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Turbulence stimulation.

The drag of an object in a mixed laminar and turbulent flow is depended on the extent of the laminar

region. The NACA 66 series of foil shapes tries to maximize this region by delaying the location of the

increase in pressure on the surface of the foil. As the extent is also dependent upon the Reynolds

number and the surface roughness these items have to be taken into account when expanding model

tests to full size. Not only must the Reynolds number of the model match that of the full size section

,but the regions of laminar and turbulent flow should be as close to identical as possible.

For example yaghts keel is believed that it operates in an almost full turbulent fashion on any but

extremely small craft. In order to simulate this in testing facilities for keels and hull shapes ,it is

common practice to install some kind of turbulence stimulator to the forward edge of the body being

tested

The primary purpose of stimulating turbulence for resistance tests on ship models is to stabilize the

models frictional resistance, so that the corresponding plank frictional resistance can be computed

with confidence. Experiments with small models has shown that neither consistent nor reproducible

test results can be expected unless turbulence is stimulated. In all model tests, transition from laminarto turbulent flow occurs relatively rther aft (due to relatively lower Rn numbers, Rn(model) ~10

6and Rn

(ship) ~109

In order to avoid this scale effect, the model is equipped with some artificial materials

(sand strip, studs, trip wires etc.) called turbulence stimulators.

Turbulence stimulators

1. Sand strips , at or near the stern

Sand strips used for turbulence stimulation will typically comprise backing strips/adhesive of 5 mm

to 10 mm width covered with sharp edged sand with grain size around 0.50 mm, with its leading

edge situated about 5% LPP aft of the FP. This kind of turbulence devise is most effective when

they introduce energy directly into the boundary layer is self, very near at the leading edge.

2. Studs ahead of the stearn

The dimensions of studs and the location of the studs as turbulence stimulators on a raked stem

of conventional type are given by Hughesand Allan (1951) and NPL Report 10/59 (1960).

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3. Rod

In this kind of stimulation it is appeared that the turbulence produced by the rod decays more

rapidly at the surface than the keel depth .It is expected that quantitative measurements of the

decay of turbulence behind rods will provide sufficient data to determine the best position of the

rod adequate stimulation.

4. Trip wires near the bow

Wires used for turbulence stimulation will be typically between 0.5 mm and 1.0 mm diameter,

depending on position and model speed, and be situated about 5% LPP aft of the FP