Multiple Free Surfaces Research Articles

Comparison of schlieren-based techniques for measurements of a turbulent and wavy free surface

The free surface at an air–water interface can provide information regarding bathymetric complexities, as well as the subsurface flow. We present a comparison of the performance of two recent advances in light-based optical techniques for free surface measurements, total internal reflection-deflectometry and moon-glade background-oriented schlieren, with the more established method of free surface synthetic schlieren. We make use of an optical flow algorithm over the more traditional digital image correlation, in order to obtain higher spatial resolution data across the imaged free surface domain. The optical flow algorithm presents additional benefits, such as computational efficiency and robustness in capturing large displacements and straining of tracked features. The three optical techniques are assembled in synchronization to image two free surface conditions: (1) a free surface being impinged upon by an underlying turbulent, free-shear flow and (2) a random and irregular wave field induced by a free jet. Using the high-resolution measurements, we provide insight on the emergence of multiple free surface dynamics for a turbulent free surface. We present a comprehensive discussion on the benefits and drawbacks of each technique, including suggestions on the suitability of each technique for several experimental constraints.Graphical abstract

Adhesion, cavitation, and fibrillation during the debonding process of pressure sensitive adhesives

Transient 3-dimensional simulations of viscoelastic fluidlike adhesives access the physics behind the debonding process of pressure sensitive adhesives (PSA). A novel finite element method is developed to simulate viscoelastic flows with multiple free surfaces and contact lines. The rheological properties of the adhesive are correlated with its tackiness and stickiness, targeting the development of materials with improved adhesive energy.

The sinking of the Ro–Ro passenger ferry SS Heraklion

On December 8, 1966 the Ro–Pax ferry SS Heraklion capsized and sank in the Aegean Sea, resulting in the death of over 200 people. The present paper tries to shed some light into the various events that led to the largest tragedy of modern Greek maritime history. It re-constructs the accidental data based on a variety of original investigation reports, ship files and legal evidence. Ship’s loading, intact and damage stability behaviour were re-investigated and the flooding/sinking of the ship was simulated by use of a time simulation method. Results of our investigation were compared with available testimonies of survivors. It was found that downflooding of large void spaces below the flooded car deck and the effect of multiple free surfaces on ship’s stability was eventually the main reason for SS Heraklion’s capsize.

URANS simulations for a high-speed transom stern ship with breaking waves

An exploratory study of high-speed surface ship flows is performed to identify modelling and numerical issues, to test the predictive capability of an unsteady RANS method for such flows, to explain flow features observed experimentally, and to document results obtained in conjunction with the 2005 ONR Wave Breaking Workshop. Simulations are performed for a high-speed transom stern ship (R/V Athena I) at three speeds Froude number (Fr) = 0.25, 0.43 and 0.62 with the URANS code CFDSHIP-IOWA, which utilizes a single-phase level set method for free surface modelling. The two largest Fr are considered to be high-speed cases and exhibit strong breaking plunging bow waves. Structured overset grids are used for local refinement of the unsteady transom flow at medium speed and for small scale breaking bow and transom waves at high-speeds. All simulations are performed in a time accurate manner and an examination of time histories of resistance and free surface contours is used to assess the degree to which the solutions reach a steady state. The medium speed simulation shows a classical steady Kelvin wave pattern without breaking and a wetted naturally unsteady transom flow with shedding of vortices from the transom corner. At higher speeds, the solutions reach an essentially steady state and display intense bow wave breaking with repeated reconnection of the plunging breaker with the free surface, resulting in multiple free surface scars. The high-speed simulations also show a dry transom and an inboard breaking wave, followed by outboard breaking waves downstream. In comparison to an earlier dataset, resistance is well predicted over the three speeds. The free surface predictions are compared with recent measurements at the two lowest speeds and show good agreement for both non-breaking and breaking waves.

Numerical study of flows with multiple free surfaces

AbstractThis paper demonstrates that a numerical method based on the generalized simplified marker and cell (GENSMAC) flow solver and Youngs' volume of fluid (Y‐VOF) surface‐tracking technique is an effective tool for studying the basic mechanics of hydraulic engineering problems with multiple free surfaces and non‐hydrostatic pressure distributions. Two‐dimensional flow equations in a vertical plane are solved numerically for this purpose. The numerical results are compared with experimental data and earlier numerical results based on a higher‐order depth‐averaged flow model available in the literature. Two classical problems, (i) flow in a free overfall and (ii) flow past a floor slot, are considered. The numerical results correspond very well with the experimental data for both sub‐critical and supercritical flows. Copyright © 2001 John Wiley & Sons, Ltd.

An experimental and numerical investigation of container filling with viscous liquids

This work is concerned with a study of container filling, with particular reference to the food industry. A computer code was developed and an experimental rig was built, the main purpose being to validate the software. The computational fluid dynamic (CFD) code, called GENSMAC, was specifically designed for relatively slow viscous flow and was capable of capturing multiple free surfaces. This paper focuses on the design of the experimental rig and how it functions. The visual output of the code is then compared with high-speed photographic shots of glucose syrup being jetted into a tub for a selected number of flow regimes

FINITE ELEMENT MODELLING OF FREE SURFACE VISCOELASTIC FLOWS WITH PARTICULAR APPLICATION TO RUBBER MIXING

Internal mixers are used extensively in industry for mixing the components of rubber compounds. In these operations, in order to achieve effective mixing, the mixer chamber is always partially filled. This inevitably results in the appearance of multiple free surfaces in flow fields inside rubber mixer chambers. Mathematical modelling of such a flow regime is not a simple task and requires a great deal of effort. Traditional free surface flow-modelling techniques, which are mainly based on the use of volume-of-fluid or pseudo-density approaches in an Eulerian framework, are not flexible enough to cope with this problem. In this paper we describe a new method for the numerical modelling of free surface flows. In this method the pseudo-density approach is extended to a special Lagrangian framework along the trajectories of the fluid particles. We show that the developed scheme can very effectively simulate viscoelastic free surface flows encountered in rubber-mixing processes.

A numerical technique for solving unsteady non-Newtonian free surface flows

A numerical method has been developed for solving two-dimensional generalized Newtonian fluid flow with multiple free surfaces. It is an extension of the GENSMAC code which solves the time-dependent Navier-Stokes equations for the primitive variables of velocity and pressure in an arbitrary domain. Like GENSMAC, it is a finite-difference technique, based on staggered grids, using (virtual) marker particles as a means of flow visualization. The code has been employed to solve three time-dependent problems: extrudate die swell, viscous jet buckling, and injection moulding in complex cavities. Both Newtonian and non-Newtonian results are displayed.

Cast filling simulations of thin-walled cavities

Computer simulation models of casting processes which couple the velocity potential approach with a transient Bernoulli equation are developed. Instead of employing three primitive variables, displacement, velocity and pressure, only one variable-velocity potential is needed. When the thickness integrated equation is also employed, the method can provide practical results with very reasonable computing time and storage for the 3-D casting of an arbitrary shape thin-walled cavity. In this study, the flexibility of the finite element method in dealing with complex geometries and the efficient algorithm of volume of fluid (VOF) approach for tracking moving free surfaces are combined for solving casting filling problems. Multiple free surface contacts are developed when the stream finally turns back and impacts the original free surface. A double-node scheme is developed to treat these multiple free surface contacts. The movement of the interface is small and an inflexible wall across the separating surface is assumed. Because the casting process involves phase change, and the interface between the solid and liquid is generally an unknown curve, the enthalpy model with fixed mesh is used to determine the temperature distribution and the thickness of the filling. Numerical examples for prediction of filling patterns, effects of solidification on patterns and parametric studies are presented. Fairly good agreement between this method and experimental results and other numerical simulations have also been obtained. The computational techniques developed in this study can provide a powerful and flexible tool for analyzing the fluid flow and heat transfer in metal casting of thin-walled cavities and can help design engineers reduce the costly and time-consuming process of designing complex molds for the manufacture of casted parts.

A simple numerical technique for turbulent flows with free surfaces

AbstractA simple technique is presented for the numerical solution of two‐dimensional time‐dependent flows, either laminar or turbulent, involving multiple free surfaces of arbitrary configuration. The governing equations are the Reynolds equations for incompressible fluids with Boussinesq closure, the k‐ and ϵ‐equations and an additional equation describing the fluid configuration. This technique can potentially describe the propagation, deformation and overturning of pre‐breaking waves and the mean flow, surface configuration and turbulence field after breaking. The properties of the method are illustrated by several calculational examples. The main parts of the algorithm are optimized for vector processing in a form suitable for installation in supercomputer facilities.

Thermally stressed multiple systems in steady state

This work is concerned with formulating the steady state linear thermoelastic problem for bodies with finite dimensions and multiple cracks. A superposition scheme is used such that the finite body problem is divided into three component problems involving the infinite region. The first and second component problem are concerned, respectively, with prescribing displacements/tractions and thermal disturbance in an infinite region at the site that conforms to the boundary of the finite body. The third component problem corresponds to an infinite region with cracks subjected to the negative of the tractions obtained from the mechanical and thermal loadings. Upon adding all three component problems the mechanical and thermal boundary conditions of the original problem with multiple free surface cracks are recovered. Numerical results are obtained for edge crack, three parallel cracks and three inclined cracks along the sides of an equilateral triangle. In all cases, a steady state temperature difference is maintained either uniformly over or across the specimen with fixed edges. Constant contours of shear stress are calculated and they compared well with the corresponding isochromatics observed in experiments.

Three-dimensional studies on bicomponent extrusion

The present work is concerned with the mathematical modelling and numerical simulation of three-dimensional (3-D) bicomponent extrusion. The objective is to provide an understanding of the flow phenomena involved and to investigate their impact on the free surface shape and interface configuration of the extruded article. A finite element algorithm for the 3-D numerical simulation of bicomponent stratified free surface flows is described. The presence of multiple free surfaces (layer interface and external free surfaces) requires special free surface update schemes. The pressure and viscous stress discontinuity due to viscosity mismatch at the interface between the two stratified components is handled with both a double node (u−v−w−P 1 −P 2 −h 1 −h 2) formulation and a penalty function (u−v−w−P−h 1 −h 2) formulation. The experimentally observed tendency of the less viscous layer to encapsulate the more viscous layer in stratified bicomponent flows of side-by-side configuration is established with the aid of a fully 3-D analysis in agreement with experimental evidence. The direction and degree of encapsulation depend directly on the viscosity ratio of the two melts. For shear thinning melts exhibiting a viscosity crossover point, it is demonstrated that interface curvature reversal may occur if the shearing level is such that the crossover point is exceeded. Extrudate bending and distortion of the bicomponent system because of the viscosity mismatch is shown. For flows in a sheath-core configuration it is shown that the viscosity ratio may have a severe effect on the swelling ratio of the bicomponent system. Modelling of the die section showed that the boundary condition imposed at the fluid/fluid/wall contact point is critical to the accuracy of the overall solution.