Flow Evolution Process Research Articles

Flow and mixing characteristics of an elevated pulsating transverse jet

Flow-evolution processes as well as the penetration, spread, and dispersion characteristics of elevated pulsating transverse jets were studied experimentally in a wind tunnel. Jet pulsations were induced by means of acoustic excitation. Streak pictures of the smoke-flow patterns, illuminated by a laser-light sheet in the median plane, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocities of instabilities in the flow. Penetration height and spread width were obtained through a binary edge identification technique. Tracer-gas concentrations were measured to provide information on jet dispersions and trajectories. Three characteristic flow modes (synchronized flapping jet, transition, and synchronized shear-layer vortices) were identified in the domain of the jet-to-crossflow momentum-flux ratio and the excitation Strouhal number. At low excitation Strouhal numbers, the jet column near the tube exit flapped back-and-forth periodically at the excitation frequency and induced large up-down motions of the deflected jet. The penetration, spread, and dispersion of the jet increased drastically compared with the non-excited jet because the up-down oscillating motions of the deflected jet transformed the axial momentum into oscillating lateral momentum. Forcing the jet into the transition and synchronized shear-layer vortices regimes caused the vortices to appear along the upwind shear layer of the deflected jet. Under these conditions, the penetration, spread, and dispersion of the jet presented insignificant increases because the entrainment effect induced by the shear-layer vortices was not as large as that produced by the jet oscillating motions in the synchronized flapping jet regime.

Day-to-Day Dynamic Model in Discrete–Continuum Transportation Networks

In a discrete–continuum transportation network, the freeway network is modeled as an ordinary discrete link–node network; the dense minor streets are approximated as a two-dimensional continuum; and the two are connected at a limited number of points (freeway ramps). This representation of a discrete–continuum network highlights the backbone role played by the freeway network in modern urban transportation systems, and the continuum approximation approach suits large-scale dense minor streets. Previous studies on continuum network modeling all focused on the concept of network equilibrium. In this paper, the disequilibrium flow evolution process is studied after some network disruption or facility change has taken place. A day-to-day dynamic model suitable for the discrete–continuum representation of urban transportation networks is proposed. This model captures travelers' cost-minimization behavior in their path finding and also captures their inertia. The fixed point of the proposed dynamical system is the classic Wardrop user equilibrium. A numerical example is provided to demonstrate the effectiveness of the model.

Modeling coupled THM processes of geological porous media with multiphase flow: Theory and validation against laboratory and field scale experiments

A FEM model for analysis of fully coupled multiphase flow, thermal transport and stress/deformation in geological porous media was developed based on the momentum, mass and energy conservation laws of the continuum mechanics and the averaging approach of the mixture theory over a three phase (solid–liquid–gas) system. Six processes (i.e. stress–strain, water flow, gas flow, vapor flow, heat transport and porosity evolution processes) and their coupling effects are considered, which not only makes the problem well-defined, but renders the governing PDEs closed, complete, compact and compatible. Displacements, pore water pressure, pore gas pressure, pore vapor pressure, temperature and porosity are selected as basic unknowns. The physical phenomena such as phase transition, gas solubility in liquid, thermo-osmosis, moisture transfer and moisture swelling are modeled. As a result, the relative humidity and other related variables in porous media can be evaluated on a sounder physical basis. A three dimensional computer code, THYME3D, was developed, with eight degrees of freedom at each node. The laboratory CEA Mock-up test and the field scale FEBEX benchmark test on bentonite performance assessment for underground nuclear waste repositories were used to validate the numerical model and the software. The coupled THM behaviors of the bentonite barriers were satisfactorily simulated, and the effects and impacts of the governing equations, constitutive relations and property parameters on the coupled THM processes were understood in terms of more straightforward interpretation of physical processes at microscopic scale of the porous media. The work developed enables further in-depth research on fully coupled THM or THMC processes in porous media.

BENCHMARK SOLUTION FOR UNSTEADY STATE CFD PROBLEMS

Unsteady convective dominated flows are Very common in engineering and science. There is a need to develop an accurate numerical scheme to predict unsteady flaw and heat transfer to understand the physics of instability and the flow evolution process. No numerical scheme is free from errors. It is essential to reduce the numerical errors to some extent, so that the prediction results are dependable. This article suggests two platform problems to test the accuracy of a time marching numerical scheme. One problem is buoyancy driven flow in a differentially heated cavity. This problem has importance in melting and solidification processes. The other problem is the forced flow in an oscillatory lid driven cavity. These problems are chosen because they have well-defined geometry, boundary, and initial conditions, yet the flow exhibits complexity and skewness with the grid line. High-order schemes are used to generate benchmark solutions for the mentioned problems. It is believed that Ike predicted results are numerically correct and oscillation of the fluid is due to physics of the flow.

Transient oscillatory conjugate natural convection in a tall water cavity

A two-dimensional numerical simulation was conducted to investigate the effects of the heat capacity of the heated wall on the transient oscillatory convection in a tall cavity. Results were particularly obtained for water (Pr=6) in a tall cavity (A=6) with constant-heat-flux heating on one side and isothermal cooling on the opposing side. Significant wall heat capacity effects were found. Specifically, a heated wall of finite heat capacity can slow down the flow evolution process to steady state at low Rayleigh number and damp the flow oscillation during the initial developing period at high Rayleigh number. In periodic flow atRa* H =4.4×1010 the wall heat capacity significantly reduces the amplitude of the temperature oscillation. A quasi-periodic flow atRa* H =4.8×1010 was found to become periodic when the wall heat capacity was included. The wall heat capacity does not shown noticeable effect when the flow is chaotic forRa* H =6.0×1010.

Flow and acoustic properties of low Reynolds number underexpanded supersonic jets

An experimental program to investigate the flow and acoustic properties of model underexpanded supersonic jets was conducted. In particular, the role played by large-scale organized fluctuations in the flow evolution and acoustic production processes was examined in detail. The experimental conditions were chosen as low Reynolds number ( Re = 8000) Mach number 1·4 and 2·1 underexpanded jets exhausting from convergent nozzles. A consequence of performing the experiments at low Reynolds number is that the broadband shock-associated noise is suppressed. The focus of the present study is on the generation of noise by large-scale instabilities in the presence of strong shock cell structures. The aeroacoustic properties of these underexpanded jets are shown to have some departures from those of perfectly expanded jets at identical Mach and Reynolds numbers. Also, the noise radiated by these model jets is shown to be very similar to the shock screech noise of conventional high Reynolds number underexpanded jets. It is demonstrated that the production of screech is related to the modulation and decay of large-scale turbulence structures. This is consistent with high Reynolds number jet flows of comparable Mach number and nozzle geometry.