Investigation of unsteady combustion of methane-air flame in a model combustion chamber with swirling flow

The present paper reports on the investigation of unsteady combustion of a methane-air mixture, including combustion at increased pressure in the combustion chamber and increased temperature of mixture heating for a model gas-turbine swirl burner based on a design by Turbomeca. To measure the velocity and OH fluorescence fields in the flows a combination of stereoscopic PIV and acetone PLIF systems is used. In all cases, the flow dynamics is associated with the movement of large-scale vortex structures in the inner and outer mixing layers and the flow structure corresponds to a swirling jet with a central recirculation zone containing combustion products. An increase in the heating temperature of the mixture and pressure in the combustion chamber leads to a periodic partial separation of the flame from the model swirl nozzle. However, the flow of fuel through the central channel will stabilize the flame.

The Emergence and Identification of Large-Scale Coherent Structures in Free Convective Flows of the Rayleigh-Bénard Type

The revealing of the turbulence archetypes is one of the fundamental problems in the study of turbulence, which is important not only from the fundamental point of view but also for practical applications, e.g., in geophysics of ocean and lakes. The paper is devoted to the study of the emergence of coherent structures and the identification of their turbulent archetypes, typical for the free convective flows of the Rayleigh-Bénard type. Using Direct Numerical Simulation, we perform a numerical study of two refined convective flows: convection in a cylinder heated from below and internally heated convection in a layer. The main purpose of the study is identifying coherent structures (CS), investigating its main features and properties, and determining the turbulence archetypes using the anisotropy invariant map (AIM). We show that, in both configurations considered, CS takes place. In a cylinder, CS is a single large-scale vortex that can rotate azimuthally in non-titled container, but is almost “fixed” in the case of slightly tilted cylinder; in a layer, CS is a quasi-2D vortex, which can arise, exist for some time, disrupt, and then re-emerge again in the orthogonal direction. Nevertheless, the turbulence archetypes represented by the AIM are quite similar for both cases, and there are the distinct CS fingerprints on AIM.

Flexible Flaps Inspired by Avian Feathers Can Enhance Aerodynamic Robustness in low Reynolds Number Airfoils

Unlike rigid rotors of drones, bird wings are composed of flexible feathers that can passively deform while achieving remarkable aerodynamic robustness in response to wind gusts. In this study, we conduct an experimental study on the effects of the flexible flaps inspired by the covert of bird wings on aerodynamic characteristics of fixed-wings in disturbances. Through force measurements and flow visualization in a low-speed wind tunnel, it is found that the flexible flaps can suppress the large-scale vortex shedding and hence reduce the fluctuations of aerodynamic forces in a disturbed flow behind an oscillating plate. Our results demonstrate that the stiffness of the flaps strongly affects the aerodynamic performance, and the force fluctuations are observed to be reduced when the deformation synchronizes with the strong vortex generation. The results point out that the simple attachment of the flexible flaps on the upper surface of the wing is an effective method, providing a novel biomimetic design to improve the aerodynamic robustness of small-scale drones with fixed-wings operating in unpredictable aerial environments.

Thermal Aeroelastic Characteristics of Inflatable Reentry Vehicle Experiment (IRVE) in Hypersonic Flow

The inflatable reentry vehicle provides a new technical way in aerospace entry, descent, and landing. The structural failure of inflatable reentry vehicle experiment caused by thermal aeroelastic effect is serious, which needs to be further studied. A traditional numerical method about flexible vehicles separates the aeroheating and aeroelastic problems, resulting in poor matching with the actual test. In this paper, a thermal-fluid-solid coupling model considering inflation gas effect was established, which associates the aeroheating and aeroelastic modules and adopts the LES to improve the depicting ability of hypersonic flow. The model was used to solve the thermal aeroelastic characteristics under extreme aeroheating load. From aeroheating results, the large-scale vortex on windward generated by the interaction of the shock layer and boundary layer has great influence on aeroheating due to the heat dissipation, and the skin deformation also increases the surface friction and local heating near depressions. From aeroelastic analysis, the flexible structure performs violent forced vibration induced by the unsteady large-scale vortex on windward, and the aeroheating effect will significantly increase the thermal stress and natural vibration properties. The thermal-fluid-solid coupling method for the flexible structure proposed in this paper provides a reasonable reference for engineering.

Diagnosis of Pre-Depression Large-Scale Vortex Instability in the Tropical Atmosphere

<p>An approach is proposed [1] for determining the precise time of the start of tropical cyclogenesis, which includes a combined analysis of data from cloud-resolving numerical modeling and GOES Imagery. The approach is based on the similarity of patterns in the fields of vertical helicity (numerical simulation) and temperature (satellite data), allowing for the localization of intense rotating convective clouds known as the Vortical Hot Towers. As a theoretical ground, we applied a hypothesized (to date) interpretation of tropical cyclogenesis as a large-scale instability caused by the mechanism of the turbulent vortex dynamo in the atmosphere [1,2], and with bearing in mind the crucial role of Vortical Hot Towers in providing the dynamo-effect [2]. In this context, birth of a hurricane is considered as an extreme threshold event in the helical atmospheric turbulence of a vorticity-rich environment of a pre-depression cyclonic recirculation zone. Helical turbulence is characterized by the broken mirror symmetry and permits an existence of inverse energy cascade in three-dimensional cases. In order to trace and analyze processes of self-organization in the tropical atmosphere, that span scales from convective clouds with horizontal dimensions of 1-5 km to mesoscale vortices of hundreds of kilometers, we used the post-processing [1-3] of data from cloud-resolving numerical simulations [4].  Implementation of the proposed approach revealed that large-scale vortex instability can begin a few hours, or even dozens of hours, before the formation of the Tropical Depression. This work was supported by the research project “Monitoring” No. 01200200164.</p><p>References</p><p>[1] Levina, G. V., 2020. Birth of a hurricane: early detection of large-scale vortex instability. J. Phys.: Conf. Ser., <strong>1640  </strong>012023,  doi:10.1088/1742-6596/1640/1/012023</p><p>[2] Levina, G. V., 2018. On the path from the turbulent vortex dynamo theory to diagnosis of tropical cyclogenesis. Open J. Fluid Dyn., <strong>8,</strong> 86–114,  https:<strong>//</strong>doi.org/10.4236/ojfd.2018.81008</p><p>[3] Levina, G. V. and M. T. Montgomery, 2015. When will Cyclogenesis Commence Given a Favorable Tropical Environment?  Procedia IUTAM, <strong>17</strong><strong>,</strong> 59–68, https://doi.org/10.1016/j.piutam.2015.06.010</p><p>[4] Montgomery, M. T., M. E.  Nicholls, T. A. Cram, and A. B. Saunders, 2006: A vortical hot tower route to tropical cyclogenesis. J. Atmos. Sci., 63, 355–386,  https://doi.org/10.1175/JAS3604.1</p>

DIMPLE GENERATOR OF VORTEX STRUCTURES

The paper presents the results of experimental studies of the space-time characteristics of the velocity and pressure field inside a hemispherical dimple on a flat surface. The features of the formation and development of vortex structures generated inside the dimple, as well as their interaction with the streamlined surface of the dimple and the boundary layer were established. Integral, spectral and correlation characteristics of the field of velocity, dynamic and wall pressure fluctuations were obtained. The velocities and directions of transfer of large-scale vortex structures and small-scale vortices inside the dimple were determined. The frequencies of rotations and ejections of large-scale vortices, the frequencies of oscillations of the vortex flow inside the dimple and self-oscillations of the vortex structures of the shear layer, their subharmonics and harmonics of higher orders were established.

Vortex Dynamo in an Obliquely Rotating Stratified Nanofluid by Small-Scale Non-Helical Forces

In this work, a large-scale instability of the hydrodynamic -effect in an obliquely rotating stratified nanofluid taking into account the effects of Brownian diffusion and particle flux under the influence of a temperature gradient (thermophoresis) is obtained. The instability is caused by the action of an external small-scale non-spiral force, which excites small-scale velocity oscillations with zero helicity and a low Reynolds number. Nonlinear equations for large-scale motions are obtained using the method of multiscale asymptotic expansions by a small parameter (Reynolds number). A linear large-scale instability of hydrodynamic -effect is investigated depending on the parameters of rotation , temperature stratification , and concentration of nanoparticles . A new effect of the generation of large-scale vortex structures in nanofluid at is associated with an increase in the concentration of nanoparticles is obtained. The maximum instability increment is reached at inclination angles for the Prandtl numbers , and for the Prandtl numbers at inclination angles . It has been found that the frequency changing of the parametric impact will make it possible to control and track the generation of large-scale vortex structures. It is shown that circularly polarized Beltrami vortices appear in nanofluid as the result of new large-scale instability development. In this paper, the saturation regime of large-scale instability in an obliquely rotating stratified nanofluid with an external small-scale non-spiral force is investigated. In the stationary regime was obtained a dynamic system of equations for large-scale perturbations of the velocity field. Numerical solutions of this system of equations are obtained, which show the existence of localized vortex structures in the form of nonlinear Beltrami waves and kinks. The velocity profile of kink tends to be constant at large Z values.

Planet Formation in Transition Discs

<p>High-resolution imaging of protoplanetary discs has revealed their wealth of substructure. Perhaps the most striking observation has been the presence of large-scale crescent-shaped features, which have been interpreted as large quantities of dust trapped in anticyclonic vortices. Such regions of high dust-to-gas ratios are expected to promote planet formation processes, so understanding their formation and evolution is of primary interest.<br />Gas-only hydrodynamics simulations have demonstrated that the thermal feedback from a planetary embryo undergoing rapid formation by pebble accretion can trigger the generation a large-scale vortex. However, the ability for such a vortex to trap dust and the impact this has on the forming planet are yet to be investigated. I will present results from hydrodynamics simulations of a disc containing both gas and dust, showing the efficiency with which dust grains accumulate in a vortex, and discuss the consequences this has for the growth of the planetary embryo.</p>