2.5 Constraints of Testing in a Wind Tunnel
35
The main wall effects that can be identified and corrected are the following:
– The presence of solid walls: the solid blockage due to the reduction of the cross
sectional area or volume of the section available for the airflow and the resulting
increase in speed and dynamic pressure. This effect is often negligible in open jets
due to the fluid boundary conditions.
– “Wake blockage” similar phenomenon to that of solid blockage but due to
displacement effect of the wake.
– Modification of the local incidence along the span of a loaded wing, mainly at the
tips (opposite effects in solid or fluid boundaries).
– Modification of the natural curvature of the streamline due to localised contraction
or divergence created between the model and walls, with direct effect on the
overall angle of incidence, the lift and the pitching moment (weaker effects in
fluid boundaries in open jets).
– Modification of the model deflection due to different aerodynamic loads on the
wing with effects on the lift, drag and static stability (opposite effects in solid or
fluid boundaries in open jets).
– More severe effects on propellers or rotors due to the large diameters. Significantly
smaller scale models are required; other complexities are then introduced due to
lower Reynolds number regime of testing (effects weaker in fluid boundaries in
open jets).
One can account for a solid wall blockage by fictitiously imposing a curvature
to the solid wall based on the mirror image of the model surface curvature. By
reproducing the process for different configurations, it is possible to simulate the
blockage effect in the wind tunnel test section (which theoretically leads to an infinite
number of configurations). Using this technique the raw or uncorrected experimental
results can be extrapolated to free flight conditions which takes place in the absence
of wall boundaries. The influence of test section confinement is discussed in more
detail in Chap. 13.
2.5.2 Model Installation and Different Kinds of Support
Another issue of parasitic interactions results from the device supporting the model
that obviously does not exist in flight. This support assembly, the size of which is a
function of the aerodynamic loads exerted on the model, should be as small as possible
while permitting model displacements under loads. Based on the dimensions of the
supports they can perturb the flow and affect the measurements on the model. In some
cases, it may be necessary to carry out tests with different support configurations.
The first precaution is to make this assembly as aerodynamically discrete as possible and to fix it to the model in the least intrusive way. When these support interference
effects are small, a correction of the raw measurements can be introduced by taking
35
The main wall effects that can be identified and corrected are the following:
– The presence of solid walls: the solid blockage due to the reduction of the cross
sectional area or volume of the section available for the airflow and the resulting
increase in speed and dynamic pressure. This effect is often negligible in open jets
due to the fluid boundary conditions.
– “Wake blockage” similar phenomenon to that of solid blockage but due to
displacement effect of the wake.
– Modification of the local incidence along the span of a loaded wing, mainly at the
tips (opposite effects in solid or fluid boundaries).
– Modification of the natural curvature of the streamline due to localised contraction
or divergence created between the model and walls, with direct effect on the
overall angle of incidence, the lift and the pitching moment (weaker effects in
fluid boundaries in open jets).
– Modification of the model deflection due to different aerodynamic loads on the
wing with effects on the lift, drag and static stability (opposite effects in solid or
fluid boundaries in open jets).
– More severe effects on propellers or rotors due to the large diameters. Significantly
smaller scale models are required; other complexities are then introduced due to
lower Reynolds number regime of testing (effects weaker in fluid boundaries in
open jets).
One can account for a solid wall blockage by fictitiously imposing a curvature
to the solid wall based on the mirror image of the model surface curvature. By
reproducing the process for different configurations, it is possible to simulate the
blockage effect in the wind tunnel test section (which theoretically leads to an infinite
number of configurations). Using this technique the raw or uncorrected experimental
results can be extrapolated to free flight conditions which takes place in the absence
of wall boundaries. The influence of test section confinement is discussed in more
detail in Chap. 13.
2.5.2 Model Installation and Different Kinds of Support
Another issue of parasitic interactions results from the device supporting the model
that obviously does not exist in flight. This support assembly, the size of which is a
function of the aerodynamic loads exerted on the model, should be as small as possible
while permitting model displacements under loads. Based on the dimensions of the
supports they can perturb the flow and affect the measurements on the model. In some
cases, it may be necessary to carry out tests with different support configurations.
The first precaution is to make this assembly as aerodynamically discrete as possible and to fix it to the model in the least intrusive way. When these support interference
effects are small, a correction of the raw measurements can be introduced by taking