An experimental study on streamwise streaks in transitional boundary layers

Abstract: The present experimental study focuses on two topics, a passive transition-delay method for different types of wave disturbances and the effect of free-stream turbulence characteristics on boundary layer transition over a at plate. In the investigations, hot-wire anemometry was employed and the experiments were performed in a well-controlled wind tunnel facility.In the past streamwise vortices, which generate high and low velocity streaks in the spanwise direction, has been employed successfully in order to damp boundary layer perturbations and eventually postpone transition to the turbulence. In the previous experiments the perturbations have been Tollmien- Schilichting waves (TSW) and the vortices, generating the streaky boundary layer, were produced by bluff obstacles, i:e: cylindrical surface roughnesses on the plate. In the first investigation of this thesis, it is shown that vortex generators originally used to control boundary layer separation, have a strong damping effect on boundary layer perturbations and are able to postpone the transition to turbulence. The present vortex generators are however miniature with respect to classical ones and are in the following denoted MVGs, for miniature vortex generators. The benefit of using MVGs is that the streaks are more stable compared to cylindrical surface roughnesses due to the fact that vortex shedding does not appear behind the MVGs. In addition, for the first time the perturbations are generated upstream of the MVGs, a configuration, which is closer to real applications. The effect of the streaks, which are generated by MVGs, can be characterized by a new integral-based amplitude definition. This amplitude definition, which scales on boundary layer parameters and geometrical parameters of the vortex generators, takes spanwise variations into account, which are neglected in the classical amplitude definition. Besides TSW, the effect of the vortex generators are investigated on other types of wave like disturbances, such as a single oblique wave (SOW) and a pair of oblique waves (POW). In the linear regime, in which the perturbations are of the order of and up to 1% of the free-stream velocity, it is observed for all the wave types that after an initial increase of the disturbance amplitude, they decay quickly in the streamwise direction in presence of the MVGs. Moreover, in the non-linear regime, a length of 1-2 meter transition delay is achieved in the presence of the MVGs. At the same time, the energy of the fluctuations are typically four orders of magnitude smaller in the modulated boundary layers compared to the uncontrolled case. It should be pointed out that there is a limitation for the streak amplitude in order to damp the disturbances. When the streak amplitude amplifies more than 30% using the new definition, the streaks become unstable and transition may occur even further upstream compared to the unmodulated boundary layer.As a second study, another transition scenario was investigated, namely when the free-stream turbulence (FST) level is high enough to cause by-pass transition. FST can be generated by mounting a grid inside the wind tunnel upstream of the leading edge. By manufacturing grids by pipes (instead of solid bars), it is possible to pressurize them and then have a secondary ow injection through orifices in the pipes into the main stream pointing in the upstream direction. With this feature using several different FST grids the turbulence intensity can be varied, and along with a relative position of the grid to the leading edge, 42 different cases are provided in terms of different turbulent intensities, Tu = urms=U?, and integral length scales, ?x, in the free-stream. In the first step, a universal streamwise distribution of turbulence intermittency, ? is introduced and then the effect of FST characteristics, such as turbulent intensity and integral length scale, are studied on the onset and length of the transition region. It is shown that the transition location is advanced by increasing turbulence intensity and goes with the exponent of -2, and the integral length scale has opposite effect on the transition onset at low and high turbulence intensities. While at low Tu, transition is postponed by decreasing the integral length scale, it is advanced at higher turbulence intensities. Moreover, it is argued that, even at high turbulence intensities, there exists a minimum distance for the turbulent boundary layer to be selfsustained.