Pipe flow

About: Pipe flow is a research topic. Over the lifetime, 13826 publications have been published within this topic receiving 351605 citations.

Mode-matching strategies in slowly varying engine ducts

Abstract: A matching method is proposed to connect the computational fluid dynamics (CFD) source region to the computational aeroacoustics propagation region of rotor-stator interaction sound produced in a turbofan engine. The method is based on a modal decomposition across three neighbouring axial interfaces adjacent to the matching interface. The modal amplitudes are determined by a least-squares fit. When slowly varying modes are taken, the interface may be positioned in a duct section of varying cross section. Furthermore, the spurious reflections back into the CFD domain, which result from imperfect reflection-free CFD boundary conditions, can be filtered out by including both left- and right-running modes in the matching. Although the method should be applicable to a wider range of acoustic models, it is implemented and favourably tested for the recently available relatively simple case of slowly varying modes in homentropic potential flow in lined ducts. Homentropic potential flow is a very relevant model for the inlet side and a good model for the bypass side if swirl or other types of vorticity are not dominant in the mean flow. By matching with density or pressure perturbations, any contamination of residual nonacoustical vorticity is avoided.

Extending the validity of squeezed-film damper models with elongations of surface dimensions

Abstract: Compact models for micromechanical squeezed-film dampers with gap sizes comparable to the surface dimensions are presented. Two different models considering both the border flow and non-uniform pressure distribution effects are first derived for small squeeze numbers. In the first 'surface extension' model the border effects are considered simply by calculating the damping with extended surface dimensions, and in the second 'border flow channel' model an additional short fictitious flow channel is placed at the damper borders. Utilizing a large amount of two-dimensional (2D) FEM simulation results by varying the damper dimensions, mainly the ratio a/h between the surface length and the air gap height, surface elongations are extracted using both elongation models. Both linear and torsional modes of motion are considered at the continuum flow regime. These results show that the 'surface extension' model is superior, since the extracted elongation Δa is almost constant (Δa = 1.3h), leading to a very simple model. Next, the rare gas effects are included in the 'surface extension' model in the slip flow regime (Knudsen number 0 4 in the linear motion and for a/h > 10 in the torsional motion. The model assumes incompressible flow and thus the maximum frequency where the models are valid is limited. In typical MEMS topologies where the elongations must be considered, this means that the models are valid below frequencies of 500 kHz. To also model rectangular 2D squeezed-film dampers, these elongations are applied directly in the surface length and width used in the compact models. Comparison with three-dimensional (3D) FEM simulations shows that the new model gives excellent results, and it extends the validity range of existing compact models. The maximum relative error of the models is smaller than 10% for a/h > 16 in the linear motion and for a/h > 16 in the torsional motion. The new surface extension model is useful in simulating both the circuit level and the system level behavior of gas-damped microelectromechanical devices with aspect ratios greater than 2 in the time and frequency domains.

Subsurface flow velocities in a hillslope with lateral preferential flow

Abstract: [1] Our understanding of hillslope subsurface flow relies on assumptions about how storm characteristics affect the hillslope runoff response. Experiments in hillslopes dominated by preferential flow features often show that runoff is dynamic and is affected by antecedent conditions, rainfall conditions, and position of the slope. We applied tracers to a hillslope under natural and steady state flow boundary conditions to determine the relationship between lateral tracer velocities and various hillslope lengths and storm indicators. Tracer velocities were similar to the fastest velocities measured in other similar experiments. The velocities were dependent on the boundary conditions and slope length, and the subsurface flow velocity was most closely related to the 1-h rainfall intensity. Unlike some studies, there was little correlation between our measured flow velocities and storm volume or antecedent conditions. We attributed this to the hillslope characteristics and the relatively consistent wet antecedent conditions during the experiments. This experiment showed that the connectivity of the hillslope preferential flow network is an important factor governing the average subsurface flow velocity.