Instabilities of nonuniform flows of acoustically active media

The analytic methods and results of investigating the acoustic instability of nonuniform steady channel flows are reviewed. The study is based on the system of equations describing the motion of an electrically conducting gas at low magnetic Reynolds numbers [25]. This makes it possible to consider the acoustic effects in plasma and nonconducting gas flows within the framework of a unified approach.Based on paper presented to the fluid mechanics sections of the Seventh Congress on Theoretical and Applied Mechanics, Moscow, August 1991.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.5, pp. 31–46, September–October, 1992.     

Discharge from a smooth stratified two-phase region through two horizontal side branches located in the same vertical plane

Experimental data are presented for the mass flow rate and quality of two-phase (air-water) discharge from a stratified region through two side branches (6.35 mm W.) with their parallel centrelines located in a vertical plane. These data correspond to different values of the interface level between the onset of gas entrainment at the upper branch to the onset of liquid entrainment at the lower branch for test-section pressures of 316 and 517 kPa, test-section-to-separators pressure difference ranging from 40 to 235 kPa, branch separating distance to diameter ratio ranging from 1.5 to 8 and different hydraulic resistances of the lines connecting the test section to the separators. Normalized plots are shown to be capable of absorbing the effects of some independent variables within the tested range of operating conditions. Empirical relations were developed for the prediction of the two-phase mass flow rate and quality in terms of normalized parameters. These relations represent the data with a high degree of correlation.     

Experimental study of gaseous detonation propagation over acoustically absorbing walls

The paper presents results from an experimental investigation of the propagation of gaseous detonation waves over tube sections lined with acoustically absorbent materials. The measurements were compared with results from control tests in a smooth wall section. The results show the increasing effectiveness of a perforated steel plate, wire mesh and steel wool in attenuating detonation. Received 25 October 2000 / Accepted 20 August 2001     

Shape optimisation of wall structures located in solitary wave flows

ABSTRACT

In this paper, we present a shape optimisation method for wall structures due to the wave force induced by a solitary wave. The fluid is assumed to be incompressible. Introducing the adiabatic assumption in addition, the acoustic velocity method presented by the author's group, the SUPG finite element method, is effectively used. To evaluate the wave force, we use the performance functional, which consists of the sum of the square of the wave force integrated between the starting and final times. The coordinates of the wall structure are regulated to obtain the minimum performance functional. The adjoint equation method is utilised to derive the gradient of the performance functional with respect to the coordinates. The simple weighted gradient method is employed as the minimisation procedure. Two numerical studies show that the results are consistent with existing structures and provide useful information on the practical design of coastal structures.     

A shape optimisation method of a body located in adiabatic flows

The purpose of this study is to derive an optimal shape of a body located in adiabatic flow. In this study, we use the equation of motion, the equation of continuity and the pressure–density relation derived from the Poisson’s law as the governing equation. The formulation is based on an optimal control theory in which a performance function of fluid force is taken into consideration. The performance function should be minimised satisfying the governing equations. This problem can be solved without constraints by using the adjoint equation with adjoint variables corresponding to the state equation. The performance function is defined by the drag and lift forces acting on the body. The weighted gradient method is applied as a minimisation technique, the Galerkin finite element method is used as a spatial discretisation and the implicit scheme is used as a temporal discretisation to solve the state equations. The mixed interpolation, the bubble function for velocity and the linear function for density, is employed as the interpolation. The optimal shape is obtained for a body in adiabatic flows.     

Effects of the side walls on starting flows in ducts

This article considers the effects of the side walls on the unsteady flow of an incompressible viscous fluid in a duct of uniform cross-section. In order to show the effects of the side walls, three illustrative examples are given. They are: the starting flow in a duct of semicircular cross-section, the starting flow in a duct of rectangular cross-section and the starting flow in a duct of circular cross-section. The velocity distributions and the volume fluxes obtained for these flows are compared and it is shown that the flow in a duct of semicircular cross-section reaches steady state earlier than those for the flow in a duct of circular cross-section and for the flow in a duct of square cross-section. It is found that there are remarkable effects of the side walls of a duct on the required time to attain the asymptotic values of flow properties.     

Numerical predictions of flows over backward-facing steps

Predictions are reported for two-dimensional, steady, incompressible flows over rearward-facing steps for both laminar and turbulent conditions. The standard k-? turbulence model was used for the turbulent flow. Attention was focused on obtaining accurate solutions to the differential equations. It is concluded that some of the serious discrepancies that have occurred between prediction and observation, and attributed in earlier studies to the inadequacy of the turbulence model, may have been due to the inaccuracy of the solution.     

Computational modelling of blood flow in side arterial branches after stenting of cerebral aneurysms

The major concern with the use of stents as flow diverters for the treatment of intracranial aneurysms is the potential occlusion of a perforating artery or other side branches which can cause ischemic strokes. This article presents image-based patient-specific models of stented cerebral aneurysms in which a small side artery has been jailed by the stent mesh. The results indicate that, because of the large resistances of the distal vascular beds which dominate the flow divisions among the different arterial branches, the flow reduction in jailed side branches is quite small even when a large percentage of the inlet area of these branches has been blocked. This suggests that unless the side branch is completely occluded, it will likely maintain its normal blood flow. Although this conclusion eases the concern of stenting cerebral aneurysms, a complete occlusion can still be caused depending on the conformability characteristics of the stents.     

Two-way coupled turbulence simulations of gas-particle flows using point-particle tracking

This paper addresses computational models for dilute gas-particle multiphase flow in which the three dimensional, time-dependent fluid motion is calculated in an Eulerian frame, and a large number of particles are tracked in a Lagrangian frame. Point forces are used to represent the back effect of the particles on the turbulence. The paper describes the early development of the technique, summarizes several experiments which show how dilute particle loadings can significantly alter the turbulence, and demonstrates how the point-particle method fails when the particles are comparable in scale to the small scale turbulence. High-resolution simulations and experiments which demonstrate the importance of the flow details around individual particles are described. Finally, opinions are stated on how future model development should proceed.     

Numerical study of turbulent flows over wavy boundaries

The fully developed turbulent flows over wavy boundaries are investigated by means of thek-ε model. Predicted flow characteristics over rigid wavy walls are in good agreement with the vailable experimental data. Moreover drag reduction has been found in a 2-dimensional channel with periodical wavy walls. The energy input from turbulent wind to regular waves is also studied in the paper by the same turbulence model with carefully posed boundary conditions at wind-wave interface. Better agreement has been obtained in the predication of the growth rates of wind waves as compared with the previous theoretical and numerical results. The project supported by the National Natural Science Foundation of China.     

Influx to a partially penetrating gallery with an impermeable barrier located symmetrically relative to the top and bottom of the stratum

The solution is presented of the problem of the flow rate of a partially penetrating gallery in a uniformly anisotropic stratum with an impermeable interstratum. The result obtained is used to calculate the limiting water-free oil flow; the effect of the barrier size is studied.The author wishes to thank I. A. Charnyi, V. A. Evdokimova, and I. N. Kochina for discussing the results of the present study.     

An ISPH simulation of coupled structure interaction with free surface flows

An incompressible smoothed particle hydrodynamics (ISPH) model is developed for the simulation of fluid–structure coupling problems, especially for moving structures. The mirror particle method is employed in the model for a moving boundary. The surface force integration and force-motion algorithms are presented to solve the body translation and rotation. An additional free surface criterion is introduced with the consideration of both the particle number density and the local particle symmetry. A series of numerical experiments are conducted to verify the applicability of the model for simulations of fluid interaction with various types of moving structures. These problems include the fluid motion by a moving body with a prescribed trajectory, such as liquid sloshing in a moving tank. Water entry problems in which the body motions are coupled with the fluid forces are also studied. In all of the cases, there is good agreement when the numerical results are compared with the available analytical, experimental and other numerical data found in the literature.     

Large-scale structures of turbulent flows over an open cavity

Low Mach number turbulent flows over an open cavity were studied to investigate the quantitative characteristics of large-scale vortical structures responsible for self-sustained oscillations. Wind tunnel experiments with particle image velocimetry (PIV) were conducted in the range of the ratio of cavity length (L) to depth (D), 1<L/D<4, when the incoming boundary layer is turbulent at Re_θ=830 and 1810. Self-sustained oscillation modes were classified by varying the conditions of L/D and Re_θ. The oscillation modes were consistent with the number of vortical structures existing between the leading and trailing edges of the cavity. Proper orthogonal decomposition (POD) was employed to the spatial distributions of vertical velocity correlations on the lip line of cavity geometry. By examining the conditionally averaged distributions of the correlation coefficients of POD, the spatial characteristics of large-scale vortical structures for self-sustained oscillations were examined.     

Self-sustained oscillations of turbulent flows over an open cavity

The mechanism of self-sustained oscillations in laminar cavity flows has been well characterized; however, the occurrence of self-sustained oscillations in turbulent cavity flows has only previously been characterized by direct observation of flows. Here, the quantitative characteristics of vortical structures in turbulent flows over an open cavity were determined, and then statistical properties were examined for evidence of self-sustained oscillations. Specifically, instantaneous velocity fields were measured using PIV and wall pressure fluctuations were determined from microphone data. Cavity geometries of L/D = 1 and 2, where L and D are the length and depth of the cavity, respectively, were used under conditions where the incoming boundary layer was turbulent at Re _θ  = 830. Statistical analyses were applied based on the instantaneous velocity fields of PIV data. The spatial distributions of vertical velocity correlations (v–v) showed alternating patterns that reflect the organized nature of the large-scale vortical structures corresponding to the modes of N = 2 for L/D = 1 and N = 3 for L/D = 2. These values were consistent with the numbers of vortical structures obtained from a modified version of Rossiter’s equation. Furthermore the numbers of vortical structures determined in the statistical analyses were consistently observed in instantaneous distributions of the swirling strength (λ _ci). The incoming turbulent boundary layer can give rise to the formation of large-scale vortical structures responsible for self-sustained oscillations.     

Numerical simulation of viscous transonic flows over an airfoil

High Reynolds number viscous transonic flow is described based on an interaction of the potential outer flow with the boundary layer and wake. Following the procedure of Lighthill (1958), the solutions in these domains are matched to each other through boundary conditions. The solution to the complete problem is obtained iteratively through successive computations of the flows in the outer and inner domains. Both old and new algorithms are used for the iteration process and subsequent problem solution. Results are given for all the airfoils from the Experimental Data Base for Computer Program Assessment (AGARD-AR-138, 1979). A comparison of these results with experimental data shows the degree of agreement between these unbounded airfoil flow simulations and real transonic flow over the central part of a straight wing.     

Numerical simulation of fully-developed compressible flows over wavy surfaces

Rough surfaces are common on high-speed vehicles, for example on heat shields, but compressibility is not usually taken into account in the flow modelling other than through the mean density. In the present study, supersonic fully-developed turbulent rough wall channel flows are simulated using direct numerical simulation to investigate whether strong compressibility effects significantly alter the mean flow and turbulence properties across the channel. The simulations were run for three different Mach numbers M = 0.3, 1.5 and 3.0 over a range of wall amplitude-to-wavelength ratios from 0.01 to 0.08, corresponding to transitionally and fully rough cases respectively. The velocity deficit values are found to decrease with increasing Mach number. It is also found that at Mach 3.0 significant differences occur in the mean flow and turbulence statistics throughout the channel and not just in a roughness sublayer. These differences are found to be due to the presence of strong shock waves created by the peaks of the roughness elements.     

Non-invasive measurements of granular flows by magnetic resonance imaging

Magnetic Resonance Imaging (MRI) was used to non-invasively measure velocity and concentration of granular flows in a partially filled, steadily rotating, long, horizontal cylinder. First, rigid body motion of a cylinder filled with granular material was studied to confirm the validity of this method. Then, the density variation and the depth of the flowing layer, where particles collide and dilate, and the flow velocity profile were obtained as a function of the cylinder rotation rate.     

Similitude of hypersonic flows of low-density gas over blunt bodies

Laws of similitude of hypersonic flows of monatomic gases have been obtained earlier from asymptotic analysis of the equations as S and confirmed by experimental data and numerical results [1], For diatomic gases, dimensionless numbers have not been deduced by analyzing the equations but by general arguments based on analogy with monatomic gases; they were used to compare experimental and calculated results in [1–3]. In the present paper, dimensionless numbers are derived on the basis of model kinetic equations for a diatomic gas, and limits of their applicability are established. Numerical calculations confirm the exact and approximate laws of similitude and permit a comparison with experimental results. The influence of the laws of viscosity on the drag for a sphere as a function of the Reynolds number Re₀ determined using the viscosity at the stagnation point is investigated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 130–135, March–April, 1981.