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

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

An Assessment of the Effect of Compressibility on Dynamic Stall

Compressibility plays a significant role in the development of separation on airfoils experiencing unsteady motion, even at moderately compressible free-stream flow velocities. This effect can result in completely changed stall characteristics compared to those observed at incompressible speed, and can dramatically affect techniques used to control separation. There has been a significant effort in recent years directed toward better understanding; of this process, and its impact on possible techniques for control of separation in this complex environment. A review of existing research in this area will be presented, with emphasis on the physical mechanisms that play such an important role in the development of separation on airfoils. The increasing impact of compressibility on the stall process will be discussed as a function of free-stream Mach number, and an analysis of the changing flow physics will be presented. Examples of the effect of compressibility on dynamic stall will be selected from both recent and historical efforts by members of the aerospace community, as well as from the ongoing research program of the present authors. This will include a presentation of a sample of high speed filming of compressible dynamic stall which has recently been created using real-time interferometry.

Carr, Lawrence W.↗
NASA NTRS1999

Review of Compressible Dynamic Stall and Its Control

A review of compressibility effects on dynamic stall of pitching airfoils and unsteady separation control by manipulation of unsteady vorticity using a deformable leading edge airfoil design is presented.

Chandrasekhara, M. S.↗
NASA NTRS1990

Schlieren studies of compressibility effects on dynamic stall of airfoils in transient pitching motion

Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0 - 60 degrees over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experimental documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers, the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 degrees.

Chandrasekhara, M. S.↗
NASA NTRS1992

Leading edge velocity field of an oscillating airfoil in compressible dynamic stall

Phase-averaged mean-velocity and turbulence data are obtained and analyzed for the leading-edge region of an oscillating airfoil under compressibility conditions. A two-component laser-Doppler velocimetry system was used to make the measurements. Results are compared for the two Mach numbers 0.3 and 0.4 at a reduced frequency of 0.05 with varying airfoil angles of attack. For a Mach number of 0.3, a separation bubble is present on the airfoil throughout the oscillation cycle and no dynamic stall occurs as the peak angle of attack is below the static stall angle. However, a slight imprint of vortical structures is seen in the shear layer enveloping the bubble at the top of the cycle, a result confirmed also by the vorticity contours and in agreement with the earlier stroboscopic schlieren studies. When the Mach number is 0.4, dynamic stall occurs with its origin in the break-up of the separation bubble. Turbulence intensities in the bubble were found to be very large.

Vandyken, R. D.↗
NASA NTRS1993

Visualizing Dynamic-Stall Airflows

Wind-tunnel apparatus facilitates experiments on airfoils undergoing dynamic stall. Possible to visualize flow on oscillating airfoils nonintrusively. In new apparatus, mounting structure also viewing window: airfoil pin-mounted between two optical-quality glass windows. Windows oscillated simultaneously, giving unobstructed view of complex flow around pitching airfoil.

Carr, L.↗
NASA NTRS1988

Fluid mechanics of dynamic stall. II - Prediction of full scale characteristics

Analytical extrapolations are made from experimental subscale dynamics to predict full scale characteristics of dynamic stall. The method proceeds by establishing analytic relationships between dynamic and static aerodynamic characteristics induced by viscous flow effects. The method is then validated by predicting dynamic test results on the basis of corresponding static test data obtained at the same subscale flow conditions, and the effect of Reynolds number on the static aerodynamic characteristics are determined from subscale to full scale flow conditions.

Ericsson, L. E.↗
NASA NTRS1974

Analysis of dynamic stall using unsteady boundary-layer theory

The unsteady turbulent boundary layer and potential flow about a pitching airfoil are analyzed using numerical methods to determine the effect of pitch rate on the delay in forward movement of the rear flow reversal point. An explicit finite difference scheme is used to integrate the unsteady boundary layer equations, which are coupled at each instant of time to a fully unsteady and nonlinear potential flow analysis. A substantial delay in forward movement of the reversal point is demonstrated with increasing pitch rate, and it is shown that the delay results partly from the alleviation of the gradients in the potential flow, and partly from the effects of unsteadiness in the boundary layer itself. The predicted delay in flow-reversal onset, and its variation with pitch rate, are shown to be in reasonable agreement with experimental data relating to the delay in dynamic stall. From the comparisons it can be concluded (a) that the effects of time-dependence are sufficient to explain the failure of the boundary layer to separate during the dynamic overshoot, and (b) that there may be some link between forward movement of the reversal point and dynamic stall.

Scruggs, R. M.↗
NASA NTRS2002

A Comparative Study of Three Methodologies for Modeling Dynamic Stall

During the past two decades, there has been an increased reliance on the use of computational fluid dynamics methods for modeling rotors in high speed forward flight. Computational methods are being developed for modeling the shock induced loads on the advancing side, first-principles based modeling of the trailing wake evolution, and for retreating blade stall. The retreating blade dynamic stall problem has received particular attention, because the large variations in lift and pitching moments encountered in dynamic stall can lead to blade vibrations and pitch link fatigue. Restricting to aerodynamics, the numerical prediction of dynamic stall is still a complex and challenging CFD problem, that, even in two dimensions at low speed, gathers the major difficulties of aerodynamics, such as the grid resolution requirements for the viscous phenomena at leading-edge bubbles or in mixing-layers, the bias of the numerical viscosity, and the major difficulties of the physical modeling, such as the turbulence models, the transition models, whose both determinant influences, already present in static maximal-lift or stall computations, are emphasized by the dynamic aspect of the phenomena.

Sankar, L.↗
NASA NTRS1986

Unsteady separation characteristics of airfoils operating under dynamic stall conditions

Unsteady viscous/inviscid interaction phenomena are investigated for airfoils operating under dynamic stall conditions, using coupling procedures between a time-dependent inviscid panel method and two-dimensional unsteady boundary layer codes. Two strategies are pursued: a coupling of the inviscid panel method with the boundary layer code, and a strong coupling of the inviscid panel method with the boundary layer code. Attention is given to the main features of the unsteady time-marching panel method and boundary layer codes, as well as to numerical stability and the phenomenon of unsteady separation.

Geissler, Wolfgang↗
NASA NTRS1982

An experimental study of dynamic stall on advanced airfoil sections. Volume 1: Summary of the experiment

The static and dynamic characteristics of seven helicopter sections and a fixed-wing supercritical airfoil were investigated over a wide range of nominally two dimensional flow conditions, at Mach numbers up to 0.30 and Reynolds numbers up to 4 x 10 to the 6th power. Details of the experiment, estimates of measurement accuracy, and test conditions are described in this volume (the first of three volumes). Representative results are also presented and comparisons are made with data from other sources. The complete results for pressure distributions, forces, pitching moments, and boundary-layer separation and reattachment characteristics are available in graphical form in volumes 2 and 3. The results of the experiment show important differences between airfoils, which would otherwise tend to be masked by differences in wind tunnels, particularly in steady cases. All of the airfoils tested provide significant advantages over the conventional NACA 0012 profile. In general, however, the parameters of the unsteady motion appear to be more important than airfoil shape in determining the dynamic-stall airloads.

Mccroskey, W. J.↗
NASA NTRS1974

Dynamic stall modeling and correlation with experimental data on airfoils and rotors

Two methods for modeling dynamic stall have been developed. The alpha, A, B method generates lift and pitching moments as functions of angle of attack and its first two time derivatives. The coefficients are derived from experimental data for oscillating airfoils. The Time Delay Method generates the coefficients from steady state airfoil characteristics and an associated time delay in stall beyond the steady state stall angle. Correlation with three types of test data shows that the alpha, A, B method is somewhat better for use in predicting helicopter rotor response in forward flight. Correlation with lift and moment hysteresis loops generated for oscillating airfoils was good for both models.

Carlson, R. G.↗
NASA NTRS1984

Application of the ONERA model of dynamic stall

A semiempirical model to predict the unsteady loads on an airfoil that is experiencing dynamic stall, is investigated. The mathematical model is described from an engineering point of view, demonstrates the procedure for obtaining various empirical parameters, and compares the loads predicted by the model with those obtained in the experiment. It is found that the procedure is straightforward, and the final calculations are in qualitative agreement with the experimental results. Comparisons between calculations and measurements also indicate that a decrease in accuracy results when the values of both the reduced frequency and the amplitude of oscillation are large. Potential quantitative improvements in the accuracy of the calculations are discussed for accounting of both the hysteresis in the static data and the effects of stall delay in the governing equations.

Mcalister, K. W.↗
NASA NTRS1991

Oscillating airfoil velocity field during large amplitude dynamic stall

The leading edge flowfield of an airfoil executing a sinusoidal oscillatory motion and experiencing dynamic stall under compressibility conditions has been studied using a two component LDV system. Phase averaged mean velocity measurements and some flow quantities derived from it are presented and discussed. The results indicate extremely large accelerations of the flow are present around the leading edge with mean velocity values 60 percent higher than and instantaneous velocities as large as 1.75 times the free stream velocity. The velocity profiles at certain locations over the airfoil resemble that of a wake.

Chandrasekhara, M. S.↗
NASA NTRS1978

Observations on the dynamic stall characteristics of advanced helicopter rotor airfoils

A significant amount of research was devoted to understanding the mechanism of dynamic stall delay as applicable to the flow environment of a helicopter rotor in forward flight. One aspect of such research deals with the unsteady characteristics of two-dimensional airfoil sections over a Mach number range from 0.3 to 9.6, since such characteristics can be meaningfully related to rotor performance and loads. This paper summarizes the results of several oscillatory tests carried out on conventional, transonic and BLC-equipped airfoils.

Dadone, L.↗
NASA NTRS1984

Laser velocimeter measurements of dynamic stall

Laser velocimeter measurements were made during the study of a two-dimensional NACA 0012 airfoil undergoing conditions of dynamic stall. The measurements, which were obtained in the Ames 2 foot wind tunnel at reduced frequencies of 0.12 and 1.2, show significant flow field hysteresis around the static stall angle. Comparisons were also made with dual-plate interferograms and good agreement was found for the attached flow cases. For separated flow, characteristic vortex shedding caused poor agreement and significantly increased the measured Reynolds shear stresses.

Owen, F. K.↗
NASA NTRS1993

Suppression of dynamic stall with a leading-edge slat on a VR-7 airfoil

The VR-7 airfoil was experimentally studied with and without a leading-edge slat at fixed angles of attack from 0 deg to 30 deg at Re = 200,000 and for unsteady pitching motions described by alpha equals alpha(sub m) + 10 deg(sin(wt)). The models were two dimensional, and the test was performed in a water tunnel at Ames Research Center. The unsteady conditions ranged over Re equals 100,000 to 250,000, k equals 0.001 to 0.2, and alpha(sub m) = 10 deg to 20 deg. Unsteady lift, drag, and pitching-moment measurements were obtained along with fluorescent-dye flow visualizations. The addition of the slat was found to delay the static-drag and static-moment stall by about 5 degrees and to eliminate completely the development of a dynamic-stall vortex during unsteady motions that reached angles as high as 25 degrees. In all of the unsteady cases studied, the slat caused a significant reduction in the force and moment hysteresis amplitudes. The reduced frequency was found to have the greatest effect on the results, whereas the Reynolds number had little effect on the behavior of either the basic or the slatted airfoil. The slat caused a slight drag penalty at low angles of attack, but generally increased the lift/drag ratio when averaged over the full cycle of oscillation.

Mcalister, K. W.↗
NASA NTRS1978

Dynamic stall: An example of strong interaction between viscous and inviscid flows

A study was done of the phenomena concerning profiles in dynamic stall configuration, and more specially those related to pitch oscillations. The most characteristic experimental results on flow separations with a vortex character, and their repercussions on local pressures and total forces were analyzed. Some aspects of the methods for predicting flows with the presence (or not) of boundary layer separation are examined, as well as the main simplified methods available to date for the calculation of total forces in such configurations.

Philippe, J. J.↗