S. G. Mironov

Modeling of a supersonic flow around a cylinder with a gas-permeable porous insert

Results of an experimental and numerical study of a supersonic (M∞ = 4.85) flow around a streamwise-aligned cylinder with a gas-permeable porous insert on the frontal face in the range of Reynolds numbers ReD = (0.1–2.0) · 105 are presented. The numerical study is performed by using the Ansys Fluent software system and a porous medium model based on a quadratic law of filtration. The parameters of the quadratic dependence are calculated on the basis of experimental data for an air flow in a porous material. Flow fields are obtained, and the wave drag of the model is calculated as a function of the porous insert length and the Reynolds number. Results of numerical simulations are compared with wind tunnel measurements.

The efficiency of the method of sound-absorbing coatings in vibrationally excited hypersonic flow

We have studied the effect of a porous sound-absorbing insert on the intensity of pressure pulsations in the high-temperature hypersonic flow of carbon dioxide past the surface of a plate oriented at a 10.2° angle of attack. The mode composition of disturbances in the flow core of a pulsed wind tunnel has been experimentally determined, which is dominated by acoustic disturbances of a fast mode. It is established that, under the action of acoustic disturbances of the fast mode on the shock layer, the sound-absorbing insert reduces the intensity of pressure pulsations on the plate surface as compared to a continuous surface. The experimental data are compared to the results of numerical simulations.

Effect of nozzle size on supersonic microjet length

The gasdynamic structure of supersonic underexpanded nitrogen microjets emitted from supersonic nozzles with diameters of 10–340 μm has been studied. Data on the mean wave-structure cell length in the initial region and the length of the supersonic region have been obtained. The relative length of the supersonic region significantly increases when the nozzle diameter is reduced below about 20 μm. The results of measurements are compared to available published data for gas macro- and microjets.

Relaminarization in supersonic microjets at low Reynolds numbers

Results of measuring the length of the supersonic portion of the air jets that flow out of axisymmetric sonic nozzles 10.4 μm-1 mm in diameter are presented. The measurements are carried out in a range of degree of jet noncalculation of 1–30 and in a wide Reynolds number range, including the laminar and turbulent flow modes. It is shown that the Reynolds number calculated from the nozzle diameter and the outlet parameters of gas is the parameter that governs jet flow. It is found that, for a laminar jet mixing layer, the length of the supersonic portion sharply increases. When the jet mixing layer becomes turbulent, the length of the supersonic portion decreases. The effect of increasing the length of the supersonic portion after its decrease due to the turbulization of flow in a jet and a growth in the Reynolds number is first discovered.

Stability of the Hypersonic Shock Layer on a Flat Plate

The stability of hypersonic viscous gas flow in a shock layer in the neighborhood of a flat plate is considered. The stability of the velocity, temperature, density, and pressure profiles calculated on the basis of the complete viscous shock layer equations is investigated within the framework of the linear stability theory with allowance for the shock wave relations. The calculated perturbation growth rates and phase velocities are compared with the experimental data obtained by means of electron-beam fluorescence.

An experimental study of the structure of supersonic flat underexpanded microjets

We have experimentally studied the structure of supersonic flat underexpanded room-temperature air jets escaping from micro nozzles with characteristic heights from 47 to 175 μm and widths within 2410–3900 μm in a range of Reynolds numbers of 1280–9460. The dimensions of the first shock cell are established. The supersonic core length of supersonic flat underexpanded air jets has been determined for the first time. A flow regime with a large supersonic core length has been observed for air jets escaping from a 47-μm-high nozzle.

A Similarity Criterion for Supersonic Flow Past a Cylinder with a Frontal High-Porosity Cellular Insert

We have experimentally and numerically studied the influence of the ratio of the diameter of a cylinder with a frontal gas-permeable porous insert made of nickel sponge to the average pore diameter in the insert on the aerodynamic drag of this model body in supersonic airflow (M∞ = 4.85, 7, and 21). The analytical dependence of the normalized drag coefficient on a parameter involving the Mach number and the ratio of cylinder radius to average pore radius in the insert is obtained. It is suggested to use this parameter as a similarity criterion in the problem of supersonic airflow past a cylinder with a frontal high-porosity cellular insert.

Fluid Transport Under Confined Conditions

The viscosity and thermal conductivity of the fluid are determined by the transport of the impulse and energy in the system considered. In turn, these transports are defined by and depend on the interaction of the fluid molecules. The situation in the fluid under confined conditions (e.g., in a nanochannel) is more complicated, because the transport of the impulse and energy in fluids is highly dependent on the interaction of the fluid molecules with the wall atoms (or molecules). Therefore, the viscosity and thermal conductivity of such a fluid are the properties of the entire “fluid+wall” system. In this chapter, the statistical theory of transport processes in fluids under confined conditions is proposed. The considered system is the specific two-fluid system consisting of fluid and wall molecules. In the chapter, the new constitutive relations for the fluid under confined conditions are proposed. As a result, the Green-Kubo formulas were generalized. Using this new formula and the molecular dynamics method, the viscosity coefficient of the fluid in a nanochannel was studied. It is shown that the viscosity coefficient depends, to a large extent, on the properties of interaction of fluid molecules with channel wall atoms.

Methods of Modeling of Microflows and Nanoflows

The development and application of methods of numerical simulation of micro- and nanoflows are urgent tasks because of the lack and inconsistency of systematic experimental data. However, interpretation of results and determination of the applicability area of particular methods of modeling such flows should also be treated carefully and cautiously. In addition, precise terminology is important, because inadequate usage of terms can lead not only to misunderstanding, but even to erroneous ideas about the physics of the phenomena being considered. The usual flows of liquids and gases are rather difficult in the general case. This is even more so for micro- and nanoflows. Therefore, such flows should be treated with different methods. The situation becomes even more complicated if multiphase fluid flows are studied. In the present chapter, all of these situations were considered consecutively. It begins with a brief classification of these flows. After that, the methods of the modeling flows of the rarefied and dense gases and liquids are described. In the following two sections, the modeling of dispersed fluids, including nanofluids, is analyzed. The last section is devoted to a brief description of the method of molecular dynamics, the application of which is necessary for the modeling of nanoflows.

Fluid Flows in Microchannels

The chapter describes the results of measurements of friction factors in microchannels of various shapes and various diameters for laminar and turbulent flows, as well as the friction factor for input regions. Much attention is paid in this chapter to technologies of fabrication of test benches, methodical aspects of experiments, and evaluation of reliability of experimental data. The chapter is organized in such a way that all aspects of microflow experiments are consecutively considered: from the development of test benches through choosing measurement techniques to estimating the error of results obtained.