Meshfree-based numerical modelling of three-dimensional (3-D) microscale deformations of plant food cells during drying

Charith Rathnayaka

doi:10.5204/thesis.eprints.118069

Numerical modelling has been a helpful tool for analysing plant cellular structure and associated dynamics. It generally consumes less time, money and other resources compared to experimenting with real plant structures. In this context, investigating the morphological changes that take place in the plant cellular structure under different circumstances has recently been an important application. Drying is one of the most common and cost effective techniques for extending the shelf life of food-plant materials (for instance, fruits and vegetables). During the drying process, food-plant cellular structure undergoes structural deformations that influence drying operations in terms of performance as well as food quality. To engineer effective and efficient food drying processes, it is important to establish a good understanding of cell morphological changes and underlying mechanisms. Grid-based approaches and meshfree methods are the two main categories of numerical modelling techniques used to analyse food-plant drying phenomena. Grid-based methods encounter drawbacks in some applications due to the inherent 'grid' behaviour and subsequent inability to successfully model problems with large deformations and multiphase phenomena. To overcome these drawbacks, meshfree (or meshless) based numerical modelling and simulation methods have been developed. There are recently reported efforts to numerically model the micro mechanics of food-plant matter using coupled Smoothed Particle Hydrodynamics (SPH) and Discrete Element Method (DEM)-based approaches. Some of these studies focus only on fresh plant cellular structures and their behaviour under external mechanical loading. There are other studies considering both fresh and dried plant cellular structures in two dimensions (2-D) along with their morphological characteristics. The overall computational approach in those investigations show a promising capacity to be further extended towards more realistic scales. However, it is difficult to describe a truly 3-D phenomenon like cellular scale drying phenomena by means of a 2-D approach. Thus, in order to approximate the morphological changes of cellular scale food-plant drying phenomena in a more detailed manner, there is a requirement to extend that approach into the 3-D level. In addition, there are conceptual constraints in using the Discrete Element Method (DEM) to represent the cell wall membrane in a completely meshfree numerical model. The literature suggests that conceptually, a Coarse-Grained (CG) approach could be more suited for this application, as there is a stronger conceptual and fundamental matching in an SPH-CG coupling than in an SPH-DEM coupling. Within this background, this investigation aimed to develop a 3-D Smoothed Particle Hydrodynamics (SPH) and Coarse Grained method (CG) coupled numerical model, which could successfully approximate the morphological behaviour of foodplant cells during drying. Initially, the fundamentals of microscale plant cellular drying phenomena were studied. The applicability of a coupled SPH-CG 3-D approach was evaluated through a basic 3-D plant cell drying model. Next, an experimental investigation was carried out to observe the real morphological changes taking place in plant cellular structure during drying. Through the learning gleaned from both the basic numerical and experimental studies, an improved 3-D SPH-CG cell drying model was developed. The 3-D nature of this model allows it to predict the morphological changes on a more realistic scale compared to the previous 2-D models developed using a SPH-DEM coupling. The numerical results are found to be well comparable, both qualitatively and quantitatively, with the experimental findings. As the next step, the developed 3-D numerical approach was successfully applied to model different types of food-plant cells (e.g. apple, potato, grape and carrot). The agreement between the model predictions and the experimental findings was found to be favourable for all four food-plant categories selected. The 3-D SPH-CG numerical model investigated in this study can successfully model dryness states of food-plant cells in a larger moisture content range with stable results compared to the recently reported Finite Element Modelling (FEM)-based and meshfree-based plant cell drying models. The computational accuracy of the numerical modelling scheme has been maintained at a high value through limiting the percentage model consistency error to less than 1%. This developed 3-D model will provide a source of guidance for industrial practitioners to optimise food drying operations in terms of final product quality, nutritious value and overall process performance. In addition, the developed computational framework has potential future applications in modelling a wide range of plant cells and animal cells.

Meshfree-based numerical modelling of three-dimensional (3-D) microscale deformations of plant food cells during drying

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