Recombination of two vortex filaments and jet noise

The recombination of two vortex filaments in a viscous incompressible fluid is analysed by the use of the vorticity equation. The analysis is confined to a local flow field, where the recombination process occurs, and is based on several assumptions, such as the conservation of the fluid impulse, spatial symmetry of the flow field etc. The flow field is expanded as polynomials of coordinates, and variations of their coefficients are obtained by the use of the vorticity equation. It is proved that the process is completed within a short time ofO (σ ²/Γ) and the viscous effect is essential;σ and Γ are the size and the circulation of the vortex filaments, respectively. This result is applied to predict the far-field noise of a circular jet by assuming that the main noise source is the recombination process in deformed vortex rings in the jet near field. The predicted noise intensity shows theU dependence and has an additional new factor (d/σ)⁶;U is the jet velocity andd is the average spacing between vortex rings.     

Numerical simulation of aerosol collection in filters with staggered parallel rectangular fibres

 In this paper, filters with rectangular fibres arranged in a staggered and parallel array and placed transverse to the flow are studied numerically. A two- dimensional flow field is obtained by solving Navier–Stokes equations with the control volume method. Periodic boundary conditions are introduced in the calculation. In order to achieve higher accuracy, a second-order upwind scheme is adopted and a fine mesh is arranged near the fibre and the symmetrical plane of the flow field where large gradients in velocity are expected. Particle trajectories are calculated by solving the corresponding Lagrangian equation of motion to obtain the collection efficiency of a single rectangular fibre, in which positions of the approaching particles on the inlet plane of the flow field are randomly distributed according to the Monte-Carlo principle. The simulation considers all the important mechanisms of particle capture including interception, inertial impaction and Brownian motion. Effects of fibre aspect ratio, filter packing density, particulate size and Reynolds number on the collection efficiency are numerically determined. The volumetric packing density ranges from 0.4 to 4% and the particle diameter is from 0.01 μm to 2 μm. Reynolds number based on the height of computational domain varies from 20 to 100 and the aspect ratio is from 0.1 to 10. Simulations with and without Brownian motion are carried out for different Reynolds numbers, packing densities and aspect ratios and the results show that Brownian effects are significant for particles smaller than 1 μm. Received 25 May 2001     

Heat exchange in turbulent flow of an incompressible liquid in a rough tube with a constant wall temperature

Turbulent flow of an incompressible liquid in a tube is considered as flow of an oriented liquid whose symmetry is determined by the director. The velocity profile and the temperature field in steady-state flow in a straight circular tube with a rough wall are determined within the framework of the model. The solutions found coincide with analogous solutions for a smooth tube for roughness equal to zero. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 89–96, September–October, 2007.     

Effect of a moving boundary on pulsatile flow of incompressible fluid in a tube

The pulsatile flow in a pipe with a moving boundary has been studied for a viscous, incompressible fluid by solving the Navier-Stokes equations numerically. The governing equations were formulated in boundary fitted curvilinear coordinates and a finite volume discretization procedure was used to solve the problem. This analysis is based on the assumption that the flow has a simple periodic pulsation and the shape of the wall changes according to the frequency of pulsation. The presence of the moving boundary causes unsteadiness in the flow behaviour as the vibrating wall has a nonlinear interaction with the flow. A detailed analysis of the flow field is presented here for a range of frequencies (5≤α≤10) where α is the reduced frequency parameter and a Reynolds number of 100.     

Vortex motion in the early stages of unsteady flow around a circular cylinder

In this paper, processes in the early stages of vortex motion and the development of flow structure behind an impulsively-started circular cylinder at high Reynolds number are investigated by combining the discrete vortex model with boundary layer theory, considering the separation of incoming flow boundary layer and rear shear layer in the recirculating flow region. The development of flow structure and vortex motion, particularly the formation and development of secondary vortex and a pair of secondary vortices and their effect on the flow field are calculated. The results clearly show that the flow structure and vortices motion went through a series of complicated processes before the symmetric main vortices change into asymmetric: development of main vortices induces secondary vortices; growth of the secondary vortices causes the main vortex sheets to break off and causes the symmetric main vortices to become “free” vortices, while a pair of secondary vortices is formed; then the vortex sheets, after breaking off, gradually extend downstream and the structure of a pair of secondary vortices becomes relaxed. These features of vortex motion look very much like the observed features in some available flow field visualizations. The action of the secondary vortices causes the main vortex sheets to break off and converts the main vortices into free vortices. This should be the immediate cause leading to the instability of the motion of the symmetric main vortices. The flow field structure such as the separation position of boundary layer and rear shear layer, the unsteady pressure distributions and the drag coefficient are calculated. Comparison with other results or experiments is also made. This work was presented at the First Asian Congress of Fluid Mechanics, Bangalore in December 1980.     

The secondary splitting of zero-gradient points in a scalar field

The mechanisms related to the secondary splitting of zero-gradient points of scalar fields are analyzed using the two-dimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnation-point flow, cf. Gibson (Phys Fluids 11:2305–2315, 1968), which is approximated using a Taylor expansion up to third order. The temporal evolution of the scalar field in the vicinity of the stagnation point is derived using a series expansion, and it is found that the splitting can only be explained when the third-order terms of the Taylor expansion of the flow field are included. The non-dimensional splitting time turns out to depend on three parameters, namely the local Péclet number Pe _δ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting casePe _δ → 0, the splitting time is found to be finite but Péclet-number independent, while for the case of Pe _δ → ∞ it increases logarithmically with the Péclet number. The physical implications of the two-dimensional mathematical solution are discussed and compared with the splitting times obtained numerically from a Taylor–Green vortex.     

Extraction of water from a capillary specimen in an acoustic field

The authors present results of experiments on drying of a model capillary specimen in a subheated air flow in an acoustic field. It is established that the acoustic field causes considerable intensification of the drying process. The influence of the specimen position relative to the front of an acoustic wave and the flow velocity is established that the acoustic field causes considerable intensification of the drying process. The influence of the specimen position relative to the front of the acoustic wave and the flow velocity is investigated. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 4, pp. 686–687, July–August, 2000.     

Some effects of an electric field on the plastic deformation of metals and ceramics

 The external parameters generally considered in the plastic deformation of metals and ceramics are the temperature, pressure or stress and time. Usually neglected are the effects of electric and magnetic fields. However, such fields can often have a significant influence, especially when applied concurrently with the more common parameters. Some examples of the effects of an electric field on the plastic deformation of metals and ceramics found by the author and his coworkers are presented. Included are the following: (a) the influence of electropulsing on the flow stress of metals at 78–300 K, (b) the effect of an external electric field (surface charge) on the superplastic deformation of the 74754 Al alloy, (c) the influence of an electric field on the flow stress and ductility of polycrystalline NaCl at 0.28–0.75 T_M and (d) the effect of an electric field on the superplastic deformation of 3Y-TZP. Mechanisms responsible for the observed effects are considered. Received: 1 January 1998 / Accepted: 1 March 1998     

Vortex electromagnetic flow meters for liquid metal coolants

Magnetohydrodynamic (MHD) effects which arise when a magnetic field interacts with a Karman vortex street in the flow of a liquid-metal coolant are considered. The features of MHD effects in light and heavy coolants (for example, sodium and lead) are analyzed. Devices and operating principles of induction and conduction vortex flow-rate converters are described. Data of experiments are presented which enable vortex electromagnetic flow meters to be recommended as an effective means of measuring the flow rate of liquid metals, for example, in nuclear power engineering. __________ Translated from Izmeritel’naya Tekhnika, No. 1, pp. 43–47, January, 2007.     

Viscous flow over a sphere with fluctuations in the free-stream velocity

 The problem of viscous flow over a sphere with fluctuations in the free-stream velocity is considered. The governing conservation equations are expressed terms of the stream function and vorticity and solved using the series truncation method where the stream function and vorticity are approximated using finite series of Legendre and first associated Legendre functions. The effects of the Reynolds number, Strouhal number, and the amplitude of the fluctuations on the flow characteristics are studied. Results are presented for periodic variation of the drag coefficient, surface vorticity and pressure distributions for Reynolds numbers ranging from 5 to 100, Strouhal numbers ranging from π/4 to π, and amplitude of fluctuations from 0.0 to 0.5. The time variation of the velocity field during one complete cycle is presented in the form of streamline and equi-vorticity patterns. The periodic variation of the angle of separation as well as the length of the separation region are also presented. Received 28 February 1999     

Hydromagnetic convection flow through a porous medium in axially varying pipe

The hydromagnetic mixed convection flow through a porous medium in a pipe of varying radius in a uniform axial magnetic field is analyzed. The pipe wall is maintained at a prescribed nonuniform temperature. The governing equations are solved analytically to obtain the velocity, temperature, and induced magnetic field. Their behaviors are evaluated for different variations in the governing parameters. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 4, pp. 96–104, July–August, 2006.     

Features of electromagnetic thermal protection of space capsule

The problem of using a magnetic field to reduce heat fluxes on the surface of a flying vehicle is discussed as a continuation of the previous investigations [1–3]. A classical electromagnetic thermal protection scheme that uses a magnetic dipole oriented opposite to the oncoming flow is considered for a space capsule of Stardust (NASA space probe) mission entering the Earth’s atmosphere. It is shown that, for fairly high magnetic fields, the distribution of heat flux on the capsule surface weakly depends on the field amplitude, while the shock-wave stand-off distance from the surface increases in proportional to the field amplitude.     

Corner flow in free liquid films

A lubrication-flow model for a free film in a corner is presented. The model, written in the hyperbolic coordinate system ξ = x ² −y ², η = 2xy, applies to films that are thin in the η-direction. The lubrication approximation yields two coupled evolution equations for the film thickness and the velocity field which, to lowest order, describes plug flow in the hyperbolic coordinates. A free film in a corner evolving under surface tension and gravity is investigated. The rate of thinning of a free film is compared to that of a film evolving over a solid substrate. Viscous shear and normal stresses are both captured in the model and are computed for the entire flow domain. It is shown that normal stress dominates over shear stress in the far field, while shear stress dominates close to the corner. This revised version was published online in October 2005 with corrected page numbers.     

“Salt in a porous matrix” sorbent and sawdust as air driers for ventilation systems

The processes of moisture desorption by a sorbent-drier of the type of “salt in a porous matrix” and sawdust in convective flow, as well as under the acoustoconvective action, have been investigated. It has been shown that at a flow velocity of 31 m ⁄ sec the sorbent has a rather prolonged drying stage with a constant rate, and at a flow velocity of 13 m ⁄ sec this drying stage ends faster. It has been established that the velocity of the convective flow strongly influences the drying process of the sorbent, and the action of the acoustic field therewith produced no marked effect on the drying process. Moisture desorption from sawdust proceeds with a variable rate, and its intensity markedly increases under the action of the acoustic field. The influence of acoustic vibrations on the rate of moisture removal has a nonmonotonic character. The kinetics of the process of humidification of the sorbent-drier has been investigated. On the basis of the investigations made, a basic circuit for ventilating closed rooms by dried air with the use of sawdust or an IK-011-1 sorbent-drier has been proposed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 252–257, March–April, 2009.     

3D fluid–structure-contact interaction based on a combined XFEM FSI and dual mortar contact approach

Finite deformation contact of flexible solids embedded in fluid flows occurs in a wide range of engineering scenarios. We propose a novel three-dimensional finite element approach in order to tackle this problem class. The proposed method consists of a dual mortar contact formulation, which is algorithmically integrated into an eXtended finite element method (XFEM) fluid–structure interaction approach. The combined XFEM fluid–structure-contact interaction method (FSCI) allows to compute contact of arbitrarily moving and deforming structures embedded in an arbitrary flow field. In this paper, the fluid is described by instationary incompressible Navier–Stokes equations. An exact fluid–structure interface representation permits to capture flow patterns around contacting structures very accurately as well as to simulate dry contact between structures. No restrictions arise for the structural and the contact formulation. We derive a linearized monolithic system of equations, which contains the fluid formulation, the structural formulation, the contact formulation as well as the coupling conditions at the fluid–structure interface. The linearized system may be solved either by partitioned or by monolithic fluid–structure coupling algorithms. Two numerical examples are presented to illustrate the capability of the proposed fluid–structure-contact interaction approach.     

Filtration of a gas-condensate mixture near the hydraulic fracture

The problem of filtration of a gas-condensate mixture near the operating well with a hydraulic fracture has been investigated. It was assumed that flow is three-dimensional in the matrix and two-dimensional on the fracture. It has been shown that for steady-state flow, the problem is split into a purely physicochemical problem on phase transitions and a nonlinear partial boundary-value problem for the pressure field. An example of numerical solution for the fracture with a prescribed conductivity field has been given. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 3, pp. 409–416, May–June, 2008.     

The process of drying of a capillary specimen subjected to an acoustic-convective effect

The process of drying of a model capillary specimen located in an intense acoustic field under the conditions of convective blowing is investigated. Video recording has registered a drop mechanism of water extraction from the capillaries of the specimen. It is shown that the driving force of this process is the difference between the acoustic pressure in the external flow and inside the cavity of the specimen. Measurements have demonstrated a certain increase in the temperature inside a capillary exposed to an acoustic field. The influence of the position of capillaries relative to the sonic wave front and of the flow velocity is investigated. It is shown that in the capillaries positioned horizontally the temperature in them is much lower than in those positioned vertically. Kinetic curves of drying of the specimen are obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 166–172, March–April, 2007.     

MHD Flow past a circular cylinder – a numerical study

 The steady, two-dimensional, incompressible MHD flow past a circular cylinder with an applied magnetic field parallel to the main flow is studied using the finite difference method. The magnetic Reynolds number is assumed to be small. Results are presented up to the Reynolds number R=500 and interaction parameter N=1.3. As N increases suppression of the separation is observed. Drag coefficient is decreasing for the small values of N and then increasing as N increases. It is found that a smaller value of far field distance is required as N increases. Received 6 March 2000     

Thermal convection in square porous cavity under transverse oscillatory micro gravity field

Two-dimensional thermal convection in saturated square porous cavity under transverse sinusoidal micro-gravity field, has been analyzed by solving mass and transient momentum and energy balance equations, using Darcy’s law and Boussinesq’s approximation. Isothermal boundary conditions are considered at the two vertical walls but at different temperatures and other two horizontal walls are adiabatic. Average Nusselt numbers are presented as a function of Darcy-Rayleigh number (Ramod) and dimensionless time. Thermally convective, transient nature of the flow field has also been presented for different Darcy-Rayleigh number (Ramod).     

Modified zonal model for calculating the thermodynamics of the gas in a fire within an atrium

A modified zonal model for calculating the thermodynamics of the gas in a fire has been developed. An equation for determining the flow rates of the gas-mixture and smoke at cross sections of a convective column is proposed. The results of numerical calculations of the parameters of the heat and mass transfer in a fire within an atrium with the use of the modified zonal model as well as zonal and field models are presented and discussed. Specifics of the use of zonal models for estimating the fire hazard in an atrium are considered. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 84–89, March–April, 2007.