Planar Doppler velocimetry (PDV) is an optical,
laser-based flow-field measurement technique that is
being developed for the large wind tunnels at Ames
Research Center. The PDV technique measures the
Doppler shift of light that is scattered from aerosol
particles that are added to the flow and illuminated
by a pulsed laser sheet. Images of the planar measurement
region are acquired with scientific-grade
digital cameras and then processed to yield instantaneous,
three-dimensional (3-D) velocity maps, which
are limited in size only by the field of view of the
cameras and any requirement to meet a particular
spatial resolution.
The PDV concept is based on the sensitive
frequency discrimination that occurs when the
scattered laser light is viewed through an optical filter
cell containing iodine vapor. Particles moving with
the flow pass through the laser sheet and scatter light
that is shifted in frequency according to the Doppler
effect. Some of this light is collected to form an image
that records intensity variations not caused by the
Doppler effect. These variations are caused primarily
by nonuniform aerosol distribution and nonuniform
illumination. Simultaneously, some of the light is
collected and imaged through the optical filter cell
containing the iodine vapor. This cell acts as a sharp
spectral filter at the laser frequency. Because the
Doppler-shifted laser light has a narrower bandwidth
than the spectral range of the iodine absorption
feature edge, Doppler shifts of the collected light are
converted to variations in recorded image intensity as
the frequency moves on the filter edge. In its full
implementation, pairs of filtered and unfiltered
images are simultaneously acquired during each laser
pulse by using three camera systems that view the
laser sheet from three different locations. The filtered
images are normalized by the unfiltered images to
eliminate the variations in intensity that are not
related to the Doppler effect. These normalized
images are processed and become quantitative
velocity map images of three separate, instantaneous
velocity-field components. An algebraic transformation
yields the complete 3-D orthogonal velocity
vector field.
At Ames Research Center, the PDV technique,
along with its related optical hardware and data
analysis software, has been shown in the laboratory
to be capable of resolving instantaneous velocities as
low as 2 meters per second from a single pulse and to
be applicable at ranges exceeding 40 meters. These
capabilities, in addition to the minimal requirements
of PDV on the optical and density properties of the
aerosols that must be added to the flow, make it
particularly attractive as a means of measuring 3-D
velocity vector fields in time-dependent flows in
large-scale facilities where full-scale rotor tests are
conducted and velocity vector-field measurements
between turning rotor blades are of interest.
The PDV system was first used in the wind tunnel
environment during the XV-15 rotor test in the Ames
80- by 120-Foot Wind Tunnel. That test was conducted
to determine fundamental differences in blade
vortex interaction (BVI) noise for this tilt rotor in
approach flight configuration. The first figure shows a
diagram of the test section and measurement geometry.
The three camera locations are shown along
with the respective measured velocity vector directions.
The camera field of view is indicated by the
square region in the laser sheet. Measurements were
acquired at various stations behind the rotor blade
using a variable time delay to synchronize the laser
pulses and camera shutters with the rotor azimuth.
The second figure shows the computed, single-pulse
3-D orthogonal velocity vector field. The view is
normal to the measurement plane; in-plane velocities
are shown as vectors, and out-of-plane velocities are
shown as color contours. Also shown is the position
of the rotor blade, which is 22.6 degrees upstream of
the measurement plane in this case. Free-stream
velocity, rotor-shaft angle, and advance ratio simulate
the tilt-rotor approach configuration.
Point of Contact: M. Reinath
(650) 604-6680
mreinath@mail.arc.nasa.gov
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