Pascal Ferrand

Parametic Study of a Fan Blade Cascade Using a New Parametric Flow Solver Turb’Opty

The newly developed parameterized CFD solver Turb’Opty™, based on a Taylor series expansion to high order derivatives of the solutions of the discretized Navier-Stokes equations, has been successfully applied to the turbulent incompressible flow field of an engine cooling fan blade cascade. Comparisons with the classical CFD results have validated the accuracy of the parameterized solutions obtained by a simple polynomial reconstruction around a reference solution with respect to two different flow parameters for two different cases: a fifth order expansion with respect to these coupled parameters for a frozen turbulence and a first order expansion with respect to each parameter for a variable turbulence. The latter is found to have a better accuracy and a larger range of application. Starting from a reference solution obtained with another commercial code has also been successfully tested. Finally, further industrial perspectives of turbomachinery global optimization are finally demonstrated by coupling this method with a simple genetic algorithm.Copyright

Unsteady RANS Computations of the Flow Past an Airfoil in the Wake of a Rod

Two-dimensional Reynolds Averaged Navier-Stokes (RANS) equations are solved in order to simulate the interaction between a Karman vortex street shed from a rod and a NACA-0012 airfoil in the wake of the rod. Two closure turbulence models are tested, a linear and a nonlinear k-ω model, for a chord based Reynolds number Rec ∼ 4.8105 . These models provide consistent results in terms of both mean and fluctuating flow quantities. Insight into the instantaneous vorticity field shows that the vortex shedding pattern near the wall is quite well predicted, despite an over-estimated frequency. Downstream, computations always exhibit head-on interactions of the vortices with the airfoil leading edge whereas the experiments show a more variable configuration.Copyright

NONLINEAR INTERACTION OF UPSTREAM PROPAGATING SOUND WITH TRANSONIC FLOWS IN A NOZZLE

A numerical simulation of the interaction of upstream propagating acoustic waves with a shock in a convergent-divergent two-dimensional nozzle is presented. Both Euler and Navier-Stokes simulations are carried out and compared with data and theories. The Navier-Stokes simulation uses a k-w model for turbulence and fully accounts for the effect of boundary-layer separation which originates at the shock foot location. It is shown that nonlinear effects are concentrated near the shock/boundary-layer interaction region where the unsteady pressure shows higher modes. The motion of the shock remains essentially harmonic driven mainly by the frequency of the outlet imposed acoustic wave. The intensity of the reflected wave is much stronger than that predicted by the inviscid analysis suggesting that the fluctuation of the separated region near the shock foot acts as the main source of sound propagating downstream.

2D Elementary Geometric Decomposition to Study Flutter Motion of a Space Turbine Blisk

Within the framework of aerospace turbines, an isolated integral bladed disk is examined. The blisk presents very high eigenfrequencies with complex deformations of the blades. A 3D steady RANS com ...

Simulation of Surge Inception and Performance of Axial Multistage Compressor

This work concerns transient multistage turbomachine modeling. The main application is the simulation of off-design regimes and unstable situations (essentially surge). Two specific approaches are developed: the first is a simple and fast model, based on a simplification of actuator disk model (0D). The second approach is based on 1D axisymmetric Navier-Stokes equations at the scale of the row. The models are tested on open literature cases of the gas turbine aircraft community. The numerical results compare favorably with these data from a qualitative point of view. The description of deep surge occurrence and the prediction of quantitative elements of compressor performance during surge are satisfactory provided few steady-state parameters are correctly determined. Otherwise the fully deterministic approach gives approximate, but acceptable results for a 0D or 1D model.Copyright

A Partitioned Strong Coupling Procedure for Simulation of Shock Wave/Flexible Structure Interaction

The prediction of FSI limit cycles involving transonic separated flows requires efficient and accurate solvers coupling techniques. Explicit partitioned strong coupling is considered in time domain, where careful attention should be paid to energy conservation at the fluid-structure interface. For all the presented results, both meshes are set up such that structural skin points and fluid boundary mesh points are collocated. The presented test case involves a shock tube in which the shock wave impinges on a cross flow flexible panel, initially at rest. Compared to experimental results, the pressure peaks and fluctuations are correctly predicted but the pressure level is over predicted as well as the displacement frequency. Results analysis explains correctly the flow physic which is shown to be weakly modified by structural damping, turbulence modeling and time discretization. This discrepancy between experimental and numerical results could been explained by the structure model, in which the panel root modeling might be questionable.Copyright

Space Infill Study of Kriging Meta-Model for Multi-Objective Optimization of an Engine Cooling Fan

The meta-model based optimization is widely used in the aerodynamical design process for rotating machines, and the main industrial cost of such techniques comes from physical evaluations of answers, either by experimental or numerical means. Design of experiment (DoE) with Latin Hypercube sampling has been studied for the design of an automotive fan system for engine cooling. Surrogate models constructed with Kriging and Co-Kriging methods are estimated with the help of a reference numerical model. The objective of the present work is to assess the necessary number of sampling points for the initial DoE for this kind of meta-model method and to study the influence brought by the sample dispersion. The objective being to execute future aerodynamic optimizations at a reduced cost in term of timeframe and CPU effort. Two parameters, camber and chord length were used to investigate geometrical changes and they are completed with a physical parameter which is the flow rate. The optimization should lead to a higher level of performances with given constraints of rotational speed, torque and packaging. A criterion was defined for the initial necessary number of evaluations and the variances for different DoE design were controlled for the sake of comparison. Starting from an initial meta-model, a variance based method was used for further training with additional points. Uncertainties due to lack of information outside the domain led the model to regularly propose new points on the borders, yielding to high sample variance. A genetic-algorithm was employed to explore the final meta-model and to conduct a multi-objective optimization. Results are presented in terms of Pareto Front and are analysed with SOM to understand the relations between factors and objectives. A final optimal design was selected, and proposed to demonstrate the relevancy of the method.Copyright

Comparison of Fluid-Structure Coupling Methods for Blade Forced Response Prediction

A time-marching coupled method has been applied to a high pressure compressor forced response case. The purpose of the study is to compare the contribution of the coupled scheme against the state of the art decoupled methodology. Although the time-marching scheme allows for a stronger coupling between the motion and aerodynamic field, results have shown good agreement with the decoupled method. The additional computing cost arising from the long transient state and the small difference in amplitude prediction with the decoupled scheme reduce the interest for the coupled one. A new transient state method is proposed to combine both coupled and decoupled schemes features. Here, the forcing and damping functions are extracted during the mechanical transient state of the coupled simulation and forced response is calculated as in the decoupled method. Results have shown good agreements with the experiment and all the methods are compared in terms of underlying assumptions and performances.Copyright

Dynamic Modeling of a Nuclear Gas Turbine Plant: Application to Surge Prediction

This work concerns the dynamic modeling of closed cycle gas turbine with a nuclear heat source. The paper focuses on a particular safety question: the consequences of a hypothetical large break accident. A model of the whole circuit of the Gas Fast Reactor (GFR) has been built using a specific turbo-machine description. The compressor modeling presented in a separate paper [1] is completed with turbine modeling. Transient simulation results point out the importance of the location of the pipe rupture: in some detailed cases, back flow through the core can occur during the first seconds. The other safety question concerns the capacity of the power conversion unit to extract the decay heat from the nuclear core after the break event.Copyright

An Analysis of the Spatial Fluctuations at the Stator-Rotor Interface in a Transonic Turbine

This paper analyses the flow in a transonic turbine stage, using numerical results from a full 3D Navier-Stokes computation over the whole stage. This analysis, based on the combination of time and space Fourier transforms, points out how a spatio-temporal periodicity phenomenon is involved in the so-called stator-rotor interaction.A first comparison is made between the computational results using geometrical approximation and the results achieved by the complete simulation. This analysis points out how the signal is enforced by the numerical conditions at the circumferential boundaries.The stator-rotor interaction produces low spatial harmonics which are identified as spinning modes by means of Fourier series. The analytical theory from Tyler and Sofrin (1962) is then used to define a general model of stator-rotor interaction. This model allows the determination of the angular speed of rotation of the spinning modes, and results are compared with the global numerical simulation.Finally, a first analysis of the axial propagation of the spinning modes is proposed, and attention is focused on the fact, that only few parameters are necessary to reproduce the unsteady signal.Copyright