Stochastic forcing of the 2D boundary layer by DBD plasma actuator

The paper describes the results of the parametric study of the broadband velocity pulsations, induced by DBD plasma actuator in 2D subsonic boundary layer. The presented data include the analysis of the disturbance growth at various pressure gradients. It is assumed that the broadband pulsations are composed of the elementary disturbances, induced by an individual microdischarges, wandering along the electrode edge. These disturbances have a streak-like structure in a near field, and evolve into a fan of packets of Tollmien-Schliechting waves as one moves downstream. The streamwise length, needed for transition to modal behavior, depends on the stability properties of the boundary layer.

Investigation of Transition Delay by Dynamic Surface Deformation

Numerical calculations were carried out to investigate control of transition on a flat plate by means of local dynamic surface deformation. The configuration and flow conditions are similar to a previous computation which simulated transition mitigation. Physically, the surface modification may be produced by piezoelectrically driven actuators located below a compliant aerodynamic surface, which have been employed experimentally. One actuator is located in the upstream plate region and oscillated at the most unstable frequency of 250 Hz to develop disturbances representing Tollmien–Schlichting instabilities. A controlling actuator is placed downstream and oscillated at the same frequency, but with an appropriate phase shift and modified amplitude to decrease disturbance growth and delay transition. While the downstream controlling actuator is two-dimensional (spanwise invariant), several forms of upstream disturbances were considered. These included disturbances which were strictly two-dimensional, those which were modulated in amplitude and those which had a spanwise variation of the temporal phase shift. Direct numerical simulations were obtained by solution of the three-dimensional compressible Navier–Stokes equations, utilizing a high-fidelity computational scheme and an implicit time-marching approach. A previously devised empirical process was applied for determining the optimal parameters of the controlling actuator. Results of the simulations are described, features of the flowfields elucidated, and comparisons made between solutions of the uncontrolled and controlled cases for the respective incoming disturbances. It is found that the disturbance growth is mitigated and the transition is delayed for all forms of the upstream perturbations, substantially reducing the skin friction.

Algebraic disturbance growth by interaction of Orr and lift-up mechanisms

Algebraic disturbance growth in spatially developing boundary-layer flows is investigated using an optimization approach. The methodology builds on the framework of the parabolized stability equations and avoids some of the limitations associated with adjoint-based schemes. In the Blasius boundary layer, non-parallel effects are shown to significantly enhance the energy gain due to algebraic growth mechanisms. In contrast to parallel flow, the most energetic perturbations have finite frequency and are generated by the simultaneous activity of the Orr and lift-up mechanisms. The highest amplification occurs in a limited region of the parameter space that is characterized by a linear relation between the wavenumber and frequency of the disturbances. The frequency of the most highly amplified perturbations decreases with Reynolds number. Adverse streamwise pressure gradient further enhances the amplification of disturbances while preserving the linear trend between the wavenumber and frequency of the most energetic perturbations.

Scaling of streamwise boundary layer streaks and their ability to reduce skin-friction drag

Spanwise arrays of miniature vortex generators (MVGs) are used to generate energetic transient disturbance growth, which is able to modulate the boundary layer flow with steady and stable streak amplitudes up to 32 % of the free-stream velocity. This type of modulation has previously been shown to act in a stabilizing manner on modal disturbance growth described by classical instability theory. In an attempt to reproduce a more realistic flow configuration, in the present experimental set-up, Tollmien–Schlichting (TS) waves are generated upstream of the MVG array, allowing for a complete interaction of the incoming wave with the array. Fifteen new MVG configurations are investigated and the stabilizing effect on the TS waves is quantified. We show that the streak amplitude definition is very important when trying to relate it to the stabilization, since it may completely bypass information on the mean streamwise velocity gradient in the spanwise direction, which is an essential ingredient of the observed stabilization. Here, we use an integral-based streak amplitude definition along with a streak amplitude scaling relation based on empiricism, which takes the spanwise periodicity of the streaks into account. The results show that, applying the integral definition, the optimal streak amplitude for attenuating TS wave disturbance growth is around 30 % of the free-stream velocity, which corresponds to ${\sim }20\hspace{0.167em} \% $ in the conventional definition when keeping the spanwise wavelength constant. The experiments also show that the disturbance energy level, based on the full velocity signal, is significantly reduced in the controlled case, and that the onset of transition may be inhibited altogether throughout the measured region in the presence of an MVG array.

The Instability of Vapor Bubble Growth within the Diesel Droplet under the Condition of Supercavitation

A new analytical form of dispersion equation which can be used to describe the disturbance growth rate of the diesel bubble growth instability is derived. The instability analysis of vapor bubble growth within the diesel droplet is carried out in this paper. Analysis results show: the disturbance growth rate is majorly influenced by six dimensionless variables. The disturbance growth rate initially decreases gradually then increases rapidly with increasing bubble volume fraction. The disturbance growth rate increases with increasing Weber number of vapor bubble growth, increasing Mach number and increasing diesel droplet Reynolds number. Both vapor bubble Reynolds number and ambient air Reynolds number have slightly influence on disturbance growth rate.

Algebraic/transcendental disturbance growth behind a row of roughness elements – ERRATUM

It has come to our attention that in the printed version of Goldstein et al. (2011) figures 8, 9, 10 and 11 were printed in black and white when they should have appeared in colour. The reader is referred to the online version of this article where the colour is correct for all figures.

Numerical Study of 3D Nonlinear Disturbance Growth in Transitional Natural Convection

Three dimensional boundary layer disturbance growth in natural convection adjacent to a uniformly heated vertical plate has been studied using a finite volume direct numerical simulation (DNS). The sequence of events associated with the intermediate stage of the spatial laminar-turbulent-transition for water, with Prandtl number Pr = 6.7, for the Rayleigh numbers in the range Rax ≈ 1 × 1010 – 4 × 1013, with perturbation parameters closely matching the experiment reported in [1], is investigated. Preliminary calculations confirm that this stage is characterized by the development of spatially periodic inner and outer span-wise vortices (shear layers), which travel in a differing fashion and speed; and a secondary mean flow system. The important route to natural convection flow breakdown, the arising of a double vortex mean longitudinal system, repeatedly suggested by past experiments, seems to be related to the complex and opposing behavior of these two layers.