Annular jets impinging on a uniformly heated flat plate with or without swirling flow by short guide vanes are experimentally characterized. With the Reynolds number fixed at a relatively low value, the variation of jet flow structures with impinging distance is characterized using the technique of particle image velocimetry (PIV). Correspondingly, the distributions of wall pressure and heat transfer on the plate are measured. At sufficiently large impinging distances, without swirling flow, the obtained flow and wall pressure/heat transfer data are consistent with the classical observation for a conventional annular impinging jet, showing the transition from annular impinging jet flow to single circular impinging jet-like flow. In contrast, no such transition occurs in the presence of flow turning by short guide vanes. At short and intermediate impinging distances, flow turning causes more non-uniform distributions of wall pressure and heat transfer on the target plate and the local heat transfer rates higher than those of the conventional annular jet. This is attributed to the vortical flow structures shed and convected downstream from the short guide vanes. In sharp contrast, at large impinging distances, the larger momentum loss due to flow turning results in lower heat transfer rates on the plate.
The impact of a rigid body(protected structure) together with cushion material(cellular metal foam) on hard ground from a fixed height was investigated.An analytical one-degree-of-freedom colliding model(ODF-CM) was established to analyze the protection ability and energy absorption by the foam under low velocity impact conditions.For validation,drop hammer experiments were carried out for high porosity closed-cell aluminum foam specimens subjected to low velocity impact loading.The dynamic deformation behavior of the specimen was observed and the velocity attenuation of the drop hammer was measured.The results demonstrated that the aluminum foam had excellent energy absorption capabilities,with its dynamic compressive behavior similar to that obtained under quasi-static loading conditions.Finite element method(FEM) was subsequently employed to obtain stress distributions in the foam specimen.As the propagating period of stress in the specimen was far less than the duration of attenuation,the evolution of the stress was similar to that under quasi-static loading conditions and no obvious stress wave effect was observed,which agreed with the experimental observation.Finally,the predicted velocity attenuation by the ODF-CM was compared with both the experimental measurements and FEM simulation,and good agreements were achieved when the stress distribution was considered to be uniform and the "quasi-static" compressive properties are employed.
The radiation of noise from a parallelly rib-stiffened skin plate of aircraft cabin fuselage in the presence of external mean flow is theoretically investigated.An aero-acoustic-elastic model is developed and used to calculate the radiated sound pressure level(SPL) versus frequency curves with reference to sound radiation of a bare plate immersed in a steady fluid.The flexural and rotational motions of the rib stiffeners are described by applying the Euler-Bernoulli beam theory and torsional wave equation,respectively.Therefore,the coupling forces and moments between the ribs and the face-panel,caused separately by flexural and rotational motion of the ribs,are both taken into account.Given the periodicity of the structure,the Fourier transform technique is employed to solve panel vibration equations and acoustic equations.Systematic parametric investigation demonstrates that the presence of mean flow as well as rib spacings play significant roles in the sound radiation behavior of parallelly rib-stiffened plates.The proposed model provides a convenient and efficient tool for the factual engineering design of this kind of periodic structures with acoustic requirements.
