Three-dimensional numerical study of acoustic streaming phenomenon in rectangular resonator

The article presents a numerical study of the large-amplitude, acoustically-driven streaming flow for different frequencies of the acoustic wave and different temperature gradients between hot and cold surfaces. The geometries studied were mainly two-dimensional rectangular resonators of different lengths, but also one three-dimensional rectangular resonator and one long and narrow channel, representative of a typical U-shaped resistance thermometer. The applied numerical model was based on the Navier–Stokes compressible equations, the ideal gas model, and finite volume discretization. The oscillating wall of the considered geometries was modeled as a dynamically moving boundary of the numerical mesh. The length of the resonators was adjusted to one period of the acoustic wave. The research confirmed that baroclinic acoustic streaming flow was largely independent of frequency, and its intensity increased with the temperature gradient between the hot and cold surface. Interestingly, a slight maximum was observed for some oscillation frequencies. In the case of the long and narrow channel, acoustic streaming manifested itself as a long row of counter-rotating vortices that varied slightly along the channel. 3D calculations showed that a three-dimensional pair of streaming vortices had formed in the resonator. Examination of the flow in selected cross-sections showed that the intensity of streaming gradually decreased as it approached the side walls of the resonator creating a quasi-parabolic profile. The future development of the research will focus on fully 3D calculations and precise identification of the influence of the bounding walls on the streaming flow.

The liquid suspension's staggered domains affect everyday life engineering. In this way, both parallel/anti-parallel movement of walls of staggered domains complicates formulation, making it difficult for researchers to detect liquid suspension flow field features. The current article is a key effort in this regard. The staggered cavity is equipped with liquid suspension. In a vertical path, a magnetic field is affected externally. To be more precise, we considered three cases for moving the top and bottom walls. In Case-I, just the top wall moves, while the other walls stay still. Case-II has parallel top and bottom walls. Case-III: anti-parallel wall movement. The left wall is chilly, the top wall is adiabatic, and the right side and bottom walls are heated consistently. The continuity, momentum, energy, and boundary constraints equations determine the physical configuration. Finite element analysis discretizes the governing equations. Every contour graphic show velocity, pressure, and temperature with an external magnetic field. Line graphs help describe velocity components. At a finer refinement level, all instances record kinetic energy versus magnetic field parameter and Reynolds number in tabular and bar graphs. Fluid flow length and velocity always decrease with magnetic field parameter. Kinetic energy is reduced with magnetic field intensity.

The main steam line break (MSLB) in a steam generator is one of the most critical accidents that can occur in a nuclear power plant. When such accidents happen, the steam generator is exposed to high pressure (7.8–0.101 MPa) and temperature differences (273–15 °C) in a short time. In such a case, the selection of the working fluid model is a very important parameter in the numerical analysis because of the steam property changes abruptly. When analyzing compressible fluids, using ideal gas model is one of the simplest ways for calculating the change of the fluid property value (density) according to state variable (temperature and pressure). However, this model becomes more inaccurate at higher pressures and lower temperatures. Therefore, a number of more accurate real gas models have been developed as like IAPWS, Redlich-Kwong, and Peng-Robinson models. In this study, we want to identify the differences in the ideal gas model and real gas models that can be used to carry out the main steam line break accident analysis. In this study, CFD analysis of the main steam line break (MSLB) was performed using the ideal gas equation model and the real gas model, which can be used as the working fluid model. The results show that the real gas models—IAPWS, Redlich-Kwong, and Peng-Robinson are more reliable than the ideal gas model.

The temperature distribution and distortion of fluid flow inside the closed cavities, square and triangle, are studied for different boundary conditions. Two different conditions of thermal boundary conditions are used for studying square cavities: (i) Left wall is hot, right wall is cold, top and bottom walls are adiabatic. (ii) Left and right walls are cold, top wall is adiabatic, bottom wall is hot. For triangular enclosure, the boundary conditions are (i) the vertical wall is insulated, bottom wall is hot. (ii) The vertical wall is hot, the bottom wall insulated and the inclined walls are kept cold in both conditions. The velocity of the flow is observed by means of stream function and the temperature distribution is displayed in the form of contours. The study is carried out in ANSYS software. The mathematical procedure for solving the nonlinear system of partial differential equations by penalty finite element method involving bi-quadratic elements is also discussed in detail.

This paper reports an experimental study of turbulent flows in variable adverse and favorable pressure gradients over transverse ribs. The pressure gradients were produced in asymmetric converging and diverging channels. The channels are 2500 mm long and 194 mm wide. The first and last 750 mm of the test channel has straight parallel bottom and top walls. The middle section of 1000 mm length has straight bottom wall and top wall that varied non-linearly from 60 mm to 90 mm for the diverging channel, and from 90 mm to 60 mm the converging channel. Transverse square ribs with pitch-to-height-ratio of 4 were glued onto 4.5 mm acrylic sheet which was then screwed onto the bottom wall of channel. A particle image velocimetry was used to conduct detailed measurements at several streamwise-transverse planes located upstream of, within and downstream of the variable sections. From these measurements, the mean velocities, turbulent intensities, and the mean and turbulent momentum fluxes at the top plane of the ribs were obtain to study the interaction between the cavities and overlying boundary layer. The profiles of mean velocity, turbulent intensities, Reynolds shear stress, and turbulence production terms were obtained across the channel to document the salient features of adverse and favorable pressure turbulent flows over transverse ribs in asymmetric channels.

Three-dimensional driven-cavity flows with a vertical temperature gradient

Steady conjugate heat transfer in an air filled electronic chassis, which is indirectly cooled by forced air along its top and bottom walls, is investigated numerically. Four printed circuit boards (PCB's), vertically located in the chassis, form five separate air compartments for natural convection to take place. Along the top and bottom walls of the chassis, cooling air, flowing inside finned channel passages, removes the heat generated by the PCBs. The effects of temperature distributions on the top and bottom walls of the chassis on the PCB temperature field are of prime interest here. The integrated system and board level modeling is done using a two-step process. First, a simplified computational model is developed for the finned channel, which calculates the nonuniform temperature distributions on the top and bottom walls of the chassis. By applying the calculated top and bottom wall temperature distributions as boundary conditions, a three-dimensional (3-D) numerical study is next performed using control volume based finite volume scheme to calculate the detailed temperature fields. This model considers the conduction within the PCBs, as well as the natural convection within the adjacent air. To elucidate the importance of an integrated system and board level analysis, the results are compared with those for conventional constant wall temperature boundary conditions on the chassis top and bottom walls.

A low Reynolds number flow in a channel obstructed by rows of ribs is of practical importance with respect to heat transfer enhancement. In order to obtain a high outlet gas temperature in a limited channel length, a flow rate should be low, whilst the flow should be turbulent to ensure an efficient heat transfer. Our preliminary study has indicated that the turbulence can be retained even at a low Reynolds number through the disturbance by the rows of the ribs in the channel. The ribs induce three-dimensional complex flow in their vicinity. The purpose of our research is to examine the influence of the ribs upon the flow and the temperature fields. Direct numerical simulation (DNS) of turbulent heat transfer in a fully developed channel flow has been performed at a friction Reynolds number of Reτ = 80 and a Prandtl number of Pr = 0.71, where Reτ is based on the friction velocity uτ0 , the channel half width δ and the kinematic viscosity ν. The friction velocity uτ0 is defined by the streamwise mean pressure gradient which drives the flow. The non-slip condition is adapted to the top and bottom walls. The periodic boundary condition is imposed in the horizontal directions. The rectangular rib is placed at the center of the computational domain (Fig. 1) and connected directly to both of the top and bottom walls. The rib is treated by the immersed boundary method. The constant wall temperature difference is applied for the temperature field. The flow rate decreases owing to the pressure drag of the rib, and the bulk Reynolds number Rem ( = um·2δ/ν ) drops down to 1120, where um is the bulk mean velocity. In the case of the plane channel without the rib, our additional calculation at this Rem has revealed that the flow becomes laminar. In the present case, the flow remains turbulent since the turbulent kinetic energy is produced in the vicinity of the rib. The instantaneous flow field is shown in Fig.1. Upstream of the rib, a horseshoe vortex is formed near the wall. The flow separates at the forward-edge of the rib. Downstream of the rib, a three-dimensional wake is formed, which contains a lot of vortices. Figure 2 shows the time-averaged local Nusselt number on the bottom wall. The heat transfer is significantly enhanced in front of the rib and the wake region. Upstream of the rib, the local Nusselt number is remarkably large. It indicates that the low temperature fluid in the central region is transported towards the bottom wall by the flow impingement induced by the rib. This region corresponds to that of the horseshoe vortex. In addition, Nu is large in the wake region as a consequence of effective mixing.

On inclined heated square obstacle in a liquid stream carried by partially heated channel: Finite element analysis

This work investigates mixed convection numerically using a heated square cylinder at the middle of a vented cavity filled with fluid-saturated porous media. External cold fluid enters into the cavity through the opening located either at the bottom wall or at the bottom and side walls, whereas the hot plume is vented through an opening at the top wall. All the walls of the cavity are insulated. The governing equations are solved by in-house code. Flow through porous medium has been modeled using Brinkman-Forchheimer-Darcy Model (BFDM). The influence of flow parameters (Reynolds number Re = 10–500, Richardson number Ri = 0.1–100) and porous-medium parameters (Darcy number Da = 10-7–10-3, porosity e = 0.1–1) and inlet openings are investigated systematically. It is observed that the heat transfer increases as Ri and Re increase.

The results of a numerical study of the effect of cold-water density inversion (Prandtl number Pr=11.59) on the flow and heat transfer in a horizontal plane-parallel channel with isothermal top and bottom walls are presented. The calculations were performed for the Grashof number Gr=3·104, the Reynolds number Re=10, and the channel segment length-to-height ratiol/d=40. The wall temperature was so varied that the temperature difference between the top and bottom walls remained constant.

The wave regime has a strong influence on the sediment transport in coastal systems. Modifications in wave regime induced by climate changes can influence the sediment dynamics of those coastal systems. To access wave regime changes it is crucial to analyse the future modifications in the wave height, period and direction. This work aims to analyse the influence of a future wave regime in the sediment budget of a coastal lagoon inlet and at the nearshore adjacent coast. To achieve this goal a morphodynamic modelling system was used, forced by present and future waves, corresponding to a typical year of present and future wave climates. A methodology to determine a typical year of each climate was developed based on the determination of correlation coefficients between each climate and corresponding year data. The comparison between present and future wave climates evidences that wave period and height are in general similar for both climates, and confirms the anticlockwise rotation of waves in the future. The morphodynamic simulations revealed analogous results for both wave climates, resulting in similar patterns for the residual sediment fluxes, but slightly more intense in the present. The consequent bathymetric changes show that the deposition trend presently observed offshore the inlet tends to increase for future waves climate. The transport budgets were also analysed for both wave regimes, evidencing that the alongshore transport slightly decreases (~1%) for future waves.

Forced condensation heat transfer characteristics of vapour in the presence of noncondensable gas in horizontal tubes operating at high pressures

In the present numerical study the effect of a vertically applied magnetic field on convection of low Prandtl number liquid metal, rotating in a cubical cavity of aspect-ratio 8:8:1 has been investigated. The bottom wall is heated while the top wall is cooled and all the other walls are kept thermally insulated. The simulations have been carried out for liquid metal flows having a fixed Prandtl number Pr = 0.01, Rayleigh number Ra = 107 and magnetic Prandtl number Pm = 4.0 × 10−4, while the Chandrasekhar number Q varies from 5.0625 × 104 to 4.9 × 105 and the Taylor number Ta is varied from 0 to 1010. It is found that the magnetic field generates a strong damping effect on flow velocities and heat transfer at low rotation rates corresponding to Ta = 106 and Ta = 2.5 × 107, though at higher rotation rates Ta = 1010 the Coriolis forces become comparable to the Lorentz forces thus generating conditions conducive for incipience of dynamo. At the same Ta, with an increase in magnetic fields corresponding to Qh = 4.9 × 105 there is a sharp fall in rms velocities near the top and bottom walls due to the formation of thin Hartmann layers. At low rotation rates, vertically stretched multi-cellular rolls perpendicular to the gravity axis are found. As the rotation increases, the rolls change their spatial orientation and align themselves parallel to the rotation axis. At the highest rotation rate Ta = 1010, a cylindrical vortical column forms at the centre of the cavity.

In this paper, the outline of the Mach 4 supersonic wind tunnel for the investigation of the supersonic internal flows in ducts was firstly described. Secondly, the location, structure and characteristics of the Mach 2 and Mach 4 pseudo-shock waves in a square duct were investigated by color schlieren photographs and duct wall pressure fluctuation measurements. Finally, the wall shear stress distributions on the side, top and bottom walls of the square duct with the Mach 4 pseudo-shock wave were investigated qualitatively by the shear stress-sensitive liquid crystal visualization method. The side wall boundary layer separation region under the first shock is narrow near the top wall, while the side wall boundary layer separation region under the first shock is very wide near the bottom wall.