G.T. Roberts

Compressibility effects on boundary layer transition induced by an isolated roughness element

Direct numerical simulation has been used to study the transition to turbulence of a high-speed boundary layer over a flat plate with an isolated roughness element. The roughness element consists of a continuous bump that is approximately half the boundary-layer thickness in height. Simulations at Mach 3 and Mach 6 have been run over a range of Reynolds numbers for two fixed wall temperatures corresponding to either an adiabatic or a cooled wall condition. In each case a lift-up of low momentum fluid away from the wall is observed leading to the formation of a detached shear layer. Acoustic disturbances are imposed to stimulate the instability of this layer and breakdown to turbulence is observed in the simulations at the highest Reynolds number in all cases except for the Mach 6 cold wall condition. The magnitude of the streamwise vorticity behind the roughness element increases with roughness height Reynolds number and is relatively insensitive to Mach number and wall temperature. Strouhal numbers associated with hairpin vortex formation are generally lower than in incompressible flow. The critical roughness height Reynolds number for transition within the computational domain is found to increase as the Mach number increases and is higher for the cooled wall condition. A correlation based on roughness height Reynolds and Mach numbers and wall temperature is found to separate laminar from transitional cases for the range of flow conditions studied.

Preliminary results of an atomic oxygen spaceflight experiment

An atomic oxygen (AO) experiment designed to measure the AO environment and the erosion rate of selected materials is currently flying on a microsatellite (STRV-la). The experiment comprised four bare silver resistance sensors and eight silver sensors with thin film overlays of polyethylene, PTFE, carbon and silica. Preliminary analysis of the results from the bare silver films shows good agreement between the measured AO environment with that of the theoretical ESABASE environmental estimates. The calculated erosion rate for PTFE is also in good agreement with that obtained by exposing an identical sensor in a laboratory simulation facility. As predicted, after 250 days into the flight, there is no change in the status of polyethylene, carbon and silica overlay sensors.

Reactions of spacecraft materials with fast atomic oxygen

Abstract Silver films with thicknesses in the range 600–3200 A were exposed to high velocity atomic oxygen in a laboratory facility at flux levels simulating those experienced by surfaces in the low Earth orbit. Reaction rates were calculated from the resistance changes as the conducting silver was converted to its non-conducting oxides. A two-stage reaction process was observed, consisting of a rapid linear rate to a depth of approximately 340 A followed by a slower, diffusion limited parabolic rate. The reaction rates of polyethylene, PTFE, carbon and silica were established by exposing sensors comprising thin film overlays of the test material on a 300 A silver substrate to atomic oxygen. The reaction rate of PTFE was found to be higher than found in orbit, whereas the rates for polyethylene, silica and carbon were similar to reported values.

Vortex shedding from a circular cylinder near a moving ground

The flow and force characteristics have been experimentally investigated of a circular cylinder with an aspect ratio of 8.33, with and without end-plates, placed near and parallel to a moving ground, on which substantially no boundary layer developed to interfere with the cylinder. Mean drag and lift measurements, surface oil flow visualization, and particle image velocimetry (PIV) measurements were carried out at two upper-subcritical Reynolds numbers of 0.4 and 1.0×105 (based on the cylinder diameter d) to investigate the mechanisms of the ground effect, i.e., the effect of the gap-to-diameter ratio h∕d, where h is the gap between the cylinder and the ground. For the cylinder with end-plates, on which the oil flow patterns were observed to be essentially two-dimensional, the drag rapidly decreased as h∕d decreased to less than 0.5 but became constant for h∕d of less than 0.35, unlike that usually observed near a fixed ground. This critical drag behavior was found to be directly related to a global chang...

The effect of Mach number on unstable disturbances in shock/boundary-layer interactions

The effect of Mach number on the growth of unstable disturbances in a boundary layer undergoing a strong interaction with an impinging oblique shock wave is studied by direct numerical simulation and linear stability theory (LST). To reduce the number of independent parameters, test cases are arranged so that both the interaction location Reynolds number (based on the distance from the plate leading edge to the shock impingement location for a corresponding inviscid flow) and the separation bubble length Reynolds number are held fixed. Small-amplitude disturbances are introduced via both white-noise and harmonic forcing and, after verification that the disturbances are convective in nature, linear growth rates are extracted from the simulations for comparison with parallel flow LST and solutions of the parabolized stability equations (PSE). At Mach 2.0, the oblique modes are dominant and consistent results are obtained from simulation and theory. At Mach 4.5 and Mach 6.85, the linear Navier-Stokes results show large reductions in disturbance energy at the point where the shock impinges on the top of the separated shear layer. The most unstable second mode has only weak growth over the bubble region, which instead shows significant growth of streamwise structures. The two higher Mach number cases are not well predicted by parallel flow LST, which gives frequencies and spanwise wave numbers that are significantly different from the simulations. The PSE approach leads to good qualitative predictions of the dominant frequency and wavenumber at Mach 2.0 and 4.5, but suffers from reduced accuracy in the region immediately after the shock impingement. Three-dimensional Navier-Stokes simulations are used to demonstrate that at finite amplitudes the flow structures undergo a nonlinear breakdown to turbulence. This breakdown is enhanced when the oblique-mode disturbances are supplemented with unstable Mack modes.

Development of hydrogen peroxide monopropellant rockets

Alta S.p.A. (Italy) and DELTACAT Ltd. (United Kingdom) are conducting a study, funded by the European Space Agency, into the development of hydrogen peroxide monopropellant thrusters using advanced catalytic beds. The present paper focuses on the design of two different demonstration thrusters with nominal ratings of 5 N and 25 N. Design requirements and specifications are presented, followed by the main results of a concept study, which was conducted to define the approximate dimensions needed. Some details about the specific design of the two prototypes and the choice of the main components are provided, with particular regard to the sensors and transducers to be used during the experimentation. Different catalytic bed configurations, including pure silver gauzes and pellets coated with manganese oxide or platinum, are going to be tested in the prototype thrusters, in order to find the optimum one for further industrial development. A dedicated test bench, designed and realized by Alta S.p.A. for tests on the thruster prototypes, is also illustrated.

Satellite and rocket-borne atomic oxygen sensor techniques

Neutral atomic oxygen (AO)—the dominant atmospheric species at typical low Earth orbit altitudes—is responsible for the erosion, or other degradation, of many satellite materials. Therefore, AO has become an important consideration for spacecraft designers and manufacturers. The study of AO is also of interest to atmospheric physicists because it is involved in many of the chemical reactions occurring naturally in the mesosphere and lower thermosphere. Both these groups rely on atmospheric models for computer-based simulation and prediction of atomic oxygen concentrations. Such models require, or are enhanced by, empirical input data—that is, actual measurements of AO number densities. A review is presented of the different measurement techniques that, to date, have been used on satellites and sounding rockets to perform AO studies. Rather than reporting results from every sensor application, this article takes a more general view of the experimental methods, using example devices to highlight their advan...

Initial results from ground-based testing of an atomic oxygen sensor designed for use in earth orbit

Many sensors have been applied to the problem of measuring neutral atomic oxygen fluxes in low Earth orbit. The techniques used to date tend to suffer from several key disadvantages, variously: large mass and power budgets, large size, high cost, the ability to make only one measurement and poor time resolution. In this article preliminary results from ground-based testing of a novel atomic oxygen sensor based on a semiconducting metal oxide are reported. Such sensors are simple and relatively cheap while also requiring small power and mass budgets and, most importantly, are reusable. The sensors have been used in laboratory experiments to investigate the axial variation of atomic oxygen flux in a pulsed laser atomic oxygen source; the results compare well with readings taken with a carbon-coated quartz crystal microbalance. A small instrument based on these sensors has been designed and built for application on the UK’s STRV-1c microsatellite.

A low cost microsatellite instrument for the in situ measurement of orbital atomic oxygen effects

A small, low-cost sensor for measuring orbital atomic oxygen fluxes and their corrosive effect on spacecraft materials has been developed. Its operation is based on the reaction between hyperthermal atomic oxygen and a thin silver film. The electrical conductivity of the silver oxides formed is negligible allowing the reaction rate to be monitored through resistance measurements of the film. Atomic oxygen effects on spacecraft coatings can be investigated by placing a sample above the silver film. The construction of the sensor emphasizes simple techniques and components to improve reliability and reduce costs, while its small size and mass will allow it to be used on a wide range of spacecraft, increasing flight opportunities. An instrument comprising 12 of the sensors has been tested in orbit on the Space Technology Research Vehicle (STRV-1a) microsatellite mission. Results from this experiment agree well with those from ground-based simulation facilities. The successful outcome of this demonstration fl...

Thin-film semiconductor sensors for hyperthermal oxygen atoms

In Earth orbit, atomic oxygen (AO) erodes and degrades many spacecraft materials. Several sensors have been employed to make in situ measurements of these oxygen atom fluxes, yet none has proved suitable for general application to microsatellites. We have investigated the potential of thin-film zinc oxide (ZnO) AO sensors. It is known that single crystal and thin-film semiconductor sensors may be used to measure fluxes of low-energy (thermal) oxygen atoms. This work extends the principle to the measurement of hyperthermal (high energy) oxygen atoms in a ground-based simulation facility. It is found that, upon exposure to AO, the conductance of the ZnO sensors decreases. Moreover, the rate of sensor conductance change is proportional to the flux of oxygen atoms. Two sensors, which were manufactured simultaneously, demonstrated very similar responses when exposed concurrently to hyperthermal AO. Further experiments showed that the sensors were not affected by fluxes of molecular oxygen, but were influenced by ultraviolet (UV) radiation. A ZnO film covered with silica — to prevent the action of AO — was used to examine the influence of UV, which was shown to cause a small, permanent change of the conductance of the semiconductor.