Aerodynamic measurement
of high-speed base flows
Background
The separated wake at the base of cylindrical bodies
presents a number of challenges to aerodynamic designers
dealing with similar geometries, such as in missiles or
rocket launchers, due to the unsteady and threedimensional character of the wake.
Likewise, the measurement of this flow is equally
challenging and requires the application of threedimensional measurement techniques capable of operating
at the high speeds characteristic of these problems.
This topic therefore concentrates on two parallel problems:
first, how to characterize the unsteady aerodynamic
phenomena occurring in the flow. Once a series of metrics
are defined, the second task is how to extend current
measurement techniques to enable measurements in this
regime.
PhD Candidate: Kyle Lynch
Department: Aerodynamics
Section: Aerodynamics
Supervisor: F. Scarano
Promoter: F. Scarano
Start date: 07-07-2011
Funding: FLOVIST / AFDAR
High-Speed,
Three-Dimensional
Measurement
Techniques
A primary complication of the unsteady aerodynamic
phenomena is the unsteady loading occurring in the base
and reattachment region. This suggests that linking the
flow phenomena to the pressure along the body can
provide insight on the flow structures or modes responsible
for undesirable effects. One method for measuring the flow
field is through particle image velocimetry (PIV), which
measures the motion of particle tracers suspended in air
and illuminated at multiple times. The particle images are
captured in cameras and analyzed to yield dense,
instantaneous information on the flow field, as shown
below.
12-camera array for acceleration measurements in high
speed flows.
Progress and Objectives
Aerospace Engineering
The progress in the first two years of the PhD was an
understanding of PIV and its extension to threedimensional flows: tomographic PIV. As part of this
process, the concept of fluid trajectory correlation (FTC)
evolved and became a major focus of my investigations as
a means to increase the accuracy of measurements
performed using tomographic PIV.
Velocity field as determined from a PIV experiment on
a base flow model.
The connection between flow field information and
pressure field information can be derived from the flow
governing equations, e.g.,
=−
Ariane 5 launcher with separated flow region
highlighted and shown in additional detail. From Deck
et al. Unsteadiness of an axisymmetric separating-reattaching
flow: Numerical investigation
Base Flow Aerodynamics
As flow encounters a base geometry, it rapidly separates to
form a mixing layer separating the free stream flow from
an inner recirculation region. Proceeding downstream, the
mixing layer grows and approaches the wall. The mixing
layer proceeds to impinge on the wall, leading to
reattachment.
D
+
Dt
Which relates the material acceleration to the pressure
gradient field. The pressure gradient field can then be
integrated to yield the pressure. This equation requires the
acceleration to be measured, which is not typically
provided in a PIV experiment. Thus, we have extended
traditional PIV by acquiring four images of data, and
developing unique algorithms to determine the acceleration
of the fluid throughout the images. This specialized
algorithm, Fluid Trajectory Correlation (FTC), predicts the
position of particles in time, and corrects the prediction
such that it tracks more accurately the true fluid trajectory.,
as shown below. By then parameterizing this trajectory into
polynomial functions, the acceleration can easily be
determined through differentiation.
A simplified depiction of this is provided in the above
figure; in reality, the impingement of the mixing layer on
structures in the afterbody is highly unsteady, leading to
high stress and the possibility of fatigue for mechanical
devices in this region. Besides the mixing layer, the
recirculation bubble also exhibits unsteady dynamics which
contribute to loading imbalances at the base, requiring
additional control authority to maintain stability. Ultimately,
the understanding of these aerodynamic phenomena can
allow control schemes to be implemented to improve
launcher performance.
With the principles of FTC established, the current year has
focused on the implementation of a tomographic PIV
system suitable for high speed flows. The approach
adopted by our group involves to use of a large array of
inexpensive cameras which provides a unique ability to
acquire four images at a speed which is suitable for high
speed flows, and additionally provides enough views of the
base of the model to describe the full three-dimensional
motion.
The upcoming year is focusing on the application of this
system to low speed experiments and high speed
experiments to examine the underlying fluid dynamics of
the base flow problem. This will culminate in a series of
publications taking advantage of the unique measurement
capabilities available with such a camera array and with
the uniquely developed algorithms. It should be stressed
that the techniques developed in this work are also
applicable to many other high-speed measurement
scenarios and may find use in additional laboratories in the
future.
Camera array performing measurements of the
azimuthal structure of the wake in a low-speed test.
Example of predictor-corrector used in FTC algorithm.
Publications
- K. Lynch, F. Scarano “Enhancing the velocity dynamic range and accuracy of time-resolved PIV through fluid trajectory correlation.” 16th Int Symp on Applications of Laser Techniques to
Fluid Mechanics. Lisbon, Portugal, 09-12 July, 2012
- K. Lynch, F. Scarano “A high-order time-accurate interrogation method for time-resolved PIV,” Meas Sci. Technol 24:035305.