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STX France is a ship building and fleet services company. The company is one of the leaders for designing, building, erecting and commissioning highly complex ships and maritime installations.
Modern cruise liners are often called “floating hotels” featuring luxury cabins, a wide range of shopping, dining and entertainment areas, and innovative outdoor swimming and leisure areas. With all of these amenities, passengers expect a luxurious experience. The largest cruise ships can accommodate up to 6,000 passengers and 2,000 staff, the equivalent of a small town floating on the ocean.
While passengers relax and enjoy the various on-ship options, many services are working behind the scenes; these include diesel engines powering the liner, kitchens supplying the food to restaurants, as well as laundry services and incinerators. An unfortunate by-product of these systems are exhaust fumes, which have to be vented to the outside of the ship. If these fumes drift back onto the ship, they can create “smell discomfort” for both passengers and crew. Any area of the ship with unpleasant odors will be avoided or unusable by passengers. The human sense of smell is so sensitive than even concentrations as low as 300 parts per million of diesel fumes will make passengers uncomfortable. Ship owners want to avoid any discomfort or wasted/dead space, but also want to maintain the overall aesthetic and design of the ship.
STX France has extensive experience designing and building cruise ships to meet the highest requirements and standards, and has delivered many vessels operating all over the world. STX France is proud of its heritage of more than 100 years of shipbuilding and of its ability to innovate and create the ultimate luxury experience. Deliveries include Harmony of the Seas – the largest cruise ship ever built – which recently received the fuel efficiency award at the Seatrade exhibition. The Mediterranean Shipping Company (MSC) Meraviglia cruise ship makes use of all the latest environmental features developed by STX France in their ECORIZON program.
As part of their drive to create the most energy-efficient and comfortable, best-in-class ships, STX France is increasing its use of computational fluid dynamics (CFD) as part of the design process. CFD has been used systematically for more than 10 years to optimize hull geometries and ship-wave interactions. More recently, STX France has started using CFD as a tool to predict exhaust plume dispersion in the vicinity of the superstructure. This method has allowed them to understand the plume behavior in detail and improve the design of funnels and vents to avoid fumes in the passenger decks. STX is using Simcenter™ STAR-CCM+® software, which is part of Siemens’ Simcenter portfolio of advanced simulation tools.
Gases are vented from the ship at different locations; the exhaust gases are generally warmer than the surrounding air so they rise in a plume away from the vent. The development of this plume is highly dependent on the environmental conditions. Cruise ships run at an average speed of 20 knots, while the prevailing wind direction can vary in both strength and direction. The speed of the exhaust gases at the vent is generally low compared to the external flow, so the plume motion and development is turbulent and highly unsteady. A typical Gaussian plume model will not correctly capture the development of the plume, as it neglects the crosswind contribution to the flow. Instead, it is better to think of the plume as a series of “puffs,” or highly unsteady winds moving in three dimensions and growing and dispersing over time.
To examine the plume dispersion in Simcenter STAR-CCM+, STX France models the entire superstructure of the ship. Each ship has a different three-dimensional computer-aided design (CAD) model, created and maintained by STX France in collaboration with the owner and the architect. This design must fulfill multiple requirements, both aesthetic and practical, and vent placement and design is just one of these. Cruise ship superstructures are complex geometries with detailed features on a range of scales. These features can have a significant impact on the flow patterns around the superstructure that affect the plume dispersion, making it vital to maintain a high level of detail in the CFD model. The Simcenter STAR-CCM+ surface wrapper automatically creates a closed starting surface from the imported CAD geometry. The wrapper does not defeature the geometry, preserving the full details of the superstructure. Using this makes setting up the model quick and easy, with little manual geometry preparation required. After being imported into Simcenter STAR-CCM+, the full-scale CAD model is wrapped, meshed and set up so that multiple vents are modeled in the same simulation. The same general mesh settings are used for all cases to ensure that the results are consistent and comparable, but additional specific mesh refinements are defined downstream of each vent, with the location depending on the wind direction being tested. The complete mesh has around 35 million cells, surrounding a typical ship size of 350 meters (m) long, 40m wide and 65m high.
Because of the highly unsteady plume dynamics, a steady Reynolds-Averaged Navier Stokes (RANS) simulation is not suitable. Instead, the detached eddy simulation (DES) hybrid modeling approach is used. The DES approach uses RANS modeling in boundary layers, but switches to a large eddy simulation (LES) model in detached (highly turbulent) flow. This gives higher accuracy in the turbulent contribution to the flow development in the areas needed, for example in the plumes. To track the concentrations of the exhausts, a multiphase approach is used, with the output from each type of exhaust defined as a different phase. This allows multiple exhaust vents to be analyzed in the same CFD simulation, gaining maximum information from each model run. In a typical simulation, the velocity field around the ship is highly turbulent, with many vortices forming and shedding from the superstructure. The plume structure can be visualized as an isosurface of the mass fraction at a specified level. Concentrations of the different exhaust gases can be monitored across the complete model, and any areas that show concentrations higher than the required level can be easily found.
When looking at an initial design, STX France focuses first on the most extreme configurations of wind speed and direction. As the design is refined, a wider range of more detailed configurations is analyzed, covering wind from all directions and at three different strengths. The exhaust speed can also be altered, depending on the ship operation profile. There are more than 25 main outlets on the superstructure of the ship. To reduce the overall number of cases, each simulation can have up to eight exhaust outlets, with four or five different phases being modeled. Even with this, there can be a large number of cases for each design. STX France runs cases in batch on its cluster and has a standard setup for analysis allowing quick comparison of results.
A standard shipbuilding contract takes up to three years, from signing to delivery of the built ship. The plume analyses are carried out as part of the initial detailed design phase in the first six months of the contract. During this phase, STX France works closely with the ship owner, sharing the results of these CFD simulations and giving feedback on the suggested design. There can be multiple design iterations before a final design is agreed upon; while aesthetics is important, ship owners will not take the risk of bad plume behavior so this feedback and iteration are a vital part of the design process.
Prior to using Simcenter STAR-CCM+, STX France performed wind tunnel tests to look at plume dispersion. While it was easy to make quick design alterations in the wind tunnel and repeat results, the scaled model size limited the level of detail that could be captured. This limitation gave some uncertainty on the behavior of the plume, particularly near small spaces on the ship sides. Using Simcenter STAR-CCM+ enables simulation of the plume behavior on the full-scale ship geometry, preserving all of its detail. The CFD analysis gives a much greater understanding of the full character of the flow, both around the ship and the plume. STX France now routinely uses Simcenter STAR-CCM+ to perform the plume dispersion studies in CFD instead of wind tunnel tests.
Because “smell comfort” is such a subjective measure, it is not easy to validate the CFD studies. STX France has, however, been able to compare their Simcenter STAR-CCM+ predictions with readings on a completed ship, and the correlation between the results gives confidence in using CFD in this way.
Ensuring efficient dispersion of exhausts and polluting gases from a cruise ship is critical in providing the experience that cruise passengers expect. A poorly devised funnel design can lead to areas with poor smell comfort, which will be underused by customers. As space is at a premium, ship owners do not want to risk passenger discomfort or create vacant real estate. By running CFD simulations in Simcenter STAR-CCM+, STX France is able to analyze the complete ship geometry at full scale, and therefore predict the plume dispersion and exhaust concentration at any point on the structure. They use this information to provide feedback to ship owners and designers during the early design stage. The process is efficient as multiple exhausts can be modeled in one simulation.
This modeling approach has given a greater understanding of the characteristics of the plumes and their potential interaction with the ship superstructure than was possible before, and has now replaced wind tunnel testing for exhaust dispersal prediction in the design phase. Using Simcenter STAR-CCM+ is helping STX France to develop and build some of the most successful cruise ships sailing today.