Matteo Angelino

Preliminary Numerical Solutions of the Evolution of Free Jets

The present paper presents preliminary numerical solutions of the flow evolution of a two dimensional rectangular free jet. The numerical simulations in a two-dimensional domain are carried out with Open-FOAM, the open-source code, and compare the numerical results with the experimental visualizations performed in the same laboratory with the shadowgraph technique. The evolution of a two-dimensional submerged free jet is reported in the literature by the presence of two regions of flow: the potential core, where the centerline velocity maintains equal to that on the slot exit, and the turbulent or mixing region, where the centerline velocity decreases with the distance from the exit. Previous anemometric measurements, carried out in this laboratory with an air jet emerging from a rectangular channel, showed the presence of a region of flow, just outside the exit and before the potential core, where velocity and turbulence remain almost equal to those measured on the exit, and it has been called “undisturbed region of flow” because is present also in turbulent conditions. Previous and present shadowgraph visualizations show a jet which has the same height along the undisturbed region of flow and increases its height afterwards. The length of the undisturbed region depends on the Reynolds number of the flow and on the presence of turbulence promoters, e.g. metallic grids, at the exit of the slot. The undisturbed region is becoming nil with the increase of the Reynolds numbers, in agreement to the literature. The present two dimensional numerical solutions, carried out at Re numbers equal to 25,000 and 60,000 confirm the results obtained with the shadow visualizations.

Experimental and Numerical Heat Transfer on a Cylinder Cooled by Two Rectangular Jets of Different Heights

The present paper presents experimental measurements of heat transfer on a cylinder, cooled by two rectangular jets of the same width but different height, H, in order to investigate the influence of the jet height on the local and the average cooling rates, because one of the problems in the heat transfer with jet flow is the convenience to choose the height of the jet in comparison to the height of the impinged object. The cylinder, heated by electric current, is set at several distances from the jet exit, x, on the symmetrical plane of symmetry, i.e. in a two-dimensional geometry, in order to find the optimal position which realizes the maximum heat transfer. The experimental heat transfer on the impingement shows that the local Nusselt number, defined with the cylinder diameter, D, is greater for the smaller slot because velocity is slightly higher but the average Nusselt numbers, defined with the cylinder diameter, D, are quite similar because the higher slot has a greater surface of impingement. Local and average Nusselt numbers are in qualitative agreement only if are compared with the same dimensionless distance, x/H, which can be interpreted as the ratio Rex/ReH, which is per unit of mass flow rate or is independent of the mass flow rate. Numerical simulations are carried out with a two-equations turbulent model using the RNG k-e approach, on a cylinder with the same thickness of the experiments or without thickness. The numerical simulations of the cylinder without thickness are in acceptable agreement for what concern the average Nusselt numbers. The local Nusselt numbers are in fair agreement only if the cylinder has the thickness of the experiments, independently of the heat flow conditions, i.e. uniformly throughout the thickness or from the inner surface.Copyright

Numerical solution of three-dimensional rectangular submerged jets with the evidence of the undisturbed region of flow

ABSTRACT The evolution of turbulent rectangular submerged free jets has been investigated numerically with a two-dimensional (2D) approach by the present authors and, by using the large eddy simulations (LES) at several Reynolds numbers. The average numerical results confirmed the presence of the undisturbed region of flow (URF) located between the slot exit and the beginning of the potential core region (PCR) previously observed experimentally at the University of Rome “Tor Vergata” by Gori and coworkers. The 2D study of the present authors carried out under the conditions previously investigated in the literature, showed that the URF has a self-similar behavior, and proposed a new law for the evolution of the momentum. The present paper extends the LES to three-dimensional (3D) rectangular submerged free jets, in the range from Re = 5,000 to Re = 40,000, showing that the self-similar behavior of URF is also present in the 3D numerical simulations, as well as in the PCR and in the fully developed region (FDR).

Numerical evidence of an undisturbed region of flow in a turbulent rectangular submerged free jet

ABSTRACT The evolution of turbulent rectangular submerged free jets is described in the literature by the presence of two regions of flow: the potential core region (PCR) and the fully developed region (FDR). However, experiments carried out in the last decade showed that a third region of flow is present, the undisturbed region of flow (URF), so-called in the average visualization, or the negligible disturbances flow (NDF) plus the small disturbances flow (SDF), so-called in the instant visualization. The URF is located between the slot exit and the beginning of the PCR. The main characteristics of URF, and NDF, are that velocity and turbulence profiles remain almost equal to those measured on the slot exit, and the height of the jet remains equal to the slot one. In the SDF the jet height undergoes small variations, i.e., contractions or expansions, but without formation of the vortex. To date, no numerical evidence of the presence of URF has been given by the literature. The present study, which concerns a two-dimensional jet, presents Large Eddy Simulations (LES), carried out at four Reynolds numbers, which are able to predict and characterize URF. The present numerical results are compared to previous theoretical approaches and confirm the presence of URF, between the slot exit and the PCR. Moreover, URF has a self-similar behavior and a new law for the evolution of the momentum is proposed.

Far-field noise prediction of round and serrated jets with increasingly refined grids

Turbulent jet large-eddy simulations (LES) are performed at Mach 0.9 and Reynolds number of 106. For subgrid scale stress modeling the σ-model is used. Solutions are obtained for a baseline axisymmetric (round) nozzle and a serrated (or chevron) nozzle with high bending and penetration, on grids ranging from 5 to 80 million grid points in order to assess the correlation between coarser and finer grid solutions. Computed mean and second-order fluctuating quantities of the turbulent near field compare favorably with measurements. The radiated far-field sound is predicted using the Ffowcs Williams and Hawkings (FW-H) surface integral method. Remarkable agreement of the predicted farfield sound directivity and spectra with measurements is obtained. A preliminary discussion is presented on the correlation and possible combination of multiple spectra from different grids.

Adaptive wall-modelled large eddy simulation of jet noise in isolated and installed configurations

The study of jet acoustics is crucial for future aeroengine designs. Although the highbypass ratio of modern turbofans can have a shielding effect on the core jet noise, there is an increased potential of interaction of the jet flow with wing and flap, and its effects on noise need to be thoroughly investigated. Wall-Modelled Large-Eddy Simulation (WMLES) is a powerful method to study the installation effects on jet noise, as it does not have strict near-wall requirements, allowing for a more uniform mesh for better noise propagation and a saving in computational cost. An adaptive wall model is here introduced and validated on channel flow, on the MD-30P/30N high-lift multi-element airfoil, and on the NASA High-Lift Common Research Model (HL-CRM). WMLES simulations, combined with the Ffowcs Williams and Hawkings (FW-H) sound extrapolation method, are performed on turbulent coaxial jets in isolated and installed configurations. Computed flow field and sound spectra present favourable agreement with experimental results, confirming key features of the installation effects on jet noise.

Emerging Patterns In Crowd Streams And The Aid Of Abm For Egress Management

The aim of this study is to illustrate the advantage of using agent based models (ABM) in order to deepen the understanding of crowd phenomena and help manage design strategies and egress operations on the field. The crowd stream, recognised as a proper nonlinear system, is in fact able to produce characteristic patterns that interact with the environment in recurring shapes. In particular, the free stream and the multi-leader stream are chosen as two mostly representative patterns that develop under typical conditions in a real life egress environment, so that the ABM engineering may bring an added value also in the development of the environment layout. The first part of the study describes the features of these emerging patterns in one geometrical enclosure typical of a Mall. Such issues as queuing, clogging generation, paths of crowd spreading are, for instance, the primary goal upon which to compare the modelling. The overall outcomes, in terms of travel times and people fluxes, complete the scene. The second part of the study describes the connected evolution of these emerging patterns in an environment typical of the landside space in an Airport terminal. Such issues as people advancement, emotions transport, and change of direction are, for instance, the primary goal for the modelling. The overall outcomes of the travel times and people fluxes are the remaining issues to be assessed. The comparison between the results coming from the various ABM packages (Evac, Pathfinder) completes the study, showing how a proper choice in modelling can be based depending on the foreseen goal of the design stage.