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.

Turbulent spots are essential in the late stage of the transition process from laminar to turbulent flow in boundary layers and exhibit complex spatial and temporal characteristics. This study presents a novel experimental approach to investigate the properties of turbulent spots. A unique measurement setup is introduced, integrating wall-mounted microphones to record wall-pressure fluctuations and hot-wire anemometry for time-resolved velocity signals. Our experimental results validate the effectiveness of the technique, demonstrating its ability to grasp the propagation characteristics of turbulent spots with high fidelity. The findings underscore the potential of this approach for advancing our understanding of boundary-layer transition processes and the nature of turbulent spots.

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 %.

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 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.

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.

An experimental investigation has been conducted to demonstrate the utility of active flow control as a disturbance generator for vortex body interaction studies. The technique is used to explore the flow physics of parallel vortex body interaction between two NACA 0012 airfoils in series. Experiments were carried out at a chord-based Reynolds number of 740,000 relative to the first airfoil. Active flow control in the form of nanosecond pulse-driven dielectric barrier discharge plasma actuation, originating close to the leading edge, was used to produce vortex shedding from the upstream (disturbance) airfoil at various frequencies (0.038 <= F+ <= 0.762). These vortices were characterized, showing reduced circulation and diameter with increasing frequency, before examining the downstream wake-airfoil interactions. Time-resolved pressure and phase-locked PIV measurements were taken on the downstream (target) airfoil for multiple angles of attack. For F+ <= 0.5, the target airfoil is subject to strong oscillations from the wake of the disturbance airfoil that lead to large fluctuations in lift and pitching moment. However, a further increase in F+ reattaches the flow over the disturbance airfoil and no major vortex body interactions are observed on the target. Governing parameters for this type of vortex body interaction are explored, and differences between isolated and non-isolated encounters as well as the presence of a viscous response are examined. Finally, means to alleviate loads caused by the incident vortex are explored.

To date, very few careful and direct comparisons between experiments (EXP) 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 being 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. From the existing literature, it is clear that the FST induced boundary-layer transition scenario in its very simplest case, i.e. for a zero-pressure gradient flow, we are still raising questions on the receptivity process, and we do not seem to have reached a consensus on the break-down process of streamwise streaks into turbulent spots. In this paper we present a detailed comparison between our EXP and available DNS data of this complex transition scenario at a turbulence intensity level of about Tu = 3% and an FST Reynolds number of about 67. The most important factors that need to be considered if one wants to compare DNS with EXP, or one EXP/DNS case with another EXP/DNS case for that matter, are elucidated. Through careful matching of these factors, we show that the agreement between EXP and DNSis satisfying and that all these factors must be important for describing the transition scenario appropriately.

The instability mechanism behind a geometrically simple roughness element continues to be a challenging topic in fluid mechanics. Significant advances towards fully understanding the phenomena in the wake of a roughness have been made in the recent years, but in order to be able to predict the type of instability in any given flow configuration, further research is required. 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 large variation of parameters (aspect ratios in the range of 0.25–7), enabled by a cylindrical roughness element that is connected to a linear traverse, has been performed in order to broaden the understanding of this type of flow. Results show good agreement with previous investigations and give further insights into the stability properties. Unexpected results arise especially for particularly low as well as very high roughness aspect ratios. For low aspect ratios, no global instability is detected even at the highest Reynolds numbers, whereas high aspect ratios suggest a delay in the onset of instability. Furthermore, information regarding the dominating frequency can be extracted from the visualization images. Results show a new scaling of the frequency as the velocity is increased. Additionally, the dominating frequency in the wake can be predicted well using a Strouhal number in a certain flow regime.