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This report is intended to reveal the apparent relationship of air seal leakage rates between 2 atmospheres (atm) to 1 atm and 1 atm to vacuum conditions. Gas dynamics analysis is provided as well as data summarizing the MSFC test report, 'Space Station Freedom (S.S. Freedom) Seal Flaw Study With Delta Pressure Leak Rate Comparison Test Report'.
New diagnostic techniques for measuring the high pressure flow fields associated with high velocity ram accelerator propulsive modes was experimentally investigated. Individual propulsive modes are distinguished by their operating Mach number range and the manner in which the combustion process is initiated and stabilized. Operation of the thermally choked ram accelerator mode begins by injecting the projectile into the accelerator tube at a prescribed entrance velocity by means of a conventional light gas gun. A specially designed obturator, which is used to seal the bore of the gun, plays a key role in the ignition of the propellant gases in the subsonic combustion mode of the ram accelerator. Once ignited, the combustion process travels with the projectile and releases enough heat to thermally choke the flow within several tube diameters behind it, thereby stabilizing a high pressure zone on the rear of the projectile. When the accelerating projectile approaches the Chapman-Jouguet detonation speed of the propellant mixture, the combustion region is observed to move up onto the afterbody of the projectile as the pressure field evolves to a distinctively different form that implies the presence of supersonic combustion processes. Eventually, a high enough Mach number is reached that the ram effect is sufficient to cause the combustion process to occur entirely on the body. Propulsive cycles utilizing on-body heat release can be established either by continuously accelerating the projectile in a single propellant mixture from low initial in-tube Mach numbers (M less than 4) or by injecting the projectile at a speed above the propellant's Chapman-Jouguet detonation speed. The results of experimental and theoretical explorations of ram accelerator gas dynamic phenomena and the effectiveness of the new diagnostic techniques are presented in this report.
The 8-Foot High Temperature Tunnel (HTT) at NASA Langley Research Center is a combustion-driven blow-down wind tunnel. A major potential failure mode that was considered during the combustor redesign was the possibility of a deflagration and/or detonation in the combustor. If a main burner flame-out were to occur, then unburned fuel gases could accumulate and, if reignited, an explosion could occur. An analysis has been performed to determine the safe operating limits of the combustor under transient explosive loads. The failure criteria was defined and the failure mechanisms were determined for both peak pressures and differential pressure loadings. An overview of the gas dynamics analysis was given. A finite element model was constructed to evaluate 13 transient load cases. The sensitivity of the structure to the frequency content of the transient loading was assessed. In addition, two closed form dynamic analyses were conducted to verify the finite element analysis. It was determined that the differential pressure load or thrust load was the critical load mechanism and that the nozzle is the weak link in the combustor system.
The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results will be compared with observations where possible and with the results from previous analytic and numerical studies. The primary progress during the first two years has been to develop codes for 2-D models in both spherical and cylindrical coordinates and to apply them to the solar wind-interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. Work has also been initiated on the development of a 3-D MHD code in spherical coordinates.
The impact of passage rotation on the gas dynamic wave processes is analyzed through a numerical simulation of ideal shock-tube flow in a closed rotating-channel. Initial conditions are prescribed by assuming homentropic solid-body rotation. Relevant parameters of the problem such as wheel Mach number, hub-to-tip radius ratio, length-to-tip radius ratio, diaphragm temperature ratio, and diaphragm pressure ratio are varied. The results suggest possible criteria for assessing the consequences of passage rotation on the wave processes, and they may therefore be applicable to pressure-exchange wave rotors. It is shown that for a fixed geometry and initial conditions, the contact interface acquires a distorted three-dimensional time-dependent orientation at non-zero wheel Mach numbers. At a fixed wheel Mach number, the level of distortion depends primarily on the density ratio across the interface as well as the hub-to-tip radius ratio. Rarefaction fronts, shocks, and contact interfaces are observed to propagate faster with increasing wheel Mach number.
Uniform high order spectral methods to solve multi-dimensional Euler equations for gas dynamics are discussed. Uniform high order spectral approximations with spectral accuracy in smooth regions of solutions are constructed by introducing the idea of the Essentially Non-Oscillatory (ENO) polynomial interpolations into the spectral methods. The authors present numerical results for the inviscid Burgers' equation, and for the one-dimensional Euler equations including the interactions between a shock wave and density disturbance, Sod's and Lax's shock tube problems, and the blast wave problem. The interaction between a Mach 3 two-dimensional shock wave and a rotating vortex is simulated.
Oxygen ion production rates above the ionopauses of Venus and Mars are calculated for photoionization, charge exchange, and solar wind electron impact ionization processes. The latter two require the use of the Spreiter and Stahara (1980) gas dynamic model to estimate magnetosheath velocities, densities, and temperatures. The results indicate that impact ionization is the dominant mechanism for the production of O(+) ions at both Venus and Mars. This finding might explain both the high ion escape rates measured by Phobos 2 and the greater mass loading rate inferred for Venus from the bow shock positions.
Recent advances in the development of a grid-independent finite volume scheme for the Euler equations of gas dynamics are described. In the proposed method, flowfield gradient data are reconstructed locally (on a triangle) using five elementary planar waves, and an upwind numerical flux function for grid-oblique waves is developed to model the effect of the passage of these waves on the data in a cell. Numerical examples for several two-dimensional test problems are included. These results show high wave resolution and nearly monotone strong-wave transitions.
We compare ab initio calculations of semiforbidden C II line profiles near 2325 A with recently published observations of the inactive red giant Alpha Tau (K5 III) obtained using the GHRS on board the Hubble Space Telescope. Our one-dimensional, time-dependent calculations assume that the chromosphere is heated by stochastic acoustic shocks generated by photospheric convection. We calculate various models using results from traditional (mixing length) convection zone calculations as input to hydrodynamical models. The semiforbidden C II line profiles and ratios provide sensitive diagnostics of chromospheric velocity fields, electron densities, and temperatures. We identify major differences between observed and computed line profiles which are related to basic gas dynamics and which are probably not due to technical modeling restrictions. If the GHRS observations are representative of chromospheric conditions at all epochs, then one (or more) of our model assumptions must be incorrect. Several possibilities are examined. We predict time variability of semiforbidden C II lines for comparison with observations. Based upon data from the IUE archives, we argue that photospheric motions associated with supergranulation or global pulsation modes are unimportant in heating the chromosphere of Alpha Tau.
A new spectral nonoscillatory interpolation scheme is proposed that achieves spectral accuracy in smooth regions and is nonoscillatory on piecewise discontinuous data. The essential idea behind the scheme is to increase the order of an ENO scheme in proportion to the number of points, wherever possible. Numerical experiments with the new scheme on interpolation, 1D advection, and 1D gas dynamics confirm the high resolution features of the scheme. Comparisons with the results of earlier spectral nonoscillatory schemes show that the new scheme is competitive both in efficiency and accuracy.
The impact of passage rotation on the gas dynamic wave processes is analyzed through a numerical simulation of ideal shock-tube flow in a closed rotating-channel. Initial conditions are prescribed by assuming homentropic solid-body rotation. Relevant parameters of the problem such as wheel Mach number, hub-to-tip radius ratio, length-to-tip radius ratio, diaphragm temperature ratio, and diaphragm pressure ratio are varied. The results suggest possible criteria for assessing the consequences of passage rotation on the wave processes, and they may therefore be applicable to pressure-exchange wave rotors. It is shown that for a fixed geometry and initial conditions, the contact interface acquires a distorted three-dimensional time-dependent orientation at non-zero wheel Mach numbers. At a fixed wheel Mach number, the level of distortion depends primarily on the density ratio across the interface as well as the hub-to-tip radius ratio. Rarefaction fronts, shocks, and contact interfaces are observed to propagate faster with increasing wheel Mach number.
Gas dynamic quantities within an underexpanded nitrogen free jet, seeded with 0.5 percent NO, were measured nonintrusively by using an intracavity-doubled, rapid-tuning, CW ring dye laser. The UV beam passed obliquely through the jet axis, and its frequency repetitively scanned across adjacent rotational lines in the NO gamma band near 225 nm at a rate of 4 kHz. Spatially resolved excitation scans were obtained by monitoring the induced broadband fluoresence. Modeling the Doppler-shifted excitation scans with Voigt profiles permitted simultaneous determinations of NO velocity, rotational temperature, and pressure. Zero Doppler shift was referenced to an absorption trace obtained across a static cell and recorded concurrently with the excitation scan. Typically, the measured and predicted axial distributions agreed within 10 percent. At high Mach numbers there was evidence of rotational freezing of NO.
An experimental investigation in the Langley Gas Dynamics Laboratory has been made of the flow separation from slender rods mounted to extend forward along the axes of symmetry of blunt-nosed bodies. The mechanism of the separation and its governing criteria are discussed. Drag data at a Mach number of 2.72 for various rod lengths and nose radii at an angle of attack of zero degrees are presented. It was found that the drag coefficients of blunt noses could be appreciably reduced by the use of protruding rods. Criteria for rod length which gives lowest drag are given. (author)
The work performed in fiscal year (FY) 1991 under the Propulsion Technology Program RTOP (Research and Technology Objectives and Plans) No. (55) 506-42-31 for Low-Thrust Primary and Auxiliary Propulsion technology development is described. The objectives of this work fall under two broad categories. The first of these deals with the development of ion engines for primary propulsion in support of solar system exploration. The second with the advancement of steady-state magnetoplasmadynamic (MPD) thruster technology at 100 kW to multimegawatt input power levels. The major technology issues for ion propulsion are demonstration of adequate engine life at the 5 to 10 kW power level and scaling ion engines to power levels of tens to hundreds of kilowatts. Tests of a new technique in which the decelerator grid of a three-grid ion accelerator system is biased negative of neutralizer common potential in order to collect facility induced charge-exchange ions are described. These tests indicate that this SAND (Screen, Accelerator, Negative Decelerator) configuration may enable long duration ion engine endurance tests to be performed at vacuum chamber pressures an order of magnitude higher than previously possible. The corresponding reduction in pumping speed requirements enables endurance tests of 10 kW class ion engines to be performed within the resources of existing technology programs. The results of a successful 5,000-hr endurance of a xenon hollow cathode operating at an emission current of 25 A are described, as well as the initial tests of hollow cathodes operating on a mixture of argon and 3 percent nitrogen. Work performed on the development of carbon/carbon grids, a multi-orifice hollow cathode, and discharge chamber erosion reduction through the addition of nitrogen are also described. Critical applied-field MPD thruster technical issues remain to be resolved, including demonstration of reliable steady-state operation at input powers of hundreds to thousands of kilowatts, achievement of thruster efficiency and specific impulse levels required for missions of interest, and demonstration of adequate engine life at these input power, efficiency, and specific impulse levels. To address these issues we have designed, built, and tested a 100 kW class, radiation-cooled applied-field MPD thruster and a unique dual-beam thrust stand that enables separate measurements of the applied- and self-field thrust components. We have also initiated the development of cathode thermal and plasma sheath models that will eventually be used to guide the experimental program. In conjunction with the cathode modeling, a new cathode test facility is being constructed. This facility will support the study of cathode thermal behavior and erosion mechanisms, the diagnosis of the near-cathode plasma and the development and endurance testing of new, high-current cathode designs. To facilitate understanding of electrode surface phenomenon, we have implemented a telephoto technique to obtain photographs of the electrodes during engine operation. In order to reduce the background vacuum tank pressure during steady-state engine operation in order to obtain high fidelity anode thermal data, we have developed and are evaluating a gas-dynamic diffuser. A review of experience with alkali metal propellants for MPD thrusters led to the conclusion that alkali metals, particularly lithium, offer the potential for significant engine performance and lifetime improvements. These propellants are also condensible at room temperature, substantially reducing test facility pumping requirements. The most significant systems-level issue is the potential for spacecraft contamination. Subsequent experimental and theoretical efforts should be directed toward verifying the performance and lifetime gains and characterizing the thruster flow field to assess its impact on spacecraft surfaces. Consequently, we have begun the design and development of a new facility to study engine operation with alkali metal propellants.
A new class of piecewise linear methods for the numerical solution of the one-dimensional Euler equations of gas dynamics is presented. These methods are uniformly second-order accurate, and can be considered as extensions of Godunov's scheme. With an appropriate definition of monotonicity preservation for the case of linear convection, it can be shown that they preserve monotonicity. Similar to Van Leer's MUSCL scheme, they consist of two key steps: a reconstruction step followed by an upwind step. For the reconstruction step, a monotonicity constraint that preserves uniform second-order accuracy is introduced. Computational efficiency is enhanced by devising a criterion that detects the 'smooth' part of the data where the constraint is redundant. The concept and coding of the constraint are simplified by the use of the median function. A slope steepening technique, which has no effect at smooth regions and can resolve a contact discontinuity in four cells, is described. As for the upwind step, existing and new methods are applied in a manner slightly different from those in the literature. These methods are derived by approximating the Euler equations via linearization and diagonalization. At a 'smooth' interface, Harten, Lax, and Van Leer's one intermediate state model is employed. A modification for this model that can resolve contact discontinuities is presented. Near a discontinuity, either this modified model or a more accurate one, namely, Roe's flux-difference splitting. is used. The current presentation of Roe's method, via the conceptually simple flux-vector splitting, not only establishes a connection between the two splittings, but also leads to an admissibility correction with no conditional statement, and an efficient approximation to Osher's approximate Riemann solver. These reconstruction and upwind steps result in schemes that are uniformly second-order accurate and economical at smooth regions, and yield high resolution at discontinuities.
Positivity preserving property of first and higher order finite volume schemes for one and two dimensional compressible Euler equations of gas dynamics is considered. A general framework is established which shows the positivity of density and pressure whenever the underlying one dimensional first order building block based on exact or approximate Riemann solver and the reconstruction are both positivity preserving. Appropriate limitation to achieve high order positivity preserving reconstruction is described.
Collisions of cosmic bodies with terrestrial planets involve many physical processes such as deceleration and ablation during their flight through an atmosphere, the impact at a surface accompanied by cratering, melting and evaporation of surface material, generation of shock waves, etc. If body velocity is high enough then a thermal radiation is very important. All these processes on Mars proceed differently than on the other planets because of the low density of its atmosphere. In particular, this leads to the fact that smaller bodies of sizes of the order of 0.1-10 m strike the planet surface without being decelerated and perform some effects which may be detected by equipment placed on a board of artificial satellites, by a network of stations at the surface of Mars and even from the Earth. These observations can be used to determine size-velocity distribution of such bodies in the Solar System. Numerical simulation of the impacts at the surface of Mars have been carried out using two-dimensional gas dynamic code with detailed consideration of the thermal radiative transfer. This work is an extension of our previous paper. We have expanded a range of projectile sizes up to r sub 0 = 100 m. For such a large-scale body, the initial stage of the impact, involving crate ring and ejection of surface material, is very important. Thus, these effects have been taken into account.
We have devised a technique that makes it possible to obtain energy estimates for initial-boundary value problems for nonlinear conservation laws. The two major tools to achieve the energy estimates are a certain splitting of the flux vector derivative f(u)(sub x), and a structural hypothesis, referred to as a cone condition, on the flux vector f(u). These hypotheses are fulfilled for many equations that occur in practice, such as the Euler equations of gas dynamics. It should be noted that the energy estimates are obtained without any assumptions on the gradient of the solution u. The results extend to weak solutions that are obtained as point wise limits of vanishing viscosity solutions. As a byproduct we obtain explicit expressions for the entropy function and the entropy flux of symmetrizable systems of conservation laws. Under certain circumstances the proposed technique can be applied repeatedly so as to yield estimates in the maximum norm.