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The flow in the vicinity of the attachment line of a swept airfoil is investigated numerically. The typical Goertler–Haemmerlin modes are recovered at the attachment line and found to be stabilised with an increase of the Mach number and reduction of the nose radius. Away from the leading edge, another instability, most probably a streamline–curvature instability as described by Itoh, 1994, was found which dominates the attachment line instability.

The study of "simple" flows such as Poiseuille flow have been for a long time studied and their simplicity has permitted us to highlight different instability mechanisms. More recently, several authors have shown, through transient growth studies and by the identification of the nonnormal character of the operator of Orr-Sommerfeld, that these two flows could have subcritical dynamics. However, the assumption of one-dimensionality of the basic flow limits the comparison with the experiment where the basic flow is not exactly one-dimensional. Although different studies have been realized on the stability of a two-dimensional basic flow, all were interested only in the most unstable mode. In this present paper, the stability of the laminar flow in a rectangular duct of an arbitrary aspect ratio is investigated numerically with for objective the computation of the complete spectrum. This study will highlight strong similarities between the 1D and 2D spectra but, in spite of a powerful numerical method, will show the numerical limitations observed to obtain a spectrum converged in a sufficiently large field in

The unsteady dynamics of laminar separation bubbles such as those arising in bluff body flows, or behind obstacles, is of considerable practical interest. Bubble formation near the leading edge of an aerofoil can dramatically alter the characteristics of the flow over the wing and induce stall. Similarly, laminar separation near the trailing edge of an aerofoil and instability of the separation bubble can in.uence the dynamics of the wake flow. The flow past a row bluff bodies placed in a uniform stream, is often used to model engine inlet flows, and thus an understanding of the unsteady dynamics of the eddies which form behind the bluff bodies and the unsteady development of the wake flow is useful in predicting the flow impinging on subsequent rotor blades.

A novel theoretical method is presented based on the theory of adjoints for analyzing the behaviour of Tollimein - Schlichting waves and other wall-based waves incident on junctions between rigid and compliant walls.

We present a theory for computing the optimal steady suction distribution in order to minimize the growth of convectively unstable disturbances, and thus delay laminar-turbulent transition on swept wings. Here, we use the optimal control theory and minimize an objective function based on a sum of the kinetic energy of an arbitrary number of disturbances. The optimization procedure is gradient-based where the gradients are obtained using the adjoint of the parabolized stability equations and the adjoint of the boundary layer equations. Results are presented for an air foil designed for medium range commercial air crafts.

A semi-automatic and active control of T-S waves and oblique waves is performed using an array of piezo-ceramic actuators attached on a flat-plate surface. The actuators generate counter waves to cancel the incoming instability waves. The actuator’s operating amplitudes and phases are successively updated using the velocity .uctuations monitored downstream by a rake of hotwires. It is shown that the system could weaken these waves when their sweep angles are less than 15 degrees. However, it has difficulty in controlling the waves of large sweep angles. Through the results from the experiment activating only one actuator, it is shown that the difficulty arises because the wave front of the waves generated by each actuator deforms into an arch.

This work focuses on the application of linear feedback control to transition to turbulence in shear flows. The controller uses wall-mounted sensor information to estimate the flow disturbances and uses wall actuators to prevent transition to turbulence. The flow disturbances are induced by external sources of perturbations described by means of a stochastic volume forcing. We show that improved performance can be achieved if the proper destabilisation mechanisms are targeted.

In this paper we present results on the behavior of a turbulent, negatively buoyant jet in an ambient fluid having a higher viscosity than the jet fluid. Our experimental results indicate that the turbulent jet undergoes a reverse transition. Large scale eddies at the interface are suppressed, and the observed entrainment rate also reduces dramatically for the jet in a higher-viscosity medium. We also present results from numerical simulations, using vortex methods with a viscous diffusion scheme, to corroborate the results from our experiments.

Numerical simulation results for the behaviour of disturbances in flows over compliant surfaces are described. Most attention is given to the propagation of Tollmien-Schlichting waves over compliant panels and the self-excited generation of such waves by very short panels. The use of compliant walls to suppress transiently growing forms of boundary-layer disturbance and the effects of surface compliance on the rotating disc boundary layer are also briefly discussed.

This paper discusses the role of hydrodynamic stability theory in understanding wall turbulence and its possible suppression by using compliant surfaces. Our work reveals that, in wall turbulent flows, there are three important ‘mode classes’; namely, the Tollmien-Schlichting (TS) mode class, the Static Divergence (SD) mode class, and the High-speed highly damped (HSHD) mode class. All these modes scale with inner wall variables and so do the material properties of the compliant surface. The general thrust should be to replace TS modes by the HSHD stable modes. Outer modes were also investigated and found to be damped.