Alexander V. Dovgal

Experiments on secondary instability of streamwise vortices in a swept-wing boundary layer

A detailed experimental study on the formation of crossflow vortex mode packets and their high-frequency secondary instability in a swept-wing boundary layer was carried out. Stationary vortex packets are most likely to be generated under natural flight conditions and transition to turbulence is quickest within these disturbances. In the present experiments, different methods of controlled excitation are used so that the crossflow vortex packets are generated by surface-roughness elements and by localized continuous suction. It is found that as the stationary disturbance reaches a significant amplitude, of about 10% of the free-stream velocity, while being below the saturation level, high-frequency secondary instabilities start to grow. Influence of the crossflow vortex packet magnitude on the development of the secondary instability is investigated in detail and below its threshold the crossflow vortex packet was found to be nearly neutrally stable. By studying the unstable packets, the frequency of natural secondary perturbations was identified and the travelling disturbances were forced in a controlled manner by periodic blowing-suction applied locally under the stationary vortex. Two modes of secondary instability were found to develop and the preferred mode was dependent on the properties of the primary stationary disturbance. Additionally, the underlying physics of the process of nonlinear formation and development of the vortices in the boundary layer is clarified. It was observed that the large-amplitude co-rotating vortices may interact, thus reducing their amplitude. Also a large-scale excitation by an isolated roughness element produced two individual stationary crossflow vortex packets at its tips, each with different preferred secondary instability modes.

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.

Secondary instability of a swept-wing boundary layer disturbed by controlled roughness elements

Wind-tunnel data on velocity perturbations evolving in a laminar swept-wing flow under low subsonic conditions are reported. The focus of the present experiments are secondary disturbances of the boundary layer which is modulated by stationary streamwise vortices. Both the stationary vortices and the secondary oscillations of interest are generated in a controlled manner. The experimental data are obtained through hot-wire measurements. Thus, evolution of the vortices, either isolated or interacting with each other, is reconstructed in detail. As is found, the secondary disturbances, initiating the laminar-flow breakdown, are strongly affected by configuration of the stationary boundary-layer perturbation that may have an implication to laminar–turbulent transition control.Graphical abstract

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.

Transition prediction and control

In this chapter we consider some applications of the unstable-flow physics for laminar—turbulent transition prediction and control. Basically the purpose of transition prediction is to clarify whether the transition takes place in a flow under consideration and to find (calculate or measure) the Reynolds number of transition, ReT. If inside the neutral stability curve the disturbance becomes strong enough at its propagation in the streamwise direction, nonlinear mechanisms come to play which lead to the flow turbulization at ReT. Below we show how the linear stability approach in combination with empirical correlations can be used to predict the location of laminar—turbulent transition with a reasonable accuracy in certain practical situations.

Fundamentals of stability theory

A general and indicative definition of stability was given by Betchov and Criminale (1967): ‘the stability can be defined as quality of immunity to small disturbances.’ An illustration of this general property to the stability of mechanical systems is served by the elementary examples shown in Fig. 1.1.

Secondary instabilities of shear layers

When a linear instability mode reaches a large-enough amplitude, it enters the region of its essentially nonlinear, but still deterministic development. Usually the disturbance amplitude saturates in this region, which resembles the formation of a new quasi-steady state that opens in some cases the door for mechanisms of secondary instabilities discussed below.

Excitation of shear flow disturbances

Turbulence in convectively unstable shear flows subjected to extrinsic dynamics results from amplification of their perturbations, which are generated by external disturbances and usually start to grow far upstream of the turbulent flow region. In the previous chapters, we considered consecutively the transitional events in far-field and near-field regions of disturbance sources and emphasized the importance of the regions for different laminar–turbulent transition scenarios. Now we concentrate on the disturbance excitation in shear layers. This process is referred to as ‘receptivity’ and is the main concern in this chapter.

Some other basic factors of shear-layer stability

This chapter describes the results of theoretical, numerical and experimental studies to show how different isolated factors affect the linear stability of parallel and quasi-parallel flows. The palette of these factors includes surface geometry, volume forces, temperature effects, presence of particles in the fluid, wall permeability and compliance. Certainly, this set is not exhaustive. In particular, the stability of magnetohydrodynamic, unsteady flows, etc. is beyond the present scope. However, the set is diverse enough to provide a general view of the basic aspects of the stability analysis as applied to some problems related to engineering applications.

Nonlinear effects during the laminar–turbulent transition

There is a variety of nonlinear processes taking place at flow breakdown to turbulence. Competing with each other, they occur more or less individually only with special adjustment of the initial conditions. Below we consider some prototypical mechanisms of the laminar flow breakdown originating from the preceding amplification of linear instability waves and streaks in boundary layers.