Jorg Schluter

Large eddy simulation inflow conditions for coupling with reynolds-averaged flow solvers

Hybrid approaches using a combination of Reynolds-averaged Navier-Stokes (RANS) approaches and large eddy simulations (LES) have become increasingly popular. One way to construct a hybrid approach is to apply separate flow solvers to components of a complex system and to exchange information at the interfaces of the domains. For the LES flow solver, boundary conditions then have to be defined on the basis of the Reynolds-averaged flow statistics delivered by a RANS flow solver. This is a challenge, which also arises, for instance, when defining LES inflow conditions from experimental data. The problem for the coupled RANS-LES computations is further complicated by the fact that the mean flow statistics at the interface may vary in time and are not known a priori but only from the RANS solution. The present study defines a method to provide LES inflow conditions through auxiliary, a priori LES computations, where an LES inflow database is generated. The database is modified to account for the unsteadiness of the interface flow statistics

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.

LES of jets in cross flow and its application to a gas turbine burner

LES computations of jets in cross flow (JICF) were performed. Experimental investigations reported in literature are reproduced with good agreement concerning the momentum field and the mixing of a passive scalar. The results validate the ability of the present LES approach to compute fuel injection of the type JICF. LES computations of fuel injection in an industrial gas turbine burner are presented.

A Framework for Coupling Reynolds-Averaged With Large-Eddy Simulations for Gas Turbine Applications

Full-scale numerical prediction of the aerothermal flow in gas turbine engines are currently limited by high computational costs. The approach presented here intends the use of different specialized flow solvers based on the Reynolds-averaged Navier-Stokes equations as well as large-eddy simulations for different parts of the flow domain, running simultaneously and exchanging information at the interfaces. This study documents the development of the interface and proves its accuracy and efficiency with simple test cases. Furthermore, its application to a turbomachinery application is demonstrated.

Computational study on the internal layer in a diffuser

We report an internal layer found in the turbulent flow through an asymmetric planar diffuser using large-eddy simulation; we discuss five issues relevant to the internal layer: definition and identification, conditions for occurrence, connection with its outer flow, similarity with other equilibrium flows, and growth. The present internal layer exists in a region with stabilized positive skin friction downstream of a sharp reduction. The streamwise pressure gradient changes suddenly from slightly favourable to strongly adverse at the diffuser throat, and relaxes in a prolonged mildly adverse region corresponding to the skin friction plateau. Development of the internal layer into the outer region is slow, in contrast to the internal layers previously identified from certain external boundary-layer flows where the sudden change in streamwise pressure gradient is from strongly adverse to mildly favourable. Signatures of the internal layer include an inflectional point in the wall-normal profiles of streamwise turbulence intensity, and a well-defined logarithmic slope in the mean streamwise velocity underneath a linear distribution extending to the core region of the diffuser. Some of these characteristics bear a certain resemblance to those existing in the C-type of Couette–Poiseuille turbulent flows. Frequency spectrum results indicate that application of strong adverse pressure gradient at the diffuser throat enhances the low-frequency content of streamwise turbulent fluctuations. Inside the internal layer, the frequency energy spectra at different streamwise locations, but with the same wall-normal coordinate, nearly collapse. Two-point correlations with streamwise, wall-normal and temporal separations were used to examine connections between fluctuations inside the internal layer and those in the core region of the diffuser where the mean streamwise velocity varies linearly with distance from the wall. Galilean decomposition of instantaneous velocity vectors reveals a string of well-defined spanwise vortices outside the internal layer. The internal layer discovered from this study provides qualified support for a conjecture advanced by Azad &

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.

Outflow Conditions for Integrated Large Eddy Simulation/Reynolds-Averaged Navier-Stokes Simulations

The numerical flow prediction of highly complex flow systems, such as the aerothermal flow through an entire aircraftgasturbineengine,requirestheapplicationofmultiplespecializedflowsolvers,whichhavetorunsimultaneously in order to capture unsteady multicomponent effects. The different mathematical approaches of different flow solvers, especially large eddy simulation (LES) and Reynolds-averaged Navier‐Stokes (RANS) flow solvers, pose challenges in the definition of boundary conditions at the interfaces. Here, a method based on a virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of an LES flow domain in order to take downstream flow effects computed by a RANS flow solver into account. This method shows good results in the test case of a swirl flow, where the influence of a flow contraction downstream of the LES domain is represented entirely by the Reynolds-averaged velocity field at the outlet of the LES domain. I. Motivation N UMERICALsimulationsofcomplexlarge-scaleflowsystems must capture a variety of physical phenomena in order to predicttheflowaccurately.Currently,manyflowsolversarespecialized to simulate one part of a flow system effectively, but are either inadequate or too expensive to be applied to a generic problem. Asanexample,theaerothermalflowthroughagasturbineengine can be considered. In the compressor and the turbine section, the flow solver has to be able to handle the moving blades, model the wall turbulence, and predict the pressure and density distributions properly. This can be done efficiently by a flow solver based on the Reynolds-averaged Navier‐Stokes (RANS) approach. On the other hand, the flow in the combustion chamber is governed by largescale turbulence, complex mixing processes, chemical reactions, and the presence of fuel spray. Experience shows that these phenomena require an unsteady approach. 1 Hence, the use of a large

Multi-Code Simulations: A Generalized Coupling Approach

We present an approach to perform multi-physics simulations by coupling multiple solvers. In order to simplify the coupling process, we have developed the software module CHIMPS (Coupler for High-performance Integrated Multi-Physics Simulations) that handles the coupling of two mesh-based solvers. The module is based on the script language python and has been developed to accommodate a wide variety of mesh based solvers. In this paper we will present previous multi-code approaches, the advantages of the new approach using the software module CHIMPS, and present some verication test-cases.

Lift Enhancement at Low Reynolds Numbers Using Self-Activated Movable Flaps

I NRECENTyears, interest in micro air vehicles (MAVs) is rising. The reason for that is the availability of microand nanotechnology necessary for the design of these vehicles, but also the emergence of new threats, especially in form of terrorism. MAVs promise to be able to be deployed in closed environments, such as buildings, subway tunnels, and dense forests, allowing reconnaissance, surveillance, and search and rescue missions, for which the use of humans is dangerous or even impossible. To be able to operate in this environment, MAVs need to be small and capable to fly at low speeds, yet they need to be agile enough to move around corners and bends. Moreover, they have to be able to transport a payload, such as video surveillance equipment or sensors for chemical agent detection. So far, no operational MAVexists that fits within these operational limits. The reason for this is that conventional aircraft designs have difficulties in providing sufficient lift and aerodynamic efficiency at low Reynolds numbers [1]. The low lift coefficients cause problems in the design of MAVs, because the often prohibitively large wings are required. Hence, novel methods to increase lift at low Reynolds numbers need to be considered. A method to increase the lift coefficient, inspired by biological flows, is investigated here. Schatz et al. [2,3] reported that the use of a passive flap near the trailing edge results in a lift increase at high angles of attack. The use of a passive flap is inspired by the feather structure of a bird on the upper side of the bird’s wing [4,5]. At high angles of attack, it can be observed that the feathers start to pop up (Fig. 1). Schatz et al. [2,3] investigated the use of a passive flap to emulate this behavior. Their force measurements at Reynolds numbers above Re 1; 000; 000 showed an increase of lift of about 10%. Kernstine et al. performed experiments on this passive flap configuration at Reynolds numbers around Re 330; 000 [6], with similar results. Kernstine et al. focused especially on the optimal placement of the flaps and determined that the flap should be located closer to the midchord rather than the trailing edge to obtain optimal results. The Cl;max increased by a maximum of 15%. The role of the flap is to capture the trailing edge flow separation and to prevent it from creeping upstream. This allows the flow to be attached on a larger portion of the wing than would be the case without and, as such, sustain higher angles of attack without stalling. The advantage of this flap is that it is a very simple and robust device that does not require complex mechanisms, such as conventional slats and flaps. It is easy to install and even retrofit existing aircraft. Here, the possibility of applying this method to low Reynolds numbers at about Re 30; 000–40; 000, which is a more suitable range for MAV, is investigated. The aerodynamics of airfoils at this Reynolds number range are especially challenging due to the presence of laminar flow separation and the nonlinearities, such as hysteresis effects, associated with it. Awater tunnel is used to visualize the flow and to measure the effect of the flap at this Reynolds number range.

Static Control of Combustion Oscillations by Coaxial Flows: A Large-Eddy-Simulations Investigation

In the design process of lean premixed combustors, the suppression of combustion instabilities plays a crucial role. Because active control measures to suppress combustion instabilities involve costs and maintenance, passive or static control measures are a preferable choice. The current work focuses on vortex-driven combustion instabilities, where large-scale vortices trigger the instability. A skillful vortex management in the combustor by passive methods- such as a coaxial flow-should be able to decrease the influence of large-scale vortices on the flame or to suppress the creation of large vortices completely. The current study investigates the possibilities of manipulating the shear layer at the burner nozzle of an axisymmetric dump combustor by a small circumferential coaxial flow. To explore these possibilities of flow control, large eddy simulations (LES) of a confined Bunsen burner flame are performed using a flow solver based on a low-Mach-number assumption and the G equation. The flow is periodically excited in order to simulate the acoustic forcing of the combustion instability. Phase averaging delivers the flow response to the forcing, and transfer functions give information about the amplification of the forcing. The success of the control method is then determined by the ability of the control method to suppress an amplification of the periodic excitation. The results of LES of the cold flow shows that the swirled coaxial flow is able to destroy large-scale structures and to suppress the periodic flow response substantially. In the reacting case a coaxial flow is able to shield the main flow and the flame front from large-scale vortices, and the influence of vortices on the flame front decreases dramatically.