Volume 54, Nº 4 (2019)

The results of numerical simulation of cavitating flow around the experimental model of a guide vane of a water turbine are given. The possibilities of the detached eddy simulation (DES) method for calculating the flow dynamics around the hydrofoil in two regimes at 3° and 9° angles of attack are analyzed. The mean flow velocity inside the boundary layer is compared with the experimental data. The dimensions and shape of the three-dimensional vapor cavity are presented in various operating regimes. The cavity dynamics and induced pulsations are analyzed.

An analytical model for calculating a viscous incompressible fluid flow in a rotating cylinder with a braking lid and the formation of turbulent boundary layers on the end surfaces is presented. The analysis is made with account for all nonlinear inertial terms in the equations of motion, within the framework of Loitsyanskii's integral relations. The approximate velocity profiles in the boundary layers are preassigned in accordance with the empirical 1/7 law. The main flow is subdivided into an inviscid quasisolid core and a lateral layer, where almost the entire upward part of a circulatory flow is concentrated. The unknown angular velocity of the core and its radial boundary are evaluated from the balance between the moments of the friction forces acting on the main rotating flow and the continuity condition for the circulatory flow.

The flow in the three-dimensional laminar boundary layer in the vicinity of the plane of symmetry of a semi-infinite planar wing having a bend of the leading edge is investigated in the case of strong interaction with the external hypersonic flow. The flow functions are asymptotically expanded in power series in the angular coordinate in the vicinity of the plane of symmetry. The corresponding boundary value problem for the principal terms of the expansion is formulated and solved. The possibility of the existence of several solutions in the vicinity of the plane of symmetry is shown. The sweep angle effect on the distinctive features of the flow is considered.

The turbulent flow structure and the heat transfer in an unsteady bubbly round impinging jet is numerically simulated for different pulse frequencies. The mathematical model is based on the Eulerian approach for describing the flow dynamics and heat transfer in the disperse phase (air bubbles). The problem is considered in the axisymmetric formulation; the system of unsteady Reynolds-averaged Navier—Stokes equations is solved. The turbulence of the carrier phase (liquid) is described invoking the model of the transport of the Reynolds stress components with account for the bubble effect on turbulence production. The pulse supply frequency effect on the flow structure and the heat transfer in an impinging gas-liquid jet is investigated. The pulsed nature of the jet causes both a considerable (almost twofold) increase in the liquid turbulence and heat transfer during the pulse interval and a considerable suppression of these parameters, when the jet is flow-off, as compared with the case of a steady impinging bubblyjet at the same time-average jet flow fluid rate.

The self-similar problem of breakdown of an arbitrary discontinuity is considered with reference to the processes of nonisothermal saline fluid flow through a porous medium. The possibility of salt precipitation in the form of a solid phase in the porous medium accompanied by reduction in its permeability is taken into account. The geometric method for solving the problem in plane is proposed under the assumption of incompressibility of the medium and neglecting thermal conductivity. It is shown that the solution should be constructed with the use of only the shock waves and zones with homogeneous parameter distributions whereas centered rarefaction waves cannot enter into the solution. The possible types of solutions of the problem are investigated. It is shown that three shocks propagate into the aquifer when the high-temperature saline fluid is injected, the temperature being discontinuous only on the inner shock.

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.

The solution of the problem of gas flow in a narrow plane or cylindrical channel with the internal rod on the axis and chemical reactions on the walls is obtained. The problem reduces to solution of an ordinary differential equation for the transverse flow parameter distribution. In a certain degree, this example simulates flows in chemical reactors used sometimes for investigation of heterogeneous chemical reactions.

Synthetic jet is a novel fluctuating time dependent flow technique that transfers linear momentum to the surroundings by ingesting and expelling fluid from a cavity containing an oscillating diaphragm and is conceivably useful for electronic cooling. In this paper numerical analysis is performed to address the effects of the variation in geometric parameters of orifice and excitation frequency on the synthetic jet fluidic. The moving piezoelectric diaphragm is modeled with constant voltage amplitude boundary condition. Computations are carried out by using COMSOL 5.3a Multiphysics software. The present study focuses on the synthetic jets which are formed from a single cylindrical cavity but with different orifices, such as single-hole, three-holes, single-rectangular slots and three-rectangular slots. The exit areas for single-hole and single rectangular-slot has been chosen as 7.0 mm², while exit areas for multiple holes and slots has been chosen as 21.0 mm₂. The velocity of the synthetic jet reaches a maximum value when the diaphragm is excited at an optimum frequency. In case of single-hole synthetic jet, the optimum frequency is nearly the same as that of single rectangular- slot type orifice; however the optimum frequency for the multi-orifices is lower than that of singleorifice synthetic jet. The quality of the simulation results is verified by grid, time and domain independence studies, and validated with the existing experimental data. The simulation results obtained in this study are remarkable as they provide primary design guidepost for the excitation frequency and orifice shape. The present investigation also indicates the maximum value of average heat transfer coefficient is 86.5 W/m² K with single rectangular-slot orifice, which is 21% higher as compared with a single-hole orifice. For single rectangular-slot orifice when distance ratio of orifice-to-heater is (Z/b) = 80 the value of heat transfer coefficient is 112.5 W/m² K which is about 20.3% higher as compared to that of single-hole at (Z/d) = 14 thereby leading to better performance.