Further research

Introduction

Dr Heller is interested in many additional problems related to experimental fluid dynamics and numerical modelling. A selection is shown here involving shallow-water vortices/turbulent flows, viscoelastic material behaviour, the development of Galinstan strain gauges and beach reflection analysis.

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Home     Students     Wave energy conversion     Landslide-tsunamis     NERC project     Scale effects     Ski jump hydraulics     Further research    Downloads     Publications

Personal research website of Dr Valentin Heller

Fig. 2. Cylinder in a round boundary to generate a vortex; the white particles are used as seeding for surface PIV

Vortices and large-scale turbulent structures are generated in many phenomena and at various scales such as in man-made marine structures (tidal energy converters, wave energy converters) and in the Ocean (landslide-tsunamis, Gulf Stream). Despite this large range of scales, they have similar characteristics, such that the photo of a river in Fig. 1 could easily be mistaken as an image of the Ocean or the atmosphere. Vortices are relevant in many physical model studies and this on-going project investigates them generically. The vortices are generated with a rotating cylinder (Fig. 2) which is pneumatically lifted and moved aside (Fig. 4). The vortex characteristics are then quantified with surface PIV (Heller 2017).

Selected publications

Journals

Chaplin, J.R., Heller, V., Farley, F.J.M., Hearn, G.E., Rainey, R.C.T. (2012). Laboratory testing the Anaconda. Philosophical Transactions of the Royal Society A 370:403-424 (http://dx.doi.org/10.1098/rsta.2011.0256).

Heller, V. (2009). Beschreibung turbulenter Strömungen. Wasser Energie Luft (4):328-336 (in German).

Others

Heller, V. (2017). Scale effects in shallow water vortices. Extended abstract for 4th International Symposium of Shallow Flows. Eindhoven, The Netherlands, June 2017.

Spinneken, J., Heller, V., Kramer, S.C., Piggott, M.D, Viré, A. (2012). Assessment of an advanced finite element tool for the simulation of fully-nonlinear gravity water waves, Proc. 22nd International Ocean and Polar Engineering Conference ISOPE, Rhodes, Greece.

Heller, V., Chaplin, J.R. (2011). Dynamic mechanical analysis of rubber used in Anaconda testing, Proc. 9th European Wave and Tidal Energy Conference, paper 373, 5. - 9. September 2011, Southampton.

 

Shallow-water vortices/turbulent flows

Fig. 1. Turbulent flow upstream of Rheinfall at Neuhausen am Rheinfall, Switzerland (Heller 2009)

Fig. 3. Stress-strain curves (left) from uniaxial quasi static tensile tests of a  vulcanized natural rubber specimen (right, 1.08 mm (T) × 20.32 mm (W) × 205 mm (L)) with load increment 2.9 N; the tests were conducted three times showing that the material behaviour is different during first and successive loading cycles (Mullins effect) (Heller and Chaplin 2011)

Fig. 4. A Galinstan strain gauge fixed on the Anaconda tube in circumference direction; a change in tube diameter changed the length and therefore the electrical resistance of the gauge (applied in Chaplin et al. 2012)

 

Galinstan strain gauges

The Anaconda project, as common in wave energy converter developments, required much more than simply measure power. Many further parameters such as the wave field, bulge wave speed, pressure or power along the tube were also required for the assessment of the performance and the development of and comparison with theory. Such measurements turned out to be challenging. However, this was resolved with self-made Galinstan strain gauges. They measured the circumference, arranged in circumference direction at ten sections along the tube, and indirectly the bulge wave pressure, speed and power along the tube.

Viscoelastic material behaviour

Rubber is a viscoelastic material, i.e. a part of the deformation energy is temporarily stored as potential energy (similar as in a spring) and the remaining fraction is dissipated as heat due to hysteresis losses (similar as in a viscous fluid). Further characteristics of rubber or rubber-like materials are stress relaxation, creeping, residual strain, recovery or Mullins effect. In order to better understand the rubber used for the Anaconda tube, quasi static stress-strain tests (Fig. 3), relaxation tests and a dynamic mechanical analysis (DMA) on a linear actuator were conducted. The DMA showed how much of the deformation energy in the rubber was dissipated in heat and therefore 'lost' for energy production. Such a value was required in the theory presented in Chaplin et al. (2012).

Beach reflection analysis

The main purpose of laboratory beaches is to minimize wave reflections, such that the surface profile and kinematics of the incident wave train are not significantly contaminated. Generic design guidelines for this essential element of hundreds of facilities are limited, such that many beaches are not performing as expected and have to be improved in laborious case studies. This study addresses the design and assessment of highly efficient parabolic laboratory beaches. An excessive physical model study is under investigation involving two wave flumes and a wave basin where the beach profile, beach perforation and roughness elements are varied. Figure 5 shows the effect of the perforation on wave breaking, run-up and down-wash of a permeable (Fig. 5(a) to (e)) and impermeable (Fig. 5(f) to (j)) parabolic beach. A peer-review article including generic design guidelines is in preparation.

Fig. 5. Effect of permeability: Comparison of wave breaking, run-up and down-wash on a permeable parabolic beach (a) to (e) and an impermeable parabolic beach (f) to (j)

 

Last modified: 24.04.2017