Impact of forward-facing steps on laminar-turbulent transition in transonic flows without pressure gradient

Edelmann, Christopher A.; Rist, Ulrich

doi:10.2514/6.2013-80

Cited by 9 publications

(10 citation statements)

References 2 publications

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“…Nonlinear mechanisms, rapidly leading to the birth of turbulent spots, start at the transition point. It is worth noting that the validity of this spatial approach was shown in the case of rapidly evolving flows, in the case of thin separation bubbles, by comparing −Imα values taken from local linear stability analyses with direct numerical simulation results (see the work by Gruber and Bestek [12] for a separation bubble and the work by Edelmann and Rist [13] in the case of a forward-facing step). In the case of deep gaps, acoustics as well as other types of instabilities may play a role.…”

Section: A Numerical Prediction 1 Theoretical Overviewmentioning

confidence: 99%

Modeling of Transverse Gaps Effects on Boundary-Layer Transition

Beguet

Perraud

Forte

et al. 2017

Journal of Aircraft

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Delaying laminar-turbulent transition is considered a crucial issue with respect to the reduction of airplanes' environmental impact, allowing the reduction of skin friction drag and thus fuel consumption. As part of this theme, boundary-layer destabilizing effects of surface imperfections (assembly gaps, steps, rivets, waviness, and holes) have been investigated by ONERA scientists for many years. Experimental, numerical, and theoretical works provided fruitful results partly described in this paper, which especially focuses on gaps effects. Geometrical triggering criteria were searched and found to reproduce transition at a realistic chord percentage on reduced scale models. More recently, predicting the impact of gaps on laminar flow transition was also studied so as to provide a numerical tool for defining tolerances compatible with natural laminar flow airfoils. A specific model based on a ΔN approach has been developed to quantify the transition displacement caused by transverse gaps with a rectangular section. Detailed experimental studies that recently contributed to its validation are also presented in this paper.

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Section: A Numerical Prediction 1 Theoretical Overviewmentioning

confidence: 99%

Modeling of Transverse Gaps Effects on Boundary-Layer Transition

Beguet

Perraud

Forte

et al. 2017

Journal of Aircraft

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“…The numerical computation of the flowfield over a forward-facing step is still resource-intensive2 , 3 , 12 , so that a parametric study of the effect of the step on boundary layer transition is typically performed only for a selected configuration and a limited variation of flow conditions3 , 12 .…”

Section: Introductionmentioning

confidence: 99%

Experimental Investigation of the Effect of Forward-facing Steps on Boundary Layer Transition

2015

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The effect of geometric forward-facing steps on boundary layer transition was experimentally investigated at a high subsonic Mach number in a blow-down wind tunnel facility in Göttingen, Germany. Boundary layer transition was detected nonintrusively by means of the Temperature-Sensitive Paint (TSP) technique. Forward-facing steps of different height were installed on a spanwise invariant wind tunnel model. Streamwise pressure gradient and high chord Reynolds number were systematically varied and their effect on boundary layer transition was studied in the presence of forward-facing steps on the model surface. At all tested stability situations, the surface imperfections were shown to reduce the extent of the laminar region. Transition was observed to move gradually towards the step location with increasing step Reynolds numbers and increasing relative step height. For a given combination of step height and chord Reynolds number, more pronounced negative pressure gradients led to an increase in transition Reynolds number. The reduction in transition Reynolds number due to the effect of the surface imperfection was more marked at larger flow acceleration. The plots of the relative change in transition location as a function of the step Reynolds number and of the relative step height gave good correlation of the results. The correlations were found to be practically independent of the streamwise pressure gradient in the examined range. Criteria for the allowable tolerances on lowsweep Natural Laminar Flow surfaces can now be derived from the functional relations determined in this work.

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“…To incorporate the impact of gaps, Perraud and Arnal proposed a ΔN that increases the N factor compared with N s , reached for a smooth surface. This is a common idea that is also applied for steps by Wang and Gaster [11] and Edelmann and Rist [12], for instance.…”

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confidence: 99%

Impact of Deep Gaps on Laminar–Turbulent Transition in Compressible Boundary-Layer Flow

Zahn

Rist

2016

AIAA Journal

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Two-dimensional direct numerical simulations are used to study the impact of deep gaps on laminar-turbulent transition in compressible boundary-layer flow. For these, the gap depth-to-width ratio is always larger than five. They are located on a flat plate without pressure gradient. A steady base flow is used with a Mach number of 0.6, freestream temperature of 288 K, and free-stream pressure of 1 bar. Subsequently, Tollmien-Schlichting waves are introduced by suction and blowing at the wall, and their growth over the gap is evaluated by N factors. The influence of the gap on laminar-turbulent transition is quantified by the difference ΔN compared with the N factor obtained for a flat plate without gap. A periodic influence of the gap depth on ΔN is observed. In the direct numerical simulations, acoustic waves enter the gap and form a standing wave due to reflections, similar as occurring in organ pipes. The feedback of the standing wave on the boundary-layer flow above is essential for the observed ΔN variations. In a second case, the influence of a specific gap placed in front of a forward-facing step is studied as well. Here, a reduction of the N factor and hence a delay of transition, relative to the flow with step alone, are reached due to the presence of the gap. Nomenclature A = dimensionless amplitude A m = dimensionless maximum amplitude in vertical direction a = normalized amplitudẽ c = speed of sound, m∕s c p = heat capacity at constant pressure, J∕K c v = heat capacity at constant volume, J∕K d = dimensionless gap depth h = dimensionless step height L ref = reference length, m Ma = Mach number N = factor describing amplification of TS waves n = factor describing the amplification of a TS wave for one frequency Pr = Prandtl number p = dimensionless pressure p w = dimensionless wall pressure Re d = Reynolds number based on the gap depth r = receptivity factor t = time, s u = dimensionless horizontal velocity v = dimensionless vertical velocity w = dimensionless gap width x = dimensionless length in horizontal direction y = dimensionless length in vertical direction α = phase shift between waves γ = heat capacity ratio ΔN = deviation of an N factor from the N factor of the flat plate Δα = constant phase shift between standing wave and new TS wave δ = boundary-layer thickness, m δ 1 = displacement thickness, m λ = dimensionless wave length μ = dynamic viscosity, kg∕s · mρ = density, kg∕m 3 τ w = dimensionless wall shear stress ω = dimensionless circular frequency ∼ = dimensional quantities Subscripts an = antinode amplitude of standing wave dis = disturbances (fluctuations) of flow quantities do = values of downward traveling waves in the gap op = values at the gap opening res = amplitude of superposition of TS waves up = values of upward traveling waves in the gap ∞ = free-stream quantities

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Impact of forward-facing steps on laminar-turbulent transition in transonic flows without pressure gradient

Cited by 9 publications

References 2 publications

Modeling of Transverse Gaps Effects on Boundary-Layer Transition

Modeling of Transverse Gaps Effects on Boundary-Layer Transition

Experimental Investigation of the Effect of Forward-facing Steps on Boundary Layer Transition

Impact of Deep Gaps on Laminar–Turbulent Transition in Compressible Boundary-Layer Flow

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