1.5µm lidar for helicopter blade tip vortex detection

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2 1.5µm lidar for helicopter blade tip vortex detection June 24 th 2009 A. Dolfi, B. Augere, J. Bailly, C. Besson, D.Goular, D. Fleury, M. Valla

3 objective of the study Context : European project AIM (Advanced In-flight Measurement technique) Objective : to assess the capability of on board lidar technique to investigate helicopter tip vortices behavior Works Design of the 1.5µm lidar for helicopter flight tests : Simulation of lidar performance using velocity flow field from aerodynamic simulation Ground based 1.5µm lidar tests on helicopter in hover flight : adaptation of a lidar dedicated to tip vortex detection - real rotor flow velocity measurements on DLR hovering helicopter 0.2 Output of Work : Specification of a future 1.5µm lidar anemometer for airborne in-flight research and industrial tests Lidar 3 CLRC June 09

4 Challenge and requirements Challenge Characterization of small phenomenal at short range: vortex core diameter = 10 mm lidar instrumental design delicate Blade tip vortex convicted and disturbed by helicopter rotor inflow Very strong wind dispersion: 80 m.s -1 of wind dynamics!!! Requirements Lidar measurement simulation End to End to optimize lidar system Real time signal acquisition and visualization Height (m) Module vitesse dans le plan Dimension en m Range (m) Dimension en m 5 Scanning angle -4 LIDAR 4 CLRC June 09

5 1.5µm lidar design 1.5µm Lidar definition Several units to simplify installation and adjustment Fiber link to off-set sensor head from the other units 10m range to avoid helicopter flow disturbances Availability of seeding particle: allow the use of classical laser power (2W) Field of view dependant on vortex size, lidar sensitivity and helicopter stabilization Seeding particles Seeding particles Θ 3 Lidar:Sensor Head Scanner Objectif Fiber link Lidar: Signal Processing Lidar: Laser+fibered architecture 5 CLRC June 09

6 Lidar description Lidar characteristics and performance Range:10m Angular resolution: 0.03 (5mm at 10 m) Scan angle: 3.8 (66 cm at 10 m) Velocity dynamics: 40m/s Axial resolution: 300mm Velocity accuracy: 1m/s Optical fibered architecture 6 CLRC June 09

7 Lidar installation in DLR airport Tests Configuration Helicopter in hover flight : bound to the ground Smoke seeding produced by flares 7 CLRC June 09

8 Ground based lidar tests 1.5µm Lidar tests Various measurement configurations: Different vortex ages: observation planes 10, 40 and 70 behind the trailing edge of the blade 3.8 of lidar scan angle Great difficulty to implement smoke seeding device : Use of DLR air compressor to inject tracer particles in the vortex flow. Even so, seeding density not high and not uniform in the region of interest Consequences of not efficient smoke seeding: Lidar data mainly without helicopter rotor thrust Lidar signal not constant (burst signal), no adjustment of lidar field of view in real time, so no optimization of vortex capture 8 CLRC June 09

9 lidar tests results Rotor flow velocities measured by 1.5µm lidar without rotor thrust (scan angle :3.8 ) Velocity (m/s) 10 observation planes 40 observation planes 70 observation planes vmax= -25m/s to 15m/s;rc=4cm vmax= -15m/s to 8m/s;rc=6cm vmax= -12m/s to 4m/s;rc=8cm Blade tip vortex Rotor inflow Scan angle ( ) Power spectral density 9 CLRC June 09

10 lidar tests results Real rotor flow velocities measured by 1.5µm lidar with rotor trust 10 observation planes, 3.8 scanning angle Inhomogeneous seeding and no seeding in the vortex core No the whole vortex but information on slope of speed Velocity (m/s) Power spectral density Scan angle ( ) 10 CLRC June 09

11 lidar measurement analysis Interpretation of wind field measured by 1.5µm lidar Vortex model to translate lidar information (projection and integration of radial speeds) in 2D vortex field distribution Hallock-Burnham vortex model : Characteristics parameters : Circulation Γ0 : Maximal velocity Vθmax: v θ Γ0 ( r) = 2 π. r r 2 r r c r v(m/s) v θ max r(m) = Γ 4π 0. r c Lidar data lidar model 11 CLRC June 09

12 lidar measurement analysis Overview of lidar data analysis lidar data fit with Hallock-Burnham model (with and without thrust) Γo, vmax of the main vortex Modeled lidar image using PIV data input (with thrust) Hallock-Burnham model fit Γo, vmax of the main vortex Comparison with lidar data Modeled lidar image using aerodynamics CFD simulations input (with thrust and without thrust) Comparison with lidar data fit 12 CLRC June 09

13 lidar measurement analysis Interpretation of wind field measured by 1.5µm lidar: lidar data fit with Hallock-Burnham model (without thrust) Lidar wind field data fit to radial speed distribution of vortex (HB model) with the following parameters: Circulation Γ0 : 2.8 m 2 /s Maximal velocity Vθmax: 29 m/s 13 CLRC June 09

14 lidar measurement analysis Comparison between lidar and PIV measurements: Modeled lidar image using PIV data input (with thrust) PIV field test conditions: velocity field measured on Bo 105 for 2T thrust at a vortex age of 3 behind the blade and for 1 of field of view Fit of PIV wind field by vortex model: Γ0= 6 m 2 /s ; Vθmax=39m/s Fit of lidar data by vortex model: Γ0= 6 m 2 /s ; Vθmax=39m/s PIV and lidar data are consistant 14 CLRC June 09

15 lidar measurement analysis Comparison between lidar measurements and CFD vortex calculations : Modeled lidar image using aerodynamics CFD simulations input (with thrust) Simulation of lidar measurement using vortex aerodynamic calculations : velocity field measured on Bo 105 at a vortex age of 10 below, for 3.8 of field of view with rotor thrust Fit of vortex aerodynamic calculations by lidar vortex model: calculations resolution is not enough accurate to see vortex core ; calculations on a finer grid will get a swirl more concentrated with stronger vorticity, stronger speeds and weaker size. 15 CLRC June 09

16 lidar summary Conclusion on lidar technique for helicopter vortex detection Lidar field tests have provided blade tip vortex images. Sensor characteristics (Field of view, spatial and spectral resolutions) were well adapted to provide relevant data. The main component of the vortex field distribution has been characterized using the Hallock-Burnham model (circulation, maximal speed). Lidar results were consistent with PIV results. The velocity measurement is direct and absolute (not calibration need) and its accuracy can be up to 0.25 m/s and commonly 1m/s A compact and reliable onboard lidar sensor with components off the self has been specified. An architecture close to AIM field tests lidar would be suitable for in flight measurements. However, it would require seeding (clouds). 16 CLRC June 09