Self-oscillation regimes in a liquid jet curtain separating gas regions with different pressures

The results of an experimental investigation of the self-oscillation regimes of liquid jet outflow into a plane channel with air injection in its dead end are presented. The effect of the volume of the cavity, or the air cushion, and its thickness (or channel width) on the flow is studied on a wide range of the gas injection rate and the liquid outflow velocity. The self-oscillation flow regimes are realized at a constant pressure of water in the supply tank and a constant mass flow rate of the air injected into the cushion. With increase in the air injection rate the self-oscillation regime realized at lower injection intensities is replaced by a higher-frequency regime. High-speed videofilming shows that the difference from the low-frequency regime consists in the absence of the direct interaction between the out flowing jet and the channel wall. It is found that in both low-frequency and high-frequency regimes the self-oscillation frequency and amplitude are independent of the cushion thickness but the moment of the regime changeover is determined by this parameter. It is established that there exists a threshold value of the cavity volume, behind which the low-frequency regime does not occur at any air injection rates.     

Effect of the feeding pipeline properties on the nature of cavitation-induced self-oscillations in the presence of a ventilated cavity with a negative cavitation number in the system

Physical modeling of a ventilated cavity with a negative cavitation number has shown that at the same flow rate and head parameters different cavitation-induced self-oscillation patterns can be realized. The generation of these patterns depends on the feeding pipeline parameters. The high-speed videofilming shows that the physics of the process are the same in different frequency regimes, the wave propagation velocity in the jet flow is mainly determined by the cavitation number, and the difference between self-oscillation patterns is characterized by the number of the waves along the cavity length. A method of estimating the self-oscillation frequencies from the given flow geometry and the cavitation number is proposed.     

Cavitation interaction between fluid counterflows

Some features of self-oscillatory regimes for the cavitation interaction of coaxial water and annular gas jets with a water counterflow are studied. The dependencies of the self-oscillation frequency, the maximum displacement of a cavity toward the incident flow, and the displacement amplitude on the pipe diameter and the Froude number are found. A conclusion on the onset of self-oscillations is substantiated. The possibility of a significant increase in the counterflow range due to the gas injection in the vicinity of the counterflow is shown.     

Self-oscillatory regimes of cavitation interaction between counter-streaming fluid flows

The effects of unsteady cavitation interaction between a coaxial system of a continuous water jet and a surrounding annular gas jet and a counter-streaming water flow in a cylindrical tube is investigated both experimentally and numerically. The distinctive features of the mechanism of the formation of regular self-oscillatory interaction regimes and their salient features are studied. The dependence of the mean self-oscillation frequency on the ratio of the counter-streaming water flows is determined at different gas jet flow rates. The Froude number effect on the self-oscillation frequency is analyzed. The dependence of the maximum value of the longitudinal displacement of the nose of a developed oscillating cavity penetrating into the counter stream and of the displacement amplitude on the flow rate of the supplied gas is determined. The idea on the pressure distribution in the cavity in different stages of flow development is obtained. The nature of the dependence of the self-oscillation frequency on the degree of tube flow blockage by the cavity is established.     

Hybrid nanomaterial flow and heat transport in a stretchable convergent/divergent channel:a Darcy-Forchheimer model

The flow behavior in non-parallel walls is an important factor of any physical model including cavity flow and canals, which is applicable for diverging/converging channel. The present communication explains that the flow of the hybrid nanomaterial subjected to the convergent/divergent channel has non-parallel walls. It is assumed that the hybrid nanomaterial movement is in the porous region. A Darcy-Forchheimer medium of porosity is considered to interpret the porosity features. A useful simila...     

Simulation of flow boiling in micro-channels: Effects of inlet flow rate and hot-spots

This paper presents the findings of a numerical study on the flow boiling in a micro-channel heat sink. The Navier-Stokes equations, energy equation, and the continuity equation are solved in a finite-volume framework using the front-tracking method. The numerical method is validated by comparison with the experimental results for a slug bubble growth, and vertical flow boiling. The numerical method is then used to study the effect of changing the inflow mass-velocity on the heat transfer coefficient, bubble size distribution, and the bubble nucleation frequency for a constant heat flux. The mean heat transfer coefficient of all the cases is found to be nearly twice that of the single-phase heat transfer coefficient. The bubble nucleation frequency is found to increase monotonically with the inflow mass-velocity. The bubble size distribution along the channel is found to become flatter as the mass-velocity is increased. We identify three distinct phases of the bubble evolution, namely the initial rapid growth phase, the boiling dominant phase, and finally the condensation dominant phase. Subsequently, the numerical method is used to study the effect of having a hot-spot near the bubble nucleation site on the heat transfer characteristics. It is found that the bubble nucleation frequency increases and the bubbles’ maximum volume decreases as the intensity of the hot-spot is increased for a fixed inlet flow rate. It is also observed that the average heat transfer coefficient does not change significantly with changing the intensity of the hot-spot, and that the bubble size distribution along the channel becomes flatter as the intensity of the hot-spot is increased.     

Combined thermal radiation and natural convection in a cavity containing a discrete heater: Effects of nature of heating and heater aspect ratio

This paper investigates the interaction between thermal radiation and natural convection in an air filled square cavity with a discrete heater placed inside. The vertical walls of the cavity are cooled while the horizontal ones are insulated. Two types of the heater, viz., isothermal and heat generating are considered. The governing coupled nonlinear partial differential equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters such as the Rayleigh number, aspect ratio of the discrete heater and surface emissivity on the heat and fluid flow characteristics are investigated in detail. In general it is found that the overall heat transfer rate is enhanced with an increase of the surface emissivity and the Rayleigh number for both isothermal and heat generating heaters. Buoyancy induced flow gets augmented for the isothermal solid body whereas it is weakened for the heat generating body in the presence of radiation exchange.     

二维水平通道内流动沸腾换热的格子Boltzmann模拟

利用格子Boltzmann方法模拟二维水平通道内水的流动沸腾过程,获得不同壁面过热度下流型特点和不同因素对换热过程的影响规律。结果表明,随着壁面过热度升高,流道内流型依次经历从泡状流、弹状流到反环流的转变,平均热流密度和平均换热系数先增大后减小。入口流速降低会使流道内出现受限气泡流,核态沸腾受到抑制。提高入口流速能够有效促进气泡脱离,壁面平均换热系数随入口流速增大而增大,但增长速率有所减小。减小通道宽度有利于汽化现象发生,核态沸腾得到强化,壁面平均换热系数有所提高。     

On the dispersion of a solute in Poiseuille flow of a third-grade fluid in a channel

Gill and Sankarasubramanian's analysis of the dispersion of Newtonian fluids in laminar flow between two parallel walls are extended to the flow of non-Newtonian viscoelastic fluid (known as third-grade fluid) using a generalized dispersion model which is valid for all times after the solute injection. The exact expression is obtained for longitudinal convective coefficient K₁(Γ), which shows the effect of the added viscosity coefficient Γ on the convective coefficient. It is seen that the value of the K₁(Γ) for Γ≠0 is always smaller than the corresponding value for a Newtonian fluid. Also, the effect of the added viscosity coefficient on the K₂(t,Γ) (which is a measure of the longitudinal dispersion coefficient of the solute) is explored numerically. Finally, the axial distribution of the average concentration C_m of the solute over the channel cross-section is determined at a fixed instant after the solute injection for several values of the added viscosity coefficient.     

开区注氮采空区自燃温度场的数值模拟研究

为了研究开区注氮采空区遗煤自燃和被抑制的力学过程,用非均质多孔介质中的渗流连续性方程、气体弥散方程和综合传热方程的联立,建立了采空区注氮非定常数值模型．结合实例,用迎风格式有限元方法求解．给出了采空区的漏风流态、氮气流态,描绘了采空区自燃过程中氧、CO浓度和温度分布的变化过程．计算考虑了瓦斯涌出和工作面推进的影响．得到注氮条件下采空区高温区形状变窄;随着注氮量的提高,使自然发火期逐渐变长,直至不自燃．理论计算与实际情况吻合．     

Experimental study of ventilated cavity flow over a 3-D wall-mounted fence

Ventilated cavity flow over a fixed height 3-D wall-mounted fence is experimentally investigated in a cavitation tunnel for a range of free-stream conditions. The impact of 3-D effects on cavity topology is examined, along with the dependence of the cavitation number and drag on the volumetric flow rate coefficient, fence height based Froude number and vapour pressure based cavitation number. Three different flow regimes are identified throughout the range of cavitation numbers for a particular free-stream condition. Generally, the cavity has a typical re-entrant jet closure the intensity of which is found to increase linearly with increasing Froude number. This increase in re-entrant jet intensity causes an increase in drag with Froude number for constant volumetric flow rate coefficient. At low Froude numbers the closure mechanism transitions from a single to a split re-entrant jet. The parameters used to characterize the cavity topology show a linear dependence on Froude number irrespective of the closure mode. The cavity topology and drag are found to be independent of vapour pressure based cavitation number.     

Specific gasdynamical features of spontaneous condensation in an unsteady rarefaction wave

On the basis of numerical modeling, the formation of an unsteady shock wave induced by a condensation shock in a rarefaction wave moving in the high-pressure channel of a shock tube filled with moist air is demonstrated. It is shown that in a fairly long channel a periodic structure consisting of an alternating sequence of condensation shocks and the shock waves they generate may be formed. This structure is a linear unsteady analog of the self-oscillation regime of type IV in the classification [1] for condensing medium flows in the subsonic section of a Laval nozzle. The specific features detected are important for planning and interpreting experiments aimed at investigating spontaneous condensation using a “condensation shock tube”.     

Turbulence manipulation in air–water flows on a stepped chute: An experimental study

In a stepped channel operating with large flow rates, the flow skims over the pseudo-bottom formed by the step edges as a coherent stream. Intense three-dimensional recirculation is maintained by shear stress transmission from the mainstream to the step cavities, while significant free-surface aeration takes place. The interactions between free-surface aeration and cavity recirculation are investigated herein with seven step cavity configurations. The experiments were conducted in a large stepped channel operating at large Reynolds numbers. For some experiments, triangular vanes, or longitudinal ribs, were placed across the step cavities to manipulate the flow turbulence to enhance the interactions between the mainstream flow and the cavity recirculation region. The results showed a strong influence of the vanes on the air–water flow properties in both free-stream and cavity flows. The findings demonstrate some passive turbulence manipulation in highly turbulent air–water flows.     

Influences of suction and blowing on vortex shedding behind a square cylinder in a channel

The structure of confined wakes behind a square cylinder in a channel subject to a locally uniform suction or blowing at both the channel walls is presented. A pressure based finite-difference technique has been used to solve the unsteady Navier-Stokes equations. It is observed that the amplitude of the lift coefficient decreases with increase in the blowing velocity. Coefficients of drag also decrease for the application of uniform blowing and for a suitable value of the blowing parameter, the flow becomes steady and symmetric. The amplitude of the lift coefficients increases up to a certain limit of suction velocity and after that it suddenly decreases and flow becomes steady. Coefficients of drag also gives the same feature. Effects of the suction and blowing on the vortex-shedding region are analyzed in detail and presented graphically.     

SELF-SUSTAINED OSCILLATION OF A SUBMERGED JET IN A THIN RECTANGULAR CAVITY

Self-sustained oscillating jet flow of water in a rectangular cavity, having thickness which is small relative to its width, is measured using LDA and PIV, and predicted using a transient two-dimensional computational fluid dynamic model which incorporates a resistance coefficient for cross-flow. The basic geometry represents a scale model of a mould typical of thin slab steel casting. The frequency of the oscillation was found to be independent of cavity thickness. It also increased as the cavity width decreased down to some critical value, after which the oscillation ceased. The frequency was observed to increase with nozzle diameter and was found to decrease with increasing length/width ratio of the cavity. The numerical model, with a fixed dimensionless cross-flow resistance coefficient, was shown to predict the Strouhal number of the oscillation and the dimensionless mean velocity profiles in the jet extremely well.     

Experimental Investigation of the Transverse Self-Oscillations of Circular Cylinders Mounted with a Narrow Clearance in a Plane Channel

The results of an investigation of the flow past and the behavior of free bluff bodies mounted in pipes and channels with a narrow clearance, conducted in the Institute of Mechanics of Moscow State University, are presented. The drag of circular cylinders of different size and mass in a circulation-free water flow in a plane channel of rectangular cross-section was studied in the transverse self-oscillation regime. The experiments were conducted for Reynolds numbers based on the cylinder diameter 1.7 ? 10⁴ ≤ Re ≤ 7.2 ? 10⁴, relative clearances \(\bar S\) based on a cross-sectional area ranging from 0.76 to 0.9, and cylinder-to-water density ratios ρ _c /ρ ranging from 1.29 to 8.2. Only the case of intense transverse self-oscillations accompanied by impact interaction with the channel wall was considered. The dependence of the period-average cylinder drag coefficient C _x on the basic dimensionless relevant parameters is obtained. The dependence of the dimensionless self-oscillation frequency determined in [1] is refined. The kinematic and dynamic features of the flows past spheres in cylindrical pipes and cylinders in plane channels are compared in the transverse self-oscillation regime.     

Three-dimensional viscoelastic flows through a rectangular channel with a cavity

Three-dimensional viscoelastic flows in a rectangular channel with a cavity were studied both numerically and experimentally. In the numerical study, computations were carried out using the Phan-Thien–Tanner (PTT) model as a constitutive equation. A finite volume method (FVM) using colocated grids was applied. A three-dimensional structure in the cavity was observed even when three-dimensional behavior was not remarkable in main flows. At high Weissenberg number, the flow in the cavity spirals to move towards the center plane of the channel. In the experiments, the flow of polymer solutions was visualized to observe three-dimensional flow behavior near the cavity part. It was confirmed that the spiraling flow moving towards the center plane emerged in the cavity.     

Pre- and post-injection flow characterization in a heavy-duty diesel engine using high-speed PIV

High-speed particle image velocimetry (HS-PIV) using hollow microspheres has been applied to characterize the flow in a heavy-duty diesel engine during and after fuel injection. The injection timings were varied in the range representing those used in premixed charge compression ignition (PCCI) regimes, and multiple injections have been applied to investigate their influence on the flow inside the combustion chamber. By injecting into pure nitrogen, combustion is avoided and the flow can be studied long after injection. The results show a sudden change of air motion at the start of injection as a result of the air entrainment at the core of the spray. Furthermore, as expected, spray injection causes a considerable increase in the cycle-to-cycle fluctuations of the flow pattern, the more so for longer injection durations.     

Numerical modeling of the self-oscillation onset near a three-dimensional backward-facing step in a transonic flow

The results of a numerical investigation of the nature of self-oscillation processes occurring in transonic flow past a backward-facing step and a cavity with a flow of the open type are presented. The turbulent flow past the above-mentioned bodies is modeled using the NOISEtte software package intended for solving problems of aerodynamics and aeroacoustics on unstructured grids. The modeling is performed using the eddy-resolving IDDES method that belongs to the class of hybrid RANS-LES approaches. The adequacy of the calculations is confirmed by means of comparing the results obtained with the available experimental data. The structure and the salient features of the self-oscillatory, hydrodynamic-in-nature process, which arises in flow past a cavity and a backward-facing step, are established.     

Effect of air gaps on ballistic resistance of targets for conical impactors

High velocity penetration of a rigid conical impactor into a ductile target with air gaps between the plates is studied using the cylindrical cavity expansion approximation describing impactor–target interaction. It is showed that the latter model predicts improvement of the ballistic performance of the target with the increase of air gaps. It is found analytically that the ballistic limit velocity of the target consisting of N plates with a fixed total thickness with large air gaps increases with the increase of N. The conditions are discussed when the predicted effects can be most pronounced.