A simple ghost fluid method for compressible multicomponent flows with capillary effects

A novel way of implementing surface tension effects in sharp-interface compressible flow models is proposed, aiming to address problems where liquid compressibility and capillarity are both important. The method is built on the principles of the grid-aligned formulation for ghost fluid techniques. In this approach, the Riemann problems at the interface are formulated along the grid rather than in a normal-to-the-interface direction; the method is thus simpler. The performance of the method is thoroughly examined following implementation in a well-established front tracking framework

Numerical simulations of the shockwave induced collapse of bubbles near surfaces

In the study of collapsing bubbles and their relation to surface erosion, two mechanisms are identified as of major significance: the high-speed liquid jet and the water-hammer shock wave that is subsequently formed. In the current work, we suggest that secondary mechanisms such as the emission of shockwaves following the collapse of the bubble remains and the late-time wave interactions may also be of importance when it comes to surface erosion. We examine our hypothesis by considering the collapse of a bubble by a shock wave that runs in a direction that is parallel to a solid surface

The effect of porosity on the drag of cylinders

It is well known that perforation of a flat plate reduces its drag when exposed to a flow. However, studies have shown an opposite effect in the case of cylinders. Such a counterintuitive result can have significant consequences on the momentum modelling often used for wind turbine performance predictions, where increased porosity is intrinsically linked to lower drag. Here, a study of the drag of various types of porous cylinders, bars and plates under steady laminar inflow is presented. It is shown that, for most cases, the drag decreases with increased porosity. Only special types of perforations can increase the drag on both cylinders and bars, either by enhancing the effect of the rear half of the models or by organizing the wake structures. These rare occurrences are not relevant to wind turbine modelling, which indicates that current momentum models exhibit the qualitatively correct behaviour

A 3D finite-volume integral boundary layer method for icing applications

A three-dimensional integral boundary layer code was developed to allow fast computations of boundary layer flows for the purpose of ice accretion modelling. The model is derived in this paper. It is based on a surface Finite-Volume approach. The unsteady equations of momentum deficit and kinetic energy deficit are solved until convergence is reached, preventing from specifying explicitly the stagnation point or separation line. A validation of the code is also presented in the present article. First, the 3D solver is cross-checked against a 2D solver on test cases of self-similar flows and on a NACA0012 configuration. The modelling of the effects of three-dimensionality is also assessed on a self-similar flow test-case. Moreover, the use of unstructured grids is also validated. Finally, an example of the use of the code for the computation of ice accretion is presented

A simplified approach for simulations of multidimensional compressible multicomponent flows: the grid-aligned ghost fluid method

In the present work the authors present a simplified formulation for the extension of a ghost fluid method in multidimensional space. In the proposed method, the Riemann problems at the interface are formulated along the grid rather than in a normal to the interface direction. The information that is required to construct these Riemann problems is acquired “on-the-fly” from the adjacent to the interface cells. With respect to existing multidimensional ghost fluid formulations, the method is computationally less expensive, as the procedures of determining ghost fluid regions, extending, interpolating and extrapolating variables and computing geometrical quantities are avoided. More importantly, it is markedly simple with respect to its implementation. By introducing the proposed formulation in a well-established front tracking framework we perform an extensive validation of the method and demonstrate that despite its simplicity it yields highly accurate results while remaining free of oscillations

A sharp-interface model for grid-resolved cavitating flows

A sharp-interface model for cavitating flows is presented in this work. The proposed model can deal with the dynamic evolution of cavitation, and considers both phase change and pre-existing gas expansion mechanisms. The interfaces between the liquid and gas phases are fully sharp, the effects of compressibility (in both phases) and surface tension are considered, and the liquid may withstand certain amounts of tension before breaking down. The formulation is in principle independent of grid discretization. The method is simple to implement, and avoids raising the computational cost as it does not require the solution of additional transport equations. The behaviour of the proposed model is thoroughly assessed through a series of numerical tests. Finally, the developed method is used to predict the nucleation and collapse of a three-dimensional cavitation bubble cloud

Analytical all-induction state model for wind turbine wakes

Analytical wind turbine wake models are an integral component of wind farm design and optimization. However, when the turbine induction factor increases, these models are prone to failure, as they do not account for the increasingly important effects of low wake pressure. To resolve this issue, this paper proposes an analytical wake model which incorporates the effect of wake pressure in its predictions. The model is based on inviscid flow theory for the initial wake region [K. Steiros and M. Hultmark. J. Fluid Mech. 853, R3 (2018)0022-112010.1017/jfm.2018.621] and an extension of Morton's [B. R. Morton. J. Fluid Mech. 10, 101 (1961)0022-112010.1017/S0022112061000093] theory for the far wake, both of which include the effects of wake pressure. Comparison with high-fidelity wind turbine simulations shows that the model is comparable to conventional ones at low induction factors, but continues to be accurate at higher induction factors where existing models break down

Energy focusing in shock-collapsed bubble arrays

During its collapse a bubble can draw and concentrate energy from its surroundings. In the present work, we investigate the behaviour of certain multibubble configurations that have the potential for achieving significant levels of energy focusing. The dynamics of these configurations are studied for the first time in three dimensions, and are shown to be significantly different from those in two dimensions. Novel observations regarding focusing regimes in collapsing arrays are presented. Finally, through a series of numerical experiments on previously unexplored arrangements, we demonstrate that substantially enhanced levels of energy concentration can be achieved

An analytical blockage correction model for high-solidity turbines

A significant challenge in the experimental or computational characterisation of porous bodies and wind turbines is the correction of the obtained flow quantities for wall interference effects. Conventional corrective models are based on the Rankine–Froude theory, which is valid when the body solidity, or turbine induction factor, is sufficiently low. To resolve this issue, this work presents a new corrective model that builds on an extension of the Rankine–Froude theory, valid at arbitrary solidities, coupled with the method of mirror images to account for the existence of channel walls. The predictions of the new model are validated using laboratory and numerical experiments of porous plates and wind turbines. The results show that the new model performs equally as well as conventional ones when the solidity is low, but becomes increasingly more accurate as the latter grows

Recirculation regions in wakes with base bleed

The appearance of detached recirculation regions in wakes with base bleed determines the aerodynamic properties of many natural organisms and technological applications. In this work we introduce an analytical model which captures certain key dimensions of the recirculation region in the wake of porous plates of infinite aspect ratio, along with the porosity range over which it exists when vortex shedding is absent or suppressed. The model is used to interpret why the recirculation region (i) emerges, (ii) migrates away from the body with increasing base bleed, (iii) disappears at a critical bleed, and (iv) is partially insensitive to variations in the Reynolds number. The model predictions show considerable agreement with data from laboratory experiments and numerical simulations