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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Analysis of Instabilities in Non-Axisymmetric Hypersonic Boundary Layers Over Cones

Hypersonic flows over circular cones constitute one of the most important generic configurations for fundamental aerodynamic and aerothermodynamic studies. In this paper, numerical computations are carried out for Mach 6 flows over a 7-degree half-angle cone with two different flow incidence angles and a compression cone with a large concave curvature. Instability wave and transition-related flow physics are investigated using a series of advanced stability methods ranging from conventional linear stability theory (LST) and a higher-fidelity linear and nonlinear parabolized stability equations (PSE), to the 2D eigenvalue analysis based on partial differential equations. Computed N factor distribution pertinent to various instability mechanisms over the cone surface provides initial assessments of possible transition fronts and a guide to corresponding disturbance characteristics such as frequency and azimuthal wave numbers. It is also shown that strong secondary instability that eventually leads to transition to turbulence can be simulated very efficiently using a combination of advanced stability methods described above.

Li, Fei↗
NASA NTRS1966

Liapunov's second method

Stability theorem based on Liapunov function, and using invariance property of limit sets of solutions to differential equations

THEOREM PROVING↗
NASA NTRS1978

Apparatus and method for stabilized phase detection for binary signal tracking loops

Apparatus and method is presented for phase detection in binary signal tracking loops wherein two bandpass detectors are alternately interchanged between electrical connection with two local code reference tracking signals in order to cancel any adverse effect of gain imbalance in the bandpass detectors and direct current offset or drift. The detectors are time shared in multiplex fashion between the two local reference signals.

Hopkins, P. M.↗
NASA NTRS2005

Computational Methods for Stability and Control (COMSAC): The Time Has Come

Powerful computational fluid dynamics (CFD) tools have emerged that appear to offer significant benefits as an adjunct to the experimental methods used by the stability and control community to predict aerodynamic parameters. The decreasing costs for and increasing availability of computing hours are making these applications increasingly viable as time goes on and the cost of computing continues to drop. This paper summarizes the efforts of four organizations to utilize high-end computational fluid dynamics (CFD) tools to address the challenges of the stability and control arena. General motivation and the backdrop for these efforts will be summarized as well as examples of current applications.

Hall, Robert M.↗
NASA NTRS1947

On the Application of the Energy Method to Stability Problems

Since stability problems have come into the field of vision of engineers, energy methods have proved to be one of the most powerful aids in mastering them. For finding the especially interesting critical loads special procedures have evolved that depart somewhat from those customary in the usual elasticity theory. A clarification of the connections seemed desirable,especially with regard to the post-critical region, for the treatment of which these special methods are not suited as they are. The present investigation discusses this question-complex (made important by shell construction in aircraft) especially in the classical example of the Euler strut, because in this case - since the basic features are not hidden by difficulties of a mathematical nature - the problem is especially clear. The present treatment differs from that appearing in the Z.f.a.M.M. (1938) under the title "Uber die Behandlung von Stabilittatsproblemen mit Hilfe der energetischen Methode" in that, in order to work out the basic ideas still more clearly,it dispenses with the investigation of behavior at large deflections and of the elastic foundation;in its place the present version gives an elaboration of the 6th section and (in its 7 th and 8th secs.)a new example that shows the applicability of the general criterion to a stability problem that differs from that of Euler in many respects.

Marguerre, Karl↗
NASA NTRS1977

Development of a transfer function method for dynamic stability measurement

Flutter testing method based on transfer function measurements is developed. The error statistics of several dynamic stability measurement methods are reviewed. It is shown that the transfer function measurement controls the error level by averaging the data and correlating the input and output. The method also gives a direct estimate of the error in the response measurement. An algorithm is developed for obtaining the natural frequency and damping ratio of low damped modes of the system, using integrals of the transfer function in the vicinity of a resonant peak. Guidelines are given for selecting the parameters in the transfer function measurement. Finally, the dynamic stability measurement technique is applied to data from a wind tunnel test of a proprotor and wing model.

Johnson, W.↗
NASA NTRS2017

Time Domain Stability Margin Assessment Method

The baseline stability margins for NASA's Space Launch System (SLS) launch vehicle were generated via the classical approach of linearizing the system equations of motion and determining the gain and phase margins from the resulting frequency domain model. To improve the fidelity of the classical methods, the linear frequency domain approach can be extended by replacing static, memoryless nonlinearities with describing functions. This technique, however, does not address the time varying nature of the dynamics of a launch vehicle in flight. An alternative technique for the evaluation of the stability of the nonlinear launch vehicle dynamics along its trajectory is to incrementally adjust the gain and/or time delay in the time domain simulation until the system exhibits unstable behavior. This technique has the added benefit of providing a direct comparison between the time domain and frequency domain tools in support of simulation validation.

Clements, Keith↗
NASA NTRS1971

Methods of obtaining stability derivatives

Various methods of obtaining aerodynamic derivatives are reviewed. These methods include wind-tunnel tests, aerodynamic theory, and extraction from flight tests, with the emphasis being on flight tests. Two techniques for extracting derivatives from flight are covered in sufficient detail to indicate the procedures involved. These methods are a strict least-squares approach and an iteration technique. References are listed for additional background and details of various techniques.

M. J. Queijo↗
NASA NTRS1988

Multigrid method for stability problems

The problem of calculating the stability of steady state solutions of differential equations is addressed. Leading eigenvalues of large matrices that arise from discretization are calculated, and an efficient multigrid method for solving these problems is presented. The resulting grid functions are used as initial approximations for appropriate eigenvalue problems. The method employs local relaxation on all levels together with a global change on the coarsest level only, which is designed to separate the different eigenfunctions as well as to update their corresponding eigenvalues. Coarsening is done using the FAS formulation in a nonstandard way in which the right-hand side of the coarse grid equations involves unknown parameters to be solved on the coarse grid. This leads to a new multigrid method for calculating the eigenvalues of symmetric problems. Numerical experiments with a model problem are presented which demonstrate the effectiveness of the method.

Ta'asan, Shlomo↗
NASA NTRS1988

Multigrid method for stability problems

The problem of calculating the stability of steady state solutions of differential equations is treated. Leading eigenvalues (i.e., having maximal real part) of large matrices that arise from discretization are to be calculated. An efficient multigrid method for solving these problems is presented. The method begins by obtaining an initial approximation for the dominant subspace on a coarse level using a damped Jacobi relaxation. This proceeds until enough accuracy for the dominant subspace has been obtained. The resulting grid functions are then used as an initial approximation for appropriate eigenvalue problems. These problems are being solved first on coarse levels, followed by refinement until a desired accuracy for the eigenvalues has been achieved. The method employs local relaxation on all levels together with a global change on the coarsest level only, which is designed to separate the different eigenfunctions as well as to update their corresponding eigenvalues. Coarsening is done using the FAS formulation in a non-standard way in which the right hand side of the coarse grid equations involves unknown parameters to be solved for on the coarse grid. This in particular leads to a new multigrid method for calculating the eigenvalues of symmetric problems. Numerical experiments with a model problem demonstrate the effectiveness of the method proposed. Using an FMG algorithm a solution to the level of discretization errors is obtained in just a few work units (less than 10), where a work unit is the work involved in one Jacobi relization on the finest level.

Taasan, Shlomo↗
NASA NTRS1995

Standard Methods for Bolt-Bearing Testing of Textile Composites

The response of three 2-D braided materials to bolt bearing loading was evaluated using data generated by Boeing Defense and Space Group in Philadelphia, PA. Three test methods, stabilized single shear, unstabilized single shear, and double shear, were compared. In general, these textile composites were found to be sensitive to bolt bearing test methods. The stabilized single shear method yielded higher strengths than the unstabilized single shear method in all cases. The double shear test method always produced the highest strengths but these results may be somewhat misleading. It is therefore recommended that standard material comparisons be made using the stabilized single shear test method. The effects of two geometric parameters, W/D and e/D, were also studied. An evaluation of the effect of the specimen width (W) to hole diameter (D) ratio concluded that bolt bearing responses were consistent with open hole tension results. A W/D ratio of 6 or greater should be maintained. The proximity of the hole to the specimen edge significantly affected strength. In all cases, strength was improved by increasing the ratio of the distance from the hole center to the specimen edge (e) to the hole diameter (D) above 2. An e/D ratio of 3 or greater is recommended.

Portanova, M. A.↗
NASA NTRS2004

COMSAC: Computational Methods for Stability and Control

The unprecedented advances being made in computational fluid dynamic (CFD) technology have demonstrated the powerful capabilities of codes in applications to civil and military aircraft. Used in conjunction with wind-tunnel and flight investigations, many codes are now routinely used by designers in diverse applications such as aerodynamic performance predictions and propulsion integration. Typically, these codes are most reliable for attached, steady, and predominantly turbulent flows. As a result of increasing reliability and confidence in CFD, wind-tunnel testing for some new configurations has been substantially reduced in key areas, such as wing trade studies for mission performance guarantees. Interest is now growing in the application of computational methods to other critical design challenges. One of the most important disciplinary elements for civil and military aircraft is prediction of stability and control characteristics. CFD offers the potential for significantly increasing the basic understanding, prediction, and control of flow phenomena associated with requirements for satisfactory aircraft handling characteristics.

Fremaux, C. Michael↗
NASA NTRS2011

Scatterometer-Calibrated Stability Verification Method

The requirement for scatterometer-combined transmit-receive gain variation knowledge is typically addressed by sampling a portion of the transmit signal, attenuating it with a known-stable attenuation, and coupling it into the receiver chain. This way, the gain variations of the transmit and receive chains are represented by this loop-back calibration signal, and can be subtracted from the received remote radar echo. Certain challenges are presented by this process, such as transmit and receive components that are outside of this loop-back path and are not included in this calibration, as well as the impracticality for measuring the transmit and receive chains stability and post fabrication separately, without the resulting measurement errors from the test set up exceeding the requirement for the flight instrument. To cover the RF stability design challenge, the portions of the scatterometer that are not calibrated by the loop-back, (e.g., attenuators, switches, diplexers, couplers, and coaxial cables) are tightly thermally controlled, and have been characterized over temperature to contribute less than 0.05 dB of calibration error over worst-case thermal variation. To address the verification challenge, including the components that are not calibrated by the loop-back, a stable fiber optic delay line (FODL) was used to delay the transmitted pulse, and to route it into the receiver. In this way, the internal loopback signal amplitude variations can be compared to the full transmit/receive external path, while the flight hardware is in the worst-case thermal environment. The practical delay for implementing the FODL is 100 s. The scatterometer pulse width is 1 ms so a test mode was incorporated early in the design phase to scale the 1 ms pulse at 100-Hz pulse repetition interval (PRI), by a factor of 18, to be a 55 s pulse with 556 s PRI. This scaling maintains the duty cycle, thus maintaining a representative thermal state for the RF components. The FODL consists of an RF-modulated fiber-optic transmitter, 20 km SMF- 28 standard single-mode fiber, and a photodetector. Thermoelectric cooling and insulating packaging are used to achieve high thermal stability of the FODL components. The chassis was insulated with 1-in. (.2.5-cm) thermal isolation foam. Nylon rods support the Micarta plate, onto which are mounted four 5-km fiber spool boxes. A copper plate heat sink was mounted on top of the fiber boxes (with thermal grease layer) and screwed onto the thermoelectric cooler plate. Another thermal isolation layer in the middle separates the fiberoptics chamber from the RF electronics components, which are also mounted on a copper plate that is screwed onto another thermoelectric cooler. The scatterometer subsystem fs overall stability was successfully verified to be calibratable to within 0.1 dB error in thermal vacuum (TVAC) testing with the fiber-optic delay line, while the scatterometer temperature was ramped from 10 to 30 C, which is a much larger temperature range than the worst-case expected seasonal variations.

McWatters, Dalia A.↗
NASA NTRS1970

Method of stabilizing flueric vortex valves and vortex amplifiers

Inducing losses in the vortex chamber of vortex valves and vortex amplifiers resolves the problem of unstable operation caused by a sufficiently large positive feedback. Induced losses also reduce pressure gain and throttling range of vortex pressure amplifier.

Erwin, L. R.↗