Vol 54, No 5 (2019)

The technique of experimental investigation of the effect of an accelerated shear flow on the development of the Rayleigh—Taylor (RT) instability at the interface of two fluids at a small Atwood number is developed. Using a counterpart of the laser sheet method (laser induced fluorescence) the data on the structure of the RT mixing zone are obtained and the instability stabilization under the action of an accelerated shear flow is confirmed.

The linear stability of combined evaporating liquid—vapor-gas mixture flow is investigated under conditions of inhomogeneous heating within the framework of the conjugate problem. The exact solution of the Navier-Stokes equations in the Boussinesq approximation is used to describe the two-layer flow. The solution takes into account the influence of both nonzero streamwise and transverse temperature gradients and the direct and inverse thermal diffusion effects in the vapor-gas layer and on the phase interface. The influence of the applied external thermal load on the main flow parameters, their structure, the volume vapor content, the mass evaporation rate, and the stability is studied for systems with various liquid-layer thicknesses. The typical forms of characteristic perturbations are distinguished and the critical loads that lead to loss of stability at small transverse temperature drops are determined.

Experiments were conducted on overexpanded equilateral triangular supersonic jets (Mach 1.8) at Reynolds numbers 6.71 × 10⁵ and 4.81 × 10⁵ to study their decay and spread characteristics by means of measuring the jet centreline and lateral total pressure distributions. Schlieren images of the jets were also taken to study the shock patterns in the jet structure. The above Reynolds numbers correspond to the nozzle inlet total pressures of 550 kPa (6.71 × 10⁵) and 360 kPa (4.81 × 10⁵). For comparison purposes the above experiments were repeated on a circular nozzle with the same exit area and area ratio (1.44) and total pressures. The observations from the experiments reveal that the triangular jet exhibits a shorter supersonic core compared with the circular jet, i.e., reduction of 34.25% at 360 kPa and 31.11% at 550 kPa. The total pressure decay is more abrupt and a greater loss of the total pressure occurs closer to the nozzle exit plane in the case of the triangular jet compared to the circular jet. The lateral total pressure distribution reveals that the jet spreading rate is greater on the flat than on the corner side of the triangular jet.

The flow in the three-dimensional laminar boundary layer on a yawed flat plate of finite length is studied in the regime of strong viscous-inviscid interaction with a hypersonic flow. In the vicinity of the leading edge the flow functions are expanded in series under the assumption that the base pressure dependent on the transverse coordinate is given at the trailing edge of the plate. It is established that the expansions obtained include an indefinite function and its derivative with respect to the transverse coordinate. The corresponding boundary value problems are formulated and numerically solved, the eigenvalues are found, and it is shown that the exponent in the third term of the expansion differs from that in the second term only by one. The plate surface temperature effect on the flow parameters and the initiation of three-dimensional disturbances is investigated.

The solution of the Buckley-Leverett problem classical for the theory of flow through a porous medium, generalized to the case of two-phase flows in fractured-porous media, is considered. In this case, immiscible displacement of fluid in the porous medium is complicated by the absence of local capillary equilibrium between pore spaces of different scales and in the generic case the solution of problem is not self-similar. The flow through a porous medium is considered in the limiting case of large time scales when capillary equilibrium is established and the flow parameter distributions, as shown in the study, tend to self-similar asymptotics. For the effective ordinary porous medium the average equations of equilibrium flow through the porous medium which describe these asymptotics are obtained

This paper investigates the unsteady aerodynamic characteristics of an oscillating flat plate and the NACA 0012 airfoil around the angle of attack (AoA) close to their static stall angle at a low Reynolds number, 3.2 × 10⁴. The kinematic oscillatory motion is described by a sinusoidal function in which the oscillation frequency and amplitude are variable. Both experimental and numerical methods are applied in the two-dimensional space. The experiment aims at measuring the aerodynamic forces and the moment directly. For numerical simulation, the SST (Shear Stress Transport) gamma theta model is employed to solve the unsteady flow field and to compute the lift coefficients (CLs). Good qualitative agreement between the experimental and numerical results for CL is obtained, which demonstrates the feasibility of the modified RANS model in the flow transition case. In general, NACA 0012 is greatly influenced by the dynamic effect in contrast with the flat plate. For a given reduced frequency, the shape of the hysteresis loop of CL shows some distinguishing features; the process of flow reattachment of NACA 0012 is slower than that of the flat plate in the downstroke phase, so that a smooth transition of CL is observed; there are still vortices shedding from the trailing edge even at small angles of attack, which results in a local instability of CL. By studying the effects of the reduced frequency (K) and the amplitude, it is found that the AoA corresponding to the maximum CL is more sensitive to the former and the reduced pitch rate (a') is the main parameter determining the dynamic stall angle for both flat plate and NACA 0012. In addition, the results for K = 0.07 show that the lift and drag coefficients at a maximum angle of attack are close to their static values for the discussed amplitudes and wing geometries.

The problem of stationary Mach reflection of a Shockwave in a plane channel is considered within the framework of the Euler model. Emphasis is placed on an investigation of the flow parameters at the triple point. In an analytical investigation the local theory for curvilinear shock waves is employed. The conditions on the input data of the problem, at which singularity is realized at the triple point, are derived. Under the singularity conditions the flow parameter gradients and the curvatures of the shock fronts and tangential discontinuity at the triple point increase without bound. In the numerical investigation the second-order Godunov method is used, together with a new technology of contracting the grid toward the triple point in combination with the discontinuity fitting. The calculations confirm the theoretically predicted singularity boundary. Additional numerical experiments show that the singularity boundary is conserved, when artificial source terms are introduced into the energy equation. These results allow one to put forward the hypothesis that the singularity within the same boundaries is also realized for other two-dimensional flows with irregular shock-wave reflection.

After publication of the paper, the authors realized that the affiliation of the fourth author (Tiberiu Esanu) was given incorrectly. Its correct version appears above. Moreover, a second funding grant was missed in the acknowledgements. We give their complete correct version below.