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This contribution proposes details about the time and phase-averaged locations of laminar-turbulent transition, separation, and reattachment on a lifting NACA 0015 hydrofoil placed in a propeller wake following a domain decoupling approach, where pieces of information at the hydrofoil's surface, supplied by experimental investigations, are supported and justified by numerical data in the flowfield, forced by the interaction with the upstream propeller. Measurements rely on time-resolved temperature maps at the wall provided by the luminescence of a Temperature Sensitive Paint applied on the hydrofoil. The maps of standard deviation and skewness obtained as averages in the time and phase domains illustrate the spatial distribution and the temporal evolution of the transition and critical loci. Numerical data about the flow field, coming from Detached Eddy Simulation, support the connections between the critical loci dynamics on the hydrofoil's surface and the phase evolution of the pressure gradients imposed by the propeller to the incoming flow. The close connection between the time-resolved temperature maps and the skin friction exerted by the fluid on the surface allows for the detailed description of the boundary layer around the hydrofoil. Statistics of temperature's time series supply a neat, non-redundant, and valuable information on laminar-turbulent transition, separation, and reattachment within and out of the propeller's stream tube. This information appears as the natural complement of the time-resolved 3D flow maps obtained with the Detached Eddy Simulation numerical approach that, relieved of the difficulties inherent in the near-wall calculus, provides a fast and accurate description of the propeller's wake.

In this contribution, three methodologies based on temperature-sensitive paint (TSP) data were further developed and applied for the optical determination of the critical locations of flow separation and reattachment in compressible, high Reynolds number flows. The methodologies rely on skin-friction extraction approaches developed for low-speed flows, which were adapted in this work to study flow separation and reattachment in the presence of shock-wave/boundary-layer interaction. In a first approach, skin-friction topological maps were obtained from time-averaged surface temperature distributions, thus enabling the identification of the critical lines as converging and diverging skin-friction lines. In the other two approaches, the critical lines were identified from the maps of the propagation celerity of temperature perturbations, which were determined from time-resolved TSP data. The experiments were conducted at a freestream Mach number of 0.72 and a chord Reynolds number of 9.7 million in the Transonic Wind Tunnel Göttingen on a VA-2 supercritical airfoil model, which was equipped with two exchangeable TSP modules specifically designed for transonic, high Reynolds number tests. The separation and reattachment lines identified via the three different TSP-based approaches were shown to be in mutual agreement, and were also found to be in agreement with reference experimental and numerical data.

DLR and CNR-INM cooperate since several years on the application and further development of the Temperature-Sensitive Paint (TSP) measurement technique for boundary-layer transition detection and skin-friction diagnostics. The common efforts originally focused on underwater experimental research on boundary-layer transition, separation and reattachment over bluff and slender bodies, with the recognized successful development and application of methodologies to optically determine skin-friction fields from TSP data, as confirmed by common INM-DLR publications [1-5]. The analysis of the TSP data enabled to study in detail the random bursting and evolution of turbulent, wedge- shaped events characterizing the laminar separation bubble over the suction side of a NACA 0015 airfoil model at both moderate Reynolds number and angles of attack [3]. The experimental observations motivated the recent numerical investigation of the flowfield by means of Large Eddy Simulations (LES), which confirmed the conjectured role of flow structures and further shed light on the flow evolution at these transitional conditions [6]. Another focus of the DLR-INM cooperation in the recent years has been on the investigation of boundary-layer transition, separation and reattachment in a compressible air flow (freestream Mach number up to 0.75) at a chord Reynolds number of 10 million [7]. In this experimental campaign on a supercritical airfoil model, boundary-layer transition was examined for a variety of pressure distributions, including cases with laminar separation bubbles and shock-wave / boundary-layer interaction.

Questo rapporto descrive lo stato dell'arte sull'utilizzo di bolle per ottenere la riduzione della resistenza all'avanzamento della nave in qualunque condizione meteomarina. Questa riduzione riguarda, in particolare, la componente d'attrito della resistenza al moto della carena. Nel rapporto si seguiranno due linee parallele: i) un approccio descrittivo relativo all'applicazione su scala nave dei sistemi di riduzione della resistenza che utilizzano l'aria; ii) un approccio fisico basato sulla descrizione dei effetti ottenuti in scala di laboratorio prodotti dalla presenza di bolle di piccole dimensioni su uno strato limite turbolento, generalmente sviluppato a ridosso di una lastra piana. Una prima bozza del rapporto `e stata redatta presso l'Universit`a di Trieste nell'anno accademico 2011/2012 sotto la guida del relatore universitario Prof. Igor Zotti e del Referente Fincantieri Ing. Gianpiero Lavini. La redazione finale `e a cura dell'Ing. Massimo Miozzi.

We experimentally characterize the incipient stall condition which develops on the suction side of a NACA 0015 hydrofoil model at Re = 180; 000, by observing the skin friction evolution on the model surface. Grounding on temperature data acquired via Temperature Sensitive Paint (TSP), the proposed approach adopts a twofold point of view. From one side, the relative skin-friction vector fields tw(x;y; t), extracted from temperature maps Tw(x;y; t) by means of an optical-flow-based algorithm, provide flow topology at the wall and feed a physics based criterion for the identification of flow separation S(y; t) and reattachment R(y; t). From the other side, a direct link between friction velocity Ut and celerity of propagation of temperature disturbances UT0 is established on the basis of currently available results in scientific literature. Then, the time averaged streamwise component of velocity of passive transport of temperature disturbancesUx;T0 is estimated after minimization of the dissimilarity between observed and ideal wave behavior suggested by the Taylor hypothesis. Eventually, a quantitative estimation of time averagedUx;t maps is gained. We describe, through the synergistic application of the proposed methods, the incipient stall evolution by firstly identifying the trailing edge separation at an angle-ofattack AoA=11:5° and then by capturing the onset of upstream oriented mushroom-like structures at AoA = 13°. The concomitant occurrence of both scenarios is reported as well (AoA = 12:2°).

We report about the feasibility to experimentally characterize the interaction between a propeller and a hydrofoil model at incidence in its wake by using a temperature-sensitive paint. The experiments were conducted in a cavitation water tunnel at a freestream speed of 3.4 m/s and at a propeller rotational speed of 17 rps for two model angles of attack (4° and 8°). Time-resolved maps of the temperature evolution on both suction and pressure sides of the model obtained by means of the temperature-sensitive paint allow to take advantage of the direct link between friction velocity U? and celerity of propagation of temperature disturbances UT. By minimizing the dissimilarity between the observed propagation and that of the ideal wave suggested by the Taylor hypothesis, a quantitative estimation of the friction velocity vector field is gained. Detached eddy simulations are also conducted for the examined problem in the case of a model angle of attack of 4°, providing numerical data that are compared with the experimental results. The analysis of time-averaged results unveil the existence of laminar separation bubbles, stretched and folded according to the constructive / destructive coupling of the pressure fields induced by the hydrofoil incidence and by the propeller streamtube.

The large variation of lift and drag around a lifting profile, as a consequence of the flow transition and/or separation, has been widely investigated in recent years. The accurate characterization of the flow separation and the laminar-turbulent transition evolution is a primary input for surface design and flow control, while accurate measurements resolved in space and time are still not available. To this aim, the skin-friction estimation provides useful insights in the understanding of laminar and turbulent boundary layer evolution and in the investigation of separation and laminar-turbulent transition. Here we deal this subject with an experimental approach whose focus is on the skin-friction data derived from time histories of temperature maps obtained using Temperature Sensitive Paint (TSP). We report about a NACA 0015 hydrofoil tested in a water cavitating tunnel (CEIMM facility) at angles of attack AoA between 1° and 10° and Re = 180000, observed at a frame rate of 1 kHz via a Photron SAX camera. A region of recirculating flow was identified confined between laminar separation and turbulent reattachment points, which is commonly referred to as Laminar Separation Bubble (LSB). We extract wall quantities which allow for a topological description of the flow at the wall and shed a light in complex interactions of flow structures with the surface. The focus is placed on the topology of the coherent structures that appear in the reverse flow inside the recirculating region at different AoA. We conjecture that disturbances from the shear layer, which develop before the incoming laminar-turbulent transition, appear as impinging jets in the reverse flow region. Then, by adopting a potential vortex model as corresponding induced flow field and coupling it with a model of the flow in the reverse region, the raise, development and set of the associated coherent structures at the wall is described and compared with the corresponding skin friction topology extracted from temperature maps. Test cases are shown about the interaction of such impinging jets against the separation line at AoA = 10° and the development of omega-shaped, hairpin-like vortices in the reverse flow region at AoA = 3°. These flow structures appear opposite to the classical hairpin scheme of turbulent boundary layer, i.e. faster flow far from the wall is pushed down against the separation line towards the leading edge, thus inducing intermittent, highly energetic bursting phenomena involving the LSB dynamics. Further examples of coherent structures will be reported in the final contribution.

We report about the topology and the coherent structures inside the recirculating region of the Laminar Separation Bubble (LSB) that appear on the suction side of a NACA 0015 hydrofoil at Angles of Attack AoA = [3 o ,10o ] and at chord Reynolds number Re = 1.8 × 105 . The non-intrusive Temperature Sensitive Paint (TSP) technique provides the thermal footprints Tw(x, y,t) of the fluid on the model surface, unveiling three different flow regimes whose complexity in time and space decreases when AoA increases. Tw(x, y,t) maps are analyzed by applying an optical-flow-based method to extract the relative skin-friction fields; the obtained ?w lines provide a topological description of the flow at the wall. The analysis of the critical points shows that, with a fair level of generality, the separation region S is composed by a sequence of manifolds which connect saddle points to converging nodes via the saddles unstable manifolds whereas, in a complementary way, the reattachment region R is composed by a sequence of manifolds which connect saddle points to converging nodes via the saddles stable manifolds. This ubiquitous configuration is common to both the examined AoA and represents an essential portrait of the investigated LSB structure which fully agrees with 3D separation theory. 2D and 3D disturbances strongly alter this scenario. In the paper we report about: o the extraction of the ?w vector fields after the application of the optical-flow based algorithm; o the evolution of the flow topology at separation S at AoA = [3 o ,10o ] and reattachment R at AoA = 10o ; o the effect of weak 2D and 3D disturbances on the topology of the LSB at AoA = 10

An experimental investigation on a horizontal two-phase air-water jet has been conducted in order to develop a technique that can provide information on the interaction between the air bubbles and turbulent flow. In order to allow a measurement of the liquid phase flow field and of the bubbles size, the Particle Image Velocimetry (PIV) has been used, with the liquid phase seeded with a fluorescent tracer consisting of Rhodamine WT mixed with a commercial polyester resin. A phase discrimination algorithm has been developed and successfully used for the detection and separation of tracers and bubbles in distinct images. Three data sets have been recorded: one for a one-phase water jet and two for a two-phase water jet. The obtained results allow a valid characterization of the flow field and an estimation of the effect of the gaseous phase in the air-water jet flow. An analysis of the bubbles size and number has been also conducted and here reported.

The present work focuses on the application of simulation-based design for the resistance optimization of waterjet propelled Delft catamaran, using integrated computational and experimental fluid dynamics. A variable physics/variable fidelity approach was implemented wherein the objective function was evaluated using both low fidelity potential flow solvers with a simplified CFD waterjet model and high fidelity RANS solvers with discretized duct flow calculations. Both solvers were verified and validated with data for the original hull. The particle swarm optimizer was used for single speed optimization at Fr = 0.5, and genetic algorithms were used for multi speed optimization at Fr = 0.3, 0.5 and 0.7. The variable physics/variable fidelity approach was compared with high fidelity approach for the bare-hull shape optimization and it showed an overall CPU time reduction of 54% and converged to the same optimal design at Fr = 0.5. The multi-speed optimization showed design improvement at Fr = 0.5 and 0.7, but not at Fr = 0.3 since the design variables were obtained based on sensitivity analysis at Fr = 0.5. High fidelity simulation results for the optimized barehull geometry indicated 4% reduction in resistance and the optimized waterjet equipped geometry indicated 11% reduction in effective pump power required at self-propulsion. Verification was performed for the optimized hull form and its reduction in powering will be validated in forthcoming experimental campaign.

Purpose of the present report is the experimental study of the water entry and water-exit phenomena of a cylindrical body. The experimental set-up and the measurements relative to the local loads and the velocity fields around the body have been described and analyzed.