High-resolution numerical simulation has been performed to study aeroacoustic noise radiated from a turbulent boundary layer at freestream Mach number Ma = 0.3, which develops on a smooth flat plate and over a small forward-facing step. Sound waves radiated from the turbulent boundary layer on the flat plate are dominant in a very-low-frequency band and have characteristics of a linear sound source. The sound waves are observed in the far-field from the boundary propagate outside of the flow region where small hydrodynamic pressure fluctuations with a significant long-wavelength appear. The instantaneous hydrodynamic pressure fields that gradually develop or decay while moving downstream with the turbulent boundary layer are associated with the evolution of various vortical structures. The characteristics of the sound wave being a linear sound source and dominant in a very-low-frequency band are similar to those of the real high-speed train. The sound waves generated from the turbulent boundary layer over the forward-facing step with a height (SH) of SH/y+ ≅ 62 are significantly larger in a full frequency band than those radiated from the turbulent boundary layer on the flat plate. Numerical results of the turbulent boundary layer over the forward-facing step with a height of SH/y+ ≅ 30 show that the sound waves are dominant in the low-frequency band, and also step-specific sound waves are generated in the high-frequency band. Further, even if the step height is SH/y+ ≅ 7.5, the sound waves unique to the small step are generated in the high-frequency band.
In general, technical methods for improving the thermal efficiency of an engine increase the heat load on peripheral components. Recently, a piston cooling gallery equipped with a flow path has been developed. The engine oil is supplied an oil jet from the nozzle, which is placed under the piston to the piston gallery entrance hall. The nozzle of the oil jet is curved to minimize its size, and the jet interface between ambient air and oil fluctuates near the nozzle exit owing to the shape. Few studies have investigated the behavior of oil jets ejecting from curved pipes. We therefore investigated the flow in two nozzles having a basic bend of 90° with radii of curvature of 15 and 60 mm. Our results clarify the effect of internal flow on the ejecting oil jet behavior. Silicone oil was used as the working fluid. The kinematic viscosity of the silicone oil at 298 K was similar to that of engine oil at 353 K. The behavior of the oil jet was investigated by visualization using background light. A light-emitting-diode displacement meter was installed to measure the jet width. We found that the width of the oil jet increased on the downstream side with large fluctuation of the interface under the condition of a small radius of curvature and large Reynolds number. Furthermore, we time-synchronously measured flow in the nozzle, two-dimensional two-component time-resolved particle image velocimetry, and visualization of the jet. The Reynolds number was set from 1000 to 3000, which is close to that of the engine oil jet. The oil flow velocity in the nozzle fluctuated in the radial direction. The fluctuation became strong under the condition of a small radius and large Reynolds number. The fluctuation propagation speed calculated from the correlation coefficient was as high as the flow speed itself. Furthermore, the jet interface fluctuation speed in the flow direction was as high as the fluctuation propagation speed in the nozzle. Our results demonstrate that the cause of the interface fluctuation is the fluctuation propagation of flow in the nozzle.
Effects of buoyancy force stabilizing the disturbances are investigated on a turbulent channel flow bounded by two walls of different temperatures. With a constant mean pressure gradient imposed, the buoyancy force related to the Grashof number (Gr) is systematically increased by increasing the temperature difference between upper and lower walls, which are perpendicular to the gravitational direction. As a result, the mean flow rate increases with an increase in the Gr, and turbulent structures become intermittent and inhomogeneous; turbulent and quasilaminar regions simultaneously appear in the same computational region. Detailed visualization for instantaneous turbulent structure shows that in the upper and lower sides of a channel, the turbulent regions appear alternatively, and local one-sided turbulence is observed. Finally, with a further increase in the Gr, the stratified channel flow becomes complete one-sided turbulence where a flow becomes turbulent on one side of a channel, while it becomes laminarized on the other side.
Based on the Moving Least Square (MLS) approximation, we propose a sharp interface direct-forcing immersed boundary method for incompressible fluid flows with fixed and moving boundaries. Since the domain of definition for the interpolation is highly flexible and the MLS approximation provides an accurate reconstructed approximation of the solution, the proposed method serves the precision and versatility required for a numerical framework to study the fluid-structure interaction problems. To alleviate the inherent spurious numerical oscillation that occurs in the calculated forces on moving boundary embedded objects, we use a two step predictor-corrector method in which the direct forcing terms are calculated after the predictor step and imposed on the whole solid domain as well as at the immediate vicinity of the solid boundary inside the fluid domain. To represent the arbitrary geometries, we adopt a signed distance function representation of the rigid body and an interpolation strategy to considerably reduce the computational cost of the re-initialization of the distance function at every time step. The potential capability of the method is demonstrated for both fixed and moving boundary problems. We also solve a sedimentation of a single cylinder to demonstrate the ability of the present method in solving fluid-structure interaction problems. These numerical experiments show that the proposed moving least square immersed boundary method can handle relatively complex moving problems while enjoying a versatile interpolation strategy and keeping the boundary conditions sharp with remarkable accuracy.
We develop a versatile and accurate structured adaptive mesh refinement (S-AMR) strategy with a moving least square sharp-direct forcing immersed boundary method (IBM) for incompressible fluid-structure interaction (FSI) simulations. The computational grid consists of several nested blocks in different refinement levels. While blocks with the coarsest grid cover the entire computational domain, the computational domain is locally refined at the location of solid boundary (moving or fixed) by bisecting selected blocks in every coordinate direction. The grid topology and data structure is managed by an extended version of Afivo toolkit (Teunissen and Ebert, 2018), where a novel technique is introduced for conservative data transfer between the coarser and the finer blocks, particularly in velocity transformation for which the mass conservation plays a crucial role. In the present study, the continuity and Navier-Stokes equations for incompressible flows are spatially discretized with a second order central finite difference method using a collocated arrangement and are time-integrated using a semi-implicit second order fractional step method, although the proposed S-AMR strategy can be used with different discretization schemes. An IBM using a moving least square approach is utilized to impose boundary conditions. To handle FSI problems, all the governing equations for the dynamics of fluid and structure are simultaneously advanced in time by a predictor-corrector strategy. Several test cases of increasing complexity are solved in order to demonstrate the robustness and accuracy of the proposed method as well as its capability in simulation-driven mesh adaptivity.
A numerical calculation model for gas-liquid unsteady two-phase flow in a micro channel was established and validated. The model focuses on flow in a channel including air gaps. It also reduces calculation cost by minimizing the number of control-volume elements compared with conventional numerical methods for gas-liquid two-phase flow. Gas-liquid two-phase flow in channels with diameters of 1 to 2 mm is important in liquid transportation in chemical processing and analysis. As for designing flow channels in chemical processing and analysis, simple numerical models are desirable to evaluate many possible patterns quickly. The model uses moving boundaries that correspond to gas-liquid interfaces, but it represents boundaries between different channels with fixed boundaries. By setting several different configurations of air gaps, the numerical model was validated in regard to position and volume of air gaps in the channel. As an application of the numerical model to the design of flow channels in chemical processing and analysis, the necessary movement conditions of a syringe pump to achieve quick liquid transportation were investigated. By applying the numerical model, the necessary conditions to minimize flow rate oscillation were determined. In simulation-based design of micro-channel, the numerical model in this research is an effective tool to determine design parameters quickly.