Wake Instabilities behind Discrete Roughness Elements in High Speed Boundary Layers

Choudhari, Meelan M.; Li, Fēi; Chang, Chau‐Lyan; Norris, Andrew T.; Edwards, Jack R.

doi:10.2514/6.2013-81

Cited by 36 publications

(55 citation statements)

References 21 publications

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“…For a k/δ k =0.44 element (Re kk =791), the roughness wake was found to be dominated by varicose mode instability driven by wall-normal shear, while sinuous mode instabilities (associated with lateral shear) were found to be relatively weaker. Results were in general agreement with previous numerical and experimental studies by Choudhari et al (2010Choudhari et al ( , 2013 and Kegerise et al (2012) on isolated diamond elements at Mach 3.5, where sinuous modes were found to experience greater amplification for shorter elements and varicose instability to become more dominant and drive the breakdown to turbulence for higher Re kk . In subsequent DNS studies on square elements by De Tullio & Sandham (2015) in a Mach 6 boundary layer, two main mechanisms responsible for the excitation of wake modes were described: a first one due to the interaction of the local reversed flow with external disturbances and a second one due to the interaction between the roughness wake and the different boundary layer modes.…”

Section: Introductionsupporting

confidence: 91%

“…Below k/δ k =0.15, x tr is fixed at the corner due to the local destabilising effects and it is then shortened with increasing height, eventually asymptoting towards x tr ≈21±1mm (∼12δ k ). The effective transition length is of a similar order as that found under 'noisy' (highdisturbance) environmental conditions in Choudhari et al (2013) and Kegerise et al (2012). In their studies, the effective transition length induced by diamond elements was shown to be over 2.5 times longer under 'quiet' (low-noise) conditions, where the stability analysis of the element wake was based on N-factor calculations; the effect of environmental disturbance levels on x tr was further found to lead to even greater differences between 'quiet' and 'noisy' conditions for shorter elements, with the breakdown of the dominant modes associated to transition onset exhibiting a strongly nonlinear dependence on roughness height.…”

Section: Resultssupporting

confidence: 67%

“…As shown in the schematic in figure 8a, the more slender geometry of diamond elements reduces the extent of local separation near the roughness element and induces enhanced spanwise gradients, in turn leading to increased local heating effects (e.g. Choudhari et al 2013). On the other hand, for square elements (figure 8b), the local separation effects induced by the blunt geometry are relatively stronger and often result in an upstream horseshoe vortex which convects further downstream (e.g.…”

Section: Resultsmentioning

confidence: 99%

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Effect of isolated roughness element height on high-speed laminar–turbulent transition

et al. 2017

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Understanding of the roughness-induced laminar-turbulent transition of supersonic and hypersonic flows is partly challenged by the intricate sensitivities presented by different correlation criteria. We investigate experimentally the effect of height for an isolated roughness element of quadrilateral planform. Heat transfer measurements document the enhancement of roughness-induced disturbances -here the associated heat flux perturbation -along a downstream axisymmetric laminar separation. With increasing element height k, a gradual intensification in wake disturbance levels is found for subcritical elements (k/δ k <0.15, where δ k is the undisturbed boundary layer thickness) while elements taller than the effective condition (k/δ k 0.32) bypass the more moderate transition mechanisms to produce a fully turbulent element wake. Results exhibit high sensitivity to flow properties at roughness height between critical and effective conditions. A reduction in wake disturbance levels with increasing height is documented within 0.23 k/δ k 0.32. This effect coincides with a decrease in kinematic viscosity at roughness height ν k (as Mach number at height M k increases from 1.52 to 1.96) and is restricted to elements with strong local separation, whereby the influence of local shear effects is enhanced.

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Section: Introductionsupporting

confidence: 91%

Section: Resultssupporting

confidence: 67%

Section: Resultsmentioning

confidence: 99%

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Effect of isolated roughness element height on high-speed laminar–turbulent transition

et al. 2017

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“…Recently Wheaton & Schneider (2012 have carried out a set of experiments on roughness-induced transition at M = 6, reporting the first quantitative measurements of the roughness wake instability at hypersonic speeds. For the same Mach number, the numerical simulations of De Tullio & Sandham (2012) and Choudhari et al (2013) show that the roughness wake can sustain the growth of a number of different instability modes, the relative importance of which seems to depend on the specific flow conditions considered.…”

Section: Introductionmentioning

confidence: 93%

Laminar–turbulent transition induced by a discrete roughness element in a supersonic boundary layer

et al. 2013

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The linear instability and breakdown to turbulence induced by an isolated roughness element in a boundary layer at Mach 2.5, over an isothermal flat plate with laminar adiabatic wall temperature, have been analysed by means of direct numerical simulations, aided by spatial BiGlobal and three-dimensional parabolized (PSE-3D) stability analyses. It is important to understand transition in this flow regime since the process can be slower than in incompressible flow and is crucial to prediction of local heat loads on next-generation flight vehicles. The results show that the roughness element, with a height of the order of the boundary layer displacement thickness, generates a highly unstable wake, which is composed of a low-velocity streak surrounded by a three-dimensional high-shear layer and is able to sustain the rapid growth of a number of instability modes. The most unstable of these modes are associated with varicose or sinuous deformations of the low-velocity streak; they are a consequence of the instability developing in the three-dimensional shear layer as a whole (the varicose mode) or in the lateral shear layers (the sinuous mode). The most unstable wake mode is of the varicose type and grows on average ∼17 % faster than the most unstable sinuous mode and ∼30 times faster than the most unstable boundary layer mode occurring in the absence of a roughness element. Due to the high growthrates registered in the presence of the roughness element, an amplification factor of N = 9 is reached within ∼50 roughness heights from the roughness trailing edge. The independently performed Navier-Stokes, spatial BiGlobal and PSE-3D stability results are in excellent agreement with each other, validating the use of simplified theories for roughness-induced transition involving wake instabilities. Following the linear stages of the laminar-turbulent transition process, the roll-up of the three-dimensional shear layer leads to the formation of a wedge of turbulence, which spreads laterally at a rate similar to that observed in the case of compressible turbulent spots for the same Mach number.

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“…The shear layers in the wake of the roughness can sustain two types of instabilities: sinuous (sometimes called odd or antisymmetric), associated with the detached wall-normal shear layer; and varicose (or even/symmetric) modes concentrated near the regions of high spanwise shear at the sides. Varicose modes are usually more unstable [14,15] than sinuous modes, but it is not completely understood how they depend on the roughness geometry and flow conditions. Choudhari et al [6] found that, for lower roughness height, the sinuous modes were dominant, whereas the varicose modes became dominant for a roughness element with larger height.…”

mentioning

confidence: 99%

Numerical Simulations of Transition due to Isolated Roughness Elements at Mach 6

Eynde

Sandham

2016

AIAA Journal

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An accurate prediction of transition onset behind an isolated roughness element has not yet been established. This is particularly important in hypersonic flow, where transition is accompanied by increased surface heating. In the present contribution, a number of direct numerical simulations have been performed of a Mach 6 boundary layer over a flat plate with isolated roughness elements. The effects of roughness shape, planform, ramps, and freestream disturbance levels on instability growth and transition onset are investigated. It is found that the frontal shape has a large effect on the transition onset, which is in agreement with previous studies, whereas the roughness element planform has a marginal influence. A new result is that the roughness shape in the streamwise direction (in particular, the aft section) is also an important characteristic, since an element with a ramped-down aft section allows the detached shear layer to spread out and weaken, leading to a lower instability growth rate. Above a critical value, the instability growth rate is found to be correlated with the amplitude of the low-speed streak formed by the roughness element, suggesting that a more physically based transition criterion should take account of the local liftup effect of the particular roughness shape.

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Wake Instabilities behind Discrete Roughness Elements in High Speed Boundary Layers

Cited by 36 publications

References 21 publications

Effect of isolated roughness element height on high-speed laminar–turbulent transition

Effect of isolated roughness element height on high-speed laminar–turbulent transition

Laminar–turbulent transition induced by a discrete roughness element in a supersonic boundary layer

Numerical Simulations of Transition due to Isolated Roughness Elements at Mach 6

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