CE1 Project 1 Title: Prediction of Hydrodynamic

Ahmed A. Swidan – Competence and Commitment - MRINA CEng 1/3

CE1

Project 1

Title:

Prediction of Hydrodynamic Impact Loads using CFD

CE1.1

Introduction Chronology: May 2014 - March 2015. Location: Australian Maritime College, Launceston, TAS 7248, Australia The name of the organisation: University of Tasmania (UTAS).

CE1.1.1

This episode describes an essential part of my postgraduate research at the National Centre for Marine Engineering & Hydrodynamics. During this project I developed, evaluated, and validated a range of numerical simulations using Computational Fluid Dynamics (CFD) under the academic supervision of the following people: • Professor Dev Ranmuthugala Position: Head, Defence Science and Technology Group, Melbourne, Australia Mobile: +61 (0) 409 957 082 Email: [email protected] • Professor Giles Thomas (Communicate via Skype and email) Position: BMT Chair, Department of Mechanical Engineering, University College London, UK Mobile: +44 (0) 776 130 2664 Email: [email protected]

CE1.2

Background

CE1.2.1

With the increasing demand for faster and lighter ferries the need for predicting motions and sea loads for efficient structural design and safe operation has become necessary.

CE1.2.2

Over the past three decades, high-speed catamarans have extended their service area to the open ocean where frequent and severe sea loads can result in structural damage and crew injuries emphasises the need for the development of reliable tools to accurately predict slam loads.

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CE1.2.3

One of the principal sea loads acting on catamarans is called wetdeck slamming. For a catamaran fitted with a centrebow, as shown in Fig. 1.1, slamming occurs when the archway between the demihull and centrebow impacts the surface at relatively high-speeds.

Figure 1.1: INCAT 112m high-speed catamaran fitted with centrebow [Incat website].

CE1.2.4

Currently classification societies (American Bureau of Shipping (ABS), 2016; Det Norske Veritas (DNV), 2015; Lloyd's Register (LR), 2016) provide designers with a range of empirical formulae that in reality may over or underestimate the actual impact pressure distributions.

CE1.2.5

I reviewed the literature pertaining to fluid-structure interaction and CFD modelling methods using a variety of resources available at UTAS.

CE1.2.6

The review allowed me to gain better knowledge about transient hydrodynamic impact loads on floating structures.

CE1.2.7

I discussed the global aim of this project with the project team, which was to provide ship designers and classification authorities with a validated numerical method to accurately predict impact loads.

CE1.2.8

My main duties and responsibilities during this project included: 1. Development of CFD simulations to accurately capture transient impact loads acting on 2D and 3D model impacting with water.

Provided designers with information on the different numerical techniques and settings required to accurately model such unsteady events.

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CE1.2.9

The organisational chart presented below addresses roles of team members: Incat Tasmania (Industrial Partner)

Australian Maritime College

Research & Development Of High-Speed Catamarans

Industrial Supervisors

Academic Supervisors

Hydrodynamic Loads

Structure

Powering

Resistance Sea-keeping

Four PhD Students (Naval Architects) Experimental

Numerical

SPH simulations

CFD Simulations

PhD Engineer

Ahmed Swidan PhD student Mechanical Eng.

CE1.3

Personal Engineering Activities

CE1.3.1

Prior to commencement, the project team (Academic and Industrial supervisors and me) analysed the needs of our Industrial partner and the available resources

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provided by the NCMEH. During this project I was committed to the privacy of Incat’s designs and I formally signed a confidentiality agreement. CE1.3.2

I computed a Work break down structure (WBS) that incorporated into a Gantt chart by means of Microsoft Project Software (MPS). I determined the project milestones to meet the project objectives and I broke down tasks to smaller elements that I can manage efficiently with the assignment of the available advanced computer power resources at UTAS and the estimated timeframe. This allowed me to manage my time efficiently throughout the project.

CE1.3.3

I estimated the required budget to perform this project, which included buying two text books, and attending two prestigious international conferences to present my findings of this project. I submitted my estimated budget to the head of the NCMEH (Professor Giles Thomas at that time) for discussion and approval. I also identified the main risk involved in this project which was concerned with the validation of the numerical with experimental data. I determined the main and minor risks and I developed strategies to mitigate its consequences on the expected progress. I also ensured that my work meets the requirements of safety and health, and research ethics at the AMC.

CE1.3.4

In this project I developed and validated quasi-2D and 3D CFD models using the commercial CFD package STAR-CCM+. I decided employing STARCCM+ to solve the complex water entry problem for the following reasons: • STAR-CCM+ is a validated code. • The University of Tasmania has 50 STAR-CCM+ licenses available. • STAR-CCM+ provides a promising technique, namely overset mesh to simulate moving bodies within high-accuracy (see CE1.3.8).

CE1.3.5

I attended courses, presentations and lectures at such as “Applied Computational Methods (ME 651) at AAST, Computational Fluid Dynamics and Hydrodynamics (JEE509) at AMC”, a training course called “Introduction to STAR-CCM+, CFD Code at AMC” and a range of webinars held by CDAdapco. In particular these three courses assisted me in performing the technical tasks allocated to this project.

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CE1.3.6

I utilised the following approach to achieve the main project objectives: • I conducted an extensive review of the international standards and regulations such as recently issued by DNV, LR, ABS, literature pertaining to fluid-structure interaction and CFD modelling methods. • I was then conducted a series of quasi-2D CFD simulations to validate computed results with existing free-fall experimental data and I compared results to previously published numerical simulations using Smoothed Particle Hydrodynamics (SPH), with my CFD results showing better agreement with the experimental data than the SPH predictions. Thus, I was then extended my CFD simulations to evaluate and validate two numerical methods in predicting the flow behaviour and transient loads acting on a numerical model impacting a 3D body of water.

CFD Approaches CE1.3.7

I reviewed the User Guide of STAR-CCM+, Steve Portal by CD-adapco, attended a range of CFD international webinars, and I consulted Mr. Kynan Maley at CD-adapco Australia in a professional way, via email, to discuss some technical issues in the computational setup and I appreciated his help.

CE1.3.8

I have recognised (from reviewing many resources) that there are several techniques that have been developed to simulate moving bodies through multiphase flows. The three main international approaches are grid re-meshing, grid deformation and overlapping/overset grid. I utilised overset mesh method in solving the water-impact problems as it enables large body motion without regenerating/deforming the old grid and hence it preserves quality of cells/results as well as save simulations time. Thus, I intended to learn it by myself through reviewing the STAR-CCM+ user guide, as it was just a new technique.

CE1.3.9

To solve water-entry problems, I employed finite-volume method, overset grids, Reynolds-Averaged Navier-Stokes (RANS) equations and Volume of Fluid (VOF) method.

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Numerical simulations CE1.3.10 To assist the validation of the 2D and 3D CFD results with the experimental data, I replicated the experimental conditions (such as dimensions, velocity, mass of model, water temperature, etc.). I also run the simulations on a number of nodes (as a factor of total cell numbers) using an advanced computing cluster at the AMC. The provided computational power helped me in performing, an extensive numerical studies within allocated timeframes. CE1.3.11 To validate the computed results against experimental data, I developed a code in STAR-CCM+ based on my understanding of its User guide for simulating the measured velocity profiles. CE1.3.12 I selected a slow growth rate for grid generation to ensure smooth transition between neighbouring cells. I also considered that the cell sizes preserve the transducer geometry to accurately predict the average applied pressure on the sensing areas. CE1.3.13 To resolve the near wall turbulence quantities I employed the advanced all y+ wall treatment method that attempts to combine the high y+ wall treatment for coarse grids and low y+ treatment for fine grids, where y+ is a nondimensional wall distance. I selected this turbulence model as it has the advantages of the k-ω model near the boundary layer and retaining the insensitivity of k-ε in the far field. CE1.3.14 I utilised the Volume of Fluid (VOF) formulation, to capture the behaviour of the free surface between the air and water fluid mixture (i.e. two phase flow) and the complex splash-up during the water-entry phase. I also used the recommended (in the STAR-CCM+ user guide) High Resolution Interface Capturing (HRIC) scheme to preserve the sharpness of the interface between the water and air, whilst ensured a Courant number of less than 0.5. CE1.3.15 I employed the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) to resolve the pressure-velocity coupling, while the linkage between the momentum and continuity equations is achieved through predictor and corrector stages.

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CE1.3.16 For the temporal integration I decided to apply the second order of the implicit Euler scheme to maximise the numerical stability based on extensive numerical simulations which I conducted. CE1.3.17 To study the sensitivity of computed results against the used sizes of cells to discretise the computational domains and models, I conducted grid-density independence studies and time independence studies for all simulations. Thus I selected the grid sizes of cells and required time-steps based on analysing the uncertainty error bounds when it ensured stability, i.e. did not enhance significantly, despite required a higher computational cost. I chose a various range of time-steps during my simulations, in order to satisfy a Courant number of less than 0.5 in the same domain (to ensure the HRIC scheme is activated). To ensure reliability of computed results, I conducted validation study with high-quality experimental data. Uncertainty analysis CE1.3.18 I assumed that the simulated models were infinitely rigid, pressure transducers were flush-mounted to the curved structure surface and the compressibility of water was neglected. I assumed a negligible effect of the frictional forces between moving parts in the drop test rigs. I assumed an adiabatic compression of air at the top of the arch, between the structure and disturbed free-surface. CE1.3.19 During simulations of the quasi-2D catamaran model, I found that simulating the same venting clearance (see Fig. 1.2) between 5 mm space as in the 2D experiment of a catamaran section resulted in an error of around 78% (i.e.

–

), as presented in Fig.1.3. I attributed

this error to boundary condition limitations due to the rotation of flow around the sharp ended edge of the wetdeck and in a very tiny space of the venting clearance.

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Figure 2.2: Symmetry, side and perspective, views of initial general arrangement of the numerical model and the tank side.

Figure 3.3: Sensitivity to the grid resolution at 5 mm venting clearance.

CE1.3.20 To solve the problem, I decided to conduct further CFD simulations for a range of venting clearances from 6mm to 8mm, and I analysed the computed results as shown in Fig. 1.4. I found that at 7 mm venting clearance or more the computed results showed good agreement with experimental data during the 2D catamaran simulations. During our weekly team meetings I shared my technical report (including proposed design) with my academic supervisors and outsourcing experts, i.e. Dr Whelan who conducted the 2D drop tests and sought feedback, communicating via Skype and email. Both my academic supervisors and Dr Whelan agreed with my assumptions and appreciated the technique I used in solving this problem. My results were validated against experimental data and showed good agreement, and is presented in the symposium of “High-Speed Marine Vehicles (HSMV)”, in Naples, Italy, 2014.

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Figure 1.4: Venting clearance error study and corresponding time record uncertainty of vertical acceleration and slamming pressures distribution.

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CE1.3.21 To validate the computed results of a 3D catamaran bow section impacting water against experimental data (see Fig. 1.5), I used the velocity profile measured during the experiment (presented in career episode 2). In addition, I utilised CFD to compare between two numerical techniques for simulation of the water-entry process as follows: • I allowed the body to move relative to the initially stationary fluid using overset grid method within STAR-CCM+; and • I applied an inlet flow at a given velocity with the body remaining stationary using the traditional fixed grid method.

Figure 1.5: Overview of the developed computational domain of a 3D catamaran hullform model.

Reporting Results and Findings CE1.3.22 For the problem of water-entry at constant speed, although both techniques (i.e. overset and fixed grid) provided close predictions in comparison with the experimental work, the overset grid technique presented significantly better prediction of the slam force magnitude when compared with experimental data. I produced a technical report and shared findings with all CFD simulation

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results were discussed in detail to my supervisors and industrial partners. In addition, I published three scientific papers including validation studies and I presented my work at two prestigious international conferences held in the USA and Italy, and I sought feedback from experts. I acknowledged the ongoing support of AMC for awarding a full-time scholarship to me and for providing cutting edge technology and financial support through this research project, i.e. to present my results in two prestigious international conferences in the Italy and the USA during my research project. I also recognised the work conducted by Whelan and for providing high-quality data that I used for validation my computed results. CE1.3.23 I reported and presented (using powerpoint software) useful information for catamaran designers (at Revolution Design and Incat) on a range of techniques and settings that I recommend (based on published results) to accurately predict two phase flow behaviour and transient loads. My supervisors and industrial partners appreciated the quality of work I produced during the course of this project. My findings would enhance the accuracy of predicting water impact loads, and hence resulting in lighter ships, lower fuel consumption, and maximum profit. I also provided consultancy advice and technical input for international researchers such as Josef Camilleri (PhD candidate at Southampton University) who appreciated my help.

CE1.4

Summary

CE1.4.1

I addressed in this project the current limitations of engineering designers to assess transient hydrodynamic loads on complex bodies impacting water using CFD.

CE1.4.2

I developed numerical CFD modelling that commenced with a quasi-2D model progressing to a 3D numerical model using a dynamic meshing technique. One of the drawbacks in employing CFD techniques for solving 3D flow regime is the deformation of the free-surface, jet evolution and multi-phase flow which require significant computer power and time to accurately capture slam event details.

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CE1.4.3

I developed and validated a 3D CFD numerical model that can accurately predict wetdeck slamming events and offer a feasible solution to the localised pressure distributions, total vertical force and flow behaviour around 3D model impacting a body of water. The application of the developed method can be applied for various engineering applications, such as any two phase flow experiencing sudden impacts.

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