Andrey V. Boiko

Physics of Transitional Shear Flows

A summary of recently published book on hydrodynamic stability and transition phenomena in incompressible shear layers with the same title as that of the present contribution to the EUCASS proceedings is given. The objective is to emphasize the milestones of the edition which is aimed, most of all, at university and postgraduate students starting with the problem and may be of interest for the experienced “transition” community, as well.

Spatial simulation of the instability of channel flow with local suction/blowing

A direct numerical simulation was made of instability in a spatially evolving channel flow. A local surface suction/blowing was imposed at the upper wall (x/h=20). A Tollmien–Schlichting (TS) wave was superimposed on the laminar channel flow at the inflow. At the outflow, the buffer domain technique was applied to suppress the reflection of outgoing waves. The influence of the local suction/blowing on the linear and nonlinear instabilities of the flow was examined. It was found that the local suction/blowing increases the disturbance energy significantly in the interaction zone for subcritical (Re=5000) and supercritical (Re=10 000) cases. The effects of the blowing strength (0⩽AS⩽0.1) and the initial TS wave amplitude (0⩽ATS⩽2.0) on the subcritical channel flow were scrutinized. Two regimes of the wave/flow interaction were found by varing AS, i.e., “monotonic” and “vortex splitting” regimes.

Instability of a backward-facing step flow modified by stationary streaky structures

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.

Stability of flat plate boundary layer over monolithic viscoelastic coatings

It is well known that compliant coatings of a wetted surface can affect the stability of hydrodynamic flows. The mechanism of this influence is related to viscoelastic properties of the coatings particularly to their reaction on the disturbing impact. However, the majority of corresponding studies are devoted to investigation of the effect of soft compliant coatings which are unpractical. Moreover, up to now only model computations of flow stability over the compliant coatings have been performed, as experimental data on viscoelastic properties (the modulus of elasticity E characterizing the elastic features, and the loss tangent μ characterizing the viscous or damping features) as functions of frequency for the coatings promising from the point of view of their practical application have been absent. Such data were obtained only last years for some materials (silicon rubbers) in a series of studies [1–3]. The coatings are prospective first of all because of their adequate stiffness (about 1 MPa). In the present paper, results of computations of boundary layer stability performed for the first time for the flow developed over compliant coatings with real properties.

Effect of riblets on the streaky structures excited by free stream tip vortices in boundary layer

In this study, experimental investigations were made regarding the effect of riblets on the streak instability in boundary layer. The streak instability is now regarded as a major source of the self-regeneration mechanism for the hairpin type coherent structures in turbulent boundary layer flow. Thus, it is important to control the instability to suppress the drag-inducing vortical structure in terms of drag reduction. Toward enhancing the measurement accuracy and spatial resolution, an enlarged version of riblets was applied to a streak which was artificially induced by a microwing in a laminar boundary layer. It is found that the riblets have attenuation effect on the streak instability, i.e., to reduce the spanwise velocity gradient of the quasi-streamwise streak in boundary layer.

Transition to turbulence in separation bubbles

This chapter focuses on instability and laminar—turbulent transition in local regions of boundary layer separation or ‘separation bubbles’ in the steady flow of an incompressible fluid. The present topic applies to aerodynamics of aerofoils and wings at low Reynolds numbers, boundary layers affected by steps, humps and other surface imperfections, flow separation at sharp edges, etc.

Modification of flow perturbations in a laminar separation bubble by heat transfer

Laminar boundary layer separation in conditions of localized heat transfer is investigated at low subsonic velocity through wind-tunnel measurements and linear stability analysis. A backward-facing step flow is subjected to a stationary temperature variation generated by Peltier elements installed on the test model directly downstream of the separation line. The experimental and theoretical data clarify the response of velocity disturbances in the separation region to the temperature variation, the latter appearing primarily as a modifier of the initial wave spectrum of the amplifying separated layer oscillations.

Towards improving efficiency of control for blowing into a boundary layer through a permeable wall

The results of experimental and numerical investigations of the efficiency of control by an incompressible turbulent boundary layer with the help air blowing through a permeable wall fabricated with maintenance of most of the necessary requirements for the quality and configuration of microapertures and having a low effective roughness are analyzed. Various cases of modeling the process of air blowing into the boundary layer through a specified hi-tech finely perforated surface are considered, and the data for average parameters and characteristics of turbulence of the flow types under investigation are presented. A substantial decrease in the skin-friction coefficient along the model length, which can achieve 90% with increasing the blowing coefficient, is shown. The estimate of the energy consumption for the process of blowing under terrestrial conditions testifies to the high potential of this method of control capable to provide 4–5% gain in the total aerodynamic drag of a simple modeling configuration.

Measurement Method of Broadband Dynamic Characteristics of Viscoelastic Material for Compliant Coating

An improved method to measure the dynamic viscoelastic properties of elastomers is proposed. The method is based on the analysis of forced oscillation of a cylindrical sample loaded with inertial mass. No special equipment or instrumentation other than the ordinary vibration measurement apparatus is required. Typical measurement of the viscoelastic properties of a silicone rubber Silastic ® S2 were measured over the wide frequency range

Longitudinal Structures in Plane Jets

The physics of mixing in jets is of considerableinterest from the standpoint of both fundamental sci-ence and practical application. The intensity and homo-geneity of mixing significantly affect the combustionefficiency, heat transfer coefficient, formation of waste,and jet noise (see, e.g., [1]).Laminar jets have inflectional mean velocity pro-files. This property leads to the formation of Kelvin–Helmholtz vortices, which represent the main instabil-ity of such shear layers. The initial stage in the develop-ment of these vortices is usually well described by lin-ear stability theory (see, e.g., review [2]). The next,nonlinear stage is characterized by amplitude saturationand vortex pairing due to the resonance of disturbanceswith subharmonics and superharmonics. Further devel-opment of nonlinear structures is often accompanied bythe occurrence of longitudinal, or streamwise, vorticalstructures. Their formation is usually attributed to theso-called secondary three-dimensional instability of theKelvin–Helmholtz vortices [3, 4]. Experiments showthat the dynamics of these structures play an importantrole in the mixing process in the far wake of a jet [5].Other longitudinal disturbances that can often bedeveloped in shear layers are formed downstream ofnozzle surface irregularities [6]. These are regions ofquasi-stationary three-dimensional deformations,mainly of the longitudinal velocity in the shear flow,and have a characteristic form of “streaks” in the visu-alization pictures [7]. Their occurrence is not due to thesecondary instability of the Kelvin–Helmholtz vortices.These structures arise as a result of stabilization effectsupon the development of compact three-dimensionaldisturbances of the normal velocity component evenwith small amplitude [7]. These longitudinal structuresintensely interact with other flow disturbances, e.g.,instability waves. This interaction usually acceleratesflow turbulization [8]. This feature of the streaky struc-tures makes them a promising means for improving themixing and control over the flow in jets.There is an extensive literature on the investigationof longitudinal structures in wall flows (see, e.g., theoverview in [7]). However, investigations into the phys-ics of similar structures in jets began only recently. Suc-cessful excitation and observation of natural longitudi-nal structures and their interaction with Kelvin–Helm-holtz vortices in circular jets [9] offer the possibility ofstudying controlled longitudinal structures in planejets. To this end, a simple but efficient smoke visualiza-tion technique using a pulsed laser sheet synchronizedwith the Kelvin–Helmholtz vortex shedding wasapplied in this study.The plane jet generator and visualization schematicsare presented in Fig. 1. The flow was produced by aDISA wind tunnel that is designed for the calibration ofsensors of a hot-wire anemometer and ensures a stableair flow rate. Smoke was supplied to the test sectionthrough a tube connected to a smoke generator. The