A proven concept of the past that yields great potential in the reduction of shipping GHG's, is wind assisted ship propulsion or for short WASP. In particular, the Magnus effect driven Flettner Rotor, a rotating cylinder concept placed upright on the deck of a ship, is on the forefront of promising candidates, mainly due to its substantial aerodynamic force producing capabilities and its relatively low power consumption compared to more conventional ship propulsion.

To accommodate aerodynamic rotating cylinder research at the Low Speed Laboratory of the TU Delft, this study presents the design, realisation and characterisation of an experimental two-dimensional rotating cylinder setup for the SLT wind tunnel facility. Additionally, the effects of testing at a high blockage ratio were investigated for this initial study using the new setup. As a result, the area model blockage ratio deliberately reached an unconventional high 33%, where an aspect ratio of 4.5 was chosen, comparable to full scale WASP applications. The aerodynamic characteristics for three test cases at subcritical Reynolds numbers equal to 62500, 125000 and 250000 with spin ratio ranges equal to 0 ≤ k ≤ 8, 0 ≤ k ≤ 4 and 0 ≤ k ≤ 2 respectively, were obtained by the utilisation of force measurements, particle image velocimetry flow visualisation and proper orthogonal decomposition techniques.

As with similar rotating cylinder experiments, the magnitudes of the presented force coefficients differ in comparison, owing to the large blockage ratio and variations in experimental setup, however the experimental trends in the force coefficient curves are fully captured up to spin ratios of 2.5. Between 0 ≤ k ≤ 0.8, the aerodynamic characteristics are affected by both the Reynolds number and spin ratio, where differences in boundary layer transition, laminar separation bubble formation, boundary layer lengths and vortex shedding behaviour are the underlying mechanisms causing a change in the obtained force coefficients. Transition effects are stronger at the higher subcritical Reynolds numbers, where reversal of the Magnus effect can occur within this range of spin ratios and Reynolds numbers. Beyond k > 0.8, advancing side boundary layer transition has taken place for all tested Reynolds numbers. The force coefficients and flow field topology showed great similarities between the different Reynolds numbers with a further increase of the spin ratio. Slight variations in turbulent kinetic energy production were observed in the retreating side boundary layer and shear layer behaviour, possibly owing to laminar to turbulent transition. For k > 2.0, the periodic shedding of vortical structures is no longer observed, which coincides with the formation of the highly turbulent rotating boundary layer, settling of the near-wake, and the knee in the force coefficient curves within the range of 2.2 < k < 2.6. For remainder of the spin ratios, k > 2.6, the flow field has completely reversed and is pointed in opposite direction of the incoming free stream, where large negative mean drag coefficients and instantaneous force fluctuations are recorded. This is believed to be the result of unsteady nozzle-model gap interactions with the separated region of the cylinder and is dictated by the large blockage ratio.

Current manufacturing techniques in the aviation industry result in a number of two-dimensional surface irregularities (e.g. panel joints, seals, seams) which can lead to an increase in skin friction due to premature boundary layer transition. Panel joints are commonly modeled in the form of two-dimensional steps. Prior studies have shown that Backward-Facing Steps (BFS) promote transition earlier than Forward-Facing Steps (FFS). Therefore, in the design of laminar flow components, FFS are preferred over BFS. However, the understanding of which mechanisms are responsible for the larger growth experienced by Tollmien-Schlichting (TS) waves in the presence of FFS remains unknown. Prior experimental works focused on parametric studies on transition location with limited measurements close to the step. In addition, studies using Direct Numerical Simulations (DNS) assume two-dimensional flow and consequently do not capture transition location. All of this makes comparison between existing experimental and numerical data rather cumbersome, hindering the problem understanding.

In light of this, the present study aims to close-examine the TS waves at the step to identify which are the relevant mechanisms that modify their growth and move transition upstream. To do so, this work presents an experimental and numerical investigation jointly conducted by TU Delft and the German Aerospace Center (DLR) on Tollmien-Schlichting (TS) waves interaction with a Forward-Facing Step (FFS). Experiments are conducted at the TU Delft low-turbulence anechoic wind tunnel (A-tunnel) on an unswept flat plate model. Single-frequency disturbances are introduced using controlled acoustic excitation. The temporal response of the flow in the vicinity of the step is measured using Hot-Wire Anemometry (HWA). In addition, the global effect of the step on laminar-turbulent transition is captured using Infrared Thermography (IR). Two-dimensional (2D) Direct Numerical Simulations (DNS) performed at DLR provide detailed information at the step. Experimental and numerical comparison is performed in subcritical step conditions. At larger step heights only experimental data is provided.

Experimental and DNS results in clean and subcritical step conditions present very good agreement. Both methods predict large distortion of the TS wave downstream of the step, where DNS results present different growth trends between streamwise and wall-normal components of the fundamental mode. Furthermore, while upstream of the step the TS waves exhibit exponential growth, downstream of it they present a complex growth behavior followed by regions where the perturbation energy production term changes sign along the streamwise direction. Finally, regions of highly negative and positive production seem to correlate with the tilting of TS waves in and against the mean shear direction, respectively. These findings point towards the presence of different growth mechanisms triggered by the step which could modify the level of amplification of disturbances far downstream.

Experimental measurements are performed on a 45 degree swept flat plate model at the low speed laboratory (LSL) at the Delft University of Technology, in a low turbulence environment to stimulate the development of stationary crossflow. The swept flat plate model is equipped with two linear manual stages to create forward and backward facing steps. Preliminary measurements characterize the pressure gradient over the swept flat plate model under study. In the preliminary study , hot-wire anemometry (HWA) measurements characterize the flow over the swept plate without steps over a large chordwise domain with and without forcing by discrete roughness elements (DREs). The DREs are spaced at a spanwise wavelength corresponding to the overall maximum N factors from LST. The mean velocity contours and N factor trends presented in these measurements reinforced the need for DREs to control flow. Spectral content is monitored and the frequency bands associated with probe vibration and travelling crossflow interaction were delineated. Infrared thermography was employed to observe the movement of transition front with varying step heights and initial crossflow amplitudes. When the DRE height increases, the transition front moves upstream consistently for all step heights. Furthermore, when the DRE height is kept a constant , but the array is moved upstream and downstream of the neutral point, the transition front moves upstream for all step heights. In order to observe the flow in the vicinity of the step, HWA was once again used to quantify the interaction of crossflow with FFS. The clean, short FFS and supercritical step height configurations identified from the IR study, are studied for two initial amplitudes. For the supercritical step configuration, bandpass filtered fluctuations are found to align with a high wall normal and spanwise shear region which has been identified in previous work. It is postulated to be associated with a vortex shedding mechanism, for which frequency bands are delineated. Estimates of the range of recirculation bubble length were made and a flapping frequency range was also demarcated. In this study, a vortex shedding scenario is used to explain the presence of these near wall fluctuations. To conclude the report, recommendations are made for extending the present study for future work.