Aerodynamic measurement of high-

speed base flowsBackgroundThe 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 three-dimensional character of the wake.

Likewise, the measurement of this flow is equally challenging and requires the application of three-dimensional 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.

Progress and ObjectivesThe progress in the first two years of the PhD was an understanding of PIV and its extension to three-dimensional 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.

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.

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.

Base Flow AerodynamicsAs 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.

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.

High-Speed, Three-Dimensional Measurement TechniquesA 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.

PhD Candidate: Kyle LynchDepartment: AerodynamicsSection: AerodynamicsSupervisor: F. ScaranoPromoter: F. ScaranoStart date: 07-07-2011Funding: FLOVIST / AFDAR

Aero

spac

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∇𝑝=−𝜌 D𝐮Dt +𝜇∇2𝐮

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

The connection between flow field information and pressure field information can be derived from the flow governing equations, e.g.,

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.

Example of predictor-corrector used in FTC algorithm.

Velocity field as determined from a PIV experiment on a base flow model.

12-camera array for acceleration measurements in high speed flows.

Camera array performing measurements of the azimuthal structure of the wake in a low-speed test.