Simulation Of Boundary Layer Transition Research Articles

High-fidelity simulations of boundary layer transition on an underwater axisymmetric body with stability theory

High-fidelity simulations of boundary layer transition on an underwater axisymmetric body with stability theory

Embedded large eddy simulation of boundary layer transition behind a micro-ramp

Boundary layer transition induced by surface roughness elements plays an important role in aerodynamic and aero-thermodynamic design of subsonic/supersonic/hypersonic vehicles. However, the effect of three-dimensional isolated roughness element in the process of promoting/suppressing boundary layer transition is far from being fully understood, particularly in the incompressible laminar flow. In the present study, the laminar-to-turbulent transition induced by a three-dimensional isolated micro-ramp element immersed in an incompressible laminar boundary layer is investigated numerically. The embedded large eddy simulation (ELES) combining the intermittency transition model and the wall-modeled large eddy simulation (WMLES) S − Ω model is employed for the first time. Numerical results on the time-averaged/instantaneous flow field and the statistical flow fluctuations are analysed and validated thoroughly by the existing experimental measurements. It is found that the interaction of the secondary and the tertiary streamwise vortices causes a high level of wall shear and an inflectional velocity distribution in the near-wall region. An additional eddy system that develops nonlinearly from the unstable inflectional flow may trigger the boundary layer transition. The present study validates that the ELES combined with the WMLES S − Ω model is an efficient simulation tool for industrial wall-bounded flows allowing effective compromise between flexibility, cost, and accuracy.

Critical study of the effects and numerical simulations of boundary layer transition in lift-based wind turbines at moderate Reynolds numbers

Early laminar-to-turbulent transition due to fouling typically degrades the lift and amplifies the drag of airfoils. Few studies of medium-size horizontal and vertical axis wind turbines (HAWTs and VAWTs, respectively) provide information on this turbulence. This reflects uncertainties in the reported power coefficients (Cp) and contradictory recommendations for computational fluid dynamics (CFD) models. This paper investigates the Cp degradation of wind turbines (Rechord = 0.7–1 × 106) and its CFD-modeling aspects. Published experimental data for the modern DU12W262 airfoil, for both the clean and dirty surface conditions, are taken as the baseline. CFD is employed to reproduce airfoil characteristics for both cases. Using the blade element momentum and computational fluid dynamics (CFD) models, the experimental and numerical HAWT and VAWT performances are assessed. Non-transition turbulence closure models [Spalart–Allmaras and Shear Stress Transport (SST)], which have been recommended against in previous works without due consideration for surface aspects, are shown to successfully mimic dirty surface conditions but to underpredict the dramatic experimental degradation of 82% in maximum CL/CD to 67% when uncalibrated. The resulting experimental Cp reductions for HAWTs and VAWTs are 34% and 65%, respectively, values that are also underpredicted by CFD as 17% and 34%, respectively, without calibration. The detailed findings may serve as pioneering data for transition effects and aspects of CFD for both HAWTs and VAWTs at moderate Reynolds numbers.

基于直接控制FFD参数化方法的跨声速层流翼身组合体稳健性设计

The geometric shape is not directly manipulated by free-form deformation (FFD) technique. A high level FFD control volume is always adopted in the refined aerodynamic design. The direct manipulated free-form deformation (DFFD) technique is developed in this paper to improve the shortcomings of the FFD technique. This method adopt a parameter identification technique which can identify the disturbance quantities and convert them into displacement values on the FFD control lattices. The DFFD technique can accomplish refined aerodynamic shape design using less design variables. The visual impression is much better than the traditional FFD technology, and the number of the design variables is not increased with using of the high level FFD control volume. The robust design system is established by DFFD technique, multi-groups collaboration particle swarm optimization (MCPSO) and adaptive sampling algorithm, combined with quaternion mesh deforming technique and the numerical simulation of boundary layer transition based on γ - Re θt transition model. A transonic laminar wing body is optimized by this system. The design results show that not only a wide range of laminar flow area is maintained, but also the development of shock is controlled.

On the relationship between the wall-shear-stress and transient-growth disturbances in a laminar boundary layer

The present study is motivated by the development of wall-based flow-state estimators for feedback control of transient-growth disturbances in a laminar boundary layer. Such estimators have been successfully demonstrated in recent numerical studies, but they rely on information that, while available in simulations, is generally not accessible in the laboratory. Here, some physical aspects of transient-growth modes and their relationship to the unsteady wall shear stress are examined to guide the development of estimation methods that perform satisfactorily yet are practical to implement. The usefulness of the resulting physical understanding for flow estimation is demonstrated using a proper orthogonal decomposition mode estimator, applied to a direct numerical simulation of boundary layer transition beneath free-stream turbulence. The results shed light on the strengths and weaknesses of employing each of the streamwise and spanwise wall-shear-stress components in estimating the disturbance wall-normal velocity and vorticity fields. It is also found that the streamwise-elongated nature of transient-growth disturbances allows coarse sampling of the wall-shear-stress information used in the estimation without substantially deteriorating the estimation accuracy. This is particularly true for estimates based on the streamwise shear stress.

Direct numerical simulation of hypersonic boundary layer transition over a blunt cone with a small angle of attack

The direct numerical simulation of boundary layer transition over a 5° half-cone-angle blunt cone is performed. The free-stream Mach number is 6 and the angle of attack is 1°. Random wall blow-and-suction perturbations are used to trigger the transition. Different from the authors’ previous work [Li et al., AIAA J. 46, 2899 (2008)], the whole boundary layer flow over the cone is simulated (while in the author’s previous work, only two 45° regions around the leeward and the windward sections are simulated). The transition location on the cone surface is determined through the rapid increase in skin fraction coefficient (Cf). The transition line on the cone surface shows a nonmonotonic curve and the transition is delayed in the range of 20°≤θ≤30° (θ=0° is the leeward section). The mechanism of the delayed transition is studied by using joint frequency spectrum analysis and linear stability theory (LST). It is shown that the growth rates of unstable waves of the second mode are suppressed in the range of 20°≤θ≤30°, which leads to the delayed transition location. Very low frequency waves (VLFWs) are found in the time series recorded just before the transition location, and the periodic times of VLFWs are about one order larger than those of ordinary Mack second mode waves. Band-pass filter is used to analyze the low frequency waves, and they are deemed as the effect of large scale nonlinear perturbations triggered by LST waves when they are strong enough.

Simulation of boundary layer transition induced by periodically passing wakes

The interaction between an initially laminar boundary layer developing spatially on a flat plate and wakes traversing the inlet periodically has been simulated numerically. The three-dimensional, time-dependent Navier–Stokes equations were solved with 5.24×107 grid points using a message passing interface on a scalable parallel computer. The flow bears a close resemblance to the transitional boundary layer on turbomachinery blades and was designed following, in outline, the experiments by Liu & Rodi (1991). The momentum thickness Reynolds number evolves from Reθ = 80 to 1120. Mean and second-order statistics downstream of Reθ = 800 are of canonical flat-plate turbulent boundary layers and are in good agreement with Spalart (1988).In many important aspects the mechanism leading to the inception of turbulence is in agreement with previous fundamental studies on boundary layer bypass transition, as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside the boundary layer evolve rapidly into longitudinal puffs during an initial receptivity phase. In the absence of strong forcing from free-stream vortices, these structures exhibit streamwise elongation with gradual decay in amplitude. Selective intensification of the puffs occurs when certain types of turbulent eddies from the free-stream wake interact with the boundary layer flow through a localized instability. Breakdown of the puffs into young turbulent spots is preceded by a wavy motion in the velocity field in the outer part of the boundary layer.Properties and streamwise evolution of the turbulent spots following breakdown, as well as the process of completion of transition to turbulence, are in agreement with previous engineering turbomachinery flow studies. The overall geometrical characteristics of the matured turbulent spot are in good agreement with those observed in the experiments of Zhong et al. (1998). When breakdown occurs in the outer layer, where local convection speed is large, as in the present case, the spots broaden downstream, having the vague appearance of an arrowhead pointing upstream.The flow has also been studied statistically. Phase-averaged velocity fields and skin-friction coefficients in the transitional region show similar features to previous cascade experiments. Selected results from additional thought experiments and simulations are also presented to illustrate the effects of streamwise pressure gradient and free-stream turbulence.

Numerical simulation of boundary-layer transition at Mach two

The linear and early nonlinear stages of boundary-layer transition at free-stream Mach number M ∞ =2.0 are investigated by direct numerical simulation of the compressible NavierStokes equations. Results from simulations with a large computational box and small-amplitude random initial conditions are compared with linear stability theory. The growth rates of oblique waves are reproduced correctly. Two-dimensional waves show a growth that is modulated in time, indicating the presence of an extra unstable mode which moves supersonically relative to the free stream. Further simulations are conducted to investigate the nonlinear development of two- and three-dimensional disturbances The transition due to oblique disturbance waves is the most likely cause of transition at this Mach number, and is found to lead to the development of strong streamwise vortices.