Comparison of free-flight experimental results with theory on the nonlinear aerodynamic effects of bluntness for slender cones at Mach number 17
Aerodynamic effects of bluntness on slender cones in free flight tests at Mach 17
Malcolm, G. N.↗
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A Whitham-Theory Sonic-Boom Analysis of the TU-144 Aircraft at a Mach Number of 2.2
NASA became interested in the Tu- 144 aircraft when it was offered for use as a "flying testbed" to study operating characteristics of a supersonic-cruise commercial airplane. In addition to the other operational tests being conducted on the Tu- 144 aircraft, it was proposed that two sets of sonic-boom pressure signature measurements be made. One set of tests would consist of ground-level pressure measurements. The second set would be measurements of pressures under the Tu- 144 aircraft in cruise. Pressure gages would be required for this second set of tests: measuring ambient and differential pressures in the flow field of a Tu-144 cruising at supersonic speeds. The range of these gages had to be determined well in advance of the scheduled flights so that they could be installed and calibrated.
Robert J Mack↗
Nonlinear effects on sound propagation through high subsonic Mach number flows in variable area ducts
A nonlinear theory for sound propagation in variable area ducts carrying a nearly sonic flow is presented. Linear acoustic theory is shown to be singular and the detailed nature of the singularity is used to develop the correct nonlinear theory. The theory is based on a quasi-one dimensional model. It is derived by the method of matched asymptotic expansions. In a nearly chocked flow, the theory indicates the following processes to be acting: a transonic trapping of upstream propagating sound causing an intensification of this sound in the throat region of the duct; generation of superharmonics and an acoustic streaming effect; development of shocks in the acoustic quantities near the throat. Several specific problems are solved analytically and numerical parameter studies are carried out. Results indicate that appreciable acoustic power is shifted to higher harmonics as shocked conditions are approached. The effect of the throat Mach number on the attenuation of upstream propagating sound excited by a fixed source is also determined.
Callegari, A. J.↗
Determination of Shapes of Boattail Bodies of Revolution for Minimum Wave Drag
By use of an approximate equation for the wave drag of slender bodies of revolution in a supersonic flow field, the optimum shapes of certain boattail bodies are determined for minimum wave drag. The properties of three specific families of bodies are determined, the first family consisting of bodies having a given length and base area and a contour passing through a prescribed point between the nose and base, the second family having fixed length, base area, and maximum area, and the third family having given length, volume, and base area. The method presented is easily generalized to determine minimum-wave-drag profile shapes which have contours that must pass through any prescribed number of points. According to linearized theory, the optimum profiles are found to have infinite slope at the nose but zero radius of curvature so that the bodies appear to have pointed noses, a zero slope at the body base, and no variation of wave drag with Mach number. For those bodies having a specified intermediate.diameter (that is, location and magnitude given), the maximum body diameter is shown to be larger, in general, than the specified diameter. It is also shown that, for bodies having a specified maximum diameter, the location of the maximum diameter is not arbitrary but is determined from the ratio of base diameter to maximum diameter.
Adams, Mac C.↗
Digital signal processing and control and estimation theory -- Points of tangency, area of intersection, and parallel directions
A number of current research directions in the fields of digital signal processing and modern control and estimation theory were studied. Topics such as stability theory, linear prediction and parameter identification, system analysis and implementation, two-dimensional filtering, decentralized control and estimation, image processing, and nonlinear system theory were examined in order to uncover some of the basic similarities and differences in the goals, techniques, and philosophy of the two disciplines. An extensive bibliography is included.
Willsky, A. S.↗
Method for calculating effects of a propfan on aircraft aerodynamics at subsonic speeds
A subsonic-flow panel code has been modified to handle the effects of a propeller wake. The effects of the propeller were modelled by a system of ring vortices of constant strength. Principles based on the blade element theory and the momentum theory were used to evaluate the swirl velocity and the pressure increase, across the propeller. Theoretical calculations are compared to experimental results at a Mach number of 0.50. The discrepancies between the theory and the experimental results are analysed. Suggestions for improvements to enhance the accuracy of the theoretical prediction are indicated.
Chandrasekaran, B.↗
An Integrated Approach to Damage Accommodation in Flight Control
In this paper we present an integrated approach to in-flight damage accommodation in flight control. The approach is based on Multiple Models, Switching and Tuning (MMST), and consists of three steps: In the first step the main objective is to acquire a realistic aircraft damage model. Modeling of in-flight damage is a highly complex problem since there is a large number of issues that need to be addressed. One of the most important one is that there is strong coupling between structural dynamics, aerodynamics, and flight control. These effects cannot be studied separately due to this coupling. Once a realistic damage model is available, in the second step a large number of models corresponding to different damage cases are generated. One possibility is to generate many linear models and interpolate between them to cover a large portion of the flight envelope. Once these models have been generated, we will implement a recently developed-Model Set Reduction (MSR) technique. The technique is based on parameterizing damage in terms of uncertain parameters, and uses concepts from robust control theory to arrive at a small number of "centered" models such that the controllers corresponding to these models assure desired stability and robustness properties over a subset in the parametric space. By devising a suitable model placement strategy, the entire parametric set is covered with a relatively small number of models and controllers. The third step consists of designing a Multiple Models, Switching and Tuning (MMST) strategy for estimating the current operating regime (damage case) of the aircraft, and switching to the corresponding controller to achieve effective damage accommodation and the desired performance. In the paper present a comprehensive approach to damage accommodation using Model Set Design,MMST, and Variable Structure compensation for coupling nonlinearities. The approach was evaluated on a model of F/A-18 aircraft dynamics under control effector damage, augmented by nonlinear cross-coupling terms and a structural dynamics model. The proposed approach achieved excellent performance under severe damage effects.
Boskovic, Jovan D.↗
Remarks on disputed numerical results in compressible boundary-layer stability theory
Past work on the influence of Mach number on the viscous and inviscid instability of flat-plate boundary layers is reviewed, and new spatial calculations are presented. These calculations support the previous view that viscosity is only stabilizing for both two- and three-dimensional first-mode waves above M1 = 3.0, and for second-mode waves at all Mach numbers. It is concluded that the calculations of Wazzan, Taghavi, and Keltner that show viscous instability at M1 = 6.0 for first-mode 50 deg waves, and at M1 = 3.0 for two-dimensional second-mode waves, are not correct.
Mack, L. M.↗
Experimental tests of FTE theories
A brief overview is presented of a number of qualitative geometrical models as well as simulation models of flux transfer events (FTEs) and an attempt is made to identify critical observational tests that would help discriminate between these models. These tests are concerned with flux tube orientation, speed and structure. Available single and multiple spacecraft observations are discussed in the context of these tests. Particular attention is given to the problem of determining flux-tube orientation and speed from a single spacecraft.
Sonnerup, B. U. O.↗
Scanning electron acoustic microscopy of residual stresses in ceramics: Theory and experiment
Several reviews have highlighted a number of applications of scanning electron acoustic microscopy (SEAM) to metals and semiconductors which show that SEAM can provide new information on surface and near-surface features of such materials, but there have been few studies attempting to determine the capabilities of SEAM for characterizing ceramic materials. We have recently observed image contrast in SEAM from residual stress fields induced in brittle materials by Vickers indentations that is strongly dependent on the electron beam chopping frequency. We have also recently developed a three-dimensional mathematical model of signal generation and contrast in SEAM, appropriate to the brittle materials studied, that we use as a starting point in this paper for modeling the effect of residual stress fields on the generated electron acoustic signal. The influence of the electron beam chopping frequency is also considered under restrictive assumptions.
Cantrell, John H.↗
Conductance of Ion Channels - Theory vs. Experiment
Transmembrane ion channels mediate a number of essential physiological processes in a cell ranging from regulating osmotic pressure to transmission of neural signals. Kinetics and selectivity of ion transport is of critical importance to a cell and, not surprisingly, it is a subject of numerous experimental and theoretical studies. In this presentation we will analyze in detail computer simulations of two simple channels from fungi - antiamoebin and trichotoxin. Each of these channels is made of an alpha-helical bundle of small, nongenomically synthesized peptides containing a number of rare amino acids and exhibits strong antimicrobial activity. We will focus on calculating ionic conductance defined as the ratio of ionic current through the channel to applied voltage. From molecular dynamics simulations, conductance can be calculated in at least two ways, each involving different approximations. Specifically, the current, given as the number of charges transferred through the channel per unit of time, can be obtained from the number of events in which ions cross the channel during the simulation. This method works well for large currents (high conductance values and/or applied voltages). If the number of crossing events is small, reliable estimates of current are difficult to achieve. Alternatively, conductance can be estimated assuming that ion transport can be well approximated as diffusion in the external potential given by the free energy profile. Then, the current can be calculated by solving the one-dimensional diffusion equation in this external potential and applied voltage (the generalized Nernst-Planck equation). To do so three ingredients are needed: the free energy profile, the position-dependent diffusion coefficient and the diffusive flux of ions into the channel. All these quantities can be obtained from molecular dynamics simulations. An important advantage of this method is that it can be used equally well to estimating large and small currents. In addition, once the free energy profile becomes available the full current-voltage dependence can be readily obtained. For both channels we carried out calculations using both approaches. We also tested the main assumptions underlying the diffusive model, such as uncorrelated nature of individual crossing events and Fickian diffusion. The accuracy and consistency of different methods will be discussed. Finally we will discuss how comparisons between calculated and measured ionic conductance and selectivity of transport can be used for determining structural models of the channels.
Pohorille, Andrew↗
Studies of leading-edge thrust phenomena
A study of practical limitations on achievement of theoretical leading-edge thrust has been made and an empirical method for estimation of attainable thrust has been developed. The method is based on a theoretical analysis of a set of two-dimensional airfoils to define thrust dependence on airfoil geometric characteristics and arbitrarily defined limiting pressures, an examination of two-dimensional airfoil experimental data to provide an estimate of limiting pressure dependence on local Mach number and Reynolds number, and employment of simple sweep theory to adapt the method to three-dimensional wings. Because the method takes into account the spanwise variation of airfoil section characteristics, an opportunity is afforded for design by iteration to maximize the attainable thrust and the attendant performance benefits. The applicability of the method was demonstrated by comparisons of theoretical and experimental aerodynamic characteristics for a series of wing-body configurations. Generally, good predictions of the attainable thrust and its influence on lift and drag characteristics were obtained over a range of Mach numbers from 0.24 to 2.0.
Carlson, H. W.↗
On the inversion of block tridiagonals without storage constraints
A strategy was developed to permit trade-offs between the number of floating point operations required and the storage requirements for the solution of certain difference problems, such as block tridiagonal systems of equations. This is done by recomputing some intermediate results instead of storing them. Reducing the storage to the square root of the current requirement roughly doubles the number of computations. Reducing the storage more than this tends to make the number of computations prohibitively large. In theory, if m is the order of each sub-matrix in the block tridiagonal matrix, one can solve any linear system with only 5m(2) + 1 temporary storage cells. In many cases m is a constant and quite small. For example, in solving a factored form of the three-dimensional Navier-Stokes equations, the size m of the block tridiagonals is 5. This method lends itself to efficient use on computers with parallel processing or vector processing architectures. On these computers the larger number of floating point operations is more than offset by the decrease in I/O and the increased percentage of vector operations made possible by this algorithm.
Merriam, M. L.↗
The Microstructure of Turbulent Flow
In 1941 a general theory of locally isotropic turbulence was proposed by Kolmogoroff which permitted the prediction of a number of laws of turbulent flow for large Reynolds numbers. The most important of these laws, the dependence of the mean square of the difference in velocities at two points on their distance and the dependence of the coefficient of turbulence diffusion on the scale of the phenomenon, were obtained by both Kolmogoroff and Obukhoff in the same year. At the present time these laws have been experimentally confirmed by direct measurements carried out in aerodynamic wind tunnels in the laboratory, in the atmosphere, and also on the ocean. In recent years in the Laboratory of Atmospheric Turbulence of the Geophysics Institute of the Soviet Academy of Sciences, a number of investigations have been conducted in which this theory was further developed. The results of several of these investigations are presented.
TURBULENCE↗
Influence of Buoyant Convection on the Stability of Enclosed Laminar Flames
An investigation of the stability limits of Enclosed Laminar Flames (ELF) was conducted in the Middeck Glovebox (MGBX) facility on the STS-87 Space Shuttle mission (November to December 1997). The primary objective of the ELF glovebox investigation is to determine the effect of buoyancy on the stability of round, laminar, gas-jet diffusion flames in a co-flow air duct. Comparison tests were conducted in normal gravity to allow isolation and identification of the influence of buoyancy. The results were used to map the lift-off and blow-out stability limits as a function of the fuel and air velocities for the two buoyancy conditions. Approximately 50 tests were conducted during the Space Shuttle mission, using a 50/50 mixture (volume basis) of methane and nitrogen as the fuel. The experimental results verified the hypothesis that substantially greater velocities are required to destabilize the flame in microgravity. The increase in air velocity required to induce lift off in microgravity (compared to normal gravity) was nearly equal to the increase required to induce blow out. Furthermore, the air velocity increase was relatively independent of the fuel flow, except at low fuel flows. At high fuel flows, it was found that the microgravity flames tend to immediately blow out after lift off. This is in agreement with the free-jet theory which suggests that stable lifted flames are not possible for fuels with a Schmidt number of 0.5<Sc<l, such as the diluted methane used in this study. However, stable lifted flames were observed in microgravity at low fuel flows, and in normal gravity, despite the fuel's low Schmidt number. The discrepancy between the experimental data and the theory of Chung and Lee is presumably due to the differences in the flow condition; namely, the ducted flow versus a free jet, and a non-similar velocity profile versus a self-similar profile.
Brooker, John E.↗
New trends in cosmology
A review of big-bang cosmology is presented, emphasizing the big-bang model, hypotheses on the origin of galaxies, observational tests of the big-bang model that may be possible with the Large Space Telescope, and the scale-covariant theory of gravitation. Detailed attention is given to the equations of general relativity, the redshift-distance relation for extragalactic objects, expansion of the universe, the initial singularity, the discovery of the 3-K blackbody radiation, and measurements of the amount of deuterium in the universe. The curvature of the expanding universe is examined along with the magnitude-redshift relation for quasars and galaxies. Several models for the origin of galaxies are evaluated, and it is suggested that a model of galaxy formation via the formation of black holes is consistent with the model of an expanding universe. Scale covariance is discussed, a scale-covariant theory is developed which contains invariance under scale transformation, and it is shown that Dirac's (1937) large-numbers hypothesis finds a natural role in this theory by relating the atomic and Einstein units.
Canuto, V. M.↗
Design and fabrication requirements for low-noise supersonic/hypersonic wind tunnels
Analyses of NASA Langley experimental results obtained from efforts to develop a low disturbance wind tunnel by means of linear stability theory have shown that the amplification of Gortler vortices on the concave walls of nozzles at Mach numbers from 3 to 5 are the cause of transition. The theory is used to design advanced nozzles for Mach numbers of 3.5 and 6 which can generate substantially longer quiet test regions. Transition on the nozzle walls is noted to be extremely sensitive to nozzle wall roughness and contamination.
Beckwith, I. E.↗
Rapid Distortion Analysis and Direct Simulation of Compressible Homogeneous Turbulence at Finite Mach Number
The effect of rapid mean compression on compressible turbulence at a range of turbulent Mach numbers is investigated. Rapid dist'ortion theory (RDT) and direct numerical simulation results for the case of axial (one-dimensional) compression are used to illustrate the existence of two distinct rapid compression regimes. These regimes - the nearly solenoidal and the 'pressure-released' - are defined by a single parameter involving the timescales of the mean distortion, the turbulence, and the speed of sound. A general RDT formulation is developed and is proposed as a means of improving turbulence models for compressible flows. In contrast to the well-documented observation that 'compressibility' (measured, for example, by the turbulent Mach number) is often associated with a decrease in the growth rate of turbulent kinetic energy, we find that under rapid distortion compressibility can produce an amplification of the kinetic energy growth rate. We also find that as the compressibility increases, the magnitude of the pressure-dilation correlation increases, in absolute terms, but its relative importance decreases compared to the magnitude of the kinetic energy production.
Cambon, C.↗