18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 A Performance Comparison between Classical Schlieren and Background-Oriented Schlieren P. Manovski1,*, J. Wehrmeyer2, K. Scott2, B. Loxton1, H. Quick1, S. Lam1, M. Giacobello1 1: Aerospace Division, Defence Science and Technology Group, Australia 2: Arnold Engineering Development Complex, Arnold Air Force Base, United States of America * Correspondent author: [email protected] Keywords: Background-oriented schlieren, Quantitative or Calibrated schlieren ABSTRACT A performance comparison was made between a classical calibrated schlieren system and a background-oriented schlieren (BOS) system at the Defence Science and Technology Group (DST Group) Transonic Wind Tunnel (TWT) in Melbourne, Australia. Comparisons between these two techniques have been made by others. This study aims to contribute to the findings by conducting a similar investigation in a large-scale facility using the latest high pixel resolution (29 MP), high sensitivity cameras to maximise spatial resolution and the ability to resolve small refraction angles. This study also aims to investigate the effects of the conical perspective of a typical BOS setup compared with the cylindrical view of schlieren. Two configurations were investigated, firstly a standard setup and then a high sensitivity setup. The sensitivity was determined experimentally and compared with theory. The minimum detectability was determined to be of the same order, approximately ±1 arc-second for both systems. The performance of each system was then investigated by imaging the compressible flow around a cone-cylinder model for a range of Mach numbers from 0.6 to 1.2. Qualitatively, a good agreement was obtained. By translating the BOS system in the streamwise direction, at 1.05 Mach, the effect of the BOS system conical perspective was evident, a relatively strong bow shock at the tip of the cone-cylinder model could be made to disappear from the BOS image. A quantitative comparison of the measured line-of-sight refraction angle showed a good agreement near the centre of the BOS lens but away from the centre discrepancies were evident and are likely due to the BOS conical perspective. The schlieren system range was also shown to be limited. 1. Introduction Both classical schlieren and background-oriented schlieren (BOS) techniques measure refractive- index variations and can be used to visualise the line-of-sight quantitative density gradient information. The techniques are widely used to study compressible flow phenomena, such as shock waves, expansion fans, and variable-density wakes and boundary layers in high speed flows. Standard schlieren imaging is used in many wind tunnels, such as the Defence Science and Technology Group (DST Group) Transonic Wind Tunnel (TWT) in Melbourne, Australia and has a long development history [1]. In contrast, BOS is a relatively new technique that relies on digital imaging cameras and computer-based image correlation software [2]. BOS is used in the Arnold Engineering Development Complex (AEDC) 16T (16 foot Transonic) wind tunnel, rather than classical schlieren, because of the latter technique’s requirement of optical access that would block a large percentage of the test section walls. Wall blockage is detrimental to test section flow quality for transonic wind tunnels, whose test section walls are 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 ventilated to minimize shock reflection. The DST Group TWT is often used with a half model mounted on the side wall which blocks the optical access required for schlieren. For the DST Group, BOS presents an alternative that is worth investigating. The purpose of this study was to obtain a relative performance comparison between the BOS and a standard z-type schlieren system installed at the DST Group TWT while considering the facility’s geometric constraints. In particular, the aim was to compare the ability of the two techniques in detecting small density gradients. Comparisons between these two techniques have been made by others [3, 4]. This study aims to contribute to the findings by conducting a similar investigation in a large scale wind tunnel facility using the latest high pixel resolution (29 MP), high sensitivity cameras to maximize spatial resolution and the ability to resolve small refraction angles. This study also aims to investigate the effects of the conical perspective of a typical BOS setup compared with the cylindrical view of schlieren. Two configurations were investigated, firstly a standard setup that is typically used for test campaigns in the TWT and then a high sensitivity setup. The sensitivity and minimum detectability were experimentally determined by introducing a wedged optic that produces a very small (known) angular refraction as the schlieren object. The performance of each system was assessed qualitatively and quantitatively by imaging the compressible flow around a cone-cylinder model for a range of Mach numbers from 0.6 to 1.2. 2. Experimental equipment 2.1 DST Group Transonic Wind Tunnel The DST Group Transonic Wind Tunnel is a closed-circuit continuous flow wind-tunnel. A schematic of the circuit is shown in Fig. 1. The tunnel is equipped with a Plenum Evacuation System (PES), used to pressurize and depressurize the tunnel circuit as well as improve the flow quality in the test section by removing air through the slotted test section walls, and re-injecting it downstream of the first corner after compression. The tunnels convergent nozzle can be operated in an under-expanded mode to produce supersonic Mach numbers by evacuating the test section to allow expansion of the sonic nozzle flow. The total pressure in the circuit can be evacuated down to 40 kPa or pressurized up to 200 kPa. When the test section is empty the tunnel has a Mach number range of 0.30 to 1.20. The test section is nominally 2700 mm long, 806 mm high and 806 mm wide. The sidewalls are interchangeable and can be either solid or slotted. Each slotted wall consists of six slots per wall giving an overall porosity of 4.97%. The main model support is downstream of the test section, incorporating a pitch and roll mechanism. The pitch mechanism has a position accuracy of ±0.02 deg and the roll drive is accurate to within ±0.1 deg. Further details of the model support system are in [5]. The Mach number is controlled by adjusting the main fan speed and plenum suction via two control valves (one for gross control, the other for fine control) in the PES inlet pipe, thus controlling the rate of air removed from the plenum. The Mach number is monitored by the tunnel control system from the total pressure in the settling chamber and static pressure in the plenum, and is controlled to within ±0.002 for subsonic operations and ±0.01 for supersonic [5]. 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 Temperature in the test section, which can be set between 30°C and 40°C, is controlled via a heat exchanger in the settling chamber to within ±2°C. Humidity is reduced to below 2000 ppmv before each test run through a dehumidifier in the PES. Fig. 1 DST Group TWT circuit schematic Fig. 2 Modified z-type schlieren system layout in the DST Group TWT. 2.2 DST Group TWT schlieren system 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 A classical schlieren system for flow visualisation is based on the deflection of a light beam as it passes through a change in refractive index caused by a density gradient in the wind tunnel test section [1]. A properly calibrated system can measure the magnitude of the density gradient, normal to the knife edge from the intensity of the light reaching the sensor [1]. The DST Group schlieren system employs a modified version of the z-type Toepler layout and is shown in Fig. 2. The modifications include the use of two plane mirrors (PM1 and PM2) that fold the light path to reduce the space necessary for the standard schlieren system. The system mirrors consist of two 406 mm parabolic schlieren mirrors, with focal lengths SM1 = 2517 mm and SM2 = 2533 mm. The z-type arrangement introduces astigmatism into the system, however, a combination of careful alignment and minimizing the off-axis angles reduces this to a minimum and the resultant effect in the classical setup with a knife edge cut-off is negligible. Further details of the TWT schlieren system can be found in [6]. In this study a classical setup was used with a rectangular light source slit and a vertical knife edge allowing the visualisation of the streamwise density gradients. A micrometer was used to accurately measure the cut-off percentage or unobstructed source height, a. A white LED light source (LED Engin 24 die LZP-DOCW00) was used to provide stable high powered uniform illumination during the test period. The LED was driven by a Garadsoft RT820F-20 controller, capable of providing continuous or pulsed operation, with pulse widths from 3 µs – 200 ms. An achromatic doublet imaging lens of focal length 100 mm was used to provide a field-of-view encompassing the entire schlieren window. The camera used for the schlieren imaging was LaVision Imager LX 29M with camera link interface, and provided a 12 bit greyscale pixel depth. To calculate the sensitivity for the schlieren system, the camera pixel intensities, E, is converted into a contrast or normalised intensity as per, ∆� � − �'() � = = , (1) �'() �'() where �'(), is the background image or reference intensity. The reference intensity is the image intensity of undisturbed (zero density gradient) region of the image.