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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
NASA NTRS1961

Effect of Nose Bluntness on Transition for a Cone and a Hollow Cylinder at Mach Numbers 1.41 and 2.01

An investigation has been made to determine the effects of nose bluntness on boundary-layer transition for a cone with an included angle of 10 degrees and for a hollow cylinder. The tests were conducted at Mach numbers of 1.41 and 2.01 for free-stream Reynolds numbers per foot ranging from 1 x 10(exp 6) to 9 x 10(exp 6). The investigation was made with the use of schlieren photography for which the models were aligned with the free stream. For the 10 degree cone, the favorable effects of nose blunting were so small at both test Mach numbers as to be lost within the experimental accuracy. For small amounts of nose blunting on the hollow cylinder, for which the ratio of bluntness height to transition distance for the sharp-leading-edge cylinder was relatively small, there was little, if any, effect of blunting on transition. For somewhat larger values of this ratio, nose blunting had a favorable effect on transition. The magnitude of the favorable effect was dependent upon the size and the shape of the bluntness, and the maximum increase in transition distance relative to the sharp-leading-edge cylinder is in good agreement with the theoretical predictions of NACA Technical Report 1312. For relatively large values of the ratio of nose bluntness to transition distance, the effects of nose blunting were adverse for both the cone and the cylinder. In general, adverse effects due to blunting were larger for the flat bluntness than for the hemispherical or the round bluntness of equal bluntness height. Increasing the Mach number increased the size of bluntness required to induce adverse effects at constant free-stream Reynolds number per foot, delayed the adverse effects to higher values of Reynolds number per foot for constant nose bluntness, and reduced the abruptness of the transition decrease.

Monta, William J.↗
NASA NTRS2015

The Effect of Backward-Facing Step Height on Instability Growth and Breakdown in Swept Wing Boundary-Layer Transition

A low-speed experiment was performed on a swept at plate model with an imposed pressure gradient to determine the effect of a backward-facing step on transition in a stationary-cross flow dominated flow. Detailed hot-wire boundary-layer measurements were performed for three backward-facing step heights of approximately 36, 45, and 49% of the boundary-layer thickness at the step. These step heights correspond to a subcritical, nearly-critical, and critical case. Three leading-edge roughness configurations were tested to determine the effect of stationary-cross flow amplitude on transition. The step caused a local increase in amplitude of the stationary cross flow for the two larger step height cases, but farther downstream the amplitude decreased and remained below the baseline amplitude. The smallest step caused a slight local decrease in amplitude of the primary stationary cross flow mode, but the amplitude collapsed back to the baseline case far downstream of the step. The effect of the step on the amplitude of the primary cross flow mode increased with step height, however, the stationary cross flow amplitudes remained low and thus, stationary cross flow was not solely responsible for transition. Unsteady disturbances were present downstream of the step for all three step heights, and the amplitudes increased with increasing step height. The only exception is that the lower frequency (traveling crossflow-like) disturbance was not present in the lowest step height case. Positive and negative spikes in instantaneous velocity began to occur for the two larger step height cases and then grew in number and amplitude downstream of reattachment, eventually leading to transition. The number and amplitude of spikes varied depending on the step height and cross flow amplitude. Despite the low amplitude of the disturbances in the intermediate step height case, breakdown began to occur intermittently and the flow underwent a long transition region.

Eppink, Jenna L.↗
NASA NTRS2026

Aerodynamic Performance and Acoustic Impacts of Varying Tip Speeds and Tripping Conditions on Small Rotors in an Anechoic Hover Chamber

Performance and acoustic measurements were taken in a hover chamber for various optimum hovering rotors (OPT2) and a commercial-off-the-shelf (COTS) rotor. A total of 10 rotors are included in this report, all of which have two blades and a tip radius of 0.1905 m (7.5 in.). For the OPT2, results for three additive manufacturing methods of fabrication are presented: stereolithography(SLA) using Accura Xtreme, SLA using FormLabs 10K resin, and selective laser sintering (SLS) using mineral-filled PA12 nylon material. All but one set of rotors were designed with a trailing edge bluntness that is 3% of the chord length, and one set was designed with a bluntness that is 1% of the chord length. Spanwise boundary layer trips were applied to the SLA, FormLabs and COTS rotors. Performance comparisons between untripped and tripped configurations demonstrate the impact of boundary layer state on rotor efficiency. Acoustic results, including periodic and broadband noise components, are presented. The effects of tripping these rotors near the leading edge are also presented. For some of the rotors, acoustic spectra of tip speed sweeps are presented to show how the frequency content changes with tip speed. Amplitude and frequency scaling methods are used to collapse broadband spectra at various tip speeds towards a unified curve. These findings contribute to the understanding of small rotor aeroacoustics and provide valuable datasets for computational model validation in urban air mobility applications.

eVTOL↗
NASA NTRS1998

Effectiveness of Flow Control for Alleviation of Twin-Tail Buffet

Effectiveness of active flow control for twin- tail buffet alleviation is investigated. Tangen- tial leading-edge blowing (TLEB) and flow suction along the vortex cores (FSVC) of the lead- ing edges of the delta wing are used to delay the vortex breakdown flow upstream of the twin tail. The combined effect of the TLEB and FSVC is also investigated. A parametric study of the effects of the spanwise position of the suction tubes and volumetric suction flow rate on the twin-tail buffet response are also investigated. The TLEB moves the path of leading-edge vortices laterally towards the twin tail, which increases the aero- dynamic damping on the tails. The FSVC effectively delays the breakdown location at high angles of attack. The computational model consists of a sharp-edged delta wing of aspect ratio one and swept-back flexible twin tail with taper ratio of 0.23. This complex multidisciplinary problem is solved sequentially using three sets of equations for the fluid flow, aeroelastic response and grid deformation, on a dynamic multi-block grid structure. The computational model is pitched at 30 deg. angle of attack. The freestream Mach number and Reynolds number are 0.3 and 1.25 million, respectively. The model is investigated for the inboard position of the twin tails, which corresponds to a separation distance between the twin tails of 33% of the wing span.

Sheta, Essam F.↗
NASA NTRS1991

Lightning protection of the Fokker 100 CFRP rudder

The construction of the structural parts of the Fokker 100 CFRP rudder is described with respect to the requirements for electrical bonding and lightning protection. Furthermore, the philosophy for the selection of a consumable trailing edge is given. A description of possible alternative designs for trailing edges and their advantages and disadvantages with respect to damage after lightning impact will also be reviewed. An overview of the tests performed on test samples and the rudder construction are presented and discussed. The effectiveness of both the selected structural provisions and trailing edge are described (and proven) by reporting the results of the simulated lightning tests performed. Proof is given that the trailing edge construction and its bonding through the structural parts of the rudder to the main aircraft structure is a solution which results in minor damage to the rudder after lightning impact. Furthermore, it is shown that the selected trailing edge construction is less favored by the structural designers due to the weight penalty.

Ruiter, A. J. M.↗
NASA NTRS1977

Theoretical prediction of over-wing-blowing aerodynamics

A theoretical method is developed for predicting the aerodynamic characteristics of wings with over-wing-blowing jet. The method allows the jet to stay above the wing surface as well as to wash the surface. It accounts for the wing-jet interaction due to differences between the jet and freestream dynamic pressures and Mach numbers, in addition to the jet entrainment. For the former effect, the quasi-vortex-lattice method is used to satisfy the jet and wing boundary conditions. For the latter, a new theory was developed to calculate the jet entrained flow for given jet properties. Comparison of predicted results with available data of various configurations shows reasonably good agreement. Further theoretical analysis indicates that it is aerodynamically advantageous to locate the jet exit near and ahead of the wing leading edge, and that the camber shape has significant effect on the induced drag.

Lan, C. E.↗
NASA NTRS1981

Experimental determination of the effects of moisture on composite-to-composite adhesive joints

The primary mode of moisture ingress into bonded composite joints is determined using a nuclear probe for deuterium (NPD) to measure the localized D2O content along the length of the adhesive (FM-300 and EA-9601) and through the thickness of bonded composite speciments. Calculated diffusivities and NPD measured equilibrium moisture contents are used to predict the moisture profiles along the length of the adhesives as a function of exposure time, temperature, and relative humidity. These results are compared with the observed moisture profiles to evaluate the extent of enhanced edge diffusion. The FM-300 adhesive exhibits good agreement between measured and predicted profiles at 49 C, 70% and 90% RH, and 77 C, 70% RH. At 77 C, 90% RH, the measured moisture content near the adhesive edge is substantially larger than the predicted level. The EA-9601 adhesive also shows good agreement at 49 C, 70% and 90% RH, but at 77 C, the concentration of D20 near the edges is enhanced at each humidity level. The effect of moisture content on the bond shear strength at room temperature and at elevated temperature is evaluated.

Deiasi, R. J.↗
NASA NTRS1986

Verification of performance results for a low-speed 15 percent elliptical circulation control airfoil

Low-speed wind tunnel tests performed by the Naval Ship Research and Development Center (NSRDC) on a circulation control airfoil model was repeated by the Joint Institute for Aerodynamics and Acoustics in an attempt to reproduce the performance results. The model used was a 15% ellipse with interchangeable trailing edges. Surface pressure measurements were taken to obtain lift and pitching moment coefficients as functions of jet blowing momentum, and the momentum deficit in the wake was measured and used to calculate the drag coefficient. The effects of spanwise slot height variation and of leading edge blowing on performance were also investigated. The performance results showed that of the three slot heights tested, a slot height/chord ratio of 0.0022 produced the most lift coefficient for a given blowing rate. Lift obtained in the current test ranged from 2 to 35% lower than the NSRDC test. However, the two data sets compared reasonably well considering wind tunnel and wall blowing scheme differences. The spanwise lift distribution showed less change in lift due to a variation in slot height than expected. The leading edge blowing results demonstrated that although lift initially decreased, a positive lift increment was possible at higher leading edge blowing rates.

Rodman, L. C.↗
NASA NTRS1987

A study of the effects of Reynolds number and Mach number on constant pressure coefficient jump for shock-induced trailing-edge separation

The influence of Mach and Reynolds numbers as well as airfoil and planform geometry on the phenomenon of constant shock jump pressure coefficient for conditions of shock induced trailing edge separation (SITES) was studied. It was demonstrated that the phenomenon does exist for a wide variety of two and three dimensional flow cases and that the influence of free stream Mach number was not significant. The influence of Reynolds number was found to be important but was not strong. Airfoil and planform geometric characteristics were found to be very important where the pressure coefficient jump was shown to vary with the sum of: (1) airfoil curvature at the upper surface crest, and (2) camber surface slope at the trailing edge. It was also determined that the onset of SITES could be defined as a function of airfoil geometric parameters and Mach number normal to the leading edge. This onset prediction was shown to predict the angle of onset to within + or - 1 deg accuracy or better for about 90% of the cases studied.

Cunningham, Atlee M., Jr.↗
NASA NTRS1992

A simplified method for thermal analysis of a cowl leading edge subject to intense local shock-wave-interference heating

Type IV shock wave interference heating on a blunt body causes extremely intense heating over a very localized region of the body. An analytical solution is presented to a heat transfer problem that approximates the shock wave interference heating of an engine cowl leading edge of the National Aero-Space Plane. The problem uses a simplified geometry to represent the leading edge. An analytical solution is developed that provides a means for approximating maximum temperature differences between the outer and inner surface temperatures of the leading edge. The solution is computationally efficient and, as a result, is well suited for conceptual and preliminary design or trade studies. Transient and steady state analyses are conducted, and results obtained from the analytical solution are compared with results of 2-D thermal finite element analyses over a wide range of design parameters. Isotropic materials as well as laminated composite materials are studied. Results of parametric studies are presented to indicate the effects of the thickness of the cowl leading edge and the width of the region heated by the shock wave interference on the thermal response of the leading edge.

Mcgowan, David M.↗
NASA NTRS1978

Flight investigation of insect contamination and its alleviation

An investigation of leading edge contamination by insects was conducted with a JetStar airplane instrumented to detect transition on the outboard leading edge flap and equipped with a system to spray the leading edge in flight. The results of airline type flights with the JetStar indicated that insects can contaminate the leading edge during takeoff and climbout. The results also showed that the insects collected on the leading edges at 180 knots did not erode at cruise conditions for a laminar flow control airplane and caused premature transition of the laminar boundary layer. None of the superslick and hydrophobic surfaces tested showed any significant advantages in alleviating the insect contamination problem. While there may be other solutions to the insect contamination problem, the results of these tests with a spray system showed that a continouous water spray while encountering the insects is effective in preventing insect contamination of the leading edges.

Peterson, J. B., Jr.↗
NASA NTRS1978

Flight experience on the need and use of inflight leading edge washing for a laminar flow airfoil

An investigation of leading-edge contamination by insects was conducted at the NASA Dryden Flight Research Center with a JetStar airplane instrumented to detect transition on the outboard leading-edge flap and equipped with a system to wash the leading edge in flight. The results of airline-type flights with the JetStar indicated that insects can contaminate the leading edge during take-off and climbout at large jet airports in the United States. The results also showed that the insects collected on the leading edges at 180 knots did not erode at cruise conditions for a laminar flow control airplane and caused premature transition of the laminar boundary layer. None of the superslick and hydrophobic surfaces tested showed any significant advantages in alleviating the insect contamination problem. While there may be other solutions to the insect contamination problem, the results of these tests with a washer system showed that a continuous water spray while encountering the insects is effective in preventing insect contamination of the leading edges.

Fisher, D. F.↗
NASA NTRS2001

Preliminary Results from Coordinated UVCS-CDS-Ulysses Observations

The June 2000 quadrature between the Sun, Earth, and Ulysses took place with Ulysses at a distance of 3.35 AU from the Sun and at heliocentric latitude 58.2 deg south, in the southeast quadrant. This provided an opportunity to observe the corona close to the Sun with Coronal Diagnostic Spectrometer (CDS) and Ultraviolet Coronograph Spectrometer (UVCS) and, subsequently, to sample the same plasma when it reached Ulysses. Here we focus on simultaneous observations of UVCS and CDS made on June 12, 13, 16 and 17. The UVCS data were acquired at heliocentric altitudes ranging from 1.6 to 2.2 solar radii, using different grating positions, in order to get a wide wavelength range. CDS data consisted of Normal Incidence Spectrometer (NIS) full wavelength rasters of 120" x 150" centered at altitudes up to 1.18 solar radii, together with Grazing Incidence Spectrometer (GIS) 4" x 4" rasters within the same field of view, out to 1.2 solar radii. The radial direction to Ulysses passed through a high latitude streamer, throughout the 4 days of observations, Analysis of the spectra taken by UVCS shows a variation of the element abundances in the streamer over our observing interval: however, because the observations were in slightly different parts of the streamer on different days, the variation could be ascribed either to a temporal or spatial effect. The oxygen abundance, however, seems to increase at the edge of the streamer, as indicated by previous analyses. This suggests the variation may be a function of position within the streamer, rather than a temporal effect. Oxygen abundances measured by SWICS on Ulysses are compared with the CDS and UVCS results to see whether changes measured in situ follow the same pattern.

Parenti, S.↗
NASA NTRS1988

Effects of surface pressures and streamline metrics on the calculation of laminar heating rates

The effect of streamline geometry and pressure distributions on surface heating rates is examined for slender, spherically blunted cones. The modifications to the approximate aeroheating code include a curve fit of pressures computed by an Euler solution over a range of Mach numbers and cone angles. The streamline geometry is then found using the surface pressures and inviscid surface properties. Previously, streamlines were determined using the inviscid properties at the edge of the boundary layer when accounting for the effects of entropy-layer swallowing. Streamline calculations are now based on inviscid surface conditions rather than boundary-layer edge properties. However, the heating rates are calculated using inviscid properties at the edge of the boundary layer. Resulting heating rates compare favorably with solutions from the viscous-shock-layer equations.

Riley, Christopher J.↗
NASA NTRS1992

Evaluation of a doubly-swept blade tip for rotorcraft noise reduction

A computational study was performed for a doubly-swept rotor blade tip to determine its benefit for high-speed impulsive (HSI) and blade-vortex interaction (BVI) noise. This design consists of aft and forward sweep. For the HSI-noise computations, unsteady Euler calculations were performed for several variations to a rotor blade geometry. A doubly-swept planform was predicted to increase the delocalizing Mach number to 0.94 (representative of a 200+ kt helicopter). For the BVI-noise problem, it had been hypothesized that the doubly-swept blade tip, by producing a leading-edge vortex, would reduce the tip-vortex effect on BVI noise. A procedure was used in which the tip vortex velocity profile computed by a Navier-Stokes solver was used to compute the inflow associated with BVI. This inflow was used by a Euler solver to compute the unsteady pressures for an acoustic analysis. The results of this study were inconclusive due to the difficulty in accurately predicting the viscous tip vortex downstream of the blade. Also, for the condition studied, no leading-edge vortex formed at the tip.

Wake, Brian E.↗
NASA NTRS1960

Static Longitudinal Stability and Control Characteristics At A Mach Number of 1.99 of a Lenticular-Shaped Reentry Vehicle

An investigation has been made in the Langley 4- by 4-foot supersonic pressure tunnel at a Mach number of 1.99 to determine the longitudinal stability and control characteristics of a reentry model consisting of a lenticular-shaped body with two fin configurations (horizontal fins with end plates). Effects of deflecting the larger size fins as pitch-control surfaces were also investigated. The results indicate that the body alone was unstable from an angle of attack of 0 deg to about 55 deg where it became stable and remained so to 90 deg. The addition of fins provided positive longitudinal stability throughout the angle-of-attack range and increased the lift-drag ratio of the configuration. Reducing the horizontal-fin area at the inboard trailing edge of the fin had only a small effect on the aerodynamic characteristics of the vehicle for the condition of no fin deflection. Deflecting the fins, appeared to be an effective means of pitch control and had only a small effect on lift-drag ratio.

Jackson, Charles M., Jr.↗
NASA NTRS2009

100-GHz Phase Switch/Mixer Containing a Slot-Line Transition

A circuit that can function as a phase switch, frequency mixer, or frequency multiplier operates over a broad frequency range in the vicinity of 100 GHz. Among the most notable features of this circuit is a grounded uniplanar transition (in effect, a balun) between a slot line and one of two coplanar waveguides (CPWs). The design of this circuit is well suited to integration of the circuit into a microwave monolithic integrated circuit (MMIC) package. One CPW is located at the input end and one at the output end of the top side of a substrate on which the circuit is fabricated (see Figure 1). The input CPW feeds the input signal to antiparallel flip-chip Schottky diodes connected to the edges of the slot line. Phase switching is effected by the combination of (1) the abrupt transition from the input CPW to the slot line and (2) CPW ground tuning effected by switching of the bias on the diodes. Grounding of the slot metal to the bottom metal gives rise to a frequency cutoff in the slot. This cutoff is valuable for separating different frequency components when the circuit is used as a mixer or multiplier. Proceeding along the slot line toward the output end, one encounters the aforementioned transition, which couples the slot line to the output CPW. Impedance tuning of the transition is accomplished by use of a high-impedance section immediately before the transition.

Gaier, Todd↗