Edward Canepa

Unsteady Aerodynamics of an Aeroengine Double Swirler Lean Premixing Prevaporizing Burner

Lean premixing prevaporizing (LPP) burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large-scale model of a double swirler aeroengine LPP burner at isothermal conditions. A three-dimensional (3D) laser Doppler velocimeter and an ensemble-averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation, which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow-mixing process is highly enhanced. Turbulence acts on a large spectrum of fluctuation frequencies, whereas the large-scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.

Further Study of the Test Configuration for Axial Flow Fans Noise Measurements

The present paper concerns the effect of the test configuration on the noise generated by axial flow fans employed for automotive cooling systems. Different test configurations may be employed depending on the design phase. Discrepancies are often observed between the results of different tests, resulting in difficulties in transferring and generalizing the measured SPL. In the present case the effect of the mounting system (free-discharge wooden panel versus test plenum with zero and design downstream pressure) on the irregular growth of the tonal noise during speed ramps is considered. The spectral decomposition method applied to the measured SPL spectra has been used to obtain the propagation function which has been compared with the scaled SPL at the blade passage harmonics. The comparison shows that the fluctuations of the SPL during the speed ramps depend on the acoustics of the surrounding environment and not on variations in the incoming flow field.

Experimental study of the effect of the rotor-stator gap variation on the tonal noise generated by low-speed axial fans

In the present paper the effect of the rotor stator axial gap on tonal noise generated by axial flow fans employed for automotive cooling systems is studied. A fan equipped with either a 18 vanes stator or a 3 struts one, positioned at several axial gaps, is tested in a hemianechoic chamber during rotational speed ramps. To analyze the acoustic pressure properties three different quantities have been employed: the velocity scaled SPL at the BPF harmonics, the phase of the acoustic pressure at the BPF harmonics and the propagation function obtained by means of the spectral decomposition. The useful properties of these quantities and their joint use are first analyzed. Then they are employed for two purposes: investigating propagation effects and source strength fluctuations and identifying contributions to tonal noise due to random sources (large scale inlet turbulent structures) and due to deterministic sources (aerodynamic rotor-stator interaction). The results of the study show that the stator causes unexpected propagation effects which affect the measured SPL. These effects depend on the axial gap and on the considered BPF harmonic and they also affect the feasibility of a SPL velocity scaling.

Influence of Aerodynamic Loading on Rotor-Stator Aerodynamic Interaction in a Two-Stage Low Pressure Research Turbine

The unsteady flow within a two-stage low-pressure research turbine equipped with high lift profiles has been investigated in detail for three different aerodynamic loading conditions. Experiments have been carried out at low speed. Velocity and turbulence intensity in the blade-to-blade plane at midspan have been measured by means of a crossed hot-wire probe, upstream and downstream of each blade row. The probe has been traversed circumferentially over 1.5 bladings pitch and the phase-locked data acquisition and ensemble average technique have been used to reconstruct the flow in space and time. The effects of multistage configuration have been identified and analyzed by considering the velocity components and turbulence intensity. Potential interaction from the downstream blading in relative motion, periodic wake perturbations from the upstream blading and preceding stage perturbations make the flow in the second stage extremely complex. Overall the flow downstream of rotors is perturbed in space by upstream and downstream stators, while flow downstream of stators is mostly perturbed in time by rotor effects. As expected, high lift profiles are significantly sensitive to incidence variation, with this effect further enhanced by the multistage cumulative interactions.

Boundary Layer Separation Control on a Flat Plate With Adverse Pressure Gradients Using Vortex Generators

The continuous tendency in modern aeroengine gas turbines towards reduction of blade count and ducts length may lead to aerodynamic loading increase beyond the limit of boundary layer separation. For this reason boundary layer separation control methods, up to now mostly employed in external aerodynamics, begin to be experimented in internal flows applications. The present paper reports the results of a detailed experimental study on low profile vortex generators used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. Inlet turbulent boundary layer conditions and pressure gradients are representative of aggressive turbine intermediate ducts. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics (AIDA). The pressure gradients on the flat plate are generated by increasing the aperture angle of a movable wall opposite to the flat plate. To avoid separation on the movable wall, boundary layer suction is applied on it. Complementary measurements (surface static pressure distributions, surface flow visualizations by means of wall mounted tufts, instantaneous and time-averaged velocity fields in the meridional and cross-stream planes by means of Particle Image Velocimetry) have been used to survey the flow with and without vortex generators. Three different pressure gradients, which induce turbulent separation in absence of boundary layer control, were tested. Vortex generators height and location effects on separation reduction and pressure recovery increase were investigated. For the most effective VGs configurations detailed analyses of the flow field were performed, that demonstrate the effectiveness of this passive control device to control separation in diffusing ducts. Particle Image Velocimetry vector and vorticity plots illustrate the mechanisms by which the vortex generators transfer momentum towards the surface, re-energizing the near-wall flow and preserving the boundary layer from separation.Copyright

Psychoacoustic Optimization of the Spacing of Propellers, Helicopter Rotors, and Axial Fans

The paper deals with the minimization of the subjective annoyance (that is, of the quality of the received sound rather than of its acoustic power) due to tonal noise generated by subsonic low-solidity axial-flow rotors. The employed method is based on the uneven spacing of the blades, is typically applied to open or nearly open rotors, and is based on acoustic interference. Thus, linear propagation of the acoustic waves is a prerequisite for its application. An optimization problem is formulated for which the objective function is derived from a psychoacoustic metric, the decision variables are the blades angular positions, the sound pressure level spectrum generated by a single blade is a parameter, and the constraints are the rotor balancing and the minimum angular distance between adjacent blades. A further intuitive criterion, based on lowering the highest peaks, is also considered. The solution of the problem is critical due to the presence of a very large number of minima in the objective function,...

An experimental investigation of the spectral properties of the tonal noise generated by axial flow fans with and without rotor-stator interaction

The present paper is aimed at studying the spectral properties of the tonal noise generated by axial flow fans. The phase of the acoustic pressure FFT is considered as a means to integrate the usual plots of SPL at BPF harmonics. Four different configurations of a fan, obtained combining an upstream bar and a downstream stator in presence of large inlet turbulent structures, have been tested in a hemi-anechoic chamber. In order to vary the strength of the acoustic source, measurements are taken during rotational speed ramps at low angular acceleration. The joint analysis of SPL and phase at BPF harmonics shows that it is possible to distinguish between tonal noise due to inlet turbulence from tonal noise due to rotor-stator aerodynamic interaction. Furthermore, information about the presence of important propagation effects, which result in a non linear phase trend, may also be gained.

UNSTEADY AERODYNAMICS OF AN AERO-ENGINE DOUBLE SWIRLER LPP BURNER

Lean premixing prevaporizing burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large scale model of a double swirler aero-engine LPP burner at isothermal conditions. A 3-D laser Doppler velocimeter and an ensemble averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow mixing process is highly enhanced. While turbulence acts on a large spectrum of fluctuation frequencies, the large scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.Copyright

Performance analysis of a motor-sailing propulsion system for control design purposes

This paper presents the simulation approach developed to evaluate the interaction between sailing and motor propulsion of a tall ship. In particular, the study is focused on the computational fluid dynamics (CFD) analysis of the wind thrust action on the sails. CFD modelling has been used because of the lack on scientific literature of the performance assessment of a square soft sail, especially if the interference among the several sails has to be considered. CFD results are discussed and then properly arranged to develop a propulsion simulator, able to provide interesting information on the whole ship performance. In sailing propulsion mode, it is worth paying attention also to the effect of the trailing propeller on the shaftline dynamics, mainly for reasons due to the efficiency reduction of the oil lubrication system at low revolutions. The final goal of the present work is to propose material and methods to find out and set up innovative control logics, in order to optimise the particular motor-sailing propulsion of the tall ships.

Experimental study and velocity scaling of the tip-leakage noise generated by low-speed axial flow-fans

The present paper reports an experimental study of the tip-leakage noise generated by axial-flow fans employed for automotive cooling systems. Two operating points and three different configurations, with and without tip-leakage flow, have been considered. The measurements have been taken in a hemi-anechoic chamber at constant rotational speed and during speed ramps. Different noise components have been identified and the properties of the generating mechanisms have been separated from the acoustic propagation effects. To this aim, the acoustic response function of the test configuration has been computed employing the spectral decomposition method, and then it has been compared with the constant-Strouhal number SPL. The low frequency broadband part of the spectra and the peaks related to the tip-leakage flow are affected by the same propagation effects but show a different dependence on the rotational speed. The scaling exponent also depends on the operating point.