Free-stream turbulence (FST) gives, undoubtedly, rise to the most complicated boundary-layer transition-toturbulence scenario. The reason for the complexity is that the boundary layer thickness grows with the downstream distance at the same time as the turbulence intensity (Tu) of the FST decays and the FST characteristic length scales grow. The FST is present everywhere in the free stream, but changes characteristics with the downstream distance. This implies that the actual forcing by the FST on the boundary layer changes gradually, which makes it an intricate receptivity problem. Today, we cannot honestly say that we are capable to accurately predict the transition location subject to FST in the simplest boundary layer flow, namely the one developing over a flat plat under zero-pressure gradient condition. Based on a set of original experimental data, consisting of 42 unique FST conditions, we here report on a semiempirical transition prediction model, which takes into account both the integral length scale and the turbulence velocity fluctuation at the leading edge. We show that the Tu, used in all existing models, is not the leading variable. Instead, our data show that the necessary ingredients in a successful transition prediction model includes, firstly, a FST Reynolds number (Refst) as leading variable, secondly, an FST parameter being the integral length scale Reynolds number (ReL) which further accounts for the effect of different length scales and, thirdly, a scale-matching model between the FST and the boundary layer. However, the importance of Tu can still be realized, since it constitutes the quotient of the two Reynolds numbers, namely Tu = Refst=ReL, even though Tu does not explicitly appear in the model.

In this paper we propose a strategy, entirely relying on available experimental data, to estimate the effect of a small control rod on the frequency of vortex shedding in the wake past a thick perforated plate. The considered values of the flow Reynolds number range between Re similar or equal to 6.6 x 10(3) and Re = 5.3 x 10(4). By means of particle image velocimetry, an experimental database consisting of instantaneous flow fields is collected for different values of suction through the body surface. The strategy proposed here is based on classical stability and sensitivity analysis applied to mean flow fields and on the formulation of an original ad hoc model for the mean flow. The mean flow model is obtained by calibrating the closure of the Reynolds averaged Navier-Stokes equations on the basis of the available experimental data through an optimisation algorithm. As a result, it is shown that the predicted control map agrees reasonably well with the equivalent one measured experimentally. Moreover, it is shown that even when turbulence effects are neglected, the stability analysis applied to the mean flow fields provides a reasonable estimation of the vortex shedding frequency, confirming what is known in the literature and extending it up to Re = 5.3 x 10(4). It is also shown that, when turbulence is taken into account in the stability analysis using the same closure that is calibrated for the corresponding mean flow model, the prediction of the vortex shedding frequency is systematically improved.

Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress the growth of unsteady disturbances that would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works demonstrated the versatility of these miniature vortex generators in zero-pressure-gradient boundary layers. In this work, mean-flow disturbances developing from miniature vortex generators in adverse and favorable pressure-gradient boundary layers are measured systemically to assess the possibility of extending miniature vortex generator-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values, and an empirical scaling is found based on existing results. The effect of streaks on transition in an adverse pressure-gradient boundary layer is also tested, and moderate increases to laminar flow extents are observed.

Discrete suction is deployed in a flat plate boundary layer to create spanwise mean velocity gradients (SVG) with the goal of delaying the onset of laminar-to-turbulent transition. It is shown that finite boundary layer suction through a set of holes in a spanwise oriented array in the flat plate is successful in setting up steady and robust streamwise streaks in the boundary layer. Today, the SVG method for transition control is known to attenuate the growth of Tollmien-Schlichting (TS) waves and delay the transition to turbulence. In this investigation, low-amplitude forced TS waves are attenuated with the implication of extending the laminar flow by at least 120% for a discrete suction of 0.8% of the free-stream velocity. The control technique is also tested successfully for natural transition, with a resulting transition delay of 30%.

A study on the relation between the wake inlet conditions and the wake characteristics of a bluff body by means of Particle Image Velocimetry is presented. The wake inlet condition, being a laminar boundary layer at the trailing edge of the body, are varied by means of wall-suction. Measurements are carried out at Reh = 6.7 × 103 based on the body thickness h. The induced radius of curvature of the streamlines is shown to be a promising parameter in explaining the increase in base drag and decrease in vortex shedding frequency associated with a thinner boundary layer.

Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress growth of unsteady disturbances which would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works have demonstrated the versatility of these miniature vortex generators (MVGs) in zero pressure gradient boundary layers. In this work, mean-flow disturbances developing from MVGs in adverse and favorable pressure gradient boundary layers are measured systemically to assess the possibility of extending MVG-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values and an empirical scaling is found in congruence with existing results. The effect of streaks on transition in an adverse pressure gradient boundary layer is also tested and moderate increases to laminar flow extents are observed.

A laminar flow control technique based on spanwise mean velocity gradients (SVGs) has recently proven successful in delaying transition in boundary layers. Here we take advantage of a well-known nonlinear effect, namely, the interaction of two oblique waves at high amplitude, to produce spanwise mean velocity variations. Against common belief we are able to fully master the first stage of this nonlinear interaction to generate steady and stable streamwise streaks, which in turn trigger the SVG method. Our experimental results show that the region of laminar flow can be extended by up to 230%.

In this numerical investigation we explore the possibility of applying free-stream vortices as a passive flow control method for delaying the transition to turbulence. The work is motivated by previous experimental studies demonstrating that stable streamwise boundary layer (BL) streaks can attenuate both two-and three-dimensional disturbances inside the BL, leading to transition delay, with the implication of reducing skin-friction drag. To date, successful control has been obtained using physical BL modulators mounted on the surface in order to generate stable streaks. However, surface mounted BL modulators are doomed to failure when the BL is subject to free-stream turbulence (FST), since a destructive interaction between the two is inevitable. In order to tackle free-stream disturbances, such as FST, a smooth surface is desired, which has motivated us to seek new methods to induce streamwise streaks inside the BL. A first step, in a systematic order, is taken in the present paper to prove the control idea of generating free-stream vortices for the attenuation of ordinary Tollmien-Schlichting waves inside the BL. In this proof-of-concept study we show that, by applying a spanwise array of counter-rotating free-stream vortices, inducing streamwise BL streaks further downstream, it is possible to alter the BL stability characteristics to such a degree that transition delay may be accomplished. For the demonstration we use direct numerical simulations along with stability analysis.

Streaky boundary layers generated by an array of miniature vortex generators (MVGs) mounted on a flat plate have recently shown to have a stabilizing effect on both two-and three-dimensional disturbances. An experimental study on the effect of the geometrical parameters of MVGs on the generated streamwise streaks in the flat plate boundary layer is carried out, and the corresponding stabilizing effect on Tollmien-Schlichting (TS) wave disturbances is quantified. The new experimental configurations have led to an improved empirical scaling law, which includes additional geometrical parameters of the MVGs compared to the previously reported relation. It is found that the MVG configuration can be optimized with respect to the attenuation of disturbances. In addition, the streamwise location of branch I of the neutral stability curve, with regard to the location of the MVG array, is found to be correlated with the initial receptivity of TS waves on the MVG array and the attenuation of the TS wave amplitude in the unstable region.