Fluid Dynamics最新文献

The model of the stability of viscous incompressible flow in a channel with thick compliant walls is developed and studied under the assumption of small disturbances. The eigenvalue problem thus obtained is solved numerically using the collocation method. The computations are carried out for several viscoelastic materials. Some new results concerning the effect of the wall thickness and the characteristic flow velocity on the flow stability are obtained. The effect of viscoelastic properties of the channel wall material on the suppression of the Tollmien–Schlichting instability is estimated.

The abnormal intensification of a separated flow and heat transfer in limited packages of inclined grooves on a heated isothermal section of a plate in a uniform air flow at Re = 3 × 10⁴ are numerically and physically modeled. The numerical predictions obtained on the basis of solutions of the Reynolds-averaged Navier–Stokes equations when closed by a model of shear stress transport and the energy equation are validated by comparing them with heat measurement data using the gradient heatmetry method of heat flux measurements in a package of four inclined grooves on a special thermophysical setup at St. Petersburg Polytechnic University (SPbPU). The influence of the number of grooves in limited packages on the intensification of the separated flow and heat transfer on a structured plate is analyzed on a previously created thermophysical setup for studying heat transfer in a single groove with the angle of inclination varying from 0° to 90°. It is shown that with an increase in the number of grooves, there is an increase in the relative heat transfer coefficients in them, caused by an increase in extraordinary pressure drops in the grooves as they move away from the leading edge of the plate, accompanied by an intensification of recirculating and swirling flows. A fundamental difference in the distribution of the relative heat transfer coefficient in the end part for the first and subsequent grooves in limited packages is established. As the number of grooves increases, heat transfer in the end sections intensifies.

The time-dependent problem of oil outflow from a defected reservoir into water is considered. The mathematical model of the joint flow of three immiscible phases (water, oil, and air) includes the continuity and Navier–Stokes equations, together with the equation for determining the interphase surface location. The problem is solved using the finite volume method. The velocity of the oil outflow from the tank, the fluid velocity profile in the orifice section, and the orifice size effect on the drop of oil level are determined. Different variants of filling the ballast space of a tanker are estimated from the standpoint of minimizing the negative effect of the petroleum products spill on the surrounding medium. It is shown that in the emergency case the presence of ballast space filled with water reduces considerably the volume of the oil seepage into the water.

The flight feather of Aquila Chrysaetos has large flexibility and usually deforms greatly during wing beating, which changes the aerodynamic performances of Aquila Chrysaetos dramatically. However, the flexible effects of flight feather on airfoil aerodynamic performances have not yet been fully conducted, nor the aeroelastic model of flexible feather ever been built. In the current study, the geometry of a wing section with a secondary flight feather of Aquila Chrysaetos was scanned using the tracking laser scanning system to establish the bionic airfoil. The flexibility of the flight feather was measured to obtain the nonlinear beam model. The radial basis function (RBF) mesh motion approach was adopted to dynamically generate the feather mesh with large deformations at each iteration. Thereafter, an aeroelastic approach was established by coupling the CFD method and the structural dynamic systems (CSD) method. In the method, airfoil aerodynamic forces were predicted by the CFD method followed by feather deformation calculations based on the CSD method, and the data was transferred via a fluid-structure interaction interface developed using the multi-point constraint (MPC) approach. Aeroelastic performances of the bionic airfoil with a flexible feather undergoing elastic deformation were simulated by the method proposed. Results showed that feather flexibility has a great influence on aerodynamics. Obtained flexible feather effects and flow mechanisms could be an inspiration for future aircraft design.

The integrated design is of vital importance to the high-speed vehicle due to appropriate aerodynamic configuration under high-speed conditions. Especially, the blended wing body (BWB) design for integration pays great attention to the constraints of various geometric components. The free-form deformation (FFD) parameterization is wildly applied in the aerodynamic shape design as it can effectively control the variation in airfoils and reflect the transition effect of the wing-body fusion area, but take disadvantages on setting constraints for global deformation. In this study, a divided FFD parameterization that clearly distinguishes between the wing, the wing-body fusion and the fuselage in integrated design while maintaining geometric continuity is proposed. This clear definition helps to optimize the aerodynamic performance of each component while ensuring the overall design coherence and consistency. The parameterization proposed is applied to aerodynamic optimization under multiple flight conditions. A comparative analysis reveals that our method can effectively output the reasonable configurations. The optimization uses Bayesian optimization, with 50 iterations, and achieves stability after about 10 iterations. After optimization, the constrained wing section retains the geometric feature of the original configuration design, while the wing-body fusion forms a geometric transition between the two components. The lift-to-drag ratio of the reusable flight vehicle improves significantly across multiple angles of attack at an average of 32.6%.

This study is aimed to investigate the control of the mechanism of unsteady dielectric barrier discharge (DBD) plasma actuation in flow separation of turbine blades and vortex shedding at the trailing edge, as well as the impact of excitation frequency on the control effectiveness. For the T106A turbine blades, the large eddy simulation (LES) method was used to analyze the evolution pattern of the flow field structure under the unsteady plasma actuation. Primary flow patterns and their interactions were identified through the proper orthogonal decomposition (POD) method. The induced vortex structures generated by plasma actuation are coupled with the vortex structures at the trailing edge, which significantly weakens the intensity of vortex structures, leading to a notable improvement in the spatiotemporal structure of the flow field. Simultaneously, plasma actuation enhances the momentum of low-energy fluid, stimulates large-scale turbulent fluctuations in the flow field, and suppresses irregular small-scale turbulent fluctuations. When the excitation frequency is set at the level of 0.8, the induced flow field exhibits better coupling with the vortex structures near the trailing edge. The total pressure loss coefficient diminishes by 20.96%. As the frequency increases, the improvement on flow control effect is not obvious and the wake loss can increase slightly.

Liquid jet flows in the presence of a ventilated cavity with a negative cavitation number are investigated. The studies carried out in the Institute of Mechanics of Moscow State University show that under certain conditions cavitation-induced self-oscillations can occur in the hydraulic system with highly intense pressure fluctuations. The results of an investigation of the axisymmetric model of a pulsed jet generator with liquid jet outflow through a central orifice in a diaphragm and gas blow from the periphery beyond the diaphragm are presented. The two-phase medium outflow was realized through a convergent conical nozzle. The influence of the generator parameters and the distance to a wall (screen) on the efficiency of its operation is investigated. A narrow range of comparatively small blowing, in which high-frequency pressure oscillations are recorded, while the amplitude of impact pressure pulses on the screen is considerably higher than the amplitude of pulses in high-frequency generation regimes, is revealed. This flow regime can be due to the development of two-phase structures on the unstable jet boundary interactioning with the convergent nozzle walls. The evidence for the possible existence of this flow regime has been given by the solution of the plane problem of interaction between a finite jet and an inclined plate for different pressures on the jet surfaces. The problem was solved exactly using the methods of theory of functions of a complex variable for quasi-doubly-periodic theta functions.

The problems of laminar jets that admit self-similar solutions are considered. A method for determining the self-similarity parameter is proposed based on the condition of existence of a solution to equations in self-similar variables under given boundary conditions with only a single self-similarity parameter. In problems of plane free and wall jets the self-similarity parameters are determined analytically. In the problem of a three-dimensional wall jet, the self-similarity parameter is determined using a neural network.

The article describes the methods and results of the study of heat transfer in a wind tunnel and shock tubes in the presence of shock waves. In the first case, the research is carried out using a thermal imager and infrared observation windows, which allow us to determine the patterns of heat transfer on the inner walls of the wind tunnel. The study is carried out for the Mach number 2.48 and the turbulent flow regime. The shock wave is created using a plane wedge mounted on the upper wall of the setup test section. A high-speed thermoelectric detector with a resolution of up to 1 microsecond is used to study heat fluxes in the shock tubes. The experiments are carried out on two diaphragm-type shock tubes at the postshock Mach number of 15.

A series of experiments are conducted with the use of a shock tube at the Institute of Mechanics, Moscow State University to determine radiation absorption spectra in shock-heated oxygen in the wavelength range of 213–260 nm at a gas pressure of 1 Torr and shock wave velocities ranging from 3.4 to 4.5 km/s. Based on a comparison of the experimental data with the results of calculations using a spectral-kinetic model, the effect of bound–bound and bound–unbound transitions in the Schumann–Runge system on the absorption properties of oxygen is analyzed.