Aeroacoustic response of diffusers and bends

Experimental results were obtained in the study of the aeroacoustic response of diffusers and abrupt expansions. An experimental study on the aeroacoustics of bends has also been carried out and shows similar results. In both cases the low-frequency aeroacoustic behaviour can be predicted by a quasi-steady flow model when the flow separation point is fixed by sharp edges in the geometry. At smooth pipe discontinuities nonlinear effects are more important and the response is essentially unsteady

Acoustics of 90 degree sharp bends. Part I: Low-frequency acoustical response

The acoustical response of 90 degree sharp bends to acoustical perturbations in the absence of a main flow is considered. The aeroacoustical response of these bends is presented in part II [1]. The bends considered have a sharp 90 degree inner edge and have either a sharp or a rounded outer corner. They are placed in pipes with either a square cross-section (2D-bends) or a circular cross-section (3D-bends). The acoustical performance of a numerical method based on the non-linear Euler equations for two-dimensional inviscid and compressible flows is checked and its ability to predict the response of 3D-bends is investigated. The comparison between 2-D and 3-D data is made for equal dimensionless frequencies f/fc where f is the frequency of the acoustical perturbations and fc is the cut-off frequency of the bends. In the case of a bend with a sharp inner edge and a sharp outer corner, the 2-D numerical predictions agree with 2-D analytical data obtained from a mode expansion technique and with 2-D experimental data from literature and our own 3-D experimental results. In the case of a bend with a sharp inner edge and a rounded outer corner, the 2-D numerical simulations predict accurately the 2-D experimental data from literature. However, the 2-D numerical predictions do not agree with our 3-D experimental data. The acoustical response of 3D-bends appears to be independent of the shape of the outer corner. This behavior is quite unexpected

Acoustics of 90 degree sharp bends. Part II: Low-frequency aeroacoustical response

The aeroacoustical response of 90 degree sharp bends is defined as the response to acoustical perturbations in the presence of a main flow. The acoustical response of bends, in the absence of a main flow, has been considered in part I [1]. Experiments are carried out for bends in pipes with circular cross-sections. These 3D-bends have a sharp inner edge and have either a sharp outer corner or a rounded outer wall. The three-dimensional experimental results are compared with results of numerical simulations, based on the Euler equations for two-dimensional inviscid and compressible flows, and with analytical data obtained by means of two-dimensional quasi-steady flow theories. As observed in the absence of a main flow (part I [1]), the two-dimensional numerical simulations provide a good prediction of the aeroacoustical response of bends with a sharp inner edge and a sharp outer corner. For bends with a rounded outer corner, the prediction is less satisfactory. The two-dimensional quasi-steady flow approximation predicts reasonably well the response of bends up to Strouhal numbers of the order of unity. However, quasi-steady flow theories do not predict the irregularities of the response as a function of the flow velocity. These irregularities are expected to be a Strouhal number effect and are observed both in experiments and numerical simulations

Large-scale directed model checking LTL

Abstract. To analyze larger models for explicit-state model checking, directed model checking applies error-guided search, external model checking uses secondary storage media, and distributed model checking exploits parallel exploration on multiple processors. In this paper we propose an external, distributed and directed on-the-fly model checking algorithm to check general LTL properties in the model checker SPIN. Previous attempts restricted to checking safety properties. The worst-case I/O complexity is bounded by O(sort(|F||R|)/p + l · scan(|F||S|)), where S and R are the sets of visited states and transitions in the synchronized product of the Büchi automata for the model and the property specification, F is the number of accepting states, l is the length of the shortest counterexample, and p is the number of processors. The algorithm we propose returns minimal lasso-shaped counterexamples and includes refinements for property-driven exploration.