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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Unresolved Problems by Shock Capturing: Taming the Overheating Problem

The overheating problem, first observed by von Neumann [1] and later studied extensively by Noh [2] using both Eulerian and Lagrangian formulations, remains to be one of the unsolved problems by shock capturing. It is historically well known to occur when a flow is under compression, such as when a shock wave hits and reflects from a wall or when two streams collides with each other. The overheating phenomenon is also found numerically in a smooth flow undergoing rarefaction created by two streams receding from each other. This is in contrary to one s intuition expecting a decrease in internal energy. The excessive amount in the temperature increase does not reduce by refining the mesh size or increasing the order of accuracy. This study finds that the overheating in the receding flow correlates with the entropy generation. By requiring entropy preservation, the overheating is eliminated and the solution is grid convergent. The shock-capturing scheme, as being practiced today, gives rise to the entropy generation, which in turn causes the overheating. This assertion stands up to the convergence test.

Liou, Meng-Sing↗
NASA NTRS2017

The Root Cause of the Overheating Problem

Previously we identified the receding flow, where two fluid streams recede from each other, as an open numerical problem, because all well-known numerical fluxes give an anomalous temperature rise, thus called the overheating problem. This phenomenon, although presented in several textbooks, and many previous publications, has scarcely been satisfactorily addressed and the root cause of the overheating problem not well understood. We found that this temperature rise was solely connected to entropy rise and proposed to use the method of characteristics to eradicate the problem. However, the root cause of the entropy production was still unclear. In the present study, we identify the cause of this problem: the entropy rise is rooted in the pressure flux in a finite volume formulation and is implanted at the first time step. It is found theoretically inevitable for all existing numerical flux schemes used in the finite volume setting, as confirmed by numerical tests. This difficulty cannot be eliminated by manipulating time step, grid size, spatial accuracy, etc, although the rate of overheating depends on the flux scheme used. Finally, we incorporate the entropy transport equation, in place of the energy equation, to ensure preservation of entropy, thus correcting this temperature anomaly. Its applicability is demonstrated for some relevant 1D and 2D problems. Thus, the present study validates that the entropy generated ab initio is the genesis of the overheating problem.

Liou, Meng-Sing↗
NASA NTRS2013

Why Is the Overheating Problem Difficult: the Role of Entropy

The development of computational fluid dynamics over the last few decades has yielded enormous successes and capabilities being routinely employed today; however there remain some open problems to be properly resolved-some are fundamental in nature and some resolvable by operational changes. These two categories are distinguished and broadly explored previously. One, that belongs to the former, is the so-called overheating problem, especially in rarefying flow. This problem up to date still dogs every method known to the author; a solution to it remains elusive. The study in this paper concludes that: (1) the entropy increase is quantitatively linked to the increase in the temperature increase, (2) it is argued that the overheating is inevitable in the current shock capturing or traditional finite difference framework, and (3) a simple hybrid method is proposed that removes the overheating problem in the rarefying problems, but also retains the property of accurate shock capturing. This remedy (enhancement of current numerical methods) can be included easily in the present Eulerian codes.

Liou, Meng-Sing↗
NASA NTRS2012

Overheating Anomalies during Flight Test Due to the Base Bleeding

In this paper we present the results of the analytical and numerical studies of the plume interaction with the base flow in the presence of base out-gassing. The physics-based analysis and CFD modeling of the base heating for single solid rocket motor performed in this research addressed the following questions: what are the key factors making base flow so different from that in the Shuttle [1]; why CFD analysis of this problem reveals small plume recirculation; what major factors influence base temperature; and why overheating was initiated at a given time in the flight. To answer these questions topological analysis of the base flow was performed and Korst theory was used to estimate relative contributions of radiation, plume recirculation, and chemically reactive out-gassing to the base heating. It was shown that base bleeding and small base volume are the key factors contributing to the overheating, while plume recirculation is effectively suppressed by asymmetric configuration of the flow formed earlier in the flight. These findings are further verified using CFD simulations that include multi-species gas environment both in the plume and in the base. Solid particles in the exhaust plume (Al2O3) and char particles in the base bleeding were also included into the simulations and their relative contributions into the base temperature rise were estimated. The results of simulations are in good agreement with the temperature and pressure in the base measured during the test.

base bleeding↗
NASA NTRS1985

Overheated Open Coronal Regions

The physics of compact, hydrostatic coronal shells formed from the collapse of overheated coronae is addressed. A large number of hydrostatic shells was computed for the usual exponential heating law with constant damping length. The boundary conditions were that on both sides of a shell the conductive flux at chromospheric temperatures is small. The boundary value problem was solved with a shooting technique. The main goal of the calculations was to determine the dependence of the shells on coronal heating flux (F sub Mo) and damping length (L); and in particular, to localize the boundary line in the parameter space (F sub Mo, L) that separates normal extended coronae from coronal shells.

Hammer, R.↗
NASA NTRS1957

Effect of Overheating on Creep-rupture Properties of S-816 Alloy at 1,500 F

The effects of overheats to temperatures of 1650, 1800, 1900, and 2000 F were evaluated in terms of the changes in creep-rupture characteristics at 1500 F of S-816 alloy under stresses within the range of rupture strengths of the alloy for 100 to 1000 hours. Overheat periods were predominantly of 2-minute duration and were applied cyclically at approximately 5- or 12-hour intervals. The possible damage from overheating was believed to include internal metal structure changes induced by exposure to the higher temperatures and loss of life by creep if stress was present during the overheats.

ALLOYS, HEAT-RESISTING↗
NASA NTRS2009

Properties of Smoke from Overheated Materials in Low-Gravity

Smoke particle size measurements were obtained under low-gravity conditions by overheating several materials typical of those found in spacecraft. The measurements included integral measurements of the smoke particles and physical sample of the particles for Transmission Electron Microscope analysis. The integral moments were combined to obtain geometric mean particle sizes and geometric standard deviations. These results are presented with the details of the instrument calibrations. The experimental results show that, for the materials tested, a substantial portion of the smoke particles are below 500 nm in diameter.

Urban, David L.↗
NASA NTRS2015

Particle Morphology and Elemental Composition of Smoke Generated by Overheating Common Spacecraft Materials

Fire safety in the indoor spacecraft environment is concerned with a unique set of fuels which are designed to not combust. Unlike terrestrial flaming fires, which often can consume an abundance of wood, paper and cloth, spacecraft fires are expected to be generated from overheating electronics consisting of flame resistant materials. Therefore, NASA prioritizes fire characterization research for these fuels undergoing oxidative pyrolysis in order to improve spacecraft fire detector design. A thermal precipitator designed and built for spacecraft fire safety test campaigns at the NASA White Sands Test Facility (WSTF) successfully collected an abundance of smoke particles from oxidative pyrolysis. A thorough microscopic characterization has been performed for ten types of smoke from common spacecraft materials or mixed materials heated at multiple temperatures using the following techniques: SEM, TEM, high resolution TEM, high resolution STEM and EDS. Resulting smoke particle morphologies and elemental compositions have been observed which are consistent with known thermal decomposition mechanisms in the literature and chemical make-up of the spacecraft fuels. Some conclusions about particle formation mechanisms are explored based on images of the microstructure of Teflon smoke particles and tar ball-like particles from Nomex fabric smoke.

Smoke↗
NASA NTRS1990

Sensor Detects Overheating Of Perishable Material

Experimental temperature sensor changes color rapidly and irreversibly when temperature rises above pre-determined level. Based on reactions of enzymes in paraffins, blended so mixture melts at temperature considered maximum safe value. Similar devices used to detect temperature abuse, whether foods or medicines refrigerated exposed to excessive temperatures during shipment and storage. By viewing sensor, receiving clerk tells immediately whether product maintained at safe temperatures and acceptable.

Dordick, Jonathan S.↗
NASA NTRS2011

MOSFET Switching Circuit Protects Shape Memory Alloy Actuators

A small-footprint, full surface-mount-component printed circuit board employs MOSFET (metal-oxide-semiconductor field-effect transistor) power switches to switch high currents from any input power supply from 3 to 30 V. High-force shape memory alloy (SMA) actuators generally require high current (up to 9 A at 28 V) to actuate. SMA wires (the driving element of the actuators) can be quickly overheated if power is not removed at the end of stroke, which can damage the wires. The new analog driver prevents overheating of the SMA wires in an actuator by momentarily removing power when the end limit switch is closed, thereby allowing complex control schemes to be adopted without concern for overheating. Either an integral pushbutton or microprocessor-controlled gate or control line inputs switch current to the actuator until the end switch line goes from logic high to logic low state. Power is then momentarily removed (switched off by the MOSFET). The analog driver is suited to use with nearly any SMA actuator.

Gummin, Mark A.↗
NASA NTRS1971

Hypersonic research engine project. Phase 2: Structures and cooling development

Testing of the structures assembly model (SAM) continued at the Langley 8-foot high-temperature structures tunnel. A total of 30 test runs were completed, involving 38 loading cycles for the SAM. Fuel injection was used during 27 of the cycles. That portion of the program specifically aimed at thermal-cycling evaluation was completed. For these runs, tunnel total conditions ranged to 2200 psia and 3000 R. The tunnel nozzle is fixed and produces Mach numbers near 7 at the test conditions. The coolant for all runs was ambient-temperature hydrogen. The condition of the SAM following these runs was satisfactory. Isolated leaks occurred at joints in the cooled structure, but they have not affected operation or resulted in local areas of overheating. Three punctures in the leading edge, caused by foreign object damage, were present during most of the run; however, they showed no signs of propagation or distress due to overheating.

Jilly, L. F.↗