Delaying the laminar-turbulent transition of a boundary layer reduces the skin-friction drag and can thereby increase the efficiency of any aerodynamic device. A passive control strategy that has reaped success in transition delay is the introduction of boundary layer streaks. Surface-mounted vortex generators have been found to feature an unstable region right behind the devices, which can be fatal in flow control if transition is triggered, leading to an increase in drag with respect to the reference case without devices. In a previous proof of concept study, numerical simulations were employed to place artificial vortices in the free stream that interact with the boundary layer and accomplish transition delay. In the current study, we present experimental results showing the feasibility of generating free-stream vortices that interact with the boundary layer, creating high- and low-speed boundary layer streaks. This type of streaky base flow can act as stabilizing if introduced properly. We confirm the success of our flow control approach by artificially introducing two-dimensional disturbances that are strongly attenuated in the presence of streaks, leading to a transition delay with respect to the reference case of approximately 40 %.

Predicting the behavior of turbulent spots is critical in modeling the late stage of laminar-turbulent transition in boundary layers. While these spots have been studied for over 70 years and are relatively well understood, their development under the influence of free-stream turbulence (FST) remains less explored. Previous investigations suggest that the characteristics of the incoming FST influence the spreading parameters of turbulent spots. The present study delves deeper into this phenomenon, utilizing a novel experimental setup featuring wall- mounted microphones to track individual, naturally occurring turbulent spots under varying FST conditions. The findings reveal variations in both, the growth rates and the number of generated spots.

The efficacy of spanwise velocity gradients (SVGs) to attenuate the growth of disturbances in a laminar boundary layer (BL) has been an active topic of research for more than 20 years. It was shown both numerically and in wind-tunnel experiments that modulating a laminar BL with steady, alternating high- and low- speed streaks can delay the transition to turbulence and hence lead to a reduction in skin friction drag. In this project, the feasibility of this mechanism to delay transition in a real-world application is explored. Wind-tunnel experiments are performed in order to design miniature vortex generators (MVGs) intended to be used on the fuselage of an aircraft during flight with the goal to delay natural transition in a realistic environment. Blade-type MVGs with a height lower than the BL thickness have been found to be effective in generating steady and stable SVGs in a BL that can accomplish transition delay.

The efficacy of spanwise velocity gradients (SVGs) to attenuate the growth of disturbances in a laminar boundary layer (BL) has been an active topic of research for more than 20 years. It was shown both numerically and in wind-tunnel experiments that modulating a laminar BL with steady, alternating high- and low- speed streaks can delay the transition to turbulence and hence lead to a reduction in skin friction drag. In this project, the feasibility of this mechanism to delay transition in a real-world application is explored. Wind-tunnel experiments are performed in order to design miniature vortex generators (MVGs) intended to be used on the fuselage of an aircraft during flight with the goal to delay natural transition in a realistic environment. Blade-type MVGs with a height lower than the BL thickness have been found to be effective in generating steady and stable SVGs in a BL that can accomplish transition delay.

In this work we study the wake of a yawed wind-turbine model immersed in an atmospheric boundary layer (ABL). The ABL is replicated in the wind tunnel by means of a barrier-spires and distributed roughness configuration and is representative of a rural terrain. We quantify the properties of the wake in the horizontal plane at hub height and compare the predictions of available wake models to our data for different yaw angles. It is found that the model based on lifting-line theory performs best in predicting the velocity deficit without the need of tuning the parameters to the current setup. However, the wake deflection is slightly underestimated, most notably at the transition between near and far wake. Furthermore, a comparison with the turbine in a uniform incoming flow highlights the enhanced downward deflection of the wake which results from its interaction with the ABL.

Wind tunnel experiments were performed to investigate the response of a wind turbine model immersed in a replicated atmospheric boundary layer to dynamic changes in the yaw angle. Both the flow field in the wake and the operating properties of the turbine, namely its thrust force, torque, and angular velocity, were monitored during repeated yaw maneuvers for a variety of yaw angles. It was observed that the characteristic time scale of the transient experienced by the turbine scalar quantities was one order of magnitude larger than that of the yaw actuation and depended primarily on the inertia of the rotor and the generator. Furthermore, a Morlet wavelet analysis of the thrust signal showed a strong peak at the rotation frequency of the turbine, with the transient emergence of high activity at a lower frequency during the yaw maneuver. The insights provided by the proper orthogonal decomposition analysis performed on the wake velocity data enabled the development of a simple reduced-order model for the transient in the flow field based on the stationary states before and after the yaw maneuver. This model was then further improved to require only the final state, extending its applicability to any arbitrary wind farm as a dynamical surrogate of the farm behavior.

The instability mechanism behind a geometrically simple cylindrical roughness element continues to be a challenging topic in fluid mechanics. Considerable progress has been made in understanding the phenomena in recent years, but more research is needed to predict the temporal nature and spatial structure of the dominant instability in a given flow configuration. This is of particular interest, as these instabilities dictate the transition to turbulence and thus are significant for large-scale effects such as skin friction drag. A smoke-flow visualization study with a large variation of parameters, featuring a cylindrical roughness element connected to a linear traverse, has been performed. Results show good agreement with previous investigations and provide further insights into the stability properties, revealing several unexpected effects. For a low roughness aspect ratio ?, no global instability is detected even at the highest roughness Reynolds number Rekk, whereas a high aspect ratio indicates a delay in the onset of instability. From the acquired visualizations, we constructed the, so far, richest instability diagram of the wake behind an isolated roughness element in the Rekk-? space, sampled in the same measurement campaign. Furthermore, information regarding the dominant frequency in the wake can be extracted from the visualization images. Our results suggest a new scaling of the frequency as the velocity is increased. Finally, it is shown that the dominant frequency in a certain flow regime can be well predicted using a Strouhal number based on the cylinder diameter and the roughness velocity.

To date, very few careful and direct comparisons between experiments and direct numerical simulations (DNS) have been published on free-stream turbulence (FST) induced boundary layer transition, whilst there exist numerous published works on the comparison of canonical turbulent boundary layers. The primary reason is that the former comparison is vastly more difficult to carry out simply because all known transition scenarios have large energy gradients and are extremely sensitive to surrounding conditions. This paper presents a detailed comparison between new experiments and available DNS data of the complex FST transition scenario in a flat plate boundary layer at turbulence intensity level about Tu = 3 % and FST Reynolds number about Re-fst = 67. The leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE are identified as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters is not enough as previously thought, which is illustrated by setting up two experimental FST cases with about the same FST integral parameters at the LE but with different energy spectra. Finally, an FST boundary layer penetration depth (PD) measure is defined using DNS, which suggests that the PD grows with the downstream distance and stays around 20 % of the boundary layer thickness down to transition onset. With this result, one cannot rule out the significance of the continuous FST forcing along the boundary layer edge in this transition scenario, as indicated in previous studies.

An experimental investigation aimed at detecting the critical roughness Reynolds number (Re-kk,Re-tr) both with and without Tollmien-Schlichting (T-S) waves is described in this paper. As a novel technique to examine Re-kk,Re-tr systematically, we employed isolated, cylindrical roughnesses with automatically adjustable height of micro-meter precision. The experiment was performed using hot-wire anemometry in flat-plate boundary layers developing under close to zero-pressure-gradient conditions in a low-turbulence level wind tunnel. The present data for Re-kk,Re-tr without T-S waves confirmed previous results and showed a strong correlation between the roughness aspect ratio and root Re-kk,Re-tr. Controlling the roughness height while keeping the free-stream velocity fixed revealed noteworthy hysteresis for Re-kk,Re-tr. As expected the critical roughness Reynolds number decreased with the presence of T-S waves. The necessary T-S wave amplitude needed for transition became smaller with increasing the roughness height with a sudden drop when approaching the critical roughness height without T-S waves.

Free-stream turbulence (FST) induced boundary layer transition is an intricate physical process that starts already at the leading edge (LE) with the LE receptivity process dictating how the broad spectrum of FST scales is received by the boundary layer. The importance of the FST integral length scale, apart from the turbulence intensity, has recently been recognized in transition prediction but a systematic variational study of the LE pressure gradient has still not been undertaken. Here, the LE pressure gradient is systematically varied in order to quantify its effect on the transition location. To this purpose, we present a measurement technique for accurate determination of flat-plate boundary layer transition location. The technique is based on electret condenser microphones which are distributed in the streamwise direction with high spatial resolution. All time signals are acquired simultaneously and post-processed giving the full intermittency distribution of the flow over the plate in a few minutes. The technique is validated against a similar procedure using hot-wire anemometry measurements. Our data clearly shows that the LE pressure gradient plays a decisive role in the receptivity process for small integral length scales, at moderate turbulence intensities, leading to variations in the transitional Reynolds number close to 40 %. To our knowledge, this high sensitivity of LE pressure gradient to transition has so far not been reported and our experiments were therefore partly repeated using another LE to ensure set-up independence and result repeatability.