The scale interactions occurring within a turbulent boundary layer are investigated in the presence of free-stream turbulence. The free-stream turbulence is generated by an active grid. The free stream is monitored by a single-component hot-wire probe while a second probe is roved across the height of the boundary layer at the same streamwise location. Large-scale structures occurring in the free-stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. It is speculated that depending on the extent of the penetration, i.e. based on the level of freestream turbulence, the near-wall turbulence production peaks at different wall-normal locations than the expected location of y + ≈15 for a canonical turbulent boundary layer.It is shown that the large-scales dominating the log region have a modulating effect on the small-scales in the near wall region; this effect becomes more significant with increasing turbulence in the free-stream, i.e. similarly increasing Re λ0 . This modulating interaction and its Reynolds number trend have similarities with canonical turbulent boundary layers at high Reynolds numbers where the interaction between the large-scales and the envelope of the small-scales exhibit a pure amplitude modulation (Hutchins & Marusic 2007a;Mathis et al. 2009). This similarity has encouraging implications toward generalising scale interactions in turbulent boundary layers.

A test coupon coated with light calcareous tubeworm fouling was scanned, scaled and reproduced for wind-tunnel testing to determine the equivalent sand grain roughness ks. It was found that this surface had a ks = 0.325 mm, substantially less than previously reported values for light calcareous fouling. Any number of variations in surface topology could account for these different report values, such as sparseness, differences in species settlement etc. The experimental results were used to predict the drag on a fouled full scale ship. To achieve this, a modified method for predicting the total drag of a spatially developing turbulent boundary layer (TBL), such as that on the hull of a ship, is presented. The method numerically integrates the skin friction over the length of the boundary layer, assuming a widely accepted analytical form for the mean velocity profile of the TBL. The velocity profile contains the roughness (fouling) information, such that the prediction requires only an input of ks, free-stream velocity (ship speed), kinematic viscosity and the length of the boundary layer (hull length). Using the equivalent sandgrain roughness height determined from experiments, a FFG-7 Oliver Perry class Frigate is predicted to experience a 23% increase in total resistance at cruise, if its hull is coated in light calcareous tubeworm fouling. A similarly fouled Very Large Crude Carrier would experience a 34% increase in total resistance at cruise. turbulent boundary layer, skin friction drag, roughness, tubeworms

In this study, the development of a boundary layer past a change in surface roughness (from rough to smooth, R→S) is examined. Measurements of the flow were made by hotwires, whereas the friction velocity was estimated by Preston tube measurements. By means of a diagnostic plot of the turbulence intensity, it is shown that above the internal layer the flow exhibits characteristics of a rough wall-bounded flow, whereas near the wall the turbulence intensity is similar to that of an isolated smooth wall. Similarly, viscous scaling of the mean streamwise velocity shows an excessive wake region downstream of the R→S wall surface change that diminishes with the fetch from the surface change. Above the internal layer a second peak in the streamwise Reynolds stress was associated with the upstream rough wall flow. Examination of the turbulent spectra revealed the presence of large scale motions within this region that gradually diminishes in strength with increasing distance from the change in surface roughness. The magnitude of the near-wall peak failed to collapse to that of a comparable smooth wall boundary layer under viscous scaling, however, the wall-normal location of the peak appears to be at y + ≈ 15 at all downstream distances. A new mixed scaling is proposed for the nearwall peak based on the corrected wake deficit and the friction velocity. This shows the importance of outer region to the growth of near-wall peak and suggests the presence of amplitude modulation of the near-wall region by the outer region in this non-equilibrium boundary layer.

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Particle image velocimetry (PIV) measurements are performed to examine the structural organisation inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 12% and 8%). The high-intensity free-stream turbulence is generated using an active grid in a wind tunnel. The statistical information of the flow regarding the wall-normal velocity and Reynolds shear stress are presented. The effect of increasing the turbulence level in the free stream for these flows has been found to have similarities with increasing Reynolds number for high Reynolds number canonical flows. Quadrant analysis is performed to determine the contributions of different Reynolds-stress-producing events. In this regard, the distribution of momentum transport events shows some similarity with channel flows, which can be justified by comparison of similar intermittency characteristics of both flows. In addition, the coherent structures found inside the boundary layer have inclined features that are consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organisation of the structures inside the boundary layer supports the growing evidence for a universal structure for wall-bounded flows. * jason.hearst@ntnu.no

Closed-loop feedback control of boundary layer streaks embedded in a laminar boundary layer and experiencing transient growth, which is inherent to bypass boundary layer transition, is experimentally investigated. Streaky disturbances are introduced by a spanwise array of cylindrical roughness elements, and a counter disturbance is provided by a spanwise array of plasma actuators, which are capable of generating spanwise-periodic counter rotating vortices in the boundary layer. Feedback is provided by a spanwise array of shear stress sensors. An input/output model of the system is obtained from measurements of the boundary layer response to steady forcing, and used to design and analyze a proportional-integral controller, which targets a specific spanwise wavenumber of the disturbance. Attention is directed towards a quasi-steady case in which the controller update is slower than the convective time scale. This choice enables addressing issues pertinent to sensing, actuation, and control strategy that are also relevant to the control of unsteady disturbances but without the full complexity of transient effects. The feedback controller and plasma actuators perform well, attenuating the streamwise streaks both in the vicinity of the sensors and farther downstream. The controller remains effective for a range of off-design flow conditions, such as when the free-stream velocity is varied.

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