Direct Numerical Simulation Of Transition Research Articles

Direct numerical simulation of transition under free-stream turbulence and the influence of large integral length scales

Under the action of free-stream turbulence (FST), elongated streamwise streaky structures are generated inside the boundary layer, and their amplitude and wavelength are crucial for the transition onset. While turbulence intensity is strongly correlated with the transitional Reynolds number, characteristic length scales of the FST are often considered to have a slight impact on the transition location. However, a recent experiment by Fransson and Shahinfar [J. Fluid Mech. 899, A23 (2020)] shows significant effects of FST scales. They found that, for higher free-stream turbulence levels and larger integral length scales, an increase in the length scale postpones transition, contrary to established literature. Here, by performing well-resolved numerical simulations, we aim at understanding why the FST integral length scale affects the transition location differently at low- and high turbulence levels. We found that the integral length scales in Fransson and Shahinfar's experiment are so large that the introduced wide streaks have substantially lower growth in the laminar region, upstream of the transition to turbulence, than the streaks induced by smaller integral length scales. The energy in the boundary layer subsequently propagate to smaller spanwise scales as a result of the nonlinear interaction. When the energy has reached smaller spanwise scales, larger amplitude streaks results in regions where the streak growth are larger. It takes longer for the energy from wider streaks to propagate to the spanwise scales associated with the breakdown to turbulence, than for those with smaller spanwise scales. Thus, there is a faster transition for FST with lower integral length scales in this case.

Direct numerical simulation of transition in a differentially heated vertical channel

The transition mechanisms of natural convection flows ensuing in a fluid layer between two differentially heated vertical plates at Prandtl number Pr = 0.71 are investigated by means of Direct Numerical Simulations. In accordance with several previous studies, results show a first bifurcation from the so-called laminar conduction regime to steady convection at Rayleigh number Ra = 5708. On the other hand, the subsequent transition to turbulence appears to be accompanied by a great sensitivity to some fundamental numerical choices such as domain size, accuracy, resolution and amplitude of the imposed perturbations. Results reveal the occurrence of a bifurcation branch which leads the system to chaos via a second bifurcation to a steady-state, a Hopf bifurcation and, seemingly, a period-doubling cascade. Although the described scenario compares well with previous findings, some doubts persist upon the possible pitfalls in the use of numerical simulation for the study of transition in this kind of systems. Indeed, several numerical aspects are found to become of crucial importance for the prediction of the dynamical behaviour and heat transfer rate of the system.

A mesoscopic modelling approach for direct numerical simulations of transition to turbulence in hypersonic flow with transpiration cooling

A rescaling methodology is developed for high-fidelity, cost-efficient direct numerical simulations (DNS) of flow through porous media, modelled at mesoscopic scale, in a hypersonic freestream. The simulations consider a Mach 5 hypersonic flow over a flat plate with coolant injection from a porous layer with 42 % porosity. The porous layer is designed using a configuration studied in the literature, consisting of a staggered arrangement of cylinder/sphere elements. A characteristic Reynolds number Rec of the flow in a pore cell unit is first used to impose aerodynamic similarity between different porous layers with the same porosity, ∊, but different pore size. A relation between the pressure drop and the Reynolds number is derived to allow a controlled rescaling of the pore size from the realistic micrometre scales to higher and more affordable scales. Results of simulations carried out for higher cylinder diameters, namely 24 μm, 48 μm and 96 μm, demonstrate that an equivalent Darcy-Forchheimer behaviour to the reference experimental microstructure is obtained at the different pore sizes. The approach of a porous layer with staggered spheres is applied to a 3D domain case of porous injection in the Darcy limit over a flat plate, to study the transition mechanism and the associated cooling performance, in comparison with a reference case of slot injection. Results of the direct numerical simulations show that porous injection in an unstable boundary layer leads to a more rapid transition process, compared to slot injection. On the other hand, the mixing of coolant within the boundary layer is enhanced in the porous injection case, both in the immediate outer region of the porous layer and in the turbulent region. This has the beneficial effect of increasing the cooling performance by reducing the temperature near the wall, which provides a higher cooling effectiveness, compared to the slot injection case, even with an earlier transition to turbulence.

A High Accuracy Preserving Parallel Algorithm for Compact Schemes for DNS

A new accuracy-preserving parallel algorithm employing compact schemes is presented for direct numerical simulation of the Navier-Stokes equations. Here the connotation of accuracy preservation is having the same level of accuracy obtained by the proposed parallel compact scheme, as the sequential code with the same compact scheme. Additional loss of accuracy in parallel compact schemes arises due to necessary boundary closures at sub-domain boundaries. An attempt to circumvent this has been done in the past by the use of Schwarz domain decomposition and compact filters in “A new compact scheme for parallel computing using domain decomposition,” J. Comput. Phys. 220, 2 (2007), 654--677, where a large number of overlap points was necessary to reduce error. A parallel compact scheme with staggered grids has been used to report direct numerical simulation of transition and turbulence by the Schwarz domain decomposition method. In the present research, we propose a new parallel algorithm with two benefits. First, the number of overlap points is reduced to a single common boundary point between any two neighboring sub-domains, thereby saving the number of points used, with resultant speed-up. Second, with a proper design, errors arising due to sub-domain boundary closure schemes are reduced to a user designed error tolerance, bringing the new parallel scheme on par with sequential computing. Error reduction is achieved by using global spectral analysis, introduced in “Analysis of central and upwind compact schemes,” J. Comput. Phys. 192, 2, (2003) 677--694, which analyzes any discrete computing method in the full domain integrally. The design of the parallel compact scheme is explained, followed by a demonstration of the accuracy of the method by solving benchmark flows: (1) periodic two-dimensional Taylor-Green vortex problem; (2) flow inside two-dimensional square lid-driven cavity (LDC) at high Reynolds number; and (3) flow inside a non-periodic three-dimensional cubic LDC with the staggered grid arrangement.

Direct numerical simulation of the transition to turbulence in a supersonic boundary layer on smooth and rough surfaces

Direct numerical simulations of instability development and transition to turbulence in a supersonic boundary layer on a flat plate are performed. The computations are carried out for moderate supersonic (free-stream Mach number M = 2) and hypersonic (M = 6) velocities. The boundary layer development is simulated, which includes the stages of linear growth of disturbances, their nonlinear interaction, stochastization, and turbulent flow formation. A laminar–turbulent transition initiated by distributed roughness of the plate surface at the Mach number M = 2 is also considered.

Direct numerical simulation of transition to turbulence in a supersonic boundary layer

Based on full unsteady compressible Navier–Stokes equations a direct numerical simulation of the linear and nonlinear stages of the laminar-turbulent transition in boundary layer of a flate plate at the freestream Mach number M = 2 is carried out.

Direct numerical simulation of transition in a sharp cone boundary layer at Mach 6: fundamental breakdown

Direct numerical simulations (DNS) were performed to investigate the laminar–turbulent transition in a boundary layer on a sharp cone with an isothermal wall at Mach 6 and at zero angle of attack. The motivation for this research is to make a contribution towards understanding the nonlinear stages of transition and the final breakdown to turbulence in hypersonic boundary layers. In particular, the role of second-mode fundamental resonance, or (K-type) breakdown, is investigated using high-resolution ‘controlled’ transition simulations. The simulations were carried out for the laboratory conditions of the hypersonic transition experiments conducted at Purdue University. First, several low-resolution simulations were carried out to explore the parameter space for fundamental resonance in order to identify the cases that result in strong nonlinear interactions. Subsequently, based on the results from this study, a set of highly resolved simulations that proceed deep into the turbulent breakdown region have been performed. The nonlinear interactions observed during the breakdown process are discussed in detail in this paper. A detailed description of the flow structures that arise due to these nonlinear interactions is provided and an analysis of the skin friction and heat transfer development during the breakdown is presented. The controlled transition simulations clearly demonstrate that fundamental breakdown may indeed be a viable path to complete breakdown to turbulence in hypersonic cone boundary layers at Mach 6.

Direct numerical simulations of transition and turbulence in smooth-walled Stokes boundary layer

Stokes boundary layer (SBL) is a time-periodic canonical flow that has several environmental, industrial, and physiological applications. Understanding the hydrodynamic instability and turbulence in SBL, therefore, will shed more light on the nature of such flows. Unlike its steady counterpart, the flow in a SBL varies both in space and time, which makes hydrodynamic instability and transition from laminar to turbulent state highly complicated. In this study, we utilized direct numerical simulations (DNS) to understand the characteristics of hydrodynamic instability, the transition from laminar to turbulent state, and the characteristics of intermittent turbulence in a smooth SBL for $Re_\Delta$ReΔ in the range of 500–1000. Simulation results show that nonlinear growth plays a critical role on the instability at $Re_\Delta = 500$ReΔ=500 and 600. However, the nonlinear growth does not warrant sustainable transition to turbulence and the outcome is highly dependent on the amplitude and spatial distribution of the initial velocity disturbance in addition to $Re_\Delta $ReΔ. Simulation results at $Re_\Delta = 500$ReΔ=500 confirm that instability and subsequent transitional flow will eventually decay. At $Re_\Delta = 600$ReΔ=600 nonlinear growth recurs at every modulation period but such transition does not evolve into fully developed turbulence at any time in the modulation cycle. At $Re_\Delta = 700$ReΔ=700, the flow shows features of fully developed turbulence during some modulation periods and the transitional character of $Re_\Delta = 600$ReΔ=600 at the remaining. Therefore, we conclude that flow in the range of $Re_\Delta = 600$ReΔ=600–700 is to be classified as self-sustaining transitional flow. For higher Reynolds number the flow indeed exhibits features of fully developed boundary layer turbulence for a portion of the wave period, which is known as the intermittently turbulent regime in the literature.

Direct numerical simulation of transition in MHD duct flow

AbstractTransition in the flow of electrically conducting fluid in a square duct with insulating walls is studied by direct numerical simulations. A uniform magnetic field is applied in the transverse direction. Moderate values of the Reynolds (Re = 5000) and Hartmann (Ha = 0 … 30) numbers are considered that correspond to the classical Hartmann & Lazarus [1] experiments. It is shown that the laminarization begins in the Hartmann layers, whereas the sidewall layers remain turbulent. Complete re‐laminarization occurs in the range of R = Re/Ha ≈︁ 220, which is in agreement with the H. & L. experiments. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-order shock-fitting methods for direct numerical simulation of hypersonic flow with chemical and thermal nonequilibrium

In recent years, much progress has been made in the direct numerical simulation of laminar-turbulent transition of hypersonic boundary layer flow. However, most of the efforts at the direct numerical simulation of transition previously have been focused on the idealized perfect gas flow or “cold” hypersonic flows. For practical problems in hypersonic flows, high-temperature effects of thermal and chemical nonequilibrium are important and cannot be modeled by a perfect gas model. Therefore, it is necessary to include the real gas models in the numerical simulation of hypersonic boundary layer transition in order to accurately predict flow field parameters. Currently most numerical methods for hypersonic flow with thermo-chemical nonequilibrium are based on shock-capturing approach at relatively low order of accuracy. Shock capturing schemes reduce to first-order accuracy near the shock and have been shown to produce spurious oscillations behind curved strong shocks. There is a need to develop new methods capable of simulating nonequilibrium hypersonic flow fields with uniformly high-order accuracy and avoid spurious oscillations near the shock. This paper presents a fifth-order shock-fitting method for numerical simulation of thermal and chemical nonequilibrium in hypersonic flows. The method is developed based on the state-of-the-art real gas models for thermo-chemical nonequilibrium and transport phenomena. Shock-fitting approach is used because it has the advantage of capturing the entire flow field with high-order accuracy and without any oscillations near the shock. The new method has been tested and validated for a number of test cases over a wide span of free stream conditions. The developed method is applied for the study of receptivity of free stream acoustic waves over a blunt cone for hypervelocity flow. Some preliminary results of the computations of the high order shock fitting method for the above mentioned study have also been presented.

Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence

The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes laminar separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the laminar separation point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it laminar or a moderate free-stream turbulence of intensityTu~ 3%. Upstream of the separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the laminar boundary layer to separate. The separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased,Tu~ 6.5%, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer separation, is observed in the wake of the turbulent events.

Regularization-based sub-grid scale (SGS) models for large eddy simulations (LES) of high-Re decaying isotropic turbulence

Regularization-based subgrid-scale (SGS) turbulence models for large eddy simulations (LES) are quantitatively assessed for decaying homogeneous turbulence (DHT) and transition to turbulence for the Taylor-Green vortex (TGV) through comparisons to laboratory measurements and direct numerical simulations (DNS). LES predictions using the Leray-α, LANS-α, and Clark-α regularization-based SGS models are compared to the classic nondynamic Smagorinsky model. Regarding the regularization models, this work represents their first application to relatively high-Re decaying turbulence with comparison to the active-grid-generated decaying turbulence measurements of Kang et al. (J. Fluid Mech., 2003) at Re λ ≈ 720 and the Re = 3000 DNS of transition to turbulence in the TGV of Drikakis et al. (J. Turbulence, 2007). For DHT the nondynamic Smagorinsky model is in excellent agreement with measurements for the turbulent kinetic energy and energy spectra but higher-order moments show slight discrepancies. For TGV too, the energy decay rates of Smagorinsky agree reasonably well with DNS. Regarding the regularization models stable results are not obtained as compared to the Smagorinsky model at the same grid resolution for various values of α; and at higher grid resolutions, they are in worse agreement. However, with additional dissipation such as in mixed α-Smagorinsky models, results are acceptable and show only slight deviations from Smagorinsky.

Direct Numerical Simulation of Hypersonic Boundary Layer Transition over a Blunt Cone

Direct numerical simulation of transition How over a blunt cone with a freestream Mach number of 6, Reynolds number of 10,000 based on the nose radius, and a 1-deg angle of attack is performed by using a seventh-order weighted essentially nonoscillatory scheme for the convection terms of the Navier-Stokes equations, together with an eighth-order central finite difference scheme for the viscous terms. The wall blow-and-suction perturbations, including random perturbation and multifrequency perturbation, are used to trigger the transition. The maximum amplitude of the wall-normal velocity disturbance is set to 1% of the freestream velocity. The obtained transition locations on the cone surface agree well with each other far both cases. Transition onset is located at about 500 times the nose radius in the leeward section and 750 times the nose radius in the windward section. The frequency spectrum of velocity and pressure fluctuations at different streamwise locations are analyzed and compared with the linear stability theory. The second-mode disturbance wave is deemed to be the dominating disturbance because the growth rate of the second mode is much higher than the first mode. The reason why transition in the leeward section occurs earlier than that in the windward section is analyzed. It is not because of higher local growth rate of disturbance waves in the leeward section, but because the growth start location of the dominating second-mode wave in the leeward section is much earlier than that in the windward section.

Direct numerical simulation of transition to turbulence in an oscillatory channel flow

In this Note, we present results of the numerical simulation of transition to turbulence for a purely oscillatory channel flow. These simulations were performed for various values of the Reynolds number, the so-called Stokes parameter being equal to 4. The methodology used for the flow simulation relies on a combination of finite element space approximations with time-discretization by operator splitting; it has shown to be very effective, even when it is applied to relatively complex domains with strong expansions at the inlet and outlet of the channel. The numerical results obtained agree qualitatively well with previous experiments by other investigators. To cite this article: L.H. Juárez, E. Ramos, C. R. Mecanique 331 (2003).

Refined Interaction Method for Direct Numerical Simulation of Transition in Separation Bubbles

In direct numerical simulations (DNS) of laminar-turbulent transition in laminar separation bubbles, the definition of a well-posed freestream boundary condition is crucial. Different, partially contradicting properties are required: e rst, an adverse pressure gradient has to be imposed to force separation. Moreover, oscillations at the freestream boundary caused by disturbance waves, which extend into the potential e ow, have to be treated accurately. Finally, displacement effects of the separation bubble on the surrounding potential e ow by the so-called viscous-inviscid boundary-layer interaction have to be captured. Usually, either the integration domain has to be sufe ciently high, or a state-of-the-art boundary-layer interaction model based on the theory of thin airfoils can be applied. At a high Reynolds number at separation, neither possibility is applicable. Therefore, an improved model has been developed that meets the just-mentioned requirements. The method is validated by variations of the height of the integration domain. It is shown that boundary-layer interaction has not only quantitative but even qualitative impact on the separation bubble. The comparison of a highly resolved DNS with an experiment proves the applicability of the present method for separation bubbles on airfoils. Nomenclature Cu, cu,ij = matrix and its coefe cients; Eqs. (15) and (16) Cv, cv,ij = matrix and its coefe cients; Eqs. (12) and (13) c = chord length ci = blending function in interaction model fc = polynomial function (interaction model )

Direct numerical simulation of transition and turbulence in compressible mixing layer

The three-dimensional compressible Navier-Stokes equations are approximated by a fifth order upwind compact and a sixth order symmetrical compact difference relations combined with threestage Ronge-Kutta method. The computed results are presented for convective Mach number Mc = 0.8 and Re = 200 with initial data which have equal and opposite oblique waves. From the computed results we can see the variation of coherent structures with time integration and full process of instability, formation of ∧ -vortices, double horseshoe vortices and mushroom structures. The large structures break into small and smaller vortex structures. Finally, the movement of small structure becomes dominant, and flow field turns into turbulence. It is noted that production of small vortex structures is combined with turning of symmetrical structures to unsymmetrical ones. It is shown in the present computation that the flow field turns into turbulence directly from initial instability and there is not vortex pairing in process of transition. It means that for large convective Mach number the transition mechanism for compressible mixing layer differs from that in incompressible mixing layer.

Refined interaction method for direct numerical simulation of transition in separation bubbles

Refined interaction method for direct numerical simulation of transition in separation bubbles

Direct numerical simulation of transition in pipe flow under the influence of wall disturbances

In this paper we consider a direct numerical simulation of transition from laminar to turbulent flow excited by wall disturbances in a cylindrical pipe. The wall disturbances are imposed by means of blowing and suction through the pipe wall. Two cases are considered: periodic suction/blowing (PSB) and random suction/blowing (RSB). The former case can be interpreted as a pipe flow in combination with a periodic roughness element of a fixed size and the latter as a pipe with randomly distributed wall roughness elements. For the PSB case the critical amplitude of the blowing/suction velocity that causes transition is obtained. The dependence of this critical amplitudes on the Reynolds number is considered. In addition the dependence of the transition time on the suction/blowing amplitudes and the Reynolds number is investigated. This latter parameter is also considered for the RSB case.

Direct numerical simulation of transition in the pipe flow with roughness

The paper investigates the development of transition in a cylindrical pipe flow with roughness. The problem is tackled here by direct numerical simulation with spectral and spectral element methods. The roughness is simulated by periodic suction/blowing (PSB) applied to the pipe wall for regular wall roughness. Randomly distributed wall roughness is achieved by putting random suction/blowing (RSB) on the pipe wall. The critical amplitude that causes transition is obtained and also the dependence of the critical amplitudes on the Reynolds number. The dependence of the transition time on the roughness amplitudes and the Reynolds numbers is also investigated.

Comparison of temporal and spatial direct numerical simulation of compressible boundary-layer transition

Two models are used in the direct numerical simulation of transitional boundary-layer flows, the spatial model and the temporal model. The spatial approach is the closest realization of a transition experiment but is significantly more expensive than the temporal approach, which employs a local parallel flow assumption in its formulation. Because of the parallel flow assumption, a temporal approach cannot take into account the nonparallel effects of boundary-layer flows properly. The consequence of this defect has never been addressed adequately. In this study, the results from various temporal approaches are compared extensively to those obtained by our two newly developed spatial direct numerical simulation codes for nonlinear subharmonic transition and oblique wave breakdown at Mach numbers 1.6 and 4.5. A recently developed new temporal approach that can include some nonparallel effects in the computation is also introduced. The use of our new formulation is essential in obtaining quantitative agreement with the spatial approach at high Mach numbers, whereas the standard temporal approach does not give satisfactory results.