Learning Scene Context Without Images

Teaching machines of scene contextual knowledge would enable them to interact more effectively with the environment and to anticipate or predict objects that may not be immediately apparent in their perceptual field. In this paper, we introduce a novel transformer-based approach called $LMOD$ ( Label-based Missing Object Detection) to teach scene contextual knowledge to machines using an attention mechanism. A distinctive aspect of the proposed approach is its reliance solely on labels from image datasets to teach scene context, entirely eliminating the need for the actual image itself. We show how scene-wide relationships among different objects can be learned using a self-attention mechanism. We further show that the contextual knowledge gained from label based learning can enhance performance of other visual based object detection algorithm

Turbulent drag reduction by spanwise wall forcing. Part 1: Large-eddy simulation

Turbulent drag reduction through streamwise travelling waves of spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction at friction Reynolds numbers $Re_\tau = 951$ and $4000$. The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where drag reduction is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence drag reduction, is found to change with the near-wall Stokes layer protrusion height $\ell_{0.01}$. A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer; in this range, the drag reduction increases as $\ell_{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in drag reduction with increasing $\ell_{0.01}$. We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any drag reduction savings. This motivates the study of the outer-scaled actuation (OSA) pathway in Part 2, where drag reduction is achieved through actuating the outer-scaled motions

Turbulent drag reduction by spanwise wall forcing. Part 2: High-Reynolds-number experiments

Here, we present measurements of turbulent drag reduction at high friction Reynolds numbers in the range of $4500 \le Re_\tau \le 15000$. The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequency structures of the near-wall cycle, and (ii) outer-scaled actuation (OSA), which was recently presented by Marusic et al. (Nat. Commun., vol. 12, 2021) for high-$Re_\tau$ flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic-PIV (particle image velocimetry) to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, drag reduction of up to 25% was achieved, but mostly with net power saving losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive net power savings of 5 - 10%, for moderate drag reductions of 5 - 15%. The results suggest that OSA is an attractive pathway for energy-efficient drag reduction in high Reynolds number applications. Both ISA and OSA strategies are found to produce complex inter-scale interactions, leading to attenuation of the turbulent fluctuations across the boundary layer for a broad range of length and time scales

Recovery of wall-shear stress to equilibrium flow conditions after a rough-to-smooth step change in turbulent boundary layers

This paper examines the recovery of the wall-shear stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia and Luxton (J. Fluid Mech., vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-wall methods for measuring wall-shear stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases at Reτ≈4100 that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ( z+≲4 ) recovers almost immediately to an equilibrium state with the new wall conditions; however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a wall-bounded flow with a rough-to-smooth wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed

Recent progress on reliability assessment of large-eddy simulation

Reliability assessment of large-eddy simulation (LES) of turbulent flows requires consideration of errors due to shortcomings in the modeling of sub-filter scale dynamics and due to discretization of the governing filtered Navier–Stokes equations. The Integral Length-Scale Approximation (ILSA) model is a pioneering sub-filter parameterization that incorporates both these contributions to the total simulation error, and provides user control over the desired accuracy of a simulation. It combines an imposed target for the ‘sub-filter activity’ and a flow-specific length-scale definition to achieve LES predictions with pre-defined fidelity level. The performance of the ‘global’ and the ‘local’ formulations of ILSA, implemented as eddy-viscosity models, for turbulent channel flow and for separated turbulent flow over a backward-facing step are investigated here. We show excellent agreement with reference direct numerical simulations, with experimental data and with predictions based on other, well-established sub-filter models. The computational overhead is found to be close to that of a basic Smagorinsky sub-filter model

Accuracy control for large-eddy simulation of turbulent mixing:integral length-scale approach

\u3cp\u3eTurbulent flow at high Reynolds numbers is currently not accessible on the basis of direct numerical simulation (DNS) of the Navier-Stokes equations - the computational complexity is too high to allow DNS in most realistic flow conditions. Instead, Large-Eddy Simulation (LES) offers an alternative in which the focus is on capturing the larger dynamic scales of a problem. However, the fundamental closure problem in LES induced by spatial filtering of nonlinear terms, and the role of discretization errors in the numerical treatment of the LES equations, induce a principal uncertainty in any LES prediction. This uncertainty requires quantification and control. We investigate error control capabilities of the Integral Length-Scale Approximation (ILSA) and apply this modeling to transitional and turbulent mixing, focussing on the achieved reliability of LES as function of the grid resolution and ‘sub-filter activity’.\u3c/p\u3

Centrifugal buoyancy driven turbulent convection in a thin cylindrical shell

Centrifugal buoyancy driven convection is closely related to Rayleigh–Bénard convection, and offers another approach to the ultimate regime of thermal convection. Here, we perform direct numerical simulations (DNSs) of centrifugal convection in a cylindrical shell rotating about its axis at constant angular velocity. The walls undergo solid-body rotation, and the flow is purely driven by the temperature difference between the cold inner wall and the hot outer wall. We invoke the thin-shell limit where radial variations in centrifugal acceleration can be neglected. The Prandtl number is 0.7 corresponding to air. For this setup we have two input parameters: 1) the Rayleigh number Ra characterising the driving by centrifugal (buoyancy) effect, and 2) the Rossby number Ro characterising the Coriolis effect. Here, we vary Ra from 107 to 1010, and the inverse Rossby number Ro−1 from 0 (no rotation) to 1. We find that the flow dynamics is subjected to an interplay between the driving buoyancy force and the stabilising Coriolis force, similar to that of Chong et al. (Phys. Rev. Lett., vol. 119, 2017, 064501), but with an important difference owing to the different axis of rotation. Instead of the formation of highly coherent plume-like structures at optimal condition that maximises heat transport, here, the formation of strong bidirectional wind at optimal condition (Ro−opt1 ≈ 0.8) minimises heat transport. By increasing Ra at Ro−opt1, the mean flow approaches the Prandtl–von Kármán (logarithmic) behaviour, yet full collapse on the logarithmic law is not reached at Ra = 1010

Roughness and Reynolds number effects on the flow past a rough-to-smooth step change

We report direct numerical simulations (DNSs) of open-channel flow with a step change from three-dimensional sinusoidal rough surface to smooth surface. We investigate the persistence of non-equilibrium behaviour beyond this step change (i.e. departures from the equilibrium smooth open-channel flow) and how this depends on (1) roughness virtual origin ϵ/hϵ/h? (scaled by the channel height h), (2) roughness size k / h?, (3) roughness shape? and (4) Reynolds number ReτReτ? To study (1), the roughness origin was placed aligned with, below (step-up) and above (step-down) the smooth patch. To study (2), the equivalent sand-grain roughness of the aligned case was decreased from k+s≃ks+≃ 160 to k+s≃106ks+≃106. To study (3) and (4) the step-down case at Reτ≃395Reτ≃395 was compared with a backward-facing step case at Reτ≃527Reτ≃527, and DNS of square rib rough-to-smooth case at Reτ≃1160Reτ≃1160 (Ismail et al., J. Fluid Mech., vol. 843, 2018, pp. 419–449). Results showed that ϵ/hϵ/h affects the departure from equilibrium by a large extent, while k / h, roughness shape and ReτReτ have a marginal influence. The departure from equilibrium was found to be related to the near-wall amplification of Reynolds shear stress, which in turn depends on ϵ/hϵ/h, i.e. higher ϵ/hϵ/h leads to higher amplification

Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy

We investigate the Reynolds analogy over riblets, namely the analogy between the fractional increase in Stanton number Ch and the fractional increase in the skin-friction coefficient Cf, relative to a smooth surface. We investigate the direct numerical simulation data of Endrikat et al. (Flow Turbul. Combust., vol. 107, 2021, pp. 1–29). The riblet groove shapes are isosceles triangles with tip angles α=30∘,60∘,90∘, a trapezoid, a rectangle and a right triangle. The viscous-scaled riblet spacing varies between s+≈10 to 60. The global Reynolds analogy is primarily influenced by Kelvin–Helmholtz rollers and secondary flows. Kelvin–Helmholtz rollers locally break the Reynolds analogy favourably, i.e. cause a locally larger fractional increase in Ch than in Cf. These rollers induce negative wall shear stress patches which have no analogue in wall heat fluxes. Secondary flows at the riblets’ crests are associated with local unfavourable breaking of the Reynolds analogy, i.e. locally larger fractional increase in Cf than in Ch. Only the triangular riblets with α=30∘ trigger strong Kelvin–Helmholtz rollers without appreciable secondary flows. This riblet shape globally preserves the Reynolds analogy from s+=21 to 33. However, the other riblet shapes have weak or non-existent Kelvin–Helmholtz rollers, yet persistent secondary flows. These riblet shapes behave similarly to rough surfaces. They unfavourably break the global Reynolds analogy, and do so to a greater extent as s+ increases.Aerodynamic