The frictional resistance of a ship is a substantial part of the total resistance. This is influenced, among other things, by the texture of the skin (e.g., type of coating, degree of fouling). To minimise the power consumption and thereby reduce costs and protect the environment, it is therefore sensible to hold frictional resistance as low as possible by special coatings or surface structures. Corresponding studies can be performed on the friction measuring system. A roughness analysis of the surface by itself is not sufficient to deduce the exact frictional resistance. Experimental studies allow for more accurate conclusions. For this purpose, two plates with the coating to be tested are installed so that these form a narrow rectangular channel which is traversed by water in the friction test section. By the simultaneous measurement of the flow rate and the pressure loss along the test section and the water temperature, the wall shear stress can be detected and finally the frictional resistance coefficient of the plates is calculated. The results are transferrable to the frictional resistance of the ship. In order to cover the largest possible range of speeds, up to 20 m/s can be run in the friction measuring system.

These studies are not limited to the shipbuilding industry, but are also applicable in the aerospace and automotive industries. The results from the friction measuring system are also transferrable for these applications and can be profitably implemented where friction plays a role.

The Potsdam Propeller Test Case (PPTC) is a program for the validation of calculation methods for propellers. The PPTC propeller has been specially designed to enable researchers to validate calculation methods for propeller cavitation. The SVA design VP1304 (PPTC-Propeller) has, beside good hydrodynamic qualities, pronounced tip vortices, suction side and pressure side cavitation, root and bubble cavitation, and therefore is well suited for validation purposes.The open water characteristics of the propeller were measured at 0 ° and 12 ° shaft inclination. In selected points of operation the cavitation was recorded on the propeller optically. Additionally, extensive velocity measurements in the area of the blade tip as well as pressure fluctuation measurements were carried out. Within the framework of the International Symposium on Marine Propulsion in 2011 and 2015 respectively, a workshop has been organised on cavitation and propeller performance. In these workshops, the results of calculations from different tools were presented, analysed, and discussed and also compared with the experimental results.

For both workshops, the geometries, measurements, evaluations, reports and presentations are available on the website of the SVA (smp’11 and smp’15). The Proceedings of smp’11 and smp’15 also include presentations of the 1st and 2nd Workshop on Cavitation and Propeller Performance (www.marinepropulsors.com). The PPTC is also used by the ITTC as a benchmark for propeller calculations.

PPTC leads to various published data on the Potsdam Propeller Test Case and related projects.

PIV is a method for measuring velocity fields in fluids. The process is purely visual and is therefore a non intrusive measurement method; the flow to be examined is not affected. The measurement of the velocity field is based on the determination of the displacement of particles (bubbles, seeding particles) in the flow by distance Δs within a period Δt. The shift Δs is detected by two photographic images of the particle images which are received in a very short time interval Δt. To this end the particles in the fluid are illuminated by very short laser flashes. From the displacement of the particle images in the period Δt the velocity vectors of the fluid at the position of the particles can be calculated using stochastic methods. By using two cameras with stereoscopic recording, a three-dimensional flow field can be determined, i.e., all three velocity components are then available in the measuring range.

The process is very versatile. So far the following measuring tasks, among others, have been undertaken:

• Flow fields in the wake of ship models with and without working propellers
• Rudder dynamics with gap flow
• Decay of vortices on a generic wing
• Vortex flow around bilge keels
• Propeller wash of a thruster on a semi-submersible platform
• Propeller wash in the cavitation tunnel
• Flow around and wake of a submarine model with tower
• Flow around profile sections in the cavitation tunnel

With PIV the whole velocity field is measured in every frame. From the individual recordings the transient evolution of the flow can be visualized and also a mean velocity field can be determined by averaging all recordings. The desired spatial resolution determines the size of the field of view and the achievable number of vectors in the measuring range. The largest achieved measuring range so far had an extension of approximately 400×600 mm, in this case about 6000 vectors. For this task, a stereoscopic PIV system from the company TSI is used. It has a modular design, so that all symmetrical, asymmetrical and independent arrangements of cameras and light sheet can be realized. Thus, for example, it is possible to measure the full depth of the towing tank.

Please read more about the technical specifications of this PIV system here.