212
9 Characterisation of Flow Properties at the Surface
of the order of a millisecond, which is often sufficient. Its main disadvantage is the
roughness which causes problem in maintaining a laminar flow.
9.4 Skin Friction Measurement
9.4.1 Floating Element Balance
Accurate measurement of skin friction is challenging especially in regions of highpressure gradient. The direct method is to use a floating element consisting of a small
insert mounted flush to the surface of the model, but isolated from it (see Fig. 9.19).
The element is attached to a force balance which measures the force exerted on it
by the flow. This is the most direct method for determining the skin friction as it is
not based on any theoretical consideration of boundary layer properties. In practice,
the technique of the floating element is difficult to implement on wing or aerofoil
models because of the curvature of the surface. In addition, an important source of
error can come from an imperfect alignment of the sensing element with the surface
and also from the influence of a pressure gradient in the flow. The floating element
method can be used to calibrate other techniques considering a flat plate situation
for which this method can be very accurate.
9.4.2 Hot Film Surface Gauge
A more convenient technique is to use surface gauges formed by a heated platinum
thin film (or wire) fixed to the surface of the model through an insulating insert.
Fig. 9.19 Direct measurement of the surface shear stress: floating element inserted in the surface
9 Characterisation of Flow Properties at the Surface
of the order of a millisecond, which is often sufficient. Its main disadvantage is the
roughness which causes problem in maintaining a laminar flow.
9.4 Skin Friction Measurement
9.4.1 Floating Element Balance
Accurate measurement of skin friction is challenging especially in regions of highpressure gradient. The direct method is to use a floating element consisting of a small
insert mounted flush to the surface of the model, but isolated from it (see Fig. 9.19).
The element is attached to a force balance which measures the force exerted on it
by the flow. This is the most direct method for determining the skin friction as it is
not based on any theoretical consideration of boundary layer properties. In practice,
the technique of the floating element is difficult to implement on wing or aerofoil
models because of the curvature of the surface. In addition, an important source of
error can come from an imperfect alignment of the sensing element with the surface
and also from the influence of a pressure gradient in the flow. The floating element
method can be used to calibrate other techniques considering a flat plate situation
for which this method can be very accurate.
9.4.2 Hot Film Surface Gauge
A more convenient technique is to use surface gauges formed by a heated platinum
thin film (or wire) fixed to the surface of the model through an insulating insert.
Fig. 9.19 Direct measurement of the surface shear stress: floating element inserted in the surface