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Stochastization of boundary-layer flow has a dramatic effect on the aerodynamic characteristics of wings, nacelles, and other objects frequently encountered in practice, resulting in higher skin-friction drag and worse aerodynamic quality. A swept-wing boundary layer encountering a transition to turbulence in the presence of two-dimensional surface reliefs is considered. The relief has the form of strips of a rectangular cross-section oriented parallel to the leading edge and located at different distances from it. The computations are performed for the angle of attack of −5° and an incoming flow velocity of 30 m/s using the ANSYS Fluent 18.0 software together with the author's LOTRAN 3 package for predicting the laminar–turbulent transition on the basis of the e N -method. New data on distributions of N factors of swept-wing cross-flow instability affected by the surface relief are presented. The data are of practical importance for engineering modeling of the transition. Also, the effectiveness of using the reliefs as a passive method of weakening the cross-flow instability up to 30% to delay the flow stochastization is shown. [ABSTRACT FROM AUTHOR]

The article deals with the downstream propagation of small–amplitude disturbances of viscous incompressible laminar boundary layers, using the linearized equations for disturbance amplitudes. Two different methods are proposed. The first one solves a two-dimensional boundary-value problem, using a buffer-domain technique to mimic the outflow boundary condition. The second one solves a streamwise initial-value problem, using a spectral parabolization at each integration step. Both methods show good performance in simulating the propagation of the Tollmien–Schlichting waves and the Görtler vortices and can be applied to compute the spatial optimal disturbances. [ABSTRACT FROM AUTHOR]

Approximations of the profiles of the longitudinal and transverse velocity components in the boundary layer calculated for the flows around a swept wing and a body of revolution by means of solving the full Navier–Stokes equations and using the boundary layer profiles from the self-similar one-parameter family of the Falkner–Skan–Cooke profiles and two-parameter family of profiles proposed by Gaster are compared. A significant advantage of using the approximation of the numerical profiles near three-dimensional separation by profiles from the two-parameter family is demonstrated. [ABSTRACT FROM AUTHOR]

Results of a numerical study of the laminar-turbulent transition in the boundary layer of a prolate spheroid obtained with the use of the ANSYS Fluent gas-dynamic software with an in-house developed module of the laminar–turbulent transition based on the -method are reported. Based on experimental data, a method of determining individual threshold N-factors for different transition mechanisms is proposed. [ABSTRACT FROM AUTHOR]

In this paper, known probabilistic methods for estimating the thickness of the boundary layer of a two-dimensional laminar flow of viscous incompressible fluid are extended to threedimensional laminar flows of a viscous compressible medium. Their applicability to the problems of boundary-layer stability is studied with the LOTRAN3 software package, which allows us to compute the position of laminar-turbulent transition in three-dimensional aerodynamic configurations. [ABSTRACT FROM AUTHOR]

A review of results obtained in recent years by the authors in studying the stability of flows with separation of the laminar boundary layer under the condition of flow modulation in space and time is presented. These results provide an insight into the influence of periodic inhomogeneities of the separation region on the development of its hydrodynamic disturbances and on the transition to the turbulent state near the body surface in a low-velocity air flow. [ABSTRACT FROM AUTHOR]

A new experimental method for panoramic contactless determination of the laminar-turbulent transition position in a three-dimensional boundary layer is described. It is demonstrated that this method allows accurate determination of the transition region boundaries at both low and enhanced levels of free-stream turbulence. [ABSTRACT FROM AUTHOR]

Results of studying stability of the Blasius boundary layer on a two-layer compliant coating in the linear formulation are reported. The computations are based on experimental parameters of viscoelasticity of a real coating, which reveal the dependences of its elasticity modulus and loss coefficient on frequency. Parametric investigations of the influence of the coating layer thicknesses and free-stream velocity on flow stability, in particular, on the critical Reynolds number, are performed. Regions of a nonmonotonic behavior of the critical Reynolds number are found, which allow one to determine the optimal thicknesses of the upper and lower layers for intense interaction with the flow. An explanation of this effect is proposed. [ABSTRACT FROM AUTHOR]

A new approach to formulation of asymptotic boundary conditions for eigenvalue problems arising in numerical analysis of hydrodynamic stability of such shear flows as boundary layers, separations, jets, wakes, characterized by almost constant velocity of the main flow outside the shear layer or layers is proposed and justified. This approach makes it possible to formulate and solve completely the temporal and spatial stability problems in the locally-parallel approximation, reducing them to ordinary algebraic eigenvalue problems. [ABSTRACT FROM AUTHOR]

Evolution equations of small disturbances aimed to compute the location of a laminar-turbulent transition in boundary layers by the eN-method taking into account the compressibility and heat transfer are described and justified for aerodynamic applications. The results of computation of N-factors and transition locations with the use of locally-parallel flow assumption for a flat plate exposed to supersonic flow at zero angle of attack are given. The results agree well with known numerical and experimental data. [ABSTRACT FROM AUTHOR]

An original technology of stability analysis of an airfoil boundary layer is presented in this paper. The technology includes an extraction of the boundary layer from the results of laminar flow computations, the computation of neutral curves and increments of Tollmien-Schlichting waves, and the computation of the location of laminar-turbulent transition zone by the eN-method for a given set of freestream turbulence levels. To be specific,we consider the flow around the NACA23012 airfoil with zero angle of attack at moderate Reynolds numbers. [ABSTRACT FROM AUTHOR]

The present paper deals with small amplitude velocity oscillations at laminar boundary layer separation behind a backward-facing step on a flat plate. The focus is the effect of a spatially periodic stationary disturbance of the nominally 2D basic configuration on the linear instability of the separated shear layer. Nonstationary perturbations of the mean flow which is modulated by roughness elements spaced along the step are examined through wind-tunnel tests and stability calculations. Noteworthy is flow destabilization in the presence of streaks behind the line of separation. [ABSTRACT FROM AUTHOR]

Efficiency of air microblowing through a permeable (microperforated) wall to reduce the turbulent skin-friction drag of a flat plate in a nominally gradientless incompressible flow was studied experimentally. The mass flow rate of the air blowing per unit area ranged from 0 to 0.0739 kg/s/m² that corresponded to the maximum blowing coefficient equal to 0.00287. Unlike the pioneering work of Hwang (Hwang, D. P., "A Proof of Concept Experiment for Reducing Skin Friction by Using a Micro-Blowing Technique," AIAA Paper 1997-0546, 1997.), the main emphasis was directed to investigate detailed boundary-layer characteristics over the permeable surface. This knowledge was necessary to make real-time decisions for the experiment setup refinement to achieve the maximum efficiency of the control method under study. A consistent reduction of the local skin-friction coefficient over the permeable insert at various values of the blowing coefficient was shown, reaching a maximum reduction of 70%. It was also established that the microblowing can provide reduction of the plate total drag of about 4.5-5%. Applicability of simple physical models to describe some characteristics of the turbulent boundary layer formed above the permeable wall was considered. [ABSTRACT FROM AUTHOR]

A response of flat plate boundary layer to a single free-stream axial vortex of limited spanwise extent with periodically modulated strength is investigated. The response is dominated by a streamwise velocity perturbation (streak), whose characteristics are in agreement with data of previous experiments performed with natural streaks developed under the effect of free-stream turbulence. This confirms that the experimental approach used is suitable for an accurate modeling of the low-frequency natural streaks in controlled conditions. A quantitative comparison of the observed streak characteristics with available theoretical approaches is performed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A single free stream axial vortex of controlled strength and position was used to investigate a vortical receptivity of Blasius boundary layer. Excited boundary-layer disturbances were dominated by streamwise velocity perturbations, that grew downstream essentially linearly with the streamwise coordinate. It was shown that the disturbance characteristics are in agreement with data of previous experiments performed under natural and control conditions concerning the ‘by-pass’ transition initiated at high free stream disturbance levels. It was proved that the role of the leading edge in the receptivity process and disturbance growth under consideration is not dominant. [Copyright &y& Elsevier]