Vol 54, No 6 (2019)

The spectrum of one-dimensional natural vibrations propagating through a two-phase layered medium in the direction of the normal to the layers is investigated. The medium considered consists of many periodically alternating layers of isotropic viscoelastic material and viscous compressible fluid. It is found that the spectrum mentioned above consists of the roots of transcendental equations whose number is proportional to the number of the layers of original medium. As the initial approximations of the roots for solving these equations numerically, it is proposed to use the points of the spectrum of one-dimensional natural vibrations of the corresponding homogenized medium. These points represent the roots of linear fractional equations. It is shown that the points at which the denominators of fractions in the linear fractional equations vanish should be also taken as the initial approximations. The accuracy of the initial approximations is proved to increase when the number of layers of the original medium increases and the layer thickness decreases simultaneously.

The paper presents the results of the comparative testing of wave propulsors of five types, including the propulsor of the underwater sail type, a rigid foil with elastic couplings, and a propulsor without movable elements using the effect of the nonlinear interaction between a wave and the working body of the propulsor. The rigid foil operation is numerically simulated using the XFlow software package. The influence of the wave propulsor location on the boat frame, the depth of its immersion, the elastic coupling parameters, and the presence of stabilizing aft devices on the propulsor performance is studied. It is shown that for a rigid swinging wave propulsor with elastic couplings mounted on the bow of a boat the velocity of the boat motion is proportional to the bow swinging.

The dynamic and thermal characteristics of unsteady near-wall flows are investigated numerically on the basis of two-parameter turbulence models under the conditions of high-turbulence free stream and the impact of disturbing blowing (suction) factors in the boundary layer through a permeable surface section. The mutual effect of time harmonic oscillations of the external inviscid stream velocity and the blowing flow rate density on the wall on the development of time-dependent heatand mass-transfer processes in turbulent flow is analyzed. The laws of variation in the flow and heattransfer properties are established when the flow rate density is given to be variable in time and constant. The basic mechanisms of the impact of blowing and suction on the permeable section and downstream are studied by comparing the calculated results.

The paper presents a global stability analysis of the two-dimensional incompressible boundary layer with the effect of streamwise pressure gradient. A symmetric wedge flow is considered at different values of the dimensionless pressure gradient parameter β_H. The pressure gradient dp/dx in the flow direction is zero, when β_H = 0, favorable (negative) for β_H > 0, and adverse (positive) for β_H < 0. The base flow is computed by numerical solution of Falkner—Skan equation. The Reynolds number is based on the displacement thickness δ* at the inflow boundary. The stability equations governing the flow are derived in body-fitted coordinates. The stability equations are discretized using the Chebyshev spectral collocation method. The discretized equations, together with boundary conditions, form a general eigenvalue problem and are solved using Arnoldi’s algorithm. The temporal global modes are computed for β_H = 0.022, 0.044, and 0.066, for favorable and adverse pressure gradients. The temporal growth rate ω_i is found to be negative for all the global modes. The ω_i value is smaller for the favorable pressure gradient (FPG) than for the adverse pressure gradient (APG) at the same Reynolds number (Re = 340). Thus, the FPG has a stabilizing effect on the boundary layer. The comparison of the spatial eigenmodes and spatial amplification rates for FPG and APG show that FPG has a stabilizing effect, whereas APG has a destabilizing effect on the disturbances.

The effects of nose radius on stagnation and surface heat transfer rate along the surface are addressed in this research paper. Experiments are carried out in hypersonic shock tunnel, at hypersonic Mach number of 6.56 for 11.38° apex angle blunt cone with nose radius of 0.2R, base radius of R. Similarly, experiments are carried out at Mach 7.32 for 13.87° apex angle blunt cone models with nose radius of 0.18R′, base radius of R′. Test is performed at stagnation enthalpy of 1.4 and 2 MJ/kg with effective test time of 3.5 ms. Convective heat transfer measurements have been carried out on the test model at two different angles of attack, namely 0° and 5° with angle of rotation of 0°, 90°,180° with platinum thin film sensors. ANSYS-Fluent used to simulate the flow over the blunt models at different Mach numbers. The measured shock standoff distance from Schlieren visualization images compared with theory and computational fluid dynamic study for both configurations. The measured stagnation heating value is compared with theoretical value estimated using Fay-Riddell expression and numerical simulation. The measured heat transfer rate is higher for configuration 1 than configuration 2. The increases in heat transfer rate is due higher density ratio across the shock wave and the reduced shock layer thickness. The measured shock layer thickness is 2.06 mm for Mach 6.56 and 3.45 mm for Mach 7.32. The heat transfer rate is higher for Mach 6.56 as compared to Mach 7.32.

The problem of plane discontinuities of relatively small amplitude propagating in a layer of nonuniformly charged gas in the variable electric and constant gravity fields is considered within the framework of ideal electrohydrodynamics model. The transport equations of variational nature are derived for their amplitudes and an exact solution of the problem of weak discontinuity is given. Then, the problem of weak shock wave is solved in the approximation to the solution obtained. Examples of propagation of a wave through an arbitrary equilibrium initial state and along a gas layer moving as a solid body in a variable electric field are considered.