Fault detection, isolation, and reconfiguration (FDIR) is an important and challenging problem in many engineering applications and continues to be an active area of research in the control community. This paper presents a survey of the various model-based FDIR methods developed in the last decade. In the paper, the FDIR problem is divided into the fault detection and isolation (FDI) step, and the controller reconfiguration step. For FDI, we discuss various model-based techniques to generate residuals that are robust to noise, unknown disturbance, and model uncertainties, as well as various statistical techniques of testing the residuals for abrupt changes (or faults). We then discuss various techniques of implementing reconfigurable control strategy in response to faults.

A Survey of Fault Detection, Isolation, and Reconfiguration Methods

Detached-eddy simulation (DES) was first proposed in 1997 and first used in 1999, so its full history can be surveyed. A DES community has formed, with adepts and critics, as well as new branches. The initial motivation of high–Reynolds number, massively separated flows remains, for which DES is convincingly more capable presently than either unsteady Reynolds-averaged Navier-Stokes (RANS) or large-eddy simulation (LES). This review discusses compelling examples, noting the visual and quantitative success of DES. Its principal weakness is its response to ambiguous grids, in which the wall-parallel grid spacing is of the order of the boundary-layer thickness. In some situations, DES on a given grid is then less accurate than RANS on the same grid or DES on a coarser grid. Partial remedies have been found, yet dealing with thickening boundary layers and shallow separation bubbles is a central challenge. The nonmonotonic response of DES to grid refinement is disturbing to most observers, as is the absence of...

Detached-Eddy Simulation

An efficient generalized Zonal Detached Eddy Simulation method (ZDES) is presented, which aims at performing hybrid Reynolds Averaged Numerical Simulation (RANS)/Large Eddy Simulation (LES) calculations for both internal and external aerodynamics problems. It is based on a zonal formulation of the hybrid length scale that allows to combine the zonal approach with the best features of Delayed Detached Eddy Simulation (DDES) (Spalart et al. Theor Comput Fluid Dyn 20:181–195, 2006). In other words, the presumed weak point of a zonal approach, namely that the location of separation has to be known in advance, is now overcome. What is more, the problem of slow LES content development in mixing layers when they are treated neither in RANS nor in LES mode is investigated. It is argued that the subgrid length scale Δmax = max(Δx, Δy, Δz) entering DDES is physically justified to shield the boundary layer but is definitely not a good subgrid length scale in LES mode. Remedies are proposed based on new zonal subgrid length scales that depend not only on the grid spacing but also on the flow solution and especially on the local vorticity vector. The method is validated on a spatially developing mixing layer as well as in a backward facing step flow and then applied to a three-element airfoil. It is argued in this latter case that a precise control of the RANS mode thanks to a zonal approach is essential. More generally, in all simulated cases in this study, ZDES has proven to be very efficient as regards the behavior in LES mode while retaining the strongest asset of DDES, namely the treatment of the attached boundary layer in RANS mode. The issue of zonal or non-zonal treatment of turbulent flows is also briefly discussed.

Recent improvements in the Zonal Detached Eddy Simulation (ZDES) formulation

A large eddy simulation (LES) of a turbulent flow past an airfoil near stall at a chord Reynolds number of 2.1 x 10 6 is performed and compared with wind-tunnel experiments. This configuration still constitutes a challenging test case for Reynolds-averaged Navier-Stokes (RANS) simulation and LES as a result of the complexity of the suction side boundary layer: an adverse pressure gradient creates successively a laminar separation bubble, a turbulent reattachment, and a turbulent separation near the trailing edge. To handle this high-Reynolds-number flow with LES on available supercomputers, a local mesh-refinement technique and a discretization of the convective fluxes are developed in a block-structured finite volume code to reduce the total number of grid points and the numerical dissipation acting on the small scales, respectively. Influence of subgrid scale modeling (SGS) is assessed through the comparisons of explicit selective mixed scale model (SMSM) and implicit monotone-integrated LES model results. Moreover, the solution sensitiveness to grid refinement and spanwise extent is investigated

Large Eddy Simulation of Flow Around an Airfoil Near Stall

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

http://www.ae.metu.edu.tr/tuncer/ae546/docs/SA-FV-Deck-02.pdf

Development and application of Spalart–Allmaras one equation turbulence model to three-dimensional supersonic complex configurations

An experimental study of supersonic inlet buzz is presented. This study was carried out on a mixed compression rectangular inlet model; tests were done at Mach numbers ranging from 1.8 to 3, with and without bleed. Inlet flows were analyzed thanks to Schlieren videos and signal processing of unsteady pressure recordings. Two kinds of buzz were observed: "little buzz," which is thought to correspond to an acoustic resonance phenomenon excited by the presence of a shear layer under the cowl lip (Ferri criterion), and "big buzz," which seems to be triggered by a boundary layer separation on the compression ramps (Dailey criterion). In some cases, little buzz could be virtually suppressed by the introduction of a bleed. The frequency of big buzz is shown to be already present in the flow before the onset of large-amplitude oscillations, which suggests that the underlying mechanism of big buzz, probably linked to acoustics, already exists before buzz starts.

Experimental Study of Supersonic Inlet Buzz

Supersonic inlet buzz in a rectangular, mixed-compression inlet has been simulated on a 20 x 10 6 points mesh using the delayed detached-eddy simulation method, a version of detached-eddy simulation that ensures the attached boundary layers are treated using Reynolds-averaged Navier-Stokes equations. The results are compared with experimental data obtained during a previous campaign of wind-tunnel experiments. The comparison of unsteady data is performed thanks to phase averages, Fourier transforms, and wavelet transforms. The buzz observed at Mach 1.8, which occurred at a frequency of 18 Hz, is well reproduced. The shock oscillations, as well as the different flow features experimentally observed, are present in the simulation. The buzz frequency, as well as higher frequencies existing in the experimental pressure signals, are correctly predicted. The data issued from the simulation (time history of pressure fluctuations, pseudo-Schlieren, and three-dimensional visualizations) allow a better investigation of the inlet flowfield during buzz and a detailed description and physical analysis of this phenomenon. A description and an explanation of the mechanism at the origin of secondary oscillations that occur at a higher frequency during buzz are proposed. The crucial role of acoustic waves moving through the duct is shown.

Delayed Detached-Eddy Simulation and Analysis of Supersonic Inlet Buzz

Supersonic inlet buzz and, in particular, its onset were studied through the analysis of pressure records. These records are issued from an experimental study that was presented in a previous paper. The onset of buzz is a nonstationary phenomenon, and so the pressure signals could not be analyzed using classical signal-processing tools such as the Fourier transform, which does not take into account the possible evolutions in time. That is why time-frequency methods (spectrogram, wavelet transform, and Wigner-Ville distribution) were used. The evolution in time of the energy levels corresponding to buzz frequencies were studied. This analysis shows the existence of precursor phenomena that can appear several tenths of seconds before the onset of buzz. The detection of these precursors could, with the appropriate activation of a control device, allow the avoidance of buzz. Two change-detection algorithms (cumulative sum and generalized likelihood ratio) were tested on experimental pressure signals and proved their ability to successfully detect these precursors.

Time-Frequency Analysis and Detection of Supersonic Inlet Buzz

The present paper presents a simulation of supersonic inlet buzz on a 20x106 points mesh, using the DDES (Delayed Detached Eddy Simulation) method. This latter approach is a version of DES that ensures the attached boundary layers to be treated in RANS. The results are compared to experimental data obtained during a previous campaign of wind-tunnel experiments. The buzz experimentally observed at Mach 1.8 is well reproduced. The buzz frequency, as well as higher frequencies existing in the experimental pressure signals, are correctly predicted. The functions of DDES prove to be robust and to perform satisfactorily.

Philippe Duveau

Papers

Development and application of Spalart–Allmaras one equation turbulence model to three-dimensional supersonic complex configurations

Experimental Study of Supersonic Inlet Buzz

Delayed Detached-Eddy Simulation and Analysis of Supersonic Inlet Buzz

Time-Frequency Analysis and Detection of Supersonic Inlet Buzz

Delayed Detached-Eddy Simulation of Supersonic Inlet Buzz