Thermophysics and Aeromechanics

An experimental fluctuation analysis is usually conducted via thermal anemometry with hot-wires. However, in the vicinity of oscillating shock waves, this kind of sensor can be destroyed due to strong mechanical loads. On the basis of a sufficient modelling, a wedge-shaped hot-film can be suitable for a quantitative fluctuation analysis across a shock wave. Within this study, the modal analysis according to Kovásznay and Morkovin was adapted from hot-wires to the used wedge hot-film. Additionally, 3-dimensional fluctuation diagrams were derived for the separate as well as for mixed modes. The freestream fluctuations were detected across an oblique shock wave with a wedge hot-film in a constant-temperature mode. The shock was caused by a ramp with a 10° ramp angle placed in a flow with a Mach number of M = 2.5 and a unit Reynolds number of Re_unit = 4.96·10⁶·m⁻¹. The recorded perturbations were decomposed according to the modal analysis and found to be dominated by the acoustic mode. The fluctuations’ amplification across the shock wave and the subsequent decay could clearly be detected. They are in good agreement with the literature.

The study of the effect of close placing of two individual elements of roughness with the cylindrical shape on the nature of laminar-turbulent transition were performed. Experiments were performed for a blunt conical model with the radius 9 mm at Mach number M = 5. These double elements of roughness with different heights and divergence angles between elements were allocated at the blunted nose of the tested model. Measurements with a hot-wire anemometer provided information about the averaged and unsteady parameters of the boundary layer in the wake behind the roughness elements. For all types of roughness, we confirmed existence of an effective height of roughness; for the heights above the effective one, we observe deformation of the boundary layer margin and the growth of non-equilibrium in the wake. Process of turbulization behind a double roughness element (similar to the case of single roughness) is accompanied by the generation of longitudinal vortices and by the deformation of the velocity profile. Depending on this deformation, pulsations in the wake either enhance or decline. In contrast to the single roughness configuration, the double element roughness decreases the mass flowrate for a narrow range of angles (and fullness of the boundary layer profile is more significant). Meanwhile, flow turbulization occurs right behind the single element of turbulization, for the case of double element turbulization, the main gain in the mass flowrate occurs with the wake (developing from the boundary between twin elements).

Roughness has significant influence on unsteady characteristics of the boundary layer (when the height of roughness element is lower than the effective height). The wake downstream the double elements roughness exhibits the interaction between vortices, and this reduces the effective Reynolds number (compared to the case of single roughness).

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

The paper considers nonlinear waves generated on a surface of a horizontal liquid layer put into an assigned stress field at the gas-liquid interface. The nature of branching for wavy modes from the undisturbed flow was studied. To accomplish this, the solution of a model nonlinear equation written for the deviation of the layer thickness from the undisturbed layer is found. Analytical solutions were constructed for nonlinear steady state-travelling solutions of this equation with the wavenumbers belonging to the vicinity of neutral wavenumbers. Steady state-travelling periodic solutions for the first family were simulated for the case of wavenumbers beyond this vicinity.

Specific features of the phenomenon of vapor bubble collapse in hot tetradecane (with a temperature of 663 K) are considered for various values of pressures in the liquid in the range from 13 to 100 bar. At the beginning of the collapse, the vapor in the bubble is in the state of saturation with a pressure of 10.3 bar, and with the initial radius of the bubble to equal 500 µm. It is shown that, at a liquid pressure lower than 13 bar, a nearly-uniform vapor compression is realized in the bubble, whereas at higher pressure values, compression is realized by means of radially converging isentropic waves (at 14–18 bar) and shock waves (starting from 19 bar). The degrees of vapor compression, estimated from the vapor pressure, density and temperature at the boundary of a small central region of the bubble of 0.25-µm radius, are compared with the degrees of vapor compression realized when a similar vapor bubble collapses in cold acetone at a temperature of 273 K (as in known experiments on acoustic cavitation of deuterated acetone). It is found that the degrees of compression comparable with those achieved in the case of acetone at a pressure of 15 bar, equal to the amplitude of the acoustic action exercised in the mentioned experiments, are achieved in the case of tetradecane at a pressure of 70 bar. In the latter case, the maximum rate of bubble collapse in tetradecane is 10 times lower than that in acetone (110 m/s versus 1100 m/s).

The developed mathematical model was applied for study of fluid dynamics in a rotational bioreactor for bone tissue engineering by in vitro technology. The research goal is finding an optimal mode for rotation ensuring proper cyclic loading from fluid upon the cell-seeded biomaterial. The basis for developing a mathematical model of a bioreactor was a design of rotational type biological reactor used in medical research; the liquid flow is generated through viscosity mechanism due to surface rotation. Mathematical description of flow in a reactor cavity was performed with Navier—Stokes equations. It was assumed that flow regime in the boundary layer is laminar. Numerical algorithm was accomplished using a fluid flow solver “Fluent” in the code package ANSYS-12. Four variants of generating the rotational motion in the reactor cavity were considered. A series of parametric computations was performed for the rotation frequency f in the range 0.05 ≤ f ≤ 0.25 Hz. The paper offers visualization of velocity fields in the vertical plane. The distributions for shear stress and pressure in the working zone of reactor were calculated and analyzed. Simulations demonstrated that a method of fluid rotation by driving the outer cylinder with an offset axis is the best for arranging a cyclic pressure and cyclic shear stress on the biological material.

A heat storage using the latent heat of the solid-liquid phase change was experimentally investigated. To intensify heat exchange in its design, hyper-heat-conducting plates were used. During the experiments, the temperature values and the volume changes of the working substance (hexadecane) were measured. The analysis of the thermal balance and phase changes in the volume of the heat storage was carried out. The efficiency of the considered design of the heat storage and high heat-distribution capacity of hyper-heat-conducting plates have been confirmed.

The paper describes the design of an experimental setup for plasma gasification of organic waste with its discrete supply to the loading unit. The experimental results of determining the percentage composition of synthesis gas and its calorific value depending on time for gasification of wood sawdust and polyethylene granules are presented. The amount of methane in synthesis gas composition has been calculated depending on temperature and oxygen flow rate. It is shown that when the waste is fed in separate portions, the synthesis gas composition, its yield and calorific value change over time significantly.