Instability Results for Cone-Cylinder-Flare and Cone-Flare Hypersonic Wind-Tunnel Configurations
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Boundary Layer Transition↗
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Boundary-Layer Instabilities over Cone-Cylinder-Flare Models at Hypersonic Speeds
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Boundary Layer Transition↗
Modal and Nonmodal Instabilities over Cone-Cylinder-Flare Hypersonic Wind-Tunnel Configurations
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Boundary Layer Transition↗
Ground Tests on the BOLT Geometry at Mach 7: Cross-Facility Comparison and Stability Analysis
Transition measurements have been obtained through two experimental campaigns conducted independently by the German Aerospace Center and the French Aerospace Lab with the French Alternative Energies and Atomic Energy Commission on subscale models of the BOLT-1 flight experiment geometry. This paper details a cross-facility comparison of measurements obtained at Mach 7, as well as subsequent computational analysis. Infrared (IR) thermography measurements obtained by both campaigns have facilitated a global comparison of the transition front across facilities at analagous conditions, which are found to be in good agreement. High-frequency surface pressure fluctuation data demonstrate significant amplification of instabilities with Mack-mode characteristics in the outboard regions of the acreage. These measurements are compared to stability analyses of varying fidelity. The computational methods employed to characterize the boundary-layer transition phenomena include the traditional line-marching implementation of the Parabolized Stability Equations (PSE), 2D eigenvalue analysis coupled with PSE, and Adaptive Mesh Refinement Wavepacket Tracking. While the 2D eigenvalue analysis is found to predict instabilities, which correlate in terms of frequency and acreage location to the experimental measurements, the predicted amplification for these instabilities is lower than would typically be expected for transition. Line-marching results for traveling crossflow produce the best match to the experimental transition front, with a consistent transition N factor of approximately 3-3.5.
Boundary Layer Transition↗
Unveiling the Journey of A Highly Inclined CME: Insights from the March 13, 2012, Event With 110° Longitudinal Separation
Context. A fast (∼2000 km s -1 ) and wide (>110°) coronal mass ejection (CME) erupted from the Sun on March 13, 2012. Its interplanetary counterpart was detected in situ two days later by STEREO-A and near-Earth spacecraft, such as ACE, Wind, and Cluster. We suggest that at 1 au the CME extended at least 110° in longitude, with Earth crossing its east flank and STEREO-A crossing its- west flank. Despite their separation, measurements from both positions showed very similar in situ CME signatures. The solar source region where the CME erupted was surrounded by three coronal holes (CHs). Their locations with respect to the CME launch site were east (negative polarity), southwest (positive polarity) and west (positive polarity). The solar magnetic field polarity of the area covered by each CH matches that observed at 1 au in situ. Suprathermal electrons at each location showed mixed signatures with only some intervals presenting clear counter streaming flows as the CME transits both locations. The strahl population coming from the shortest magnetic connection of the structure to the Sun showed more intensity. Aims. The aim of this work is to understand the propagation and evolution of the CME and its interaction with the surrounding CHs, to explain the similarities and differences between the observations at each spacecraft, and report what one of the most longitudinal expanded CME structures measured in situ would be. Methods. Known properties of the large-scale structures from a variety of catalogues and previous studies were used to have a better overview of this particular event. In addition, multipoint observations were used to reconstruct the 3D geometry of the CME and determine the context of the solar and heliospheric conditions before the CME eruption and during its propagation. The graduated cylindrical shell model (GCS) was used to reproduce the orientation, size and speed of the structure with a simple geometry. Also, the Drag-Based Model (DBM) was utilised to understand the conditions of the interplanetary medium better in terms of the drag undergone by the structure while propagating in different directions. Finally, a comparative analysis of the different regions of the structure through the different observatories was carried out in order to directly compare the in situ plasma and magnetic field properties at each location. Results. The study presents important findings regarding the in situ measured CME on March 15, 2012, detected at a longitudinal separation of 110° in the ecliptic plane despite its initial inclination being around 45° when erupted (March 13). This suggests that the CME may have deformed and/or rotated, allowing it to be observed near its legs with spacecraft at a separation angle greater than 100°. The CME structure interacted with high-speed streams generated by the surrounding CHs. The piled-up plasma in the sheath region exhibited an unexpected correlation in magnetic field strength despite the large separation in longitude. In situ observations reveal that at both locations there was a flank encounter – where the spacecraft crossed the first part of the CME – then encountered ambient solar wind, and finally passed near the legs of the structure. Conclusions. A scenario covering all evidence is proposed for both locations with a general view of the whole structure and solar wind conditions. Also, the study shows the necessity of having multipoint observations of large-scale structures in the heliosphere.
methods: data analysis↗
Leading-edge transition and relaminarization phenomena on a subsonic high-lift system
Boundary-layer transition and relaminarization may have a critical effect on the flow development about multielement high-lift systems of subsonic transport aircraft with swept wings. The purpose of this paper is a study of transition phenomena in the leading-edge region of the various elements of a high-lift system. The flow phenomena studied include transition of the attachment-line flow, relaminarization, and crossflow instability. The calculations are based on pressure distributions measured in flight on the NASA Transport Systems Research Vehicle (Boeing 737-100) at a wing station where the flow approximated infinite swept wing conditions. The results indicate that significant regions of laminar flow can exist on all flap elements in flight. In future flight experiments the extent of these regions will be measured, and the transition mechanisms and the effect of laminar flow on the high-lift characteristics of the multi-element system will be further explored.
Van Dam, C. P.↗
A method for the evaluation of damage on the surface of carbon-carbon ablation samples
The effect of weave geometry on the onset of transition from laminar to turbulent flow around a sample of an orthogonally woven fine weave carbon-carbon material tested in an arc-jet is investigated. Seventeen post-test models which had undergone only partial transition around the circumference were examined as to location of turbulent regions relative to the weave axes. It was statistically shown that the weave axes had the lowest probability of being a position of initial transition onset. The probability generally increased with distance from a major axis, with some rays, notably the 45 deg rays, showing better transition resistance than the adjacent rays. The incidence of initial transition onset correlated well with the number of X or Y fibers that the flow passes over from stagnation point to the shoulder.
Heinonen, E. W.↗
Flow and fracture behavior of NiAl in relation to the brittle-to-ductile transition temperature
NiAl has only three independent slip systems operating at low and intermediate temperatures whereas five independent deformation mechanisms are required to satisfy the von Mises criterion for general plasticity in polycrystalline materials. Yet, it is generally recognized that polycrystalline NiAl can be deformed extensively in compression at room temperature and that limited tensile ductility can be obtained in extruded materials. In order to determine whether these results are in conflict with the von Mises criterion, tension and compression tests were conducted on powder-extruded, binary NiAl between 300 and 1300 K. The results indicate that below the brittle-to-ductile transition temperature (BDTT) the failure mechanism in NiAl involves the initiation and propagation of cracks at the grain boundaries which is consistent with the von Mises analysis. Furthermore, evaluation of the flow behavior of NiAl indicates that the transition from brittle to ductile behavior with increasing temperature coincides with the onset of recovery mechanisms such as dislocation climb. The increase in ductility above the BDTT is therefore attributed to the climb of the 001 line type dislocations which in combination with dislocation glide enable grain boundary compatibility to be maintained at the higher temperatures.
Noebe, R. D.↗
Low-dimensional chaos in turbulence
Direct numerical simulations are being performed on two different fluid flows in an attempt to discover the mechanism underlying the transition to turbulence in each. The first system is Taylor-Couette flow; the second, two-dimensional flow over an airfoil. Both flows exhibit a gradual transition to high-dimensional turbulence through low-dimensional chaos. The hope is that the instabilities leading to chaos will be easier to relate to physical processes in this case, and that the understanding of these mechanisms can then be applied to a wider array of turbulent systems.
Vastano, John A.↗
Numerical Simulation of a Spatially Evolving Supersonic Turbulent Boundary Layer
The results from direct numerical simulations of a spatially evolving, supersonic, flat-plate turbulent boundary-layer flow, with free-stream Mach number of 2.25 are presented. The simulated flow field extends from a transition region, initiated by wall suction and blowing near the inflow boundary, into the fully turbulent regime. Distributions of mean and turbulent flow quantities are obtained and an analysis of these quantities is performed at a downstream station corresponding to Re(sub x)= 5.548 x10(exp 6) based on distance from the leading edge.
Gatski, T. B.↗
Analysis of flow properties in relation to molecular parameters for polymer melts.
Shear rate at which transition from Newtonian to non-Newtonian flow occurs for polystyrene, polyethylene and polydimethylsiloxane
POLYETHYLENE↗
The use of oil for in-flight flow visualization
Oil was used to visualize inflight aerodynamic characteristics such as boundary layer transition, shock wave location, regions of separated flow, and surface flow direction. The technique, which is similar to wind tunnel oil-flow testing, involves an oil mixture to test aircraft before takeoff. After takeoff, the airplane climbs immediately to the test altitude and photographs are taken. The developmental experience is summarized, several examples of inflight oil-flow photographs are presented and discussed, and an approach for potential users of the technique is presented.
Curry, R. E.↗
Analysis of airfoil transitional separation bubbles
A previously developed local inviscid-viscous interaction technique for the analysis of airfoil transitional separation bubbles, ALESEP (Airfoil Leading Edge Separation), has been modified to utilize a more accurate windward finite difference procedure in the reversed flow region, and a natural transition turbulence model has been incorporated for the prediction of transition within the separation bubble. Numerous calculations and experimental comparisons are presented to demonstrate the effects of the windward differencing scheme and the natural transition turbulence model. Grid sensitivity and convergence capabilities of this inviscid-viscous interaction technique are briefly addressed. A major conclusion of this paper is that a second, counter-rotating eddy has been found to exist in the wall layer of the primary separation bubble with the use of windward differencing.
Davis, R. L.↗
On the origin of the streak spacing in turbulent shear flows
It is shown that the ideas of selective amplification and direct resonance, based on linear theory, can not provide an explanation for the well-defined streak spacing of about 100 wall units (referred to as 100(+) hereafter) in wall-bounded turbulent shear flows. In addition, for the direct resonance theory, the streaks are created by the non-linear self-interaction of the vertical velocity rather than of the directly forced vertical vorticity. In view of the failure of these approaches, it is then proposed that the selection mechanism must be inherently non-linear and correspond to a self-sustaining mechanism. The 100(+) value should thus be considered as a critical Reynolds number for that mechanism. Indeed, in the case of Poiseuille flow, this 100(+) criterion for transition to turbulence corresponds to the usually quoted value of 1000 based on the half-width and the centerline velocity. In Couette flow, it corresponds to a critical Reynolds number of about 400 based on the half width and half velocity difference.
Waleffe, Fabian A.↗
Wind and Rotation Enhanced Escape from the Early Terrestrial Atmospheres
The earliest atmospheres of the terrestrial planets are thought to have been hotter, have stronger winds and rotate faster than atmospheres of today. Since these primitive atmospheres were weakly bound, they evolved rapidly because atmospheric escape was very strong, often referred to as "blowoff." Such escape has been treated as hydrodynamic, transonic flow; similar to solar wind flow dynamics. However, in many cases, although the outward flow is hydrodynamic at low altitudes, it becomes collisionless at higher altitudes, before sonic speeds are ever attained. Recent models dealing with the transition from fluid to kinetic flow have applied the Jeans escape flux at the exobase. This approach leads to escape rates that are too low, because thermospheric winds and planetary rotation are known to increase the escape flux above the corresponding Jeans flux. In particular, for a given density and temperature at the exobase, the escape flux increases as the wind speed and/or the rotation rate increase. Also, for a given wind speed and rotation rate, the escape flux enhancement over the Jeans flux increases as the mass of an escaping constituent increases, an important factor in isotope fractionation, especially the enrichment of deuterium on Mars. Accounting for a range of possible temperatures, thermospheric wind speeds and planetary rotation rates in the primitive atmospheres of the terrestrial planets, estimates are made of light constituent escape flux increases over the corresponding Jeans fluxes.
Hartle, Richard E.↗
Chasing Fast Dynamos in the Plasma Lab
Self-generating magnetic field research is performed through the operation of the newly built Madison Plasma Dynamo Experiment (MPDX). MPDX is designed to explore a hitherto unexplored part of parameter space where dynamos operate in nature. Dynamos are systems which continuously transform kinetic energy from plasma flow into magnetic energy. Before this project, MPDX had demonstrated the ability to create steady-state, unmagnetized high conductivity, low viscosity plasmas in the lab and shown that sheared flows can be driven from the plasma boundary using an electrostatic stirring technique; the resulting Reynolds numbers and Alfvén Mach numbers easily match those required for our dynamo scenarios. The research activities included: (1) systematic undertaking of a set of plasma hydrodynamics experiments, exploring several different flow geometries, measuring plasma viscosity, how toroidal and poloidal flows self-consistently interact, observing the transition to turbulence and experimentally understanding the boundary conditions (on flow and magnetic field) that make this system unique, (2) searching for slow, laminar dynamos, and (3) searching for fast, turbulent dynamos. Discovering the conditions under which dynamos self-generate magnetic fields and then understanding how this field changes plasma dynamics is one of the most compelling questions in all of plasma astrophysics.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Quiet Supersonic Wind Tunnel Development
The ability to control the extent of laminar flow on swept wings at supersonic speeds may be a critical element in developing the enabling technology for a High Speed Civil Transport (HSCT). Laminar boundary layers are less resistive to forward flight than their turbulent counterparts, thus the farther downstream that transition from laminar to turbulent flow in the wing boundary layer is extended can be of significant economic impact. Due to the complex processes involved experimental studies of boundary layer stability and transition are needed, and these are performed in "quiet" wind tunnels capable of simulating the low-disturbance environment of free flight. At Ames, a wind tunnel has been built to operate at flow conditions which match those of the HSCT laminar flow flight demonstration 'aircraft, the F-16XL, i.e. at a Mach number of 1.6 and a Reynolds number range of 1 to 3 million per foot. This will allow detailed studies of the attachment line and crossflow on the leading edge area of the highly swept wing. Also, use of suction as a means of control of transition due to crossflow and attachment line instabilities can be studied. Topics covered include: test operating conditions required; design requirements to efficiently make use of the existing infrastructure; development of an injector drive system using a small pilot facility; plenum chamber design; use of computational tools for tunnel and model design; and early operational results.
King, Lyndell S.↗
Effects of Sonic Line Transition on Aerothermodynamics of the Mars Pathfinder Probe
Flow field solutions over the Mars Pathfinder Probe spanning the trajectory through the Martian atmosphere at angles of attack from 0 to 11 degrees are obtained. Aerodynamic coefficients derived from these solutions reveal two regions where the derivative of pitching moment with respect to angle of attack is positive at small angles of attack. The behavior is associated with the transition of the sonic line location between the blunted nose and the windside shoulder of the 70 degree half-angle cone in a gas with a low effective ratio of specific heats. The transition first occurs as the shock layer gas chemistry evolves from highly nonequilibrium to near equilibrium, above approximately 6.5 km/s and 40 km altitude, causing the effective specific heat ratio to decrease. The transition next occurs in an equilibrium flow regime as velocities decrease through 3.5 km/s and the specific heat ratio increases again with decreasing enthalpy. The effects of the expansion over the shoulder into the wake are more strongly felt on the fustrum when the sonic line sits on the shoulder. The transition also produces a counter-intuitive trend in which windside heating levels decrease with increasing angle of attack resulting from an increase in the effective radius of curvature. Six-degree-of-freedom trajectory analyses utilizing the computed aerodynamic coefficients predict a moderate, 3 to 4 degree increase in total angle of attack as the probe, spinning at approximately 2 revolutions per minute, passes through these regions.
Gnoffo, Peter A.↗