№ 5 (2024)

We present the results of experimental investigations of the efficiency of the direct flow wave propulsor and the propulsor of the underwater sail type on the models of semisubmerged catamaran boat with a small area of the section along the waterline. The effects observable in the case of the propulsor of the underwater sail type are qualitatively the same as for the previously studied swinging wing, namely, the greatest efficiency is observable, when the propulsor is situated near the free surface, while its efficiency is rapidly reduced with propulsor immersion. The propulsor efficiency varies only slightly with ship hull immersion, whereas the working frequencies are considerably reduced as compared with those of floating structures. In this case, the working frequencies vary in proportion to the model boat scale. Comprehensive studies of the direct-flow wave propulsor efficiency were performed, an inclined flat plate being fixed at the hulls of a semisubmerged ship as a working element. The optimal parameters of the propulsor were noted, when it efficiently operates on the waves with the greatest steepness (storm waves). Experiments showed that the efficiency of the wave propulsors of the swinging wing or underwater sail types in their operation ranges is somewhat higher than that of the direct-flow propulsor. However, under the rough water conditions the direct-flow propulsor has its own advantages, since precisely in this case it demonstrates its greatest efficiency, while the other versions considered are efficacious in the wavelength range dependent on the ship length and, generally speaking, do not coincide with the storm wave length.

The results of experimental studies of steady-state swirling flow in the area of channel branching, that imitates the proximal end-to-side anastomosis of the human femoral artery, are given. The experiments were carried out at a Reynolds number of 1460. This corresponds to the range of physiological values when estimating by the maximum blood flow rate in the artery during the period of cardiac contractions. For both branches, an equal ratio of the flow rates was maintained. At the inlet to the branching area, the degree of flow swirl was equal to 0.125. Using the SIV (Smoke Image Velocimetry) technique, flow was visualized and the instantaneous vector flow velocity fields of each branch were measured. The main patterns of the influence of swirl on the vortex structure of flow in the main artery below the branching area and in the shunt have been revealed. The possibility of using flow swirl to create more favorable hemodynamic conditions in the anastomotic area is being considered. A particular attention is paid to the appearance of signs of local flow turbulization in the presence and absence of swirl.

The radii of the orifice of a capillary channel and a jet flowing out of it are generally different. Fluid friction on the channel walls leads to the parabolic velocity distribution, while small shear stresses at the free jet boundary are responsible for the velocity profile equalization. Dissipation has an effect on both the length of the region, where the velocity profile is settled, and its radius-average value, as well as on the steady-state jet radius. Previously, the corresponding problem was theoretically solved in the axisymmetric approximation. However, the symmetry condition is not fulfilled in the case of small Reynolds numbers, owing to the occurrence of a bend flow region. Moreover, in the jets flowing out at a low velocity there occur the phenomena of global and boundary instability of the capillary flow. The totality of the nonlinear, mutually-agreed effects leads to velocity profile deformation, such that it becomes asymmetric with respect to the axis in the region, where its value is settled, and the non-uniqueness of the Reynolds-number dependence of the jet radius. The results of an experimental investigation of the dependence of the steady radius of highly viscous jets on the outflow velocity are for the first time presented for the case in which the bend flow region arises.

Impingement of drops of water and polymer solutions on a thin plate with a solitary round orifice is studied. The drop diameter before the impingement d_i coincides with that of the orifice d_t and is equal to 3 mm. The drops fell from heights of 5, 10, and 20 mm, their velocities before the impingement amounting to 0.31, 0.44, and 0.63 m/s. The drops flew through the orifice touching slightly its edges. High-speed photography was used to fix different stages of the collision between a drop and the obstacle. It is found that a considerable deceleration of the jet by the orifice can be observable for the impingement parameters considered, down to the complete stopping of the drop flight. The mechanisms of the observable phenomena and the effect of different factors are discussed.

The instantaneous and average structure of a supersonic underexpanded jet is studied numerically and experimentally. The photos obtained in experiments with different exposure times and the Pitot pressure measurements are compared with the results of the numerical modeling performed using an implicit large-eddy method. We note that the jet flow instability, disturbance growth, and transition to turbulence lead to the situation, in which the instantaneous flow structure can be considerably different from the average structure. The flow pattern observable in the calculations is in good agreement with that presented in the experimental photos obtained with short exposure times. Both calculated and experimental data indicate that an important role in the jet flow dynamics is played by large-scale vortex structures that exist against the background of small-scale turbulence. The calculated and experimental Pitot pressure distributions are similar with each other, up to a certain distance from the nozzle exit section. Further downstream, the experimental and calculated Pitot pressures start to increase rapidly but the calculations predict the onset of this growth at a greater distance from the nozzle than it is observable in the experiments.

The effect of taking into account stream swirl when calculating the flow of a subsonic air plasma jet past a cylindrical model of 50 mm in diameter when the jet flows out from the discharge channel into the test chamber of the VGU-4 IPMech RAS HF plasmatron is studied. A comparison has been made of calculations of axisymmetric flow past the model based on the full Navier-Stokes equations taking into account (new results) and without taking into account (old results) the tangential velocity component w under the experimental conditions at a pressure of 80 mbar in a wide range of anode supply power at various distances from the plasmatron channel outlet to the model. It is shown that when calculating the VGU-4 plasmatron for a low power with taking into account flow swirl, the pattern of flow past the frontal part of the model is modified, namely, a vortex region is formed in front of the model instead of a relatively thin boundary layer. For moderate and high plasmatron power, the effect of taking into account swirl on the isolines of the dimensionless stream function and on the isotherms is small in the jet core region in front of the model, but is significant in the outer flow region in the test chamber.

Numerical simulation of the flow of multicomponent nonequilibrium dissociated air past a water-cooled cylindrical model in underexpanded supersonic jets of high-enthalpy air is carried out within the framework of the Navier-Stokes equations using the stage-by-stage heterogeneous kinetics of interaction of dissociated air with the surfaces of β-cristobalite and copper and taking into account chemical reactions in flow for the conditions of experiments on heat transfer in the induction HF plasmatron VGU-4 (IPMech RAS). Numerical solutions for the chemical composition of the gas and for heat fluxes to the surface of the sensors are compared for various catalysis models. The contribution of diffusion and heat conduction processes to the heat flux to the surface is shown in various regimes of the interaction of gas with the surface material. The dependence of the flow characteristics (the chemical composition of the gas on the surface, the degree of filling of the surface, and the heat flux to the surface) on the density of adsorption sites has been determined. The modification of this dependence makes it possible to describe the entire range of the boundary conditions from non-catalytic to full catalytic ones.