乱流予混合火炎のDirect Numerical Simulation

The unsteady flowfields generated by convoluted aeroengine intakes are sources of instabilities that can compromise the performance of the downstream turbomachinery. Hence, there is a need for synchronous high-spatial-resolution measurements that will allow a greater understanding of the aerodynamics. In this work, stereo particle image velocimetry is used in a new application to characterize the distorted flow at the outlet of complex intakes. A suite of measurements and analyses for two S-duct configurations across a range of inlet Mach numbers is presented. The work demonstrates the feasibility of using stereo particle image velocimetry techniques for determining the flowfield at the exit of convoluted intakes with a higher spatial resolution than typical pressure measurements. Analysis of the distortion descriptors quantifies the dependency upon the S-duct configuration and highlights that the more high-offset duct generates greater levels of swirl distortion relative to the low-offset configuration. ...

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Flow Distortion Measurements in Convoluted Aeroengine Intakes

For many embedded engine systems, the intake duct geometry introduces flow distortion and unsteadiness, which must be understood when designing the turbomachinery components. The aim of this work i...

/pdf/complex-aeroengine-intake-ducts-and-dynamic-distortion-333z63j593.pdf

Complex Aeroengine Intake Ducts and Dynamic Distortion

This paper presents an experiment of flow control by supersonic pulsed jets in a very aggressive S-duct typical of modern fighter aircraft engine inlet ducts. The Mach number in the aerodynamic interface plane ranges from 0.2 to 0.4, which is realistic from an application point of view. It was shown that, with continuous blowing, the flow could be fully reattached. The sensitivity of the distortion coefficient to the jet frequency was also clearly evidenced. Surprisingly, it is recommended to avoid actuating the jets at the natural frequencies present in the separated zone of the uncontrolled flow. Besides, it is observed that pulsed jets actuation increases the unsteady flow distortion. Furthermore, in the uncontrolled flow, a lateral movement of the structures associated with high total pressure losses in the aerodynamic interface plane was highlighted.

Flow Control by Pulsed Jet in a Curved S-Duct: A Spectral Analysis

A computational large eddy simulation (LES) study is presented of the interaction between a turbulent boundary layer separating from a rounded ramp in a duct and a pair of spanwise-periodic, round synthetic jets, actuated upstream of the nominal separation line. Several scenarios are considered, for different injection angles and velocity ratios. In all cases, the actuation frequency corresponds to the shedding-instability mode of the separated shear layer. Experimental data, available for both the baseline flow and one actuated configuration, are used to verify the validity of the computational solutions. The analysis includes a separation of coherent and stochastic contributions to the time-averaged statistics of the auto- and cross-correlations of the fluctuations. The control authority is examined by reference to the effects of the actuation on the size of the separated zone, the momentum thickness of the boundary layer, the velocity field, various turbulence quantities and phase-averaged properties. The study demonstrates that the principal aspect of the interaction, at mean-flow level, is an increase in mixing provoked by the formation of strong streamwise vortices away from the wall, the induction of much weaker streamwise vortices close to the wall, and the extra production of stochastic turbulence caused by unsteady straining. The coherent stresses arising from the periodic perturbations are high – typically 5 times the levels of the unperturbed flow – but only within about 5–7 diameters of the jet orifice, and 2 orifice diameters on each side of the jet, and these are dominant primarily in the outer parts of the boundary layer. Stochastic turbulence is also elevated, but more modestly. The global effect of the actuation is a reduction of 10–20 % in the length of the separated region and 20–40 % in the thickness of the reverse-flow layer, depending on the actuation scheme, counter-flow actuation being the most effective. This reduction is mainly associated with a delay in separation. These results highlight the need for synthetic jets to be placed close to the separation zone and for the inter-jet distance to be of order 5 or lower to achieve a high level of separation-control authority.

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The interaction of round synthetic jets with a turbulent boundary layer separating from a rounded ramp

This paper presents an experiment of flow control by pulsed jets in an S-duct. One of the novelties of this study comes from the fact that PIV has been employed to characterize the flow in the duct. Specific techniques have been used to deal with optical accesses and particle seeding issues. The Mach number in the Aerodynamic Interface Plane ranges from 0.2 to 0.45. This range is realistic from an application point of view and it has necessitated the design of supersonic pulsed jet actuators. Finally, it is shown that the flow can be fully reattached even in a very aggressive duct. A sensitivity of the distortion coefficient to the jet frequency has been clearly identified. Surprisingly, it is recommended to avoid using natural frequencies present in the separated zone of the uncontrolled flow.

Flow Control By Pulsed Jet In A Highly Bended S-Duct

A high-fidelity simulation of the shock/transitional boundary layer interaction caused by a axisymmetrical compression ramp is performed at a free stream Mach number of 5 and a transitional Reynolds number. The inlet of the computational domain is perturbed with a white noise in order to excite convective instabilities. Coherent structures are extracted using spectral proper orthogonal decomposition (SPOD), which gives a mathematically optimal decomposition of spatio-temporally correlated structures within the flow. The mean flow is used to perform a resolvent analysis in order to study non-normal linear amplification mechanisms. The comparison between the resolvent analysis and the SPOD results provides insight on both the linear and nonlinear mechanisms at play in the flow. To carry out the analysis, the flow is separated into three main regions of interest: the attached boundary layer, the mixing layer and the reattachment region. The observed transition process is dependent on the linear amplification of oblique modes in the boundary layer over a broad range of frequencies. These modes interact nonlinearly to create elongated streamwise structures which are then amplified by a linear mechanism in the rest of the domain until they break down in the reattachment region. The early nonlinear interaction is found to be essential for the transition process.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112020008332

Transition scenario in hypersonic axisymmetrical compression ramp flow

A high-fidelity simulation of the shock/transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a freestream Mach number of 5 and a transitional Reynolds number. The inlet of the computational domain is perturbed with a white noise in order to excite convective instabilities. Coherent structures are extracted using Spectral Proper Orthogonal Decomposition (SPOD), which gives a mathematically optimal decomposition of spatio-temporally correlated structures within the flow. The mean flow is used to perform a resolvent analysis in order to study non-normal linear amplification mechanisms. The comparison between the resolvent analysis and the SPOD results provides insight on both the linear and non-linear mechanisms at play in the flow. To carry out the analysis, the flow is separated into three main regions of interest: the attached boundary layer, the mixing layer and the reattachment region. The observed transition process is dependent on the linear amplification of oblique modes in the boundary layer over a broad range of frequencies. These modes interact nonlinearly to create elongated streamwise structures which are then amplified by a linear mechanism in the rest of the domain until they break down in the reattachment region. The early nonlinear interaction is found to be essential for the transition process.

A microscale low power high temperature gradient calorimetric (HTGC) sensor measuring both mean and fluctuating bidirectional wall shear stress is presented. The micromachined sensor is composed of three free-standing $$3\,\upmu \text {m}\times 1$$
  mm micro-wires mechanically supported using perpendicular micro-bridges. The static and dynamic characterisations were performed in a turbulent boundary layer wind tunnel on a flat plate configuration, and compared to the one obtained with a conventional hot-film probe. The results demonstrated that the calorimetric sensor behaves similarly to the hot-film in constant temperature anemometry with nonetheless lower power consumption and better spatial resolution and temporal response. Additionally, its calorimetric measurement detected the direction of the wall shear stress component orthogonal to the wires, corresponding to the shear stress sign in 2D flows. The calibrated HTGC micro-sensor was then used for unsteady flow separation detection downstream a 2D square rib for $$Re_h= 2.56\times 10^4$$
 . The calorimetric micro-sensor enabled self-correlated measurements and consequently successfully achieved the detection of flow separation and the reattachment point around $$x/h=10.7$$
 .

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Unsteady flows measurements using a calorimetric wall shear stress micro-sensor

Large eddy simulations have been conducted to gain further insight into the drag-reducing mechanisms of riblets in zero-pressure gradient turbulent boundary layer. The retained groove geometry achieves 9.8 % drag reduction on the controlled zone developing from $Re_{\theta } = 670$ to 975. It is shown that the turbulent contribution to the drag—as defined by Fukagata et al. Phys. Fluids, 14(11):L73, 2002 [7]—is the most affected. In the light of the obtained results, energy and enstrophy budgets will finally conduct to isolate a key mechanism involved in the riblets drag reduction.

https://hal.archives-ouvertes.fr/hal-01087636/document

Eric Garnier

Papers

Flow Control By Pulsed Jet In A Highly Bended S-Duct

Transition scenario in hypersonic axisymmetrical compression ramp flow

Transition scenario in hypersonic axisymmetrical compression ramp flow

Unsteady flows measurements using a calorimetric wall shear stress micro-sensor

Riblets Induced Drag Reduction on a Spatially Developing Turbulent Boundary Layer