Fluid Dynamics最新文献

Radiation is one of the factors of external influence on the structural elements of the aircraft that perform high-speed flight in dense layers of the atmosphere and the units of traction power plants. It occurs during the combustion of large masses of combustible substances, and the artificial sources of radiation in the form of incandescent lamps and based on the principles of electric discharge in gases are widely used in technical devices. The article presents the main provisions and relations of the theory of heat-radiation exchange and considers the features of the interaction of radiation with materials, including those that exhibit partial transparency in relation to radiation and the features of heat exchange processes associated with this factor. The article also focuses on the use of radiation sources in thermal testing of structures.

Measurement of the parameters of external thermal impact on the test object and the results of this impact — temperature at control points — have required the development of appropriate methods and technical means. In this case, it has been necessary to take into account the features associated with both the conditions of heating the test object by radiation and its physical properties and the range of change of the recorded values. Below, some methods examined during high-temperature thermal testing of structures using radiation sources have been considered.

The great advantage of tubular radiation sources is the possibility of their arrangement in blocks to heat complex profile constructions by a given spatiotemporal law of heat flux density variation. However, the development of designs of such blocks is associated with the solution of a number of problems caused by discreteness and geometric parameters of the arrangement of individual radiation sources in the heating blocks, which leads to the influence of these factors on regularities of heating of the test object, as well as by the peculiarities of mutual influence of the arrangement of individual radiation sources on their performance. Thus, in the general case, rational use of tubular radiation sources requires solving the problems of radiative and radiative-conductive heat transfer in a closed system of radiating and absorbing bodies.

Tubular radiation sources are convenient for conducting thermal tests of structures, as they allow arrangement of the sources into blocks applicable to testing objects of geometrically simple or complex shape to ensure the necessary law of space-time distribution of heat flux density. In addition, radiation sources allow to have them easily combined with heating sources of other physical nature and thus implement the conditions of combined, for example, radiation-convective heating.

Heat exchange processes in objects heated by radiation are characterized by the fact that the effect from the radiation flux depends not only on its magnitude, but also on the qualitative characteristics of the material in relation to the radiation, namely, its possible partial transparency (semi-transparency). This effect manifests itself in the volumetric nature of heating, and such integral characteristics as the reflectivity and emissivity of the heated object depend on the volumetric physical properties of the material in relation to the acting radiation and the temperature field formed by it. The thermal state of the structural elements exhibiting the property of partial transparency is determined by the adjoint system of equations containing the energy equation, in the particular case of a condensed medium (material)—the heat conductivity equation, and the integro-differential equation of radiation transfer. In this paper, one of the approximate methods for solving the radiation transfer equation and some of its applications are considered.

The preparation of thermal tests of materials and structural elements involves a preliminary analysis of the influence of a number of factors on the temperature condition of the heated object. The most significant are the scale ratios of the characteristic dimensions of the test object and the heating source, their mutual orientation, the radiation spectra of the heating source and absorption of the test object, as well as the spatial distribution of the radiation flux on the heated surface in the field of discretely located radiators. This article discusses some approaches to solving these problems.

Radiation sources are promising for thermophysical studies, thermal and thermal strength tests of materials and structural elements subjected to high-temperature heating. Of the existing types of radiation sources, tubular radiation sources play a special role, making it possible to create test installations for heating large-sized fragments and full-scale aircraft structures by assembling them into multi-chamber heating units. For these purposes, incandescent radiation sources and high-current gas-discharge radiation sources can be used. Experimental data on the characteristics of the most common radiation sources are presented, on the basis of which heating installations have been created in relation to thermal tests of aerospace engineering structures.

Serpentine microchannels have the extensive application potential and the increasing research value due to their compact structure and the high heat transfer performance. Serpentine microchannels with the hydraulic diameters of 0.65 mm and the turning curvature radii of 1.2 and 2.4 mm are used to study the influence of the centrifugal force, the vapor mass quality, and the mass velocity on the vapor–liquid flow pattern maps of adiabatic and boiling flow. Bubbly flow, slug flow and annular flow were studied in adiabatic and boiling flow. The patterns of adiabatic flow and boiling flow in serpentine microchannels were found. The centrifugal force is an important factor in the flow pattern transition in the case of two-phase flow in serpentine microchannels. The centrifugal force existing in the bend promotes transition of slug flow to annular flow and wavy-annular flow to stable annular flow. Experiment data is compared with the flow regime maps existing in the literature, finding that the classification of slug flow and annular flow is inconsistent. For flow boiling, heat transfer elongates the flow pattern transition channel length, due to the increasing vapor mass quality. Bubbles are usually initiated on the inner wall just out of turning sections, where a low-pressure zone exists.

Since the reduced-order model techniques can reduce the computational burden of numerical simulation while retaining the most important features of flow physics, the reduced-order model plays a crucial role in the optimization and control for the unforced round jet flow. In this work, a deep neural network method or neural ordinary differential equation (ODE) was applied to the reduced-order model for a free round jet. In this model, the output or proper orthogonal decomposition (POD) coefficient of the reduced-order model is calculated using an ODE solver. The method is exemplified for classic shear flow such as a jet and numerically demonstrated for a round jet generated by large-eddy simulation (LES). The Reynolds number Re of the round jet is calculated based on the diameter of nozzle exit D and averaged streamwise velocity along the spanwise distribution. The reduced-order model accurately reconstructs the free jet velocity field based on the original snapshots. These results revealed that the employment of neural ODEs will significantly improve the availability and efficiently of the reduced-order model, which may supply crucial instruction on future studies using the reduced-order model improved by machine learning algorithms. We expect the proposed method to be applicable for a model-based flow control in future.

The bubbly suspension is a gas–liquid mixture such that the bubbles are dispersed in a continuous liquid phase; it is widely used in industrial applications. When the bubbles happen to deform under shear, the flow structure of suspensions is altered. In turn, this can further change the rheological properties of bubbly suspensions. In this paper, the effects of the capillary number Ca and the gas–liquid viscosity ratio λ on bubble deformation and rheological properties are investigated based on a three-dimensional parallel plate Couette flow model using the volume-of-fluid method. When Ca = 0.05 and 0.1, the deformation parameter D is less affected by λ, D ≈ 0.05 and 0.1, so after stabilization the bubble shape is approximately spherical. When Ca is in the range of 0.4–2.5, D first increases and then decreases with increase in λ, and its deflection angle decreases first and then remains stable. For large viscosity ratios (λ = 5 and 10), the effect of Ca on D is small. As λ decreases (i.e., λ ≤ 2), the effect of Ca on D increases. At a small λ, the deformation parameter D gradually increases as the capillary number Ca increases. However, when Ca is large enough to reach a certain value of D ≈ 1, the bubble shape does not show a significant change. The relative viscosity of bubbly suspension η_r is closely related to the bubble shape (characterized by Ca) and the viscosity of gas inside the bubble (characterized by λ). When λ is certain, η_r decreases with increase in Ca (in the range of 0.05–2.5), and the larger Ca is, the smaller η_r. For 1 < λ < 10, although η_r decreases with increase in Ca, the minimum value of η_r is always greater than 1. As λ decreases, η_r is closer to 1. When λ ≤ 0.5, η_r decreases with increase in Ca, and the minimum value of η_r is less than 1. When Ca is certain, the shear stress at the gas–liquid interface increases as λ becomes large, and the resistance to the flow of the surrounding fluid increases; and thus the larger λ, the larger η_r. The injection of bubbles changes the rheological properties of suspensions and causes the first normal stress N₁. The first normal stress N₁ follows the distribution pattern of increasing first and then decreasing. At the same Ca, the larger λ, the smaller N₁.