Georgi Kalitzin

Unsteady Turbomachinery Computations Using Massively Parallel Platforms

The CPU power offered by the latest generation of supercomputers is such that the unsteady simulation of the full wheel of a compressor or turbine is now within reach. This CPU power is mainly obtained by increasing the number of processors beyond several thousands, e.g. the BlueGene/L computer of Lawrence Livermore National Laboratory has approximately 130,000 processors. Consequently extreme care must be taken when the simulation codes are ported to these platforms. This paper discusses the computer scientific aspects of simulating unsteady turbomachinery flows on massively parallel systems when multi-block structured grids are used. Load balance, parallel IO and search algorithms are addressed, especially for the case where the number of processors is larger than the number of blocks, i.e. when blocks must be split during runtime. Preliminary results for cases with more than 200 million nodes running on 1,800 processors are presented.

DNS of fully turbulent flow in a LPT passage

Abstract This work addresses the pattern of turbulent kinetic energy generated by distortion and the effect of external disturbances on boundary layer transition. This is investigated with direct numerical simulation of grid turbulence convected through a linear turbine blade cascade. Comparisons are made with results from earlier computations of flow through the same cascade with a turbulence free inflow and an inflow with migrating wakes. The distribution of turbulence in the passage strongly depends on the mean flow field and can partly be explained by the travel time needed for the inlet turbulence to drift to a certain location. This results in a local amplification of turbulence near the leading edge stagnation region and in the passage on the pressure side near the trailing edge. The penetration of disturbances into the blade boundary layers induces bypass transition. In particular, the transition pattern on the suction side of the blade differs significantly for the three types of inflow.

An eddy-viscosity based near-wall treatment for coarse grid large-eddy simulation

An eddy-viscosity-based near-wall treatment is proposed to enable large-eddy simulations (LES) to be performed on coarse grids. This formulation consists of imposing wall stress boundary conditions and an eddy viscosity in the near-wall region. The wall stress and eddy viscosity have a Reynolds-averaged Navier-Stokes-like character and are obtained from an averaged velocity profile of a resolved LES of channel flow at Reτ=395. Both are tabulated and are used for the instantaneous quantities. The tabulated eddy viscosity is further corrected using the resolved turbulent stress. Numerical results for flow in a channel at several Reynolds numbers ranging from Reτ=395 to Reτ=10000 are presented.

CHIMPS: A high-performance scalable module for multi-physics simulations

As computational methods attempt to simulate ever more complex physical systems the need to couple independently-developed numerical models and solvers arises. This often results from the requirement to use different physical or numerical models for various portions of the domain of interest. In many situations it is also common to use different physical models that interact within the same domain of interest. The interaction between these models normally requires an exchange of information between the participating solvers. When the solvers that exchange information are distributed over a large number of processors in a parallel computer, the problem of exchanging information in an efficient and scalable fashion becomes complicated. This paper describes our efforts to develop a Coupler for High-Performance Integrated Multi-Physics Simulations, CHIMPS, that can enable the exchange of information between solvers and that automates the search, interpolation and communication processes in order to allow the developer to focus on appropriate strategies to couple solvers in an accurate and stable fashion. Our basic approach, the underlying technology and a number of examples are presented. A series of appendices are included with actual sample code and a description of the full CHIMPS API at the time of writing.

LOCAL GRID REFINEMENT FOR AN IMMERSED BOUNDARY RANS SOLVER

A RANS solver based on the Immersed Boundary technique is extended to handle locally refined grids in order to increase the resolution close to the boundaries for high Reynolds number simulations. A novel data management architecture is introduced to take advantage of the quasi-structured nature of the grids and to obtain fully implicit, fast and robust solutions when several levels of refinement are introduced. The mesh refinement is fully anisotropic, and can handle n-to-one cell connectivity. It is generated in a fully automatic way by coarsening a fine structured grid. A conjugate gradient-based algorithm is used to solve the Navier-Stokes equations and validation cases include a twoand a three-dimensional problem.

Integrated RANS/LES Computations of an Entire Gas Turbine Jet Engine

The interaction between different components of a jet engine represents a very important aspect of the engine design process. Sudden mass flow-rate changes induced by flow separation and pressure waves, interaction of the unsteady wakes originating from the fan blades with the low-pressure compressor, high temperature streaks interacting with the first stages of the turbine are all complex unsteady phenomena that cannot be simply accounted for through boundary conditions of a single component simulation. Only simulations that integrate multiple engine components can describe these flow features accurately. Today’s use of Computational Fluid Dynamics (CFD) in gas turbine design is usually limited to component simulations. The demand on the models to represent the large variety of physical phenomena encountered in the flow path of a gas turbine mandates the use of a specialized and optimized approach for each component. The flow-field in the turbomachinery portions of the domain is characterized by both high Reynolds numbers and high Mach numbers. The prediction of the flow requires the precise description of the turbulent boundary layers around the rotor and stator blades, including tip gaps and leakage flows. A number of flow solvers that have been developed to deal with this kind of problem have been in use in industry for many years. These flow solvers are typically based on the Reynolds-Averaged Navier-Stokes (RANS) approach. Here, the unsteady flow-field is ensemble-averaged, removing all the details of the small scale turbulence; a turbulence model becomes necessary to represent the effects of turbulence on the mean flow. The flow in the combustor, on the other hand, is characterized by multi-phase flow, intense mixing, and chemical reactions. The prediction of turbulent mixing is greatly improved using flow solvers based on Large-Eddy Simulations (LES). While the use of LES increases the computational cost, LES has been the only predictive tool able to simulate consistently these complex flows. LES resolves the large-scale turbulent motions in time and space, and only the influence of the smallest scales, which are usually more universal and hence, easier to represent, has to be modeled. 2 Since the energy-containing part of the turbulent scales is resolved, a more accurate description of scalar mixing is achieved, leading to improved predictions of the combustion process,. LES flow solvers have been shown in the past to be able to model simple flames and are currently being adapted for use in gas turbine combustors. 5 In order to compute the flow in the entire jet engine, one needs to couple RANS and LES solvers. We have developed a software environment that allows a simulation of multi-component effects by executing multiple solvers simultaneously. Each of these solvers computes a portion of a given flow domain and exchanges flow data at the interfaces with its peer solvers (see Figure 1). The approach to couple two or more existing flow solvers has the distinct advantage of building upon the experience and validation that has been put

Integrated computations of an entire jet engine

In this paper, we present a framework that we have developed for integrated computations of an entire jet engine. It is based on a software environment, CHIMPS, that allows a simulation of multi-component effects by executing multiple solvers simultaneously. Each of these solvers computes a portion of a given flow domain and exchanges flow data at the interfaces with its peer solvers. We demonstrated this approach in a simulation of a 20 degree sector of the entire gas turbine jet engine, encompassing the fan, low and high pressure compressor, combustor, high and low pressure turbine and the exit nozzle. We will show that such a simulation can deliver important insight into the physics of interaction between different engine components, within a reasonable turnover time, which is necessary for it to be useful in the design process of an engine.

UNSTEADY FLOW SIMULATIONS OF WHEEL-WHEELHOUSE CONFIGURATIONS

Numerical simulations of the unsteady flow around stationary and rotating wheels are carried out with the objective of establishing the accuracy of a newly developed numerical code. A simplified isolated wheel and a realistic truck configuration are considered; measured pressure distributions are available for the first case and the comparison of the computed and the experimental data shows a satisfactory agreement. The effect of the wheelhouse cavity on the time evolution of the aerodynamic drag forces is also presented; the dominant feature is the unsteady motion within the cavity and no evidence of coherent vortex shedding (or wake motion) is observed.

DNS OF FULLY TURBULENT FLOW IN A LPT PASSAGE

The questions being addressed in this work are the pattern of turbulent kinetic energy generated by distortion and the effect of external disturbances on boundary layer transition. This is investigated with direct numerical simulation of grid turbulence convected through a low pressure turbine cascade on a mesh with 86 million grid nodes. Comparisons are made with results from earlier computations with a turbulence free inlet and with migrating wakes through the same passage. The distribution of turbulence in the passage strongly depends on the mean flow field and can partly be explained by the travel time needed for the inlet turbulence to drift to a certain location. A local amplification of turbulence is caused by the mean flow strain field. Bypass transition is induced by external disturbances penetrating into the boundary layer near the blade surfaces. Grid turbulence induces a significantly different transition pattern on the suction side of the blade than migrating wakes in the passage.

On Coupling of RANS and LES for Integrated Computations of Jet Engines

Large scale integrated computations of jet engines can be performed by using the unsteady RANS framework to compute the flow in turbomachinery components while using the LES framework to compute the flow in the combustor. This requires a proper coupling of the flow variables at the interfaces between the RANS and LES solvers. In this paper, a novel approach to turbulence coupling is proposed. It is based on the observation that in full operating conditions the mean flow at the interfaces is highly non-uniform and local turbulence production dominates convection effects in regions of large velocity gradients. This observation has lead to the concept of using auxilliary ducts to compute turbulence based on the mean velocity at the interface. In the case of the RANS/LES interface, turbulent fluctuations are reconstructed from an LES computation in an auxiliary three-dimensional duct using a recycling technique. For the LES/RANS interface, the turbulence variables for the RANS model are computed from an auxilliary solution of the RANS turbulence model in a quasi-2D duct. We have demonstrated the feasibility of this approach for the integrated flow simulation of a 20° sector of an entire jet engine.Copyright