A combined numerical and experimental study of the 3D tumble structure and piston boundary layer development during the intake stroke of a gasoline engine

Due to its positive effect on flame propagation in the case of a well-defined breakdown, the formation of a large-scale tumble motion is an important goal in engine development. Cycle-to-cycle variations (CCV) in the tumble position and strength however lead to a fluctuating tumble breakdown in space and time and therefore to combustion variations, indicated by CCV of the peak pressure. This work aims at a detailed investigation of the large-scale tumble motion and its interaction with the piston boundary layer during the intake stroke in a state-of-the-art gasoline engine. To allow the validation of the flow near the piston surface obtained by simulation, a new measurement technique called “Flying PIV” is applied. A detailed comparison between experimental and simulation results is carried out as well as an analysis of the obtained flow field. The large-scale tumble motion is investigated based on numerical data of multiple highly resolved intake strokes obtained using scale-resolving simulations. A method to detect the tumble center position within a 3D flow field, as an extension of previously developed 2D and 3D algorithms, is presented and applied. It is then used to investigate the phase-averaged tumble structure, its characteristics in terms of angular velocity and the CCV between the individual intake strokes. Finally, an analysis is presented of the piston boundary layer and how it is influenced by the tumble motion during the final phase of the intake stroke

On the behavior of a large scale tumbling vortex flow submitted to compression

An analysis of the disruption of a compressed tumbling vortex is proposed here. This situation is typical of in-cylinder flows developing in combustion chambers of premixed spark ignition engines. In an inviscid fluid, we discuss analytically the motion of a point vortex of constant intensity submitted to compression in a time dependant rectangular domain. A relevant non-dimensional number comparing the vortex time scale and the compression time scale is obtained. We show that the compression unsteadiness is expected to delay the viscous response of the near-wall flow. The temporal evolution of a tumbling motion is then measured with high-speed particle image velocimetry in a model square piston compression machine. The experimental set-up and the measurement procedure are presented. The data provide a clear picture of the evolution and disruption of the compressed vortex. A physical analysis is proposed and we prove that elliptical instability and interaction with walls are the main mechanisms that lead to the generation of a three-dimensional unstructured velocity field

Aeroacoustic performance of fractal spoilers

One of the major environmental problems facing the aviation industry is that of aircraft noise. The work presented in this paper, done as part of the European Union’s Optimisation for Low Environmental Noise Impact Project, looks at reducing spoiler noise while maintaining aerodynamic performance, through means of large-scale fractal porosity. It is hypothesized that the highly turbulent flow generated by fractal grids from the way the multiple-length scales are organized in space, would reduce the impact of the recirculation region and, with it, thelow-frequency noise it generates. In its place, a higher frequency noise is introduced, which is more susceptible to atmospheric attenuation and is less offensive to the human ear. A total of nine laboratory-scaled spoilers were looked at, seven of which had a fractal design, one with a regular grid design, and one solid for reference. The spoilers were inclined at an angle of 30 deg. Force, acoustic, and flow visualization experiments on a flat plate were carried out and it was found that the present fractal spoilers reduce the low-frequency noise by 2.5 dB. Results show that it is possible to improve the acoustic performance by modifying a number of parameters defining the fractal spoiler, some of them very sensitively. From these experiments, two fractal spoilers were chosen for a detailed aeroacoustic study on a three element wing system, where it was found that the fractal spoilers had a reduction of up to 4 dB in the sound pressure level, while maintaining similar aerodynamic performances as conventional solid spoilers on the measured wing syste

Drag reduction using longitudinal vortices on a flat-back Ahmed body

AbstractThe paper presents different cavity wall designs at the rear of a flat-back Ahmed body to achieve drag reduction. The wake balance is assessed using the variance of the base pressure gradient and correlated to the obtained drag reduction. Without the rear cavity, the flat-back Ahmed body is subject to a steady instability producing a strong wake imbalance in the horizontal direction. The low drag rear design consists of a $$5^\circ$$ 5 ∘ inclined spoiler at the bottom (refered to as a diffuser) that first provokes a vertical imbalance and a non-inclined top spoiler with central imprints that re-balance the wake. The drag reduction is 11.5%, the wake structure is observed to respect the symmetry of the body base in both lateral and vertical directions suggesting a full stabilization of the initial steady instability. Both the wake balance and the stabilization are associated with the formation of a longitudinal vortex pair initiated at the imprint edges and penetrating the recirculating area.</jats:p

Flow sources of wall pressure fluctuations resolved by DDES in a reattaching flow region

International audienceThis paper is concerned with the analysis of the flow structures responsible for the wall pressure fluctuations predicted by DDES near a reattachment. To this aim, a simulation of the flow over a thick plate is considered. It observed that the linear part of sources is dominant upstream of the reattachment, and that quadratic sources dominate downstream. This is associated with the rapid loss of coherence of the vortices shed from the separated shear layer in the downstream region. Furthermore, it is shown that linear stochastic estimation of the velocity field from the wall pressure well reproduce the linear sources, with negligible quadratic sources

Representativeness of geometrical details during wind tunnel tests. Application to train aerodynamics in crosswind conditions

International audienceThis work aims to highlight the influence of the scale of reproduction of geometric details on models during wind tunnel tests. An application is presented concerning the reproduction of high voltage lines present on the roof of trains. It is shown that the presence of these geometric artefacts deeply modifies the forces and moments measured under cross-wind. It is also shown that the reproduction of geometric details with the scale of the model is not a sufficient condition to ensure a good representativeness of the tests. It is necessary to ensure that the changes in the flow dynamics caused by the presence of the details are correctly reproduced (separation-reattachment process on the roof of the train in this study). The influence of the scale of reproduction of the geometrical details is also studied and a method is proposed to extrapolate the results obtained without roof-lines in order to predict the rolling moment in the presence of roof-lines

Forcing three-dimensional large-scale flow asymmetries in the wake of blunt body : wake equilibrium and drag reduction

International audienceWe experimentally study the effect of high-frequency fluidic forcing on variousasymmetric turbulent wakes behind a blunt body at ReH = 5 .105. Different wake asymmetriesare obtained by the use of passive perturbations around the body modifying the wakeequilibrium. High-frequency forcing of the wake along the rear edges is then applied toincrease the base pressure. Depending on the initial unforced wake vertical equilibrium,localized forcing along the top or bottom edges may strongly increase or decrease the basepressure. The efficiency of spatially localized forcing and the fluidic boat-tailing thus createdis linked to the global wake equilibrium and its modificatio