Short title : LES, acoustics, experiments in gas turbines. AbstractThe turbulent flow within a complex swirled combustor is studied with compressible LES (Large Eddy Simulation), acoustic analysis and experiments for both cold and reacting flows. Detailed fields of axial, tangential and radial velocities (average and RMS) given by LES are compared to experimental values measured by LDV. The unsteady activity is identified using LES and acoustic tools for the whole geometry from inlet (far upstream of the swirler) to the atmosphere (far downstream of the chamber exhaust). Concerning comparisons between experiments and LES, this nose-to-tail procedure removes all ambiguities related to the effects of boundary conditions. Results for the cold flow show that the second acoustic mode at 360 Hz dominates in the plenum while a hydrodynamic mode at 540 Hz due to a Precessing Vortex Core (PVC) is found in the combustion chamber. With combustion, the PVC mode is damped and the main mode frequency dominating all unsteady activity is 500 Hz. Acoustic analysis shows that this mode is still the second acoustic mode observed in the cold flow: its frequency shifts from 360 Hz to 500 Hz when combustion is activated. More generally, these results illustrate the power of combined numerical tools (LES and acoustic analysis) to predict mean flow as well as instabilities in combustors.

International audienceThe aim of this work is to study the role of the liquid phase in the thermo-acoustic coupling which subsequently leads to combustion instabilities. Experimental investigations were performed on an actual multipoint spray injector geometry used in real aeronautical combustors. A test bench was specifically designed with continuously changeable acoustics conditions; which allows obtaining a stable or an unstable flame for identical flow conditions. Different laser-based visualization techniques were used to analyze the kerosene spray (both liquid and vapor phases) and the heat released from the flame. A phase-averaged data processing of the Planar Laser-Induced Fluorescence (PLIF) images reveals the complex unsteady behavior of the liquid phase and its coupling with pressure fluctuations in the chamber and the heat released from the flame. The origins of the spray fluctuations are also analyzed. Nomenclature = surface of the control system boundary, m 2 = volume of the control system, m 3 = heat capacity ratio = characteristic time, s dA = surface integration variable, 1 m 2 dt = time integration variable, 1 s dV = volume integration variable, 1 m 3 D REF = reference diameter F = Flame Describing Function FSP = Fuel Split Parameter GER = Global Equivalent Ratio I = light intensity over camera dynamics, # counts IEL = Inner Exhaust Length, mm J = momentum ratio air m  = air mass flow rate, g/s P f m ,  = fuel mass flow rate on the pilot system, g/s MP f m ,  = fuel mass flow rate on the multipoint system, g/s p' = acoustic pressure, Pa p = averaged pressure, Pa q' = unsteady heat release, W/m 3 Ra = local Rayleigh index T = instability cycle period, s T air = air inlet temperature, K u' = acoustic velocity, m/

The design of a clean combustion technology based on lean combustion principles will have to face combustion instability. This oscillation is often discovered late in engine development when unfortunately only a few degrees of freedom still exist to solve the problem. Individual component test rigs are usually not useful in detecting combustion instability at an early stage because they do not have the same acoustic boundary conditions as the full engine. An example of this unsteady activity phenomenon observed during the operation of a high-pressure core is presented and analyzed. To support the investigation work, two numerical tools have been extensively used: (1) experimental measurement of unsteady pressure and the results of a multidimensional acoustic code are used to confirm that the frequency variations of the observed modes within the operating domain of the high pressure core are due to the excitation of the first and second azimuthal combustor modes. The impact of acoustic boundary conditions for the combustor exhaust is shown to control the appearance and mode transition of this unsteady activity. (2) 3D reacting and nonreacting Large Eddy Simulations (LES) for the complete combustor and for the injection system cup alone suggest that the aerodynamic instability of the flow passing through the cup could be the noise source exciting the azimuthal acoustic modes of the chamber. Based on these results, the air system (cup) was re-designed in order to suppress this aerodynamic instability and experimental combustion tests confirm that the new system is free of combustion instability.

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The scour at bridge foundations caused by supercritical flows is reviewed and knowledge gaps are analyzed focusing on the flow and scour patterns, available measuring techniques for the laboratory and field, and physical and advanced numerical modeling techniques. Evidence suggests that the scour depth caused by supercritical flows is much smaller than expected, by an order of magnitude compared to that found in subcritical flows, although the reasons for this behavior remain still unclear. Important questions on the interaction of the horseshoe vortex with the detached hydraulic-jump and the wall-jet flow observed in supercritical flows arise, e.g., does the interaction between the flow structures enhance or debilitate the bed shear stresses caused by the horseshoe vortex? What is the effect of the Froude number of the incoming flow on the flow structures around the foundation and on the scour process? Recommendations are provided to develop and adapt research methods used in the subcritical flow regime for the study of more challenging supercritical flow cases.Large-scale supercritical free-surface flows can occur in different environments. Some examples can be found in cases of flooded urban streets, fish-ways, tsunami inland flows, coastal channels, and mountain rivers. This paper focuses on the flow and scouring patterns at bridge foundations in rivers with supercritical conditions. The occurrence of supercritical flows in rivers is defined by high longitudinal slopes (>1%) and/or rapid flood waves. Commonly, steep rivers present gravel beds or mixtures of fine and coarse sediments, containing all possible sizes, from clay and silt up to boulders tens of centimeters in size. In dentritic networks, streams with a low Strahler's order (i.e., <3) are steep and produce flash floods but normally possess a small cross-sectional width. Therefore, deck bridges without foundations in these riverbeds are usually selected. At piedmont, however, rivers widen and it is common to observe cross sections with widths over 50 m, where bridge foundations may have to be included. Salient examples of such configurations are often encountered in steep watersheds subjected to heavy rains, such as on the Panamericana Route along Perú and Chile, La Réunion Island (Indian Ocean) in Taiwan or Japan, and also in a few European Alpine piedmont rivers (Figure 1). The examples in Figure 1 clearly highlight that supercritical flows are associated with a significant amount of energy for scouring and dynamic loading of the superstructure. Wood debris can also enhance the risk of pier stability. Such flows thus produce among the worst hydraulic conditions for bridge design. A recent bridge collapse due to scour in a supercritical flow occurred at the Rivière Saint Etienne in the La Réunion island due to cyclone Gamède. This bridge, which connected a road with traffic of 65,000 vehicles per day, collapsed and modified the terrestrial transport route for a long time (Figure 1f,g), thereby producing large economic losses. This event motivated, i...