Properties of wakes behind bluff bodies such as shedding frequency, vortex spacing, and drag change with the presence of cavitation. As extent of cavitation in the wake increases from inception to an attached super-cavity, wake shedding Strouhal number increases, reaching a peak value before decreasing as the wake forms a super cavity. The physical mechanism responsible for this observed change in shedding dynamics is yet to be fully understood. In the current study, we employed time resolved X-ray densitometry and high-speed videography to study the cavitation dynamics in the wake of a triangular, nominally two-dimensional wedge in a re-circulating water tunnel to understand the underlying mechanisms responsible for cavity formation and shedding. Mach numbers of the bubbly mixture, obtained based on base pressure and void fraction measurements, revealed that the flow transition from subsonic to supersonic coincides with the increase in Strouhal number suggesting the importance of compressibility effects on the wake dynamics.

TVC on marine propellers in behind-hull conditions is a challenging problem for numerical predictions, because it is deformed in turbulent viscous interactions with hull wake, propeller-induced flows and rudder disturbance. DES is made for cavitating flows on a propeller and a rudder of a military vessel with a 3.5° shaft inclination in non-uniform hull wake. While a cavitation tunnel test shows TVC extending over the rudder, only a short extent of TVC is reproduced in DES on the initial grid. After an adaptive grid refinement based on the Q-criterion, a larger extent of TVC is simulated and the first- and second-order pressure pulses on two points above the propeller are closer to the experimental measurements. The effects of hull wake, propeller-induced flow, rudder disturbance and shaft inclination on TVC are analyzed by looking into the deformation of the helical trajectory identified by a Q-criterion iso-surface.

Waid and Kermeen completed static experiments using cylinders planing on free liquid surfaces. Their data represents the foundation for characterization of forces imparted on a planing cylinder. Several authors have derived simplified or analytic formulations of the impact forces on a cylindrical body interacting with a curved free surface. This article presents a Computational Fluid Dynamics (CFD) based approach to modeling such dynamic forces. A curved free surface is generated in a CFD water tunnel by ventilating a gas filled wake aft of a curved deflector. The computational planing cylinder is then repeatedly plunged into the curved free surface. The CFD force record is used to reveal the dynamically added mass of the cylinder. Results indicate that the modeling approach taken accurately replicates the complex physics of the dynamic motion of the cylinder interacting with the surface. When added mass of the dynamic interaction is significant, a phase difference appears between the cylinder position and force time histories. Phase offsets between cylinder position and force have been observed across different plunging frequencies, suggesting a significant added mass effect. With the success of this approach (obtaining dynamic forces based on CFD), reduced-order models of the added mass can be created.

A study of the existing literature on flow past circular cylinder reveals that less work has been reported in the upper sub-critical flow regime which corresponds to Reynolds number ( Re D ) in the range 2x10 4 < Re D < 2x10 5 . Hence, this work brings out the flow features in the near wake of a cylinder for a sub-critical flow regime under non-cavitating as well as under different cavitating conditions. A three-dimensional numerical study of flow around a circular cylinder was carried out at Re D =6.4x10 4 for a wide range of cavitation number ( 1.06 ≥ σ/σ i ≥ 0.5 ). Unsteady Reynolds averaged Navier-Stokes (URANS) approach coupled with Schnerr-Sauer cavitation model available in ANSYS-Fluent was used for this purpose. With the help of fine mesh around the cylinder and choice of suitable time step size, this study identifies factors influencing the occurrence of cavitation and portrays dynamics of cavities downstream of the cylinder. As a part of the validation of the numerical approach, variations of mean pressure coefficient around the cylinder as well as mean and fluctuating coefficients of base pressure have been compared with experimental data available in literature and a good agreement between the two have been observed. With a reduction in cavitation number, cavitation activity extends downstream. Based on the fast Fourier transformation (FFT) of pressure and vapor fraction data, it can be concluded that vortex shedding is suppressed due to the formation of extended cavities in the wake. Another interesting observation is the interplay between flow turbulence and cavitation structures. Simulations indicate that with increasing downstream distance from x/d=0.3 (near the cylinder) to x/d=3 (away from the cylinder), pressure recovers and turbulent kinetic energy decreases in case of non-cavitating flow. However, in case of cavitating flow, turbulent kinetic energy is higher at the region of collapse of vapor pockets than that at near the cylinder which is lower than that corresponding to the non-cavitating case at the same location.

The propeller-induced hull pressure fluctuation is one of the main sources of the noise and vibration on ships. It has two main contributors which are equally important: the pressure induced by the displacement of mass, including the blade thickness and the cavitation bubble, and the pressure induced by the blade loading. The work in this paper is a continuation of previous work which introduced a sequence of numerical methods, which, in combination, can efficiently predict the propeller-induced hull pressures given the hull/rudder/propeller geometries, the ship speed, and the propeller RPM. A hybrid BEM/RANS method is first used to evaluate the effective wake on the propeller plane. Then, a BEM solver is used to calculate the propeller-induced hull pressures. The importance of the proper wake alignment model on the accuracy of the predicted pressure field in non-cavitating situations, was addressed before. In this paper, the method is extended to cavitating propellers. First, the wake alignment model is adjusted and improved for cavitating propeller conditions. Then, the unsteady wake alignment model is used to predict the propeller-induced hull pressures. The results are compared to experimental data, and the influence of different wake alignment models on the results is investigated.

Cavitation nuclei are added to the flow to achieve adequate scaling of cavitation phenomena in model-scale facilities with respect to full-scale conditions. During propeller cavitation studies in MARIN’s Depressurized Wave Basin (DWB), cavitation nuclei are added to the flow using electrolysis. In the present study, Interferometric Particle Imaging (IPI) was applied to determine the size and the concentration of nuclei in the wake of a ship model. The dependence of the nuclei size and concentration spectra on the electrolysis current and the basin pressure showed good agreement with theory. One limitation of the present technique was identified: the minimum size of nuclei that can be resolved is approximately 40 μm. This also limits the basin pressure at which nuclei measurements can be performed to a maximum of approximately 100 mbar.

In this chapter we will discuss the detailed characteristics of the lifting surfaces which we know as airfoils , in order to provide guidance in the selection of airfoil shapes that will perform satisfactorily over the broad operating environment of a wind turbine. In addition, the fundamental fluid-dynamic principles involved in modeling the structure of rotor wakes and integrating wake-induced effects over an array of turbines are presented in this chapter. Wind turbines operate by the action of the relative wind (the natural wind plus wind caused by rotor motion and rotor-induced flow), which creates aerodynamic forces on the rotating blades. These can normally be grouped into lift-like forces and drag-like forces . Lift forces operate through the generation of circulation and do not involve large viscous losses in the flow and the associated loss of total head , while drag forces function through flow separation on the blade and the loss of total head. In the previous chapter, the relative merits of lift and drag power devices were discussed, with lift being the clear preference for wind turbine rotors. When considering the potential impact of wakes on the total output of a wind power station, a lift-type device is again preferred, since a drag-type unit develops a greater wake and less energy is available to downwind units. Thus, the array efficiency of a cluster of drag-type units will be lower than that of lift-type turbines.