Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
Local wind speed measurements are performed by means of Irwin probes: these instruments are driven by means of a pressure scanner and allow measuring the magnitude of the wind speed at a given position at the desired height from the floor level (0.6 [m] height at full scale in the present case). Figure 5 shows an example of the Irwin probes layout chosen for the
yacht sun deck and the relevant
instrumentation set-up on the yacht model, while in Figure 6 an Irwin probe is represented.
The Irwin probes have been completely characterized by means of high velocity wind tests with their tubing system in order to characterise also the system transfer function, as stated in [18].
Aerodynamic forces and speed-up measurements are generally performed at two different wind speeds in order to check any potential Reynolds effect.
certain number of sampling ports are realized on the yacht model, placing them on decks or in areas that can be potentially critical for the dispersion of the tracer gas.
Figure 8 shows an example of the layout of sample ports used for the sun deck level on the yacht model.
Figure 7: Stacks of the yacht model.
Figure 6: Irwin probe.
4. POLLUTANT DILUTION MEASUREMENT 4.1 WIND TUNNEL EXPERIMENT ON SCALE MODELS
The main requirement for the pollutants dilution
lab‐scale experiments is related to release from the stacks of a gas having well defined properties in terms of
density (to reproduce a particular buoyancy effect) and content of a suitable tracer gas, which is used to gain information about its dilution and dispersion in the ship wake [19]. In the wind tunnel environment, having to reproduce the emission of exhausts produced by diesel fuel combustion, a mixture consisting of air and carbon dioxide, at room temperature, with a CO2 concentration equal to 20% by volume, is fed to the yacht model stacks.
The tracer flow‐rate is controlled by means of a mass flow meter. The CO2 concentration into the
feed
flow‐rate is generally checked prior to the start of each experimental session by sampling a small amount of gas
directly from one of the stacks (Figure 7) and analysing it directly so as to verify the tracer gas composition. The gas analysis is carried out in real time by means of an
on‐line analyser, equipped with several detectors, including a non‐dispersive IR for detecting continuously the concentration of CO2. In order to obtain a quantitative
measure of the pollutant dilution conditions and the potential presence of the tracer gas into the ship wake or its possible accumulation in some parts of the vessel, a
© 2014: The Royal Institution of Naval Architects
Figure 8: Map of sampling points SCALING RULES
for
dilution analysis and experimental set up. 4.2
As previously said the wind tunnel reproduces the
atmospheric boundary layer characteristics at model scale; therefore, the conditions necessary to ensure the flow similarity are obtained from the dimensional analysis of exhausts flow motion equations.
Geometric similarity must be ensured by the
homogeneous scaling of all lengths that can be relevant for the system under investigation (e.g.
those of the
model). The dynamic similarity is obtained by acting on the equations of conservation of mass, momentum and
B-75
the pollutant
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