L. di Mare

Synthetic turbulence inflow conditions for large-eddy simulation

Direct numerical or large-eddy simulations of the majority of spatially inhomogeneous turbulent flows require turbulent inflow boundary conditions. A potential implication is that any results computed may be strongly influenced by the prescribed instantaneous inlet velocity profiles. Such profiles are practically never available, and a usual practice is to generate synthetic inflow data satisfying certain statistical properties, which may, for example, be known from experimental data or empirical correlations. The present paper describes a new method for generating turbulent inflow data based on digital filters that is capable of reproducing specified statistical data. Two variants of the approach are presented: a simple method in which the Reynolds stresses and a single length scale are prescribed, and a more detailed approach that is able to reproduce the complete Reynolds-stress tensor as well as any given, locally defined, spatial and temporal correlation functions. The application of the methods to a...

LES of turbulent flow past a swept fence

Abstract The present study deals with the large-eddy simulation of the span-wise invariant turbulent flow past a swept fence at low Reynolds numbers. The swept fence geometry was introduced by McCluskey et al. [Eighth Symposium on Turbulent Shear Flows, 1991, p. 9.5.1] as a way of gathering information on the behaviour of near wall flows beneath a separation region in the presence of a significant cross flow. The configuration avoids both the lack of generality of typical statistically two-dimensional flows and the difficulties of fully three-dimensional flows. In the present case, large-eddy simulation is both a challenging and promising approach. On the one hand, as in most low-Reynolds number and recirculating flows, simple wall models cannot be applied [Engineering Turbulence Modelling and Experiments 2, Florence, Italy, p. 303] on the other hand, due to the relatively low Reynolds of the flow and the type of scaling suggested in Hardman and Hancock [Exp. Fluids 27 (2000) 653], accurate resolution of the near wall region can be achieved without incurring prohibitively high computational costs. A variant of the dynamic SGS model of Germano et al. [Phys. Fluids A 3 (1991) 1760] with a smooth and reliable numerical behaviour is also tested in this flow. The agreement with the experimental data of Hardman [Moderately three-dimensional separated and reattaching turbulent flow. Ph.D. thesis, University of Surrey, 1998] is found to be satisfactory.

A Numerical Study of Labyrinth Seal Flutter

A numerical study of a labyrinth-type turbine seal flutter in a large turbofan engine is described. The flutter analysis was conducted using a coupled fluid-structure interaction code, which was originally developed for turbomachinery blade applications. The flow model is based on an unstructured, implicit Reynolds-averaged Navier-Stokes solver. The solver is coupled to a modal model for the structure obtained from a standard structural finite element code. During the aeroelasticity computations, the aerodynamic grid is moved at each time step to follow the structural motion, which is due to unsteady aerodynamic forces applied onto the structure by the fluid. Such an integrated time-domain approach allows the direct computation of aeroelastic time histories from which the aerodynamic damping, and hence, the flutter stability, can be determined. Two different configurations of a large-diameter aeroengine labyrinth seal were studied. The first configuration is the initial design with four fins, which exhibited flutter instability during testing. The second configuration is a modified design with three fins and a stiffened ring. The steady-state flow was computed for both configurations, and good agreement was reached with available reference data. An aeroelasticity analysis was conducted next for both configurations, and the model was able to predict the observed flutter behavior in both cases. A flutter mechanism is proposed, based on the matching of the structural frequencies to the frequencies of waves traveling in the fluid, in the interfin cavities and in the high- and low-pressure cavities.

Lip Stall Suppression in Powered Intakes

This work describes a computational study into lip stall in subsonic civil aircraft intakes and its alleviation by action of the fan. Beyond a certain flow incidence, the lower lip of a civil aircraft intakes stalls. This phenomenon causes entropy and vortical distortions at the fan face. Consequently, it has detrimental effects on vibration levels and performance of the low-pressure compressor system. The most important parameters influencing lip separation are the flight Mach number, the Reynolds number, and altitude. Fully three-dimensional simulations have been performed on a flight intake in current service for which the experimental data are available. Steady and time-resolved simulations have been performed. Distortion coefficients have been evaluated as functions of incidence and have been compared with experimental results. A comparison between an isolated intake and a powered intake shows that the fan stage has the beneficial effect of increasing tolerance to flow incidence and decreasing distor...

Optimal placement of piezoelectric plates for active vibration control of gas turbine blades: experimental results

It is well known that the gas turbine blade vibrations can give rise to catastrophic failures and a reduction of the blades life because of fatigue related phenomena[1]-[3] . In last two decades, the adoption of piezoelectric elements, has received considerable attention by many researcher for its potential applicability to different areas of mechanical, aerospace, aeronautical and civil engineering. Recently, a number of studies of blades vibration control via piezoelectric plates and patches have been reported[4]-[6] . It was reported that the use of piezoelectric elements can be very effective in actively controlling vibrations. In one of their previous contributions[7] , the authors of the present manuscript studied a model to control the blade vibrations by piezoelectric elements and validated their results using a multi-physics finite elements package (COMSOL) and results from the literature. An optimal placement method of piezoelectric plate has been developed and applied to different loading scenarios for realistic configurations encountered in gas turbine blades. It has been demonstrated that the optimal placement depends on the spectrum of the load, so that segmented piezoelectric patches have been considered and, for different loads, an optimal combination of sequential and/or parallel actuation and control of the segments has been studied. In this paper, an experimental investigation carried out by the authors using a simplified beam configuration is reported and discussed. The test results obtained by the investigators are then compared with the numerical predictions [7] .

Optimal placement of piezoelectric plates to control multimode vibrations of a beam

Damping of vibrations is often required to improve both the performance and the integrity of engineering structures, for example, gas turbine blades. In this paper, we explore the possibility of using piezoelectric plates to control the multimode vibrations of a cantilever beam. To develop an effective control strategy and optimize the placement of the active piezoelectric elements in terms of vibrations amplitude reduction, a procedure has been developed and a new analytical solution has been proposed. The results obtained have been corroborated by comparison with the results from a multiphysics finite elements package (COMSOL), results available in the literature, and experimental investigations carried out by the authors.

Numerical Investigation of Labyrinth Seal Aeroelastic Stability

Labyrinth seals, used to prevent flow leakage between rotating and stationary components in aero engines, may be prone to aeroelastic instabilities. Current work investigates the aeroelastic stability of flexible seals using a coupled fluid-structure code. The aim is to determine the key parameters and to explain the flutter mechanism with a view to establish simple design rules. The unsteady flow field due to seal vibration is solved in a time accurate manner to determine the aerodynamic damping of the seal. The fluid model is based on non-linear time-accurate unsteady Reynolds-averaged Navier-Stokes equations. The vibrating structure is modelled using a classical modal approach. The parameters investigated are the influence of the seal natural frequencies and the support position for the seal. The geometry considered is a multi-finned straight-through seal. A ten-degree sector is meshed for the simulations, and cyclic symmetry boundary conditions are used. The stability is found to be affected by both the mechanical/acoustic frequency ratio and the support side of the seal. The aerodynamic work depends mainly on the cavity shape, the mode shape and the unsteady pressure phase distribution. It is found that the phase variation from one cavity to another is of primary importance to assess the relative contribution of each cavity to the aerodynamic work. The results agree qualitatively with those available in the literature.Copyright

A numerical study of high pressure turbine forced response in the presence of damaged nozzle guide vanes

This paper reports results from numerical computations of low engine order and blade-passing forced response on the rotor of a high pressure turbine due to severe damage to a single nozzle guide vane. The computations are performed using a time-domain, non-linear viscous compressible flow simulation code. The flow and the levels of forcing for a few selected modes are compared for the undamaged and the damaged configurations. The results show that the response in various modes is affected to a different extent by the damage. The main blade-passing response was found to be largely unaffected, if not marginally reduced. On the other hand, the vibration levels for some modes were seen to be up to eight times higher because of the low-order excitation harmonics created by the damaged passage.

Minimisation of Ducted Flow Non-Uniformity Caused by Downstream Blockages

Flow in annular ducts is sensitive to the presence of downstream blockages which can cause flow non-uniformities propagating far upstream of the blocking body. These effects can be exacerbated in swirling flows where a cascade of uniform guide vanes is present upstream of the blockage. This work uses two- and three-dimensional boundary singularity methods to model and optimise a guide vane cascade geometry to minimise the upstream velocity distortion. Starting from a uniform cascade, the geometry is modified to provide a uniform upstream velocity distribution and minimised blade-to-blade loading in two dimensions. The new geometry is then extrapolated to a three-dimensional annulus. A three-dimensional tool is used to further modify the geometry in three dimensions to minimise the velocity distortion in the whole annulus upstream of the cascade.Copyright

Comparison Between a CFD Code and a Three-Control-Volume Model for Labyrinth Seal Flutter Predictions

Labyrinth seals are extensively used in turbomachinery to control flow leakage in secondary air systems. While a large number a studies have been performed to investigate the leakage and rotordynamics characteristics of these seals, the studies on their aeroelastic stability remain scarce. Little is known about this phenomenon and the design methods are limited to a stability criterion which does not take into account many of the parameters which are known to influence labyrinth seal aeroelastic stability. As a consequence the criterion can be unreliable or overly pessimistic. The alternative to this criterion is the use of CFD methods which, although reliable, are computationally expensive. This paper presents a three-control-volume (3CV) bulk-flow model specifically developed for flutter calculations in labyrinth seals. The model is applied to a turbine labyrinth seal of a large diameter aero-engine and the results are compared to those of a CFD analysis. Conclusions are drawn on the potential of this 3CV model for design purposes.Copyright