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At least 901 records · Page 50

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NASA NTRS1975

Augmentation of vortex lift by spanwise blowing

An investigation has been conducted to evaluate the aerodynamic effects associated with blowing a jet spanwise over a wing's upper surface in a direction parallel to the leading edge. Experimental pressure and force data were obtained on wings with sweep angles of 30 and 45 degrees and showed that spanwise blowing aids in the formation and control of the leading-edge vortex and, hence, significantly improves the aerodynamic characteristics at high angles of attack. Full vortex section lift is achieved at the inboard span station with a small blowing rate, but successively higher blowing rates are necessary to attain the full vortex-lift level at increased span distances. Spanwise blowing generates large increases in lift at high angles of attack, improves the drag polars, and extends the linear pitching moment to high lifts.

Campbell, J. F.↗

NASA NTRS1975

Hazard criteria for wake vortex encounters

A piloted, motion-base simulation was conducted to evaluate the ability of simulators to produce realistic vortex encounters and to develop criteria to define hazardous encounters. Evaluation of the simulation by pilots experienced in vortex encounters confirmed the capability of the simulator to realistically reproduce wake vortex encounters. A boundary for encounter hazard based on subjective pilot opinion was identified in terms of maximum bank angle. For encounter altitudes from 200 to 500 ft (61.0 to 152.4 m), tentative hazard criteria established for visual flight conditions indicated that the acceptable upset magnitude increased nearly linearly with increasing altitude. The data suggest that the allowable upsets under instrument conditions no greater than 50 percent of that allowable under visual conditions.

Sammonds, R. I.↗

NASA NTRS1975

Development and flight tests of vortex-attenuating splines

The ground tests and full-scale flight tests conducted during development of the vortex-attenuating spline are described. The flight tests were conducted using a vortex generating aircraft with and without splines; a second aircraft was used to probe the vortices generated in both cases. The results showed that splines significantly reduced the vortex effects, but resulted in some noise and climb performance penalties on the generating aircraft.

Hastings, E. C., Jr.↗

NASA NTRS1975

Low speed wind tunnel investigation of span load alteration, forward-located spoilers, and splines as trailing-vortex-hazard alleviation devices on a transport aircraft model

The effectiveness of a forward-located spoiler, a spline, and span load alteration due to a flap configuration change as trailing-vortex-hazard alleviation methods was investigated. For the transport aircraft model in the normal approach configuration, the results indicate that either a forward-located spoiler or a spline is effective in reducing the trailing-vortex hazard. The results also indicate that large changes in span loading, due to retraction of the outboard flap, may be an effective method of reducing the trailing-vortex hazard.

Croom, D. R.↗

NASA NTRS1976

Some recent applications of the suction analogy to vortex-lift estimates

An extension of the suction analogy for the estimation of vortex lift along the side edge of wings is reviewed along with the concept of an augmented vortex lift to account for the effect of the leading-edge vortex passing downstream over an aft part of the model. Applications of these extensions have resulted in an improved estimating capability for a wide range of isolated sharp-edge planforms and also for multiple lifting surfaces. Hence, the suction analogy concept can now have wider applicability at both subsonic and supersonic speeds, especially in the preliminary design cycle.

Lamar, J. E.↗

NASA NTRS1976

Minimum trim drag design for interfering lifting surfaces using vortex-lattice methodology

A new method has been developed by which the mean camber surface can be determined for trimmed noncoplanar planforms with minimum vortex drag under subsonic conditions. The method uses a vortex lattice and overcomes previous difficulties with chord loading specification; it uses a Trefftz plane analysis to determine the optimum span loading for minimum drag, then solves for the mean camber surface of the wing which will provide the required loading. Pitching-moment or root-bending-moment constraints can be employed as well at the design lift coefficient. Sensitivity studies of vortex-lattice arrangement have been made with this method and are presented. Comparisons with other theories show generally good agreement. The versatility of the method is demonstrated by applying it to (1) isolated wings, (2) wing-canard configurations, (3) a tandem wing, and (4) a wing-winglet configuration.

Lamar, J. E.↗

NASA NTRS1976

Comparison of vortex lattice predicted forces with wind tunnel experiments for the F-4E(CCV) airplane with a closely coupled canard

The F-4E (CCV) wind tunnel model with closely coupled canard control surfaces was analyzed by means of a version of a vortex lattice program that included the effects of nonlinear leading edge or side edge vortex lift on as many as four individual planforms. The results were compared with experimental data from wind tunnel tests of a 5% scale model tested at a Mach number M = 0.6. They indicated that a nonlinear vortex lift developed on the side edges due to tip vortices, but did not appear to develop on the leading edges within the range of angles of attack that were studied. Instead, substantial leading edge thrust was developed on the lifting surfaces. A configuration buildup illustrated the mutual interference between the wing and control surfaces. On the configuration studied, addition of the wing increased the loading on the canard, but the additional load on the canard due to adding the stabilator was small.

Gross, L. W.↗

NASA NTRS1976

A subvortex technique for the close approach to a discretized vortex sheet

The close-approach problem associated with vortex-lattice methods was examined numerically with the objective of calculating velocities at arbitrary points, not just at midpoints, between the vortices. The objective was achieved using a subvortex technique in which a vortex splits into an increasing number of subvortices as it is approached. The technique, incorporated in a two-dimensional potential flow method using "submerged" vortices and sources, was evaluated for a cambered Joukowski airfoil. The method could be extended to three dimensions, and should improve non-linear methods, which calculate interference effects between multiple wings and vortex wakes, and which include procedures for force-free wakes.

Maskew, B.↗

NASA NTRS1976

Vortex noise from nonrotating cylinders and airfoils

An experimental study of vortex-shedding noise was conducted in an acoustic research tunnel over a Reynolds-number range applicable to full-scale helicopter tail-rotor blades. Two-dimensional tapered-chord nonrotating models were tested to simulate the effect of spanwise frequency variation on the vortex-shedding mechanism. Both a tapered circular cylinder and tapered airfoils were investigated. The results were compared with data for constant-diameter cylinder and constant-chord airfoil models also tested during this study. Far-field noise, surface pressure fluctuations, and spanwise correlation lengths were measured for each configuration. Vortex-shedding noise for tapered cylinders and airfoils was found to contain many narrowband-random peaks which occurred within a range of frequencies corresponding to a predictable Strouhal number referenced to the maximum and minimum chord. The noise was observed to depend on surface roughness and Reynolds number.

Schlinker, R. H.↗

NASA NTRS1976

Vortex simulation of the pressure field of a jet

Fluctuations of the pressure field of a jet are simulated numerically by a flow model consisting of axisymmetric vortex rings with viscous cores submerged in an inviscid uniform stream. Vortex shedding time intervals, randomly created to imitate the time-history characteristics of the pressure signals of a jet, are generated based on a probability distribution of the intervals between successive pressure peaks obtained from experiments. It is found that, up to five diameters downstream of the jet exit, the characteristics of the pressure fluctuations and the most probable time intervals between experimental and numerical results show good qualitative agreements. The role played by the axisymmetric vortex model in pressure field as well as extensions of the model is also discussed.

Fung, Y. T.↗

NASA NTRS1976

Some experiments in swirling flows: Detailed velocity measurements of a vortex breakdown using a laser Doppler anemometer

The results of an experimental study of spiraling flows in a slightly diverging, circular duct are reported. Seven types of flow disturbances were observed. In addition to the spiral and axisymmetric vortex breakdowns and the double helix mode, four other forms were identified and are reported. The type and axial location of the disturbance depended on the Reynolds and circulation numbers of the flow. Detailed velocity measurements were made by using a laser Doppler anemometer. Measurements made far upstream of any disturbance showed that the introduction of swirl resulted in the formation of a high axial velocity jet centered around the vortex center. A mapping of the velocity field of a so-called axisymmetric breakdown, formed at a Reynolds number of 2560, revealed that the recirculation zone is a two-celled structure, with four stagnation points on the vortex axis marking the axial extremes of the concentric cells. The dominant feature of the flow inside the bubble was the strong, periodic velocity fluctuations. Existing theoretical models do not predict the two-celled structure and the temporal velocity fluctuations that were observed.

Faler, J. H.↗

NASA NTRS1977

Rolling moments in a trailing vortex flow field

Pressure distributions are presented which were measured on a wing in close proximity to a tip vortex of known structure generated by a larger, upstream semispan wing. Overall loads calculated by integration of these pressures are checked by independent measurements made with an identical model mounted on a force balance. Several conventional methods of wing analysis are used to predict the loads on the following wing. Strip theory is shown to give uniformly poor results for loading distribution, although predictions of overall lift and rolling moment are sometimes acceptable. Good results are obtained for overall coefficients and loading distribution by using linearized pressures in vortex-lattice theory in conjunction with a rectilinear vortex. The equivalent relation from reverse-flow theory that can be used to give economic predictions for overall loads is presented.

Mcmillan, O. J.↗

NASA NTRS1976

A numerical study of flow phenomena within a vortex sink rate sensor

A numerical investigation of a viscous incompressible fluid with constant properties within a vortex sink rate sensor is undertaken. A computer program was developed, for obtaining the numerical solution of the Navier-Stokes equations, assuming laminar flow, having general prescribed inlet conditions and axisymmetric boundary conditions. Computational results for various viscous flows and assorted boundary conditions have been obtained. In the vortex chamber, the radial momentum is conserved, whereas the axial momentum is negligible. In the sink region, the radial and tangential velocities, as well as the tangential vorticities, have significantly high values. In the sink tube, the radial momentum is negligible. The radial momentum within the vortex chamber is converted to the axial momentum in the sink tube. A change in the flow rate had an appreciable effect within the sensor and particularly near the sink tube entrance. As the flow rate was increased, both the tangential velocity and tangential vorticity increased rapidly. At the higher flow rates, vortices were produced at the corner of the entrance section of the sink tube.

Patel, D. K.↗

NASA NTRS1977

Rolling moments in a trailing vortex flow field

Mcmillan, O. J.↗

NASA NTRS1977

Lift enhancement by an externally trapped vortex

A theoretical study is made of the performance capabilities of a lift concept that utilizes a spanwise vortex over the upper surface of the wing. The vortex is generated by a vertical flap near the leading edge of the wing and maintained by suction through orifices in endplates at the wingtip. The analysis approximates the three-dimensional flow field with a two-dimensional configuration that is mapped by conformal transformation into the flow about a circle. Theoretical solutions for a range of flap and orifice configurations predict that section lift coefficients up to around 10 can be achieved. It is concluded that such a lift concept is applicable to STOL aircraft if the vortex can be adequately stabilized and if the endplate suction can be generated efficiently.

Rossow, V. J.↗

NASA NTRS1977

Ground development and flight correlation of the vortex attenuating spline device

Data presented indicate that the wing-mounted spline is a effective vortex-attenuating device. A comparison of the vortex induced rolling moment results at a separation scale distance of 0.70 km with those measured in full scale flight indicate good agreement for the unattenuated vortex configuration. The comparison also indicates that the spline effectiveness in flight was greater than in the ground facility test. The results of an applications study show that, for the heavy commercial jet aircraft studied, use of the splines does result in some degradation of the climb gradient and rate of climb, but the aircraft should meet certification requirements.

Patterson, J. C., Jr.↗

NASA NTRS1977

Vortex attenuation flight experiments

Flight tests evaluating the effects of altered span loading, turbulence ingestion, combinations of mass and turbulence ingestion, and combinations of altered span loading turbulance ingestion on trailed wake vortex attenuation were conducted. Span loadings were altered in flight by varying the deflections of the inboard and outboard flaps on a B-747 aircraft. Turbulence ingestion was achieved in flight by mounting splines on a C-54G aircraft. Mass and turbulence ingestion was achieved in flight by varying the thrust on the B-747 aircraft. Combinations of altered span loading and turbulence ingestion were achieved in flight by installing a spoiler on a CV-990 aircraft and by deflecting the existing spoilers on a B-747 aircraft. The characteristics of the attenuated and unattenuated vortexes were determined by probing them with smaller aircraft. Acceptable separation distances for encounters with the attenuated and unattenuated vortexes are presented.

Barber, M. R.↗

NASA NTRS1977

Investigation of compressible vortex flow characteristics

The nature of intense air vortices was studied and the factors which determine the intensity and rate of decay of both single and pairs of vortices were investigated. Vortex parameters of axial pressure differential, circulation, outflow rates, separation distance and directions of rotation were varied. Unconfined vortices, generated by a single rotating cage, were intensified by an increasing axial pressure gradient. Breakdown occurred when the axial gradient became negligible. The core radius was a function of the axial gradient. Dual vortices, generated by two counterrotating cages, rotated opposite to the attached cages. With minimum spacing only one vortex was formed which rotated in a direction opposite to the attached cage. When one cage rotated at half the speed of the other cage, one vortex formed at the higher speed cage rotating in the cage direction.

Muirhead, V. U.↗