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The Influence of Scale Resolving Simulations in Predictions of Vortex Interaction about a Generic Missile Airframe
Abstract
View Video Presentation: https://doi.org/10.2514/6.2022-1685.vid Hybrid Reynolds Average Navier-Stokes (RANS) - Large Eddy Simulations (LES) have been applied to predict the rolling moment coefficient of a generic missile at high angle of incidence in supersonic flow. The missile airframe was rolled which generated unsymmetrical vortices affecting the downstream tail fin section. Traditional RANS indicated difficulties predicting the rolling moment. This challenge was accepted by the NATO Science and Technology Organization (STO), Applied Vehicle Technology (AVT) panel forming a devoted Task Group AVT-316 “Vortex Interaction Effects Relevant To Military Air Vehicle Performance”. The paper describes the Missile Facets work on scale resolving simulations with spatial and temporal resolution strategy, quality index for LES and comparison with industry standard RANS methods. The hybrid RANS-LES results provided additional insights into the nature of the complex vortex interactions, including shocks, associated with slender body aerodynamics not detected with RANS. Scale resolving simulations drastically reduced vortex dissipation and resulted in significant shift of rolling moment magnitude.
... The Missile Facet was established to (i) Assess the current capabilities of CFD to predict missile aerodynamic characteristics for flows containing multiple vortex interactions; (ii) Share and seek to learn from comparable experience of applying CFD to other classes of NATO vehicles (combat aircraft, in particular); and (iii) Consolidate lessons learned and any attendant future requirements [1]. This paper is one of a series being presented at this conference to provide a technical overview of the activities and accomplishments of the AVT-316 Missile Facet [2][3][4][5][6][7][8][9][10][11]. The work is still ongoing: a final output, constituting a more detailed and consolidated technical record, will be published by NATO STO towards the end of 2022 [68]. ...
... Several different turbulence closures have been investigated. In a companion paper [6] the results of scale-resolving approaches, such as DES, are reported. ...
... Fig. 32 presents iterative convergence histories of the maximum turbulent Reynolds number obtained using the SA-noft2 model and two different scale resolving models and show that the use of a simple turbulence model results in a maximum turbulent Reynolds number an order of magnitude larger compared to that seen in the scale-resolving simulations. The application of scale-resolving approaches to the OTC1 test case is explored more fully in a companion paper [6]. ...
View Video Presentation: https://doi.org/10.2514/6.2022-0416.vid Within the framework of the NATO Science and Technology Organization Applied Vehicle Technology Task Group AVT316 calculations have been made of the supersonic flow around a slender body with wings and fins. In this paper a synthesis of the results obtained using the Reynolds Averaged Navier-Stokes equations are presented. The results show significant sensitivity to the choice of turbulence model. Whilst the gross features of the flow are similar, details of the development of the leeward wake are different. Simple linear eddy viscosity models predict vortices that rapidly decay, resulting in weak interactions with the downstream fins and relatively small rolling moments. This is attributed to an over production in turbulence quantities that results in excessive effective turbulent viscosity. Interventions that limit the production of turbulence, for example the SST limiter or curvature corrections, results in vortices that grow more slowly, changing the nature of the downstream interactions resulting in increased rolling moment. The use of more complex formulations, such as Reynolds stress models, that are inherently more capable for highly strained flows, further limits the rate of growth of the vortex cores leading to rolling moment predictions that are 2-3 times greater than those obtained with the simplest models.
View Video Presentation: https://doi.org/10.2514/6.2022-2308.vid Time-accurate, turbulence scale resolving simulations of a transonic missile at high incidence and roll angle were performed and compared to simulations using standard Reynolds-Averaged Navier-Stokes turbulence models. The scale resolving simulations showed improved prediction of the total roll moment observed in wind tunnel tests compared to the RANS simulations, but still did not accurately predict the trend in roll moment with incidence angle in the range 15.0°≤σ≤17.5°. The scale resolving simulations predicted less turbulence energy in the vortex structures and larger separation regions over the wings. Spectral analysis of the roll moment signal showed dominant frequency modes consistent with a leading-edge vortex breakdown over two wings.
View Video Presentation: https://doi.org/10.2514/6.2022-1178.vid Vortex dominated flows provide a challenge for modern computational fluid dynamics (CFD) solvers to accurately predict and capture the true physics of the flow. Modeling the formation and propagation of vortices downstream, as well as interactions with other vortices and shocks, are affected by a number of decisions including but not limited to the mesh generated, numerical scheme employed, and modelling assumptions. A research program has been underway for four years to study vortex interaction aerodynamics that are relevant to military air vehicle performance. The program has been conducted under the auspices of the NATO Science and Technology Organization (STO), Applied Vehicle Technology (AVT) panel by a Task Group with the identification of AVT-316. The paper at hand will look at the OTC1 test case [1], which is comprised of a generic missile configuration in supersonic flow, and the influence of the numerical scheme on the predicted results. The decision of the spatial discretization scheme, order of the turbulent flux, and choice of limiter were all shown to have strong influence on the predicted rolling moment of the OTC1 test case.
View Video Presentation: https://doi.org/10.2514/6.2022-0002.vid A research program has been underway for four years to study vortex interaction aerodynamics that are relevant to military air vehicle performance. The program has been conducted under the auspices of the NATO Science and Technology Organization (STO), Applied Vehicle Technology (AVT) panel by a Task Group with the identification of AVT-316. The Task Group was split into two facets: an aircraft facet and a missile facet, each focusing on the vortex interactions associated with airframes of direct interest to NATO. This paper is one of a series being presented at this conference to provide a technical overview of the activities and accomplishments of the AVT-316 missile facet. A substantial body of work has been established pertaining to two generic missile airframes: OTC1 and LK6E2. The test cases studied incorporate multiple vortices and associated interactions and pose a variety of challenges for CFD. Significant progress has been made in consolidating understanding of these challenges and in assessing the current capabilities of CFD to simulate such flows. In so doing, the importance of solution verification has been reinforced, with fine resolution of the flow being required to avoid excessive dissipation. The use of multi-fidelity analysis has also afforded various insights into the physical models being used: for instance, this has allowed compelling demonstrations of the limitations of linear eddy viscosity turbulence models to be provided without recourse to physical test data. As a result of the progress made in these fundamental areas with OTC1, it has been possible to identify features of the overall aerodynamic characteristics of LK6E2 that had been obscured in previous CFD analyses and to subject them to better informed – and therefore more incisive – analysis. The work of the missile facet is ongoing: a final output, constituting a more detailed and consolidated technical record, will be published by NATO STO towards the end of 2022.
View Video Presentation: https://doi.org/10.2514/6.2022-0001.vid A research program has been underway for four years to study vortex interaction aerodynamics that are relevant to military air vehicle performance. The program has been conducted under the auspices of the NATO Science and Technology Organization (STO), Applied Vehicle Technology (AVT) panel by a Task Group with the identification of AVT-316. The Task Group was split into two facets: an aircraft facet and a missile facet, each focusing on the vortex interactions associated with airframes of direct interest to NATO. This paper, one of a series being presented at this conference to provide a technical overview of the activities and accomplishments of the AVT-316 missile facet, provides an overview of its formation, objectives and manner of working. The work of the missile fact is still ongoing: a final output, constituting a more detailed and consolidated technical record, will be published by NATO STO towards the end of 2022.
View Video Presentation: https://doi.org/10.2514/6.2022-0025.vid A research program has been underway for four years to study vortex interaction aerodynamics that are relevant to military air vehicle performance. The program has been conducted under the auspices of the NATO Science and Technology Organization (STO), Applied Vehicle Technology (AVT) panel by a Task Group with the identification of AVT-316. Seven special sessions have been established to highlight accomplishments from the AVT-316 research. An overview of the AVT-316 program is presented in this paper.
View Video Presentation: https://doi.org/10.2514/6.2022-0160.vid The NATO Science and Technology Organization Applied Vehicle Technology Task Group AVT316 study vortical flow for missiles and combat aircraft. RANS and DDES simulations were performed for both cases and show similar physics of vortical development between the two cases. CFD simulations are compared to averaged aircraft experimental data showing that DDES better captures the details of the flow than RANS. When increasing the Reynolds number, the boundary layer flow become complex and interacts with the shear layer feeding the primary vortex in a nonlinear way creating steady substructures. In return, the primary vortex interacts with the boundary layer creating a closed feedback loop. In the missile case, the coefficients are directly impacted by the unsteadiness created at the wing travelling downstream within the vortical structure before hitting the fins.
The AG54 project (RaLESin) has been dedicated to a collaborative effort in conducting validation and verification of hybrid RANS-LES modelling approaches of both zonal and non-zonal types. The general objective of the RaLESin project has been, by means of a systematic computational exploration of hybrid modelling methods, to address the major problem associated to RANS-LES interface using a variety of different approaches to enable effective scale-resolving simulations of typical flow phenomena in aeronautical applications. The project work has consequently led to a set of improved modelling and simulation methods incorporated into partners CFD solvers, serving industrial needs and further facilitating aerodynamic applications of these methods.
Kestrel version 5.0 allows users to employ a background Cartesian mesh for better resolution of flow features. The automatic mesh refinement techniques and core solver are extensions of the SAMCart Cartesian flow solver used with the HPCMP CREATE-AV Helios code. The primary enhancements are integration under the Common Scalable Infrastructure (CSI), the ability to use implicit time-stepping, and the addition of upwind flux schemes. Within these broad categories, we detail the strategy for multi-solver communication through the PUNDIT domain connectivity software, benefits gained from CSI, and the new algorithm options available to users. Finally, we present several examples that validate the added options and demonstrate the dual-mesh approach on various use cases of interest to the defense acquisition community.
In the present paper we study the influence of weak and strong no-slip solid wall boundary conditions on the convergence to steady-state. Our Navier-Stokes solver is edge based and operates on unstructured grids. The two types of boundary conditions are applied to no-slip adiabatic walls. The two approaches are analyzed for a simplified model problem and the reason for the different convergence rates are discussed in terms of the theoretical findings for the model problem. Numerical results for a 2D viscous steady state low Reynolds number problem show that the weak boundary conditions often provide faster convergence. It is shown that strong boundary conditions can prevent the steady state convergence. It is also demonstrated that the two approaches converge to the same solution. Similar results are obtained for high Reynolds number flow in two and three dimensions.
A large-eddy simulation database of homogeneous isotropic decaying turbulence is used to assess four different LES quality
measures that have been proposed in the literature. The Smagorinsky subgrid model was adopted and the eddy-viscosity ‘parameter’
C < sub > S < /sub >CS and the grid spacing hh were varied systematically. It is shown that two methods qualitatively predict the basic features
of an error landscape including an optimal refinement trajectory. These methods are based on variants of Richardson extrapolation
and assume that the numerical error and the modelling error scale with a power of the mesh size. Hence they require the combination
of simulations on several grids. The results illustrate that an approximate optimal refinement strategy can be constructed
based on LES output only, without the need for DNS data. Comparison with the full error landscape shows the suitability of
the different methods in the error assessment for homogeneous turbulence. The ratio of the estimated turbulent kinetic energy
error and the ‘true’ turbulent kinetic energy error calculated from DNS is studied for different Smagorinsky parameters and
different grid sizes. The behaviour of this quantity for decreasing mesh size gives further insight into the reliability of
these methods.
View Video Presentation: https://doi.org/10.2514/6.2022-2308.vid Time-accurate, turbulence scale resolving simulations of a transonic missile at high incidence and roll angle were performed and compared to simulations using standard Reynolds-Averaged Navier-Stokes turbulence models. The scale resolving simulations showed improved prediction of the total roll moment observed in wind tunnel tests compared to the RANS simulations, but still did not accurately predict the trend in roll moment with incidence angle in the range 15.0°≤σ≤17.5°. The scale resolving simulations predicted less turbulence energy in the vortex structures and larger separation regions over the wings. Spectral analysis of the roll moment signal showed dominant frequency modes consistent with a leading-edge vortex breakdown over two wings.
View Video Presentation: https://doi.org/10.2514/6.2022-2307.vid In this study, the influence of the turbulence model, grid resolution and flow solvers on the results of RANS simulations is investigated. The work is part of the NATO AVT Task Group AVT-316 (Vortex Interaction Effects Relevant to Military Air Vehicle Performance). Simulations have been carried out for a generic transonic missile configuration by various organizations and compared with data from wind tunnel experiments for a Mach number of M = 0.85, a roll angle of and total incidences in the range of . In the first part of the study each organization used their own computational mesh and best practice for the grid generation. In the second part a common mesh family (three grids) were used for the simulations. It was shown that even for the common mesh family, the results of the different flow solvers in some cases showed large variations. This applies in particular to the rolling moment. Here, the accurate flow solver-dependent prediction of the complicated flow topology on the large wings and the accurate prediction of the location of the leeward vortices have a particularly strong effect. Only one flow solver predicted this flow in a form that gave good agreement with the wind tunnel results over the entire angular range ().
View Video Presentation: https://doi.org/10.2514/6.2022-1177.vid The complex interaction of forebody and wing vortices significantly impacts missile aerodynamics. The formation of these vortices involves smooth regions of the geometry or geometric discontinuities like leading edges, trailing edges, tips, and corners. Regions of supersonic flow and complex shock topologies interact with boundary layers and vortices. Smooth-body separation and 3D viscous effects strain current Reynolds-averaged Navier-Stokes (RANS) techniques. The quantification and control of discretization error is critical to obtaining reliable simulation results and often turbulence model assessments are made in the presence of unquantified (and potentially large) discretization errors. Two mesh adaptation schemes are applied to steady RANS simulations. Multiscale unstructured mesh adaptation is applied to control interpolation error estimates of the Mach field, which resolves boundary layers, vortices, and shocks. A dual-mesh approach with overset communication is applied between an expert-crafted near-body unstructured mesh and an adaptive off-body Cartesian mesh refined with Q-criterion scaled by the strain tensor magnitude. A generic missile configuration is examined in a supersonic flow field to show the interaction of mesh adaptation and turbulence model. Turbulence model modifications for rotational correction and a quadratic constitutive relationship show a strong influence on adaptive mesh refinement and predicted rolling moment.
View Video Presentation: https://doi.org/10.2514/6.2022-1686.vid Predicting the flowfield around a supersonic store at a high incidence angle is challenging due to the presence of vortices and shocks that interact with each other. The complexity of the problem is further increased by the presence of wing-body and wing-tail junctions giving rise to secondary flows. Given that the flow is turbulent, linear eddy-viscosity turbulence models are unable to account for the secondary flows and are often more dissipative than their non-linear counterparts. The high incidence angle further increases the complexity. This work investigates the effect of grid refinement and turbulence modelling on three store configurations - one with wings and fins, one without fins and wings, and one with wings only. The in-house CFD solver of the University of Glasgow is used to perform simulations at different angles of incidence and roll. Grids consisting of approximately 80 × 10e6 cells or less were found to be inadequate to capture the flow features. This shows that even if a high-order spatial method is employed, a grid of sufficient density must be used to accurately capture the aerodynamic loads of the store. In addition, grid converged results were difficult to obtain for the full configuration due to the interaction of the wing vortices with the store’s fins. Improved convergence was observed for the simplified store configurations. This further showed that the difficulty in grid convergence is related to the wing vortex interactions with the store’s fins.
View Video Presentation: https://doi.org/10.2514/6.2022-0416.vid Within the framework of the NATO Science and Technology Organization Applied Vehicle Technology Task Group AVT316 calculations have been made of the supersonic flow around a slender body with wings and fins. In this paper a synthesis of the results obtained using the Reynolds Averaged Navier-Stokes equations are presented. The results show significant sensitivity to the choice of turbulence model. Whilst the gross features of the flow are similar, details of the development of the leeward wake are different. Simple linear eddy viscosity models predict vortices that rapidly decay, resulting in weak interactions with the downstream fins and relatively small rolling moments. This is attributed to an over production in turbulence quantities that results in excessive effective turbulent viscosity. Interventions that limit the production of turbulence, for example the SST limiter or curvature corrections, results in vortices that grow more slowly, changing the nature of the downstream interactions resulting in increased rolling moment. The use of more complex formulations, such as Reynolds stress models, that are inherently more capable for highly strained flows, further limits the rate of growth of the vortex cores leading to rolling moment predictions that are 2-3 times greater than those obtained with the simplest models.
A new low-dissipation low-dispersion second-order scheme suitable for unstructured finite volume flow solvers is presented that is designed for vortical flows and for scale-resolving simulations of turbulence. The idea is that, by optimizing its dispersion properties, a standard second-order method can be improved significantly for such flows. The key is to include gradient information for computing face values of the fluxes and to use this additional degree of freedom to improve the dispersion properties of the scheme. The scheme is motivated by a theoretical consideration of the dispersion properties for a one-dimensional scalar transport equation problem. Then, the new scheme is applied using the DLR TAU-code for compressible flows and the DLR THETA-code for incompressible flows for simulations of the moving vortex problem and the Taylor–Green vortex flow. The improved accuracy for small-scale transportation and the easy implementation make this scheme a promising candidate for efficient scale-resolving simulations of turbulent flows.
Existing approximate Riemann solvers do not perform well when the grid is not aligned with strong shocks in the flow field. Three new approximate Riemann algorithms are investigated to improve solution accuracy and stability in the vicinity of strong shocks. The new algorithms are compared to the existing upwind algorithms in OVERFLOW 2.1. The new algorithms use a multidimensional pressure gradient based switch to transition to a more numerically dissipative algorithm in the vicinity of strong shocks. One new algorithm also attempts to artificially thicken captured shocks in order to alleviate the errors in the solution introduced by "stair-stepping" of the shock resulting from the approximate Riemann solver. This algorithm performed well for all the example cases and produced results that were almost insensitive to the alignment of the grid and the shock.
doi:10.2514/6.1997-333
The development and preliminary validation of a new k-omega turbulence model based on explicit algebraic Reynolds-stress modeling are presented. This new k-omega model is especially designed for the requirements typical in high-lift aerodynamics. Attention is especially paid to the model behavior at the turbulent/laminar edges, to the model sensitivity to pressure gradients, and to the calibration of the model coefficients for appropriate flow phenomena. The model development is based on both analytical studies and numerical experimenting. The developed;model is assessed and validated for a set of realistic flow problems including high-lift airfoil flows.
Some new developments of explicit algebraic Reynolds stress turbulence models
(EARSM) are presented. The new developments include a new near-wall treatment
ensuring realizability for the individual stress components, a formulation for
compressible flows, and a suggestion for a possible approximation of diffusion terms in
the anisotropy transport equation. Recent developments in this area are assessed and
collected into a model for both incompressible and compressible three-dimensional
wall-bounded turbulent flows. This model represents a solution of the implicit ARSM
equations, where the production to dissipation ratio is obtained as a solution to a
nonlinear algebraic relation. Three-dimensionality is fully accounted for in the mean
flow description of the stress anisotropy. The resulting EARSM has been found to
be well suited to integration to the wall and all individual Reynolds stresses can be
well predicted by introducing wall damping functions derived from the van Driest
damping function. The platform for the model consists of the transport equations for
the kinetic energy and an auxiliary quantity. The proposed model can be used with
any such platform, and examples are shown for two different choices of the auxiliary
quantity.