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Airfoil dynamic stall in incompressible flow is characterized by two interacting viscous flow phenomena: time-varying trailing-edge separation and the shedding of intermittent leading-edge-vortex structures. In the current work, a physics based low-order method capable of modeling the interactions between the two flow phenomena is developed with the aim of predicting dynamic stall with only a few empirical tuning parameters. Large computational datasets are used to understand the flow physics of unsteady airfoils so as to augment an inviscid, unsteady airfoil theory to model the time-dependent viscous effects. The resulting model requires only three empirical coefficients for a given airfoil and Reynolds number, which could be obtained from a single moderate-pitch-rate unsteady motion for that airfoil/Reynolds number combination. Results from the low-order model are shown to compare excellently with computational and experimental solutions, in terms of both aerodynamic loads and flow-pattern predictions. In addition to formulating a method with limited empirical dependencies, the current research provides valuable insights into the flow physics of unsteady airfoils and their connection to rapidly predictable theoretical parameters.

Hydrocarbon fuels offer optimal high energy per volume for scramjet applications for sustained hypersonic flight but require additional residence time due to slower ignition delays (compared with h...

This paper develops a new approach for analysis of subgrid closures for turbulent combustion as modeled using direct quadrature (finite-rate chemistry) techniques. The approach, termed multiresolut...

Aerated-liquid atomization involves the injection of a small amount of gas into a liquid stream to initiate primary breakup within the injector itself. The sprays that result have good penetration ...

A characteristic feature of flows past many oscillating airfoils and wings is the leading-edge vortex (LEV). Although considerable progress has been made in the low-order modeling of LEV formation ...

Hypersonic air-breathing engines rely on scramjet combustion processes, which involve high-speed, compressible, and highly turbulent reacting flows. The combustion environment and the turbulent flames at the heart of these engines are difficult to simulate and study in the laboratory under well controlled conditions. Typically, wind-tunnel testing is performed that more closely approximates engine development rather than a careful investigation of the underlying physics that drives the combustion process. The experiments described in this paper, along with companion data sets, aim to isolate the chemical kinetic effects and turbulence-chemistry interaction from the fuel-air mixing process in a dual-mode scramjet combustion environment. A unique fuel injection approach is adopted that produces a uniform fuel-air mixture at the entrance to the combustor and results in premixed combustion. This approach relies on the mixing enhancement of a precombustion shock train upstream of the dual-mode scramjet’s combustor. For the first time a stable flame, anchored on a cavity flameholder, is reported for a scramjet combustor operating in premixed fuel-air mode. The new experimental capability has enabled numerous companion studies involving advanced diagnostics such as coherent anti-Stokes Raman scattering (CARS), particle image velocimetry (PIV), and planar laser induced fluorescence (PLIF).

Leading-edge-vortex (LEV) formation often characterizes the unsteady flows past airfoils and wings. Recent research showed that initiation of LEV formation on airfoils in two-dimensional flow is closely tied to the criticality of the so-called leading-edge suction parameter (LESP). To characterize the LEV initiation on wings in three-dimensional flow, a large set of pitching wings was studied using Reynolds-averaged Navier–Stokes computations (computational fluid dynamics, CFD). The CFD results showed that the pitch angle and spanwise location for LEV initiation varied widely between the different wings. The same cases were also analysed using an unsteady vortex-lattice method (UVLM), which assumes attached flow. Low-order prediction of LEV initiation is assumed to occur at the pitch angle when the UVLM-calculated LESP at any point on the wing span first becomes equal to the pre-determined critical LESP for the airfoil. For all the cases, the predicted pitch angles and spanwise locations for LEV initiation from the low-order method agreed excellently with the corresponding CFD predictions. These observations show that LEV initiation on finite wings is governed by criticality of leading-edge suction, enabling the prediction of LEV initiation on an unsteady finite wing using attached-flow wing theory and the critical LESP values for the airfoil sections.

The relative stability of anatase and rutile in stagnation flame synthesis with stoichiometric mixtures is investigated experimentally. The measurements reveal a high sensitivity of anatase-rutile composition to the flame dilution. It is demonstrated that anatase formation is favoured in more dilute (colder) flames while rutile is favoured in less dilute (hotter) flames. A particle model with a detailed description of aggregate morphology and crystal phase composition is applied to investigate the anatase-rutile stability. A phase transformation model is implemented in which rutile is formed for particles larger than a “crossover” size while anatase is formed for those smaller. Two formation mechanisms/pathways are discussed and evaluated. In the first pathway, the nascent particles are assumed to be stoichiometric and the crossover size is determined solely by the surface free energy. This hypothesis captures the general trend in the measured anatase-rutile composition but fails to explain the sensitivity. In the second pathway, non-stoichiometric TiO 2 − x oxide intermediates are assumed and the crossover size is hypothesised to be composition-dependent. This shows an excellent agreement with the experimental data. However, this hypothesis is found to be strongly influenced by assumptions about the initial particle growth stages. This study demonstrates the importance of a better description of the high-temperature chemistry and initial clustering mechanism in order to understand the crystal phase formation.

The suction force at the leading edge of a round-nosed aerofoil is an important indicator of the state of the flow over the leading edge and, often, the entire aerofoil. The leading-edge suction parameter (LESP) is a non-dimensional version of this force. In recent works, the LESP was calculated with good accuracy for attached flows at low Reynolds numbers (10 000–100 000) from unsteady aerofoil theory. In contrast to this ‘inviscid’ LESP, results from viscous computations and experiments are used here to calculate the ‘viscous’ LESP on aerofoils undergoing pitching motions at low subsonic speeds. The LESP formulation is also updated to account for the net velocity of the aerofoil. Spanning multiple aerofoils, Reynolds numbers and kinematics, the cases include motions in which dynamic stall occurs with or without leading-edge vortex (LEV) formation. Inflections in the surface pressure and skin-friction distributions near the leading edge are shown to be reliable indicators of LEV initiation. Critical LESP, which is the LESP value at LEV initiation, was found to be nearly independent of pivot location, weakly dependent on pitch rate and strongly dependent on Reynolds number. The viscous LESP was seen to drop to near-zero values when the flow is separated at the leading edge, irrespective of LEV formation. This behaviour was shown to correlate well with the loss of streamline curvature at the leading edge due to flow separation. These findings serve to improve our understanding and extend the applicability of the leading-edge suction behaviour gained from earlier works.

Computational and experimental results for pitching and plunging airfoils were used to study the time lag associated with boundary-layer convection and to develop a model that can be used to augmen...

This paper is written in honor and memory of my esteemed friend and colleague, Meng-Sing Liou, with whom I worked closely in the 1990s on the development of AUSM-family schemes and their extensions to reactive, “all-speed,” and multi-phase flows. The purpose of this paper is to revisit the thought processes and concepts that led to the rapid evolution of the AUSM family as a highly efficient, highly accurate method for discretization of the Euler equations and their various extensions. No new results are presented; rather, the focus is on (re-)discovering the common threads that link the various members of the AUSM family. Special attention is given to the strategies that lead to accurate results at all flow speeds, and some reviews of more current work in this area are presented. The paper concludes with a few reflections, personal and otherwise, relating to my interactions with Meng over the years and to my view of the essential elements of AUSM-family schemes.

This work summarizes the development and testing of a new family of chemical closure models for large-eddy simulation (LES) of turbulent combustion using finite-rate chemistry. The goal of this research is to provide a simple, yet effective model that provides a correction to the ‘laminar chemistry’ prediction formed by evaluating chemical production terms using filtered-mean data. The general model takes the form $\overline {\dot {{\omega }}_{s} (q)} =f(\overline {{q}},{\Delta } ,{\ldots } )\dot {{\omega }}_{s} (\overline {{q}})$ , where the enhancement factor, f, accounts for the effects of the subgrid fluctuations on apparent reactivity as expressed at a given mesh level. A form for the enhancement factor is derived by least-squares minimization (LSM) of a ‘reactivity functional’ connecting information at different mesh levels. A modified a priori analysis, in which simultaneous large-eddy simulations are performed on fine and coarse mesh levels, is used to identify candidate modeled forms for the enhancement factor. In the modified a priori analysis, coarse-mesh realizations are constrained by the filtered fine-mesh velocity, allowing eddy structures to be highly correlated. Several LSM variants are described and tested through comparisons with experimental data. The test cases include three experiments conducted at the University of Virginia’s supersonic combustion facility involving non-premixed hydrogen and partially-premixed ethylene combustion as well as a premixed propane-air flame in the Volvo Validation Rig. The results demonstrate the capability of the models to provide a consistent (if modest in some cases) improvement in predictive capability, relative to ‘laminar chemistry’, in a cost-effective manner.

Reynolds-averaged Navier–Stokes and hybrid large-eddy/Reynolds-averaged Navier–Stokes simulations of turbulent flow past an Aerospatiale A-Airfoil near stall at Rec=2.1×106, M=0.15, α=13.3 and a NA...

A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes turbulence model is applied to compute the wave-front aberrations in an optical beam passing through a supersonic open cavity flow. Th...

A leading-edge suction parameter (LESP) that is derived from potential flow theory as a measure of suction at the airfoil leading edge is used to study initiation of leading-edge vortex (LEV) formation in this article. The LESP hypothesis is presented, which states that LEV formation in unsteady flows for specified airfoil shape and Reynolds number occurs at a critical constant value of LESP, regardless of motion kinematics. This hypothesis is tested and validated against a large set of data from CFD and experimental studies of flows with LEV formation. The hypothesis is seen to hold except in cases with slow-rate kinematics which evince significant trailing-edge separation (which refers here to separation leading to reversed flow on the aft portion of the upper surface), thereby establishing the envelope of validity. The implication is that the critical LESP value for an airfoil–Reynolds number combination may be calibrated using CFD or experiment for just one motion and then employed to predict LEV initiation for any other (fast-rate) motion. It is also shown that the LESP concept may be used in an inverse mode to generate motion kinematics that would either prevent LEV formation or trigger the same as per aerodynamic requirements.

The further development of an immersed-boundary method for general flow applications is outlined in this paper. A cell-classification procedure based on a signed distance to the nearest surface is used to separate the computational domain into cells outside the immersed object (field cells), cells outside but adjacent to the immersed object (band cells), and cells within the immersed object (interior cells). Interpolation methods based on laminar/turbulent boundary layer theory are used to prescribe the flow properties within the band cells. The method utilizes a decomposition of the velocity field near embedded surfaces into normal and tangential components, with the latter handled using power-law or log-law interpolations to mimic the energizing effects of turbulent boundary layers. Procedures for generating motion events using rendering technologies are described as methods for directly embedding sequences of stereo-lithography files representing frames of motion as immersed objects in the computational domain. Extensions of the methodology to zero-thickness immersed surfaces are discussed. Described applications center on human motion events, with a focus on understanding the effect of human motion on agent transport in confined environments.

The combustion efficiency of a scramjet is a metric that evaluates the overall performance of the engine. Until recently, combustion efficiency was measured using indirect approaches such as a one-dimensional control volume calculation or a calorimeter and wall pressure tap measurements. A novel nonintrusive direct approach for the measurement of combustion efficiency is presented that combines the optical diagnostic techniques tunable diode laser absorption tomography and stereoscopic particle image velocimetry. Experimental results are presented for measurements of the University of Virginia’s Supersonic Combustion Facility in both the scram and ram-modes of operation. The tunable-diode-laser-absorption-tomography/stereoscopic-particle-image-velocimetry method directly measures the converted hydrogen (via water vapor) mass flow rate exiting the dual-mode scramjet and compares this to the facility-measured injected hydrogen fuel mass flow rate. A complementary computational fluid dynamics study was perfo...

Iterative methods based on block incomplete LU (BILU) factorization are considered highly effective for solving large-scale block-sparse linear systems resulting from coupled PDE systems with n equations. However, efforts on porting implicit PDE solvers to massively parallel shared-memory heterogeneous architectures, such as general-purpose graphics processing units (GPGPUs), have largely avoided BILU, leaving their enormous performance potential unfulfilled in many applications where the use of implicit schemes and BILU-type preconditioners/solvers is highly preferred. Indeed, strong inherent data dependency and high memory bandwidth demanded by block matrix operations render naive adoptions of existing sequential BILU algorithms extremely inefficient on GPGPUs. In this study, we present a fine-grained BILU (FGBILU) scheme which is particularly effective on GPGPUs. A straightforward one-sweep wavefront ordering is employed to resolve data dependency. Granularity is substantially refined as block matrix operations are carried out in a true element-wise approach. Particularly, the inversion of diagonal blocks, a well-known bottleneck, is accomplished by a parallel in-place Gauss–Jordan elimination. As a result, FGBILU is able to offer low-overhead concurrent computation at O ( n 2 N 2 ) scale on a 3D PDE domain with a linear scale of N . FGBILU has been implemented with both OpenACC and CUDA and tested as a block-sparse linear solver on a structured 3D grid. While FGBILU remains mathematically identical to sequential global BILU, numerical experiments confirm its exceptional performance on an Nvidia GPGPU.

In this study, a hybrid large-eddy/Reynolds-averaged Navier–Stokes (LES/RANS) method is used to simulate ethylene combustion inside a cavity flameholder. The cavity flameholder considered is Configuration E of University of Virginia’s Scramjet Combustion Facility, which consists of a Mach 2 inlet nozzle, a constant-area isolator, a combustor, and an extender, through which the exhaust gases are vented to the atmosphere. To increase the fuel-residence time, a cavity is fitted along the upper wall inside the combustor section of the flameholder. The configuration has the capability of injecting ethylene through a series of ports located upstream of and inside the cavity along the upper wall the combustor. In the simulations, ethylene combustion is modeled using a 22-species ethylene oxidation mechanism. Also, a synthetic eddy method is used to introduce turbulence at the inflow plane of the flameholder. For an equivalence ratio of 0.15, a cavity stabilized flame is predicted. Predictions are compared with line-of-sight temperature, water column-density, water mole-fraction, CO column-density, and CO 2 column-density measurements at three stations within and downstream of the cavity. Agreement with experiment is generally good within the cavity. Downstream of the cavity, the simulations predict higher temperatures near the wall. Analysis of the flame structure predicted by the LES/RANS method indicates that the flame propagates into a stoichiometric to fuel-rich mixture near the cavity. Flame angles captured in the simulation are in close agreement with those predicted through classical premixed turbulent flame-speed estimates. Further downstream, the flame structure is non-premixed in character, and near complete conversion of CO to CO 2 is observed by the time the flame reaches the combustor exit.

Here, an OpenACC directive-based graphics processing unit (GPU) parallel scheme is presented for solving the compressible Navier–Stokes equations on 3D hybrid unstructured grids with a third-order reconstructed discontinuous Galerkin method. The developed scheme requires the minimum code intrusion and algorithm alteration for upgrading a legacy solver with the GPU computing capability at very little extra effort in programming, which leads to a unified and portable code development strategy. A face coloring algorithm is adopted to eliminate the memory contention because of the threading of internal and boundary face integrals. A number of flow problems are presented to verify the implementation of the developed scheme. Timing measurements were obtained by running the resulting GPU code on one Nvidia Tesla K20c GPU card (Nvidia Corporation, Santa Clara, CA, USA) and compared with those obtained by running the equivalent Message Passing Interface (MPI) parallel CPU code on a compute node (consisting of two AMD Opteron 6128 eight-core CPUs (Advanced Micro Devices, Inc., Sunnyvale, CA, USA)). Speedup factors of up to 24× and 1.6× for the GPU code were achieved with respect to one and 16 CPU cores, respectively. The numerical results indicate that this OpenACC-based parallel scheme is an effective and extensible approachmore » to port unstructured high-order CFD solvers to GPU computing.« less

An assessment of the ignition characteristics of several silane/hydrogen combustion models under conditions that are relevant to hypersonic combustion applications is presented. Numerical predictions of silane/hydrogen ignition characteristics are compared to experimental measurements of explosion limits and shock-tube ignition delay times. Furthermore, the ignition properties of silane/hydrogen mixtures are evaluated at conditions extracted from representative streamlines calculated in a computational-fluid-dynamics simulation of a model scramjet configuration tested at the Alliant Techsystems/General Applied Science Laboratory at Mach 10. It is found that the analyzed silane chemical kinetic models are extremely sensitive to temperature and pressure at key combustor conditions. Moreover, the presence of many inconsistencies between experiments and chemical models show that the current knowledge of silane combustion pathways is not sufficient to perform accurate computational-fluid-dynamics simulations o...

A large-eddy simulation/Reynolds-averaged Navier–Stokes model is applied to three high-speed mixing layers and three sonic injection flows to generate data suitable for evaluating two current Reynolds-averaged Navier–Stokes models for turbulent mass diffusivity. These models solve transport equations for concentration variance and dissipation rate and differ in the constitutive relation for the turbulent mass diffusivity and the form of the evolution equation for the dissipation rate. The predictive capability of the large-eddy simulation/Reynolds-averaged Navier–Stokes model is assessed through simulations of an air–air mixing-layer experiment and a sonic ethylene injection into a Mach 2.0 airstream. This model provides good predictions of the mean velocity, turbulence intensity, and rms temperature fluctuation throughout the shear layer flowfield but slightly over-predicts the spreading rate of the mixing layer. The simulation of the sonic injection is in generally good agreement with the experiment but...

A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes turbulence model is used to simulate the Mach 6 flow around a scaled model similar to NASA’s Orion multipurpose crew vehicle. The results for surface pressure and heat transfer are compared with experimental data from previous base flow experiments conducted at the Calspan—University at Buffalo Research Center. Using the highest Reynolds number test case (11×106 based on capsule diameter), different numerical aspects of the hybrid approach are addressed, such as use of a low-dissipation scheme, a modification to the eddy-viscosity blending function, time-averaging results, filtering computational results, and sensitivity to grid resolution. In addition, results are compared with Reynolds-Averaged Navier–Stokes using Menter’s two-equation baseline model and to detached-eddy simulation predictions. By introducing a new modification to the blending from Reynolds-averaged Navier–Stokes to large-eddy simulation within boundary layers, very good agr...

The reactive flow through a model scramjet combustor is simulated using a hybrid large-eddy/Reynolds-averaged Navier–Stokes technique. The scramjet configuration considered is similar to the one investigated at the Institute for Chemical Propulsion of the DLR, German Aerospace Center. The model scramjet consists of 15 fuel-injecting holes, located on the base of a wedge-shaped fuel injector, through which hydrogen is injected at sonic conditions. In the present study, only five of the 15 fuel-injecting holes are considered, and periodicity is assumed in the spanwise direction. Several parametric studies are conducted with a view toward determining the sensitivities of the predictions to modeling and algorithmic variations. Different grids (two different topologies), flux reconstruction methods (total variation diminishing and piecewise parabolic method), reaction mechanisms, and inflow boundary conditions (uniform and nonuniform) are used. To enhance fuel–air mixing, a synthetic eddy method is used to gen...

Unsteady aerofoil flows are often characterized by leading-edge vortex (LEV) shedding. While experiments and high-order computations have contributed to our understanding of these flows, fast low-order methods are needed for engineering tasks. Classical unsteady aerofoil theories are limited to small amplitudes and attached leading-edge flows. Discrete-vortex methods that model vortex shedding from leading edges assume continuous shedding, valid only for sharp leading edges, or shedding governed by ad-hoc criteria such as a critical angle of attack, valid only for a restricted set of kinematics. We present a criterion for intermittent vortex shedding from rounded leading edges that is governed by a maximum allowable leading-edge suction. We show that, when using unsteady thin aerofoil theory, this leading-edge suction parameter (LESP) is related to the $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}A_0$ term in the Fourier series representing the chordwise variation of bound vorticity. Furthermore, for any aerofoil and Reynolds number, there is a critical value of the LESP, which is independent of the motion kinematics. When the instantaneous LESP value exceeds the critical value, vortex shedding occurs at the leading edge. We have augmented a discrete-time, arbitrary-motion, unsteady thin aerofoil theory with discrete-vortex shedding from the leading edge governed by the instantaneous LESP. Thus, the use of a single empirical parameter, the critical-LESP value, allows us to determine the onset, growth, and termination of LEVs. We show, by comparison with experimental and computational results for several aerofoils, motions and Reynolds numbers, that this computationally inexpensive method is successful in predicting the complex flows and forces resulting from intermittent LEV shedding, thus validating the LESP concept.

Advanced computational models of hypersonic air-breathing combustion processes are being developed to better understand and predict the complex flows within a dual-mode scramjet combustor. However, the accuracy of these models can only be quantified through comparison to experimental databases. Moreover, the quality of computational results is dependent on accurate and detailed knowledge of the combustor inflow and boundary conditions. Toward these ends, this paper describes results from a collaboration of experimental and computational investigators. Detailed computational fluid dynamics and finite element analyses were performed throughout the design and implementation of experiments involving a direct-connect scramjet combustor operating at steady state during long duration testing. The test section hardware was designed to provide substantial access for optical laser diagnostics. Measurement locations included the inflow plane and several locations downstream of fuel injection. A suite of advanced in-...