Biomimicry or biomimetics is the examination of nature, its models, systems, processes, and elements to emulate or take inspiration from in order to solve human problems. Bio-inspiration and biomimicry has become a core consideration of this project as we seek to identify creative and innovative solutions.
For example, we have looked at using non-linear leading edge fins similar to that of the tubercules on the pectoral flippers of the humpback whale. These increase the stall angle without reducing performance at low angles of attack, whilst providing greater lift than conventional aerofoils during sharp turns.
Wind tunnel tests of the model humpback flippers have demonstrated the fluid dynamic improvements which the tubercules make such as a staggering:
- 32% reduction in drag,
- 8% improvement in lift,
- 40% increase in angle of attack over smooth flippers before stalling.
Chassis and Test Rig
Our submarine includes a novel chassis design that acts to safely and adjustably constrain the diver for optimum power transfer, whilst housing the drive train and providing mounting surfaces to the hull and the rest of the internal components.The chassis is also intended to work as a test rig assembly when mounted to a stand so that practical testing on ergonomics, clearances, the transmission, propellers and hull can be performed. As such, the manufacture of just this component should constitute a highly significant legacy to this project and its future.
Hull Shape and Fluid Dynamics
The two forms of drag acting on a submerged body are pressure drag and skin friction drag, with the combination of both giving an ideal length to width ratio of a submarine to be between 4:1 and 8:1.Using ratios in this range and considering the packaging of components within the hull, laminar teardrop shapes were developed and tested iteratively within SolidWorks flow simulation to determine the most efficient shape.
The left image depicts the pressure distribution, where a lower pressure difference around the submarine indicates better hydrodynamics.
Human Power and Ergonomics
Gaining maximum power output from the diver is governed by several factors, including the fitness of the individual, the specificity of the training, and the design of the submarine to take into account positioning, ergonomic load transfer and body part clearances.Cycling was chosen due to increased efficiency over a stepping motion, whilst the prone position was chosen according to research indicating greater power transfer under water as well as the ability to fit within a hydrodynamic shape. Anthropometrics and ergonomics will be analysed within CATIA to design optimal parameters whilst the chassis and design will allow for a range of individuals and tolerances.
Manufacturing and Materials
Some of the main hull material considerations are:
- Materials must be lightweight to maximise acceleration
- Strong enough to withstand stress concentrations and the movement of internal water masses
- Have a high impact strength avoiding damage in an accidental situation
- Able to retain properties during long periods of time underwater
Ashby plots suggest composite materials and aluminium alloys may be suitable for this application. This is consistent with the materials used for previous racing subs. Warwick Sub are keen to explore the possibility of using sustainable materials, building on a key area of interest and expertise within WMG. Research into marine applications using bamboo reinforced polymers highlights high moisture absorptivity using natural fibres. The use of thermoplastic resins may provide a more sustainable solution.
Contra-Rotating Propellers: These are found to give a typical efficiency increase of 10 to 15% compared to separate props whilst mitigating torque effect that causes many competition submersibles to spin. However, the improved efficiency and handling are achieved with significantly higher mechanical complexity and cost.
Speed: The design speed is equal to the speed of maximum propulsive efficiency, which was determined to be 4 m/s.
Design: Propeller design involves the multivariable optimisation of parameters including pitch, blade number, propeller speed, thrust operating range, submarine speed, hub diameter and the propeller’s cross sectional profile.
Marine engineering requires careful safety considerations as failure to do so can result in serious injury or death. Several aspects of the design will be devoted to mitigating these risks:
- A simple, easy to open escape hatch for swift egress of the submersible
- The incorporation of a 'dead man switch' into the steering mechanism in the form of a cycling brake. If the pilot is incapacitated, the lever releases and deploys a safety buoy to alert safety staff
- A high range of visibility to avoid collision and a strobe diving light mounted on the body for easy location of the sub
Stearing and Control
The introduction of a slalom course has required innovative steering designs to prevent compromises on speed whilst increasing agility.Several control surfaces have been designed for 3-dimensional movement:
- A dorsal fin to aid trim and mitigate any roll torque
- Pectoral fins controlled by a geared hand crank control vertical movement
- A rear rudder tail arrangement controlled by wiring and a steering wheel to control horizontal movements
Transmission and Gearing
An optimised transmission system aims to transmit power efficiently from the diver to the propellers. The input power can be transmitted through pedaling or stepping motions, whereby pedaling is more efficient with propellers as the motion is continuous. Stepper systems tend to jam in water, causing inefficiencies and breakdowns.
The power train consists of a chain or belt attached to gears to transfer the rotational motion. A gear ratio of 1:5 allows the propellers to rotate at 300 rpm through a bevel gearing.
There are two drive options; one using pulleys and a toothed belt, or gears in a traditional bicycle chain. Toothed belt drives are more efficient and give less slippage, however are more expensive than bicycle chains. Chained systems have suffered ‘kick-off’ derailment in past competitions, whilst lubrication issues and rust provide further concern.