Towards Primary Breakup Simulation of a Complete Aircraft Nozzle at Realistic Aircraft Conditions

Predicting details of aircraft engine combustion by means of numerical simulations requires reliable information about spray characteristics from liquid fuel injection. However, details of liquid fuel injection are not well documented. Indeed, standard droplet distributions are usually utilized in Euler-Lagrange simulations of combustion. Typically, airblast injectors are employed to atomize the liquid fuel by feeding a thin liquid film in the shear zone between two swirled air flows. Unfortunately, droplet data for the wide range of operating conditions during a flight is not available. Focusing on numerical simulations, Direct Numerical simulations (DNS) of full nozzle designs are nowadays out of scope. Reducing numerical costs, but still considering the full nozzle flow, the embedded DNS methodology (eDNS) has been introduced within a Volume of Fluid framework (Sauer et al., Atomization and Sprays, vol. 26, pp. 187–215, 2016). Thereby, DNS domain is kept as small as possible by reducing it to the primary breakup zone. It is then embedded in a Large Eddy Simulation (LES) of the turbulent nozzle flow. This way, realistic turbulent scales of the nozzle flow are included, when simulating primary breakup. Previous studies of a generic atomizer configuration proved that turbulence in the gaseous flow has significant impact on liquid disintegration and should be included in primary breakup simulations (Warncke et al., ILASS Europe, Paris, 2019). In this contribution, an industrial airblast atomizer is numerically investigated for the first time using the eDNS approach. The complete nozzle geometry is simulated, considering all relevant features of the flow. Three steps are necessary: 1. LES of the gaseous nozzle flow until a statistically stationary flow is reached. 2. Position and refinement of the DNS domain. Due to the annular nozzle design the DNS domain is chosen as a ring. It comprises the atomizing edge, where the liquid is brought between inner and outer air flow, and the downstream primary breakup zone. 3. Start of liquid fuel injection and primary breakup simulation. Since the simulation of the two-phase DNS and the LES of the surrounding nozzle flow are conducted at the same time, turbulent scales of the gas flow are directly transferred to the DNS domain. The applicability of eDNS to full nozzle designs is demonstrated and details of primary breakup at the nozzle outlet are presented. In particular a discussion of the phenomenological breakup process and spray characteristics is provided.

Assessment of turbulent scales over complex terrain in central Spain

<p>In this work, a micrometeorological assessment of Atmospheric Boundary Layer paremeters is carried out in order to determine the characteristic turbulent scales over complex terrain in the Sierra de Guadarrama, a range in central Spain. Observational data series of temperature and wind velocity measured at high frecuency (10 Hz) are available. These data come from two different stations located in the Bosque de La Herrería and belonging to GuMNet (2020) (Guadarrama Monitoring Network).</p><p>Integral scales, both time and spatial, have been determined for different atmospheric conditions, defined by parameters such as wind direction or stability of stratification. Also, energy cascade phenomenon occurence is assessed. In order to carry this out, different time series analysis tools are used, such as autocorrelation functions in time, and normalised power spectra or wavelets. Results obtained are compared with previous works.</p><p>In general, results show that under no synoptic forcing there is a clear dependency on diurnal cycle, giving rise to the development of big integral scales at nighttime, while they are small during the day. When synoptic forcing prevails, the scales are also small, both at daytime and nighttime. Moreover, a correlation patterns method has been implemented for scales obtained at two different heights (4 and 8 meters) on the one hand and at two locations on the other. In the first case, integral scales are highly correlated, exceeding the threshold of 0.5. In the second case, temporal scales show high correlation values, but spatial ones do not.  In addition, the slopes of the spectra in the inertial subrange have  been obtained and compared to those over homogeneous terrain (Kaimal et al., 1972), getting similar results for velocity turbulent components but not in case of vertical kinematic momentum and heat fluxes.</p><p> </p><p><strong>References</strong></p><p>GuMNet: Guadarrama Monitoring Network (UCM), https://www.ucm.es/gumnet/, 2020.</p><p>Kaimal, J. C., Wyngaard, J. C., Izumi, Y., and Coté, O. R.: Spectral characteristics of surface- layer turbulence, Quart. J. R. Met. Soc., 98, 563–589, 1972.</p>

LES-based simulation of the time-resolved flow for rotor-stator interactions in axial fan stages

Purpose The purpose of this paper is the development of a CFD methodology based on LES computations to analyze the rotor–stator interaction in an axial fan stage. Design/methodology/approach A wall-modeled large eddy simulation (WMLES) has been performed for a spanwise 3D extrusion of the central section of the fan stage. Computations were performed for three different operating conditions, from nominal (Q_N) to off-design (85 per cent Q_N and 70 per cent Q_N) working points. Circumferential periodic conditions were introduced to reduce the extent of the computational domain. The post-processing procedure enabled the segregation of unsteady deterministic features and turbulent scales. The simulations were experimentally validated using wake profiles and turbulent scales obtained from hot-wire measurements. Findings The transport of rotor wakes and both wake–vane and wake–wake interactions in the stator flow field have been analyzed. The description of flow separation, particularly at off-design conditions, is fully benefited from the LES performance. Rotor wakes impinging on the stator vanes generate a coherent large-scale vortex shedding at reduced frequencies. Large pressure fluctuations in the stagnation region on the leading edge of the vanes have been found. Research limitations/implications LES simulations have shown to be appropriate for the assessment of the design of an axial fan, especially for specific operating conditions for which a URANS model presents a lower performance for turbulence description. Originality/value This paper describes the development of an LES-based simulation to understand the flow mechanisms related to the rotor–stator interaction in axial fan stages.

The Effect of Turbulent Scales on Low-Pressure Turbine Aerodynamics: Part B — Scale Resolving Simulations

Turbulence contains a wide range of scales which form the turbulent spectrum. In low-pressure turbines (LPT) these scales of the turbulent free-stream influence large-scale mixing, the decay of turbulence kinetic energy and the transition of boundary layers through their reception of the small scales. Although these mechanisms are known in principle, the effect of turbulent scales on LPT aerodynamics has not been quantified and analyzed in detail yet. By means of Large Eddy Simulations (LES) applying the Incompressible Divergence-Free Synthetic Eddy Method (I-DFSEM) — introduced in Part A of this two-part paper — at the domain inlet to impose any desired turbulent boundary condition, the MTU-T161 LPT cascade is investigated under low-speed conditions. The simulations are successfully validated by the experimental results of the turbulent spectrum. In order to separate the effect of turbulence intensity and length scale on the cascade aerodynamics, the turbulent length scale is systematically varied while ensuring similar turbulence intensity at the profile’s leading edge. The results show an influence of the turbulent spectrum on separation-induced boundary layer transition. It is shown that the separated shear layer is amplified by integral length scales corresponding to frequencies close to the Kelvin-Helmholtz instability. Consequently it affects the turbulent mixing such that the transition point and lengths differ.