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At least 127 records · Page 7

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NASA NTRS1991

A computational study of the F-5A forebody emphasizing directional stability

CFD has been used to study the F-5A forebody flowfield at high angle-of-attack combined with sideslip. The classic wind tunnel experiment demonstrating the dominant contribution of the F-5A forebody to directional stability at high angle-of-attack has been simulated computationally over an angle-of-attack range from 10 deg to 45 deg. The key wind tunnel trend for C (n-beta) was obtained computationally using the code known as cfl3d to solve the Reynolds' equations using a modified form of the Baldwin-Lomax turbulence model. The computations provide detailed and fascinating insights into the physics of flowfield. The results of the investigation show that CFD has reached a level of development where computational methods can be used for high angle-of-attack aerodynamic design.

Mason, William H.↗

NASA NTRS1991

Evaluation of a technique to generate artificially thickened boundary layers in supersonic and hypersonic flows

The feasibility of using a contoured honeycomb model to generate a thick boundary layer in high-speed, compressible flow was investigated. The contour of the honeycomb was tailored to selectively remove momentum in a minimum of streamwise distance to create an artificially thickened turbulent boundary layer. Three wind tunnel experiments were conducted to verify the concept. Results indicate that this technique is a viable concept, especially for high-speed inlet testing applications. In addition, the compactness of the honeycomb boundary layer simulator allows relatively easy integration into existing wind tunnel model hardware.

Porro, A. R.↗

NASA NTRS2015

NASA's Aeroacoustic Tools and Methods for Analysis of Aircraft Noise

Aircraft community noise is a significant concern due to continued growth in air traffic, increasingly stringent environmental goals, and operational limitations imposed by airport authorities. The ability to quantify aircraft noise at the source and ultimately at observers is required to develop low noise aircraft designs and flight procedures. Predicting noise at the source, accounting for scattering and propagation through the atmosphere to the observer, and assessing the perception and impact on a community requires physics-based aeroacoustics tools. Along with the analyses for aero-performance, weights and fuel burn, these tools can provide the acoustic component for aircraft MDAO (Multidisciplinary Design Analysis and Optimization). Over the last decade significant progress has been made in advancing the aeroacoustic tools such that acoustic analyses can now be performed during the design process. One major and enabling advance has been the development of the system noise framework known as Aircraft NOise Prediction Program2 (ANOPP2). ANOPP2 is NASA's aeroacoustic toolset and is designed to facilitate the combination of acoustic approaches of varying fidelity for the analysis of noise from conventional and unconventional aircraft. The toolset includes a framework that integrates noise prediction and propagation methods into a unified system for use within general aircraft analysis software. This includes acoustic analyses, signal processing and interfaces that allow for the assessment of perception of noise on a community. ANOPP2's capability to incorporate medium fidelity shielding predictions and wind tunnel experiments into a design environment is presented. An assessment of noise from a conventional and Hybrid Wing Body (HWB) aircraft using medium fidelity scattering methods combined with noise measurements from a model-scale HWB recently placed in NASA's 14x22 wind tunnel are presented. The results are in the form of community noise metrics and auralizations.

Rizzi, Stephen A.↗

DOE OSTI2024

Studying Wind Loading on CSP Collectors to Improve Performance and Reliability

Electricity generation through Concentrating Solar-thermal Power (CSP) adds the advantage of thermal energy storage and heat production. CSP collector costs constitute nearly one-third of total plant costs. Managing wind loading, especially dynamic wind loading caused by the turbulent wind flow, is a significant design challenge. Traditional collector designs rely on wind tunnel experiments and numerical simulations, which do not fully capture dynamic effects. Here, we provide an overview of NREL's activities on wind loading on CSP collectors, aiming to improve their reliability and cost-efficiency. We present insights from field measurements at operational parabolic trough and power-tower CSP plants. Over two years, we measured turbulent wind fields and resulting structural loads in a parabolic trough plant, revealing wind, turbulence, and collector field interactions. Similar measurements are ongoing at the Crescent Dunes heliostat field. These campaigns show how atmospheric turbulence, wind direction, and collector orientation affect dynamic wind loading. Upwind collector structures can generate turbulent structures, causing fluctuating loads at downstream collectors, impacting fatigue lifetime and optical efficiency. Preliminary results indicate that interior collectors experience higher turning moments than those at the edges, suggesting higher drive wear. At Crescent Dunes, we use instrumented heliostats at the field's edge and interior to study the translation of turbulent wind to dynamic structural loads. The campaign is still ongoing, and we will present first findings. Another focus of our work is tying the experimental wind loading findings to optical performance of the collectors, using simulation tools. For example, we derive torsional errors of the parabolic troughs - the angular offset from the sun position along a collector row. This error is impacted by wind-induced mirror deformations. Future work will focus on continuing the measurement efforts and combining them with computational fluid dynamic simulations. By providing detailed field measurements and validated models, our efforts aim to enhance understanding of wind loading on CSP collectors, improving their structural integrity and optical performance.

CSP↗

NASA NTRS1988

An experimental study of an adaptive-wall wind tunnel

A series of adaptive wall ventilated wind tunnel experiments was carried out to demonstrate the feasibility of using the side wall pressure distribution as the flow variable for the assessment of compatibility with free air conditions. Iterative and one step convergence methods were applied using the streamwise velocity component, the side wall pressure distribution and the normal velocity component in order to investigate their relative merits. The advantage of using the side wall pressure as the flow variable is to reduce the data taking time which is one the major contributors to the total testing time. In ventilated adaptive wall wind tunnel testing, side wall pressure measurements require simple instrumentation as opposed to the Laser Doppler Velocimetry used to measure the velocity components. In ventilated adaptive wall tunnel testing, influence coefficients are required to determine the pressure corrections in the plenum compartment. Experiments were carried out to evaluate the influence coefficients from side wall pressure distributions, and from streamwise and normal velocity distributions at two control levels. Velocity measurements were made using a two component Laser Doppler Velocimeter system.

Celik, Zeki↗

NASA NTRS1993

Effect of a rotating propeller on the separation angle of attack and distortion in ducted propeller inlets

The present study represents an extension of an earlier wind tunnel experiment performed with the P&W 17-in. Advanced Ducted Propeller (ADP) Simulator operating at Mach 0.2. In order to study the effects of a rotating propeller on the inlet flow, data were obtained in the UTRC 10- by 15-Foot Large Subsonic Wind Tunnel with the same hardware and instrumentation, but with the propeller removed. These new tests were performed over a range of flow rates which duplicated flow rates in the powered simulator program. The flow through the inlet was provided by a remotely located vacuum source. A comparison of the results of this flow-through study with the previous data from the powered simulator indicated that in the conventional inlet the propeller produced an increase in the separation angle of attack between 4.0 deg at a specific flow of 22.4 lb/sec-sq ft to 2.7 deg at a higher specific flow of 33.8 lb/sec-sq ft. A similar effect on separation angle of attack was obtained by using stationary blockage rather than a propeller.

Boldman, D. R.↗

NASA NTRS1993

Effect of a rotating propeller on the separation angle of attack

The present study represents an extension of an earlier wind tunnel experiment performed with the P&W 17-in. Advanced Ducted Propeller (ADP) Simulator operating at Mach 0.2. In order to study the effects of a rotating propeller on the inlet flow, data were obtained in the UTRC 10- by 15-Foot Large Subsonic Wind Tunnel with the same hardware and instrumentation, but with the propellar removed. These new tests were performed over a range of flow rates which duplicated flow rates in the powered simulator program. The flow through the inlet was provided by a remotely located vacuum source. A comparison of the results of this flow-through study with the previous data from the powered simulator indicated that in the conventional inlet the propeller produced an increase in the separation angle of attack between 4.0 deg at a specific flow of 22.4 lb/sec-sq ft to 2.7 deg at a higher specific flow of 33.8 lb/sec-sq ft. A similar effect on separation angle of attack was obtained by using stationary blockage rather than a propeller.

Boldman, D. R.↗

NASA NTRSDate not supplied

Blade Mistuning Analysis for Boundary Layer Inlet Flow Distortion-Tolerant Fan Blades

NASA Glenn Research Center has tested the BLI2DTF propulsor including the fan rotor. BLI2DTF stands for Boundary Layer Ingesting Inlet/Distortion-Tolerant Fan. A comparison of the vibratory response between analysis results and experimental wind tunnel test results yielded reasonable agreement, yet some differences were noted. Blade mistuning effects have been identified as a potential cause for these differences. A tuned forced response analysis in a turbomachinery engine rotor system assumes that all of the blades are identical, so that the natural frequencies of individual blades are the same. In contrast to this assumption, the wind tunnel tests of the BLI2DTF rotor system revealed that the individual blade response frequencies varied. These variations in individual blade frequencies may be attributed to blade mistuning due to potential differences in the structural and material properties of individual blades. For an understanding of these variations, it is necessary to analyze the influence of blade mistuning on the BLI2DTF rotor system, which will provide a more precise assessment of its vibratory stresses and high cycle fatigue life. The purpose of the study was to investigate the influence of blade mistuning on the dynamic characteristics of this rotor system. At the same fan operation condition, a comparative forced response analysis was performed for both tuned and mistuned BLI2DTF rotor systems. A comparison of the results from these analyses found that the mistuning effects on the present BLI2DTF rotor system are notable. As a result, fan blade mistuning is to be considered a contributor to the levels of vibratory dynamic stresses observed during the wind tunnel experiment.

Blade mistuning↗

NASA NTRS2001

Effects of Rotor Design Variations on Tiltrotor Whirl-Mode Stability

Ultimate tiltrotor performance is constrained by coupled wing/rotor whirl-mode aeroelastic instability. A multifaceted research effort to alleviate those constraints is currently underway at NASA Ames Research Center. Design changes to the XV-15 rotor created large increases in the predicted stability boundary; alternatively, they allowed large increases in pitch-flap coupling (delta-three) for a given stability margin. A wind-tunnel experiment has been proposed to demonstrate the improvements at minimum cost. The design methods are also being applied to the V-22. This report summarizes the results to date for the V-22 and compares them to those for the XV-15. A proposed small-scale wind-tunnel model of the XV-15 proprotor is described.

Acree, C. W., Jr.↗

NASA NTRS2015

Flexible Micropost Arrays for Shear Stress Measurement

Increased fuel costs, heightened environmental protection requirements, and noise abatement continue to place drag reduction at the forefront of aerospace research priorities. Unfortunately, shortfalls still exist in the fundamental understanding of boundary-layer airflow over aerodynamic surfaces, especially regarding drag arising from skin friction. For example, there is insufficient availability of instrumentation to adequately characterize complex flows with strong pressure gradients, heat transfer, wall mass flux, three-dimensionality, separation, shock waves, and transient phenomena. One example is the acoustic liner efficacy on aircraft engine nacelle walls. Active measurement of shear stress in boundary layer airflow would enable a better understanding of how aircraft structure and flight dynamics affect skin friction. Current shear stress measurement techniques suffer from reliability, complexity, and airflow disruption, thereby compromising resultant shear stress data. The state-of-the-art for shear stress sensing uses indirect or direct measurement techniques. Indirect measurements (e.g., hot-wire, heat flux gages, oil interferometry, laser Doppler anemometry, small scale pressure drag surfaces, i.e., fences) require intricate knowledge of the studied flow, restrictive instrument arrangements, large surface areas, flow disruption, or seeding material; with smaller, higher bandwidth probes under development. Direct measurements involve strain displacement of a sensor element and require no prior knowledge of the flow. Unfortunately, conventional "floating" recessed components for direct measurements are mm to cm in size. Whispering gallery mode devices and Fiber Bragg Gratings are examples of recent additions to this type of sensor with much smaller (m) sensor components. Direct detection techniques are often single point measurements and difficult to calibrate and implement in wind tunnel experiments. In addition, the wiring, packaging, and installation of delicate micro-electromechanical devices impede the use of most direct shear sensors. Similarly, the cavity required for sensing element displacement is sensitive to particulate obstruction. This work was focused on developing a shear stress sensor for use in subsonic wind tunnel test facilities applicable to an array of test configurations. The non-displacement shear sensors described here have minimal packaging requirements resulting in minimal or no disturbance of boundary layer flow. Compared to previous concepts, device installation could be simple with reduced cost and down-time. The novelty lies in the creation of low profile (nanoscale to 100 μm) micropost arrays that stay within the viscous sub-layer of the airflow. Aerodynamic forces, which are related to the surface shear stress, cause post deflection and optical property changes. Ultimately, a reliable, accurate shear stress sensor that does not disrupt the airflow has the potential to provide high value data for flow physics researchers, aerodynamicists, and aircraft manufacturers leading to greater flight efficiency arising from more in-depth knowledge on how aircraft design impacts near surface properties.

Wohl, Christopher J.↗

NASA NTRS1990

Low-dimensional modelling and prediction in fluid dynamics

Research into the development of a simplified, low-dimensional, state-variable model for a fluid dynamics system is discussed. The model would be capable of predicting the state of the system in as nearly real time as possible and would be suitable for implementation in a control loop. It is noted that state-variable modeling has an advantage for real-time health monitoring and control application over the Navier-Stokes equations since the latter require an enormous amount of real time. In this study, a dynamic procedure is developed for modeling and prediction using the CFD data generated for this model centerbody-combustor configuration. Some experiences with the NASA data available for the SSME are also reported. Application of this procedure to pressure and velocity measurements in an actual wind-tunnel experiment is described, and the applicability of the approach to health monitoring is considered.

Ghia, U.↗

NASA NTRS1976

Numerical computation of space shuttle heating and surface streamlines

Exact inviscid flow-field codes are used together with a quasi-three-dimensional boundary-layer analysis to provide estimates of the windward surface heating and streamline patterns of the shuttle orbiter vehicle. The accuracy and limitations of the methods are established by comparison with available wind-tunnel experiments and with more exact numerical solutions for simple flows. Flight predictions are presented showing the effects of finite-rate (nonequilibrium) chemical reactions, and the effects of varying boundary-layer edge conditions due to the growth of the boundary-layer into the inviscid flow (entropy layer swallowing). Differences between flow-field predictions at wind-tunnel and nominal flight conditions are discussed.

Rakich, J. V.↗

NASA NTRS2013

Aerodynamic Characterization of New Parachute Configurations for Low-Density Deceleration

The Low Density Supersonic Decelerator project performed a wind tunnel experiment on the structural design and geometric porosity of various sub-scale parachutes in order to inform the design of the 110ft nominal diameter flight test canopy. Thirteen different parachute configurations, including disk-gap-band, ring sail, disk sail, and star sail canopies, were tested at the National Full-scale Aerodynamics Complex 80- by 120-foot Wind Tunnel at NASA Ames Research Center. Canopy drag load, dynamic pressure, and canopy position data were recorded in order to quantify there lative drag performance and stability of the various canopies. Desirable designs would yield increased drag above the disk-gap-band with similar, or improved, stability characteristics. Ring sail parachutes were tested at geometric porosities ranging from 10% to 22% with most of the porosity taken from the shoulder region near the canopy skirt. The disk sail canopy replaced the rings lot portion of the ring sail canopy with a flat circular disk and wastested at geometric porosities ranging from 9% to 19%. The star sail canopy replaced several ringsail gores with solid gores and was tested at 13% geometric porosity. Two disk sail configurations exhibited desirable properties such as an increase of 6-14% in the tangential force coefficient above the DGB with essentially equivalent stability. However, these data are presented with caveats including the inherent differences between wind tunnel and flight behavior and qualitative uncertainty in the aerodynamic coefficients.

parachute↗

NASA NTRS2015

Fuselage-Induced Velocity Model for LRTA, RTA, and HART Fuselages

In order to make the aerodynamic fuselage-rotor interference effects available to comprehensive rotor codes, a simple analytical model of the fuselage-induced velocities within the volume of rotor blade operation above the fuselage is developed here for the following bodies used in wind tunnel experiments: the Large Rotor Test Apparatus (LRTA), the Rotor Test Apparatus (RTA), and the Higher Harmonic Control Aeroacoustic Rotor Test (HART).While the first two are used in the National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames, California, the third one is used by DLR in the large low-speed facility of the German-Dutch wind tunnel in the Netherlands. The fuselage-induced velocity model is based on parameter identification of isolated fuselage-induced velocity data (computed by means of computational fluid dynamics, CFD) and is intended to be generic enough to be used for real helicopter fuselages as well. The accuracies obtained in reproducing the CFD data show a remaining average error of less or equal 5 of the peak-to-peak induced velocity range, which is considered sufficient for comprehensive code analysis.

HART↗

NASA NTRS1993

Flight and wind-tunnel calibrations of a flush airdata sensor at high angles of attack and sideslip and at supersonic Mach numbers

A nonintrusive airdata-sensing system was calibrated in flight and wind-tunnel experiments to an angle of attack of 70 deg and to angles of sideslip of +/- 15 deg. Flight-calibration data have also been obtained to Mach 1.2. The sensor, known as the flush airdata sensor, was installed on the nosecap of an F-18 aircraft for flight tests and on a full-scale F-18 forebody for wind-tunnel tests. Flight tests occurred at the NASA Dryden Flight Research Facility, Edwards, California, using the F-18 High Alpha Research Vehicle. Wind-tunnel tests were conducted in the 30- by 60-ft wind tunnel at the NASA LaRC, Hampton, Virginia. The sensor consisted of 23 flush-mounted pressure ports arranged in concentric circles and located within 1.75 in. of the tip of the nosecap. An overdetermined mathematical model was used to relate the pressure measurements to the local airdata quantities. The mathematical model was based on potential flow over a sphere and was empirically adjusted based on flight and wind-tunnel data. For quasi-steady maneuvering, the mathematical model worked well throughout the subsonic, transonic, and low supersonic flight regimes. The model also worked well throughout the angles-of-attack and -sideslip regions studied.

Moes, Timothy R.↗

NASA NTRS1993

Flight and wind-tunnel calibrations of a flush airdata sensor at high angles of attack and sideslip and at supersonic Mach numbers

Moes, Timothy R.↗

NASA NTRS1994

Science objectives and performance of a radiometer and window design for atmospheric entry experiments

This paper describes the techniques developed for measuring stagnation-point radiation in NASA's cancelled Aeroassist Flight Experiment (AFE). It specifies the need for such a measurement; the types and requirements for the needed instruments; the Radiative Heating Experiment (RHE) developed for the AFE; the requirements, design parameters, and performance of the window developed for the RHE; the procedures and summary of the technique; and results of the arc-jet wind tunnel experiment conducted to demonstrate the overall concept. Subjects emphasized are the commercial implications of the knowledge to be gained by this experiment in connection with the Aeroassisted Space Transfer Vehicle (ASTV), the nonequilibrium nature of the radiation, concerns over the contribution of vacuum-ultraviolet radiation to the overall radiation, and the limit on the flight environment of the vehicle imposed by the limitations on the window material. Results show that a technique exists with which the stagnation-point radiation can be measured in flight in an environment of interest to commercial ASTV applications.

Craig, Roger A.↗

DOE OSTI2024

Measurement of Wind Loading on Heliostats at the Crescent Dunes Power Plant: An Overview

The cost of solar collectors constitutes almost one third of the total cost of a CSP plant. One of the ongoing challenges in the design of these collectors is wind loading on mirrors, support structures, and drives. A particular challenge is dynamic wind loading, caused by the turbulent wind flow. To date, the design of solar collector structures has relied on wind tunnel experiments and numerical simulations that do not entirely capture the dynamic effects observed at scale. The CSP industry has shown increased interest in validating the idealized assumptions with measurements obtained in operational settings to improve wind load assumptions and increase reliability and cost-efficiency of the collector design. Further, performance models need realistic assumptions about wind loading and its impact on optical performance. In a parabolic trough field campaign, NREL successfully collected a wealth of long-term, high-resolution wind and loads data [2], that are publicly available and that can be used for the above-mentioned purposes. Heliostats are impacted differently by wind than parabolic troughs, due to their different shape, size, and field layout. To study the impact of wind and turbulence on heliostats, we initiated another field campaign in an operational power- tower plant, Crescent Dunes, in Nevada, USA. In this work, we present an overview of the measurements, first results, and potential implications of wind driven loads on Heliostats.

concentrated solar power↗