Aldo Rona

The acoustic resonance of rectangular and cylindrical cavities

An investigation is conducted on the acoustic resonance that develops in rectangular and cylindrical cavities with rigid walls. An analytical model for the small amplitude acoustic perturbations inside an enclosure with rigid walls is developed from classical linerized acoustics. The method is first applied to a rectangular geometry and the normalized mode shapes and frequencies of the first six standing wave modes are given. The results are used to diagnose whether coupling is likely to occur between the first two Rossiter modes and the acoustic standing waves, which may lead to a reinforcement of the flow instability. At the selected test conditions, the method indicates that the second Rossiter mode can couple with the first longitudinal acoustic mode. The acoustic resonant mode predictions for a cylindrical cavity of length to depth ratios 0.71 and 2.5, tested at near-incompressible speeds, suggest that the acoustic resonant modes and the main fluid dynamic instability in the enclosure are sufficiently apart in frequency not to strongly interact with one another. The parametrized analytical solutions developed in this study enable the aero-acoustic engineer to diagnose whether coupling between a given fluid dynamic instability and acoustic resonance is likely to affect a rectangular or cylindrical cavity component.

Encounters with Vortices in a Turbine Nozzle Passage

Experiments were conducted on the flow through a transonic turbine cascade. Secondary flows and a wide range of vortex types were encountered, including horseshoe vortices, shock-induced passage vortices, and streamwise vortices on the suction surface. In the separation region on the suction surface, a large rollup of passage vorticity occurred. The blunt leading edge gave rise to strong horseshoe vortices and secondary flows. The suction surface had a strong convex curvature over the forward portion and was quite flat further downstream. Surface flow visualization was performed and this convex surface displayed coherent streamwise vorticity. At subsonic speeds, strong von Karman vortex shedding resulted in a substantial base pressure deficit. For these conditions, time-resolved measurements were made of the Eckert-Weise energy separation in the blade wake. At transonic speeds, exotic shedding modes were observed. These phenomena all occurred in experiments on the flow around one particular turbine nozzle vane in a linear cascade.

An analytical model for over-shroud leakage losses in a shrouded turbine stage

This paper was presented at the GT 2007 ASME Turbo-EXPO 2007: Power for Land, Sea and Air, May 14-17, 2007, Montreal, Canada and published in the Proceedings. The published version is available from http://www.asme.org/.

POD Analysis of Cavity Flow Instability

This paper was published as Proceedings of the 41st Aerospace Sciences Meeting and Exhibit, Reno, NV, USA, 6-9 January, 2003, pp. 1-9. It is also available from http://www.aiaa.org/content.cfm?pageid=2

Investigation of Streamwise and Transverse Instabilities on Swept Cylinders and Implications for Turbine Blading

The starting point for this investigation was the observation of robust streamwise streaks in flow visualization on the suction surfaces of blades in a turbine cascade at subsonic and transonic speeds. The spanwise wavelength of an array of streamwise vortices had been predicted and is here confirmed experimentally. Observations of streaks on unswept turbine blades and on circular cylinders confirmed these earlier predictions, providing a firm basis for referencing the new measurements of vortical behavior. The observations made it clear that the boundary layers are highly three dimensional. In this paper observations of streamwise and transverse instabilities on swept circular cylinders, over a range of inclinations, are presented. The circular cylinder is a canonical case and observations relate the streamwise vorticity of the unswept case to the more aggressive crossflow instability at high sweep angles. Introducing sweep brings consideration of a wide range of instabilities. Prominent is crossflow instability resulting from the inflectional behavior of a three-dimensional boundary layer.

Organized Streamwise Vorticity on Convex Surfaces With Particular Reference to Turbine Blades

This paper was published as AIAA Conference Paper 2010-904, 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, Florida, 4-7 January 2010. Copyright the American Institute of Aeronautics and Astronautics. It is also available from http://www.aiaa.org/content.cfm?pageid=534&id=1818&luPubID=497. This paper appears in the LRA with the permission of the AIAA.

Noise sources from a cylindrical cavity

This paper was published as Proceedings of the 13th CEAS/AIAA Aeroacoustics Conference, Rome, Italy, 21-23 May 2007, pp. 1-12. It is also available from http://www.aiaa.org/

A FLOW-RESONANT MODEL OF TRANSONIC LAMINAR OPEN CAVITY INSTABILITY

A transonic air stream flowing over a rectangular cavity or enclosure is unsteady. At certain flow conditions, large scale oscillations develop in the shear layer which, for an open cavity, spans across the whole enclosure. The convected shear layer instabilities interact with the cavity geometry, they generate resonance, and induce large amplitude wall pressure fluctuations, aerodynamic pressure drag and noise radiation. A numerical method has been developed to analyse the physics of the cavity flow instability. A specific interest of this study is to investigate the nature of the feed-back loop in the transonic laminar regime, where a flow-resonant feed-back may be complementing the flow-acoustic resonance documented in past work. A Mach 1.5 laminar cavity flow is modelled in which the selected numerical method gives low dispersion and dissipation. Discrete solutions of the short-time averaged laminar Navier-Stokes equations, the flow governing equations, are obtained by a finite volume integral approach. A monotone upwind flux interpolation method by Mensink is used to obtain second order accurate solutions in space. This is based on the Roe approximate Riemann solver. A fourth order Dispersion Relation Preserving scheme time-advances the flow history; this is a multi-step Runge-Kutta type integration method. The available results indicate that the method is able to reproduce the unsteady self-sustained character of the flow. A dominant cavity mode characterises the instability. The dominant mode frequency is determined by (i) the cavity streamwise length, (ii) the shear layer convection speed, (iii) the feed-back pressure and momentum wave phase speeds in the enclosure, and (iv) the shear layer receptivity phase delay at the upstream cavity edge. A comparison of phase matched ‘instantaneous’ density fields from uniform (baseline) and uniform refined computational grid models shows a similar shear layer motion and Lecturer, Department of Engineering. Doctoral Candidate, Department of Aeronautics. Copyright c

Boundary Layer Trips for Low Reynolds Number Wind Tunnel Tests

The generation of a thick fully turbulent boundary layer is investigated in a lowspeed wind tunnel at a nominal zero pressure gradient over the Reynolds number range 0.145× 10 6 · Rex · 0.58× 10 6 . The wind tunnel floor natural boundary layer is laminar with thickness ± between 5.76 mm and 8.13 mm. Different tripping devices are tested to trigger transition so to double the boundary layer thickness and provide a fully established turbulent velocity profile. Using a trip wire significantly increases ± but leads to an unsatisfactory velocity profile. Using a sandpaper strip slightly increases ± but keeps the boundary layer laminar. Using a strip of sharp-edged silicon granules doubles the boundary layer thickness that increases up to 20 mm and the mean velocity profiles are a good fit to the logarithmic law of the wall over the outer region of the boundary layer. The spectral decay of turbulent kinetic energy in this outer layer is exponential and close to i5/3, indicating turbulence equilibrium. This work is of practical interest to wind tunnel practitioners for generating equilibrium thick turbulent boundary layers at low Reynolds numbers.

A Selective Review of Transition Modelling for CFD.

This paper was presented at the 39th AIAA Fluid Dynamics Conference, San Antonio, Texas, June 22-25, 2009. Copyright