Showing papers on "Airfoil published in 1990"

Unsteady Euler airfoil solutions using unstructured dynamic meshes

Abstract: Two algorithms for the solution of the time-dependent Euler equations are presented for unsteady aerodynamic analysis of oscillating airfoils. Both algorithms were developed for use on an unstructured grid made up of triangles. The first flow solver involves a Runge-Kutta time-stepping scheme with a finite-volume spatial discretization that reduces to central differencing on a rectangular mesh. The second flow solver involves a modified Euler time-integration scheme with an upwind-biased spatial discretization based on the flux-vector splitting of Van Leer. The paper presents descriptions of the Euler solvers and dynamic mesh algorithm along with results which assess the capability.

Airfoil design and data

Abstract: This book reflects the author's experience in the development of a computer program for the application of potential flow and boundary layer theories to the design and analysis of subsonic airfoils, including the evaluation of total lift, drag and moment coefficients. The design allows readers to compute the airfoil shape by means of a simple mathematical method from the properties of their velocity distribution, which can be specified in such a way that the boundary layer flow fulfills some given requirements. The most significant boundary layer phenomena such as laminar turbulent transition, laminar separation bubbles and boundary layer separation are considered. The first five chapters of this book describe a procedure for the design and analysis of subsonic airfoils. The data section contains 116 new airfoils for a wide range of Reynolds numbers and application requirements, including general aviation aircraft, tailless airplanes, gliders and propellers. The design features are explained and the input data for computer codes are given for all airfoils.

State-space representation of unsteady airfoil behavior

Abstract: A method is presented to model the unsteady lift, pitching moment, and drag acting on a two-dimensional airfoil operating under attached-flow conditions in a compressible flow. Starting from suitable generalizations and approximations to aerodynamic indicial functions, the unsteady airloads due to an artibrary forcing are represented in a state-space (differential equation) form. This model is in a form compatible with the aeroelastic analyses of both fixed-wing and rotary-wing systems

Control of wall-separated flow by internal acoustic excitation

Abstract: We explore the control of wall-separated flow on a NACA 63 3 -018 airfoil and a circular cylinder by using the internal acoustic excitation technique. Experimental study of the characteristics of the flow under internally emanating acoustic waves is performed in an open-type, suction wind tunnel. Tests are carried out at the Reynolds number ranging from 6.3 × 10 3 to 5.0 × 10 5 based on the relevant characteristic lengths, the airfoil chord, and the cylinder diameter

Thrust generation by an airfoil in hover modes

Abstract: A small airfoil is operated in combined harmonic plunging and pitching motions to generate thrust in a still air environment. By full utilization of dynamic stall vortices large thrust coefficients were attained. The vortical signature of thrust is a simple vortex street with the character of a jet stream.

NASA supercritical airfoils: A matrix of family-related airfoils

Abstract: The NASA supercritical airfoil development program is summarized in a chronological fashion. Some of the airfoil design guidelines are discussed, and coordinates of a matrix of family related supercritical airfoils ranging from thicknesses of 2 to 18 percent and over a design lift coefficient range from 0 to 1.0 are presented.

Dynamic stall experiments on the NACA 23012 aerofoil

Abstract: An experimental investigation was conducted to examine the dynamic stall characteristics of a NACA 23012 aerofoil section at a Reynolds number of 1.5 million. Time-dependent data were obtained from thirty miniature pressure transducers and three hot film gauges situated at the mid-span of the wing. The static stall mechanism of the NACA 23012 was determined to be via abrupt upstream movement of trailing edge separation. Under dynamic conditions, stall was found to occur via leading edge separation, followed by a strong suction wave that moved across the aerofoil. This suction wave is characteristic of a strong moving vortex disturbance. Evidence of strong secondary vortex shedding was also found to occur, and this appears symptomatic of dynamic stall only at low Mach numbers. Some evidence of flow reversals over the trailing edge of the aerofoil were indicated prior to the development of leading edge separation and dynamic stall.

Vortex formation around an oscillating and translating airfoil at large incidences

Abstract: The starting flows past a two-dimensional oscillating and translating airfoil are investigated by visualization experiments and numerical calculations. The airfoil, elliptic in cross-section, is set in motion impulsively and subjected simultaneously to a steady translation and a harmonic oscillation in pitch. The incidence of the airfoil is variable between 0° and 45° and the Reynolds number based on the chord length is between 1500 and 10000. The main object of the present study is to reveal some marked characteristics of the unsteady vortices produced from the oscillating airfoil set at large incidences in excess of the static stall angle. Another purpose is to examine, in some detail, the respective and combined effects of the major experimental parameters on the vortex wake development. It is shown that, in general, the dominant parameter of the flow is the reduced frequency not only when the airfoil oscillates at incidences close to the static stall angle but also at larger incidences. It is also demonstrated that, as the pitching frequency is increased, the patterns of the vortex wake are dependent on the product of the reduced frequency and the amplitude rather than on the frequency itself. It is noted that the combined effect of a high reduced frequency and a large amplitude can give rise to cyclic superposition of leading-edge vortices from which a gradually expanding standing vortex is developed on the upper surface.

Transonic flow around airfoils with relaxation and energy supply by homogeneous condensation

Abstract: Steady two-dimensional flow of vapor/carrier gas mixtures (moist air) with nonequilibrium condensation is investigated in theory and experiment. Aside from Laval nozzles, transonic flow over airfoils at M

Dynamic stall of a constant-rate pitching airfoil

Abstract: A computational study is presented for the dynamic stall of an airfoil that is pitched at a constant rate from zero incidence to a high angle of attack. The unsteady flow is simulated employing the mass-averaged NavierStokes equations and an algebraic turbulent eddy viscosity model. The approach is first validated by comparison of computed and experimental results for a pitching airfoil at low freestream Mach numbers. The computed dynamic stall events, as well as the computed effects of pitch rate and axis location, are found in qualitative agreement with experimental observations. The effect of compressibility on dynamic stall is investigated. As the freestream Mach number increases, the appearance of a supersonic region provides—through the shock/boundarylayer interaction—an additional mechanism in the dynamic stall process. The main effects of compressibility are found to be 1) a change from trailing-edge stall to leading-edge stall and 2) a reduction in the stall delay and in the attained maximum lift.

Control of flow separation by acoustic excitation

Abstract: To control the leading-edge flow separation on an airfoil by means of acoustic excitation, the response to the incident sound and the resulting flow instability are studied experimentally and therically on the basis of the linear stability theory, for a flat-plate airfoil, at a chord Reynolds number R c =4 x 10 4 .

Airfoil for the compression section of a rotary machine

Abstract: An airfoil for a compression section 12 of a rotary machine 10 is disclosed. Various construction details are developed to increase the efficiency of the compression section 12. In one detailed embodiment, the airfoil has a spanwise axis 52 or stacking line which extends in a generally radial direction. The stacking line or spanwise axis 52 is straight over the mid-section of the airfoil and is angled circumferentially toward the radial direction in the end wall regions 58, 66 of the airfoil.

Multi-airfoil Navier-Stokes simulations of turbine rotor-stator interaction

Abstract: An accurate numerical analysis of the flows associated with rotor-stator configurations in turbomachinery can be extremely helpful in optimizing the performance of turbomachinery In this study the unsteady, thin-layer, Navier-Stokes equations in two spatial dimensions are solved on a system of patched and overlaid grids for a rotor-stator configuration from an axial turbine The governing equations are solved using a finite-difference, upwind algorithm that is set in an iterative, implicit framework Results in the form of pressure contours, time-averaged pressures, unsteady pressure amplitudes and phase are presented The numerical results are compared with experimental data and the agreement is found to be good The results are also compared with those of an earlier study which used only one rotor and one stator The current study uses multiple rotors and stators and a pitch ratio that is much closer to the experimental ratio Consequently the results of this study are found to be closer to the experimental data

Computer-aided preform design in forging of an airfoil section blade

Abstract: This study attempts to establish systematic procedures for the preform design in forging of an airfoil section blade as a two-dimensional plane-strain problem. Forward loading and backward tracing simulations by the finite element method are used. A circular shape is selected as an original stock, and a side-pressed preform of the circular shape is adopted in view of preliminary simulations using rectangular preforms. The optimal slope angle of the die parting line and the position of the preform within the die, which satisfy the final design condition of flashless forging, are determined from the results of the simulations.

Measurements in a separation bubble on an airfoil using laser velocimetry

Abstract: An experimental investigation was conducted to measure the reverse flow within the transitional separation bubble that forms on an airfoil at low Reynolds numbers. Measurements were used to determine the effect of the reverse flow on integrated boundary-layer parameters often used to model the bubble. Velocity profile data were obtained on an NACA 663-018 airfoil at angle of attack of 12 deg and a chord Reynolds number of 140,000 using laser Doppler and single-sensor hot-wire anemometry. A new correlation is proposed based on zero velocity position, since the Schmidt (1986) correlations fail in the turbulent portion of the bubble.

Effects of a Fillet on the Flow Past a Wing-Body Junction

Abstract: Measurements are presented to demonstrate the effects of a single fillet on the flow of a turbulent boundary layer past an idealized wing-body junction. The time-averaged flow structure in the vicinity of the wing and in the wake of the wing-body junction is considered, as well as the unsteadiness of the horseshoe vortex. The effects of angle of attack and approach boundary-layer thickness are also examined.

Theoretical and numerical studies of oscillating airfoils

Abstract: Unsteady flowfields around airfoils oscillating in pitch and associated dynamic stall phenomena are investigated. A viscous flow analysis and a simplified vortical flow analysis, both based on an integro-differential formulation of the Navier Stokes equations are developed and calibrated. The formulation of the viscous flow analysis confines computations only to the viscous flow zone and leads to an efficient zonal solution procedure. In the simplified vortical flow analysis, computational demands are greatly reduced by partial analytic evaluations. Simulated flowfields and computed aerodynamic loads are in good agreement with available experimental data.

Surface for low drag in turbulent flow

Abstract: A nonsmooth surface of an object in a fluid flowstream permits a reduction of turbulent drag. The surface shape is designed to approximate a minimal surface and satisfies the equation (V·∇)∇Φ=0, where V is the relative velocity between the fluid and the surface, ∇Φ is the velocity of the strain tensor. The improved surface shape may be an array of diamond shaped elements having an airfoil like diagonal along the streamline, or it may be a series of wavelets juxtaposed to one another. The specific shape of the various types of elements to be used on the surface is selected based upon the acceleration or deceleration of the fluid at the particular position on the surface, as well as the intensity of the turbulence at that position.

Flow visualization studies of the Mach number effects on dynamic stall of an oscillating airfoil

Abstract: Compressibility effects on the dynamic stall of a NACA 0012 airfoil undergoing sinusoidal oscillatory motion were studied using a stroboscopic schlieren system. Schlieren pictures and some quantitative data derived from them are presented and show the influence of free-stream Mach number and reduced frequency on the dynamic-stall vortex. This study shows that a dynamic stall vortex always forms near the leading edge and convects on the airfoil upper surface at approximately 0.3 times the free stream velocity for all cases studied. The results also demonstrate that initiation of the dynamic stall vortex is delayed to higher angles of attack with increased reduced frequency, but that dynamic stall occurs at lower angles of incidence with increasing Mach numbers.

Euler flutter analysis of airfoils using unstructured dynamic meshes

Abstract: Modifications to a two-dimensional unsteady Euler code for the aeroelastic analysis of airfoils are described. The modifications involve including the structural equations of motion and their simultaneous time-integration with the governing flow equations. A novel aspect of the capability is that the solutions are obtained using unstructured grids made up of triangles. Comparisons are made with parallel calculations performed using linear theory and a structured grid Euler code to assess the accuracy of the unstructured grid Euler results. Results are presented for a flat plate airfoil and the NACA 0012 airfoil to demonstrate applications of the Euler code for generalized force computations and aeroelastic analysis. In these comparisons, two different finite-volume discretizations of the Euler equations on unstructured meshes were employed. Sensitivity of the Euler results to changes in numerical parameters were also investigated.

Time domain flutter analysis of cascades using a full-potential solver

Abstract: A time domain approach is used to determine the dynamic aeroelastic stability of a cascade of blades. The structural model for each blade is a typical section with two degrees of freedom. The aerodynamic model is the unsteady, two-dimensional, full-potential flow through the cascade of airfoils. The unsteady equations of motion for the structure and the fluid are integrated simultaneously in time starting with the steady flowfield and a small initial disturbance applied to the airfoils. The motion of each blade is analyzed to determine the aeroelastic stability of the cascade. The effect of interblade phase angle is included in the analysis by allowing each blade to have an independent motion and considering a number of blade passages. Calculations are made using an airfoil section and structural parameters that are representative of a propfan. The results are compared with those from a separate frequency domain analysis. Good agreement between the results is observed. With the time domain approach, it is possible to consider nonlinear structural models and nonlinear force-displacement relations. The method allows a realistic simulation of the motion of the fluid and the cascade blades for a better physical understanding and it also has the potential for saving computational time when compared to the frequency domain approach for the flutter analysis of cascades.

Various sources of wing rock

Abstract: Limit cycle oscillations in roll of advanced aircraft can result from three different fluid mechanical flow processes The so-called slender wing rock is caused by asymmetric vortex shedding from highly swept wing leading edges A completely different flow mechanism causes wing rock for aircraft with moderately swept leading edges In this case, the causative mechanism is dynamic airfoil stall

Acoustic radiation from lifting airfoils in compressible subsonic flow

Abstract: The far field acoustic radiation from a lifting airfoil in a three-dimensional gust is studied The acoustic pressure is calculated using the Kirchhoff method, instead of using the classical acoustic analogy approach due to Lighthill The pressure on the Kirchhoff surface is calculated using an existing numerical solution of the unsteady flow field The far field acoustic pressure is calculated in terms of these values using Kirchhoff's formula The method is validated against existing semi-analytical results for a flat plate The method is then used to study the problem of an airfoil in a harmonic three-dimensional gust, for a wide range of Mach numbers The effect of variation of the airfoil thickness and angle of attack on the acoustic far field is studied The changes in the mechanism of sound generation and propagation due to the presence of steady loading and nonuniform mean flow are also studied

Reinforced full-spar composite rotor blade

Abstract: A composite axial flow rotary machine blade (10) and method of blade construction is disclosed. A substantially full length composite spar (30) is spanwisely disposed within an airfoil planform (18) which supports an airfoil surface (11a). A pad of reinforcing fibers (40) is sandwiched between the spar's periphery and the airfoil surfaces' proximate inner surfaces (24a, b) corresponding to the pressure (22) and suction (24) sides of the airfoil planform in a spanwise distribution. A quantity of structural foam filler (50) is disposed through the balance of the blade's cavity (16).

Aerodynamic characteristics of two rotorcraft airfoils designed for application to the inboard region of a main rotor blade

Abstract: A wind tunnel investigation was conducted to determine the 2-D aerodynamic characteristics of two new rotorcraft airfoils designed especially for application to the inboard region of a helicopter main rotor blade. The two new airfoils, the RC(4)-10 and RC(5)-10, and a baseline airfoil, the VR-7, were all studied in the Langley Transonic Tunnel at Mach nos. from about 0.34 to 0.84 and at Reynolds nos. from about 4.7 to 9.3 x 10 (exp 6). The VR-7 airfoil had a trailing edge tab which is deflected upwards 4.6 degs. In addition, the RC(4)-10 airfoil was studied in the Langley Low Turbulence Pressure Tunnel at Mach nos. from 0.10 to 0.44 and at Reynolds nos. from 1.4 to 5.4 x 10 (exp 6) respectively. Some comparisons were made of the experimental data for the new airfoils and the predictions of two different theories. The results of this study indicates that both of the new airfoils offer advantages over the baseline airfoil. These advantages are discussed.

Pressure fluctuations in the tip region of a blunt-tipped airfoil

Abstract: The characteristics of turbulence generated in the tip region of a blunt-tipped airfoil were studied using surface pressure measurements. The model was a NACA 0012; tests were performed at flow speeds of 75, 55, and 35 m/s and angles of attack of 6, 12, and 16 deg. Reynolds numbers based on the wing chord were 1.9, 3.0, and 4.1 million. Pressure fluctuations measured near the primary tip-vortex on the upper, low pressure side of the wing tip were uncorrelated with those on the blunt tip. Fluctuations on the high pressure side of the wing were strongly correlated with those on the flat tip, but 10-20 dB less intense. Spectra measured on the flat tip displayed pronounced peaks at dimensionless frequencies of 0.8 to 1.3. Cross correlations between some of the flat-tip pressures displayed two echolike groupings. A model is proposed that explains these correlations.

Implicit flux-split Euler schemes for unsteady aerodynamic analysis involving unstructured dynamic meshes

Abstract: Improved algorithms for the solution of the time-dependent Euler equations are presented for unsteady aerodynamic analysis involving unstructured dynamic meshes. The improvements have been developed recently to the spatial and temporal discretizations used by unstructured grid flow solvers. The spatial discretization involves a flux-split approach which is naturally dissipative and captures shock waves sharply with at most one grid point within the shock structure. The temporal discretization involves an implicit time-integration shceme using a Gauss-Seidel relaxation procedure which is computationally efficient for either steady or unsteady flow problems. For example, very large time steps may be used for rapid convergence to steady state, and the step size for unsteady cases may be selected for temporal accuracy rather than for numerical stability. Steady and unsteady flow results are presented for the NACA 0012 airfoil to demonstrate applications of the new Euler solvers. The unsteady results were obtained for the airfoil pitching harmonically about the quarter chord. The resulting instantaneous pressure distributions and lift and moment coefficients during a cycle of motion compare well with experimental data. The paper presents a description of the Euler solvers along with results and comparisons which assess the capability.