Fluid Dynamics最新文献

In the present study, the RANS-SST approach was employed for numerical modeling and gradient heatmetry, enabling the examination of convective heat transfer during turbulent airflow around a 45-angled groove on an isothermal steam-heated section of a flat plate under the following conditions: a Reynolds number (Re) of 3 × 10⁴ with the groove’s depth varying from 0.05 to 0.35 relative to the groove’s width. Through experimental and numerical analysis, a twofold increase in relative heat flux was established at the inlet region of the bottom groove in the central longitudinal section within the high-intensity recirculation flow zone. Our investigation revealed the increase in separated flow, recirculation, downward, upward, and swirling flows within the groove with increased depth. In particular, a critical relative depth of 0.2 was identified, marking the onset of abnormal enhancement of separated flow and heat transfer in the inclined groove’s entrance on the heated plate. As the swirling flow progressed along the groove, its velocity escalated, accompanied by a considerable thinning of the near-wall layer in the entrance segment. In contrast, the near-wall layer thickened, especially toward the groove’s center and outlet.

Direct numerical simulation of supersonic (Mach number is 1.5) viscous heat-conducting gas flow past a body of revolution (standard HB-2 model) at an angle of attack 29° is carried out. The calculation results make it possible to resolve the general structure of shocks around the body and to obtain a good agreement of the stagnation parameters and the aerodynamic coefficients with theoretical and experimental data. The simulation was carried out on the basis of a quasi-gas dynamic (QGD) algorithm, that describes not only stationary and symmetric, but also non-stationary and asymmetric vortex zones that develop in flow. In this case, no additional turbulent viscosity is used in the calculations. It is shown that reducing the artificial dissipation coefficients in the QGD algorithm increases the accuracy of the modeling, bringing the calculated aerodynamic coefficients closer to the experimental data.

Flows in the VGU-4 high-frequency plasmatron at the Ishlinsky Institute for Problems in Mechanics of the Russian Academy of Sciences are studied numerically. The study includes calculations of the electromagnetic field and flows in the discharge channel, flows past a cylindrical model in the pressure chamber, and boundary layer flows on the symmetry axis ahead of the model’s stagnation point. The formulation of the problem for calculating a chemically nonequilibrium boundary layer of finite thickness is supplemented with a second-order ordinary differential equation that describes swirl on the symmetry axis and the corresponding boundary conditions. A source term accounting for the symmetric swirl of the incoming jet is included in the radial momentum conservation equation, and a dimensionless parameter characterizing the swirl intensity in the boundary layer is introduced. A criterion is established from the numerical solution of the problem of a nonequilibrium boundary layer near the stagnation point in dissociated air flow. In accordance with this criterion, swirl must be taken into account when calculating the heat fluxes. Taking swirl into account reduces the heat flux at the stagnation point of the model by up to 30% at low plasmatron operating powers, whereas the swirl has almost no effect on the heat flux at high powers.

The present experimental study focuses on exploring the decay characteristics of a Mach 0.6 jet ejected from a rectangular orifice of the aspect ratio (AR) equal to 2 under the influence of secondary injections in two configurations: injection along minor (IAM) and injection along diagonal (IAD). The secondary injections were introduced at three Mach numbers, namely, M_s = 0.8, 0.9, and 1.0. The total pressure decay along the streamwise direction (X), along longer (Y: major) and along shorter (Z: minor) directions have been measured to evaluate the jet decay and spreading characteristics. Axis-switching locations for the free and controlled jets were determined based on the evolution of the non-dimensional jet half-width (B*). The rectangular orifice jet displayed a shorter potential core (2.8D_eq) when compared to that of a rectangular nozzle jet at the same aspect ratio and Mach number. The IAD is found to shorten the potential core and accelerate jet decay, whereas IAM destroys the potential core entirely, leading to rapid near-field mixing and early axis-switching, with a maximum upstream shift of 77%, observed at M_s = 1.0.

Modeling the interaction with the atmosphere of an entering cosmic body and its fragments after breakup is carried out within the framework of the meteor physics equations. The uncertainty, existing at high velocities, of the heat transfer coefficient and the specific heat of mass loss, which determine the ablation model in the equations used, leads to an uncertainty in the modeling results. To study this uncertainty, the meteor physics equations are solved numerically using various ways of setting the heat transfer coefficient and the effective (due to various processes) heat of ablation. The possible error in modeling the meteoroid trajectory, energy deposition and mass loss, which can occur due to the uncertainty of the specified ablation model, is estimated for various atmospheric entry parameters.

The subsonic air plasma flow in a sectioned discharge channel of the VGU-3 plasmatron is numerically investigated on the basis of Navier−Stokes equations, together with two-dimensional equations governing a vortex electromagnetic field. The calculations are performed for the pressures of 100 and 50 hPa and the anode power supply of the HF generator ranging from 100 to 300 kW. The plasma flow parameters, namely, the velocity components, the enthalpy, the temperature, the molar composition, and the Mach and Reynolds numbers are determined at the channel exit in these regimes. The radial profiles of the gas parameters at the channel exit, the parameter distributions along the axis of symmetry from the entry section to the channel exit, and the stream function and temperature contours within the channel are presented. It is shown that the use of a sectioned discharge channel in the VGU-3 plasmatron instead of a simple channel results in the fourfold increase of the longitudinal velocity component and a 10–20% reduction of the flow enthalpy at the axis of symmetry in the exit section for the regimes considered, which makes it possible to extend considerably the range of modeling the full-size aerodynamic heating in the experiments performed at VGU-3.

The size and number of corners of a polygonal hydraulic jump depend on various parameters, including the fluid flow rate, the jet diameter, the obstacle height, and the physical properties of fluid. In other words, the size and shape of the polygonal hydraulic jump depend on the Reynolds number, the Weber number, and the Bond numbers. This study investigates the effect of the surface roughness on the shape and region of stability of polygonal hydraulic jumps. Sandpapers of different degrees of roughness are glued on the surface of the target plate to make it rough. The study reveals that, in addition to the Reynolds and Weber dimensionless numbers, the surface roughness of the target plate affects the stability of polygonal hydraulic jump. Based on the conditions of this study, including the thin-film flow, surface roughness leads to flow slip on the surface and, consequently, leads to a higher mean velocity and flow momentum. This study demonstrates that, in general, at given values of the Reynolds and Weber numbers, a rougher surface leads to a greater number of corners in the polygonal hydraulic jump. Furthermore, the rougher the surface, the smaller the extent of the region of stability of polygonal hydraulic jumps. At last, the Taguchi method is used to derive relations for estimating the number of corners of polygonal hydraulic jumps with respect to the jet diameter, the flow rate, the height of downstream obstacle, and the roughness of the target plate for both modes of increasing and decreasing flow rates.

The description of heat transfer processes in physical and chemical gas dynamics within the framework of RANS (Reynolds-averaged Navier–Stokes) turbulence models involves determination of the turbulent thermal conductivity coefficient. Historically, the turbulence models make it possible to find the turbulent viscosity distribution, from which the turbulent thermal conductivity is determined using the turbulent Prandtl number (TPN). However, the TPN can depend on the problem parameters and vary within the flow domain. The applicability of the models proposed for calculating spatial variations in the TPN is restricted to specific flows. For example, the Kays–Crawford model describes the growth of the TPN in the boundary layer near a rigid wall. To validate and improve these models, it is necessary to use experimental verification. In the present study, an experiment carried out for the impact jet of heated gas is considered. The average temperature field, measured using the background oriented schlieren (BOS), contains information on the turbulent thermal conductivity coefficient. The experiment also includes velocity measurements at individual points using a hot-wire anemometer. The physics-informed neural network (PINN) combines the experimental data with equations for reconstructing the fields of hydrodynamic quantities, including the turbulent viscosity and the turbulent thermal conductivity. It is shown that the standard condition of constant turbulent Prandtl number can be used at the center of the jet, but the TPN decreases toward the periphery. The obtained TPN distributions are compared with available studies, both experimental and numerical, using the large eddy simulation (LES) method. The proposed method expands the capabilities for studying various flows in which the temperature field or the concentration field (to determine the turbulent Schmidt number) can be measured, including flows of chemically reacting media.

The thermal processes accompanying the unsteady interaction between a plane shock wave with the Mach numbers 2.0 to 4.5 and a blunt cylinder is experimentally investigated in the test section of the channel of a shock tube. The relationship between the gasdynamic and thermal processes in the stages of shock wave diffraction and the cocurrent flow past the model and the channel walls is established on the basis of high-speed shadowgraphy (150 000 frames per sec) and infrared thermography (1.5 to 2.8 μm). It is shown that the time of recording infrared radiation from the inner quartz channel walls in the complicated unsteady flow is not greater than 500 to 700 μs and that of radiation from the surface of the cylinder model in flow is not greater than 40 ms.