Gas Dynamics Laboratory
|Faculty Researchers||Lab Location||Phone||Website|
|312 Aerodynamics Research Laboratory||217-265-9211|
The gas dynamics laboratory is a state of the art facility used for the experimental study of complex problems in compressible fluid dynamics. All wind tunnels are blowdown type, with wind speeds ranging from subsonic to Mach 4. A few wind tunnels are of a more general design and can be used for the study of numerous flow fields while others are more specialized for the study of specific flows including axisymmetric and planar base flows, planar mixing layers, and transverse jet injection into supersonic crossflow. Additionally, an open exhaust supersonic jet facility is housed within an anechoic chamber for the study of jet noise. Research performed in this lab includes studies of fundamental compressible flow fields utilizing non-intrusive optical diagnostic techniques such as tomographic and stereoscopic PIV and high-speed Schlieren photography. Studies of the modulation of these flow fields using both active flow control, such as plasma discharge, and passive flow control methods are also a primary research objective. Specifically, the most recent work in this laboratory includes tomographic PIV measurements of a Mach 2.5 axisymmetric base flow, stereoscopic PIV measurements of a compressible planar mixing layer, plasma actuation control of a Mach 2.5 axisymmetric base flow, and the fluid-structure interaction of an underexpanded jet flowing across a compliant surface. Research in this facility has received funding from numerous sources, including the Army Research Office, the Office of Naval Research, the Air Force Office of Scientific Research, as well as corporate sponsors including Rolls-Royce and Gulfstream.
Facility and Equipment Description
High Pressure and High Volume Compressed Air Systems
Two separate compressed air systems to drive supersonic blowdown wind tunnels are available. The first is a low-pressure high volume (150 psia, 132 m3) air storage system to drive relatively lower speed supersonic wind tunnels (Mach ≤ 2.5). There is also a high-pressure low volume (2000 psia, 7.8 m3) air storage facility capable of supplying air for the higher speed wind tunnels (Mach 3+). Both air storage systems allow for several minutes of continuous wind tunnel operation with minimal recharge time. Moisture from the compressed air is removed by a Zander twin tower air drier to minimize the effects of condensation in experiments.
Supersonic Wind Tunnels
The supersonic wind tunnels allow for the study of many complex fluid dynamics problems at high Reynolds numbers. The axisymmetric base flow wind tunnel (shown below) simulates the near wake of a blunt-faced, 2.5 in. diameter, cylindrical projectile in a Mach 2.5 flight. A unique 4 window test section design allows for adequate optical access for advanced flow diagnostic techniques to be employed, such as tomographic PIV. The shear layer supersonic wind tunnel (shown below) produces a planar, compressible, free shear layer formed by two separate air streams of different speeds. The nozzles of this tunnel are easily interchangeable and can produce a range of convective Mach numbers allowing for the study of the effect of compressibility on the development of free shear layers. The Mach 4 and the 5x5 supersonic tunnel (named for the size of the test section, 5” x 5”, capable of Mach 1.4 to Mach 3) wind tunnels are of a more general design and can be used for a wide variety of applications.
Axisymmetric Jet Anechoic Chamber Facility
An axisymmetric, open exhaust, supersonic jet facility is housed within a 2.1 m x 2.3 m x 2.5 m anechoic chamber. Although it is of relatively small size the chamber has been shown to exhibit anechoic characteristics down to a cut-off frequency of 400 Hz. The facility was originally constructed for the study of passive jet noise mitigation techniques. More recently, the facility was used to study the fluid-structure interaction of an underexpanded jet flowing adjacent to a compliant surface.
Advanced Flow Diagnostics
Advanced, non-intrusive, optical diagnostic techniques are utilized in this laboratory for the study of complex fluids problems. Velocity field measurements are acquired using planar, stereoscopic, and tomographic PIV techniques (shown below). High resolution cameras (LaVision Imager sCMOS and PCO 2000 CCD) allow for large field velocity measurements with high vector resolution (typically sub millimeter vector spacing). Vector field processing for PIV measurements is performed using LaVision’s DaVis 8.4 software. Several double-pulsed Nd:YAG lasers of varying output pulse energy (up to 400 mJ/pulse at 532 nm wavelength) are available depending on experimental requirements. High speed (i.e., time-resolved) diagnostic techniques include Schlieren photography using a Photron SA-5 (up to 1MHz frame rate), pressure sensitive paint, and high-speed pressure measurements using Kulite transducers.
Tomographic PIV of Compressible Base Flow
The video depicts a series of non-time-correlated tomographic PIV measurements in the wake of an axisymmetric compressible base flow downstream of reattachment. The green surfaces depict a constant valued low-speed velocity contour and the red surfaces depict vortical structures educed by the Q-criterion. Hairpin-shaped vortical structures, like those observed in turbulent boundary layers, are commonly identified throughout this region of the flow field. Prior to these tomographic PIV experiments, the existence of these complex vortical structures in this flow field could only be speculated. (Student: Braden Kirchner, Support: ARO)
Compressible Shear Layer
Two dimensional compressible shear layers represent a canonical flow field to study the effects of compressibility on turbulence and are fundamental to many high-speed aerospace applications. The two dimensional compressible mixing layer is formed as two freestreams separated by a thin splitter plate come together. For the compressible case one of the Mach numbers of the free streams is supersonic or in the high subsonic compressible regime. Although this flow has been study in the past, the goal of the current research is to utilize state-of-the-art laser diagnostics to quantify the effect of compressibility on the turbulent structure and growth of the mixing layer. Utilizing a new supersonic missing layer tunnel the turbulence of the flow field has been measured using a variety of optical and laser based techniques to create a validation quality data set. For example, below are instantaneous stereo-PIV image of the velocity field at convective Mach numbers of 0.19 and 0.53. The images of the velocity field show the larger turbulent structures and higher growth rate at the lower convective Mach number in high detail. Other convective velocities are currently being investigated as this data base is built up and more advanced analysis techniques are being employed to better characterize the compressibility effects. (Student: Kevin Kim, Support: NASA)
Plasma Flow Control of Compressible Base Flow
There are a variety of passive and active actuators that have been proposed to control different aspects of flow phenomena such as flow separation, mixing, and turbulence. One class of active flow control utilizes plasmas due to their ability to place a large amount of energy localized into the flow at a high repetition rate. The image below shows a the difference in the radial velocity with and without forcing using 8 plasma actuators operated at 40 kHz in a helical mode for a supersonic (Mach 2.49) axisymmetric base flow. Observed is the alternating pattern in the shear layers demonstrating the effectiveness of the flow control. In order to determine the flow field resulting from a single plasma actuator, micro-Particle Image Velocimetry was utilized to obtain the movie of the plasma actuator process. Shown below is a move of the resulting phase averaged velocity field from a pulse plasma jet during the activation process. Observed are the propagating toroidal vortexes that are formed from the high speed pulsed jet and at the end the cavity refilling process is clearly seen. (Student: Todd Reedy, Support: ARO)
Axisymmetric Underexpanded Jet Fluid/Structure Interaction
Fluid structure interactions combine aspects of two fields, solid structures and fluid dynamics, into a complex coupled problem. A canonical geometry that can be used to study these interactions is an exisymmetric jet exhausted parallel to a compliant surface. The images below show the streamwise velocity measured using particle image velocimetry and the simultaneous surface deflection measured using a stereo digital image correlation correlation technique. Observed is the deflection of the plate upward near the jet exit due to the lower pressure expansion followed by the shock/boundary layer interaction that results in a small recirculation region and deflection of the plate downward. For the rigid plate tomographic PIV was utilized to take the full 3D velocity field which is shown below to map out the complex flow field of the fluid structure interaction. (Student: Ruben Hortensius, Support: ARO)