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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Design of a hybrid laminar flow control nacelle

Consideration is given to the potential application of hybrid-laminar-flow control to the external surface of a modern, high-bypass-ratio (HBR) turbofan engine nacelle. With the advent of advanced ultra-HBR fans (with bypass ratios of 10-15), the wetted areas of these nacelles approach 10 percent of the total wetted area of future commercial transports. A hybrid-laminar-flow-control pressure distribution is specified and the corresponding nacelle geometry is computed utilizing a predictor/corrector design method. Linear stability calculations are conducted to provide predictions of the extent of the laminar boundary layer. Performance studies on an advanced twin-engine transport configuration are presented to determine potential benefits in terms of reduced fuel consumption.

Wie, Yong-Sun↗
NASA NTRS1991

Analysis for the application of hybrid laminar flow control to a long-range subsonic transport aircraft

The FLOPS aircraft conceptual design/analysis code has been used to evaluate the effects of incorporating hybrid laminar flow control (HLFC) in a 300-passenger, 6500 n. mi. range, twin-engine subsonic transport aircraft. The baseline configuration was sized to account for 50 percent chord laminar flow on the wing upper surface as well as both surfaces of the empennage airfoils. Attention is given to the additional benefits of achieving various degrees of laminar flow on the engine nacelles, and the horsepower extraction and initial weight and cost increments entailed by the HLFC system. The sensitivity of the results obtained to fuel-price and off-design range are also noted.

Arcara, P. C., Jr.↗
NASA NTRS1993

Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames

Gas jet diffusion flames have been a subject of research for many years. However, a better understanding of the physical and chemical phenomena occurring in these flames is still needed, and, while the effects of gravity on the burning process have been observed, the basic mechanisms responsible for these changes have yet to be determined. The fundamental mechanisms that control the combustion process are in general coupled and quite complicated. These include mixing, radiation, kinetics, soot formation and disposition, inertia, diffusion, and viscous effects. In order to understand the mechanisms controlling a fire, laboratory-scale laminar and turbulent gas-jet diffusion flames have been extensively studied, which have provided important information in relation to the physico-chemical processes occurring in flames. However, turbulent flames are not fully understood and their understanding requires more fundamental studies of laminar diffusion flames in which the interplay of transport phenomena and chemical kinetics is more tractable. But even this basic, relatively simple flame is not completely characterized in relation to soot formation, radiation, diffusion, and kinetics. Therefore, gaining an understanding of laminar flames is essential to the understanding of turbulent flames, and particularly fires, in which the same basic phenomena occur. In order to improve and verify the theoretical models essential to the interpretation of data, the complexity and degree of coupling of the controlling mechanisms must be reduced. If gravity is isolated, the complication of buoyancy-induced convection would be removed from the problem. In addition, buoyant convection in normal gravity masks the effects of other controlling parameters on the flame. Therefore, the combination of normal-gravity and microgravity data would provide the information, both theoretical and experimental, to improve our understanding of diffusion flames in general, and the effects of gravity on the burning process in particular.

Bahadori, M. Yousef↗
NASA NTRS1993

The Effects of Gravity on Wrinkled Laminar Flames

The effects of gravity are significant to the dynamics of idealized unconfined open premixed flames. Moderate to low turbulence Reynolds number flames, i.e., wrinkled laminar flames, of various unconfined geometries have been used extensively for investigating fundamental processes of turbulent flame propagation and to validate theoretical models. Without the wall constraints, the flames are free to expand and interact with surrounding ambient air. The flow field in which the flame exists is determined by a coupling of burner geometry, flame orientation and the gravity field. These complex interactions raise serious questions regarding the validity of comparing the experimental data of open flames with current theoretical and numerical models that do not include the effects of gravity nor effects of the larger aerodynamic flowfield. Therefore, studies of wrinkled laminar flame in microgravity are needed for a better understanding of the role of gravity on flame characteristics such as the orientation, mean aerodynamics stretch, flame wrinkle size and burning rate. Our approach to characterize and quantify turbulent flame structures under microgravity is to exploit qualitative and quantitative flow visualization techniques coupled with video recording and computer controlled image analysis technologies. The experiments will be carried out in the 2.2 second drop tower at the NASA Lewis Research Center. The longest time scales of typical wrinkled laminar flames in the geometries considered here are in the order of 10 msec. Hence, the duration of the drop is sufficient to obtain the amount of statistical data necessary for characterize turbulent flame structures.

Kostiuk, Larry W.↗
NASA NTRS1992

Laminar flow flight experiments - A review

A development history is presented for flight tests of flow-laminarizing technologies, from the wooden-structure wing glove used with the B-18 of about 1940 to the X-21 of 1963-1965 and the wing incidence-varying tests conducted with gloved-wing F-111 and F-14 variable-sweep aircraft in more recent years. Natural laminar flight control testing has also been conducted with a B 757 airliner. Over the years, increasing attention has been given to the treatment of the leading edge of laminar flow wings in order to prevent flow instabilities that precipitate flow transition to turbulence farther aft.

Wagner, R. D.↗
NASA NTRS1935

An Application of the Von Karman-millikan Laminar Boundary-layer Theory and Comparison with Experiment

The von Karman-Millikan theory of laminar boundary layers presented in NACA Technical Report No. 504 is applied to the laminar boundary layer about an elliptic cylinder on which boundary-layer and pressure-distribution measurements were made. An outline of the procedure of the von Karman-Millikan method is given. Good agreement is obtained between the calculated and experimental results, indicating that the method may be applied generally to the laminar boundary layer about any body provided that an experimentally determined pressure distribution is available. It appears that for all Reynolds Number above 24,000 the separation point for the elliptic cylinder should occur at a constant distance behind the point of minimum pressure, provided that the boundary layer does not become turbulent.

Von Doenhoff, Albert E↗
NASA NTRS1994

Transonic flight test of a laminar flow leading edge with surface excrescences

A flight experiment, conducted at NASA Dryden Flight Research Center, investigated the effects of surface excrescences, specifically gaps and steps, on boundary-layer transition in the vicinity of a leading edge at transonic flight conditions. A natural laminar flow leading-edge model was designed for this experiment with a spanwise slot manufactured into the leading-edge model to simulate gaps and steps like those present at skin joints of small transonic aircraft wings. The leading-edge model was flown with the flight test fixture, a low-aspect ratio fin mounted beneath an F-104G aircraft. Test points were obtained over a unit Reynolds number range of 1.5 to 2.5 million/ft and a Mach number range of 0.5 to 0.8. Results for a smooth surface showed that laminar flow extended to approximately 12 in. behind the leading edge at Mach number 0.7 over a unit Reynolds number range of 1.5 to 2.0 million/ft. The maximum size of the gap-and-step configuration over which laminar flow was maintained consisted of two 0.06-in. gaps with a 0.02-in. step at a unit Reynolds number of 1.5 million/ft.

Zuniga, Fanny A.↗
NASA NTRS1994

Parametric study on laminar flow for finite wings at supersonic speeds

Laminar flow control has been identified as a key element in the development of the next generation of High Speed Transports. Extending the amount of laminar flow over an aircraft will increase range, payload, and altitude capabilities as well as lower fuel requirements, skin temperature, and therefore the overall cost. A parametric study to predict the extent of laminar flow for finite wings at supersonic speeds was conducted using a computational fluid dynamics (CFD) code coupled with a boundary layer stability code. The parameters investigated in this study were Reynolds number, angle of attack, and sweep. The results showed that an increase in angle of attack for specific Reynolds numbers can actually delay transition. Therefore, higher lift capability, caused by the increased angle of attack, as well as a reduction in viscous drag, due to the delay in transition, can be expected simultaneously. This results in larger payload and range.

Garcia, Joseph Avila↗
NASA NTRS1995

Study of Turbulent Premixed Flame Propagation using a Laminar Flamelet Model

The laminar flamelet concept in turbulent reacting flows is considered applicable to many practical combustion systems (Linan & Williams 1993). For turbulent premixed combustion, the laminar flamelet regime is valid when turbulent Karlovitz number is less than unity, which is equivalent to stating that the characteristic thickness of the flame is less than that of a Kolmogorov eddy; this is known as the Klimov-Williams criterion (Williams 1985). In such a case, the flame maintains its laminar structure, and the effect of turbulent flow is merely to wrinkle and strain the flame front. The propagating wrinkled premixed flame can then be described as an infinitesimally thin surface dividing the unburnt fresh mixture and the burnt product.

Im, H. G.↗
NASA NTRS1995

An approach to the constrained design of natural laminar flow airfoils

A design method has been developed by which an airfoil with a substantial amount of natural laminar flow can be designed, while maintaining other aerodynamic and geometric constraints. After obtaining the initial airfoil's pressure distribution at the design lift coefficient using an Euler solver coupled with an integml turbulent boundary layer method, the calculations from a laminar boundary layer solver are used by a stability analysis code to obtain estimates of the transition location (using N-Factors) for the starting airfoil. A new design method then calculates a target pressure distribution that will increase the larninar flow toward the desired amounl An airfoil design method is then iteratively used to design an airfoil that possesses that target pressure distribution. The new airfoil's boundary layer stability characteristics are determined, and this iterative process continues until an airfoil is designed that meets the laminar flow requirement and as many of the other constraints as possible.

Green, Bradford Earl↗
NASA NTRS1996

Wing Leading Edge Joint Laminar Flow Tests

An F-104G aircraft at NASA's Dryden Flight Research Center has been equipped with a specially designed and instrumented test fixture to simulate surface imperfections of the type likely to be present near the leading edge on the wings of some laminar flow aircraft. The simulated imperfections consisted of five combinations of spanwise steps and gaps of various sizes. The unswept fixture yielded a pressure distribution similar to that of some laminar flow airfoils. The experiment was conducted at cruise conditions typical for business-jets and light transports: Mach numbers were in the range 0.5-0.8, and unit Reynolds numbers were 1.5-2.5 million per foot. Skin friction measurements indicated that laminar flow was often maintained for some distance downstream of the surface imperfections. Further work is needed to more precisely define transition location and to extend the experiments to swept-wing conditions and a broader range of imperfection geometries.

Drake, Aaron↗
NASA NTRS1997

Invariance of Hypersonic Normal Force Coefficients with Reynolds Number and Determination of Inviscid Wave Drag from Laminar Experimental Results

Observations have been made and reported that the experimental normal force coefficients at a constant angle of attack were constant with a variation of more than 2 orders of magnitude of Reynolds number at a free-stream Mach number M(sub infinity) of 8.00 and more than 1 order of magnitude variation at M(sub infinity) = 6.00 on the same body-wing hypersonic cruise configuration. These data were recorded under laminar, transitional, and turbulent boundary layer conditions with both hot-wall and cold-wall models. This report presents experimental data on 25 configurations of 17 models of both simple and complex geometry taken at M(sub infinity) = 6.00, 6.86, and 8.00 in 4 different hypersonic facilities. Aerodynamic calculations were made by computational fluid dynamics (CID) and engineering methods to analyze these data. The conclusions were that the normal force coefficients at a given altitude are constant with Reynolds numbers at hypersonic speeds and that the axial force coefficients recorded under laminar boundary-layer conditions at several Reynolds numbers may be plotted against the laminar parameter (the reciprocal of the Reynolds number to the one-half power) and extrapolated to the ordinate axis to determine the inviscid-wave-drag coefficient at the intercept.

Hawkins, Richard↗
NASA NTRS1959

Charts and Tables for Estimating the Stability of the Compressible Laminar Boundary Layer with Heat Transfer and Arbitrary Pressure Gradient

The minimum critical Reynolds numbers for the similar solutions of the compressible laminar boundary layer computed by Cohen and Reshotko and also for the Falkner and Skan solutions as recomputed by Smith have been calculated by Lin's rapid approximate method for two-dimensional disturbances. These results enable the stability of the compressible laminar boundary layer with heat transfer and pressure gradient to be easily estimated after the behavior of the boundary layer has been computed by the approximate method of Cohen and Reshotko. The previously reported unusual result (NACA Technical Note 4037) that a highly cooled stagnation point flow is more unstable than a highly cooled flat-plate flow is again encountered. Moreover, this result is found to be part of the more general result that a favorable pressure gradient is destabilizing for very cool walls when the Mach number is less than that for complete stability. The minimum critical Reynolds numbers for these wall temperature ratios are, however, all larger than any value of the laminar-boundary-layer Reynolds number likely to be encountered. For Mach numbers greater than those for which complete stability occurs a favorable pressure gradient is stabilizing, even for very cool walls.

Tetervin, Neal↗
NASA NTRS1998

Overview of Laminar Flow Control

The history of Laminar Flow Control (LFC) from the 1930s through the 1990s is reviewed and the current status of the technology is assessed. Early studies related to the natural laminar boundary-layer flow physics, manufacturing tolerances for laminar flow, and insect-contamination avoidance are discussed. Although most of this publication is about slot-, porous-, and perforated-suction LFC concept studies in wind tunnel and flight experiments, some mention is made of thermal LFC. Theoretical and computational tools to describe the LFC aerodynamics are included for completeness.

Joslin, Ronald D.↗
NASA NTRS1999

High Reynolds Number Hybrid Laminar Flow Control (HLFC) Flight Experiment: Aerodynamic Design - Report 2

This document describes the aerodynamic design of an experimental hybrid laminar flow control (HLFC) wing panel intended for use on a Boeing 757 airplane to provide a facility for flight research on high Reynolds number HLFC and to demonstrate practical HLFC operation on a full-scale commercial transport airplane. The design consists of revised wing leading edge contour designed to produce a pressure distribution favorable to laminar flow, definition of suction flow requirements to laminarize the boundary layer, provisions at the inboard end of the test panel to prevent attachment-line boundary layer transition, and a Krueger leading edge flap that serves both as a high lift device and as a shield to prevent insect accretion on the leading edge when the airplane is taking off or landing.

Source record↗
NASA NTRS1999

Structure of the Soot Growth Region of Laminar Premixer Methane/Oxygen Flames

Soot is a dominant feature of hydrocarbon/air flames, affecting their reaction mechanisms and structure. As a result, soot processes affect capabilities for computational combustion as well as predictions of flame radiation and pollution emissions. Motivated by these observations, the present investigation extended past work on soot growth in laminar premixed flames, seeking to evaluate model predictions of flame structure. Xu et al. report direct measurements of soot residence times, soot concentrations, soot structure, gas temperatures and gas compositions for premixed flames similar to those studied by Harris and Weiner and Ramer et al. respectively. It was found that predictions of major stable gas species concentrations based on mechanisms of Leung and Lindstedt and Frenklach and coworkers, were in good agreement with the measurements. The results were also used to evaluate the hydrogen-abstraction/carbon-addition (HACA) soot growth mechanisms of Frenklach and coworkers and Colket and Hall. It was found that these mechanisms were effective using quite reasonable correlations for the steric factors appearing in the theories. The successful evaluation of the HACA mechanism of soot growth in Refs. 1 and 2 is encouraging but one aspect of this evaluation is a concern. In particular, H-atom concentrations play a crucial role in the HACA mechanism and it was necessary to estimate these concentrations because they were not measured directly. These estimates were made assuming local thermodynamic equilibrium between H, and H based on measured temperatures and H2 concentrations and the equilibrium constant data of Kee et al.. This approach was justified by the flame structure predictions; nevertheless, direct evaluation of equilibrium estimates of H-atom concentrations in the soot growth regions of laminar premixed flames is needed to provide more convincing proof of this behavior. Thus, the objective of the present investigation was to complete new measurements of the structure of the soot growth region of laminar premixed flames and to use these results to evaluate whether H and H2 are in thermodynamic equilibrium and to extend the earlier evaluation of predictions of concentrations of major gas species.

Xu, F.↗
NASA NTRS1999

Accounting for Laminar Run & Trip Drag in Supersonic Cruise Performance Testing

An improved laminar run and trip drag correction methodology for supersonic cruise performance testing was derived. This method required more careful analysis of the flow visualization images which revealed delayed transition particularly on the inboard upper surface, even for the largest trip disks. In addition, a new code was developed to estimate the laminar run correction. Once the data were corrected for laminar run, the correct approach to the analysis of the trip drag became evident. Although the data originally appeared confusing, the corrected data are consistent with previous results. Furthermore, the modified approach, which was described in this presentation, extends prior historical work by taking into account the delayed transition caused by the blunt leading edges.

Goodsell, Aga M.↗
NASA NTRS1998

Effects of Buoyancy on Lean Premixed V-Flames Part I: Laminar and Turblent Flame Structure

Laser schlieren and planar laser-induced fluorescence techniques have been used to investigate laminar and turbulent v-flames in +g, -g, and micro g under flow conditions that span the regimes of momentum domination (Ri < 0. 1) and buoyancy domination (Ri > 0.1). Overall flame features shown by schlieren indicate that buoyancy dominates the entire flow field for conditions close to Ri = 1. With decreasing Ri, buoyancy effects are observed only in the far-field regions. Analyses of the mean flame angles demonstrate that laminar and turbulent flames do not have similar responses to buoyancy. Difference in the laminar +g and -g flame angles decrease with Ri (i.e., increasing Re) and converge to the microgravity flame angle at the momentum limit (Ri - 0). This is consistent with the notion that the effects of buoyancy diminish with increasing flow momentum. The +g and -g turbulent flame angles, however, do not converge at Ri = 0. As shown by OH-PLIF images, the inconsistency in +g and -g turbulent flame angles is associated with the differences in flame wrinkles. Turbulent flame wrinkles evolve more slowly in +g than in -g. The difference in flame wrinkle structures, however, cannot be explained in terms of buoyancy effects on flame instability mechanisms. It seems to be associated with the field effects of buoyancy that stretches the turbulent flame brushes in +g and compresses the flame brush in -g. Flame wrinkling offers a mechanism through which the flame responds to the field effects of buoyancy despite increasing flow momentum. These observations point to the need to include both upstream and downstream contributions in theoretical analysis of flame turbulence interactions.

Cheng, Robert K.↗