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STREAmS-2.0: Supersonic turbulent accelerated Navier-Stokes solver version 2.0
No full textWe present STREAmS-2.0, an updated version of the flow solver STREAmS, first introduced in Bernardini et al. (2021) [1]. STREAmS-2.0 has an object-oriented design which separates the physics equations from the specific back-end, making the code more suitable for future expansions, such as porting to novel computing architectures or implementation of additional flow physics. Similarly to the previous version, STREAmS-2.0 supports NVIDIA-GPU and CPU back-ends. Additionally, this version features improvements of the input/output data management, new energy and entropy preserving schemes for the discretization of the convective fluxes, recycling/rescaling inflow boundary condition, and a model for thermally perfect gases with variable specific heats. New version program summary: Program Title: STREAmS CPC Library link to program files: https://doi.org/10.17632/hdcgjpzr3y.2 Developer's repository link: https://github.com/STREAmS-CFD/STREAmS-2 Licensing provisions: GPLv3 Programming language: Fortran, CUDA Journal reference of previous version: M. Bernardini, D. Modesti, F. Salvadore, and S. Pirozzoli. STREAmS: a high-fidelity accelerated solver for direct numerical simulation of compressible turbulent flows. Comput. Phys. Commun. 263 (2021) 107906. Does the new version supersede the previous version?: Yes. Reasons for the new version: New code structure and release of new features. Summary of revisions: • The original solver [1] has been rewritten following an object-oriented design implemented through Fortran derived types that include variables and type bound procedures. The new software architecture has been designed to increase modularity and extensibility of the code, allowing users to add new back-ends and physics equations while maintaining the same code structure. This allows users to reuse portions of the code that are independent of the physics equations, the back-end, or both. The layer of computing procedures maintains a lean structure that can be highly optimized with respect to the implemented back-end. • Input handling is now based on the classic.ini format improving both user readability and input data management. • A family of new kinetic energy and entropy preserving schemes (KEEP) are now available and can be selected for stable, non-dissipative and accurate spatial discretization of the convective terms of the Navier–Stokes equations in smooth flow regions [2]. Concerning the shock-capturing flux, the improved low-dissipative WENO-Z scheme proposed by [3] is now available. • New inflow boundary conditions based on the recycling/rescale approach [4] have been implemented for the simulation of spatially evolving compressible turbulent boundary layers. Moreover, a new inflow condition based on the solution of the compressible Blasius equation is available to take into account the case of laminar boundary layers. • The constitutive relations have been generalized to take into account thermally perfect gases with variable specific heats, approximated with polynomial functions of the temperature that can be specified by the user [5]. • A new stretching function has been implemented to improve the distribution of grid nodes for the computation of wall-bounded turbulent flows. The formulation blends uniform near-wall spacing with uniform resolution in terms of Kolmogorov units in the outer wall layer, guaranteeing accuracy with higher computational efficiency [6]. Nature of problem: The code solves the compressible Navier–Stokes equations in Cartesian coordinates for a thermally perfect gas. The solver is designed for direct numerical simulation (DNS) of compressible supersonic turbulent boundary layers and various canonical configurations are supported, including turbulent channel flow, laminar and turbulent boundary layer and shock-wave/boundary layer interaction. Solution method: The equations are discretized using high-order finite difference approximations with hybrid low-dissipative/shock-capturing capabilities and the time advancement is performed using a Runge–Kutta scheme. References: [1] M. Bernardini, D. Modesti, F. Salvadore, S. Pirozzoli, STREAmS: A high-fidelity accelerated solver for direct numerical simulation of compressible turbulent flows, Comput. Phys. Commun. 263 (2021) 107906. [2] Y. Tamaki, Y. Kuya, S. Kawai, Comprehensive analysis of entropy conservation property of non-dissipative schemes for compressible flows: KEEP scheme redefined, J. Comput. Phys. 468 (2022) 111494. [3] R. Borges, M. Carmona, B. Costa, W. Don, An improved weighted essentially non-oscillatory scheme for hyperbolic conservation laws, J. Comput. Phys. 227 (6) (2008) 3191–3211, https://doi.org/10.1016/j.jcp.2007.11.038 [4] S. Pirozzoli, M. Bernardini, F. Grasso, Direct numerical simulation of transonic shock/boundary layer interaction under conditions of incipient separation, J. Fluid Mech. 657 (2010) 361–393. [5] B. J. McBride, M. J. Zehe, S. Gordon, NASA Glenn coefficients for calculating thermodynamic properties of individual species, NASA/TP 211556, NASA, 2002. [6] S. Pirozzoli, P. Orlandi, Natural grid stretching for DNS of wall-bounded flows, J. Comput. Phys. 439 (2021) 110408.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic
Direct numerical simulation of forced thermal convection in square ducts up to
Full text linkWe carry out direct numerical simulation (DNS) of flow in a turbulent square
duct by focusing on heat transfer effects, considering the case of unit Prandtl
number. Reynolds numbers up to are considered which are
much higher than in previous studies, and which yield clear scale separation
between inner- and outer-layer dynamics. Close similarity between the behavior
of the temperature and the streamwise velocity fields is confirmed as in
previous studies related to plane channels and pipes. Just like the mean
velocity, the mean temperature is found to exhibit logarithmic layers as a
function of the nearest wall, however with a different slope. The most
important practical implication is the validity of the traditional hydraulic
diameter as the correct reference length for reporting heat transfer data, as
we rigorously show here. Temperature and velocity fluctuations also have
similar behavior, but apparently logarithmic growth of their inner-scaled peak
variances is not observed here unlike in canonical wall-bounded flows. Analysis
of the split contributions to the heat transfer coefficient shows that mean
cross-stream convection associated with secondary motions is responsible for
about of the total. Finally, we use the DNS database to highlight
shortcomings of traditional linear closures for the turbulent heat flux, and
show that substantial modeling improvement may be in principle obtained by
retaining at least the three terms in the vector polynomial integrity basis
expansion
Permeability and Turbulence Over Perforated Plates
No full textWe perform direct numerical simulations of turbulent flow at friction Reynolds number Reτ≈ 500 - 2000 grazing over perforates plates with moderate viscous-scaled orifice diameter d+≈ 40 - 160 and analyse the relation between permeability and added drag. Unlike previous studies of turbulent flows over permeable surfaces, we find that the flow inside the orifices is dominated by inertial effects, and that the relevant permeability is the Forchheimer and not the Darcy one. We find evidence of a fully rough regime where the relevant length scale is the inverse of the Forchheimer coefficient, which can be regarded as the resistance experienced by the wall-normal flow. Moreover, we show that, for low porosities, the Forchheimer coefficient can be estimated with good accuracy using a simple analytical relation.In this article the author name Stefan Hickel was incorrectly written as Hickel Stefan. The original article has been corrected.Aerodynamic
Acoustic liners and their induced drag
No full textIn order to reduce the noise emitted by aircraft engines, the nacelle is coated with acoustic liners. An undesirable effect of these surfaces is that they increase the aerodynamic drag. In the present work, we characterize this type of surface roughness by performing Direct Numerical Simulations of fully resolved acoustic liner geometries. We find evidence of a fully rough regime, whose onset is determined by the value of the viscous-scaled Forchheimer coefficient. Moreover, the intensity of the wall-normal velocity fluctuations at the wall also scales with the viscous-scaled wall-normal permeability, leading to a relation between fluctuations and added drag.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic
Direct numerical simulation of forced thermal convection in square ducts
No full textWe carry out direct numerical simulation (DNS) of flow in a turbulent square duct by focusing on heat transfer effects, considering the case of unit Prandtl number. Reynolds numbers up to Reτ ≈ 2000 are considered which are much higher than in previous studies, and which yield clear scale separation between inner- and outer-layer dynamics. Close similarity between the behavior of the temperature and the streamwise velocity fields is confirmed as in previous studies related to plane channels and pipes. We find good agreement between the Nusselt number of square duct and circular pipe flow when the Reynolds number based on the hydraulic diameter is used, thus corroborating the common engineering practice. Popular engineering correlations for the heat transfer reveal deviations up to 5% with respect to DNS data, which are nicely fitted by a power law.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic
Direct Numerical Simulation of a Turbulent Boundary Layer over Acoustic Liners
No full textThe nacelle of aircraft engines is coated with acoustic liners to reduce engine noise. An undesirable effect of these liners is that they increase aerodynamic drag. We study this drag penalty by performing Direct Numerical Simulations of a turbulent boundary layer over an acoustic liner array at friction Reynolds number, Re τ ≈ 850–2500. We use this simulation to confirm several findings that we recently brought forward using a simpler channel flow setup. We show that acoustic liners lead to high wall-normal velocity fluctuations that can be directly correlated with a modulation of the classical near-wall cycle and to an increase in drag. We also confirm that the acoustic liners act as permeable surface roughness and the non-linear Forchheimer coefficient is the relevant permeability parameter for scaling the drag increase.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic
A priori tests of eddy viscosity models in square duct flow
No full textWe carry out a priori tests of linear and nonlinear eddy viscosity models using direct numerical simulation (DNS) data of square duct flow up to friction Reynolds number Re τ= 1055. We focus on the ability of eddy viscosity models to reproduce the anisotropic Reynolds stress tensor components aij responsible for turbulent secondary flows, namely the normal stress a22 and the secondary shear stress a23. A priori tests on constitutive relations for aij are performed using the tensor polynomial expansion of Pope (J Fluid Mech 72:331–340, 1975), whereby one tensor base corresponds to the linear eddy viscosity hypothesis and five bases return exact representation of aij. We show that the bases subset has an important effect on the accuracy of the stresses and the best results are obtained when using tensor bases which contain both the strain rate and the rotation rate. Models performance is quantified using the mean correlation coefficient with respect to DNS data C~ ij, which shows that the linear eddy viscosity hypothesis always returns very accurate values of the primary shear stress a12 (C~ 12> 0.99), whereas two bases are sufficient to achieve good accuracy of the normal stress and secondary shear stress (C~ 22= 0.911 , C~ 23= 0.743). Unfortunately, RANS models rely on additional assumptions and a priori analysis carried out on popular models, including k–ε and v2–f, reveals that none of them achieves ideal accuracy. The only model based on Pope’s expansion which approaches ideal performance is the quadratic correction of Spalart (Int J Heat Fluid Flow 21:252–263, 2000), which has similar accuracy to models using four or more tensor bases. Nevertheless, the best results are obtained when using the linear correction to the v2–f model developed by Pecnik and Iaccarino (AIAA Paper 2008-3852, 2008), although this is not built on the canonical tensor polynomial as the other models.Aerodynamic
Direct numerical simulation of one-sided forced thermal convection in plane channels
No full textWe carry out direct numerical simulations (DNS) of turbulent flow and heat transfer in pressure-driven plane channels, by considering cases with heating on both walls, as well as asymmetric heating limited to one of the channel walls. Friction Reynolds numbers up to are considered, and Prandtl numbers from to, the temperature field being regarded as a passive scalar. Whereas cases with symmetric heating show close similarity between the temperature and the streamwise velocity fields, with turbulent structures confined to either half of the channel, in the presence of one-sided heating the temperature field exhibits larger regions with coherent fluctuations extending beyond the channel centreline. Validity of the logarithmic law for the mean temperature is confirmed, as well as universality of the associated von Kármán constant, which we estimate to be. Deviations from the logarithmic behaviour are much clearer in cases with one-sided heating, which feature a wide outer region with parabolic mean temperature profile. The DNS data are exploited to construct a predictive formula for the heat transfer coefficient as a function of both Reynolds and Prandtl number. We find that the reduction of the thermal efficiency in the one-sided case is approximately at unit Prandtl number; however, it can become much more significant at low Prandtl number. </p
Reynolds-Averaged Navier–Stokes Simulations of Unyawed and Yawed Rotating Wheels
No full textAerodynamics has been an important aspect of the automotive industry for decades with the wheels being a notable contributing factor. They are responsible for up to 25% of the drag in the case of a general passenger car and up to 30%-50% for an open-wheeled race car. In this research, the aerodynamic characteristics of an isolated rotating wheel in contact with the ground will be investigated using RANS simulations. Open-source software OpenFOAM is used for the simulations and the mesh is generated using cfMesh. The wheel geometry used in this work is the "Fackrell A2" and the contact region is modelled using the step size approach. Firstly, the sensitivity of the step size, mesh fineness and domain size is assessed for an unyawed wheel and the k −ω SST, Realizable k − ε and Spalart-Allmaras are tested. Among these models, the Realizable k −ε model is chosen to investigate the effect of yaw and Reynolds number. The yaw is investigated up to 10° in increments of 2° and the Reynolds number effect is investigated for the Reynolds number range of 10 000 — 1 000 000. The results show that yawing the wheel yields a fairly linear increase in the drag coefficient and the side force coefficient. Furthermore, only a significant increase of the lift coefficient is observed when going from a yaw of 4° to 6°. Moreover, the wake becomes asymmetric with increasing yaw. The vortex at the leeside on the ground becomes bigger while the vortex at the windward side becomes smaller. Additionally, a new vortex in the upper part of the wake further downstream is formed and the wake becomes shorter. Increasing the Reynolds number, the value of the drag coefficient decreases and of the lift coefficient stays approximately the same. Moreover, the Reynolds number seems to affect the pressure peaks upstream and downstream of the contact patch. A lower value results in larger magnitude peaks. Finally, in the investigated Reynolds number range the wake structures are the same. However, the wake is bigger when the Reynolds number is smaller and asymmetry was observed in the wake for ReD = 1 000 000, which can be caused by the asymmetry of the wheel.Aerospace Engineerin
Rotating Discs Actuators: Direct Numerical Simulation for Turbulent SkinFriction Drag Reduction
No full textIn the aviation, maritime and oil&gas sector, the potential benefit in yearly savings deriving from the reduction of turbulent skin-friction drag through active flow control are estimated in the millions of U.S. Dollars and million tonnes of CO2 emissions. Research in this topic has brought forward numerous techniques, offering enticing drag reduction performance. However, no technique has yet proven to be suitable for large scale application in the aviation sector. A small research stream has been initiated in the field of rotating disc actuators for skin-friction drag reduction. Currently, research on this topic is limited to low Reynolds number numerical simulations, that are not representative of flight conditions.This thesis project aims at investigating the disc actuator performance in terms of drag reduction and power balance at friction Reynolds numbers higher than current literature on disc actuators. This has been achieved through direct numerical simulations, using an incompressible channel flow solver adapted for the implementation of the disc actuators. In total two disc configurations have been tested at friction Reynolds number Reτ=180, 550 and Reτ =1000. The disc diameter and tip velocity scaled in viscous units are kept constant across the Reτ range. The simulation results confirm the positive drag reduction performance observed in previous literature at Reτ =180. With increasing Reτ, a decrease in drag reduction performance is observed, with skin-friction drag reduction going from 21.6% at Reτ =180 to 16.01% Reτ =1000. The net power saving of the disc actuators show small variation with increasing Reτ, with an optimal net power saving at Reτ =1000 at -3.2%. Visualizations of local velocity profiles and local time averaged velocity have highlighted the disc influence to be limited in the region below y+< 400. Thus, the performance of disc actuators at friction Reynolds number higher than Reτ =1000 should be less influenced by the Reynolds number. Comparison with experimental testing of disc actuators from a parallel thesis study have shown good qualitative agreement between experimental and computational results. Lastly, the consistency in the drag reduction, net power balance result and disc velocity distribution within the boundary layer seem to reinforce the hypothesis that the disc performance scales with the disc diameter and tip velocity expressed in wall units.The outlook of this research shows that the performance of disc actuators at friction Reynolds numbers more representative for flight applications puts disc actuators on par with other passive skin-friction drag reduction techniques such as riblets in terms of net power saving. However, the reinforcement of the hypothesis that disc performance scales in wall units may result in too small actuators for flight conditions, resulting in more difficult full scale implementation.Aerospace Engineerin
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