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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

CFD Optimization on Network-Based Parallel Computer System

Combining multiple engineering workstations into a network-based heterogeneous parallel computer allows application of aerodynamic optimization with advance computational fluid dynamics codes, which is computationally expensive in mainframe supercomputer. This paper introduces a nonlinear quasi-Newton optimizer designed for this network-based heterogeneous parallel computer on a software called Parallel Virtual Machine. This paper will introduce the methodology behind coupling a Parabolized Navier-Stokes flow solver to the nonlinear optimizer. This parallel optimization package has been applied to reduce the wave drag of a body of revolution and a wing/body configuration with results of 5% to 6% drag reduction.

Cheung, Samson H.↗
NASA NTRS2008

Multidisciplinary Design, Analysis, and Optimization Tool Development using a Genetic Algorithm

Multidisciplinary design, analysis, and optimization using a genetic algorithm is being developed at the National Aeronautics and Space A dministration Dryden Flight Research Center to automate analysis and design process by leveraging existing tools such as NASTRAN, ZAERO a nd CFD codes to enable true multidisciplinary optimization in the pr eliminary design stage of subsonic, transonic, supersonic, and hypers onic aircraft. This is a promising technology, but faces many challe nges in large-scale, real-world application. This paper describes cur rent approaches, recent results, and challenges for MDAO as demonstr ated by our experience with the Ikhana fire pod design.

Pak, Chan-gi↗
NASA NTRS1982

Development of an LSI maximum-likelihood convolutional decoder for advanced forward error correction capability on the NASA 30/20 GHz program

The particular coding option identified as providing the best level of coding gain performance in an LSI-efficient implementation was the optimal constraint length five, rate one-half convolutional code. To determine the specific set of design parameters which optimally matches this decoder to the LSI constraints, a breadboard MCD (maximum-likelihood convolutional decoder) was fabricated and used to generate detailed performance trade-off data. The extensive performance testing data gathered during this design tradeoff study are summarized, and the functional and physical MCD chip characteristics are presented.

Clark, R. T.↗
NASA NTRS1987

STAEBL/general composites with hygrothermal effects (STAEBL/GENCOM)

A computer code has been developed to perform structural optimization of turbine blades made from angle ply fiber composite laminates. Design variables available for optimization include geometric parameters such as blade thickness distribution and root chord, and composite material parameters such as ply angles and numbers of plies of each constituent material. Design constraints include resonance margins, forced response margins, maximum stress, and maximum ply combined stress. A general description of this code is given. Design optimization studies for typical blades are presented.

Rubenstein, Robert↗
NASA NTRS1988

STAEBL/General composites with hygrothermal effects (STAEBL/GENCOM)

A computer code has been developed to perform structural optimization of turbine blades made from angle ply fiber composite laminates. Design variables available for optimization include geometric parameters such as blade thickness disribution and root chord, and composite material parameters such as ply angles and numbers of plies of each constituent material. Design constraints include resonance margins, forced response margins, maximum stress, and maximum ply combined stress. A general description of this code is given. Design optimization studies for typical blades are presented.

Rubinstein, R.↗
NASA NTRS1992

A hybrid approach to near-optimal launch vehicle guidance

This paper evaluates a proposed hybrid analytical/numerical approach to launch-vehicle guidance for ascent to orbit injection. The feedback-guidance approach is based on a piecewise nearly analytic zero-order solution evaluated using a collocation method. The zero-order solution is then improved through a regular perturbation analysis, wherein the neglected dynamics are corrected in the first-order term. For real-time implementation, the guidance approach requires solving a set of small dimension nonlinear algebraic equations and performing quadrature. Assessment of performance and reliability are carried out through closed-loop simulation for a vertically launched 2-stage heavy-lift capacity vehicle to a low earth orbit. The solutions are compared with optimal solutions generated from a multiple shooting code. In the example the guidance approach delivers over 99.9 percent of optimal performance and terminal constraint accuracy.

Leung, Martin S. K.↗
NASA NTRS1994

Development of an adaptive hp-version finite element method for computational optimal control

In this research effort, the usefulness of hp-version finite elements and adaptive solution-refinement techniques in generating numerical solutions to optimal control problems has been investigated. Under NAG-939, a general FORTRAN code was developed which approximated solutions to optimal control problems with control constraints and state constraints. Within that methodology, to get high-order accuracy in solutions, the finite element mesh would have to be refined repeatedly through bisection of the entire mesh in a given phase. In the current research effort, the order of the shape functions in each element has been made a variable, giving more flexibility in error reduction and smoothing. Similarly, individual elements can each be subdivided into many pieces, depending on the local error indicator, while other parts of the mesh remain coarsely discretized. The problem remains to reduce and smooth the error while still keeping computational effort reasonable enough to calculate time histories in a short enough time for on-board applications.

Hodges, Dewey H.↗
NASA NTRS2006

Performance, Loads and Stability of Heavy Lift Tiltrotors

Summaries of rotor performance are presented for a 124,000-lb Large Civil Tilt Rotor (LCTR) design, along with isolated-rotor and fully-coupled wing/rotor aeroelastic stability. A major motivation of the present research is the effect of size on rotor dynamics. Simply scaling up existing rotor designs to the vehicle size under study would result in unacceptable rotor weight. The LCTR was the most promising of several large rotorcraft concepts produced by the NASA Heavy Lift Rotorcraft Systems Investigation. It was designed to carry 120 passengers for 1200 nm, with performance of 350 knots at 30,000 ft altitude. Design features included a low-mounted wing and hingeless rotors, with a very low cruise tip speed of 350 ft/sec. The LCTR was sized by the'RC code developed by the U. S. Army Aeroflightdynamics Directorate. The rotor was then optimized using the CAMRAD II comprehensive analysis code. The blade and wing structures were designed by Pennsylvania State University to meet the rotor loads calculated by CAMRAD II and wing loads required for certification. Aeroelastic stability was confirmed by further CAMRAD II analysis, based on the optimized rotor and wing designs.

Acree, Cecil W., Jr.↗
NASA NTRS1995

Parallel CFD design on network-based computer

Combining multiple engineering workstations into a network-based heterogeneous parallel computer allows application of aerodynamic optimization with advanced computational fluid dynamics codes, which can be computationally expensive on mainframe supercomputers. This paper introduces a nonlinear quasi-Newton optimizer designed for this network-based heterogeneous parallel computing environment utilizing a software called Parallel Virtual Machine. This paper will introduce the methodology behind coupling a Parabolized Navier-Stokes flow solver to the nonlinear optimizer. This parallel optimization package is applied to reduce the wave drag of a body of revolution and a wing/body configuration with results of 5% to 6% drag reduction.

Cheung, Samson↗
NASA NTRS2001

Specimens and Reusable Fixturing for Testing Advanced Aeropropulsion Materials Under In-Plane Biaxial Loading: Results of Conceptual Design Study - Part 1

A design study was undertaken to investigate the feasibility of using simple specimen designs and reusable fixturing for in-plane biaxial tests planned for advanced aeropropulsion materials. Materials of interest in this work include: advanced metallics, polymeric matrix composites, metal and intermetallic matrix composites, and ceramic matrix composites. Early experience with advanced metallics showed that the cruciform specimen design typically used in this type of testing was impractical for these materials, primarily because of concerns regarding complexity and cost. The objective of this research was to develop specimen designs, fixturing, and procedures which would allow in-plane biaxial tests to be conducted on a wide range of aeropropulsion materials while at the same time keeping costs within acceptable limits. With this goal in mind. a conceptual design was developed centered on a specimen incorporating a relatively simple arrangement of slots and fingers for attachment and loading purposes. The ANSYS finite element code was used to demonstrate the feasibility of the approach and also to develop a number of optimized specimen designs. The same computer code was used to develop the reusable fixturing needed to position and grip the specimens in the load frame. The design adopted uses an assembly of slotted fingers which can be reconfigured as necessary to obtain optimum biaxial stress states in the specimen gage area. Most recently, prototype fixturing was manufactured and is being evaluated over a range of uniaxial and biaxial loading conditions.

Ellis, J. R.↗
NASA NTRS1990

Influence of structural and aerodynamic modeling on optimization with flutter constraint

The influences of structural and aerodynamic modeling on multidisciplinary optimization in an aeroelastic environment are not well understood. Therefore, optimizations with flutter and frequency constraints were performed to investigate the effects these modeling factors have on various representative wings. To this end, the Automated Structural Optimization System (ASTROS) was used as a tool to minimize the weight of various fully built-up finite element wing models in subsonic and supersonic flow under given flutter and frequency constraints. First, the performance of the optimization module was tested against results from other codes on a straight and uniform wing widely used for optimization with flutter constraints. Then, fully built-up finite element models of various wings with different aspect ratios were investigated for the influence on the structural optimization for minimum weight of the following modeling factors: finite element selection, structural grid refinement; number of selected modes, retention of breathing modes; selection of reduced frequencies to be used in flutter analysis; aerodynamic panel size and placement; splining of the aerodynamic grid to the structural grid selection of extra points of the structural wing box for splining; and number of constraints to be retained. Knowledge of these influences as well as of the program behavior is important, since optimization can be made more efficient by the selection of reasonable initial models. Also, it was shown previously that modeling has an impact on the results of modal and aeroelastic analyses. Thus, if modeling errors can negatively affect the analyses, a minimum weight optimization can be jeopardized and result in an optimal design that is rather unreliable. In the following, selected results are presented and the influences of some modeling parameters on optimization are pointed out.

Striz, Alfred G.↗
NASA NTRS2014

Protograph-Based Raptor-Like Codes

Theoretical analysis has long indicated that feedback improves the error exponent but not the capacity of pointto- point memoryless channels. The analytic and empirical results indicate that at short blocklength regime, practical rate-compatible punctured convolutional (RCPC) codes achieve low latency with the use of noiseless feedback. In 3GPP, standard rate-compatible turbo codes (RCPT) did not outperform the convolutional codes in the short blocklength regime. The reason is the convolutional codes for low number of states can be decoded optimally using Viterbi decoder. Despite excellent performance of convolutional codes at very short blocklengths, the strength of convolutional codes does not scale with the blocklength for a fixed number of states in its trellis.

Divsalar, Dariush↗
NASA NTRS1989

Model formulation of non-equilibrium gas radiation for hypersonic flight vehicles

Several radiation models for low density nonequilibrium hypersonic flow are studied. It is proposed that these models should be tested by the 3-D VRFL code developed at NASA/JSC. A modified and optimized radiation model may be obtained from the testing. Then, the current VRFL code could be expanded to solve hypersonic flow problems with nonequilibrium thermal radiation.

Chang, Ing↗
NASA NTRS2000

Performance of the OVERFLOW-MLP and LAURA-MLP CFD Codes on the NASA Ames 512 CPU Origin System

The shared memory Multi-Level Parallelism (MLP) technique, developed last year at NASA Ames has been very successful in dramatically improving the performance of important NASA CFD codes. This new and very simple parallel programming technique was first inserted into the OVERFLOW production CFD code in FY 1998. The OVERFLOW-MLP code's parallel performance scaled linearly to 256 CPUs on the NASA Ames 256 CPU Origin 2000 system (steger). Overall performance exceeded 20.1 GFLOP/s, or about 4.5x the performance of a dedicated 16 CPU C90 system. All of this was achieved without any major modification to the original vector based code. The OVERFLOW-MLP code is now in production on the inhouse Origin systems as well as being used offsite at commercial aerospace companies. Partially as a result of this work, NASA Ames has purchased a new 512 CPU Origin 2000 system to further test the limits of parallel performance for NASA codes of interest. This paper presents the performance obtained from the latest optimization efforts on this machine for the LAURA-MLP and OVERFLOW-MLP codes. The Langley Aerothermodynamics Upwind Relaxation Algorithm (LAURA) code is a key simulation tool in the development of the next generation shuttle, interplanetary reentry vehicles, and nearly all "X" plane development. This code sustains about 4-5 GFLOP/s on a dedicated 16 CPU C90. At this rate, expected workloads would require over 100 C90 CPU years of computing over the next few calendar years. It is not feasible to expect that this would be affordable or available to the user community. Dramatic performance gains on cheaper systems are needed. This code is expected to be perhaps the largest consumer of NASA Ames compute cycles per run in the coming year.The OVERFLOW CFD code is extensively used in the government and commercial aerospace communities to evaluate new aircraft designs. It is one of the largest consumers of NASA supercomputing cycles and large simulations of highly resolved full aircraft are routinely undertaken. Typical large problems might require 100s of Cray C90 CPU hours to complete. The dramatic performance gains with the 256 CPU steger system are exciting. Obtaining results in hours instead of months is revolutionizing the way in which aircraft manufacturers are looking at future aircraft simulation work. Figure 2 below is a current state of the art plot of OVERFLOW-MLP performance on the 512 CPU Lomax system. As can be seen, the chart indicates that OVERFLOW-MLP continues to scale linearly with CPU count up to 512 CPUs on a large 35 million point full aircraft RANS simulation. At this point performance is such that a fully converged simulation of 2500 time steps is completed in less than 2 hours of elapsed time. Further work over the next few weeks will improve the performance of this code even further.The LAURA code has been converted to the MLP format as well. This code is currently being optimized for the 512 CPU system. Performance statistics indicate that the goal of 100 GFLOP/s will be achieved by year's end. This amounts to 20x the 16 CPU C90 result and strongly demonstrates the viability of the new parallel systems rapidly solving very large simulations in a production environment.

Taft, James R.↗
NASA NTRS1987

The performance of trellis coded multilevel DPSK on a fading mobile satellite channel

The performance of trellis coded multilevel differential phase-shift-keying (MDPSK) over Rician and Rayleigh fading channels is discussed. For operation at L-Band, this signalling technique leads to a more robust system than the coherent system with dual pilot tone calibration previously proposed for UHF. The results are obtained using a combination of analysis and simulation. The analysis shows that the design criterion for trellis codes to be operated on fading channels with interleaving/deinterleaving is no longer free Euclidean distance. The correct design criterion for optimizing bit error probability of trellis coded MDPSK over fading channels will be presented along with examples illustrating its application.

Simon, Marvin K.↗
NASA NTRS1988

The performance of trellis coded multilevel DPSK on a fading mobile satellite channel

The performance of trellis-coded multilevel differential phase-shift-keying (MDPSK) modulation over Rician and Rayleigh fading channels is discussed. For operation at L-band, this signalling technique leads to a more robust system than the coherent system with dual pilot tone calibration previously proposed for UHF. The results are obtained using a combination of analysis and simulation. The analysis shows that the design criterion for trellis codes to be operated on fading channels with interleaving/deinterleaving is no longer free Euclidean distance. The correct design criterion for optimizing bit error probability of trellis coded MDPSD over fading channels is presented along with examples illustrating its application.

Simon, Marvin K.↗
NASA NTRS1998

Acoustics and Aeroperformance of Nozzles With Screwdriver Shaped and Axisymmetric Plugs

The recent experimental and numerical tests of corrugated nozzles have shown some acoustic and thrust benefits relative to traditional round nozzles. For example, a Bluebell nozzle which was obtained by 3D nozzle design incorporating a corrugated cross section nozzle shape with a sinusoidal lip line nozzle edge, can provide an acoustic benefit up to 4dB with about a 1% thrust augmentation. In references, this effect was explained as being the result of the corrugated design producing more efficient mixing of the exhausted jet with ambient air. Based on this argument, the authors have proposed the application of this concept for a centerbody (plug) which can form several vortices downstream from the centerbody. Several different corrugated designs are proposed and described in detail in this paper. The main design is a Screwdriver shaped centerbody or plug (SCR) which was tested experimentally and numerically. The acoustic tests were conducted in the anechoic chamber of the Central AeroHydrodynamics Institute (TsAGI, Moscow) under Civilian Research and Development Foundation (CRDF) grant. These experiments have shown an essential acoustic benefit of about 10-13% with the application of the co-annular nozzles by comparison with the reference round nozzle with the same mass flow rate. However, the expected acoustic benefits with the application of the 4-petal Screwdriver shaped centerbody were not obtained by comparison with the reference axisymmetric centerbody (CON) having the same length and the same cross section areas at the same distance from the nozzle throat. Moreover, for some angles (Theta = 60 deg and 90 deg) noise increase was observed (about 1-3%). These tests will be continued with the goal of obtaining better acoustic results. In particular, acoustic characteristics are hoped to be improved by moving t lie centerbody into the nozzle and using penetrable walls for the SCR and/or for the main nozzle. Preliminary results for such approach are very promising. Aeroperformance effects were analyzed numerically. The nozzle thrust, calculations were based on a full Navier--Stokes equations solver (NSE), and both full and marching Euler codes: CFL3D, CRAFT, and Krayko-Godunov . Grid preparation and its optimization were conducted using GRIDGEN and our own codes. The general conclusion of this numerical analysis is some thrust loss with the application of SCR design (about 1 - 1.5%). But, again, some constructive features of SCR design give some promising perspectives for its application in aviation and domestic industries.

Gilinsky, M.↗
NASA NTRS1990

A robust CELP coder with source-dependent channel coding

A CELP coder using Source Dependent Channel Encoding (SDCE) for optimal channel error protection is introduced. With SDCE, each of the CELP parameters are encoded by minimizing a perceptually meaningful error criterion under prevalent channel conditions. Unlike conventional channel coding schemes, SDCE allows for optimal balance between error detection and correction. The experimental results show that the CELP system is robust under various channel bit error rates and displays a graceful degradation in SSNR as the channel error rate increases. This is a desirable property to have in a coder since the exact channel conditions cannot usually be specified a priori.

Sukkar, Rafid A.↗