Calibration and data reduction for X-hotwires using cross validation

A strategy for calibration of X-wire probes and data inversion is described in this article. The approach used has elements of full velocity vs yaw-angle calibration with robust curve fitting. The responses of an X-wire probe placed in a calibration jet are recorded for a set of velocity and yaw inputs followed by fitting cross-validated splines. These spline functions trained from calibration data are evaluated for the probe responses during measurement. X-wire probes are calibrated for low to moderate velocities (0.65 m/s to 32 m/s) and yaw angles in the range −40° to 40° and comparisons with conventional interpolation schemes are made. The proposed algorithm can be extended to calibration of other multiple wire probes and for higher velocities. Some measurements in a single round turbulent jet flow at high Reynolds number using the proposed inversion algorithm are also presented. The present scheme is found to perform better particularly at low flow magnitudes and/or extreme flow angles than the schemes used previously.

Transient Ignition of Premixed Methane/Air Mixtures by a Pre-chamber Hot Jet: a DNS Study

The present study investigates the transient processes controlling ignition by a hot jet issued from a pre-chamber. Direct numerical simulations (DNS) have been performed to study the characteristics of the turbulent jet flow and of the associated flame during the whole ignition process, quantifying the relevant physicochemical interactions between pre-chamber and main chamber. Thanks to a detailed analysis of the DNS results, the transient ignition is found to consist of three main sequential processes: (1) near-orifice local ignition in the main chamber; (2) further flame development supported by the jet flow; and (3) global ignition and propagation of a self-sustained flame in the main chamber, independently from the hot jet. The characteristic time-scale of the hot jet as well as jet-induced effects (local enrichment, supply of radicals and heat) are found to be essential for successful ignition in the main chamber. A more intense turbulence in the main chamber appears to support local ignition. However, it also induces local quenching, thus delaying global ignition. An ignition threshold based on a critical Damköhler number is a promising concept, but is not sufficient to describe the process in all its complexity.

Analysis of Coupled CFD-DEM Simulations in Dense Particle-Laden Turbulent Jet Flow

In this paper, coupled CFD-DEM simulations of dense particle-laden jet flow are performed using CFDEM® coupling interface that couples LAMMPS-based LIGGGHTS® DEM engine with OpenFOAM CFD framework. Suspensions of mono-sized spherical glass particles with 80 microns diameter and a mass loading of 0.23 and 0.86 are considered. Three different CFD meshes are used with an average mesh resolution dimension of 3.06, 2.67, and 1.86 particle diameters and it is determined that mesh resolution does not change results for void fraction calculation (using the divided model) of the CFD-DEM equations. Samples of particle flux are taken at 0.1, 10, and 20 nozzle diameters along the axial direction of the jet region. The numerical results for particle flux are compared with a well cited experimental data found in literature. The CFD-DEM simulations in turbulent jet flow are found to be highly sensitive to initial particle velocity inputs but the experimental data provide sufficient information to produce comparable results.

Modelling of jet noise: a perspective from large-eddy simulations

In the last decade, many research groups have reported predictions of jet noise using high-fidelity large-eddy simulations (LES) of the turbulent jet flow and these methods are beginning to be used more broadly. A brief overview of the publications since the review by Bodony & Lele (2008, AIAA J. 56 , 346–380) is undertaken to assess the progress and overall contributions of LES towards a better understanding of jet noise. In particular, we stress the meshing, numerical and modelling advances which enable detailed geometric representation of nozzle shape variations intended to impact the noise radiation, and sufficiently accurate capturing of the turbulent boundary layer at the nozzle exit. Examples of how LES is currently being used to complement experiments for challenging conditions (such as highly heated pressure-mismatched jets with afterburners) and guide jet modelling efforts are highlighted. Some of the physical insights gained from these numerical studies are discussed, in particular on crackle, screech and shock-associated noise, impingement tones, acoustic analogy models, wavepackets dynamics and resonant acoustic waves within the jet core. We close with some perspectives on the remaining challenges and upcoming opportunities for future applications. This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.

Experimental Results on a Gaseous Jet Into a Turbulent Air Crossflow: Effect of the Shape of Injector Inlet on Fuel/Air Mixing

This paper presents experimental results on a gaseous jet into an air crossflow. A plain-orifice type injectors are employed for this research. It is mounted flush with a bottom wall of a test rig with a rectangular cross-section. The main emphasis is to investigate the effect of aspect ratio of the injector on fuel/air mixing. Injector diameter is fixed at 1.59 mm and two injectors with the orifice aspect ratio (1/d) of 1 and 5 are chosen for the purpose. Velocity ratios of a fuel jet to an air crossflow, Vj/Va, are chosen as 3, 5 and 7, and the mean crossflow velocity ranges 2 to 40 m/sec, which results in a wide range of Reynolds numbers for the gaseous jet encompassing laminar to turbulent flow. Acetone planar laser-induced fluorescence measurement is used for the representation of fuel concentration. The decay rate of scalar concentration, fuel jet penetration and cross-sectional distributions of the fuel plume are determined for the two injectors. First, the effect of the aspect ratio of the injector in quiescent surroundings is considerable at low jet velocities, i.e. laminar jet flow, but not at jet velocities corresponding to turbulent jet flow. Second, the influence of the aspect ratio of the injector in the air crossflow is similar to that observed in the free jet. At the laminar jet condition, the difference in the turbulent intensity produced by the aspect ratio of the injector changes the jet core characteristics of each injector and causes a difference in jet penetration. However, in the turbulent jet flow, it is found that the effect of the aspect ratio on the jet penetration is insignificant. Third, for the concentration decay, the concentration decreases with the slope of −7/9 in the laminar jet flow and the concentration decreases with the slope of −5/9 in the turbulent jet flow.