Influence Of Vortex Disturbances On Laminar-Turbulent Transition In Supersonic Boundary Layer On A Swept Wing At M = 2.5

Experimental investigations of the receptivity of a supersonic boundary layer on a swept wing to the vortex perturbations were made at Mach 2.5. A symmetrical wing model with a 45 sweep angle and a 3-percent-thick circular-arc airfoil was used in experiments. The investigation of the vortex disturbances structure generated by a wire was done. It has been shown that the level of disturbances in the free flow increases with increasing wire diameter. The positions of the laminarturbulent transition at different diameters of wire were determined and data of the disturbances development in the model boundary layer were obtained. It was found that an increase in the intensity of vortex disturbances in the free flow leads to an earlier transition. Receptivity coefficients were determined.

The singing vortex

Marine propellers display several forms of cavitation. Of these, propeller-tip vortex cavitation is one of the important factors in propeller design. The dynamic behaviour of the tip vortex is responsible for hull vibration and noise. Thus, cavitation in the vortices trailing from tips of propeller blades has been studied extensively. Under certain circumstances cavitating vortices have been observed to have wave-like disturbances on the surfaces of vapour cores. Intense sound at discrete frequencies can result from a coupling between tip vortex disturbances and oscillating sheet cavitation on the surfaces of the propeller blades. This research article focuses on the dynamics of vortex cavitation and more in particular on the energy and frequency content of the radiated pressures.

Hydrodynamic turbulence in unbounded shear flows

A new conception of subcritical transition to turbulence in unbounded smooth shear flows is discussed. According to this scenario, the transition to turbulence is caused by the interplay between the four basic phenomena: (a) linear “drift” of spatial Fourier harmonics (SFH) of disturbances in wave-number space (k-space); (b) transient growth of SFH; (c) viscous dissipation; (d) nonlinear process that closes a feedback loop of transition by angular redistribution of SFH in k-space; The key features of the concept are: transition to turbulence only by the finite amplitude vortex disturbances; anisotropy of the process in k-space; onset on chaos due to the dynamic (not stochastic) process. The evolution of 2D small-scale vortex disturbances in the parallel flows with uniform shear of velocity is analyzed in the framework of the weak turbulence approach. This numerical test analysis is carried out to prove the most problematic statement of the conception—existence of positive feedback caused by the nonlinear process (d). Numerical calculations also show the existence of a threshold: if amplitude of the initial disturbance exceeds the threshold value, the self maintenance of disturbances becomes realistic. The latter, in turn, is the characteristic feature of the flow transition to the turbulent state and its self maintenance.