Computational Fluid Dynamics (CFD)

The wake fields between model and full-scale show differences because of the underlying physics. In model-scale the boundary layer is thicker relative to full-scale and the bilge vortex more pronounced. This results in differences in the inflow to the propeller between model and ship, which should be considered when designing the propeller. Through the use of numerical methods, the wake field can be calculated for both the model and the ship. Thus it is possible, already while designing the ship lines, to evaluate the wake field and the influence of geometry modifications or appendages. Further, the wake field can be made available for propeller design.

The numerical simulations offer the following advantages:

• Calculation of the wake field in the preliminary design stage
• Calculation of the wake field for the Reynolds number of the vessel
• Evaluation of the velocity components of the wake for any radius and angular increments
• Visualising the inflow and presentation of the causes (for example, separations) for potential irregularities

Along with resistance and stability considerations resulting from wind loads, the focus of aerodynamic investigations of the hull above water is also on the problems of exhaust and intake from ventilation and air conditioning systems and the analysis of exhaust gas concentrations. The latter applies primarily to yachts and passenger ships, where comfort aspects are an important design criterion.

Due to the complex geometry of superstructures or masts, turbulence and recirculation zones can arise which can be examined in terms of gas distribution. To process such inquiries, numerical methods can offer possible solutions in order to verify structural aspects and to carry out calculations on variants.

Through simulation the following aspects may be considered:

• Calculation of wind, exhaust gas, intake and exhaust flow, and detection of mutual interactions
• Detailed visualisation of the flow and fluid concentrations, i.e. the exhaust gas
• Calculation of the flow around the whole ship (not linked to specific measurement positions)
• Temperature distribution for the detection of “hotspots”
• Accounting for the wind profile
• Calculation for full-scale version

An initial ship design is rarely optimal in terms of its hydrodynamic efficiency. Often, a new design is derived from geometries of existing ships and “manually” adjusted for the new mission profile. To maximise the economic, environmental, and in particular, fuel efficient operation, the vessel geometry can be optimised without effecting other design criteria such as displacement distribution, load capacity, speed, stability, wake distribution, etc. This optimisation is performed with the use of CFD methods, either by means of a mathematically based, automatic optimisation, or knowledge based on the groundwork of decades of experience of engineers.
For this optimisation, among other things, various parametric models of the “CAESES” software are used. The optimisation is done on the basis of viscous CFD calculations, since these are considerably more reliable in optimising than the potential theory method. The savings benefits are generally considerable and give, over the life of the vessel, a significant increase of the “return on investment”. Classical optimisation tasks are the optimisation of bows, bulbous bows, and sterns. While on new designs an optimal concept is at once available, in the area of retrofitting, especially a bulbous bow, optimisation makes sense. Generally, this is done for the customer’s predefined mission profile fully parametrically. The example shows the development of the drag resistance of a ship geometry depending on the variation of the bulbous bow.
As a secondary condition of minimum resistance or propeller thrust, optimisation of the flow into a propeller (homogeneity of the wake) can be considered.

Propeller struts, stabiliser fins and other appendages can make a significant contribution to the ship’s resistance. Through the optimised shape and alignment of the appendages, the resistance and the loading of the attachments are minimised. Accordingly the propeller wash becomes homogeneous, improving the quality and efficiency of propulsion and contributing to the reduction of vibration and noise.

The use of numerical methods offers many opportunities to examine ships at sea. The SVA can use the program system UTHLANDE or RANSE Method depending on the application. UTHLANDE provides the ability to perform seaway calculations based on linear and non-linear strip method. Using this method, monohulls and catamarans can be studied over a wide range of applications and short- and long-term statistics can be calculated. For special cases or the simulation of non-linear applications, RANSE Method (ANSYS Fluent) is used.

With numerical calculations, issues can be examined in terms of added resistance or the acceleration and motion behavior of ships. Added resistance at sea is an essential aspect in order to determine power requirements and operating costs of vessels correctly. The motion behaviour at sea, however, is an essential aspect of comfort and determines the operating limits of ships.

The numerical seaway simulations with RANSE procedures include the following applications:

• Seaway from any direction, stationary and in motion
• Calculation of regular and irregular sea states, with the focus on the calculation of regular seaway
• Determination of the added resistance
• Determination of accelerations

The use of numerical calculation methods offers many possibilities to investigate ship manoeuvres. In general, numerical calculations analogous to PMM-experiments (Planar Motion Mechanism) are carried out in which static and dynamic simulations are performed to determine the forces and moments as a function of a specific movement of the ship. From this, the hydrodynamic coefficients can be derived and fed into a mathematical model. With the calculation of a complete set of hydrodynamic coefficients, any manoeuvre can be simulated. To verify the quality of the numerical calculations, the calculation results are continually validated with the corresponding measured values.

Numerical simulations allow to:

• Simulate manoeuvring behaviour in model and full-scale
• Simulation of static and dynamic tests
• Visualisation of flow, detection of separating flow
• Design of control elements such as rudders, thrusters, etc.

The rudder is by far the most frequently used control element; it operates in the wash of the propeller. Below, the pressure distribution on the rudder for a rudder angle of δ_R = 20° with a rotating propeller is shown.

Shallow water has a significant influence on the handling of ships. The most obvious effect is the changing shape of waves in shallow water. Due to the different wave propagation and wave group velocities of deep and shallow water at the same wave length, the interaction between the different wave systems of a ship changes. This is manifested, among other things, through strong changes in KELVIN angle.

To illustrate the shallow water effects the Froude depth number is generally used; wherein the driving regimes are subvided in a subcritical (Fr_h < 0.9), critical (0.9 < Fr_h < 1.1) and a supercritical (Fr_h > 1.1) ranges. Normally, ships operate in the subcritical range. For critical Froude depth numbers, a strong increase in resistance and a large change in the dynamic flotation position can be expected depending on the ship type since, in this region, the transverse waves move at the speed of the vessel. As a special case, soliton waves can occur in this area. In the case of supercritical Froude depth number, the ship is faster than the maximum wave speed and the transverse waves disappear in the secondary wave system.

Numerical methods have extensive applications in the calculations of shallow water effects.

• Calculation of the resistance and the flotation position at varying depths, velocities and ground topologies
• Calculation of waves / wave heights on banks and shores.

The SVA uses ANSYS CFD for this.

Fast moving ships are more affected by shallow water effects than slow moving ships. The areas of the Baltic Sea are represented in the image below for three different speeds where the vessel would be running in a critical Froude depth number (0.9 <Fr_h<1.1) speed range. Near the coast the boat is moving in supercritical range, further out on the Baltic Sea in the subcritical Froude depth number speed range.