Film cooling is a common technique to manage turbine blade thermal environment. The geometry of the holes which are used to generate the cooling film is known to play a very important role on thermal performances and finding the most optimized shape involves rigorous experimental as well as numerical investigations to probe the many parameters at play. For the current study an automatic optimization tool is developed and then probed with the capability of performing hole shape optimization based on Large Eddy Simulation (LES) predictions. To do so, the particular geometry called shaped cooling hole is chosen as a baseline geometry for this optimization process. Relying on the response surface evaluation based on a reduced model approach, the use of a Design of Experiments (DOE) method allows probing a discrete set of values from the parameter space used to define the present shaped cooling hole. At first only two parameters are chosen out of the seven parameters defining the hole shape. This is followed by the automatic generation of the hole geometry, the corresponding computational domain and the associated meshes. Once the geometries and meshes are created, the numerical setup is autonomously completed for each of the cases including a first guess of the flow field to increase convergence of the simulation towards an exploitable solution. To finish, the LES fluid flow prediction is used to evaluate the discrete value of the problem response function which can then participate in the reduced model construction from which the optimization is derived.

Large Eddy Simulation (LES) of turbomachinery stages has been recently brought to attention due to its potential increased prediction fidelity and its reduced dependency to modeling. Such simulations are however often very CPU intensive, with potentially long return times and only possible for reduced periodic sectors. For real applications, such limitations are prohibitive for a daily use in a design phase. Indeed, most industrial turbomachinery applications rely on designs where at least one of the blade rows has a prime number of blades. Full 360° simulations are in such a case required for appropriate flow dynamics predictions, which implies prohibitive computational costs although recent demonstrations prove these feasible. To make LES affordable in an industrial context, it is clearly necessary to find ways to reduce its cost and return time, one approach being the reduction of the computational domain size. The Profile Transformation Approach (PTA) is one of such specific methods that allows to simulate down to a single blade passage per blade row, thus decreasing the domain size of the problem and its CPU cost. PTA has been devised and validated in a URANS context and its limits are well known in this specific context. In terms of development and implementation in a code, PTA essentially consists in re-scaling the flow field at the rotor/stator interface to comply with the geometrical constraints on both sides of the interface since these often have different angular extents. Thanks to this flow re-scaling, periodic flow conditions can be applied on the azimuthal limits of both domains while retaining only one passage per row. In the following, the method is assessed in the context of fully unsteady LES simulations in an attempt to identify generated approximations and errors. This LES approach is then used to address a set of cases of increasing complexity ranging from the academic problem focusing first on the convection of a vortex across an interface and finishing with simulations of industrial relevance.

Optimizing the design of aviation propulsion systems using computational fluid dynamics is essential to increase their efficiency and reduce pollutant as well as noise emissions. Nowadays, this design process is increasingly aided by computational fluid dynamic methods for which and with the adequate modeling approach it is possible to perform meaningful unsteady computations of the various components of a gas-turbine engine. However, these simulations are often carried out independently of each other and only share averaged quantities at the component interfaces minimizing the impact and interactions between components. The present work investigates the interactions between fan, compressor and annular combustion chamber at takeoff conditions by simulating a 360 azimuthal degrees large-eddy simulation of over 2100 million cells of the DGEN-380 demonstrator. In that case, the domain includes: 14 fan blades; 42 outlet-guide vanes (OGV); the impeller with 11 main blades and 11 splitter blades; a radial and an axial diffuser with 22 and 55 vanes, respectively; and the annular combustion chamber with a contouring casing and 13 swirlers on the back of the chamber. At take-off conditions it is found that the compressor operates in transonic conditions in the rotating frame of reference of the impeller and a shock is formed at the leading edge of the main blades which propagates upstream towards the fan and it is perceived at half the impeller blade-passing frequency (BPF). Preliminary results also show that pressure fluctuations at the impeller BPF generated by the interaction of the impeller blades with the diffuser vanes are propagated through the axial diffuser and enter the combustion chamber through the dilution holes and the swirler. The objective of this paper is to provide a deeper analysis of the interactions between components through the use of the novel operator-based analysis called dynamic mode tracking method (DMT). Indeed, this method facilitates the analysis of three-dimensional results despite the billion-size mesh and the complexity of the simulation, since it extracts modes at specific frequencies on-the-fly within the code. The frequencies corresponding to the fan, impeller and half the impeller BPF are analyzed in the domain and compared against traditional and more computationally demanding methods like the well-known Dynamic Mode Decomposition or the Direct Fourier transform.

The coupling between different components of a turbomachinery is becoming more widely studied especially by use of Computational Fluid Dynamics. Such simulations are of particular interest especially at the interface between a combustion chamber and a turbine, for which the prediction of the migration of hotspots generated in the chamber is of paramount importance for performance and life-duration issues. Despite this need for fully integrated simulations, typical turbomachinery simulations however often only consider isolated components with either time-averaged constant value, radial profile or least frequently 2D maps imposed at their inlet boundaries preventing any accurate two-way coupling. The objective of the present study is to investigate available solutions to perform isolated simulations while taking into account the effect of multi-component coupling. Investigations presented in the paper focus on the FACTOR configuration. The fist step of the proposed method is to record conservative variables solved by the LES code at the interface plane between the chamber and the turbine of a reference simulation. Then, using the Spectral Proper Orthogonal Decomposition (SPOD) method, the recorded data is analysed and can be partially reconstructed using different numbers of frequencies. Using the partial reconstructions, it is then possible to replicate a realistic inlet boundary condition for isolated turbine simulations with both velocity and temperature fluctuations, while reducing the storage cost compared to the initial database. The integrated simulation is then compared to the isolated simulations as well as against simulations making use of averaged quantities with or without synthetic turbulence injection at their inlet. The isolated simulations for which the inlet condition is reconstructed with a large number of frequencies show very good agreement with the fully integrated simulation compared to the typical isolated simulation using average quantities at the inlet. As expected, decreasing the number of frequencies in the reconstructed signal deteriorates the accuracy of the resulting signal compared to the full recorded database. However, isolated simulations with a low number of frequencies still perform better than standard boundary conditions, especially from an aero-thermal point of view.

Complex unsteady phenomena can appear in turbomachinery components and result in the self-sustained oscillatory motion of the fluid as found in aeronautical engines or rocket turbopumps for example. The origin of these oscillations often results from the complex coupling between flow non linearities and structure motion generating major risks for the operation of the engine and even undermining its components. For instance, in turbines, the internal components that are most liable to vibrate are the blades and discs. In this context, it is critical to understand the effect of the vibrating components on the flow stability in rotor/stator cavities. In order to address this problem, an academic rotor/stator cavity subject to periodic wall oscillations is investigated in the current paper where the frequency of the vibrations are imposed and correspond to the previously identified unstable fluid modes inside the cavity. The objective is to understand the behavior of the flow when subject to a periodic forcing imposed by the rotor motion. To do so, predictive numerical strategies are established based on Large Eddy Simulation (LES) in conjunction to a global stability analysis which seem to be a promising method to capture flow instabilities. Focus is here brought to the underlying pressure fluctuations found inside the cavity using spectral analysis complemented with the global stability analysis, demonstrating that such tools can address forced flow problems. More specifically and for all simulations, the results of the global stability analysis are compared to a Dynamic Mode Decomposition (DMD) of LES predictions by reconstructing the corresponding modes through a spatio-temporal approach showing that the new fluid limit cycles present modes that shift or completely disappear compared to the unforced case, the forcing mechanism altering the stability of the entire system.

In most turbomachinery CFD simulations, a large heterogenity of mesh refinement exists because specific flow regions are known to require small cell sizes for proper prediction. This leads to large cell volume ratios between the smallest and largest cells. In the context of explicit solvers where the timestep is imposed by stability constraints and smallest cell size, large difference in cell sizes may directly lead to a waste of CPU time since this timestep is much smaller than required in the majority of cells. The introduction of a Local Time-Stepping (LTS) is a potential solution to such an observation. The computational domain is divided in multiple sub-domains and each sub-domain will meet its local stability constraints, relaxing the limit of maximal allowed timesteps. In the following, this technique is applied to a wall-resolved Large-Eddy Simulation (LES) of the LS89 turbine vane cascade dividing the domain in two parts: one part containing the near-wall region of the blade (with prism layers at the blade surface leading to a small timestep) and a second part composed of the remaining volume. To evaluate the strategy, a standard wall-resolved simulation and its LTS counterpart are compared to experimental data. Overall, for the LS89 configuration that is known as challenging, there is an excellent agreement between simulations and experiment. More importantly, a 31% speedup is achieved on this configuration while giving identical results when comparing the single domain and the LTS cases despite the significant simulation cost reduction for the latter.

This paper aims at addressing design issues of turbomachinery configurations by use of Large-Eddy Simulation (LES). To do so, a research state-of-the-art high-pressure turbine stage, without technological details and for which experimental data are available, is computed with the three methods: i.e. RANS, URANS and LES. Starting from the nominal operating design, a database is acquired varying the design space (three Zweifel numbers), load (three pressure rates) and rotation speed (three reduced speeds). The analysis of the database is carried out incrementally from a design perspective. Numerical results are systematically compared to experimental ones. Main conclusions are threefold: 1/ Calibrated RANS provides excellent results at the nominal operating point but lacks of accuracy at off design conditions. Only unsteady methods (both URANS and LES) allow a good agreement with experiment along the whole database. 2/ Although very good on the overall performances, LES provides radial profiles and 2D maps leaving room for improvement in comparison with the URANS predictions. 3/ LES and standard law-of-the-wall is validated against experiments in a high-pressure turbine without technological details but still representative of a realistic and recent industrial design. From an aero design point, this paper shows the interest in using URANS for off design conditions. It also represents a milestone for LES that had to be passed before addressing more complex issues which URANS hardly addresses.

The design optimization of aviation propulsion systems by means of computational fluid dynamics is key to increase their efficiency and reduce pollutant and noise emissions. The recurrent increase in available computing power allows nowadays to perform unsteady high-fidelity computations of the different components of a gas turbine. However, these simulations are often made independently of each other and they only share average quantities at interfaces. In this work, the methodology and first results for a sectoral large-eddy simulation of an integrated high-pressure compressor and combustion chamber of a typical turbine engine architecture is proposed. In the simulation, the compressor is composed of one main blade and one splitter blade, two radial diffuser vanes and six axial diffuser vanes. The combustion chamber is composed of the contouring casing, the flame-tube and a T-shaped vaporizer. This integrated computation considers a good trade-off between accuracy of the simulation and affordable CPU cost. Results are compared between the stand-alone combustion chamber simulation and the integrated one in terms of global, integral and average quantities. It is shown that pressure perturbations generated by the interaction of the impeller blades with the diffuser vanes are propagated through the axial diffuser and enter the combustion chamber through the dilution holes and the vaporizer. Due to the high amplitude of the pressure perturbations, several variables are perturbed at the blade-passing frequency and multiples. This is also reflected on combustion where two broadband peaks appear for the global heat release.

Film cooling is commonly used to protect turbine vanes and blades from the hot gases produced in the combustion chamber. The design and optimization of these systems can however only be achieved if a precise prediction of the fluid mechanics and film efficiency is guaranteed at a level where induced losses are fully mastered. Such a prerequisite induces at the numerical level to be able to identify and assess losses. In this context, the present study addresses loss assessment in a wall-resolved Large Eddy Simulation (LES) of the film-cooled high-pressure turbine blade cascade T120D from the European project AITEB II. The objectives are twofolds: (1) to evaluate the capacity of LES to predict adiabatic film cooling effectiveness in a mastered academic case; and (2) to investigate loss generation mechanisms in a fully anisothermal configuration. When it comes to LES predictions of T120D, the flow structure around the blade and the coolant jet organization are coherent with literature findings. Satisfactory agreements are furthermore retrieved for the pressure load prediction as well as the adiabatic film effectiveness if compared to the experiment. Loss generation is then investigated illustrating the fact that aerodynamics losses dominate mixing losses which are mainly located in the coolant film. This is in line with the temperature difference between the hot and coolant flows that is low for this experimental condition. Distinct contributions can however be made available by studying the local loss generation maps by means of Second Law Analysis if recast in the specific context of anisothermal flows when simulated by LES.

The use of numerical simulations to design and optimize turbine vane cooling requires precise prediction of the fluid mechanics and film cooling effectiveness. This results in the need to numerically identify and assess the various origins of the losses taking place in such systems and if possible in engine representative conditions. Large-Eddy Simulation (LES) has shown recently its ability to predict turbomachinery flows in well mastered academic cases such as compressor or turbine cascades. When it comes to industrial representative configurations, the geometrical complexities, high Reynolds and Mach numbers as well as boundary condition setup lead to an important increase of CPU cost of the simulations. To evaluate the capacity of LES to predict film cooling effectiveness as well as to investigate the loss generation mechanisms in a turbine vane in engine representative conditions, a wall-modeled LES of the FACTOR film-cooled nozzle is performed. After the comparison of integrated values to validate the operating point of the vanes, the mean flow structure is investigated. In the coolant film, a strong turbulent mixing process between coolant and hot flows is observed. As a result, the spatial distribution of time-averaged vane surface temperature is highly heterogeneous. Comparisons with the experiment show that the LES prediction fairly reproduces the spatial distribution of the adiabatic film effectiveness. The loss generation in the configuration is then investigated. To do so, two methodologies, i.e, performing balance of total pressure in the vanes wakes as mainly used in the literature and Second Law Analysis (SLA) are evaluated. Balance of total pressure without the contribution of thermal effects only highlights the losses generated by the wakes and secondary flows. To overcome this limitation, SLA is adopted by investigating loss maps. Thanks to this approach, mixing losses are shown to dominate in the coolant film while aerodynamic losses dominate in the coolant pipe region.

Turbulent ribbed channels are extensively used in turbomachinery to enhance convective heat transfer in internally-cooled components like turbine blades. One of the key aspect of such a problem is the distribution of the Heat Transfer Coefficient (HTC) in fully developed flows and many studies have addressed this problem by use of Computational Fluid Dynamics (CFD). In the present document, Large Eddy Simulation (LES) is performed for a configuration from a test-rig at the Von Karman Institute representing a square channel with periodic square ribs. The whole channel is computed in an attempt to better understand HTC maps in this specific configuration. Resulting mean and unsteady flow features are captured and predictions are used to further explain the obtained HTC distribution. More specifically turbulent structures are seen to bring cold gas from the main flow to the wall. A statistical analysis of these events using the joint velocity-temperature PDF and quadrant method allows to define 4 types of events happening at every location of the channel and which can then be linked to the HTC distribution. First the HTC is very high where the flow impacts the wall with cold temperature whereas it is lower where the hot gas is ejected to the main flow. In an attempt to link the HTC trace on the channel wall with structures in the flow field far-off the wall, the main modes are identified performing Power Spectral Density (PSD) analysis of the velocity along the channel. Dynamic Mode Decomposition (DMD) of the flow field data is then used to present the spatio-temporal characteristics of two of the identified most dominant modes: a vortex-street mode linked to the first rib and a rib-to-rib mode appearing because of the quasi-periodicity of the configuration. However DMD analysis of the HTC trace on the wall does not emphasize any dominant mode. This indicates a weak link between the main flow large scale features and the instantaneous and more local HTC distribution.

Global warming, climate change and pollution are burning environmental issues. To reduce the carbon footprint of the aviation sector, aeronautical companies have been striving to lower engine emissions via the development of reliable lean combustors. In this context, effort has been devoted to the better understanding of various flame dynamics with emphasis on thermoacoustic instabilities, lean blow-off and extinctions. In line with this effort, Safran Helicopter Engines has recently developed and patented the revolutionary spinning combustion technology (SCT) for its next generation of combustors. This technology has indeed great flexibility when it comes to ignition and blow-off capabilities. To better understand the various physical mechanisms occurring in a SCT combustor, a joint numerical and experimental analysis of the flame stabilization in this spinning combustion technology framework has been devised. On the experimental side, the NTNU atmospheric annular combustor has been modified to introduce a relevant azimuthal component of velocity while operating under premixed fuel conditions, following the SCT concept. Note that to reduce temperature at the backplane of the chamber, film cooling is incorporated to avoid fuel injector damage. On the numerical side, high fidelity Large Eddy Simulations of the test bench have been carried out with the AVBP code developed at CERFACS, providing insights on the flame stabilization in this unique SCT geometry. In particular, it is noted that there is a strong interaction between the cooling film and the highly swirled flames exiting from the fuel injector bend. In that respect, changing the injector or global equivalence ratios while operating the SCT is shown to affect the combustion of this design.

Film cooling is a common technique to manage turbine vane and blade thermal environment. Optimizing its cooling efficiency is furthermore an active research topic which goes in hand with a strong knowledge of the flow associated with a cooling hole. The following paper aims at developing deeper understanding of the flow physics associated with a standard cooling hole and helping guide future cooling optimization strategies. For this purpose, Large Eddy Simulations (LES) of the 7-7-7 fan-shaped cooling hole [1] is performed and the flow inside the cooling hole is studied and discussed. Use of mathematical techniques such as the Fast Fourier Transforms (FFT) and Dynamic Mode Decomposition (DMD) is done to quantitatively access the flow modal structure inside the hole based on the LES unsteady predictions. Using these techniques, distinct vortex features inside the cooling hole are captured. These features mainly coincide with the roll-up of the internal shear layer formed at the interface of the separation region at the hole inlet. The topology of these vortex features is discussed in detail and it is also shown how the expansion of the cross-section in case of shaped holes aids in breaking down these vortices. Indeed upon escaping, these large scale features are known to not be always beneficial to film cooling effectiveness.

Understanding the flow from a cooling hole is very important to be able to properly control film cooling of turbine blades. For this purpose, large eddy simulation (LES) investigation of the flow inside a cylindrical film cooling hole is presented in this paper. Two different geometries, with different hole metering lengths, are investigated at a blowing ratio of 0.5. The main flow structure in the hole are the hairpin vortices that originate from a shear layer formed due to flow separation near the hole entry. The comparison of these hairpin vortices in the two cases with different hole metering length is presented in detail. The results show that in case of the hole with longer length the hairpin vortices dissociate within the hole itself. In such a case a uniform flow is seen at the hole exit. However, when the hole length is significantly decreased, it is shown that these vortices exit the hole and effect the vortex structures outside the hole, thereby accounting for the reduction in film cooling effectiveness. Overall, these results bring forth one other major reason for the reduction in film cooling effectiveness with reduction in hole length, i.e. the exit of in-hole hairpin vortices into the crossflow.

Inlet an outlet boundary conditions are essential elements of any CFD predictions and this is even more so for turbomachinery Large Eddy Simulations, either applied to academic or industrial configurations. For compressible solvers, non-reflecting, characteristic inlet boundary condition imposing total pressure, total temperature and flow direction is usually needed, while an outlet relaxation methodology that automatically adapts the outlet static pressure as a function of the desired mass-flow rate rate is used for turbomachinery flow predictions. Establishing such a framework is clearly desirable especially for industrial use of LES. Development and validations remain necessary in such a fully unsteady context as detailled hereafter.