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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Galileo inflight friction identification for the scan platform control loop

Two types of friction identification algorithms (position-based and rate-based) are investigated in an attempt to better understand cone actuator inflight behavior in the Galileo scan platform pointing system. This investigation involves the measurement of the scan platform position (or rate) with respect to the stator following a platform slew maneuver. Inflight data collection by the spacecraft's sensors and subsequent ground-based data processing comprise the two major steps in the identification experiment. The proposed algorithms were capable of obtaining friction estimates within + or - 10 percent accuracy; hence, it is concluded that this inflight system identification technique can sufficiently tune the Galileo cone controller.

Ianculescu, G. D.↗
NASA NTRS1987

Shaft Coupler With Friction and Spline Clutches

Coupling, developed for rotor of lift/cruise aircraft, employs two clutches for smooth transmission of power from gas-turbine engine to rotor. Prior to ascent, coupling applies friction-type transition clutch that accelerates rotor shaft to speeds matching those of engine shaft. Once shafts synchronized, spline coupling engaged and friction clutch released to provide positive mechanical drive.

Thebert, Glenn W.↗
NASA NTRS1986

Maximum lift/drag ratio of flat plates with bluntness and skin friction at hypersonic speeds

Newtonian theory is used to derive a simple expression for the maximum lift/drag ratio of flat plates with bluntness and skin friction at hypersonic speeds. The bluntness drag is assumed to be independent of angle of attack. Because the effect of skin friction is of second order over the angle of attack range for maximum lift/drag ratio, it was assumed constant. As an example, the expression is applied to the Space Shuttle.

Tauber, M. E.↗
NASA NTRS1986

A constitutive law for finite element contact problems with unclassical friction

Techniques for modeling complex, unclassical contact-friction problems arising in solid and structural mechanics are discussed. A constitutive modeling concept is employed whereby analytic relations between increments of contact surface stress (i.e., traction) and contact surface deformation (i.e., relative displacement) are developed. Because of the incremental form of these relations, they are valid for arbitrary load-deformation histories. The motivation for the development of such a constitutive law is that more realistic friction idealizations can be implemented in finite element analysis software in a consistent, straightforward manner. Of particular interest is modeling of two-body (i.e., unlubricated) metal-metal, ceramic-ceramic, and metal-ceramic contact. Interfaces involving ceramics are of engineering importance and are being considered for advanced turbine engines in which higher temperature materials offer potential for higher engine fuel efficiency.

Plesha, M. E.↗
NASA NTRS1986

Analytical formulation of orbiter-payload coupled by trunnion joints with Coulomb friction

An orbiter and its payload substructure are linked together by five trunnion joints which have thirty degrees-of-freedom. Geometric compatibility conditions require fourteen of the interface physical coordinates of the orbiter and payload to be equal to each other and the remaining sixteen are free to have relative motions under Coulomb friction. The component modes synthesis method using fourteen inertia relief attachment modes for the formulation of the coupled system is presented. The exact nonlinear friction function is derived based on the characteristics of the joints. Formulation is applicable to an orbiter that carries any number of payload substructures.

Liu, Frank C.↗
NASA NTRS1986

Force-free magnetic fields - The magneto-frictional method

The problem under discussion is that of calculating magnetic field configurations in which the Lorentz force j x B is everywhere zero, subject to specified boundary conditions. We choose to represent the magnetic field in terms of Clebsch variables in the form B = grad alpha x grad beta. These variables are constant on any field line so that each field line is labeled by the corresponding values of alpha and beta. When the field is described in this way, the most appropriate choice of boundary conditions is to specify the values of alpha and beta on the bounding surface. We show that such field configurations may be calculated by a magneto-frictional method. We imagine that the field lines move through a stationary medium, and that each element of magnetic field is subject to a frictional force parallel to and opposing the velocity of the field line. This concept leads to an iteration procedure for modifying the variables alpha and beta, that tends asymptotically towards the force-free state. We apply the method first to a simple problem in two rectangular dimensions, and then to a problem of cylindrical symmetry that was previously discussed by Barnes and Sturrock (1972). In one important respect, our new results differ from the earlier results of Barnes and Sturrock, and we conclude that the earlier article was in error.

Yang, W. H.↗
NASA NTRS1987

Skin friction measurements following manipulation of a turbulent boundary layer

Results are presented from three experiments in which direct, local measurements of the skin friction reduction due to flat-plate turbulence manipulators for overall viscous drag reduction were obtained. The results suggest that only a very small overall net drag reduction will be possible for such devices at moderate momentum thickness-derived Reynolds number values, since maximum skin friction drag is neither large nor sustained.

Nguyen, V. D.↗
NASA NTRS1987

Analytical formulation of orbiter-payload models coupled by trunnion joints with Coulomb friction

An orbiter and its payload substructure are linked together by five trunnion joints which have thirty degrees-of-freedom. Geometric compatibility conditions require fourteen of the interface physical coordinates of the orbiter and payload to be equal to each other and the remaining sixteen are free to have relative motions under Coulomb friction. The component modes synthesis method using fourteen inertia relief attachment modes for the formulation of the coupled system is presented. The exact nonlinear friction function is derived based on the characteristics of the joints. Formulation is applicable to an orbiter that carries any number of payload substructures.

Liu, Frank C.↗
NASA NTRS1986

Riblets for aircraft skin-friction reduction

Energy conservation and aerodynamic efficiency are the driving forces behind research into methods to reduce turbulent skin friction drag on aircraft fuselages. Fuselage skin friction reductions as small as 10 percent provide the potential for a 250 million dollar per year fuel savings for the commercial airline fleet. One passive drag reduction concept which is relatively simple to implement and retrofit is that of longitudinally grooved surfaces aligned with the stream velocity. These grooves (riblets) have heights and spacings on the order of the turbulent wall streak and burst dimensions. The riblet performance (8 percent net drag reduction thus far), sensitivity to operational/application considerations such as yaw and Reynolds number variation, an alternative fabrication technique, results of extensive parametric experiments for geometrical optimization, and flight test applications are summarized.

Walsh, Michael J.↗
NASA NTRS1988

The determination of the topological structure of skin friction lines on a rectangular wing-body combination

A short tutorial in the application of topological ideas to the intepretation of oil flow patterns is presented. Topological concepts such as critical points, phase portraits, topological stability, and indexing are discussed. These concepts are used in an ordered procedure to construct phase portraits of skin friction lines with oil flow patterns for a wing-body combination and two angles of attack. The relationship between the skin friction phase portrait and planar cuts of the velocity field is also discussed.

Yates, Leslie A.↗
NASA NTRS1987

Preliminary investigation of inertia friction welding B2 aluminides

An attempt is made to achieve inertia friction-welding in FeAl and NiAl samples, taking into account their intermetallics' compositions, extrusion parameters, and microstructural data. The energy required for the weld is stored in a rotating flywheel mass attached to one of the two pieces to be joined; when enough energy is introduced, the flywheel is disconnected and an axial load is applied which forces the spinning piece against the stationary one, converting the energy into heat by means of friction. Due to the inherent brittleness of the aluminides, a step-load program was used in which an initial, low-pressure heat buildup increased the work pieces' ductility.

Whittenberger, J. Daniel↗
NASA NTRS1988

Friction of ice

Frictional sliding experiments were performed on saw-cut samples of laboratory-made polycrystalline water ice, prepared in the same way as the material used by Kirby et al. (1987) in ice deformation experiments. The data show that the maximum frictional stress is a function of the normal stress but is not measurably dependent on temperature or sliding rate over the ranges covered in these experiments (77-115 K and 0.0003-0.03 mm/s, respectively). The sliding behavior was invariably stick slip, with the sliding surfaces exhibiting only minor gouge development. In samples with anomalously low strength, a curious arrangement of densely packed short vertical fractures was observed. The results of these experiments were applied to a model of near-surface tectonic activity on Ganymede, one of Jupiter's icy moons. The results indicate that a global expansion on Ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 + or - 3 km.

Beeman, M.↗
NASA NTRS1988

Some effects of finite spatial resolution on skin friction measurements in turbulent boundary layers

The effects of finite spatial resolution often cause serious errors in measurements in turbulent boundary layers, with particularly large effects for measurements of fluctuating skin friction and velocities within the sublayer. However, classical analyses of finite spatial resolution effects have generally not accounted for the substantial inhomogeneity and anisotropy of near-wall turbulence. The present study has made use of results from recent computational simulations of wall-bounded turbulent flows to examine spatial resolution effects for measurements made at a wall using both single-sensor probes and those employing two sensing volumes in a V shape. Results are presented to show the effects of finite spatial resolution on a variety of quantitites deduced from the skin friction field.

Westphal, Russell V.↗
NASA NTRS1988

Development of a two-dimensional skin friction balance nulling circuit using multivariable control theory

Measurement of planar skin friction forces in aerodynamic testing currently requires installation of two perpendicularly mounted, single-axis balances; consequently, force components must be sensed at two distinct locations. A two-axis instrument developed at the Langley Research Center to overcome this disadvantage allows measurement of a two-dimensional force at one location. This paper describes a feedback-controlled nulling circuit developed for the NASA two-axis balance which, without external compensation, is inherently unstable because of its low friction mechanical design. Linear multivariable control theory is applied to an experimentally validated mathematical model of the balance to synthesize a state-variable feedback control law. Pole placement techniques and computer simulation studies are employed to select eigenvalues which provide ideal transient response with decoupled sensing dynamics.

Tripp, John S.↗
NASA NTRS1989

Adaptive methods, rolling contact, and nonclassical friction laws

Results and methods on three different areas of contemporary research are outlined. These include adaptive methods, the rolling contact problem for finite deformation of a hyperelastic or viscoelastic cylinder, and non-classical friction laws for modeling dynamic friction phenomena.

Oden, J. T.↗
NASA NTRS1989

Skin-friction measurements by laser interferometry

The measurement of skin friction in rapidly distorted compressible flows is difficult, and very few reliable techniques are available. A recent development, the laser interferometer skin friction (LISF) meter, promises to be useful for this purpose. This technique interferometrically measures the time rate of thinning of an oil film applied to an aerodynamic surface. Under the proper conditions the wall shear stress may thus be found directly, without reference to flow properties. The applicability of the LISF meter to supersonic boundary layers is examined experimentally. Its accuracy and repeatability are assessed, and conditions required for its successful application are considered.

Kim, K.-S.↗
NASA NTRS1988

Friction-induced ignition of metals in high-pressure oxygen

Data are presented on friction-induced metal ignition (such as occurring as a result of the possible rubbing of oxygen-pressurized hydrostatic bearings and turbine blade tips) in a high-presure oxygen environment. Friction heating tests were carried out at oxygen pressures from 1 to 300 atm and surface speeds from 10 to 33 m/sec, using the NASA/White Sands Test Facility. Test results are presented on the rubbing of like material pairs spanning a wide range of burn factors and on that of dissimilar metal pairs having significantly different burn factors, indicating that the burn factor is a suitable index for rank ordering in ignition resistance for the most, but not all, materials.

Schoenman, Len↗
NASA NTRS1989

A skin friction law for compressible turbulent flow

An algebraic skin friction law is derived for adiabatic, compressible, equilibrium, turbulent boundary layer flow. An outer solution in terms of the Clauser defect stream function is matched to an inner empirical expression composed of compressible laws of the wall and wake. The modified Crocco temperature-velocity relationship and the Clauser eddy viscousity model are used in the outer solution. The skin friction law pertains for all pressure gradients in the incompressible through supersonic range and for small pressure gradients in the hypersonic range. Excellent comparisons with experiment are obtained in the appropriate parameter ranges. The application to numerical computation is discussed.

Barnwell, Richard W.↗