The Mach 4 Ludwieg Tube at UTSI is housed in the TALon facility, and was activated in March 2018. This type of wind tunnel is a short-duration facility (i.e. impulse facility) that relies on a finite volume of pressurized gas exhausting through a nozzle into a vacuum reservoir once a diaphragm separating the sections (pressurized and vacuum) is ruptured. The Mach 4 tunnel has a diaphragm installed upstream of the nozzle and test section in order to reduce the number of components requiring pressure ratings. A 105’ long, 24” OD driver tube comprises the pressurized section which is mounted on large frames with rollers. The driver tube can be pressurized to +150 psig. At the end of the driver tube, a 20” long metallic expansion joint is designed to absorb some of the impact loads and to support the diaphragm. Hydraulic actuators are attached to the expansion joint such that the diaphragms can be installed/removed. Immediately downstream of the diaphragm is a 3’ long, 36” OD pipe that allows the flow to transition from the circular driver tube into the two-dimensional de Laval nozzle.
The 8’ long nozzle expands the flow to Mach 4 in the test section through a constant-width of 24”. The test section is 6’ long and has a square cross-section of 24” × 24”. These components make this facility one of the largest of its type in academia. Downstream of the test section is a 36” OD pipe that acts as a diffuser for the flow. A model support sting is in development for installation in this pipe. The 10’ long diffuser is connected to a 14.5” long, 36” OD metallic expansion joint that is also designed to absorb some of the impact loads. The expansion joint connects the diffuser to the vacuum receiver tank which is 20’ long and 8’ in diameter. The total internal volume of the vacuum tank is 8,200 gal. It is pressure rated to -15 psig and +40 psig, and the saddles supporting it are designed to withstand 24,000 lbf of thrust loading along the central axis of the tank. The facility’s typical run conditions will use +5 psig in the driver tube and a vacuum of about 1 Torr in the downstream components. The maximum capabilities of the Ludwieg Tube are a +150 psig pressurized driver tube and a vacuum of 0.1 Torr. With total temperatures equal to the ambient room temperature of approximately 295 K, the model Reynolds numbers for the UTSI Mach 4 Ludwieg Tube can range from 1–16 million / ft.
Due to the nature of the Ludwieg Tube design, overall run durations are typically very short (approximately 2-3 seconds). Additionally, the stagnation conditions in the test section follow a stair-step decrease as the pressures in the entire facility are equalizing. The time between these condition changes is proportional to the length of the driver tube and the speed of sound in that section. Commissioning studies have determined that the facility has steady-state run times of about 125 ms. A diffuser is currently being designed to enable increased testing conditions.