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A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case. [ABSTRACT FROM AUTHOR]

Ammonia is a promising alternative fuel, which is considered to have the potential to substitute conventional fossil fuels. In the present work, auto-ignition characteristics of ammonia substitution on methane are investigated both experimentally and numerically. The auto-ignition procedure of ammonia-substituted methane/air mixtures are measured behind the reflected shock wave in a shock tube experiment system over temperatures from 1355 to 1877 K, pressure up to 5 atm and an equivalence ratio from 0.5 to 2. Numerical simulation studies using a detailed kinetics mechanism are also performed to gain a deep insight into the auto-ignition procedure of ammonia-substituted methane fuel mixtures. The established numerical model is verified with the measured auto-ignition delay time data by experiments. Then, the auto-ignition delay times are predicted under a wider range of conditions such as equivalence ratio, pressure, temperature, etc. In this way, combustion characteristics of such mixtures are investigated. It is found that adding ammonia fuel to methane will not change the autoignition delay time of methane a lot, while it can effectively benefit the reduction of carbon emissions. Finally, sensitivity analyses are performed to provide essential information for the elementary reaction sensitive to the ignition characteristics. The results present in this work can provide fundamental information for combustion application of ammonia-based fuels. [ABSTRACT FROM AUTHOR]

• A numerical model for simulation of sub-critical cryogenic jets is constructed. • The proposed model is able to solve multi-physics processes involving moving interface, phase transition and combustion. • Hybrid thermodynamic closure is proposed for the mixture of liquid and reactive species under the sub-critical condition. • A low dissipation Riemann solver and a homogenous reconstruction strategy are adopted. In order to simulate cryogenic H 2 − O 2 jets under subcritical condition, a numerical model is constructed to solve compressible reactive multi-component flows which involve complex multi-physics processes such as moving material interfaces, shock waves, phase transition and combustion. The liquid and reactive gaseous mixture are described by a homogeneous mixture model with diffusion transport for heat, momentum and species. A hybrid thermodynamic closure strategy is proposed to construct an equation of state (EOS) for the mixture. The phase transition process is modeled by a recent fast relaxation method which gradually reaches the thermo-chemical equilibrium without iterative process. A simplified transport model is also implemented to ensure the accurate behavior in the limit of pure fluids and maintain computational efficiency. Last, a 12-step chemistry model is included to account for hydrogen combustion. Then the developed numerical model is solved with the finite volume method where a low dissipation AUSM (advection upstream splitting method) Riemann solver is extended for multi-component flows. A homogeneous reconstruction strategy compatible with the homogeneous mixture model is adopted to prevent numerical oscillations across material interfaces. Having included these elements, the model is validated on a number of canonical configurations, first for multiphase flows, and second for reactive flows. These tests allow recovery of the expected behavior in both the multiphase and reactive limits, and the model capability is further demonstrated on a 2D burning cryogenic H 2 − O 2 jet, in a configuration reminiscent of rocket engine ignition. [ABSTRACT FROM AUTHOR]

• Implicit large eddy simulation of compressible turbulence is performed with newly proposed PnTm-BVD schemes. • More accurate predictions of the turbulence evolution are obtained compared with WENO schemes. • Faithful solutions to capture discontinuities are realized with BVD algorithm. • The superiority of PnTm-BVD is more notable in high wave-number region. Implicit large eddy simulation (ILES) of compressible turbulence with shock capturing schemes requires wide investigations and numerical experiments. In this study, a newly proposed P n T m − BVD (polynomial of n -degree and THINC function of m -level reconstruction based on BVD algorithm) shock capturing scheme is introduced to simulate compressible turbulence flow with ILES. The new scheme is designed by employing high-order linear-weight polynomials and THINC (Tangent of Hyperbola for INterface Capturing) functions with adaptive steepness as the reconstruction candidates. The final reconstruction function in each cell is determined with a multi-stage BVD (Boundary Variation Diminishing) algorithm so as to effectively control numerical oscillation and dissipation. Numerical tests involving shock waves and broadband turbulence are conducted in comparison with WENO (Weighted Essentially Non-oscillatory) schemes which are widely used in ILES. The results demonstrate performing ILES with P n T m − BVD scheme is able to obtain higher resolution and more faithful results than WENO does. Importantly, the superiority of P n T m − BVD becomes more notable in high wave-number region. Thus this paper provides and verifies a new scheme which is promising in providing high-resolution results for real-case ILES of compressible turbulence flow. [ABSTRACT FROM AUTHOR]

The scale-resolving simulation of high speed compressible flow through direct numerical simulation (DNS) or large eddy simulation (LES) requires shock-capturing schemes to be more accurate for resolving broadband turbulence and robust for capturing strong shock waves. In this work, we develop a new paradigm of dissipation-adjustable, shock capturing scheme to resolve multi-scale flow structures in high speed compressible flow. The new scheme employs a polynomial of n-degree and non-polynomial THINC (Tangent of Hyperbola for INterface Capturing) functions of m-level steepness as reconstruction candidates. These reconstruction candidates are denoted as P n T m. From these candidates, the piecewise reconstruction function is selected through the boundary variation diminishing (BVD) algorithm. Unlike other shock-capturing techniques, the BVD algorithm effectively suppresses numerical oscillations without introducing excess numerical dissipation. Then, an adjustable dissipation (AD) algorithm is designed for scale-resolving simulations. This novel paradigm of shock-capturing scheme is named as P n T m − BVD − AD. The proposed P n T m − BVD − AD scheme has following desirable properties. First, it can capture large-scale discontinuous structures such as strong shock waves without obvious non-physical oscillations while resolving sharp contact, material interface and shear layer. Secondly, the numerical dissipation property of P n T m − BVD − AD can be effectively adjusted between n+1 order upwind-biased scheme and non-dissipative n+2 order central scheme through a simple tunable parameter λ. Thirdly, with λ = 0.5 the scheme can recover to n+2 order non-dissipative central interpolation for smooth solution over all wavenumber, which is preferable for solving small-scale structures in DNS as well as resolvable-scale in explicit LES. Finally, the under-resolved small-scale can be solved with the dissipation adjustable algorithm through the so-called implicit LES (ILES) approach. Through simulating benchmark tests involving multi-scale flow structures and comparing with other central-upwind schemes, the superiority of the proposed scheme is evident. For instance, the simulation results of the supersonic planar jet show that P n T m − BVD − AD schemes can achieve competitive results as P n + 2 T m − BVD schemes which utilize a higher degree of reconstruction polynomial. Thus, in comparison with the previous work, the proposed P n T m − BVD − AD schemes have the benefit of a more compact stencil and lower cost. In summary, this work provides an alternative scheme for solving multi-scale problems in high speed compressible flows. • Novel dissipation-adjustable schemes for scale-resolving simulation. • Numerical dissipation is effectively adjusted between high order upwind and central scheme. • Capture discontinuous large-scale structures such as strong shocks without oscillations, and contact with less dissipation. • Small-scale is resolved by non-dissipative central interpolation which can be retrieved over all wavenumber. • Under-resolved scale can be solved with dissipation-adjustable algorithm through ILES approach. [ABSTRACT FROM AUTHOR]

• Novel hybrid schemes of less numerical dissipation based on BVD algorithm for unstructured grids. • Applicable to single and multiple phase compressible flows involving shocks and material interfaces. • Superior solution quality for both vortical and discontinuous flow structures in comparison with conventional schemes. • Easy to implement on unstructured grids and of significant practical impact. Solving compressible flows containing both smooth and discontinuous flow structures still remains a big challenge for finite volume methods, especially on unstructured grids where one faces more difficulties in building high-order polynomial reconstruction and limiting projection to suppress numerical oscillations in comparison with the case of structured grids. As a result, most of the current finite volume schemes on unstructured grids are of second order and too dissipative to resolve fine structures of complex flows. In this paper, we report two novel hybrid schemes to resolve vortical and discontinuous solutions on unstructured grids by reducing numerical dissipation. Different from conventional shock capturing schemes that use polynomials and limiting projections for reconstruction, the proposed schemes employ two second-order schemes, i.e. a polynomial and a sigmoid function as candidate reconstruction functions to approximate smooth and discontinuous solutions respectively. As the polynomial function, the MUSCL (Monotone Upstream-centered Schemes for Conservation law) scheme with the MLP (Multi-dimensional Limiting Process) slope limiter is adopted, while being a sigmoid function, the multi-dimensional THINC (Tangent of Hyperbola for INterface Capturing) function with quadratic surface representation and Gaussian quadrature, so-called THINC/QQ, is used to mimic the discontinuous solution structure. With these candidates for reconstruction, a single-step boundary variation diminishing (BVD) algorithm, which aims to minimize numerical dissipation, is designed on unstructured grids to select the final reconstruction function. The resulting two variant schemes, MUSCL-THINC/QQ-BVD schemes with two and three candidates respectively, are algorithmically simple and show great superiority to other existing schemes in capturing discontinuous and vortical flow structures for single and multiphase compressible flows on unstructured grids. The performance of the proposed schemes has been extensively verified through benchmark tests of single and multi-phase compressible flows, where discontinuous and vortical flow structures, like shock waves, contact discontinuities and material interfaces, as well as vortices and shear instabilities of different scales, coexist simultaneously. The numerical results show that the proposed schemes that hybrids two second-order schemes are capable of capturing sharp discontinuous profiles without numerical oscillations and resolving vortical structures along shear layers and material interfaces with significantly improved solution quality superior to other schemes of even higher order reconstructions. [ABSTRACT FROM AUTHOR]