Andrea Dadone

Progressive optimization of inverse fluid dynamic design problems

Abstract An efficient formulation for the robust design optimization of compressible fluid flow problems is presented. The methodology has three essential ingredients. First, the procedure permits the use of highly accurate flow solvers which need no additional modification. Second, efficient design sensitivities are obtained using a discrete adjoint formulation. The adjoint problem is based on a dissipative flow solver in order to obtain robust sensitivity derivatives in the presence of noise or other non-smoothness associated with objective functions on many high-speed flow problems. The third ingredient of the procedure involves what we term a progressive optimization, whereby a sequence of operations—containing a partially converged flow solution, followed by an adjoint solution—followed by an optimization step is performed. Furthermore, the progressive optimization involves the use of progressively finer grids. This approach has been tested on several inviscid, two-dimensional diffuser, nozzle, channel and airfoil flow inverse design problems in transonic, subsonic and supersonic flow. The methodology is shown to be robust and highly efficient, with a converged design optimization produced in no more than the amount of computational work to perform from one to four flow analyses.

Estimator based adaptive fuzzy logic control technique for a wind turbine–generator system

The control of a wind power plant, operating as an isolated power source, is analyzed. The plant consists of a wind turbine and a three phase synchronous electric generator, connected by means of a gear box. The mathematical models of the wind turbine and of the electrical generator are indicated. The use of an estimator based adaptive fuzzy logic control technique to govern the system is proposed. The results of a control test case are shown in order to demonstrate the reliability of the proposed control technique.

Fast convergence of inviscid fluid dynamic design problems

Abstract A new procedure for robust and efficient design optimization of inviscid flow problems has been developed and implemented on a wide variety of test problems. The methodology involves the use of an accurate flow solver to calculate the objective function and an approximate, dissipative flow solver, which is used only in the solution of the discrete quasi-time-dependent adjoint problem. The resulting design sensitivities are very robust even in the presence of noise or other non-smoothness associated with objective functions in many high-speed flow problems. The design problem is solved using what we term progressive optimization, whereby a sequence of a partially converged flow solution, followed by a partially converged adjoint solution followed by an optimization step is performed. This procedure is performed using a sequence of progressively finer grids for the solution of the flow field, while only using coarser grids for the adjoint equation solution. This approach has been tested on numerous inverse and direct (constrained) design problems involving two- and three-dimensional transonic nozzles and airfoils as well as supersonic blunt bodies. The methodology is shown to be robust and highly efficient, with a converged design optimization produced in no more than the amount of computational work to perform from 0.5 to 2.5 fine-mesh flow analyses.

Smoothed Sensitivity Equation Method for Fluid Dynamic Design Problems

We consider shape optimization problems involving compressible fluid flows, which are characterized by non-smooth and/or noisy objective functions. Such functions are difficult to optimize using derivative-based techniques. To overcome such a difficulty, we suggest an approach for estimating the sensitivity derivatives, based on a suitable smoothing of the sensitivity equations. The smoothing affects only the sensitivity derivatives and not the accuracy of the analysis. The basic mechanism by which the smoothing process achieves this result is illustrated with the help of an inverse design problem involving an inviscid quasi-one-dimensional flow having a closed-form solution. The convergence properties and the computational efficiency of the approach are demonstrated on two inverse design problems involving two-dimensional inviscid, compressible flows

One step ahead adaptive control technique for wind systems

The control of a wind system considered as an isolated source of power and composed of a horizontal-axis wind-turbine connected to an induction generator is analyzed. Appropriate mathematical models for both the horizontal-axis wind-turbine and the induction generator are used. The one-step-ahead adaptive control technique is presented and used to regulate the wind system. A sensitivity analysis of the induction generator performances with respect to control and disturbance variables is presented in order to evaluate the control flexibility. The results of three control problems are finally shown in order to prove the reliability of the suggested control technique.

Efficient design optimization of duct-burners for combined-cycle and cogenerative plants

This article proposes an efficient gradient-based optimization procedure for black-box simulation codes and its application to the thermo-fluid-dynamic design optimization of a duct-burner for combined-cycle and cogenerative plants. The article also provides a discussion on some criteria that should drive the design optimization of these components, almost neglected by the scientific literature. Using a widely employed commercial (black-box) code, a new enhanced-mixing duct-burner has been first devised. Before looking at its design optimization, experimental investigations have been performed to assess the reliability of the modelling and the accuracy of the numerical predictions. Then, a finite-difference gradient-based optimization procedure that can be combined with black-box analysis codes has been developed: its efficiency relies on the simultaneous convergence of the flow solution and of the optimization process, as well as on the use of nested grid levels. After its validation, the proposed progressive optimization technique has been applied to two examples of thermo-fluid-dynamic design optimization of the new duct-burner: the first application aims at minimizing the outlet temperature gradient, whereas the second application aims at reducing the near-wall temperatures and shortening the flame, so as to strengthen its anchorage, while reducing the body heating and the thermal NO x formation.

Towards a Better Understanding of Vorticity Confinement Methods in Compressible Flow

Vorticity confinement methods have been shown to be very effective in the computation of flows involving the convection of thin vortical layers. Indeed, these are the only Eulerian methods whereby simulations of these layers remain very thin and persist long distances without significant dissipation. Initially developed by Steinhoff and coworkers for incompressible flow, these methods have been used successfully to predict complex flows, particularly involving helicopter rotors. The confinement procedure has also been utilized to compute the flow about complex geometries using non-body fitted Cartesian coordinates by means of a body confinement technique. Recently, the method has been extended to a compressible finite-volume form, which will enable the methods to be used for a much broader class of problems. Also recently, the method has been applied to complex three-dimensional flows involving massive separation. The results have been surprisingly good and agree with experimental data as well as state-of-the-art LES and RANS calculations, but using significantly fewer grid cells. The present research was undertaken in order to better understand the applicability and limitations of vorticity confinement, particularly the compressible formulation. We have considered several simple model problems to evaluate accuracy and consistency issues. Classical accuracy has been evaluated using a supersonic shear layer problem computed on several grids and over a range of values of confinement parameter. The flow over a flat plate was utilized to study how vorticity confinement can serve as a crude turbulent boundary layer model. Then we utilized numerical experiments with a single vortex in order to evaluate a number of consistency issues related to the numerical implementation of compressible confinement. Finally, we discuss a first attempt at extending the body confinement technique to compressible flows.

SYMMETRY TECHNIQUES FOR THE NUMERICAL SOLUTION OF THE 2D EULER EQUATIONS AT IMPERMEABLE BOUNDARIES

SUMMARY The implementation of boundary conditions at rigid, fixed wall boundaries in inviscid Euler solutions by upwind, finite volume methods is considered. Some current methods are reviewed. Two new boundary condition procedures, denoted as the symmetry technique and the cur6ature-corrected symmetry technique are then presented. Their behaviour in relation to the problem of the subsonic flow about blunt and slender elliptic bodies is analysed. The subsonic flow inside the Stanitz elbow is then computed. The symmetry technique is proven to be as accurate as one of the current methods, second-order pressure extrapolation technique. Finally, for arbitrary curved geometries, dramatic advantages of the cur6aturecorrected symmetry technique over the other methods are shown.

Rapid Aerodynamic Optimization Using Far-Field Coarsened Cartesian Grids

An efficient formulation for the robust design optimization of two-dimensional transonic inviscid fluid flow problems on far-field coarsened Cartesian grids is presented. The farfield coarsening has been introduced in the Cartesian grid flow solver to avoid unnecessarily extending grid clustering near the body to the far field boundary. The far-field coarsening is based on the iblanking approach, which facilitates maintaining the structured nature of the grid while computing only the active cell centers. This Cartesian grid approach is particularly well-suited for implementation in rapid, conceptual-design tools. The design approach has several essential ingredients. First, the procedure utilizes an accurate flow solver tailored for use on far-field coarsened Cartesian grids. Approximate but efficient design sensitivities are obtained using a discrete adjoint formulation. This adjoint problem is based on an artificially dissipative flow solver in order to obtain robust sensitivity derivatives. We solve the quasi-time-dependent adjoint equations in conjunction with what we term progressive optimization, whereby a sequence of operations, containing a partially converged flow solution, followed by a partially converged adjoint solution followed by an optimization step is performed. Finally, we use progressively finer grids for the progressive solution of the flow field and of the adjoint equations. Additional efficiency is obtained by solving the adjoint equations on a reduced computational domain. The approach is shown to be accurate, robust and highly efficient, with converged inverse design optimizations of airfoils produced in less than the amount of computational work to perform two flow analyses. A detailed investigation of the reduced domain used to solve the adjoint equations, indicates that the external boundaries of this domain can be located at only one chord from the airfoil without noticeably influencing the sensitivity derivatives of the objective function. Additionally, this research considered the direct design of airfoils with some preliminary results reported.

Three-dimensional flow computations with shock fitting

Abstract The present paper provides a shock-fitting technique for solving inviscid transonic three-dimensional (3-D) flows. The continuous flow field is computed by means of an implicit fast Euler solver, which separately integrates compatibility conditions, written in terms of generalized Riemann variables along appropriate bicharacteristic lines. The continuous 3-D flow problem is thus reduced to a sequence of simple quasi 1-D problems. The shock wave is computed by means of a shock-fitting technique, which enforces the proper shock jumps by an explicit use of the Rankine-Hugoniot equations. The computed shock thus develops into a discontinuity as it is in reality. The merits of the present approach are demonstrated by means of a few simple applications and by comparison with corresponding results computed using a flux-difference splitting methodology.