Effects of large computational time steps on the computed turbulence were investigated using a fully implicit method. In turbulent channel flow computations the largest computational time step in wall units which led to accurate prediction of turbulence statistics was determined. Turbulence fluctuations could not be sustained if the computational time step was near or larger than the Kolmogorov time scale.

Effects of the computational time step on numerical solutions for turbulent flow

Inlet conditions for large eddy simulation: A review

Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor. The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities, the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations.

/pdf/efficient-generation-of-inflow-conditions-for-large-eddy-2n9xwuouai.pdf

Efficient Generation of Inflow Conditions for Large Eddy Simulation of Street-Scale Flows

Low Speed Aerodynamics

The paper presents the detailed formulation and validation results of simple and robust procedures for the generation of synthetic turbulence aimed at providing artificial turbulent content at the RANS-to-LES interface within a zonal Wall Modelled LES of attached and mildly separated wall-bounded flows. There are two versions of the procedure. The aerodynamic version amounts to a minor modification of a synthetic turbulence generator developed by the authors previously, but the acoustically adapted version is new and includes an internal damping layer, where the pressure field is computed by “weighting” of the instantaneous pressure fields from LES and RANS. This is motivated by the need to avoid creating spurious noise as part of the turbulence generation. In terms of pure aerodynamics, the validation includes canonical shear flows (developed channel flow, zero pressure gradient boundary layer, and plane mixing layer), as well as a more complex flow over the wall-mounted hump with non-fixed separation and reattachment, with emphasis on a rapid conversion from modeled to resolved Reynolds stresses. The aeroacoustic applications include the flow past a trailing edge and over a two-element airfoil configuration. In all cases the methodology ensures a very acceptable accuracy for the mean flow, turbulent statistics and, also, the near- and far-field noise.

Synthetic Turbulence Generators for RANS-LES Interfaces in Zonal Simulations of Aerodynamic and Aeroacoustic Problems

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 plane jet flow and to a developing wall boundary layer serve to demonstrate the applicability of the approach.

Synthetic turbulence inflow conditions for large-eddy simulation

LES of turbulent flow past a swept fence

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.

A numerical study of labyrinth seal flutter

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...

Lip Stall Suppression in Powered Intakes

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] .

https://www.comsol.com/paper/download/151837/botta_presentation.pdf

L. di Mare

Papers

Synthetic turbulence inflow conditions for large-eddy simulation

LES of turbulent flow past a swept fence

A numerical study of labyrinth seal flutter

Lip Stall Suppression in Powered Intakes

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