Modelling of dimensional stability of fiber reinforced composite materials

Various methods of predicting the expansion and diffusion properties of composite laminates are reviewed. The prediction equations for continuous fiber composites can be applied to SMC composites as the effective fiber aspect ratio in the latter is large enough. The effect of hygrothermal expansion on the dimensional stability of composite laminates was demonstrated through the warping of unsymmetric graphite/epoxy laminates. The warping is very sensitive to the size of the panel, and to the moisture content which is in turn sensitive to the relative humidity in the environment. Thus, any long term creep test must be carried out in a humidity-controlled environment. Environmental effects in SMC composites and bulk polyester were studied under seven different environments. The SMC composites chosen are SMC-R25, SMC-R40, and SMC-R65

Simulations of a Liquid Hydrogen Inducer at Low-Flow Off-Design Flow Conditions

The ability to accurately model details of inlet back flow for inducers operating a t low-flow, off-design conditions is evaluated. A sub-scale version of a three-bladed liquid hydrogen inducer tested in water with detailed velocity and pressure measurements is used as a numerical test bed. Under low-flow, off-design conditions the length of the separation zone as well as the swirl velocity magnitude was under predicted with a standard k-E model. When the turbulent viscosity coefficient was reduced good comparison was obtained a t all the flow conditions examined with both the magnitude and shape of the profile matching well with the experimental data taken half a diameter upstream of the leading edge. The velocity profiles and incidence angles a t the leading edge itself were less sensitive to the back flow length predictions indicating that single-phase performance predictions may be well predicted even if the details of flow separation modeled are incorrect. However, for cavitating flow situations the prediction of the correct swirl in the back flow and the pressure depression in the core becomes critical since it leads to vapor formation. The simulations have been performed using the CRUNCH CFD(Registered Trademark) code that has a generalized multi-element unstructured framework and a n advanced multi-phase formulation for cryogenic fluids. The framework has been validated rigorously for predictions of temperature and pressure depression in cryogenic fluid cavities and has also been shown to predict the cavitation breakdown point for inducers a t design conditions

Simulation of Cavitation Instabilities in Inducers

The cavitating performance of a sub-scale configuration of the SSME low pressure fuel pump (LPFP) has been simulated at off-design flow conditions where a back-flow vortex is generated at the leading edge. The numerical simulations have been compared with measured experimental data both for velocity profiles upstream of the inducer as well dynamic pressure traces on the shroud at the leading edge. Velocity profiles in the back-flow vortex for flow rates down to 70 percent of design were quantified; the swirl velocity comparisons were good while the axial velocity profile were reasonable but slightly over predicted the core velocity. Dynamic cavitating performance was modeled at a moderate Nss number of 20000 for 90 percent of design flow coefficient where rotational cavitation modes are present The source of this instability resulted from the interaction of the cavity with the neighboring blade leading to the detachment of the cavity that rotates relative to the blade and generates an asymmetric cavity pattern. The asymmetrical cavities generate a large radial load on the shaft which rotates at the fundamental mode of the rotational cavitation. For the sub-scale configuration the radial force amplitude was 186 lb-f which gives a non-dimensional force factor of 0.0116. Spectral analyses of the dynamic pressure traces on the shroud, at the leading edge plane, were compared with experimental measurements. The fundamental rotational cavitation mode was observed to be 125 Hz which is approximately 1.29 N (rotational frequency is 96. Hz); both the frequency and relative amplitude compared well with the unsteady measurements. In addition to the fundamental rotation cavitation mode the data shows substantial energy with multiple peaks in the 5 -7.5 N range. This range was reasonably represented in the numerical results although the spectrum was not as rich. A helical pressure wave at the fundamental mode is found to propagate upstream and a potential for interaction with structural elements was identified.http://deepblue.lib.umich.edu/bitstream/2027.42/84299/1/CAV2009-final122.pd

Numerical Simulations of Instabilities in Single-Hole Office Elements

An orifice element is commonly used in liquid rocket engine test facilities either as a flow metering device, a damper for acoustic resonance or to provide a large reduction in pressure over a very small distance in the piping system. While the orifice as a device is largely effective in stepping down pressure, it is also susceptible to a wake-vortex type instability that generates pressure fluctuations that propagate downstream and interact with other elements of the test facility resulting in structural vibrations. Furthermore in piping systems an unstable feedback loop can exist between the vortex shedding and acoustic perturbations from upstream components resulting in an amplification of the modes convecting downstream. Such was the case in several tests conducted at NASA as well as in the Ariane 5 strap-on P230 engine in a static firing test where pressure oscillations of 0.5% resulted in 5% thrust oscillations. Exacerbating the situation in cryogenic test facilities, is the possibility of the formation of vapor clouds when the pressure in the wake falls below the vapor pressure leading to a cavitation instability that has a lower frequency than the primary wake-vortex instability. The cavitation instability has the potential for high amplitude fluctuations that can cause catastrophic damage in the facility. In this paper high-fidelity multi-phase numerical simulations of an orifice element are used to characterize the different instabilities, understand the dominant instability mechanisms and identify the tonal content of the instabilities

Failure Mode Analysis of V-Shaped Pyrotechnically Actuated Valves

Current V-shaped stainless steel pyrovalve initiators have rectified many of the deficiencies of the heritage Y-shaped aluminum design. However, a credible failure mode still exists for dual simultaneous initiator (NSI) firings in which low temperatures were detected at the booster cap and less consistent ignition was observed than when a single initiator was fired. In order to asses this issue, a numerical framework has been developed for predicting the flow through pyrotechnically actuated valves. This framework includes a fully coupled solution of the gas-phase equation with a non-equilibrium dispersed phase for solid particles as well as the capability to model conjugate gradient heat transfer to the booster cap. Through a hierarchy of increasingly complex simulations, a hypothesis for the failure mode of the nearly simultaneous dual NSI firings has been proven. The simulations indicate that the failure mode for simultaneous dual NSI firings may be caused by flow interactions between the flame channels. The shock waves from each initiator interact in the booster cavity resulting in a high pressure that prevents the gas and particulate velocity from rising in the booster cap region. This impedes the bulk of the particulate phase from impacting the booster cap and reduces the heat transfer to the booster cap since the particles do not impact it. Heat transfer calculations to the solid metal indicate that gas-phase convective heat transfer may not be adequate by itself and that energy transfer from the particulate phase may be crucial for the booster cap burn through

Generalized conductance sum rule in atomic break junctions

When an atomic-size break junction is mechanically stretched, the total conductance of the contact remains approximately constant over a wide range of elongations, although at the same time the transmissions of the individual channels (valence orbitals of the junction atom) undergo strong variations. We propose a microscopic explanation of this phenomenon, based on Coulomb correlation effects between electrons in valence orbitals of the junction atom. The resulting approximate conductance quantization is closely related to the Friedel sum rule.Comment: 4 pages, 1 figure, appears in Proceedings of the NATO Advanced Research Workshop ``Size dependent magnetic scattering'', Pecs, Hungary, May 28 - June 1, 200

Simulations of Instabilities in Complex Valve and Feed Systems

CFD analyses are playing an increasingly important role in identifying and characterizing flow induced instabilities in rocket engine test facilities and flight systems. In this paper, we analyze instability mechanisms that range from turbulent pressure fluctuations due to vortex shedding in structurally complex valve systems to flow resonance in plug cavities to large scale pressure fluctuations due to collapse of cavitation induced vapor clouds. Furthermore, we discuss simulations of transient behavior related to valve motion that can serve as guidelines for valve scheduling. Such predictions of valve response to varying flow conditions is of crucial importance to engine operation and testing

Efficient Evaluation of Low Degree Multivariate Polynomials in Ring-LWE Homomorphic Encryption Schemes

Homomorphic encryption schemes allow to perform computations over encrypted data. In schemes based on RLWE assumption the plaintext data is a ring polynomial. In many use cases of homomorphic encryption only the degree-0 coefficient of this polynomial is used to encrypt data. In this context any computation on encrypted data can be performed. It is trickier to perform generic computations when more than one coefficient per ciphertext is used. In this paper we introduce a method to efficiently evaluate low-degree multivariate polynomials over encrypted data. The main idea is to encode several messages in the coefficients of a plaintext space polynomial. Using ring homomorphism operations and multiplications between ciphertexts, we compute multivariate monomials up to a given degree. Afterwards, using ciphertext additions we evaluate the input multivariate polynomial. We perform extensive experimentations of the proposed evaluation method. As example, evaluating an arbitrary multivariate degree-3 polynomial with 100 variables over Boolean space takes under 13 seconds