Hull Design with Computational Fluid Dynamics (CFD), Hydrodynamics, & Aerodynamics

By November 4, 2015 Uncategorized No Comments

^ Planing of a War Boat: Note the Rise of the Bow  – Image Courtesy of Royal Navy Official Photographer at the Imperial War Museum Retrieved from https://en.wikipedia.org/wiki/File:Royal_Navy_MTB_5.jpg

Green Advances on the Blue Frontier

Structurally, technically, and economically, hulls are and have been the most important design parameter for ships. Gobbling up around 20% of a ship’s total cost, hulls demand a lion’s share in the resources allocated for shipbuilding.

With the runaway juggernaut of Global Warming and Climate Change threatening to endanger our planet like none has done before, the eminence of hulls has jumped further. Even as we speak, ship design is getting more and more environment friendly.

It has to. Exhaust emission norms are tightening by the day even as rising fuel costs hike operational expenses and falling charter rates and freights cut down profit margins to a bare minimum. All this add to the already critical importance of ship efficiency.

Now, hulls determine the hydrodynamic quality and the efficacy of the entire onboard system. Appropriately designed hulls solve two pressing challenges that haunt ship designers viz. cutting energy use without reducing design speed and boosting speed with the same available power.

CFD Analysis of Jet Vehicle Image Courtesy of NASA at http://www.dfrc.nasa.gov/Gallery/Photo/X-43A/HTML/ED97-43968-1.html Retrieved from https://en.wikipedia.org/wiki/File:X-43A_(Hyper_-_X)_Mach_7_computational_fluid_dynamic_(CFD).jpg

CFD Analysis of Jet Vehicle
Image Courtesy of NASA at http://www.dfrc.nasa.gov/Gallery/Photo/X-43A/HTML/ED97-43968-1.html
Retrieved from https://en.wikipedia.org/wiki/File:X-43A_(Hyper_-_X)_Mach_7_computational_fluid_dynamic_(CFD).jpg

Reality Check

Although ships are the most environment friendly mode of transport, the sheer fact that shipping carries over 90% of the global trade means that it emits an enormous amount of CO2. If international shipping were a country, it would be the sixth largest emitter.

Data from the Network for Transport and the Environment reveals that a 10,000TEU container ship emits only 10 grams of carbon dioxide (CO2) to transport 1 ton-kilometer of cargo. Diesel trains emit 21 grams, trucks 59 grams, and airplanes an astronomical 470 grams.

Designers use Computational Fluid Dynamics (CFD) to analyze the hydrodynamics and aerodynamics of a set of hull shapes and arrive at the most optimum shape. They then test the scale model of this shortlisted hull in towing tanks before moving to full scale production.

Computational Fluid Dynamics (CFD)

Advances in CFD have empowered designers to build lighter, faster, stronger, more streamlined, and more efficient hulls at a fraction of the cost that conventional processes would have otherwise entailed.

Recently, designers have combined CFD with advanced parallel computation and databases containing information on actual and model testing of ships.

This has allowed more accurate estimation of the propulsive performance of ships at a very early design stage. Please remember, decisions made in the early phase of a shipbuilding project influence 70-80% of the project costs.

Conventional Computer Assisted Drawing (CAD) methods are inefficient when it comes to modifying hull geometries. In contrast, CFD software enables the virtual testing of novel hull designs with the option of easily changing the design as necessary.

Generation of Hydrodynamic / Aerodynamic Lift Image Courtesy of NASA at en:File:Equal_transit-time_NASA_wrong1.gif Retrieved from https://en.wikipedia.org/wiki/File:Equal_transit-time_NASA_wrong1.gif

Generation of Hydrodynamic / Aerodynamic Lift
Image Courtesy of NASA at en:File:Equal_transit-time_NASA_wrong1.gif
Retrieved from https://en.wikipedia.org/wiki/File:Equal_transit-time_NASA_wrong1.gifAnd although CFD has not completely replaced testing in towing tanks, it has made testing a whole lot easier and infinitely less wasteful. For the chosen hull is virtually checked before. Tow tank testing is used for additional comparison and validation.

CFD allows better wave pattern prediction and appendage alignment. It also enables improved forecasting of scale effects on the wake field and wave resistance when used on scale models.

Through all this, you arrive at an optimized hull form with minimum wave resistance, maximum propulsion efficiency, and zero zones of high turbulence. This:

  • can save as much as 5% fuel
  • improves sailing speed
  • lowers the noise created by propeller wakes
  • cuts down repair costs associated with hull and aft equipment

CFD also enables better planning of systems integration that slashes production costs. Now, you can establish the location of onboard equipment long before production starts.

This eliminates cumbersome and expensive rework while trimming down the number of man hours. Systems Integration is the combination of diverse elements of a system into one coherent whole.

Diagram for Deriving Bernoulli’s Equation Image Courtesy of MAnnyMax at Image:BernoullisLawDerivationDiagram.png Retrieved from https://en.wikipedia.org/wiki/File:BernoullisLawDerivationDiagram.svg

Diagram for Deriving Bernoulli’s Equation
Image Courtesy of MAnnyMax at Image:BernoullisLawDerivationDiagram.png
Retrieved from https://en.wikipedia.org/wiki/File:BernoullisLawDerivationDiagram.svg

Hydrodynamic optimization of the ship hull contour is a multi-stage, repetitive process that requires the consultants, owners, and designers to combine their economic, hydrodynamic, operational, information technology, geometric modeling, and optimization resources.

It is only after completing this cumbersome process of virtual modeling that designers move to the next phase of physical model testing. Not only does CFD improve power, fuel, and emission performance of ships, it also boosts their cost-effectiveness, safety, and structural integrity.

Mitsubishi Heavy Industries Ltd. (MHI) for example utilizes this combination for its Hull Form Design Aid System. Designers choose an initial hull from the database and prepare CAD designs for the same.

Then, they modify the design to suit their design strategy and create multiple hull forms. Parallel computing enables the estimation of the performance of each of these designs, linked as computing is to the databases on test data.

Simply speaking, CFD solves the Navier-Stokes Equations that are the crux of fluid flow modeling. When you solve them for a given set of conditions, you estimate the fluid pressure and velocity for a specified geometry.

MHI uses CFD to predict forces exerted by the fluid on an object and in the wake distribution around the object. Solving Navier-Stokes Equations is somewhat easy for simple geometries. You have to solve them numerically for complex geometries though.

Representing the principle of conservation of momentum, Navier-Stokes Equations are the extension of the Newton’s Second Law of Motion to fluids. These are always solved together with the continuity equation that manifests the principle of conservation of mass.

Hydrodynamics & Aerodynamics

Hydrodynamics is a more important branch than Aerodynamics in hull design simply because water is 784 times as dense as air. Water therefore offers more resistance to the motion of the hull. This makes designers focus more on hydrodynamic resistance.

Bulbous Bow lowers Hydrodynamic Resistance Image Courtesy of Lommer at https://en.wikipedia.org/wiki/File:USS_Ronald_Reagan_Bulbous_Bow.jpg

Bulbous Bow lowers Hydrodynamic Resistance
Image Courtesy of Lommer at https://en.wikipedia.org/wiki/File:USS_Ronald_Reagan_Bulbous_Bow.jpg

Aerodynamics is critical in the design of high-speed vessels because air resistance is important here. Such crafts include fast ferries and fast patrol boats that can clock as much as 50knots. Testers use wind tunnel technologies after CFD analysis of the above-waterline hull shape.

Usually, hulls with pointed bows are more hydrodynamically efficient because water moves around them more easily than it does around square or round hulls.

Therefore, pointed hulls have to push lesser mass of water out of the way. Plus they have smaller surface area that creates less drag i.e. resistance that a fluid exerts on an object when the object moves through the fluid.

Such easier movement of water around the hull generates less drag and gives a faster boat. In the final analysis, this improves the efficiency of the vessel while also cutting down on fuel consumption and noxious emissions.

But that may not always be so. According to the study The Shapes of Boat Hulls Matter by Brian P Hoover for the California State Science Fair 2004, a model boat with flat bow and rounded bottom experienced lesser drag when compared to boats with:

  • flat bow and flat bottom
  • pointed bow and flat bottom
  • pointed bow and rounded bottom

Hydrofoils & Tunnel Hulls

Let us consider two kinds of water crafts that incorporate cutting edge hydrodynamics and aerodynamics. These are:

  • Tunnel Hull Boats
  • Hydrofoils

Tunnel Hull Boats employ a combination of air and water force dynamics for ‘flying’ through air. Water is 784 times denser than air. Movement through air is therefore easier than through water. We all know this – walking is much easier than swimming.

These boats consist of two twin hulls connected by a solid center. The upper deck surface and the tunnel roof respectively form the upper and lower surfaces of an aerofoil shaped structure.

When the boat moves at high speed, this aerofoil structure generates lift that raises the front part of the boat above the water surface. This happens due to Bernoulli’s Principle.

Based on the Principle of Conservation of Energy, the Bernoulli Principle states that the total mechanical energy (viz. fluid pressure, kinetic energy, and potential energy of elevation) of a fluid flowing steadily along a streamline remains constant.

Class I Offshore Powerboat: Tunnel Hull boats are often used as Power boats Image Courtesy of Rennbootarchiv Schulze at https://en.wikipedia.org/wiki/File:Class1_Oostende.jpg

Class I Offshore Powerboat: Tunnel Hull boats are often used as Power boats
Image Courtesy of Rennbootarchiv Schulze at https://en.wikipedia.org/wiki/File:Class1_Oostende.jpg

In other words, any increase in velocity is accompanied by a decrease in pressure and/or elevation. If the change in elevation is negligible as is the case with an aerofoil moving on the water surface, increase in velocity is always associated with a decrease in pressure and vice versa.

Air moves faster over the upper surface of an aerofoil than over the lower. The pressure on the lower side is therefore greater than that on the upper surface. The greater pressure from below pushes the aerofoil upward. This is the aerodynamic lift. Airplanes use this same lift for takeoff.

Hydrodynamic Lift gets generated on account of the same reasons as is aerodynamic lift except that the fluid in this case is water, not air. The central part of tunnel hull boats traps air to create aerodynamic lift while the twin hulls create hydrodynamic lift.

These twin hulls are planning hulls. Planing is the technique of operation of a boat in a manner that its weight is supported more by the hydrodynamic lift than by hydrostatic lift (buoyancy).

Patrol boats, ambulance crafts, sport vessels, service crafts, recreational boats, and sport-fishing vessels are normally made in accordance with the planning design.

Hydrofoil Boats or simply Hydrofoils are water vessels that use water wings. These wings are similar to that of an aerofoil and operate on the same principle. Water flows faster over their upper surface and this creates lift.

At low speeds, the hull of hydrofoils operates as a displacement hull that removes water out of the way. As the vessel gains speed, hydrodynamic lift elevates the vessel out of the water. Only the hydrofoils remain in touch with water.

Flying Dolphin Zeus: A Hydrofoil Image Courtesy of ArnoWinter at https://en.wikipedia.org/wiki/File:Hydrofoil_near_Piraeus.JPG

Flying Dolphin Zeus: A Hydrofoil
Image Courtesy of ArnoWinter at https://en.wikipedia.org/wiki/File:Hydrofoil_near_Piraeus.JPG

Because the hydrofoils have lesser area that that of the hull, this minimizes the drag exerted on the vessel by water. We have already noted how the drag exerted by air is far less than that by water. With less drag, the vessel can cruise at high speeds of around 50knots.

Hydrofoils must not generate more than necessary lift, for this will elevate the boat out of the water. Then, air will create aerodynamic lift that is way too less than the hydrodynamic life due to the density difference. This will bring the vessel crashing down in the water again.

Finally

Research and innovation is a never ending process. In the near future, we are set to witness astounding advances in hull designs. And who knows, emissions may actually fall and take down the ogre of global temperature rise.

Visit our blog to know more on the subtleties of ship design and construction. Contact Kemplon Engineering for first rate marine fabrication services, marine pipe fitting, and large scale custom metal fabrication.