Jet Flapping Research Articles

Spatial flow velocity distribution around an inlet port/valve annulus

An experimental investigation of the airflow through inlet port of a standard production direct injection diesel engine is presented. The investigation comprises mass flow rate and pressure drop measurements at fixed valve lifts, and spatial distributions of mean and r.m.s. velocity components around the port/valve annulus measured under steady flow conditions by special designed miniature hot-wire anemometer X probe. The results show that the velocity field is distributed non-uniformly across the valve gap and around valve periphery. Nonuniformity is more expressed at higher valve lifts. Flow instability in a jet leaving the port (jet flapping) is also evident since the skewness and kurtosis of the velocity probability distribution function depart from the Gaussian form. The presented experimental method, based on the application of miniature multiple hot-wire probes, makes possible investigation of flow performances of an inlet port and poppet valve assembly of a production engine head without any modification for ensuring an optical access within the port/cylinder.

Coherent structures of the near field flow in a self-oscillating physical model of the vocal folds

Current theories of voice production depend critically upon knowledge of the near field flow which emanates from the glottis. While most modern theories predict complex, three-dimensional structures in the near field flow, few investigations have attempted to quantify such structures. Using methods of flow visualization and digital particle image velocimetry, this study measured the near field flow structures immediately downstream of a self-oscillating, physical model of the vocal folds, with a vocal tract attached. A spatio-temporal analysis of the structures was performed using the method of empirical orthogonal eigenfunctions. Some of the observed flow structures included vortex generation, vortex convection, and jet flapping. The utility of such data in the future development of more accurate, low-dimensional models of voice production is discussed.

Three-dimensional simulation of the self-oscillating flow and side-loads in an over-expanded subscale rocket nozzle

Time-accurate three-dimensional Reynolds-averaged Navier-Stokes simulations have been performed to explore the flow unsteadiness of the subscale J-2S thrust optimized contour rocket nozzle in over-expansion at prescribed air supply conditions in the chamber. The results show the development of periodic low-frequency, high-amplitude, self-sustained oscillations at the simulated nozzle pressure ratios (NPRs). The cap-shock pattern is responsible for the two-coaxial jet topology that appears. The complex three-dimensional evolution of the flowfield is characterized by a periodical jet flapping, axial oscillations, and azimuthal fluctuations with or without swirl, depending on the NPR. According to the simulations, jet flapping quasi-restricted to a preferred plane may occur during the firings, being supported by the reported thrust and side-load measurements. The computed dominant frequencies corresponding to the longitudinal and transverse oscillation modes show an excellent agreement with the experimental counterpart. The analysis of the forces spectra and flowfields has identified that synchronization between side-load (transverse) and thrust (longitudinal) oscillations develops in consonance with the observations. Besides, their strong coupling is favoured by the response of the shocks to the pulsating pressure near the large recirculation bubble that arises behind the cap-shock. A discussion on the self-oscillating, high-coherent flow phenomenon is addressed. The present three-dimensional study has captured for the first time the phenomenon of cyclic transition between reattached flow separation mode and non-reattached mode, where the alternating bursting and formation of a separation bubble is accompanied by the large amplitude axial displacement of the shock pattern. Furthermore, the role of the separation bubble dynamics on triggering high side-load peaks at fixed NPRs is numerically confirmed.

Observations of the near-field structures of the glottal flow

The three-dimensional glottal jet was measured downstream from a self-oscillating physical model of the vocal folds using several techniques, including grid-based hotwire anemometry, high-speed flow visualization, and particle image velocimetry. Coherent flow structures were extracted from the spatio-temporal data using principal component analysis (PCA). The glottal jet resembled planar jets through static slitlike orifices. The direction of the jet core axis oscillated in the medial-lateral direction. Also, there was a positive correlation between the dynamics of shear layers around the laminar jet core and flow structures in the transitional region. This finding was similar to the dynamics of planar jets where jet flapping was observed previously. This can be understood in terms of an antisymmetric array of large-scale vortices being convected downstream. This antisymmetric mode of coherent vortex structures acts as an acoustic dipole that could effect the dynamics of the shear layers in the laminar core region. Thus, our observations help to understand the dynamics of the acoustic near-field of the glottal oscillator and suggest a possible feedback mechanism for source-tract coupling.

Flapping Instability of Vertically Impinging Turbulent Plane Jets in Shallow Water

A submerged, vertical, turbulent plane jet impinging onto a free water surface will be self-excited into a flapping oscillation when the jet velocity, exiting the jet orifice, exceeds a critical value. The dependence of the critical velocity W0c and the flapping frequency f0 on the water depth H and the jet orifice width d was investigated in detail in this study. The jet flapping motion was visualized by a laser induced fluorescence technique and measured with a laser Doppler velocimeter; a supplemental measurement of the displacement of water surface by a surface wave gauge was made. The jet flapping characteristics are interpreted in terms of the effective water depth given by He = H-z0, where z0 is the virtual origin of the jet. The critical jet exit velocity was found to increase linearly with the effective water depth and to decrease with the square root of the jet orifice width. The flapping frequency decreased with the root square of the effective water depth and independent of the jet orifice width. These results led to a critical condition for the onset of instability as St = 0.929((H 2 z0)/d) 23/2 , where St is the critical Strouhal number defined by St = f 0 d/W0c.

Flow Regimes of Interaction of a Turbulent Plane Jet into a Rectangular Cavity: Experimental Approach and Numerical Modelling

The present work is devoted to the experimental and numerical study ofthe interaction of a turbulent plane jet with a rectangular cavity.Several flow regimes have been found to occur: the non-oscillationregime, the stable oscillation regime and an unstable oscillationregime. The first two regimes have been particularly considered. Theexperimental study has been carried out using hot wire anemometry andsome visualisations. The numerical predictions based on statisticalmodelling have been made using on the one hand the standard k–e model and on the other hand a two-scales split spectrum model. The structuralproperties of the flow have been described for the different situations.For the oscillatory regime, a parametrical study allowed to determinethe influence of the jet exit location and the Reynolds number on thefrequency of the jet flapping. The one point closures have been able topredict the oscillatory regime, and in particular the two-scales modelled to improved results because better account is taken of lag effectsin unsteady non-equilibrium situations.

The effect of swirl on the annular flow past an axisymmetric poppet valve

This paper describes an experimental investigation of the swirling flow through an axisymmetric port and poppet valve assembly under steady flow conditions. This work is an extension of the preceding paper, which is a study of the non-swirling flow in the same flow configuration [1]. Three different swirl rates were investigated with the aim to study the effects of swirl on the mean flow and turbulence characteristics of the flow field. The flow structure was studied by means of laser sheet flow visualization and the three velocity components and the associated Reynolds stresses were measured by ensemble-averaged laser Doppler anemometry techniques. The results are compared with those obtained with the non-swirling flow. The addition of the swirl was found to alter the flow structure, particularly below the valve where a new counter-rotating vortex is formed. A toroidal recirculation zone is also formed in the valve passage. The growth and entrainment of the jet emanating from the valve gap increase and its trajectory angle changes. The flow visualization studies showed evidence of precession of the swirl centre and jet flapping. Swirl increases the Reynolds stresses and the turbulence production rate in most regions of the flow.

Fluid Flow and Heat Transfer in an Opposed Plane Jet in an Uniform Stream (Analysis in Unsteady Motions)

The purpose of this report is to investigate the fluid flow and heat transfer in an opposed plane jet in a uniform stream. In the previous paper, experimental results indicated that there were unsteady motions of low frequency and large scale, which are known as jet flapping. In the present paper, the unsteady motions for the Reynolds number of a jet, Re0=1500, and the ratio of the jet to uniform stream velocity, α=3.75, are studied numerically by the finite difference method. Previous experimental results are simulated in outline by numerical calculations. Time histories and instantaneous or time-averaged contours for velocity, pressure coefficient and temperature are obtained, and the mechanisms of fluid flow and heat transfer are clarified.

Instantaneous velocity measurements in a vane-excited plane jet

The effects of oscillating a vane in the potential core of a turbulent jet have been investigated for the low Strouhal number range of 0.0016-0.0048 by making instantaneous velocity measurements with phase-averaging techniques. Significant unsteady effects, in the form of jet flapping, in the presence of harmonics, and nonsimilar profiles, are described. At the measured stream wise stations, entrainment and jet spreading increases significantly with increasing frequency and amplitude of vane oscillation compared with the corresponding steady jet values.

A Universal Strouhal Law

A universal relationship between the frequency of oscillation induced in a restrained fluid body by an external free flow, the velocity of the latter, and a typical width of the body, is established. Evidence is offered of the accuracy of this “Strouhal law” in such phenomena as wing autorotation, intermittent formation of vortices behind weirs and orifices, jet flapping and puffing, wake vortex shedding, oscillation of shock waves, cavitation bubbles and vortex breakdowns. A corollary of the Strouhal law shows that travelling waves of a well‐defined length may associate with the fluid‐body oscillation. Corroborations are offered concerning the spacing of wind waves, internal waves and dunes and Taylor‐cell waves. The usefulness of the law as a heuristic tool is stressed.