On identification of flow separation and vortices in internal periodic flows

To permit simplified analysis of complex time-dependent flows, possible relationship between the near-wall flow, flow separation and vortices are studied numerically for a flow in a constricted two-dimensional channel. The pulsating incoming wave-form consists of a steady flow, followed by a half-sinus flow superimposed on the steady component. One pair of vortices is created in each cycle, one vortex near each wall. The vortices propagate downstream in the next cycles, promoting flow separation as they move. Existing flow separation criteria were not found to be uniformly valid. A relation between the near-wall flow and the vortical system exists only during the steady incoming flow phase of the cycle. It seems that local criteria of flow separation cannot be found for complex internal pulsating flow fields. However, the vorticity field can be utilized, even in complex time-periodic flows, for identifying vortices that have been formed by the roll-up of shear layers.     

明渠交汇口分离区三维几何特性的大涡模拟研究

为更准确地把握交汇角对分离区三维几何特性的影响,建立了不同角度的交汇水槽模型并进行数值模拟。采用大涡模拟(LES)方法求解交汇区的湍流流场,并基于平衡层模型的Werner壁面函数法处理近壁区流场。模拟所得垂向流速分布及分离区尺寸等结果与实测资料吻合程度较高。以90°交汇水槽为例较详尽地分析了分离区的三维几何特性,并从流场结构角度对其形成机理进行了剖析。随后聚焦于不同交汇角度下分离区的三维几何特性,给出了交汇角对不同水深层面分离区各几何参数的影响。研究结果显示:交汇口的分离区三维几何特性与流场结构有较强的关联性,分离区宽度沿水深的分布规律是分离区内外的流向涡及与之伴随的象限事件作用的结果。随着交汇角的增大,各水深层面的分离区长度基本呈增大趋势;当交汇角达到105°时,各水深层面分离区长度达到最大值,其原因是受干流顶冲作用,支流的实际入流角将小于交汇角,当交汇角为90°时,支流入汇对分离区长度的影响达到最大;随着交汇角的增大,各水深层面的分离区宽度总体呈增大趋势,显示出与90°交汇水槽相似的分布特性;随着交汇角的增大,各水深层面的分离区对称系数逐渐减小,这是分离区附近水平绕流涡的三维取向和分离区的长度变化共同作用的结果。     

An analysis of the structure of laminar separation bubbles in swept infinite geometries

The influence of sweep on the general structure of short separation bubbles in strictly laminar flow fields on swept infinite geometries is investigated by theoretical analysis and direct numerical simulations (DNS). In this situation the ‘independence principle’ of the Navier–Stokes equations for incompressible flow enforces a unique topology for the mean flow, which includes the much better understood unswept separation bubbles as a special case: swept laminar separation bubbles form leading edge parallel streamtubes with a spanwise outflow and a helical motion inside directed parallel to the separation line. If chordwise inflow conditions are kept constant, their cross-sections stay independent of a rising sweep angle, as the spanwise velocity field is then merely superimposed over the unchanged flow of the corresponding unswept case. Their mean flow field follows strict scaling rules that may be derived analytically from the generic 45°-solution, as confirmed by DNS-results for a series of pressure-induced separation bubbles subjected to a systematic variation of the sweep angle between 0° and 60°.     

Control of flow separation from a model wing at low Reynolds numbers

The results of an experimental investigation of the structure of the flow separated from the model of a straight wing with point sources of disturbances (bulges) made on its surface are presented. The variations in the three-dimensional flow pattern are analyzed as functions of the bulge shapes and positions. It is found that the flow can be controlled by means of mounting the bulges downstream of the separation line, in the return flow region, since in this case they hinder large-scale vortex formation in the separation zone. The results obtained show that there is an intimate connection between the vortices and the separation zone as a whole. Impeding the vortex structure formation can result in considerable variations in the separation zone structure, up to its complete disappearance.     

Flow instability in the laminar boundary layer separation zone created by a small roughness element

The development of disturbances of the laminar flow in the separation zone behind a surface projection in the boundary layer on a flat plate has been experimentally investigated. The linear instability characteristics of the separated flow are determined and the interaction between the oscillations growing in the separation zone and the average flow is studied.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 16–22, January–February, 1990.     

Supersonic Flow Past a Circular Cylinder with an Isothermal Surface

Supersonic perfect-gas flow past a circular cylinder with an isothermal surface is investigated at the Mach number 5 and Reynolds numbers ranging from 30 to 500,000. It is shown that two branches of the numerical solution of the problem can exist. On the first branch the following flow patterns are successively realized as Re is increased: separationless flow, flow with formation of a local separation zone, and flow with formation of a global separation zone. On the second branch the flow pattern with a local separation zone is observed at all Reynolds numbers; at a certain value of Re this solution jumps to the first branch.     

Vortex structure of separated flows on model wings at low freestream velocities

Flow past model wings is experimentally investigated in a subsonic wind tunnel at large angles of attack at which the laminar boundary layer separates near the leading edge of the wing (flow stall). The object of the study was the flow structure within the separation zone. The carbon-oil visualization of surface streamlines used in the experiments showed that in the separation zone there exist one or more pairs of large-scale vortices rotating in the wing plane. Certain general properties of the vortex structures in the separation zone are found to exist, whereas the flow patterns may differ depending on the model aspect ratio, the yaw angle, and other factors.     

Experimental flow analysis of three-dimensional separation zones in front of weirs

Some results of experimental studies are shown concerning subsonic flow in separation zones of three-dimensional turbulent boundary layers formed in front of cylindrical weirs and rectangular parallelepipeds or dashboards. The width to height ratio of the weirs was varied from 0.25 to 24, and the boundary layer thickness to weir height ratio at separation was varied from 0.2 to 2.0. Flow patterns are shown along with the effects of the setup ge-ometry, of the weir width to height ratio, of the boundary layer parameters, and of the Euler and Reynolds numbers on the flow pattern and on the coordinates of characteristic points in the separation zone. Data are furnished for determining the dimensions of three-dimensional separation zones in front of weirs. The flow and the heat transfer in three-dimensional separation zones at subsonic velocities have not yet been explored adequately. The separation data published in [1, 2, 3] are not sufficient for determining the flow pattern, the static pressure distribution, and the characteristic dimensions of a separation zone — all of which are needed for calculating the heat transfer in the separation zone [4].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 50–54, January–February, 1972.The authors thank V. S. Avduevskii for reviewing the results.     

Flow separation through annular constrictions in tubes

The separation of a flow through a tube having a local annular constriction was studied experimentally. Empirical expressions for the Reynolds number at which separation occurs and for the length of the separation zone were determined in terms of the flow and of the constriction characteristics. The implications of this phenomenon are discussed in relation to an occlusive vascular stenosis.     

Effect of unsteady disturbances on the flow in a forward separation zone

The effect of low-frequency disturbances of the three-dimensional separation zone formed in supersonic flow over a sphere with a capped spike on the flow in the forward separation zone has been systematically analyzed on the basis of a large series of experiments. The separation zone was disturbed by rotating the spike about its own axis at various angular velocities. The investigation was carried out using motion-picture records of the flow pattern around the model and the pressure and heat flux distributions on the surface of the sphere.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.2, pp. 185–188, March–April, 1992.     

Jet Flow Past a Plate with a Spoiler in the Presence of a Stagnation Zone

An exact analytical solution of the problem of the jet flow past a flat plate with a spoiler is obtained for the case in which there is a stagnation zone near the spoiler. The Chaplygin method of singular points and the theory of elliptic theta-functions are used to construct the solution. The pressure in the stagnation zone is determined from the Brillouin-Villat condition of smooth separation. It is found that the lift and drag coefficients, considered as functions of the stagnation zone length, have extrema at points corresponding to the smooth separation condition.     

Particle and droplet deposition in turbulent swirled pipe flow

In this work we study deposition of particles and droplets in non-rotating swirled turbulent pipe flow. We aim at verifying whether the capability of swirl to enhance particle separation from the core flow and the capability of turbulence to efficiently trap particles at the wall can co-exist to optimize collection efficiency in axial separators. We perform an Eulerian–Lagrangian study based on Direct Numerical Simulation (DNS) of turbulence, considering the effect of different swirl intensities on turbulence structures and on particle transfer at varying particle inertia. We show that, for suitably-chosen flow parameters, swirl may be superimposed to the base flow without disrupting near-wall turbulent structures and their regeneration mechanisms. We also quantify collection efficiency demonstrating for the first time that an optimal synergy between swirl and wall turbulence can be identified to promote separation of particles and droplets.     

Formation of laminar lateral separation caused by a sustainer engine jet

The early stage of separation flow formation on a cylindrical surface immediately after the onset of jet exhaust from the base is investigated numerically for various values of the jet-to-ambient pressure ratio on the basis of the complete time-dependent Navier-Stokes equations, the energy equation and the equations for the transfer and diffusion of the exhaust gas concentration. The time dependence of the geometrical and gasdynamic parameters in the separation flow region is obtained for various flow regimes. It is found that the process is periodic with transition from the flow regime with an open separation zone to that with a closed zone, and vice versa, until the final flow regime is attained, which may be open, closed, or periodic.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 122–130, July–August, 1995.     

Characteristics of flow and liquid distribution in a gas–liquid vortex separator with multi spiral arms

Fischer–Tropsch (F–T) synthesis is an important route to achieve the clean fuel production. The performance of gas–liquid separation equipment involving in the progressive condensation and separation of light and heavy hydrocarbons in the oil-gas products has become a bottleneck restricting the smooth operation of the F–T process. In order to remove the bottleneck, a gas–liquid vortex separator with simple structure, low pressure drop and big separation capacity was designed to achieve the efficient separation between gas and droplets for a long period. The RSM (Reynolds Stress Model) and DPM (Discrete Phase Method) are employed to simulate the flow characteristics and liquid distribution in the separator. The results show that the separation efficiency is influenced by the flow field and liquid phase concentration in the annular zone. The transverse vortex at the top of spiral arm entrains the droplets with small diameter into the upper annular zone. The entrained droplets rotate upward at an angle of about 37.4°. The screw pitch between neighbor liquid threads is about 0.3 m. There is a top liquid ring in the top of annular zone, where the higher is the liquid phase concentration, the lower is the separation efficiency. It is found that by changing the operating condition and the annular zone height the vortex can be strengthened but not enlarged by the inlet velocity. The screw pitch is not affected by both inlet velocity and annular zone height. The liquid phase concentration in the top liquid ring decreases with both the increases of inlet velocity and annular zone height. The total pressure drop is almost not affected by the annular zone height but is obviously affected by the inlet velocity. When the height of annular zone is more than 940 mm, the separation efficiency is not changed. Therefore, the annular zone height of 940 mm is thought to be the most economical design.     

Stokes Plane-Parallel Vortex Systems in Channels

Plane-parallel vortex systems in a viscous incompressible fluid in channels with parallel walls and in a corner with no-slip conditions on the walls are investigated on the basis of exact solutions of the biharmonic equation. It is found that separation zones and paired (joined) vortices are formed when the fluid flows through these channels and the fluid flow path between these vortex formations is traced. The flow structures are considered in the region of intrusion of the Poiseuille or Hamel flow into the zone of predominance of the intense vortex formations and for outflow of the fluid from this zone.     

On asymptotic theory of separated flows past bodies

Symmetric two-dimensional steady flow past a body in a homogeneous incompressible fluid stream at high Reynolds numbers is considered. A slow motion in the reverse flow zone is investigated and the solution for the flow in the external region is obtained in the second approximation. Additional considerations of the fact that the flow in the closure region of the separation zone and in the wake behind this zone is turbulent are presented. The laminar-turbulent transition in the mixing layer is analyzed and an analogy between this process and the propagation of perturbations upstream of the boundary layer interaction regions is revealed.     

Effect of a local stationary flow disturbance on separated boundary layer stability

The separated flow past a transverse barrier on a plate surface is modeled in a wind tunnel. The linear stability of the two-dimensional laminar flow in the separation zone is investigated in the presence of a stationary disturbance imposed on the flow and concentrated in a narrow spanwise region. It is experimentally shown that the local flow nonuniformity leads to a change in the flow stability features, such as the frequencies of the growing oscillations, their growth rate, and the dispersion characteristics. As a result, the transverse velocity gradients induced in the separation zone exert a strong destabilizing influence on the flow. Novosibirsk. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 174–178, January–February, 2000. The study was carried out with the support of the INTAS Foundation under grant No, 96-2225.     

Methods of Solving Nonlinear Problems of Hydrodynamic Impact in Bounded Regions

The plane problem of separation impact on a plate floating on the surface of an ideal incompressible fluid in a bounded vessel is considered. In this problem the zone of contact between the body and the fluid is not known in advance and must be determined together with the fluid flow. As a result, the problem formulated is nonlinear and belongs to the class of free-boundary problems. The effect of vessel walls of different shapes on the fluid particle separation zone formed on the plate surface is studied. As examples, the problems for a layer or a truncated circular meniscus are considered.     

Numerical study of magnetohydrodynamic laminar flow separation in a channel with smooth expansion

The flow of an electrically conducting incompressible viscous fluid in a plane channel with smooth expansion in the presence of a uniform transverse magnetic field has been analysed. A solution technique for the governing magnetohydrodynamic equations in primitive variable formulation has been developed. A co‐ordinate transformation has been employed to map the infinite irregular domain into a finite regular computational domain. The governing equations are discretized using finite‐difference approximations in staggered grid. Pressure Poisson equation and pressure correction formulae are derived and solved numerically. It is found that with increase in the magnetic field, the size of the flow separation zone diminishes and for sufficiently large magnetic field, the separation zone disappears completely. The peak u‐velocity decreases with increase in the magnetic field. It is also found that the asymmetric flow in a symmetric geometry, which occurs at moderate Reynolds numbers, becomes symmetric with sufficient increase in the transverse magnetic field. Thus, a transverse magnetic field of suitable strength has a stabilizing effect in controlling flow separation, as also in delaying the transition to turbulence. Copyright © 2008 John Wiley & Sons, Ltd.     

Flow behind the boundary layer separation point in a supersonic stream

The flow structure behind the separation point of a laminar boundary layer in a supersonic stream has been investigated. Analytic and numerical solutions are obtained for simple semiinfinite separation zones starting from the leading edge or a point on the smooth surface. The question of the pressure plateau in a separation zone of finite length is discussed and its value is calculated on the basis of asymptotic theory. The asymptotic theory of flow [1, 2] in the neighborhood of the separation point of the laminar boundary layer in a supersonic gas stream (region of free interaction) is employed. The local solution obtained is subsequently used to construct the flow pattern in the separation zone [3]. An analysis is made of the behavior of the solution for the free-interaction region on transition to the region of reverse flows. The results make it possible actually to compute (in the first approximation) the pressure in the plateau region after establishing the mathematical significance of this concept, previously introduced on the basis of the experimental results. At the same time relatively simple solutions are obtained for semiinfinite separation zones.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 19–25, May–June, 1971.