170
7 Flow Visualisation Techniques
the numerical treatment makes it possible to identify the nature (of attachment or
separation) of the singular points and singular lines. The approach, coupled with
an interactive numerical model, can be used as a quick way to validate the visual
analysis of the surface flow visualisations.
7.2.2 Visualisation by Sublimating Product
This method, consist in depositing a product on the model which sublimates under
heat transfer from the flow; it is mainly used to detect the laminar to turbulent
transition. Indeed, the heat transfer coefficient being higher in turbulent than in
laminar flows, sublimation of the product will be faster on the parts of the model
where the boundary layer is turbulent (this temperature difference is also used by
infrared techniques and TSP, see Sect. 9.5). The most common product is naphthalene
diluted with acetone. Before testing, the model is covered with a thin white layer
of naphthalene which gradually disappears with the establishment of the flow, the
turbulent parts losing their whiteness before the remaining laminar regions. Since the
entire product will sublimate after a while, the snap shots should be taken at the right
instants. Infrared thermography, presented in Sect. 9.5.3 as a quantitative method
for measuring surface heat flux, is also used to visualise the laminar to turbulent
transition (see Sect. 1.7).
7.3 Visualisation in Water Tunnels
Water tunnels are valuable tools for visualising low velocity flows weakly dependent
on the Reynolds number (see Sect. 3.4). The technique involves the introduction of
dye tracers into the flow in the form of filaments of a liquid whose density is very close
to that of water, such filaments may be of different colours to distinguish the various
structures of the flow. The injection is done either upstream of the visualisation region
or through holes located on the model. Fluorescein is sometimes used as tracer and
gives spectacular images. Figure 7.7 shows a visualisation by coloured dye filaments
of the flow on a model of combat aircraft highlighting the breakdown of the intense
vortices forming on the upper surface of the delta wing.
Another technique is to generate small diameter air bubbles by means of a foaming
product upstream of the model (see Fig. 7.8), these bubbles are illuminated by a light
sheet to visualise a plane of the flow. A snapshot is then taken with a certain exposure
time and trajectories of the bubbles are captured as trails.
The images obtained are very instructive. The method is the origin of PIV which
experienced a major development thanks to the progress in the fields of laser-optics
and signal processing. Figure 7.9 shows a view, from downstream, of the flow in
a vertical plane highlighting the vortices on the upper surface of the wings of a
Concorde model.
7 Flow Visualisation Techniques
the numerical treatment makes it possible to identify the nature (of attachment or
separation) of the singular points and singular lines. The approach, coupled with
an interactive numerical model, can be used as a quick way to validate the visual
analysis of the surface flow visualisations.
7.2.2 Visualisation by Sublimating Product
This method, consist in depositing a product on the model which sublimates under
heat transfer from the flow; it is mainly used to detect the laminar to turbulent
transition. Indeed, the heat transfer coefficient being higher in turbulent than in
laminar flows, sublimation of the product will be faster on the parts of the model
where the boundary layer is turbulent (this temperature difference is also used by
infrared techniques and TSP, see Sect. 9.5). The most common product is naphthalene
diluted with acetone. Before testing, the model is covered with a thin white layer
of naphthalene which gradually disappears with the establishment of the flow, the
turbulent parts losing their whiteness before the remaining laminar regions. Since the
entire product will sublimate after a while, the snap shots should be taken at the right
instants. Infrared thermography, presented in Sect. 9.5.3 as a quantitative method
for measuring surface heat flux, is also used to visualise the laminar to turbulent
transition (see Sect. 1.7).
7.3 Visualisation in Water Tunnels
Water tunnels are valuable tools for visualising low velocity flows weakly dependent
on the Reynolds number (see Sect. 3.4). The technique involves the introduction of
dye tracers into the flow in the form of filaments of a liquid whose density is very close
to that of water, such filaments may be of different colours to distinguish the various
structures of the flow. The injection is done either upstream of the visualisation region
or through holes located on the model. Fluorescein is sometimes used as tracer and
gives spectacular images. Figure 7.7 shows a visualisation by coloured dye filaments
of the flow on a model of combat aircraft highlighting the breakdown of the intense
vortices forming on the upper surface of the delta wing.
Another technique is to generate small diameter air bubbles by means of a foaming
product upstream of the model (see Fig. 7.8), these bubbles are illuminated by a light
sheet to visualise a plane of the flow. A snapshot is then taken with a certain exposure
time and trajectories of the bubbles are captured as trails.
The images obtained are very instructive. The method is the origin of PIV which
experienced a major development thanks to the progress in the fields of laser-optics
and signal processing. Figure 7.9 shows a view, from downstream, of the flow in
a vertical plane highlighting the vortices on the upper surface of the wings of a
Concorde model.