1. Specialization in Ocean Energy MODELLING OF WAVE ENERGY CONVERSION
António F.O. Falcão
Instituto Superior Técnico,
Universidade de Lisboa
2017

2. PART 6
MODEL TESTING

3. Model testing of 3-unit array in the wave tank of Plymouth University, UK, July 2014. Scale 1:32.

4. Dimensional analysis applied to wave energy conversion
We may apply dimensional analysis techniques to model testing of wave energy converters in wave tank or wave flume.

5. This will ensure the equality of all the other dimensioless variables.
Assuming the device to be an exact geometric representation of the full-sized device
Applying Buckingham theorem, we may replace this equation by another one with only three independent (dimensionless) variables
If dynamical symilarity is to be ensured, the three dimensionless variables on the rhs must take the same values in model and full size.
This will ensure the equality of all the other dimensioless variables.
Note that viscosity effects and Reynolds number were ignored.
However, this analysis is valid even if wave amplitude and body motion amplitude are not small.

7. In the case of model tests of an oscillating-water- column converter, there are special issues concerning how to model:
Volume of the air chamber
Size of the air turbine
Rotational speed of the turbine

8. Frequency domain analysis
The system is linear
Decompose
Note: radiation conductance G cannot be negative

9. From frequency domain analysis
Model testing of OWCs and air turbines

10. Model testing: similarity laws for air chamber and air turbine
Correct dynamic similarity requires all terms in equation to take equal values in similar conditions at model size 1 and full size 2 . 1
air chamber 2

11. Turbine rotational speed
The two turbines are geometrically similar
Turbine dimensionless parameters (representing the turbine aerodynamic performance) take equal values for similar conditions of the air pressure cycle in the chamber of the model and the full-sized converter. We take such conditions as those of maximum air pressure .
Turbine size
Turbine rotational speed

12. Model testing of a cylindrical fixed-structure OWC in a wave flume (Instituto Superior Técnico, 2013). To appropriately reproduce the air compressibility effect in the chamber, the top of the tube is connected by a pipe to an air reservoir placed above.

13. Model at 1:16th scale of a spar-buoy OWC developed at Instituto Superior Técnico being tested at the large wave tank of the National Renewable Energy Centre (NAREC), Blyth, England, in The turbine was simulated by an orifice. The tank was filled with sea water.

14. Backward Bent Duct Buoy (1:4th of full scale) equipped with a Wells turbine being tested in Galway Bay, Ireland, about 2008.

15. Exercise
Consider the OWCs integrated into the breakwater of Mutriku (northern Spain). Each OWC is equipped with a Wells turbine with rotor diameter D = 0.75 m and rated power 18.5 kW. The dimensions of the OWC structure are given in the figure below. The chamber width in the direction perpendicular to the drawing is 4 m.
The plant is to be model tested at scale 1:5 in a large wave tank filled with fresh water. The turbine with be simulated in the tests by a geometrically similar turbine. Assume that the aerodynamic efficiency of the turbine is not affected by the scale (in reality this is not true, since Reynolds numbers are quite different).
a) Determine the incident wave power (W/m) in the laboratory that simulates 30 kW/m at full size.
b) Determine the air volume of the model chamber if air compressibility effects are to be adequately simulated. Divide it by the area of the model inner free-surface and compare with the height of the prototype air chamber.
c) Determine the diameter of the model turbine rotor.
d) Determine the rotational speed ratio between model and prototype under similar performance conditions.
e) Determine the model turbine rated power corresponding to 18.5 kW for the prototype.

16. TURBINE

17. OWC breakwater, Mutriku, northern Spain, 2008-12,
16 x 18.5 kW OWCs

18. END OF PART 6
MODEL TESTING

19. Sinusoidal waves in water of arbitrary, but uniform, depth h

20. irregular waves