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An analytical technique is described for predicting the plume impingement force during tandem stage separation, including the reversed flow effects. Comparisons of predicted and measured impingement forces for various geometric and flow conditions show good correlation between the analytical technique and experimental data.
This paper discusses the application of the constraint force equation methodology and its implementation for multibody separation problems using three specially designed test cases. The first test case involves two rigid bodies connected by a fixed joint, the second case involves two rigid bodies connected with a universal joint, and the third test case is that of Mach 7 separation of the X-43A vehicle. For the first two cases, the solutions obtained using the constraint force equation method compare well with those obtained using industry- standard benchmark codes. For the X-43A case, the constraint force equation solutions show reasonable agreement with the flight-test data. Use of the constraint force equation method facilitates the analysis of stage separation in end-to-end simulations of launch vehicle trajectories
An experimental study was conducted to investigate the effect of acoustic forcing on flow fields near the trailing edge of a symmetric airfoil at zero angle of attack. At low chord Reynolds numbers, the boundary layers separate from the surfaces in the rear part of the airfoil and create recirculation regions near the trailing edge. It is shown that with the introduction of acoustic forcing through a slot in the vicinity of the separation point, periodic large-scale structures are generated in the trailing edge region. Significant reduction of trailing edge separation is achieved. It is also found that the most effective forcing frequency to control trailing edge separation is the wake vortex shedding frequency. As a result of forcing, applied only on the upper surface, the upper boundary layer is accelerated and the flow over the lower surface decelerated. Consequently, an asymmetric wake is formed. The results presented indicate that the development of the near wake varies with forcing conditions.
Centrifugal force causes buffer, chosen to be denser than particles, to move outward and particles to move inward. Electrophoresis force can be made to equal centrifugal force. System tends not to be affected by convection and other disturbances that are so troublesome in conventional electrophoresis systems.
A pressure limiting propellant activating system for simultaneously limiting the output force while maintaining a constant output pressure from the combustion chamber is described. The propellant actuated system includes an outer barrel, outer housing and a combustion chamber. A main piston is movable in the barrel housing when gas pressure is developed in the combustion chamber. A relief piston is concentrically mounted and fixedly movable with the main piston when gas pressure is exerted from the combustion. A relief piston has a force-activated separation mechanism for limiting the output force while simultaneously maintaining constant output pressure on the main piston from the combustion chamber.
This paper is one of a series of five papers in a special session organized by the NASA Fundamental Aeronautics Program that addresses uncertainty assessments for CFD simulations in hypersonic flow. Simulations of a shock emanating from a compression corner and interacting with a fully developed turbulent boundary layer are evaluated herein. Mission relevant conditions at Mach 7 and Mach 14 are defined for a pre-compression ramp of a scramjet powered vehicle. Three compression angles are defined, the smallest to avoid separation losses and the largest to force a separated flow engaging more complicated flow physics. The Baldwin-Lomax and the Cebeci-Smith algebraic models, the one-equation Spalart-Allmaras model with the Catrix-Aupoix compressibility modification and two-equation models including Menter SST, Wilcox k-omega 98, and Wilcox k-omega 06 turbulence models are evaluated. Each model is fully defined herein to preclude any ambiguity regarding model implementation. Comparisons are made to existing experimental data and Van Driest theory to provide preliminary assessment of model form uncertainty. A set of coarse grained uncertainty metrics are defined to capture essential differences among turbulence models. Except for the inability of algebraic models to converge for some separated flows there is no clearly superior model as judged by these metrics. A preliminary metric for the numerical component of uncertainty in shock-turbulent-boundary-layer interactions at compression corners sufficiently steep to cause separation is defined as 55%. This value is a median of differences with experimental data averaged for peak pressure and heating and for extent of separation captured in new, grid-converged solutions presented here. This value is consistent with existing results in a literature review of hypersonic shock-turbulent-boundary-layer interactions by Roy and Blottner and with more recent computations of MacLean.
The mixing of magnetic lines of force, as represented by their rate of separation, as a function of distance along the magnetic field, is considered with emphasis on neighboring lines of force. This effect is particularly important in understanding the transport of charged particles perpendicular to the average magnetic field. The calculation is carried out in the approximation that the separation changes by an amount small compared with the correlation scale normal to the field, in a distance along the field of a few correlation scales. It is found that the rate of separation is very sensitive to the precise form of the power spectrum. Application to the interplanetary and interstellar magnetic fields is discussed, and it is shown that in some cases field lines, much closer together than the correlation scale, separate at a rate which is effectively as rapid as if they were many correlation lengths apart.
Despite the extensive experimental and computational data base in the literature on passive porosity, no clear explanation of the governing flow physics exists. It is theorized that the positive porosity concept modifies the external pressure loading by allowing communication between high- and low-pressure regions on the external surface. This study determines the dominant flow phenomena that govern the effectiveness of passive porosity. It aims to assess the contribution of each phenomenon as related to a porous slender axisymmetric forebody. To assess the influence of the mass transfer and pressure equalization phenomena on the effectiveness of passive porosity on slender axisymmetric forebodies, strakes were attached to the 5.0-caliber solid and porous forebodies to force crossflow separation. Longitudinal force and moment data were obtained at a Mach number of 0.1 over an angle-of-attack range of 0 to 55 deg.
Three separate Air Force Flight Test Center (AFFTC) tests were conducted in 1980 and 1981 on two models of the peripheral vision horizon displays (PVHD) (Malcolm Horizon). A fixed base simulator test was conducted with twenty test pilot subjects using the Flight Simulator Demonstration Model which incorporated a Helium Neon laser as the light bar medium. Two separate flight tests were conducted by the Test Pilot School classes 80A and 80B in a Twin Otter commuter aircraft using the Stage A Model PVHD. The Xenon lighted A Model was tested in its original configuration by class 80A. Class 80B used a modified configuration which incorporated an AFFTC designed and manufactured hood. With the hood, the PVHD projected a thinner, distinct light bar. Only a few general remarks concerning the tests and unrestricted, overall conclusions reached by the author are presented. The conclusions of all three AFFTC evaluations of the PVHD concept were that it has not yet been adequately evaluated. There seems to be a significant learning curve associated with the PVHD and the project pilots for Test Pilot School Class 80B only got a good start on the learning curve. A lengthy learning curve for the PVHD should be anticipated in view of the training period required for the attitude display indicator (ADI). This does seem to point out that the PVHD, in its present form, is simply not as compelling as the natural horizon. It can also be concluded that any attempt at a valid evaluation of the PVHD concept can be done only under instrument meteorological conditions (IMC) or validly simulated IMC conditions. The knee in the learning curve, however, may be reached without full IMC, although it may take much longer to reach.
Anthropogenic climate change is unfolding rapidly, yet its regional manifestation can be obscured by internal variability. A primary goal of climate science is to identify the externally forced climate response from among the noise of internal variability. Separating the forced response from internal variability can be addressed in climate models by using a large ensemble to average over different possible realizations of internal variability. However, with only one realization of the real world, it is a major challenge to isolate the forced response directly in observations. In the Forced Component Estimation Statistical Method Intercomparison Project (ForceSMIP), contributors used existing and newly developed statistical and machine learning methods to estimate the forced response over 1950–2022 within individual realizations of the climate system. Participants used neural networks, linear inverse models, fingerprinting methods, and low-frequency component analysis, among other approaches. These methods were trained using large ensembles from multiple climate models and then applied to observations. Here, we evaluate method performance within large ensembles and investigate the estimates of the forced response in observations. Our results show that many different types of methods are skillful for estimating the forced response in climate models, though the relative skill of individual methods varies depending on the variable and evaluation metric. Methods with comparable skill in models can give a wide range of estimates of the forced response pattern in observations, illustrating the epistemic uncertainty in forced response estimates. ForceSMIP gives new insights into the forced response in observations, its uncertainty, and methods for its estimation.
We investigate the dependence of charge yields on the driving force for photoinduced electron transfer in a series of all-small-molecule, semiconducting films made from indacenodithiophene nonfullerene acceptors (IDT NFAs). In contrast to reports of efficient, barrierless charge separation at near zero driving force for NFA-containing organic photovoltaics, we find that barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential, =300 meV, based on time-resolved microwave conductivity and PL quenching measurements of 5 mmolal sensitized and 700 mmolal blended films comprising IDT NFAs paired with three different donors. This discrepancy with recent literature is caused by a difference in the way that we calculate driving force. Far from being semantic, the driving force calculation is crucial because its value controls the mechanisms needed to explain experimental observations. We provide a candid assessment of the uncertainties for our methods and popular ones used in the literature and emphasize the importance of standardizing methods across this field.
Abstract Operation-induced surface reconstruction of photoelectrodes is underexplored as a path to control stability and performance. We show how adaptive junctions form via surface reconstruction of BiVO4 during glycerol photooxidation and how these surfaces affect electrolyte-dependent kinetics and durability. Preferential V dissolution in both acidic and alkaline media forms a Bi-rich layer. In situ measurements through a dual-working-electrode platform quantify the changes in photovoltage and charge-transfer resistance derived from adaptive junction formation, while enabling quantitative separation of the driving forces for charge separation and interfacial catalysis. The reconstructed surface in acidic media improves hole-transfer kinetics, functions as a glycerol-oxidation catalyst, and imparts photostability. Surface reconstruction in alkaline media exhibits the opposite behavior, impeding hole injection. This instability is mitigated by trace Ni2+ ions, which drive in situ surface activation without cocatalyst pre-deposition. This study shows the significance of surface reconstruction in the design of durable photoelectrodes for applications targeting organic electro-oxidation.
A dynamic force sensing method is proposed for a robot manipulator. A 6-axis wrist force sensor which is commonly used for robot force sensing does not take the direct measurement of the force and moment applied to an end effector externally from the environment of the manipulator, but takes the resultant of the gravity, the inertial force and moment on the end effector, and the external force and moment. The separation of gravitational and inertial force and moment from the measurement of the force sensor is crucial to achieving dynamically accurate force sensing and fast manipulator motion under force feedback control. A mathematical model is derived for the force sensing process and the separation of the force and moment as a nonlinear optimal state estimation problem formulated. Then an approximate solution is given using extended Kalman filtering techniques.
A small friction-stir-welding tool has been developed for use in tack welding of aluminum-alloy workpieces. It is necessary to tack-weld the workpieces in order to hold them together during friction stir welding because (1) in operation, a full-size friction-stir-welding tool exerts a large force that tends to separate the workpieces and (2) clamping the workpieces is not sufficient to resist this force. It is possible to tack the pieces together by gas tungsten arc welding, but the process can be awkward and time-consuming and can cause sufficient damage to necessitate rework. Friction stir tack welding does not entail these disadvantages. In addition, friction stir tack welding can be accomplished by use of the same automated equipment (except for the welding tool) used in subsequent full friction stir welding. The tool for friction stir tack welding resembles the tool for full friction stir welding, but has a narrower shoulder and a shorter pin. The shorter pin generates a smaller workpiece-separating force so that clamping suffices to keep the workpieces together. This tool produces a continuous or intermittent partial-penetration tack weld. The tack weld is subsequently consumed by action of the larger tool used in full friction stir welding tool.
Four booster separation motors (BSM) located at each end of every solid rocket booster (SRB) provide the needed side force to separate the boosters from the external tank at booster burnout. Four BSMs at the top of the SRB are located in a box pattern in the nose cone frustum. The four BSMs at the aft end of the SRB are arranged side-by-side on the SRB aft skirt. Aspects of BSM design and performance are considered, taking into account a motor design/performance summary, the case design, the insulation, the grain design, the nozzle/aft closure design, the ignition system, the propellant, and the motor assembly. Details of motor testing are also discussed, giving attention to development testing, qualification testing, and flight testing.
The results of an analytical study to determine the feasibility of a novel two-phase cold plate/hot plate (CPHP) are presented. A key feature of the CPHP is the use of capillary forces to separate the liquid and vapor phases and distribute the liquid over the evaporating/condensing surface. The liquid phase is held by capillary forces in a reservoir and is carried to the evaporating surface by a wick. The reservoir is replenished at intervals from a valved external liquid supply line. In the hot plate mode, liquid accumulates in the reservoir and is removed by an external condensate line. Performance requirements for the device were capability of handling a power density of 4 watts/sq cm, an outlet quality (percentage of vapor flow to total flow) greater than 90 percent, and operation in a 0-g environment. An analytical model of CPHP operation was developed which concentrated on the liquid and vapor flows. It was found that the liquid carrying capillary grooves of rectangular cross-section gave significantly better predicted performance than V-shaped grooves. Using the analytical model, capillary groove width and length, vapor channel dimensions, and other parameters were selected to meet the performance requirements with a 100-percent margin.
Active flow control (AFC) subscale experiments were conducted at the Lucas Wind Tunnel of the California Institute of Technology. Tests were performed on a generic vertical tail model at low speeds. Fluidic oscillators were used at the trailing edge of the main element (vertical stabilizer) to redirect the flow over the rudder and delay or prevent flow separation. Side force increases in excess of 50% were achieved with a 2% momentum coefficient (C(sub μ)) input. The results indicated that a collective C(sub μ) of about 1% could increase the side force by 30–50%. This result is achieved by reducing the spanwise flow on the swept back wings that contributes to early flow separation near their tips. These experiments provided the technical backdrop to test the full-scale Boeing 757 vertical tail model equipped with a fluidic oscillator system at the National Full-scale Aerodynamics Complex 40-by 80-foot Wind Tunnel, NASA Ames Research Center. The C(sub μ) is shown to be an important parameter for scaling a fluidic oscillator AFC system from subscale to full-scale wind tunnel tests. The results of these tests provided the required rationale to use a fluidic oscillator AFC configuration for a follow-on flight test on the Boeing 757 ecoDemonstrator.
The introduction of transverse velocity fluctuations into a separated shear layer on an airfoil at high angles of attack is presently demonstrated to be an effective separation-control technique. Airfoil aerodynamic characteristics, including poststall lift and drag as well as maximum lift coefficient and stall angle, all exhibited improvements controlled forcing at 20 deg angle of attack led to an increased spreading of the mean velocity profile, together with increased turbulence activity; separation moved from the leading edge to about 80 percent of chord.