Showing papers on "Freestream published in 1980"

Influence of Freestream Turbulence on Boundary-Layer Development

Abstract: The influence of the wind-tunnel turbulence on the development of a turbulent boundary was studied. The experiments were carried out in the low-turbulence wind tunnel of the DFVLR-AVA at a freestream velocity of U^ = 20 m/s. The turbulence level (Tut -0.06%) was increased up to Tuj «1% by means of various grids at different positions in the settling chamber or nozzle. For a fixed transition and constant distance from the nozzle throat, the effect of the wind-tunnel turbulence on the wall shear stress was investigated. In particular, an attempt was made to separate the effects which result from the turbulence intensity and from the turbulence structure, which is different in each wind tunnel.

Freestream Turbulence Effects on Galloping

Abstract: Galloping is the term used to describe large amplitude single degree-of-freedom motions of a structure associated with a sectional aerodynamic force characteristic which produces a force in the direction of and in phase with the cross-wind motion. Detailed experimental investigations show that freestream turbulence has profound effects on the galloping behavior of a square tower. It is generally believed that increase in freestream turbulence increases the turbulence mixing in the separated shear layers and the rate of entrainment from the wake, and decreases the radius of curvature of the shear layers. These effects significantly alter the transverse force characteristic and thus the galloping behavior of the tower. It is also shown that the fine-scale turbulence produced by a thin rod upstream of the stagnation streamline of the tower is sufficient to cause these effects.

Design considerations of advanced supercritical low drag suction airfoils

Abstract: Supercritical low drag suction laminar flow airfoils were laid out for shock-free flow at design freestream Mach = 0.76, design lift coefficient = 0.58, and t/c = 0.13. The design goals were the minimization of suction laminarization problems and the assurance of shock-free flow at freestream Mach not greater than design freestream Mach (for design lift coefficient) as well as at lift coefficient not greater than design lift coefficient (for design freestream Mach); this involved limiting the height-to-length ratio of the supersonic zone at design to 0.35. High design freestream Mach numbers result with extensive supersonic flow (over 80% of the chord) on the upper surface, with a steep Stratford-type rear pressure rise with suction, as well as by carrying lift essentially in front- and rear-loaded regions of the airfoil with high static pressures on the carved out front and rear lower surface.

Experimental investigation of drag on a compliant surface

Abstract: The feasibility of reducing turbulent skin-friction drag by means of surface compliance was studied experimentally in a fully developed flat-plate turbulent boundary layer in air, using a membranous surface backed by a thin cavity containing a layer of polyurethane foam. Surface motion characteristics, boundary layer structure, and overall drag were measured over a range of freestream speeds from 7 to 30 m/sec, and a range of membrane tensions from 44 to 350 N/m. Low-amplitude long-wavelength motions predominate, and no significant change from the rigid surface skin-friction coefficients was observed.

Aerodynamic interference between two Darrieus wind turbines

Abstract: The effect of aerodynamic interference on the performance of two curved bladed Darrieus-type vertical axis wind turbines has been calculated using a vortex/lifting line aerodynamic model. The turbines have a tower-to-tower separation distance of 1.5 turbine diameters, with the line of turbine centers varying with respect to the ambient wind direction. The effects of freestream turbulence were neglected. For the cases examined, the calculations showed that the downwind turbine power decrement (1) was significant only when the line of turbine centers was coincident with the ambient wind direction, (2) increased with increasing tipspeed ratio, and (3) is due more to induced flow angularities downstream than to speed deficits near the downstream turbine.

Aerodynamic heating for gaps in laminar and transitional boundary layers

Abstract: The presence of gaps, slots, and/or steps in a surface may significantly perturb a supersonic boundary layer. The paper discusses heat-transfer distributions for a variety of gap configurations which were obtained by placing instrumented inserts in a flat-plate structural carrier. The data were obtained in a 3.50 foot hypersonic wind tunnel at a freestream Mach number of 5.10 over a range of Reynolds number from 2,570,000 to 8,110,000. The variables of the test program included the freestream Reynolds number and the gap configuration, e.g., width, depth, step height, number, and orientation.

Resonance Frequencies of Ventilated Wind Tunnels

Abstract: EXPERIMENTS suggest that the theory widely used to predict the transverse resonance frequencies in slotted tunnels is in error in the 0-0.5 Mach number range. One reason for the error is that the theory is based on an unrepresentative wall boundary condition. Moreover, the theory implies that the plenum chamber depth is generally less than twice the tunnel height. An improved theory is developed which shows that the resonance frequencies of ventilated tunnels are influenced by the depth of the plenum chamber for Mach numbers up to about M=0.6. Although the theory is approximate, it agrees well with experiments for slotted and perforated walls (with both normal and 60 deg inclined holes) in a small pilot wind tunnel (100 x 100 mm). The earlier theory was only valid for slotted working sections. The results are consistent with other experiments, which show that plenum chamber design can influence the flow unsteadiness within the working section of a ventilated tunnel. Contents The previous theory for the resonance frequencies1 assumed that the oscillatory pressure difference across the equivalent homogeneous wall (which replaced the slotted wall) was independent of the plenum chamber size and proportional to the streamline curvature. The new theory2 includes a plenum chamber and uses the notation shown in Fig. 1. The two-dimensional working section extends from x= — oo to + 00 and has a uniform flow velocity U at a Mach number M. It is surrounded by two plenum chambers, each of depth dH/2, with zero mean flow. The working section is separated from the plenum chambers by homogeneous ventilated walls, which are thin, rigid, and have no boundary layers. For simplicity, we assume that the mean pressure and static temperature are the same in the working section and plenum chambers. Hence, the densities in the freestream and plenum chambers are the same. We know from the measurements of Smith and Shaw,3 however, that the static temperature within a cavity is close to the freestream total temperature, not the freestream static temperature; but the error in density is trivial at Mach numbers up to M= 1.0. For the oscillatory flow we seek compatible solutions for the velocity potentials c/> and \l/ in the freestream and plenum chambers. The boundary condition on the outer walls of the plenum chamber is simply that the normal velocity should be zero. Thus, plenum chamber, so that

Boundary-layer transition on a body of revolution

Abstract: The laminar-to-turbulent transition region was measured over a body length Reynolds number range of 9.55- 47.7 million on a large, streamlined, axisymmetric body sting mounted in the Applied Research Laboratory's 1.22 m diameter water tunnel. Theoretical calculations of the transition Reynolds numbers for this body shape were carried out and the results were correlated with the experimental data. It is shown that good correlation can be achieved if en is chosen as the transition criterion in the linear stability calculations, where n is an empirical number, which depends on the freestream turbulence intensity. It was found that this number approached 9 for those test velocities where the freestream turbulence intensity is on the order of 0.1%. OUNDARY-layer transition on axisymmetric bodies is of practical importance because its location influences the total skin frictional drag of the body. It is well recognized that moving the transition point downstream to larger body arc length positions results in drag reduction. Consequently, much of the current research is dealing with boundary-layer control concepts such as temperature differentials at the wall and surface suction. The Applied Research Laboratory at The Pennsylvania State University is actively engaged in transition research from both the analytical and experimental points of view. Experiments are generally designed for the 1.22 m diameter water tunnel located at this laboratory. This facility is par- ticularly suited for such work because of the high water velocities that can be attained (up to 19.8 m/s), the ability to test large bodies ( — 0.50 m in diameter and -3.5 m long), and the fact that settling section turbulence management results in test section turbulence intensities that are quite low ( — 0.1 %). Under normal operating water temperatures (25 °C) this water tunnel has an upper unit Reynolds number of approximately 21 million/m. By way of preliminary preparation for planned experiments on bodies with various types of boundary-laye r control, the work described in this paper was carried out. After a can- didate body shape was selected and a test model fabricated, experiments were performed in the 1.22 m diameter water tunnel to acquire baseline transition data and in-tunnel body pressure distributions. Except for the hydrodynamic shape of this body, no boundary-laye r control was employed in these experiments. The results of the pressure distribution measurements are presented elsewhere.! In addition to the experimental results, theoretical calculations for the growth of linear disturbances in the laminar boundary layer are also described for the test body. These calculations were performed using the methods of Gentry and Wazzan,2 where the body pressure distribution (which is a required input) was calculated from potential flow theory and corrected for inviscid tunnel interference ef- fects.3'4 The transition location may be deduced from this

Turbulent boundary layer heat transfer experiments: Convex curvature effects, including introduction and recovery

Abstract: Measurements were made of the heat transfer rate through turbulent and transitional boundary layers on an isothermal, convexly curved wall and downstream flat plate The effect of convex curvature on the fully turbulent boundary layer was a reduction of the local Stanton numbers 20% to 50% below those predicted for a flat wall under the same circumstances The recovery of the heat transfer rates on the downstream flat wall was extremely slow After 60 cm of recovery length, the Stanton number was still typically 15% to 20% below the flat wall predicted value Various effects important in the modeling of curved flows were studied separately These are: the effect of initial boundary layer thickness, the effect of freestream velocity, the effect of freestream acceleration, the effect of unheated starting length, and the effect of the maturity of the boundary layer An existing curvature prediction model was tested against this broad heat transfer data base to determine where it could appropriately be used for heat transfer predictions

Turbulence Amplification in Flow About an Airfoil

Abstract: An experimental investigation of the evolution of freestream turbulence in flow about an airfoil was conducted in order to ascertain its selective amplification induced by the stretching mechanism according to the vorticity-amplification theory Significant amplification of the streamwise turbulent energy transpired even in the limiting flow situation studied of a symmetric airfoil at zero angle of attack where the stretching is the least Substantiation of the stretching effect was provided by the almost 100 percent amplification of turbulence with respect to its background level in the absence of the airfoil Realization of preferred amplification at scales larger than the neutral scale of the stagnation flow was clearly indicated by the variation of the discrete streamwise turbulent energy Particularly important was the detection of a most amplified scale which is characteristic of the coherent substructure near the airfoil stagnation zone and, concurrently, commensurate with the boundary-layer thickness

Aerodynamic interference between two Darrieus wind turbines

Abstract: The effect of aerodynamic interference on the performance of two curved bladed Darrieus-type vertical axis wind turbines has been calculated using a vortex/lifting line aerodynamic model. The turbines have a tower-to-tower separation distance of 1.5 turbine diameters, with the line of turbine centers varying with respect to the ambient wind direction. The effects of freestream turbulence were neglected. For the cases examined, the calculations showed that the downwind turbine power decrement (1) was significant only when the line of turbine centers was coincident with the ambient wind direction, (2) increased with increasing tipspeed ratio, and (3) is due more to induced flow angularities downstream than to speed deficits near the downstream turbine.

Experimental investigation of the asymmetric body vortex wake

Abstract: An experimental investigation of the asymmetric body vortex wake of a circular cylinder in high subsonic flow is presented. Laser velocimeter, force and moment, and surface hot wire measurements were obtained for a freestream Mach number of 0.6 and Reynolds number (based on body diameter) of 0.62 x 10 to the 6th. Two component laser velocimeter measurements were made at three body cross-flow planes, x/d = 4, 8, and 12, and angles of attack of 25, 35, and 45 deg. Laser vapor screen photographs were also obtained at these body stations and angles of attack. Surface hot wire measurements were used to determine if any vortex switching occurred at various angles of attack of the body. The laser velocimeter measurements are related to the vapor screen photographs and side force measurements. These results show that more than one asymmetric body vortex wake configuration can exist for the same angle of attack and body roll angle.

Time Dependent Navier-Stokes Solution of a Turbulent Gas Jet Ejected from a Rectangular Orifice into a High-Subsonic Crossflow

Abstract: : High temperature exhaust gases from an airborne chemical laser ejected at a jet to freestream dynamic pressure ratio (Q) of 0.15 from an aspect ratio 1.75 rectangular diffuser exit aligned parallel to the ambient crossflow was numerically simulated. The time dependent, three-dimensional Navier-Stokes equations and a species conservation equation were solved. Diffusive flux effects caused by concentration gradients as well as variable transport and thermodynamic properties were incorporated into the numerical model. Turbulence closure was achieved by a locally varying velocity defect eddy viscosity model. Chemical reactions betwen the exhaust gases and the crossflow were proscribed. The trajectory of the jet plume, the extent of recirculation zones, and regions with high rates of heat transfer were defined. Simplified analyses demonstrated that essential flow phenomena were replicated. Convective processes dominated the low Q jet-crossflow interaction. Thermal diffusion had significantly greater effect than molecular diffusion for the jet-crossflow gases simulated. Jet penetration was dependent upon the molecular weight of the injectant for the constant Q constraint. A molecular weight correction factor was empirically used to synthesize the trajectory of one gas from that of another gas and to correct empirical trajectory formulae for molecular weight variances. Sensitivity analyses relating heat transfer to the injection surface from the jet plume with the magnitude of the turbulence diffusivities were conducted.

Turbulent Flows Produced by Perforated Plate Generators in Wind Tunnels

Abstract: ANEW application of perforated sheet metal plates has been shown to provide an efficient method of producing a large range of turbulent flows for experimental investigation. Fabrication of a standard uniformly perforated grid was done quickly, and different flow geometries were produced accurately without the use of special equipment. The standard grid and two geometric variations were used to produce a range of turbulent flows: i.e., isotropic, wake, and uniform shear flows comparable in quality to similar flows produced experimentally by other methods. Contents The system described here utilizes a perforated sheet metal plate to produce three different flows. Although this type of grid has been used to reduce the turbulence intensity in wind tunnels, in this new application a large variety of experimentally useful flow configurations can be quickly fabricated. Making easy variations of a basic grid geometry, an \"isotropic\" flowfield can be changed into a simulated \"wake\" flow or mean shear flowfields without resorting to the complication of introducing new structure inside the tunnel. These flows have the added benefit of an external turbulent flowfield that simplifies hot wire anemometry measurements by eliminating the intermittancy region. The perforated plate generator is an efficient solution to the usual constraints on flow manipulators such as permissible pressure drop, structural and spatial requirements, and cost of dif

Freestream turbulence effects on the development of a rotor wake

Abstract: The effect of freestream turbulence on the development of a three-dimensional wake of a compressor rotor blade was studied experimentally. The turbulence level at the inlet of a rotor was varied systematically using grids upstream of the rotor. The rotor wake was measured with inlet turbulence intensities of 0.5, 3, and 5%. The experimental results indicate that the maxium change in the mean velocity defect is 4% over the range of inlet turbulence levels employed, while the turbulence structure in the wake is altered more substantially. The freestream turbulence effect was also analyzed, numerically, using the modified Reynolds stress closure model. The comparison between numerical prediction and experimental data shows that the freestream turbulence effect can be represented successfully with the turbulence closure model employed in this paper.

Aerodynamic Performance of a 5-m-Diameter Darrieus Turbine

Abstract: A 5-m-diam vertical-axis wind turbine has undergone continued testing since 1976 at the Sandia National Laboratories Wind Turbine Site. The latest tests of this machine have been with extruded aluminum blades of NACA-0015 airfoil cross section. The results of these tests at several turbine rotational speeds are presented and compared with earlier test results. A performance comparison is made with a vortex/lifting line computational code. The performance of the turbine with the extruded blades met all expectations. Nomenclature A s = turbine swept area c = blade chord CdQ - zero wind drag coefficient Cp = power coefficient, Qco/ ttp^ V3(XAS J = advance ratio, V^Ru Kp = power coefficient, Q^/1ApQOAs (Ru)3 L = blade length N = number of blades Q = turbine aerodynamic torque (T+ Qf) Qf = friction tare torque R = turbine maximum radius Rec = chord Reynolds number, T = turbine shaft torque V^ = average freestream velocity X = turbine tip-speed ratio, /to/ V^ fji^ = freestream viscosity px = freestream density o) = turbine rotational speed a = solidity, NcL IA s

Coherent substructure of turbulence near the stagnation zone of a bluff body

Abstract: The evolution of freestream turbulence in crossflow about a circular cylinder was studied in order to identify the existence of a coherent substructure which is the outcome of the amplification of freesteam turbulence by the stretching mechanism in diverging flow about a bluff body. Visualization of the flow events revealed the selective stretching of cross-vortex tubes and the emergence of an organized turbulent flow pattern near the cylinder stagnation zone. Significant amplification of the total turbulent energy of the streamwise fluctuating velocity was consistently monitored. Realization of selective amplification at scales larger than the neutral scale of the stagnation flow was indicated by the variation of the discrete streamwise turbulent energy. A most amplified scale, characteristic of the energy containing eddies within the coherent substructure and commensurate with the boundary-layer thickness, was detected. Penetration of the amplified turbulence into the cylinder boundary layer led to the retardation of separation and to a concurrent decrease in the drag coefficient at subcritical cylinder-diameter Reynolds numbers.

Flowfield calculations past an entry probe with ablated nose shape

Abstract: The effects of ablated nose shapes on the flowfield solutions are studied, using a time-dependent finite-difference method developed by Kumar, et al. (1979). Solutions are obtained for the laminar flow of a radiating mixture of H-He in chemical equilibrium past a blunt axisymmetric body at zero angle of attack. The freestream conditions correspond to a point on a typical Jovian entry trajectory, and the initial probe shape is a 45-deg half-angle spherically blunted cone. It is found that as nose bluntness increases, the following occur: in the nose region, shock standoff distances and radiative heating rates increase substantially; surface pressure level increases, but convective heating rates decrease.

Complete viscous flowfield solutions about a blunt parabolic body in a supersonic stream

Abstract: A hybrid computational technique which splits the flowfield into inviscid and viscous regions is used to investigate the complete flowfield about axisymmetric parabolic blunt bodies in a supersonic stream. The solutions are carried out on the CDC CYBER-203 computer which, with its extensive memory, allows for the use of a large number of finite-difference mesh points, allowing resolution of important flowfield features. A range of freestream Mach number of 2-5 and a range of Re number based on nose radius of 500-125,000 was run for a sonic corner body. Contour plots of density, pressure, and Mach number, velocity vector plots, and surface distributions of pressure, heat transfer, and shear stress are presented. Also, correlations of the downstream extent of the base recirculation region with Re number based on nose radius are given.

Experimental study of the reynolds number effect on the length of the trailing edge region

Abstract: In order to determine the Reynolds number effect on the length of the trailing edge region, thin flat plates are stud- ied for various freestream flow conditions. The Reynolds number range, 260

Transonic flow over airfoils with tangential injection

Abstract: The influence of tangential slot injection in an attached, turbulent boundary layer on the pressure distribution of an airfoil is investigated for transonic freestream Mach numbers. The flow field is calculated by matching finite-difference solutions of the transonic small disturbance equation and the boundary layer equations. The results, obtained for different slot positions and injection rates, indicate that the surface pressure distribution can significantly be changed by tangential injection.