The relative motion between fluids and structures generates aerodynamic drag which leads to energy consumption. This increases when functional constraints limit the geometry to bluff body shapes. Although there exist solutions for reducing the drag of such bodies, nowadays devices must satisfy multiple functionalities. The 3D Woven, 3DW, material adheres to this necessity through a periodic, porous architecture, manufactured via 3D weaving techniques. Initially, 3DW was developed to perform heat transfer tasks whilst maintaining the required structural properties. The present work, investigated 3DW material to reduce the aerodynamic drag of circular cylinders. The customization of 3DW structural properties supports the replacement of a solid portion of the cylinder with a porous insert, providing an additional saving in power consumption through weight reduction, provided that the stiffness and strength are maintained. Three configurations with the 3DW placed at the Stagnation, the Separation, and at the Leeward regions were assessed. The direct drag measurements and the static pressure readings at the inner and at the outer surfaces of 3DW material suggested suitable mechanisms for the drag reduction. When placed at the Leeward region, the mean streamlines were deflected within the medium, delaying the separation and increasing the base pressure recovery. Despite the flow similarities flow, the Separation configuration generated detrimental results suggesting the importance of the configuration geometry on the performance. Alternatively, when the coating was placed at the Stagnation region, the incoming flow penetrated the substrate, outflowing near the coating boundaries. Here, it promoted turbulence within the boundary layer and delayed the flow separation. Subsequent modifications of the material topology confirmed the insights on the physics and highlighted the relevance of the internal flows for drag reduction. Besides, the near-wake analysis suggested the impact of the boundary layer velocity profile on the Strouhal number value, additionally to the base pressure coefficient.
High-resolution particle image velocimetry data obtained in rough-wall boundary layer experiments are re-analysed to examine the influence of surface roughness heterogeneities on wind resource. Two different types of heterogeneities are examined: (i) surfaces with repeating roughness units of the order of the boundary layer thickness (Placidi & Ganapathisubramani. 2015 J. Fluid Mech. 782, 541–566. (doi:10.1017/jfm.2015.552)) and (ii) surfaces with streamwise-aligned elevated strips that mimic adjacent hills and valleys (Vanderwel & Ganapathisubramani. 2015 J. Fluid Mech. 774, 1–12. (doi:10.1017/jfm.2015.228)). For the first case, the data show that the power extraction potential is highly dependent on the surface morphology with a variation of up to 20% in the available wind resource across the different surfaces examined. A strong correlation is shown to exist between the frontal and plan solidities of the rough surfaces and the equivalent wind speed, and hence the wind resource potential. These differences are also found in profiles of Ū2 and Ū3 (where U is the streamwise velocity), which act as proxies for thrust and power output. For the second case, the secondary flows that cause low- and high-momentum pathways when the spacing between adjacent hills is beyond a critical value result in significant variations in wind resource availability. Contour maps of Ū2 and Ū3
Wind-tunnel experiments were carried out on fully-rough boundary layers with large roughness (δ/h≈10 δ/h≈10, where h is the height of the roughness elements and δ δ is the boundary-layer thickness). Twelve different surface conditions were created by using LEGO™ bricks of uniform height. Six cases are tested for a fixed plan solidity (λ P λP) with variations in frontal density (λ F λF), while the other six cases have varying λ P λP for fixed λ F λF. Particle image velocimetry and floating-element drag-balance measurements were performed. The current results complement those contained in Placidi and Ganapathisubramani (J Fluid Mech 782:541–566, 2015), extending the previous analysis to the turbulence statistics and spatial structure. Results indicate that mean velocity profiles in defect form agree with Townsend’s similarity hypothesis with varying λ F λF, however, the agreement is worse for cases with varying λ P λP. The streamwise and wall-normal turbulent stresses, as well as the Reynolds shear stresses, show a lack of similarity across most examined cases. This suggests that the critical height of the roughness for which outer-layer similarity holds depends not only on the height of the roughness, but also on the local wall morphology. A new criterion based on shelter solidity, defined as the sheltered plan area per unit wall-parallel area, which is similar to the ‘effective shelter area’ in Raupach and Shaw (Boundary-Layer Meteorol 22:79–90, 1982), is found to capture the departure of the turbulence statistics from outer-layer similarity. Despite this lack of similarity reported in the turbulence statistics, proper orthogonal decomposition analysis, as well as two-point spatial correlations, show that some form of universal flow structure is present, as all cases exhibit virtually identical proper orthogonal decomposition mode shapes and correlation fields. Finally, reduced models based on proper orthogonal decomposition reveal that the small scales of the turbulence play a significant role in assessing outer-layer similarity.
Experiments on the stability of a 3D boundary layer were performed in a very low turbulence wind tunnel (Tu ͌ 0:006%U¥). The effect of different shapes of surface steps (of h = 200 mm) located at 20% chord were investigated by looking into the crossflow modes evolution and growth. Stable crossflow vortices were generated by the means of discrete roughness elements (DREs) positioned upstream of the steps. Preliminary results seem to suggest that the different step geometries have a severe influence on both the maximum disturbance growth and the excitation of the primary mode and its harmonics. These different surface imperfections also seem to play a critical role on the appearance of the non-linear phase of the instability. Finally, the different step geometries are shown to influence the transition front location by up to 9%, which results in performance degradation. The softer and more gradual geometrical disturbance (i.e. Pyramid-type step) was found to minimise the performance loss, providing that each step comprising the complex geometry is designed to be conservatively subcritical.
Introduction and background Vegetation in both fresh and sea waters is not only ubiquitous in natural habitats but also instrumental for a variety of reasons. It provides the foundation for many food chains , contributes to the thriving of fish and corals , plays a role in reducing coastal erosion  and drastically improves the water quality by producing oxygen . Furthermore, many engineering applications rely upon and would benefit from a better understanding of the flow physics characterising these problems. Despite the numerous reviews [2, 5, 6] that have attempted to capture different aspects of canopy flows over flexible vegetation, a satisfactory understanding of this topic is still elusive. For this reason, a simple controlled experiment aimed at comparing wall-bounded flows over rigid and flexible roughness was designed and carried out. Experimental facility and details Three different surfaces are considered in this work: a smooth wall and two rough-wall cases. The first rough surface is characterised by rigid roughness (i.e. conventional rough wall), while in the second case the flow develops over flexible roughness elements (i.e. aquatic vegetation). Experiments were designed to compare the statistical properties of flexiblerough beds as opposed to their rigid counterpart when the roughness height under wind loading, heff , is matched. The tests were carried out in the Donald Campbell wind tunnel at Imperial College London (freestream turbulence Tu < 0:5%U1). The tunnel working section measures 2:98 m in length, with a 1:37 m x 1:12 m cross section. The conditions were set to represent a nominally zero-pressure gradient at a freestream velocity of 12 ms
Placidi Marco, Atkin Chris J. (2018)Effect of imperfection geometry on the stability of 3d boundary layers, In: Proceedings of The 6th European Conference on Computational Mechanics (Solids, Structures and Coupled Problems) (ECCM 6) and the 7th European Conference on Computational Fluid Dynamics (ECFD 7)
European Community on Computational Methods in Applied Sciences (ECCOMAS)
Interest in laminar flow flight due to both economic and environmental factors has recently seen a resurgence (Tufts et al., 2017). Within this topic, the study of the detailed effect of surface excrescences on laminar/turbulent transition has received significant attention (Fage, 1943; Schlichting, 1979). However, most of the previous work has focused on the effect of steps in 2D environments (i.e. in the absence of a pressure gradient), while the effect of steps on a 3D wings has received less attention (Bender et al., 2005). Therefore, experiments on the stability of 3D boundary layers were performed in a very low turbulence wind tunnel by examining the effect of different excrescences, of a height of approximately one-third of the local displacement thickness, δ*, located at 20% chord. Three different stepped geometries (see figure 1) are considered in order to mimic the leading edge to wing box joint characterising new concepts of laminar flow wings. Results show, as expected, that all surface imperfections reduce the extent of the laminar flow region when compared to the case in the absence of a step. However, despite the severity of the excrescences, this reduction is very moderate, which suggests scope to relax current laminar flow wing tolerances. The pyramidal geometry (in figure 1c), with more gradual forward- and aft-facing steps is it found to be optimum, as the performance degradation is the lowest. Results also suggest that the different step geometries have an influence on both the excitation of the primary modes (and its harmonics) and the onset of the nonlinear phase of the instability. Further analysis will follow in the full paper.
Experiments on the receptivity of two-dimensional boundary layers to acoustic disturbances from two-dimensional roughness strips were performed in a low-turbulence wind tunnel on a at plate model. The freestream was subjected to a plane acoustic wave so that a Stokes Layer (SL) was created on the plate, thus generating a Tollmien-Schlichting (T-S) wave through the receptivity process. An improved technique to measure the T-S component is described based on a retracting two-dimensional roughness, which allowed for phase-locked measurements at the acoustic wave frequency to be made. This improved technique enables both protuberances and cavities to be explored in the range 30m < jhj < 750m (equivalent to 0:025 < jhj=B < 0:630 in relative roughness height to the local unperturbed Blasius boundary layer displacement thickness). These depths are designed to cover both the predicted linear and non-linear response of the T-S excitation. Experimentally, cavities had not previously been explored. Results show that a linear regime is identifiable for both positive and negative roughness heights up to 150 m (jhj=B 0:126). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of signicant depth, the non-linear behaviour is found to be milder than in the case of protuberances - this is attributed to the flow physics in the near field of the surface features. Nonetheless, results for positive heights agree well with previous theoretical work which predicted a linear disturbance response for small-height perturbations.
A triple slotted aerofoil following the Handley Page 44F design was tested at City University London T-2 wind tunnel. The model allowed the study of a fixed triple slotted wing as well as investigation of the effects of isolated slots at different locations along the chord. PIV measurements were performed within the chord Reynolds number range in between approximately 200,000-400,000. The model was tested at an angle of attack of 22o. Measurements of mean streamwise velocity, velocity fluctuations and shear stress were analysed. The study shows how an isolated slot is more favourable when it is placed closest to the leading edge, although slow moving fluid regions can still be found close to the trailing edge. Fully attached flow was only achievable by using all three slots. In addition, the fully slotted profile is shown to generate channel exit velocities in the order of 1.4U∞, which highly energise the boundary layer on the suction side.
Wind tunnel experiments were conducted in a low-turbulence environment (Tu < 0:006%) on the stability of 3D boundary layers. The effect of two different distributions of discrete roughness elements (DREs) on crossfl ow vortices disturbances and their growth was eval uated. As previously reported, DREs are found to be an effective tool in modulating the behaviour of crossfl ow modes. However, the effect of 24μm DREs was found to be weaker than previously thought, possibly due to the low level of environmental disturbances here with. Preliminary results suggest that together with the height of the DREs and their spanwise spacing, their physical distribution across the surface also intimately affects the stability of 3D boundary layers. Finally, crossfl ow vortices are tracked along the chord of the model and their merging is captured. This phenomena is accompanied by a change in the critical wavelength of the dominant mode.
Experiments were conducted in the fully rough regime on surfaces with large relative roughness height (h/δ ≈ 0.1, where h is the roughness height and δ is the boundary layer thickness). The surfaces were generated by distributed LEGOr bricks of uniform height, arranged in different configurations. Measurements were made with both floating-element drag balance and high-resolution particle image velocimetry on six configurations with different frontal solidities, λF, at fixed plan solidity, λP, and vice versa, for a total of twelve rough-wall cases. The results indicated that the drag reaches a peak value λF ≈ 0.21 for a constant λP = 0.27, while it monotonically decreases for increasing values of λP for a fixed λF = 0.15. This is in contrast to previous studies in the literature based on cube roughness which show a peak in drag for both λF and λP variations. The influence of surface morphology on the depth of the roughness sublayer (RSL) was also investigated. Its depth was found to be inversely proportional to the roughness length, y0. A decrease in y0 was usually accompanied by a thickening of the RSL and vice versa. Proper orthogonal decomposition (POD) analysis was also employed. The shapes of the most energetic modes calculated using the data across the entire boundary layer were found to be self-similar across the twelve rough-wall cases. However, when the analysis was restricted to the roughness sublayer, differences that depended on the wall morphology were apparent. Moreover, the energy content of the POD modes within the RSL suggested that the effect of increased frontal solidity was to redistribute the energy towards the larger scales (i.e. a larger portion of the energy was within the first few modes), while the opposite was found for variation of plan solidity.
An experimental investigation on the influence of the spatial frequency content of roughness distributions on the development of crossflow instabilities has been carried out. From previous research it is known that micro roughness elements can have a large influence on the crossflow development. When the spanwise spacing is chosen such that it is the most unstable wavelength (following linear stability analysis), stationary crossflow waves are amplified. While in earlier studies the focus was on the height or spanwise spacing of roughness elements, in the present study it is chosen to vary the shape of the elements. Through the modification of the shape the forcing at the critical wavelength is increased, while the forcing at the harmonics of the critical wavelength is damped. Experiments were carried in the low turbulence wind tunnel at City University London (Tu=0.006%) on a swept flat plate in combination with displacement bodies to create a sufficiently strong favourable pressure gradient. Hot wire measurements across the plate tracked the development of stationary and travelling crossflow waves. Initially, stronger crossflow waves were found for the elements with stronger forcing, while further downstream the effect of forcing diminished. Spatial frequency spectra showed that the stronger forcing at the critical wavelength (via the roughness shape) dominates the response of the flow while low forcing at the harmonics has no notable effect. Additionally, high resolution streamwise hot wire scans showed that the onset of secondary instability is not significantly influenced by the spatial frequency content of the roughness distribution.