1,721,000 research outputs found
Direct numerical simulation of heat transfer of round subsonic impinging jets at high Reynolds number
No full textImpinging jets provide an effective cooling method for various applications such as the cooling of aircraft turbine blades. The latest generation of high performance computers allows us to investigate those at practically relevant Reynolds numbers Re by means of direct numerical simulations. In order to analyse the heat transfer of a confined round impinging jet, two direct numerical simulations are performed at Re=3300 and Re=8000 using a grid of 512 x 512 x 512 respectively 1024 x 1024 x 1024 points. Each configuration is fully turbulent. The first one features two annular regions with local maxima of heat transfer at the impinging plate. These effects are related to high wall-normal turbulent heat fluxes caused by vortical structures of the turbulent flow field. The second simultion is ongoing. Its results will also be presented on the conference
Interaction between the shear layer, shock-wave and vortex ring in a starting free jet injecting into a plenum
No full textWhile continuous free jets have been investigated and optimised during the last 60 years, the impulsively started jet is still relatively unexplored. We focus here upon the very first stage of a compressible free round jet, when the flow is only few diameters long and the vortex ring generated by the sudden expansion interacts with the shock-waves and the shear layer. Direct numerical simulations with more than 2*10^9 grid points are carried out, discretising the compressible Navier-Stokes equations to compute both the fluid flow and the noise radiated by the interaction of the shear layer, the shock-waves and the vortex ring in a compressible free round jet. As a result of the mentioned interaction, a sound level of 111[dB] at a distance of 100 diameters from the jet has been computed. An interaction between the shear layer, the shock-waves and the vortex ring has been investigated using numerical methods in an impulsively started supersonic free round jet and noise levels of order of the loudest acoustic phenomenon in the continuous jet have been identified and quantified
Effect of wall curvature of an asymmetric subsonic impinging jet: DNS study
No full textWe report on Direct Numerical Simulation (DNS) of a subsonic confined jet impinging on a curved surface. The configuration resembles a turbine leading edge cooling system. The bulk Reynolds number (based on bulk velocity Ub and jet diameter D) is 3300. The impinging wall was kept at a constant temperature higher than the jet bulk temperature. Comparisons with the jet impinging on a flat plate are carried out.
Our major findings are that the turbulent flow field is affected by the shape of the impinging wall, leading to an asymmetric development of turbulent structures. The frequencies which dominates the excess heat transfer mechanism change with respect to the reference case, changing the average heat transfer into the hot wall
DNS study of fusion reactor dust particle mobilization induced by a transonic jet incoming in a vacuum container
No full textThe flow induced motion of wall deposited particles
is highly linked with the instantaneous fluid structures.
Here we perform a two-phase flow DNS to analyze
the resuspension of solid particles from a surface
hit by a transonic jet into a low pressure container
in conditions similar to those which occur in a fusion
reactor vacuum vessel during a Loss of Vacuum Accident
(LOVA). The initial condition of pressure and
temperature were set to 49.5 mbar and 373 K, with
a Reynolds number of 3300 on a 512 512 512
grid properly refined in regions where high gradients
are present. The Thornton and Ning impact/adhesion
model is adopted, whereas an advanced resuspension
model, which also takes into account the dynamics
(rolling and sliding) of particles at the wall, is here
implemented. The initial deposited particles follow a
log-normal distribution with a count median diameter
of 2.21 μm, geometric standard deviation of 2.93 and
constant density of 8527 kg/m3. It has been found that
the resuspension phenomenon mostly affect particles
of the biggest diameters. Moreover, the jet-deposit interaction
is for the most part confined within a circumference
around the jet of radius equal to the jet diamete
DNS of an oblique jet in a particle-laden crossflow: study of solid phase preferential concentration and particle-wall interaction
No full textA DNS study of the interaction between an oblique laminar jet and a particle-laden crossflow is presented. The delivery tube is included in the simulation and jet in crossflow blowing ratio is set equal to 0.5, typical for gas turbine film cooling applications. The solid phase is polydisperse and it is simulated by adopting a Lagrangian two-way coupling point-particle approach. Wall-particle interaction is also taken into account. The fluid flow in the jet outflow region was found to be dominated by a strong vorticity field and by hairpin shaped vortices that are shed periodically in the crossflow. Hairpin legs are associated to a counter-rotating vortex pair that persist in the far-field of the jet. The spatial distribution of the dispersed phase is strongly influenced by these large-scale coherent structures. A three-dimensional Voronoi analysis demonstrated a particle preferential concentration induced by hairpin vortices downstream from the jet exit. Volume fractions curves are presented as a function of the spanwise direction for different transversal sections of the crossflow region. A void particle region is induced by vortices in the central near-wall zone downstream of the jet exit. Particles tend to accumulate along two symmetric regions placed on the lateral side of the structures generated by the jet injection. By an analysis of particle impacts on the wall it was observed that particles characterized by lower values of the St number, whose response time is comparable with the characteristic time of the hairpin vortices, tend to impact on the wall on two symmetric side of the jet exit in the proximity of hairpin legs. This demonstrated that the generated large-scale coherent structures play a major role in the not homogeneous dispersion of the solid phase
Effects of wall curvature on the dynamics of an impinging jet and resulting heat transfer
Full text linkThe effects of wall curvature on the dynamics of a round subsonic jet impinging on a concave surface are investigated for the first time by direct numerical solution of the compressible Navier-Stokes equations. Impinging jets on curved surfaces are of interest in several applications, such as the impingement cooling of gas turbine blades. The simulation is performed at Reynolds and Mach numbers respectively equal to 3, 300 and 0.8. The impingement wall is kept at a constant temperature, 80 K higher than that of the jet at the inlet. The nozzle-to-plate distance (measured along the jet axis) is set to 5D, with D the nozzle diameter. In order to highlight the curvature effects, the present results are compared to a previous study of jet impinging on a flat plate. The specific influence of wall curvature is investigated through a frequency analysis based on discrete Fourier transform and dynamic mode decomposition. We found that the peak frequencies of the heat transfer also dominate the dynamics of primary vortices in the free jet region and secondary vortices produced by the interaction of primary vortices and the target plate. These frequencies are approximately 30% lower than those found in the reference study of impinging jet on a flat plate. Imperceptible differences were instead found in the time-averaged integral heat transfer
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Direct numerical simulation of an oblique jet in a particle-laden crossflow
Full text linkJet in crossflow is a classic fluid dynamics problem widely studied in the last decades because of the big quantity of natural and industrial processes in which it is encountered (Mahesh in Annu Rev Fluid Mech 45(1):379–407, 2013 [6]). The present study focuses on the interaction between solid suspended particles and gas turbines film cooling that is a commonly used coolant technique aiming at generating a protective film of cold fluid around the blade profile. Effective cooling systems are crucial to increase turbine inlet gas temperature and to protect turbine blade surfaces from the huge thermal stress generated
- …
