Experimental Study on Turbulent Canopy Flow by means of Laser Doppler Anemometry

Velocity investigation is fundamental in order to understand flow characterization. Maps of velocity field allow the determination of the flow structure and its turbulence properties. To obtain velocity measurements a Laser Doppler Anemometer (LDA), or more generally a Laser Doppler Velocimeter (LDV), has been chosen instead of Hot Wire Anemometer (HWA) or Particle Image Velocimeter (PIV), those represent the main alternatives. PIV might be an appropriate choice too but IRPHE laboratories are furnished only with stereo PIV devices.

After several tests executed with two 2-D stereo cameras we understood that this technology demonstrates a certain diculty to penetrate into vegetation, making most of the time spatial averaging not realizable. Laser Anemometry is an optical technique which locally measures instantaneous flow velocity. This technique is well-suited for canopy flow surveys, many previous researches already showed that it has a particular ability to get measurements at the vegetation level and well into the region where tree wakes interactions take place. In fact LDA permits to attain good data even in the recirculating areas where velocity may even assume negative values.
In those zones HWA technology could not be used.
Here are reported several other LDA advantages:

* is non-invasive;
* has linear graduation curve;
* does not need calibration;
* has well-defined directional response;
* has high spatial and temporal resolution;
* permits multi-component bi-directional measurement;
* is not fragile as HWA.

Principal disadvantages are high costs, necessity to obtain optical access and seeding into the measurement field. With this method we are able to perform measurements of the 3-D velocity field:
u streamwise velocity,
v traverse velocity,
w vertical velocity.
Following paragraphs provide a short overview of the theory that lies behind the LDA method.