Author: anschau

Shallow Water

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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 (Frh < 0.9), critical (0.9 < Frh < 1.1) and a supercritical (Frh > 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 <Frh<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.

 

CFD_flachw_wavecontours_m_RahmenCFD_flachw_widerstand_flachw_m_RahmenCFD_flachw_5_wavesystems

 

Context Related References / Research Projects

[1] Nietzschmann, T.: Untersuchungen zum Widerstandsverhalten von schnellen Schiffen bei veränderter Bodentopologie, 6. SVA-Forschungsforum „Theoria cum praxi“, Potsdam, 31.01.2013
[2] Lübke, L.: Fast Ship Hydrodynamics on Shallow Water, 8th International Conference on High-Performance Marine Vehicles (HIPER), Duisburg 27. – 28.09.2012

Ducted Propellers

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A nozzle is a hydrodynamically optimised shell encircling the blade tips of a propeller. The combination of propeller and nozzle is called a ducted propeller. There are two types of nozzles, the acceleration and the deceleration nozzle. The deceleration nozzle causes a reduction in flow velocity and an increase in pressure in the propeller plane. Deceleration nozzles are therefore used to reduce the cavitation risk for fast ships.

The accelerating nozzle causes an increase in flow velocity and hence a discharge of the propeller. Ducted propellers with an acceleration nozzle are used for highly loaded propellers and propellers with restricted diameters.

Since its foundation in 1953, the SVA Potsdam has been working on the development and optimisation of ducted propellers for inland and fishing vessels as well as tugs as well as thrusters. In addition to conventional ducted propellers (Wageningen, Schuschkin, OST), unconventional, partially integrated nozzle and controllable pitch propellers have been studied extensively in particular.

Based on a series of tests in the context of an R & D project with a combination of propellers of the Wageningen B-series and OST nozzles, polynomial coefficients were developed for ducted propeller systems with OST nozzles [10]. At lower thrust loads, the OST nozzle profile increases the mass flow through the propeller disc and causes a jet expansion at the nozzle outlet. The characteristics of ducted propellers with OST nozzles can be calculated with polynomial coefficients and used for propulsions prediction.

In the field of CFD calculation, research projects were conducted in close cooperation with ANSYS Germany GmbH for ducted propeller calculation. Based on the developed methods, systematic numerical calculations of ducted propellers were implemented in the R & D project “Correlating Z-Drives with Nozzles” to develop a method for Reynolds number correction (conversion of model test results to the full-size version).

The continued development and validation of the bollard pull and propulsion prediction for tugs with large power capacity has been the subject of R & D projects and industry projects. In the R & D projects “Increasing the Design Safety of Ducted Propeller Systems at Bollard Pull Conditions” and “Reynolds Number Effects on Bollard Pull Predictions” [5], [6] the influences of cavitation and scale on the bollard pull of tugs with ducted propellers were presented in detail. With these results, the risk of thrust break down of ducted propeller can be found in the design stage.

In the R & D project “Forecasting Reliability for the Power Requirement of Tugs with Ducted Propeller Systems”, GeoSim tests and calculations were carried out for tugs at the point of propulsion. The results of these studies have been incorporated into the propulsion forecasting methods for ships with ducted propellers.

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Context Related References / Research Projects

[1] Abdel-Maksoud, M.: Convergence Study of Viscous Flow Computations Around a High Loaded Nozzle Propeller, Numerical Towing Tank Symposium NuTTS 2000, Tjärnö, Sweden, September 2000
[2] Abdel-Maksoud, M., Heinke, H.-J.: Scale Effects on Ducted Propellers, 24th Symposium on Naval Hydrodynamics, Fukuoka, Japan, July 2002
[3] Gutsche, F.: Düsenpropeller in Theorie und Experiment, Jahrbuch der STG, Bd.53, 1959
[4] Heinke, H.-J., Philipp, O.: Development of a skew blade shape for ducted controllable pitch propeller systems”, Proceedings, PROPCAV’95, Newcastle, 1995
[5] Heinke, H.-J., Hellwig, K.: Aspekte der Pfahlzugprognose für Schlepper großer Leistung, 104. Hauptversammlung der STG, Hamburg, November 2009
[6] Mertes, P., Heinke, H.-J.: Aspects of the Design Procedure for Propellers Providing Maximum Bollard Pull, ITS 2008, Singapore, May 2008
[7] Philipp, O., Heinke, H.-J., Müller, E.: Die Düsenform – ein relevanter Parameter der Effizienz von Düsen-Propeller-Systemen, STG-Sprechtag „Hydrodynamik schneller Schiffe und ummantelter Propeller“, Berlin, Potsdam, September 1993
[8] Philipp, O., Heinke, H.-J., Binek, H.: Contribution of Hydrodynamics for the Calculation of Ducted Units for Ships at Shallow Water, HYDRONAV’ 95, Gdansk, November 1995
[9] Schroeder, G.: Wirkungsgrad von Düsenpropellern mit unterschiedlicher Düsen- und Propellerform, Schiffbautechnik 17 (1967) 8
[10] Schulze, R., Manke, H.: Propellersysteme mit Ostdüsen, HANSA, 137 (2000) 2

Open Water Manoeuvrability Testing

Investigations on manoeuvring behaviour that cannot be carried out at the testing facilities of the SVA because of the limited space and high speed occur in the field. The model’s behaviour is detected via a GPS system and by a LASER-optic gyroscope. All the necessary manoeuvres can be either manually or automatically controlled. In addition, so-called source manoeuvres are carried out in order to derive the mathematical model by means of a movement identification system.

 

freilandversuch2_galerieFreiland_2

 

Context Related References / Research Projects
[1] Steinwand, M.: System identification of manoeuvring ship models, SVA-CTO-Meeting, Juni 2004
[2] Steinwand, M.: Optimierung des Stoppmanövers von Schiffen mit Verstellpropellern und Hybridantrieben, 9. SVA-Forschungsforum „Theoria cum praxi“, Potsdam, Januar 2016

3D Printer

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The Eden350V 3D printer is available for the manufacture of components with complex geometries and very small tolerances. It builds synthetic material up in 16 micron layers which are cured by UV light. This method enables the manufacturing of objects of any geometry with the highest precision.
Technical data
Max. component length x axis [mm] 340
y axis [mm] 340
z axis [mm] 340
Resolution Layer thickness [µm] 16
x axis [dpi] 600
y axis [dpi] 600
z axis [dpi] 1600
Accuracy Components >= 50 mm [µm] 200
Components < 50 mm [µm] 20…85

 

3D Scanner

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The SVA owns a 3D scanner system (ATOS Core 300) with the following parameters:

  • Accuracy of 18 µm
  • Object size with high accuracy (18 µm): 300 x 230 x 300 mm³
  • Object size at a lower accuracy (50 µm): 600 x 600 x 600 mm³
  • Manual two axes adjustment (lifting table, rotary table)
  • Projection of 3D elements
  • Automatic report creation

The ATOS Core 300 is used in production and quality control as follows:

  • Scanning of the geometry after milling (while model is still in the milling block). When the predetermined allowance is not reached locally, the milling process is continued.
  • Scanning of the model after the removal from the milling block and the evaluation of the target – actual comparison with colleagues of the workshop in order to specify the steps in the manual finishing.
  • Scanning of the model after completion, analysis of target – actual comparison, preparation of proof of compliance with the quality criteria and creation of an examination report.

Furthermore, components, test objects and geometries from existing measurement systems are recorded with the 3D scanner and provided for use in new experimental arrangements.

 

GOM_AuswertungGOM_Flaechenvergleich

Metal Workshop

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