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

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

A smooth attitude-stabilizing control law is derived from which known limits on the control authority of the system are rigorously enforced. Unknown disturbance torques, assumed to be of lesser magnitude than the control limits, are included in the formulation. A smooth control signal containing hyperbolic tangent functions that rigorously obeys a known maximum-torque constraint is introduced. The controller can be viewed as a smooth analog of the variable-structure approach, with the degree of sharpness of the control permitted to vary with time according to a set of user-defined parameters. Lyapunov analysis is employed to ensure global stability, and asymptotic convergence of the angular velocity is guaranteed via the Barbalat lemma. Attitude errors, expressed as Euler parameters, are shown via simulation to vanish whenever certain design parameters are selected appropriately, and guidelines for selection of those parameters are provided in depth.

Globally stabilizing saturated attitude control in the presence of bounded unknown disturbances

We consider the “velocity-free” feedback control problem associated with attitude tracking for a rigid body subject to torque-magnitude and rate-saturation limits. Whereas no angular rate measurements are utilized within the feedback structure, the controller rigorously enforces actuator-magnitude and rate-saturation constraints. The associated stability proof is constructive and accomplished by the development of a novel Lyapunov function candidate. Although control smoothness is preserved at all times, it is important to note that the results of this paper that are derived with respect to magnitude saturation place no additional restrictions on the body inertias and make no other small-angle assumptions. The closed-loop performance of the new control solution derived here is evaluated extensively through numerical simulations in which we also include realistic levels of sensor noise in the feedback signals.

Velocity-Free Attitude Controllers Subject to Actuator Magnitude and Rate Saturations

An experimental study was conducted to investigate the effects of ramp-type vortex generators in a Wheeler doublet configuration, vortex generator jets, and their combination on a supersonic inlet unstart and to achieve active control of inlet unstart. Actuators were placed on the inner side walls of a floor-mounted inlet-isolator model in a Mach 5 flow. Unstart was initiated in the inlet-isolator model by raising a flap at the downstream end of the isolator, which increased the back pressure. The combination of the passive Wheeler doublets and vortex generator jets allowed the isolator’s back pressure to be raised about 32% higher than the maximum mean pressure achieved by the baseline case as well as resulted in a 34% reduction in the isolator rms pressure fluctuations. The Wheeler doublet plus vortex generator jet combination was successful because the presence of the Wheeler doublets mitigated the unstart-inducing effect the vortex generator jets had on the flow, when used separately. Once the Wheeler...

Control of Supersonic Inlet-Isolator Unstart Using Active and Passive Vortex Generators

Unstart detection techniques based on high-frequency pressure measurements made in a hypersonic inlet―isolator model are investigated. In this study, data that were acquired in a previous study of backpressure-induced unstart were examined. The data were acquired in a simplified-geometry inlet―isolator model that consisted of a 6 deg compression ramp inlet followed by a constant-area duct that was 25.4 mm high by 50.8 mm wide by 227.1 mm long. Fluctuating wall pressures were measured along the length of the model. A downstream flap was used to induce unstart in the model. Beyond a certain flap angle, unstart was induced, and the shock system propagated upstream and out of the inlet. The wall-pressure data, acquired as the flap was raised, were postprocessed for spectral and statistical content to evaluate different unstart detection criteria. Three shock leading-edge detection criteria are examined based on the following observations as the shock system passes over a pressure transducer: 1) a rise in pressure, 2) an increase in standard-deviation of the pressure signal, and 3) an increase in power in the 300―400 Hz frequency band. After calibrating the algorithms based on runs with no active control, a comparison of the times detected for unstart onset and unstart arrest was made based on runs with active control. Results indicate that the power-spectrum-based algorithm implemented close to the isolator exit is more sensitive to the onset of unstart, whereas the pressure-magnitude-change criterion gives earlier detection in many cases. A combination of the two, a pressure-magnitude criterion close to the inlet entrance and a spectral-power criterion near the isolator exit, therefore seems to be the most robust scheme for unstart active control. The appropriate choice of sampling frequency significantly improved the computation speed of the spectral-power-based algorithm without delaying the unstart detection times.

Unstart Detection in a Simplified-Geometry Hypersonic Inlet-Isolator 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

A. Valdivia

Papers

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

Velocity-Free Attitude Controllers Subject to Actuator Magnitude and Rate Saturations

Control of Supersonic Inlet-Isolator Unstart Using Active and Passive Vortex Generators

Unstart Detection in a Simplified-Geometry Hypersonic Inlet-Isolator Flow

Active control of supersonic inlet unstart using vortex generator jets