WakeNet Job report
COMPARISON OF WIND-TUNNEL VORTEX DATA WITH FIELD TRIAL

MEASUREMENTS BY LASER RADAR

 The range of techniques that has been employed to study wake vortices includes

theoretical analysis, computational fluid dynamics (CFD) modelling, wind tunnel

measurements, instrumented probe aircraft and instrumented towers,

anemometer arrays, catapult propulsion of scaled models and laser radar.The

various techniques contribute information to one or more of the many important

aspects of vortex behaviour, e.g., formation, evolution and decay, and response

to the environment. There is a clear need to bring together the results of different

techniques and in particular those deriving from modelling and reduced-scale

measurements in controlled conditions (e.g. CFD and wind tunnel data) and full-

scale measurements in the real atmosphere (e.g. lidar).

 

In a collaboration between DASA Airbus and DERA Malvern, a comparison has

been made of measurements on aircraft wake vortices obtained using two very

different techniques: 1) Five-hole probe measurements on a 1/13.6 (7.35%) scale

half-model of an Airbus A321 were made in the Bremen wind tunnel, and 2)

Coherent laser radar (lidar) measurements were made by DERA in full-scale field

trials at Toulouse Blagnac Airport. The lidar measurements provide line-of-sight

tangent velocities across the vortex pair to over half the peak core velocity,

whereas the wind tunnel measurements cover the full core region, extending

nearly to the mid point. Measurements from the two systems may thus be

compared over the overlap region of the vortex profile. Over this range the

measured tangent velocity profiles show good agreement for vortices of

comparable age. Vortex disturbance is brought about by atmospheric turbulence,

and the effects are evident in the lidar data, leading to significant distortion of

older vortices. This analysis emphasises the complementary character of the

information that may be derived from the two techniques. Full details of the

analysis will be reported in a paper recently submitted for publication. 

 


Figure 1. Lidar measurement of wake vortices. The vertical scale is line-of-sight velocity in ms-1, with positive values indicating motion away from the lidar. The horizontal axis represents time in seconds (with the aircraft overhead approximately at t = 0), which is converted to a distance using the scan information. The laser scan angle is indicated by the zigzag line; three clear intersections with the vortex pair have been achieved.

Figure 2. Wake vortex data obtained by both lidar and wind tunnel. The lidar data are derived from the outline plotted in figure 1. The wind tunnel data have been converted to line-of-sight velocities for direct comparison. The velocities are directly measured in each case, and contain no scaling factor. The distance scale (horizontal axis) has been scaled to optimally line up the vortex cores. The vortex separation obtained from the lidar analysis (Table 1) is consistent with this scaling to within the measurement error.

 

Comparison between wind tunnel and lidar techniques

Wind tunnel

Lidar

Measures full 3-D vector wind field.

 

Measures line-of-sight velocity only.

 

Permits very detailed measurements of early vortex development (<5seconds).

 

Early time data less precise, but evolution to maturity and decay can be observed (>60s) and evaluated.

 

Full control over experimental conditions ensures reproducibility and flexibility.

 

Examines effects of real weather conditions; stable conditions are required for comparison with wind tunnel.

 

Possible errors in scaling to full size.

 

Measures effects from real aircraft.

 

Jet engine effects not usually included. Tunnel wall effects may perturb flow.

 

Engine and undercarriage effects are incorporated.