Inter-turbine diffusers provide continuity between H.P. and LP turbines whilst diffusing the flow upstream of the L.P. turbine. Increasing of the mean turbine diameter offers the potential advantage of reducing the flow factor in the following stages leading to increased efficiency. The flows associated with these inter-turbine diffusers differ from those in simple annular diffusers both as a consequence of their high-curvature S-shaped geometry and of the presence of wakes created by the upstream turbine. It is shown that even the simplest two-dimensional wakes result in significantly modified flows through such ducts. These introduce strong secondary flows demonstrating that full three-dimensional, viscous analysis methods are essential for correct performance modelling.

No abstract

Inter-turbine diffusers offer the potential advantage of reducing the flow coefficient in the following stages leading to increased efficiency. The flows associated with these ducts differ from those in simple annular diffusers both as a consequence of their high-curvature S-shaped geometry and of the presence of wakes created by the upstream turbine. Experimental data and numerical simulations clearly reveal the generation of significant secondary flows as the flow develops through the diffuser in the presence of cross-passage pressure gradients. The further influence of inlet swirl is also demonstrated. Data from experimental measurements with and without an upstream turbine are discussed and computational simulations are shown not only to give a good prediction of the flow development within the diffuser but also to demonstrate the importance of modelling the fully three-dimensional nature of the flow.

Two rectangular cavity configurations, with and without bay doors, at Mach 0.85 are investigated with the objective of assessing the ability of 3D CFD with advanced turbulence modelling to predict narrowband and broadband flow noise. A non-linear, two-equation, eddy-viscosity model run in unsteady mode (URANS) is compared with Detached Eddy Simulation (DES) on a cavity with a L/D ratio of 5. In a thorough evaluation, comparisons are made between DES variants published in the literature, namely Spalart-Allmaras, k-ε and k-ω-SST. We also assess the effect on the noise spectra of using different CFD prediction time-samples of approximately 100 flow passes compared with 250 flow passes. Detailed experimental data for both cavity configurations provide a valuable opportunity to compare the predicted spectra at many points along the cavity ceiling and band-limited amplitude along the cavity length. We conclude that for such cavity flows, all DES models perform similarly well and are superior to unsteady RANS due to their inherent ability to resolve broadband structures.

Numerical simulations of the flow inside a cavity with a length-to-depth ratio (L/D) of 5 and a width-to-depth ratio (W/D) of 1 have been conducted using Large-Eddy Simulation (LES) and Detached-Eddy Simulation (DES). The cavity is exposed to a free-stream of zero incidence, M ∞ = 0.85 and Re = 1 × 10 6 (based on the cavity length). Previous numerical simulations of 3D cavities using Unsteady Reynolds-Averaged Navier-Stokes (URANS) have proved difficult in accurately predicting the noise level and frequency content (and hence flow features) inside cavities. Simulation techniques such as LES and DES are therefore applied to the 3D cavity and this paper demonstrates its superior effectiveness relative to URANS in the analysis of cavity flows. Plots depicting the level of noise, frequency content and velocity profiles inside the cavity are presented. Comparisons are made with experimental unsteady pressure and PIV measurements. It was found that both DES and LES fare much better than URANS in resolving the higher frequencies and velocity distributions inside the cavity. Nomenclature A Cross-sectional area D Cavity depth dt CFD time-step h Height of control device L Cavity length LE Cavity leading-edge (lip) M Mach number Re Reynolds number, R e = U L ν SP L Sound Pressure Level T E Cavity trailing-edge (downstream corner) U Velocity W Cavity width w Width of control device x,y,z X-, y-and z-coordinates Subscripts ∞ Free-stream rms Root-mean square value j Jet