James J. McGuirk

Jet Noise: Acoustic Analogy informed by Large Eddy Simulation

A novel approach to the development of a hybrid prediction methodology for jet noise is described. Modeling details and numerical techniques are optimized for each of the three components of the model. Far-field propagation is modeled by solution of a system of adjoint linear Euler equations, capturing convective and refraction effects using a spatially developing jet mean flow provided by a Reynolds-averaged Navier―Stokes computational fluid dynamics solution. Sound generation is modeled following Goldsteins acoustic analogy, including a Gaussian function model for the two-point cross correlation of the fourth-order velocity fluctuations in the acoustic source. Parameters in this model describing turbulent length and time scales are assumed to be proportional to turbulence information also taken from the Reynolds-averaged Navier―Stokes computational fluid dynamics prediction. The constants of proportionality are, however, not determined empirically, but extracted by comparison with turbulence length and time scales obtained from a large eddy simulation prediction. The large eddy simulation results are shown to be in good agreement with experimental data for the fourth-order two-point cross-correlation functions. The large eddy simulation solution is then used to determine the amplitude parameter and also to examine which components of the cross correlation are largest, enabling inclusion of all identified dominant terms in the Gaussian source model. The acoustic source description in the present approach is therefore determined with no direct input from experimental data. This model is applied to the prediction of sound to the experimental configuration of the European Union JEAN project, and gives encouraging agreement with experimental data across a wide spectral range and for both sideline and peak noise angles. This paper also examines the accuracy of various commonly made simplifications, for example: a locally parallel mean flow approximation rather than consideration of the spatially evolving mean jet flow and scattering from the nozzle; the assumption of small radial variation in Green function over the turbulence correlation length; the application of the far-field approximation in the Green function; and the impact of isotropic assumptions made in previous acoustic source models.

A depth-averaged mathematical model for the near field of side discharges into open-channel flow

A two-dimensional mathematical model is described for the calculation of the depth-averaged velocity and temperature or concentration distribution in open-channel flows, an essential feature of the model being its ability to handle recirculation zones. The model employs the depth-averaged continuity, momentum and temperature/concentration equations, which are solved by an efficient finite-difference procedure. The ‘rigid lid’ approximation is used to treat the free surface. The turbulent stresses and heat or concentration fluxes are determined from a depth-averaged version of the so-called k , e turbulence model which characterizes the local state of turbulence by the turbulence kinetic energy k and the rate of its dissipation e. Differential transport equations are solved for k and e to determine these two quantities. The bottom shear stress and turbulence production are accounted for by source/sink terms in the relevant equations. The model is applied to the problem of a side discharge into open-channel flow, where a recirculation zone develops downstream of the discharge. Predicted size of the recirculation zone, jet trajectories, dilution, and isotherms are compared with experiments for a wide range of discharge to channel velocity ratios; the agreement is generally good. An assessment of the numerical accuracy shows that the predictions are not influenced significantly by numerical diffusion.

Finite Volume Discretization Aspects for Viscous Flows on Mixed Unstructured Grids

A solution method for compressible turbulent flows on unstructured grids in two dimensions is described. The method can be used on grids consisting of triangular and/or quadrilateral cells. Control volumes are constructed from dual cells, and the solution variables are stored at the vertices of the grid. Grid-transparent algorithms are developed that do not require knowledge of cell types, leading to simple discretization schemes on mixed grids. The inviscid fluxes are computed from limited high-resolution schemes originally developed for unstructured triangular grids. They are easily applied to quadrilateral or mixed grids and are grid transparent. The discretization of the viscous fluxes is studied in detail. A positive, grid-transparent discretization of Laplaces equation is developed. The existence of tangential derivatives in the viscous terms prevents grid transparency. By neglecting tangential derivatives, an approximate form of the viscous fluxes is developed, which recovers grid transparency. The approximate form is shown to be similar to the thin-shear-layer approximation. Results are obtained for a transonic inviscid flow, a laminar separated flow, and a transonic turbulent flow

On the use of fluorescent dyes for concentration measurements in water flows

An experiment was performed to evaluate the characteristics of various fluorescent dyes used as tracers for concentration measurements in water flows, by laser induced fluorescence. Three common fluorescent dyes (fluorescein, rhodamine B and rhodamine 6G) were used, to select the most suitable fluorescent dye and identify its range of linear response. The results showed that, in terms of the stability of the solution, fluorescein is inferior to either rhodamine B or rhodamine 6G and that for concentrations of rhodamine B less than 0.08 mg/1 the response of fluorescent to the incident light is linear.

The Calculation of Three-Dimensional Turbulent Free Jets

The paper presents an application of the two-equation k—є model to the problem of three-dimensional free jets issuing from rectangular orifices. The turbulence model has been modified so that plane and round jets may be predicted with the same empirical input. The continuity, momentum, and turbulence equations are solved using the finite difference procedure of Patankar and Spalding for three-dimensional parabolic flows and results are presented for aspect ratios of 1, 5, 10, and 20. The decay of axial velocity is well predicted. The behavior of the half-widths, however, is not well predicted when no lateral velocities are specified at the orifice; the measured crossover of jet major and minor axes is not obtained. The possible existence of a lateral velocity field at the orifice cross section is examined and its ability to produce the observed jet inversion is demonstrated. Profile shapes in the orifice short-axis direction are in good agreement with measurements, and, when inlet lateral velocities are specified, the long-axis profiles are also predicted fairly well. The measured “saddle-shape” of the profiles in this direction is, however, not obtained; this will require further changes to the turbulence model.

Impingement of single and twin turbulent jets through a crossflow

Laser-Doppler measurements of velocity characteristics of the flowfield resulting from the impingement of single and twin jets against a wall through a low-velocity crossflow are presented and discussed together with visualization of the flow. The experiments have been carried out for a velocity ratio between the jet and the crossflow of 30, for a Reynolds number based on the jet exit between 60,000 and 105,000, and for the jet exit 5 jet diameters above the ground plate. In addition, calculations based on a two-equation turbulence model are presented for the three-dimensional flow characterized by the measurements, and comparison between experimental and numerical results show that the mean flowfield is well predicted. The calculation of the turbulent field requires, however, consideration of the individual stresses. 21 refs.

Shock capturing using a pressure-correction method

A new pressure-correction scheme has been developed, which is suitable for the calculation of a flow containing a wide range of Mach numbers such as a transonic impinging jet. The method uses equations based on properties per unit volume so that momentum is retained as a basic dependent variable rather than velocity. This simplifies the discretization of the time-dependent flow equations and allows a direct relationship to be determined between pressure and mass flux. The hyperbolic nature of the system of equations is obtained by using the retarded pressure approach. This is a transformation of the real pressure based on local Mach number and is used in the momentum and pressure-correction equations. The shocked quasi-one-dimensional flow in a nozzle is used as a test of shock capturing properties and speed of computation. The new method gives precise shock capturing over two nodes with no over or under-shoots; it is also significantly faster than the MacCormack and Jameson explicit schemes tested for this problem. Finally, a turbulent, under-expanded, axisymmetric, impinging jet calculation is presented. The correct periodic under/over-expansions of the jet are predicted, and the normal standoff shock is cleanly captured.

Unsteady flow structures in radial swirler fed fuel injectors

Many fuel injector geometries proposed for lean-premixed combustion systems involve the use of radial swirlers. At the high swirl numbers needed for flame stabilization, several complex unsteady fluid mechanical phenomena such as vortex breakdown and recirculation zone precession are possible. If these unsteady aerodynamic features are strongly periodic, unwanted combustion induced oscillation may result. The present paper reports on an isothermal experimental study of a radial swirler fed fuel injector originally designed by Turbomeca, and examines the dynamical behavior of the unsteady aerodynamic flow structures observed. Particle Image Velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures both internal to the fuel injector, and externally in the main flame-stabilizing recirculation zone. Multiple vortex structures are observed. Vector field analysis is used to identify specific flow structures and perform both standard and conditional time averaging to reveal the modal characteristics of the structures. This allows analysis of the origin of high turbulence regions in the flow and links between internal fuel injector vortex breakdown and external unsteady flow behavior. The data provide a challenging test case for Large Eddy Simulation methods being developed for combustion system simulation.

Isothermal flow in a gas turbine combustor — a benchmark experimental study

An experimental investigation of the three-dimensional flow field within a water model of a can-type gas turbine combustion chamber is presented. Flow visualisation demonstrated that internal flow patterns simulated closely those expected in real combustors. The combustor comprised a swirl driven primary zone, annulus fed primary and dilution jets and an exit contraction nozzle. LDA measurements of the three mean velocity components and corresponding turbulence intensities were obtained to map out the flow development throughout the combustor. Besides providing information to aid understanding of the complex flow events inside combustors, the data are believed to be of sufficient quantity and quality to act as a benchmark test case for the assessment of the predictive accuracy of computational models for gas-turbine combustors.

Numerical investigation of transient buoyant flow in a room with a displacement ventilation and chilled ceiling system

This paper presents the major findings of the PhD work of Rees, who wrote the paper and is the lead author.