Air ejector znalysis in extended operational perimeter for control oriented simulation of bleed-air aircraft systems

The integration of large size Ultra High Bypass Ratio (UHBR) engines introduces new restrictions of space for their integration with the aircraft wing. Under this scenario, a reduction of the size of the bleed air system and particularly of the pre-coolers is requested. The current research aims to introduce air-air ejectors as a potential solution. A combined methodology based on experimental and CFD analyses is proposed to obtain physical insights and validation material for the development of a robust and accurate lumped model, capable of simulating the static and dynamic ejector behaviour in normal and abnormal conditions. In this paper, an overview on the performed initial activities on the dynamic aspects is provided.Peer ReviewedPostprint (published version

Direct and large-eddy simulation of turbulent wall-bounded flows : further development of a parallel solver, improvement of multiscale subgrid models and investigation of vortex pairs in ground effect

This thesis is devoted to the numerical simulation of turbulent wall-bounded flows. Their accurate prediction is still challenging because of the nature of turbulence which is characterized by a very broad range of time and length scales. Therefore, fully resolved simulations of turbulent flows, called Direct Numerical Simulations (DNS), are unaffordable for practical applications but are an important research tool, since they provide a detailed insight into the physics of turbulence at moderate Reynolds numbers. In this thesis, direct simulations are used to study the interaction of a vortex pair with a wall and, more specifically, the development of instabilities which eventually lead to the turbulent decay of the system. That study is part of an important research effort devoted to aircraft wake vortices. The Reynolds number of the DNS is however much lower than that of aircraft wake vortices. This calls for investigations at higher Reynolds numbers using the Large-Eddy Simulation (LES) technique, which consists in simulating as small turbulent structures as the grid allows while modeling the effect of the unresolved scales using a subgrid-scale model. Performing LES of wake vortices in ground effect requires a subgrid-scale model that can handle both vortical flows and wall-bounded flows. The second part of this thesis deals with the development of such models. The proposed approach is based on the Regularized Variational Multiscale (RVM) model which is known to handle properly vortical flows thanks to its good spectral behavior. It is however shown in this thesis that the existing RVM model is not suitable for wall-bounded flows. New wall-adapting multiscale models are here obtained by using wall-adapting viscosity scalings in the regularized multiscale formalism. These models are successfully assessed in a turbulent channel flow and in a more complex test case consisting in a turbulent half channel with blowing and suction, in order to generate a non-uniform pressure gradient in the streamwise direction.(FSA 3) -- UCL, 200

Modeling the law of the wake using an offset from the wall

We focus on the wake function F(Y), where Y=y/δ: the excess velocity above the logarithmic profile in wall-bounded turbulence. It is first measured using direct numerical simulation data of turbulent channel flow at Reτ≃5200, which has a distinguishable overlap layer. The function Q(Y)=YF′(Y) is seen to be significant only beyond Yc≃0.16, and also linear (with a slope α≃1.15) up to Y≃0.5. Our simplest Q(Y) model is then a linear ramp function that starts at the measured intercept. Our Q(Y) model is furthermore extended up to the channel center, by subtracting a quadratic term which satisfies the boundary condition at Y=1. The analytical integration of Q(Y) provides the model for the wake function with offset Yc, and which covers the full range; it is seen to fit very well the DNS data. Our Q(Y) model is also compared to the “extended law of the wall” model without offset. Furthermore, and to comply with some recent literature, a version of our model is also developed with some small slope α0≪α contribution to Q(Y) within the overlap layer. The DNS of a zero pressure gradient boundary layer at Reτ≃2300 is considered next: the amplitude of the wake function is much larger than in channel flow (α≃7.3) and the intercept is smaller (Yc≃0.11). The obtained F(Y) model also compares very well with the DNS data over the full range. Lastly, we also revisit the Coles wake function model and we show that adding the offset improves it significantly

A multiscale subgrid model for both free vortex flows and wall-bounded flows

A new subgrid-scale (SGS) model which has an adequate behavior in both vortical flows and wall-bounded flows is proposed. In wall-bounded flows with "wall-resolved" large eddy simulation (LES), the theory predicts that the SGS dissipation should vanish as y(+3) near the wall. In free vortex flows, one needs to have models which do not dissipate energy in the strongly vortical and essentially laminar part of the flow, e. g., in the vortex core regions. The wall adapting local eddy (WALE) viscosity model of Nicoud and Ducros (Flow, Turbul. Combust. 62, 183 (1999)] has the correct near-wall behavior. It is, however, demonstrated here that it produces values of effective eddy viscosity which are too high in vortical flows: this constitutes a major drawback for LES of vortex flows. On the other hand, the regularized variational multiscale models are suitable to simulate vortical flows as demonstrated by Cocle et al. [Complex Effects in LES (Springer, New York, 2007), p. 56], but they do not have a correct behavior in wall-bounded flows as shown by Jeanmart and Winckelmans [Phys. Fluids 19, 055110 (2007)]. The model presented here aims at combining the strengths of the two models: it is a multiscale model, thus acting on the high pass filtered LES field, and for which the SGS viscosity is evaluated using the WALE model, itself also computed using the high pass filtered field. Hence, this model is only active when there is locally a significant high wavenumber content in the flow and it has a natural near-wall damping behavior. The ability of this model to simulate vortex and wall-bounded flows is demonstrated on three test cases. The first case is the turbulent channel flow at Re-tau=395 and Re-tau=590. The second test concerns a counter-rotating four-vortex system at Re-Gamma=20 000. The third case concerns a two-vortex system in ground effect at Re-Gamma=20 000. It is shown that the model allows to perform successfully the LES of these flows with the proper dissipative behavior in both the near-wall and the vortical regions. (C) 2009 American Institute of Physics. [doi:10.1063/1.3241991

Dynamics of the Wake Vortices of a Two-aircraft Formation, Hazard Assessment at Large Distances and Sensitivity Analysis

Extended formation flight is foreseen to be a viable technique for fuel consumption reduction in commercial aviation. It is based on a follower tracking the wake of a leader while maintaining a constant adequate separation. This leads however to different wake dynamics compared to those of the wake of an isolated aircraft. The uncertainty on the relative position between the follower and the wake of the leader further increases the analysis of the underlying vortex dynamics. Using Large Eddy Simulations, this paper presents a methodology to quantify the impact of that uncertainty on the propagation of the formation wake. Midway between a vortex method and grid-based CFD, the Vortex Particle-Mesh method combined the Immersed Lifting Lines is particularly well suited for the simulation of such long-lasting structures. The proposed methodology combines 3D Space Developing and 3D Time Developing simulations to capture the roll-up of the vortex sheets originating from the leader and the follower, their mutual interactions and their long term evolution in a turbulent environment. Simulations are performed for several formation flight configurations around a reference in order to produce a probabilistic representation for the wake propagation. Statistics on the wake vortices characteristics are deduced and are compared to the wake properties of an isolated aircraft

NUMERICAL STUDY OF THE COMPLETE OPERATING MAP OF EJECTORS IN ULTRA HIGH BYPASS RATIO ENGINE BLEED SYSTEMS

New bleed systems are needed for the aircraft based on Ultra High Bypass Ratio (UHBR) engines to improve its overall efficiency, and one of the solutions is to utilize air ejectors to optimize the bleed air system. The present numerical study investigates the behavior of the ejector via entrainment ratio, limiting pressure ratios, and the performance dependency on different operating regimes. Within each regime, the ejector's performance depends on the operating conditions, such as total pressures and temperatures given at inlets and outlets. To explore the whole operating map at a fixed temperature ratio, the numerical simulations are classified according to the primary flow and the secondary flow. Combining these two classifications allows us completely to characterize the flow in the ejector. Furthermore, variation in the inlet temperatures show a prominent effect on the mass flow rates and entrainment ratio