Research group models spacecraft module in Texas
*Click on slideshow for larger view of photos/captions
This summer, AEM Professor Ellen Longmire, a TSI collaborator, and members of Longmire’s research group spent two weeks in Texas modeling a spacecraft crew module. The goal of the project is to understand wake flow behind a model of a spacecraft crew module and determine how it interacts with a drogue parachute used for deceleration upon re-entry into the atmosphere. The project was sponsored by the NASA Jet Propulsion Laboratory (JPL).
As the module re-enters the atmosphere, eddies intermittently shed off the capsule. It is important that these eddies do not have adverse effects on the parachute opening or stability. NASA hopes to be able to predict the flow numerically in order to accurately model the situation, but their models require validation against experimental data.
Two researchers accompanied Professor Longmire. Erik Haugen, a current graduate student in the AEM Masters program, worked with setting up optics and data processing and analysis. Mitch Ryan, a recent graduate of the AEM Masters program, worked with set up, processing, and results on site. TSI provided support for the project and TSI collaborator Dan Bissell, who completed his AEM Bachelor’s degree at the University of Minnesota in 2010, also accompanied the group to lend expertise with the TSI equipment and software used in the measurement process.
Professor Longmire, Ryan, Bissell, and Haugen made the trip to Texas to experimentally test wake/parachute models in the Oran W. Nicks Wind Tunnel Facility at Texas A&M University. The results will eventually be compared with numerical predictions. The wind tunnel facility allowed for a larger size and higher flow speeds, with a maximum flow speed of 90 m/s and a cross section of approximately 2.1 m by 3 m. In comparison, the large closed return wind tunnel housed in the AEM department at the University of Minnesota has a maximum flow speed of 38 m/s and a cross section of 1 m by 1.25 m.
The Oran W. Nicks Wind Tunnel is propeller-powered and ear protection is required during use. The environment was also challenging because the tunnel was not air conditioned, and the temperature in the test section rose steadily each day beyond 120 degrees F. This was beyond the outdoor temperatures that reached about 105 degrees F.
A one-tenth scale model of the spacecraft crew module was tested in the tunnel and various data sets were collected by engineers from NASA Johnson, NASA Ames, JPL, and Airborne Systems: force measurements for lift and drag, high speed video of parachute motion, and infrared temperature visualization of laminar/turbulent transition on the model.
“The purpose of this project is to investigate the relation between the parachute dynamics and the forces on the crew module,” Haugen said. “For example, the stability of the parachute may be affected by its interaction with the module wake. This could potentially cause a failure of the parachute to inflate, parachute collapse, or unsteady motion.”
The University of Minnesota team specifically worked on obtaining velocity field measurements in the wake of the model using stereo particle image velocimetry (PIV). For this method, smoke is spread throughout the tunnel to track the fluid motion in the wake. Laser sheets are pulsed in the wake zone and their light is scattered by the smoke particles. The particle images are captured by two high resolution CCD (charge-coupled device) cameras and the images are used to determine variations in the local particle velocity and therefore the local fluid velocity. Data sets included 1400 dual-image captures, with no special computer equipment needed. TSI’s PIV software, Insight4G, was used for data acquisition and processing.
The flow was examined downstream of the model with and without a parachute and at multiple angles of attack.
The researchers also had a lot of down time between experiments. No data was collected the first couple of days as equipment setup was underway both by the UMN/TSI collaboration team and the wind tunnel staff. The UMN researchers and the other research teams were typically collecting data simultaneously during the wind tunnel experiments.
The first week held a few challenges, as well. The UMN researchers were trying to capture data immediately upstream of the parachute, but they found that the parachute rotated steadily and also moved around quite a bit. Thus, the cords were in constant motion and frequently scattered intense amounts of light into the cameras. Since this scattering could damage the camera CCD arrays, it was decided that the parachute needed to be constrained. This worked fairly well, although a few parachutes were ruined during the process. Parts of each model parachute were hand-sewn, so it took some time to replace each one.
Ryan said that this experiment went at a faster pace than many he’d worked on at the University of Minnesota.
“There were more people working on it, it was quicker to bounce ideas around,” Ryan said. “If everyone’s right there, they can all understand the problem and work together to fix it.”
As is true with many experiments, however, only a fraction of the time is spent setting up and taking data. Most of the time goes into analyzing the data. Further measurements were also obtained using the closed return wind tunnel at the University of Minnesota using a one-fifteenth scale model.
“Currently we are processing the captured images to ensure the velocity fields are accurately representing the physical flow,” Haugen said. “When that's finished, the data will be sent to NASA to be compared with CFD (computational fluid dynamics) simulations."
Overall, the researchers found the experience very rewarding.
“Having the opportunity to further the fluid mechanics side of space travel is really an honor,” Bissell said. “It was a fantastic privilege to work with Professor Longmire.”
Erik Haugen is researching the flow field behind a model space capsule (similar to those used in the Apollo missions) that would carry astronauts through atmospheric deceleration. NASA, who initially proposed this study, aims to quantify the deceleration of the capsule by investigating the interaction between capsule dynamics and those of a trailing drogue parachute. Stereo PIV (particle image velocimetry) measurements are acquired in a wind tunnel to obtain planes of data with three velocity components. With this data he tries to understand what is happening in the capsule wake as well as the flow around the porous parachute by looking at time averaged and instantaneous velocity fields, RMS velocity, and vortical structures, for example. The geometry of the capsule and parachute and the capsule wake/parachute interaction make the flow field highly complex and three-dimensional.
Figure: Flow field just downstream of the drogue parachute. The parachute pores act to break up vorticity into smaller structures. Streamwise (U) and vertical (V) velocity components are shown as vectors and the normalized vorticity as contours. Spatial domain is normalized by the capsule diameter (d = 0.5 m).