P. A. Jacobs

Adaptive slicing with sloping layer surfaces

Presents an adaptive slicing procedure for improving the geometric accuracy of layered manufacturing techniques which, unlike previous procedures, uses layers with sloping boundary surfaces that closely match the shape of the required surface. This greatly reduces the stair case effect which is characteristic of layered components with square edges. Considers two measures of error, and outlines a method of predicting these measures for sloping layer surfaces. To cater for different manufacturing requirements, presents a method to produce parts with either an inside or outside tolerance, or a combination of both. Finally, considers some problems associated with surface joins, vertices, and inflection points and proposes some solutions.

Rapid prototyping with sloping surfaces

TruSurf is a new system for building solid objects from layers with sloping surfaces that closely match the designed surface shape. The advantages of using sloping surfaces over stepped edges are improved surface finish, and decreased build time through the use of thicker layers. TruSurf uses B‐spline surfaces to describe the part, and calculate the sloped path of the layer cutting medium. Describes operation of the system in detail and presents results from the production of some test parts. Discusses some ways for improving accuracy, including using principal directions of minimum surface curvature, and using a curved cutting medium to produce layers.

Quasi-one-dimensional modeling of a free-piston shock tunnel

A Lagrangian formulation for the quasi-one-dimensional modeling of free-piston-driven shock tunnels is described. Three simulations of particular conditions for the T4 shock tunnel are then presented and compared with experimental measurements. The simulations provide very good estimates for both the shock speed and the nozzle-supply pressure obtained after shock reflection and also provide detailed information on the gasdynamic processes over the full length of the facility. This detailed information may be used to identify some of the causes for observed variations in nozzle-supply pressure.

Nonequilibrium radiation intensity measurements in simulated Titan atmospheres

This paper details the experimental work conducted at the University of Queensland to measure the nonequilibrium radiation intensity behind a shock in simulated Titan atmospheres, as would be seen during planetary entry. Radiation during Titan entry is more important at lower speeds (about 5-6 km/s) than other planetary entries due to the formation of cyanogen in above equilibrium concentrations in the shock layer, which is a highly radiative species. The experiments were focused on measuring the nonequilibrium radiation emitted from cyanogen between the wavelength range of 310-450 nm. This paper includes experimental results for radiation and spectra found in the postshock region of the flow. Experiments have been conducted at various ambient pressures, shock speeds, and chemical compositions. This leads to a comprehensive benchmark data set for Titan entry, which will be useful for validation of theoretical models. Spectra were recorded at various axial locations behind the shock, enabling the construction of radiation profiles for Titan entry. Furthermore, wavelength profiles can also be constructed to identify various radiating species, in this case, predominately cyanogen violet. Furthermore, this paper includes comparisons with experiments performed at NASA Ames Research Center on their electric arc-driven shock tube in Titan compositions. Excellent quantitative agreement has been obtained between the two facilities.

Numerical simulation of transient hypervelocity flow in an expansion tube

Abstract Several numerical simulations of the transient flow of helium is an expansion tube are presented in an effort to identify some of the basic mechanisms which cause the noisy test flows seen in experiments. The calculations were performed with an axisymmetric Navier-Stokes code based on a finite-volume formulation and upwinding techniques. Although laminar flow and ideal bursting of the diaphragms was assumed, the simulations showed some of the important features seen in the experiments. In particular, the discontinuity in the tube diameter at the primary-diaphragm station introduced a transverse pertubation to the expanding driver gas and this perturbation was seen to propagate into the test gas under some flow conditions. The disturbances seen in the test flow can be characterized as either small-amplitude, low-frequency noise, possibly introduced during shock-compression, or large-amplitude, high frequency noise, associated with the passage of the reflected-head of the unsteady expansion.

Experimental expansion tube study of the flow over a toroidal ballute

An experimental investigation of high-enthalpy flow over a toroidal ballute (balloon/parachute) was conducted in an expansion tube facility. The ballute, proposed for use in a number of future aerocapture missions, involves the deployment of a large toroidal-shaped inflatable parachute behind a space vehicle to generate drag on passing through a planetary atmosphere, thus, placing the spacecraft in orbit. A configuration consisting of a spherical spacecraft, followed by a toroid, was tested in a superorbital facility. Measurements at moderate-enthalpy conditions (15-20 MJ/kg) in nitrogen and carbon dioxide showed peak heat transfer rates of around 20 MW/m(2) on the toroid. At higher enthalpies (>50 MJ/kg) in nitrogen, carbon dioxide, and a hydrogen-neon mixture, heat transfer rates above 100 MW/m(2) were observed. Imaging using near-resonant holographic interferometry showed that the flows were steady except when the opening of the toroid was blocked.

Inviscid evolution of stretched vortex arrays

The nonlinear evolution of an array of pairs of inviscid counter-rotating vortices, subjected to an applied stretching strain field, has been studied numerically using the contour-dynamics method. The array configuration is effectively the Corcos-Lin model of streamwise vortices in the braid region of a nominally two-dimensional mixing layer. For each individual vortex the simulations elucidate the strong interaction between the vortex self-induction, the vorticity amplification of the stretching strain, and the local in-plane strain applied by all other members of the array. When the initial vorticity distribution is modelled by a non-uniform piece-wise-constant vorticity field defined over a nested set of non-intersecting contours, the dynamical evolution reveals fine structure consisting of strong vortex roll-up accompanied by trailing, filament-like spiral vortex sheets, and the presence of tertiary instabilities. It is shown by a particular example that these features are largely absent in an equivalent computation in which array members are modelled by the commonly used uniform-vortex approximation.

Expansion Tubes in Australia

Expansion tubes are hypersonic impulse facilities which are able to produce chemically correct, high enthalpy test flows. They are ideally suited to reproducing super-orbital flight through different atmospheres, and provide an increasingly important tool for aerothermodynamic studies of planetary entry vehicles. Furthermore, their uniquely high total pressure capability makes them the only class of facility currently able to reproduce the free-stream total pressures associated with high Mach number scramjet powered access-to-space. In 1987 The University of Queensland was the first research group to use a free-piston driver to power an expansion tube, and presently operates two such facilities, X2 and X3. While the free-piston driver can maximise the performance of the expansion tube, this mode of operation relies on complex flow processes, provides short test duration, and raises many other challenges in terms of test flow characterisation, instrumentation, and so forth. In the process of developing its own facilities, The University of Queensland has identified and addressed many of these challenges, and in X2 and X3 it has established high performance and reliable capabilities for routine ground testing at hypersonic flight conditions.

Application of genetic algorithms to hypersonic flight control

The paper presents an application of genetic algorithms to the design of a longitudinal flight controller for a hypersonic accelerator vehicle which is to be used to launch small satellites. A feature of hypersonic air-breathing flight vehicles is the high level of engine integration with the airframe. As a result, maintenance of vehicle attitude is not simply an issue of stability, but also one of propulsive effectiveness, which itself varies with flight conditions and the vehicle attitude. There is therefore limited scope for departure from optimum operating conditions. This, together with the extreme flight conditions, performance uncertainty, and the inherent instability of the vehicle, contributes to a demanding control task. We examine the capacity of a genetic algorithm in designing a fuzzy logic controller for the task of closed loop flight control. With a fixed, preset control structure, the design task is to configure the control surface through selection of the rule consequents and input scaling. The genetic algorithm uses a collection of simulated flight response in its formulation of the objective function. This allows the generation of a controller design without linearization of the vehicle model and dynamics. Stability augmentation is shown through flight simulation at the low-speed end of the hypersonic trajectory and also at a higher flight speed.

Approximate Riemann solver for hypervelocity flows

We describe an approximate Riemann solver for the computation of hypervelocity flows in which there are strong shocks and viscous interactions. The scheme has three stages, the first of which computes the intermediate states assuming isentropic waves. A second stage, based on the strong shock relations, may then be invoked if the pressure jump across either wave is large. The third stage interpolates the interface state from the two initial states and the intermediate states. The solver is used as part of a finite-volume code and is demonstrated on two test cases. The first is a high Mach number flow over a sphere while the second is a flow over a flow over a slender cone with an adiabatic boundary layer. In both cases the solver performs well.