation of a Novel Combustor Model for Gas Turbines

The present study describes the performance of a novel combustor model for gas turbines. The working principle of this combustor is based on the establishment of a large recirculation zone in the combustion chamber, where part of the inlet air is mixed with the combustion products. Efficient mixing is achieved by the use of high-velocity inlet air jets. Six different inlet air geometries have been analyzed under nonreacting and reacting conditions at atmospheric pressure. Laser-Doppler anemometry was employed to characterize the mean velocity and turbulent kinetic energy fields as a function of the air mass flow rate and geometry configuration. Measurements of mean gas species concentration (O 2 , CO 2 , CO, HC, and NO x ) at the model exhaust are reported as a function of the equivalence ratio for all configurations. The isothermal data revealed that the recirculation ratio is mainly a function of the geometry, with a minor dependence on the experimental conditions. Under reacting conditions, the data revealed that NO x emissions are low regardless of the combustor operating conditions and geometry. However, the air inlet configuration has a strong effect in combustor efficiency.

A distributed system for fast timing and event management on the mast experiment

This paper describes an expandable and distributed system that produces the timing signals for the correct operation of the MAST diagnostic and data acquisition systems and performs the broadcast, processing and recording of the occurrence time of externally generated events for real-time control purposes. The hardware will be implemented using the VME and CAMAC standards. The software control interface will be incorporated in the MAST control and data acquisition system, allowing for an easy database access of the system timing parameters.

The 32 bit timing unit of a real-time event-based control system for a nuclear fusion experiment

This paper describes the innovative timing unit of a distributed, expandable, real-time, event-based control system for a nuclear fusion experiment. This system is being designed in a tree-type topology, based in VME and CAMAC format modules that produce the timing signals required for the safe operation of the experiment diagnostics and digitizers. It also performs the broadcasting, processing and recording of the occurrence of externally generated events for real-time control purposes. The timing unit has been implemented in a field programmable gate array, providing flexibility and the ability to upgrade the design. The timing signals can be defined with 32 bit resolution to produce signals from 100 ns width and to about 430 s maximum duration with 500 ns accuracy, and will provide the possibility of immediate event processing.