Nuclear modification factor in intermediate-energy heavy-ion collisions

The transverse momentum dependent nuclear modification factors (NMF), namely $R_{CP}$, is investigated for protons produced in Au + Au at 1$A$ GeV within the framework of the isospin-dependent quantum molecular dynamics (IQMD) model. It is found that the radial collective motion during the expansion stage affects the NMF at low transverse momentum a lot. By fitting the transverse mass spectra of protons with the distribution function from the Blast-Wave model, the magnitude of radial flow can be extracted. After removing the contribution from radial flow, the $R_{CP}$ can be regarded as a thermal one and is found to keep unitary at transverse momentum lower than 0.6 GeV/c and enhance at higher transverse momentum, which can be attributed to Cronin effect.Comment: 8 pages, 5 figures; aceepted by Physics Letters

Simulation of Sound Absorption by Scattering Bodies Treated with Acoustic Liners Using a Time-Domain Boundary Element Method

Reducing aircraft noise is a major objective in the field of computational aeroacoustics. When designing next generation quiet aircraft, it is important to be able to accurately and efficiently predict the acoustic scattering by an aircraft body from a given noise source. Acoustic liners are an effective tool for aircraft noise reduction, and are characterized by a complex valued frequency-dependent impedance, Z(w). Converted into the time-domain using Fourier transforms, an impedance boundary condition can be used to simulate the acoustic wave scattering of geometric bodies treated with acoustic liners. This work uses an admittance boundary condition where the admittance, Y(w), is defined to be the inverse of impedance, i.e., Y(w) = 1/Z(w). An admittance boundary condition will be derived and coupled with a time domain boundary integral equation. The solution will be obtained iteratively using spatial and temporal basis functions and will allow for acoustic scattering problems to be modeled with geometries consisting of both unlined and soft surfaces. Stability will be demonstrated through eigenvalue analysis

Excitation Energy as a Basic Variable to Control Nuclear Disassembly

Thermodynamical features of Xe system is investigated as functions of temperature and freeze-out density in the frame of lattice gas model. The calculation shows different temperature dependence of physical observables at different freeze-out density. In this case, the critical temperature when the phase transition takes place depends on the freeze-out density. However, a unique critical excitation energy reveals regardless of freeze-out density when the excitation energy is used as a variable insteading of temperature. Moreover, the different behavior of other physical observables with temperature due to different $\rho_f$ vanishes when excitation energy replaces temperature. It indicates that the excitation energy can be seen as a more basic quantity to control nuclear disassembly.Comment: 3 pages, 2 figures, Revte

On the Implementation and Further Validation of a Time Domain Boundary Element Method Broadband Impedance Boundary Condition

A time domain boundary integral equation with Burton-Miller reformulation is presented for acoustic scattering by surfaces with liners in a uniform mean flow. The Ingard-Myers impedance boundary condition is implemented using a broadband multipole impedance model and converted into time domain differential equations to augment the boundary integral equation. The coupled integral-differential equations are solved numerically by a March-On-in-Time (MOT) scheme. While the Ingard-Myers condition is known to support Kelvin-Helmholtz instability due to its use of a vortex sheet interface between the flow and the liner surface, it is found that by neglecting a second derivative term in the current time domain impedance boundary condition formulation, the instability can be effectively suppressed in computation. Proposed formulation and implementation are validated using NASA Langley Research Center Grazing Flow Impedance Tube (GFIT) experimental dataset with satisfactory results. Moreover, a minimization procedure for finding the poles and coefficients of the broadband multiple impedance model is formulated in this paper by which, unlike the commonly used vector-fitting method, passivity of the model is ensured. Numerical tests show the proposed minimization approach is effective for modeling liners that are commonly used in aeroacoustics applications

In-situ X-ray tomographic imaging of microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste

This paper presents an in-situ investigation of the microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste using X-ray microcomputed tomography. Results indicate that the dissolution of fly ash and slag particles is not uniform especially for the particles with large size due to the heterogeneous distribution of chemical composition and initial defects. The dissolution of the particles with small size is faster than that of large particles owing to the relatively high specific area. The formation of reaction products (i.e., inner products) is mostly accumulated within the boundary of original particles, which become a barrier to prevent the further dissolution of unreacted particles. The fly ash-slag interaction in terms of microstructure evolution is not obvious at early ages (1–3 d) but becomes apparent at later ages (7–28 d), which can be attributed to the continuous transport of dissolved ions between fly ash and slag

Micromechanical analysis of interfacial transition zone in alkali-activated fly ash-slag concrete

This paper systematically investigates the micromechanical properties of interfacial transition zone (ITZ) in alkali-activated fly ash-slag (AAFS) concrete using nanoindentation, backscattered electron microscopy and energy dispersive spectroscopy. Results indicate that the micromechanical properties of ITZ depend on the chemical composition of reaction products and its microstructural characteristics. The ITZ with high proportion of N-C-A-S-H and C-A-S-H gels tends to have high elastic modulus because of their superior micromechanical properties. The formation of reaction products would refine the microstructure of ITZ and improve its elastic modulus. The evolution of micromechanical properties of ITZ can be divided into three stages: (i) accelerated growth stage via fast chemical reactions (<12 h); (ii) stationary stage via stable chemical reactions (12 h-7 d); (iii) decrement stage via microcrack propagation (7 d-28 d). ITZ is not the weakest region in AAFS concrete due to its desired micromechanical properties and compact microstructure compared to paste matrix