Flow control of an oblique shock wave reflection with micro-ramp vortex generators: Effects of location and size

Scarano

2016

J. Fluid Mech.

The early stage of three-dimensional laminar-to-turbulent transition behind a micro-ramp is studied in the incompressible regime using tomographic particle image velocimetry. Experiments are conducted at supercritical micro-ramp height h based Reynolds number Re h = 1170. The measurement domain encompasses 6 ramp widths spanwise and 73 ramp heights streamwise. The mean flow topology reveals the underlying vortex structure of the wake flow with multiple pairs of streamwise counter-rotating vortices visualized by streamwise vorticity. The primary pair generates a vigorous upwash motion in the symmetry plane with a pronounced momentum deficit. A secondary vortex pair is induced closer to the wall. The tertiary and even further vortices maintain a streamwise orientation, but are produced progressively outwards of the secondary pair and follow a wedge-type pattern. The instantaneous flow pattern reveals that the earliest unstable mode of the wake features arc-like Kelvin-Helmholtz (K-H) vortices in the separated shear layer. Under the influence of the K-H vortices, the wake exhibits a high level of fluctuations with a pulsatile mode for the streamwise momentum deficit. The K-H vortices are lifted up due to the upwash induced by the quasi-streamwise vortex pair, while they appear to undergo pairing, distortion and finally breakdown. Immediately downstream, a streamwise interval of relatively low vortical activity separates the end of the K-H region from the formation of new hairpin vortices close to the wall. The latter vortex structures originate from the region of maximum wall shear, induced by the secondary vortex pair causing strong ejection events which transport low-speed flow upwards. The whole pattern features a cascade of hairpin vortices along a turbulent/non-turbulent interface. The wedge-shaped cascade signifies the formation of a turbulent wedge. The turbulent properties of the wake are inspected with the spatial distribution of the velocity fluctuations and turbulence production in the developing boundary layer. Inside the wedge region, the velocity fluctuations approach quasi-spanwise homogeneity, indicating the development towards a turbulent boundary layer. The wedge interface is characterized by a localized higher level of velocity fluctuations and turbulence production, associated to the deflection of the shear layer close to the wall and the onset of coherent hairpin vortices inducing localized large-scale ejections.

Section: Introductionmentioning

confidence: 99%

Boundary layer transition mechanisms behind a micro-ramp

Scarano

2016

J. Fluid Mech.

“…The micro-ramp geometry has been widely used as a flow control device to enhance boundary layer mixing, promote transition and avoid unwanted separation (Berry et al 2001;Lin 2002). In the special case of shock wave boundary layer interactions (SWBLI) with an incoming supersonic turbulent boundary layer, the micro-ramp has also gained substantial research attention due to its effectiveness in reducing separation (Dolling 2001;Babinsky, Li & PittFord 2009;Giepman, Schrijer & van Oudheusden 2014).…”

Section: Introductionmentioning

confidence: 99%

On Reynolds number dependence of micro-ramp-induced transition

Scarano

2018

J. Fluid Mech.

The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number (Re h = 1170, 730, 460 and 320) with tomographic particle image velocimetry. The effect of Re h on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical Re h , the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing Re h . In the instantaneous flow, at Re h < 1000, the hairpin vortices induced by Kelvin-Helmholtz (K-H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K-H instability and the onset of transition. The breakdown of K-H vortices as observed at Re h = 1170, does not occur at lower Re h . Decreasing Re h , the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that Re h is the key factor determining the importance of the symmetric mode. At Re h = 1170, the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing Re h to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.

“…The incompressible shape factor H i (see Eq.1) is a measure of the fullness of the boundary layer and is often used to assess the 'health' of the boundary layer and its resilience to separation. 25 The added momentum flux E (see Eq.2) is a metric that was introduced in the work of Giepman et al 9 and provides a measure of the momentum flux that has been added to the near-wall region (<0.43δ 99 ) of the flow by the action of the micro-ramp. It was found that the added momentum flux scales linearly with the micro-ramp height h and that x scales with the boundary layer thickness δ 99 instead of the micro-ramp height h. The added momentum flux was found to increase upto x ∼ 5.7δ 99 , after which a plateau is reached and virtually no extra momentum is added or removed.…”

Section: B Piv Resultsmentioning

confidence: 99%

“…However, besides transporting highmomentum fluid towards the wall, micro-ramps also incur a drag penalty in the form of a low-momentum wake, which steadily lifts off from the surface when moving downstream. The location and strength of the wake were also found 5,[8][9][10][11] to scale proportionally with micro-ramp height.…”

Section: Introductionmentioning

confidence: 96%

“…5 These vortices transport high-momentum fluid towards the wall, thus creating a fuller boundary layer profile, allowing the flow to pass through the shock system with less separation. [6][7][8][9] In the work of Babinsky et al 8 and Giepman et al 9 it was found that larger micro-ramps are more effective at preventing boundary layer separation. In the study of Giepman et al this was related to the momentum flux added to the lower portion of the incoming boundary layer.…”

Section: Introductionmentioning

confidence: 98%

See 1 more Smart Citation

The effects of Mach and Reynolds number on the flow mixing properties of micro-ramp vortex generators in a supersonic boundary layer

Giepman

Srivastava

45th AIAA Fluid Dynamics Conference

et al. 2015

A parametric study has been conducted into the effects of device height (h = 6 − 10 mm), Mach number (M ∞ = 1.5−2.5) and Reynolds number (Re ∞ = 28−63×10 6 m −1 ) on the effectiveness of micro-ramp vortex generators as flow control devices. The control authority of the micro-ramp comes from the two counter-rotating vortices it introduces into the boundary layer. The vortices transport high-momentum fluid towards the wall, thus creating a fuller velocity profile which is less prone to separation. This momentum transport, however, comes at the price of a low-momentum wake that is formed downstream of the ramp. The induced flow field has been studied by means of particle image velocimetry, oil-flow and Schlieren visualizations. Most of the geometric flow features were found to scale linearly with micro-ramp height h: the wake height, wake strength, vortex core height and the momentum flux added to the near-wall region of the flow all increase linearly with h. The control effectiveness of the micro-ramp is reduced for higher Mach numbers, as less momentum is added to the near-wall region of the flow and a stronger wake is recorded. Only a weak Reynolds number effect is observed in the results, with the micro-ramp becoming slightly more effective at higher Reynolds numbers.