List of Tables
Chapter 2 Facility
2.1 Description and Test Procedure
The facility used for all experiments in the present study is the T5 Hypervelocity Reflected Shock Tunnel located at the Graduate Aerospace Laboratories of the Cali- fornia Institute of Technology; see Hornung (1992) and Hornung and Belanger (1990) for details on the design and operation of this facility, a rendering of which is presented in Figure 2.1. For a good explanation of the basic principles underlying piston-driven reflected shock tunnels, see Section 16.2 in Tropea et al. (2007).
2.1.1 Overview
Figure 2.2 is a labeled schematic diagram of the relevant T5 components. Prior to each experiment, atmospheric air is compressed up to a maximum of 4500 psi into holding tanks outside the laboratory. A stainless steel diaphragm, scored to an empirically determined depth to burst at a given pressure differential, is inserted in the primary diaphragm position between the compression tube and the shock tube. A thin Mylar diaphragm is placed in the secondary diaphragm position between the shock tube and the nozzle. A 120 kg aluminum piston is loaded into the piston space, which is at the end of the compression tube just downstream of the junction with the secondary reservoir. All sections of the tunnel are evacuated with vacuum pumps.
Figure 2.1: Rendering of the T5 Hypervelocity Shock Tunnel. (Based on drawings prepared by Bahram Valiferdowsi.)
Shock Tube
Secondary Diaphragm
Nozzle Test Section
Model Compression
Tube
Primary Diaphragm Piston
2R
Figure 2.2: Simplified schematic diagram of the T5 Hypervelocity Shock Tunnel. The section labeled “2R” is the secondary reservoir. See also Figure 2.5.
At the beginning of the experiment, the shock tube is filled with the desired test gas; in the present experiments, this is always N2, air, CO2, or a mixture of CO2 and air premixed in the tank described and pictured in Section 2.1.2. The compression tube is filled sequentially with a mixture of He and Ar, and the secondary reservoir is filled with pressurized atmospheric air from the external holding tanks.
The piston is released and propelled down the compression tube by the compressed air in the secondary reservoir, in turn adiabatically compressing and heating the He and Ar mixture behind the primary diaphragm. When the burst pressure of the
primary diaphragm is reached, it fails quasi-instantaneously and a strong shock wave is created at the contact surface between high-pressure driver gas and lower-pressure test gas in the shock tube. This shock wave propagates through the shock tube, accelerating, compressing, and heating the test gas, and is reflected off the end wall, simultaneously vaporizing the Mylar secondary diaphragm. The reflected shock wave propagates back through the already-shocked gas, further compressing and heating it, and also bringing it to rest. This stagnant, high-temperature, high-pressure slug of test gas serves as the reservoir for a contoured 100:1 area ratio converging-diverging nozzle, which accelerates the test gas to hypervelocity before it flows over the model.
Chapter 3 provides a further description of the test article and its instrumentation, and Section 2.5 presents analysis of the uncertainty in flow conditions at the end of the nozzle and over the test article.
2.1.2 Gas Premixing Tank
To ensure complete mixture in the shock tube for air and CO2 mixture experiments, a mixing tank was constructed from an internally cleaned and wire-brush polished former combustion vessel of approximately 400 L volume. The mixing tank has connections for two standard gas bottles and is attached to the T5 shock tube fill manifold. The tank is pictured in its position next to the shock tube in Figure 2.3.
The vessel is rated to 612 psi or 4.22 MPa. However, the maximum pressure rating for the M-30 compressed air filter, which is attached to the tank, is 125 psi or 862 kPa, so the system should not in any case be operated at a higher pressure. This filter is located in between the tank and the shock tube manifold, and is designed to remove particulates, moisture, and oil aerosol contaminants down to 0.01μm from the mixed gas. The pressure gauge currently attached to the tank has a full-scale value of 100 psig or 690 kPa. The highest fill pressure used for the mixing tank in any of the present work was 45 psia or 207 kPa. As the volume of the shock tube is about
76 L, and fill pressures for most cases are rarely higher than 100 kPa, this quantity of premixed gas is usually sufficient for at least five experiments.
Figure 2.3: Gas premixing tank.
In the present series of tests, the mixing tank was filled sequentially to the desired partial pressure of each gas several hours prior to any experiments. To ensure complete mixing and a uniform distribution of gas species, two 120 mm 12 VDC brushless computer fans were installed in the mixing tank and are run continuously prior to the experiment. These fans, pictured in Figure 2.4, are wired in parallel to an external transformer through a switch located on the mixing tank control panel.
For safety, a CO2 alarm that triggers at 5000 ppm, meeting OSHA specifications, is positioned near the tank.
Figure 2.4: Gas premixing tank internal fans.
2.1.3 Shock Tube Cleaning Procedure
At the conclusion of each experiment, care must be taken to thoroughly clean the shock tube, nozzle, and model, each of which is to a varying degree coated with soot and other small particles carried in the driver gas. This cleaning step is especially critical for work on laminar to turbulent transition, as particulates in the freestream can destabilize the boundary layer and can lead to early instability (Fedorov and Koslov, 2011, Fedorov, 2013), including intermittent broad-band density disturbances as described in Parziale (2013) and transition to turbulence.
Prior to shot 2703, the standard T5 cleaning procedure consisted of pulling a bundle of clean white towels through the length of the shock tube once or twice, and propelling a bucket covered in towels down the length of the compression tube test section. This procedure was eventually found to be insufficient for performing repeat- able experiments, in that inconsistent stability and transition results were sometimes obtained, and clouds consisting of dark particulate contamination were sometimes observed in schlieren movies during the test time. Both of these results were more likely in the next experiment performed after a shot with CO2 present in the shock tube. A variety of more ambitious cleaning procedures, involving solvents, abrasives, power tools, and multiple passes, were tested in an attempt to reduce particulate contamination. We observed that soot-like dust was present primarily in the shock
tube, and secondarily in the compression tube near its junction with the shock tube.
We also determined that the most important segment of the tunnel to clean was the end of the shock tube immediately before the nozzle, where the “slug” of reservoir gas resides.
The cleaning procedure stabilized by shot 2760, and consists of the following steps:
first, the final 2 m of the compression tube is dry polished with a wire wheel mounted on an electric drill, prior to propelling the towel-covered bucket through the length of the compression tube. Next, using an appropriate ventilator, gloves, and goggles, the final 2.1 m of the shock tube is polished with a wire wheel around which is wrapped a fresh 3M ScotchBrite Ultra-Fine Hand Pad (#7448) moistened with acetone, with additional acetone sprayed directly into the end of the tube. Over several polishing cycles, black, soot-laden solvent flows out of the end of the shock tube and is collected in a towel placed under the mouth. Next, a fresh towel is wrapped around a mop mounted at the end of a length of 1 in diameter aluminum conduit pipe, with a total pole length of 4 m. This towel is sprayed with acetone and, while twisting the pole, pushed into and then pulled out of the final 4 m of the shock tube. This is repeated at least eight times, inverting each towel once and then replacing it so that a clean surface is always exposed, until the towels return clean. Next, a bundle of several towels is rolled, sprayed with acetone, and pulled with a rope down the length of the shock tube from the nozzle end to the primary diaphragm end (so that any debris is drawn further away from the already cleaned test gas stagnation region). This process is repeated with fresh towel surfaces until the towels come through the tunnel clean, which usually takes at least 20 cycles. Finally, clean towels sprayed with denatured ethyl alcohol are pulled through the shock tube twice in the same manner in an effort to remove any acetone residue, and a single, balled dry towel is pulled through with a shop vacuum.
In addition to the extensive effort devoted to cleaning the shock tube (a total
of ∼3 man-hours per experiment was devoted to cleaning), the primary diaphragm holder is cleaned with acetone and denatured ethyl alcohol. The test article, throat, and nozzle also accumulate dark soot-like dust, although not to the same degree as the shock tube, and are cleaned with Kimwipes sprayed with denatured ethyl alcohol.
Rather than using standard “Industrial”-quality gas bottles to fill the shock tube, as had been the previous practice, reduced-contaminant “Breathing Air” was used from shot 2739. Finally, only Air Liquide “ALPHAGAZ” research-quality gas bottles were used from shot 2757, for all gas types. In this line of gas bottles, the supplier specifies tight tolerances on the O2 vs. N2 partial pressure (±0.5%) for air, and to- tal hydrocarbon contamination is less than 0.05 ppm. All of these measures, taken together, were successful in mitigating the effects of particulate contamination and resulted in more consistent, clean, repeatable transition measurements. It is recom- mended that these cleaning and contaminant minimization measures be maintained for all future T5 experiments where flow purity, optical measurements, and avoiding particulate contamination are important.