Direct Numerical Simulations of Transitional Boundary Layer over a Flat Plate in Hypersonic Free-Stream

Новиков, А. В.; Егоров, И. В.

doi:10.2514/6.2016-3952

Cited by 6 publications

(5 citation statements)

References 18 publications

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“…The boundary conditions are similar to those used in the 2D simulations at the inflow, outflow, wall and farfield boundaries, and the spanwise direction is assumed to be periodic. 3D wavepackets are introduced though wall-blowing-suction, using the following actuator model 43,44 :…”

Section: V1 General Features Of 3d Response To Wavepacket Forcingmentioning

confidence: 99%

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Instability characteristics of cooled hypersonic boundary layers

Unnikrishnan

Gaitonde

2020

AIAA Scitech 2020 Forum

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Wall cooling has substantial qualitative and quantitative effects on the development of instabilities and subsequent transition processes in hypersonic boundary layers (HBLs). A sequence of linear stability theory, two-dimensional and non-linear three-dimensional direct numerical simulations is used to analyze Mach 6 boundary layers, with wall temperatures ranging from near-adiabatic to highly cooled conditions, where the second-mode instability is accompanied by radiation of energy. Decomposition of linear stability modes into their fluid-thermodynamic (acoustic, vortical and thermal) components shows that this radiation comprises both acoustic as well as vortical waves. Furthermore, in these cases, 2D simulations show that the conventional "trapped" nature of second-mode instability is ruptured. A quantitative analysis indicates that although the energy efflux of both acoustic and vortical components increases with wall-cooling, the destabilization effect is much stronger and no significant abatement of pressure perturbations is realized. The direct impact of these mechanisms on the transition process itself is examined with high-fidelity simulations of three-dimensional second-mode wavepacket propagation. In the near-adiabatic HBL, the wavepacket remains trapped within the boundary layer and attenuates outside the region of linear instability. However, wavepackets in the cooled-wall HBLs amplify and display nonlinear distortion, and transition more rapidly. The structure of the wavepacket also displays different behavior; moderately-cooled walls show bifurcation into a leading turbulent head region and a trailing harmonic region, while highly-cooled wall cases display lower convection speeds and significant wavepacket elongation, with intermittent spurts of turbulence in the wake of the head region. This elongation effect is associated with a weakening of the lateral jet mechanism due to the breakdown of spanwise coherent structures. These features have a direct impact on wall loading, including skin friction and heat transfer. In moderately cooled-walls, the spatially-localized wall loading is similar to those in near-adiabatic walls, with dominant impact due to coherent structures in the leading turbulent head region. In highly-cooled walls, the elongated near-wall streaks in the wake region of the wavepacket result in more than twice as large levels of skin friction and heat transfer over a sustained period of time.

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Section: V1 General Features Of 3d Response To Wavepacket Forcingmentioning

confidence: 99%

“…The effects on wall heat transfer are reported in figure 21, which plots the surface heat transfer coefficient, c h , at x NC = 4. c h is defined as follows 44 :…”

Section: V4 Wall Loadingmentioning

confidence: 99%

Instability characteristics of cooled hypersonic boundary layers

Unnikrishnan

Gaitonde

2020

AIAA Scitech 2020 Forum

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“…They reported that both second-mode fundamental and oblique breakdown lead to strong nonlinear interactions, and are viable candidates to affect transition on the cone geometry. Novikov & Egorov (2016) performed a numerical study of laminar-turbulent transition over a flat plate at Mach 5.37. In that effort, perturbations were introduced into the boundary layer through a small round hole on the surface of the plate by forced pulsations of the vertical velocity component.…”

Section: Transition In High-speed Boundary Layersmentioning

confidence: 99%

“…In order to highlight the importance of the present nonlinear approach in determining the inflow disturbance spectrum, we examine other possible inflow conditions that are selected based on linear theory alone. The starting point is to evaluate the linear evolution of all the Orr-Sommerfeld and Squire instability waves that are part of the inlet condition, using the linear parabolized instability equations (Park & Zaki 2019). The N-factor associated with every F, k z wave was obtained as follows:…”

Section: Prediction From Linear Stability Theorymentioning

confidence: 99%

Nonlinearly most dangerous disturbance for high-speed boundary-layer transition

Jahanbakhshi

Zaki

2019

J. Fluid Mech.

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Laminar-to-turbulence transition in zero-pressure-gradient boundary layer at Mach 4.5 is studied using direct numerical simulations. For a given level of total disturbance energy, the inflow spectra was designed to correspond to the nonlinearly most dangerous condition that leads to the earliest possible transition Reynolds number. The synthesis of the inlet disturbance is formulated as a constrained optimization, where the control vector is comprised of the amplitudes and relative phases of the inlet modes; the constraints are the prescribed total energy and that the flow evolution satisfies the full nonlinear compressible Navier-Stokes equations; the cost function is the average wall friction which, once maximized, corresponds to the earliest possible transition location. An ensemble variational (EnVar) technique is developed to solve the optimization problem. EnVar updates the estimate of the control vector at the end of each iteration using the gradient of the cost function, which is computed from the outcome of an ensemble of possible solutions. Two inflow conditions are computed, each corresponding to a different level of energy, and their spectra are contrasted: the lower energy case includes two normal acoustic waves and one oblique vorticity perturbation, whereas the higher energy condition consists of oblique acoustic and vorticity waves. The focus is placed on the former case because it can not be categorized as any of the classical breakdown scenarios, while the higher energy condition undergoes a typical second-mode oblique transition. At the lower energy level, the instability wave that initiates the rapid breakdown to turbulence is not present at the inlet plane. Instead, it appears at a downstream location after a series of nonlinear interactions that spur the fastest onset of turbulence. The results from the nonlinearly most potent inflow condition are also compared to other inlet disturbances that are selected solely based on linear theory, and which all yield relatively delayed transition onset. †

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“…Also, the excitation of periodical disturbances is used in direct numerical simulation studies (DNS) of mechanisms of laminar-turbulent transition. [16][17][18][19] Experiments with the controlled periodic pulsations are very important and play a vital role in validating theoretical results for the linear and the nonlinear stages of transition to turbulence. However, it only provides limited insight into a natural transition scenario where a broad disturbances spectrum is excited by freestream pulsations and where complex wave interactions between all disturbance modes are possible.…”

Section: Introductionmentioning

confidence: 99%

Evolution of localized artificial disturbance in 2D and 3D supersonic boundary layers

Yatskikh

Yermolaev

Kosinov

et al. 2018

Proceedings of the Institution of Mechanical Engineers, Part G:

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The evolution of a controlled broadband wave packet in flat-plate and swept-wing supersonic boundary layers was experimentally investigated at Mach number M = 2. The wave packet was introduced into the boundary layer by a localized pulse electrical discharge. The structure and evolution downstream of the wave packet were studied by hot-wire measurements. It was found that the wave packet has a symmetric shape in a flat-plate boundary layer, whereas there is asymmetry in case of a swept-wing one. The spectral analysis of the development of different modes of the wave packet was provided.

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Direct Numerical Simulations of Transitional Boundary Layer over a Flat Plate in Hypersonic Free-Stream

Cited by 6 publications

References 18 publications

Instability characteristics of cooled hypersonic boundary layers

Instability characteristics of cooled hypersonic boundary layers

Nonlinearly most dangerous disturbance for high-speed boundary-layer transition

Evolution of localized artificial disturbance in 2D and 3D supersonic boundary layers

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