Experimental Investigation of the Periodic Unsteady Transonic Flow Field Around a Compressor Blade by Means of Particle Image Velocimetry (PIV)
Authors: Lehr A. , Bölcs A. 
Published: March 2007

In the present article, a study of the time‐dependent transonic
flow field around an isolated airfoil in a Laval nozzle
is presented. In order to obtain instantaneous flow field information
all over the flow around the blade, the Particle
Image Velocimetry (PIV) method has been employed allowing
one to obtain velocity data in an illuminated light sheet
with a high data acquisition rate and high spatial resolution.
From these instantaneous PIV measurements the mean velocity
field and turbulence quantities of the flow easily can
be obtained by statistical means.

A single compressor blade with a NACA 3506 profile and
a chord length of 78 mm is centrally mounted in the test
section. A vibration system driven by a hydraulic motor is
externally attached to the blade. The rotational motion of
the motor is translated to a plunging blade vibration perpendicular
to the chord via an eccentric shaft and connecting
arm. The vibration frequency can be varied between 0 and
100 Hz, and the average amplitude is approximately 0.4 mm.
The entire blade‐motor assembly is mounted to the tunnel
sidewall via a circular disc to allow easy adjustment of the
inlet flow incidence angle. The exit pressure of the nozzle can
be varied by a rotating flat plate (15 mm high, 39 mm wide,
and 2 mm thick) that is mounted approximately 5 times the
chord length downstream of the blade mid‐chord. The rotational frequency of this assembly can be varied between 0 and
100 Hz, producing upstream‐running waves at the doubled
frequency (i.e., 0 to 200 Hz). The above excitation systems can
be precisely synchronized and the phase lag between them
can be freely varied. This allows for unsteady measurements
to be conducted in the presence of only the downstream perturbation,
only the blade vibration, or a combination of the
two for different phase angles.

For the first test series the existing PIV measurement system
at Turbomachinery Institute (LTT) has been applied to
measure the transonic steady flow field around the compressor
blade. In another test series, measurements of the time‐dependent
periodic flow field were conducted by means of
PIV. This measurement series consisted of three subtasks:
the measurement of the unsteady flow field with only back
pressure variation, with only blade vibration, and finally
with a combination of both back pressure variation and
blade vibration. These results quantify not only the unsteady
motion of the normal shock for each case on the suction side
of the blade, but also give a detailed insight into the instantaneous
transonic flow field around the compressor blade.
Furthermore, the results of the set of unsteady measurements
with both back pressure variation and blade vibration
(3) is compared with the combination of the results of the sets
of unsteady measurements of only back pressure variation
(1) and only blade vibration (2) to validate the superposition
principle.

Finally, phase averaging of the instantaneous flowquantities
produced a large database for statistical treatment (e.g.,
turbulence) and thus the ability to compare the averaged results
with those from traditional measurement techniques.