Progress in shock wave/boundary layer interactions

This paper describes the history, objectives, structure, and current capabilities of the Stanford University Unstructured (SU 2 ) tool suite. This computational analysis and design software collection is being developed to solve complex, multi-physics analysis and optimization tasks using arbitrary unstructured meshes, and it has been designed so that it is easily extensible for the solution of Partial Differential Equation-based (PDE) problems not directly envisioned by the authors. At its core, SU 2 is an open-source collection of C++ software tools to discretize and solve problems described by PDEs and is able to solve PDE-constrained optimization problems, including optimal shape design. Although the toolset has been designed with Computational Fluid Dynamics (CFD) and aerodynamic shape optimization in mind, it has also been extended to treat other sets of governing equations including potential flow, electrodynamics, chemically reacting flows, and several others. In our experience, capabilities for computational analysis and optimization have improved considerably over the past two decades. However, the ability to integrate the resulting software packages into coupled multi-physics analysis and design optimization solvers has remained a challenge: the variety of approaches chosen for the independent components of the overall problem (flow solvers, adjoint solvers, optimizers, shape parameterization, shape deformation, mesh adaption, mesh deformation, etc) make it difficult to (a) expand the range of applicability to situations not originally envisioned, and (b) to reduce the overall burden of creating integrated applications. By leveraging well-established object-oriented software architectures (using C++) and by enabling a common interface for all the necessary components, SU 2 is able to remove these barriers for both the beginner and the seasoned analyst. In this paper we attempt to describe our efforts to develop SU 2 as an integrated platform. In some senses, the paper can also be used as a software reference manual for those who might be interested in modifying it to suit their own needs. We carefully describe the C++ framework and object hierarchy, the sets of equations that can be currently modeled by SU 2 , the available choices for numerical discretization, and conclude with a set of relevant validation and verification test cases that are included with the SU 2 distribution. We intend for SU 2 to remain open source and to serve as a starting point for new capabilities not included in SU 2 today, that will hopefully be contributed by users in both academic and industrial environments.

Stanford University Unstructured (SU 2 ): An open-source integrated computational environment for multi-physics simulation and design

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

The relationship between the upstream boundary layer and the low-frequency, large-scale unsteadiness of the separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field particle image velocimetry (PIV) and planar laser scattering (PLS) measurements in streamwise–spanwise planes. Planar laser scattering measurements in the upstream boundary layer indicate the presence of spanwise strips of elongated regions of uniform momentum with lengths greater than 40?. These long coherent structures have been observed in a Mach 2 supersonic boundary layer (Ganapathisubramani, Clemens & Dolling 2006) and they exhibit strong similarities to those that have been found in incompressible boundary layers (Tomkins & Adrian 2003; Ganapathisubramani, Longmire & Marusic 2003). At a wall-normal location of y/?=0.2, the inferred instantaneous separation line of the separation region is found to oscillate between x/?=?3 and ?1 (where x/?=0 is the ramp corner). The instantaneous spanwise separation line is found to respond to the elongated regions of uniform momentum. It is shown that high- and low-momentum regions are correlated with smaller and larger size of the separation region, respectively. Furthermore, the instantaneous separation line exhibits large-scale undulations that conform to the low- and high-speed regions in the upstream boundary layer. The low-frequency unsteadiness of the separation region/shock foot observed in numerous previous studies can be explained by a turbulent mechanism that includes these elongated regions of uniform momentum

Effects of upstream boundary layer on the unsteadiness of shock induced separation

Recent research progress on unstart mechanism, detection and control of hypersonic inlet

DOI: 10.2514/1.40966 The dynamics of unstart of a floor-mounted inlet/isolator model in a Mach 5 flow are investigated experimentally. The inlet section contains a 6-deg compression ramp, and the isolator is a rectangular straight duct that is 25.4-mm high by 50.8-mm wide by 242.3-mm long. Measurements made include 8-kHz schlieren imaging and simultaneous fast-response wall pressures along the length of the inlet/isolator. Unstart is initiated by deflecting a flap at the downstream end of the isolator. The shock system, induced by unstart, initially propagates upstream through the isolator at a velocity of about 35 m=s (in the laboratory frame of reference), then decelerates to about 20 m=s near the isolator entrance, and then accelerates to a velocity of about 74 m=s within the inlet. Throughout the isolator, unstart is seen to be strongly associated with boundary-layer separation. Once the inlet has unstarted, a highamplitudeoscillatory(periodic)unstarted flowensues,forwhichtheoscillationfrequencyisabout124Hz.However, under some conditions, an 84-Hz oscillatory unstarted flow mode, with lower pressure fluctuations, is observed. Under other conditions, a nonoscillatory unstarted flow, with much lower pressure fluctuations, is observed.

Experimental Investigation of Unstart in an Inlet/Isolator Model in Mach 5 Flow

An experimental study was conducted to investigate the effects of Wheeler Doublets (WDs), Vortex Generator Jets (VGJs), and their combination on supersonic inlet unstart. The final objective of the investigation was to achieve active control of unstart through the use of an unstart detection technique and the most effective of these actuators. Actuators were placed in the inner side wall of a floor-mounted inlet-isolator model in a Mach 5 flow. Unstart is initiated by increasing the back pressure by deflecting a flap at the downstream end of the isolator. Exclusive use of either WDs or VGJs was seen to have some effect on the back pressure but did not have a significant impact on delaying unstart. The combination of WDs and VGJs allowed the isolator’s back pressure to be raised steadily about 32% higher than the maximum mean pressure achieved by the baseline case as well as resulted in a reduction of isolator pressure unsteadiness of about 34%. The WDs+VGJs combination was successful because the presence of the WDs mitigated the unstart-inducing effect the VGJs had on the flow. Once the WDs+VGJs combination had proven to be the most effective actuator, an unstart detection technique and control logic were developed to form a closed loop control scheme aimed at preventing unstart. An important part of the control scheme was the identification of a wall-pressure-signal precursor to unstart that reliably predicted unstart onset. Experimental tests of this control system resulted in the achievement of active control of supersonic inlet unstart at a 50% success rate. In addition, areas that could significantly increase the success rate for the current unstart control technique were found, such as better unstart detection techniques and faster acting valves. The investigation showed that once the inlet-isolator system had unstarted, re-start was not achieved by activating the WDs+VGJs.

http://enu.kz/repository/2009/AIAA-2009-4022.pdf

Active control of supersonic inlet unstart using vortex generator jets

An experimental investigation of supersonic inlet unstart

Surface temperatures on a hemisphere-cylinder body with a nose cavity in a Mach 4.9 airflow have been measured using an infrared camera. Fluctuating surface pressures have also been measured at the cavity base. The cavity diameter D was fixed at one-half the cylinder diameter and the length L of the cavity was varied. If the cavity lip is sharp and the cavity is "shallow" (0.15 1) an axisymmetric, nominally steady "cool ring" forms on the external surface downstream of the lip. Flow visualization shows that the cool ring is caused by separation at the lip. Rounding the cavity lip eliminates or reduces separation and temperatures return to levels characteristic of the model without the cavity. For "intermediate deep" cavities (0.40 < LID < 0.70) the cavity pressure signals switch from a low-amplitude to high-amplitude level at random intervals resulting in an unstable, nonaxisymmetric temperature field downstream of the lip. Changes in cavity base shape from spherical to flat have little effect on the temperature history for shallow and very deep cavities, whereas for intermediate depth cavities the effects are more significant.

Nose cavity effects on blunt body pressure and temperatures at Mach 5

The dynamics of unstart and unstarted flows in a floor-mounted inlet-isolator model in a Mach 5 flow are investigated experimentally using PIV, fast response wall pressures and 8 kHz schlieren imaging. The inlet compression is obtained with a 6-degree ramp and the isolator is a rectangular straight duct that is 25.4 mm high by 50.8 mm wide by 242.3 mm long. Unstart is initiated by deflecting a motorized flap at the downstream end of the isolator. PIV measurements are made in a streamwise-transverse plane as well as a streamwise-spanwise plane. With the flap fully-down, the PIV data of the started flow capture the characteristics of the isolator boundary layers and are consistent with shockexpansion theory. It is observed that the unstart process is associated with shock-induced separation, which leads to large reverse flow velocities up to about 300 m/s and the formation of strong shear layers. Furthermore, unstart is clearly three-dimensional as separation of the sidewall boundary layers is observed. In addition, the results indicate that the inlet geometry and the resulting initial shock system play a critical role in the flow structure of the unstart process. Once the model unstarts, a high-amplitude oscillatory unstarted flow with a dominant frequency of 124 Hz ensues. At times during this oscillatory unstarted flow when the isolator pressure is decreasing, the flow velocities in the isolator are relatively low. However, during cycles times when the isolator pressure is increasing the flow is associated with propagating shock systems and the isolator entrance flow is supersonic.

Kemal Yuceil

Papers

Experimental Investigation of Unstart in an Inlet/Isolator Model in Mach 5 Flow

Active control of supersonic inlet unstart using vortex generator jets

An experimental investigation of supersonic inlet unstart

Nose cavity effects on blunt body pressure and temperatures at Mach 5

PIV Measurements of Unstart of an Inlet-Isolator Model in a Mach 5 Flow