Dr Placidi obtained both a BEng. in Aerospace Engineering and a MEng. in Aeronautics Engineering from La Sapienza - University of Rome (Italy). He obtained his Ph.D. in Engineering Sciences from the University of Southampton (UK) in 2015 for his work on wall-bounded turbulence in urban environments. He then joined City, University of London, where he worked as a Research Fellow on a series of projects in close collaboration with the aeronautical industry (e.g. Airbus, Airbus CR&T, Airbus Defence and Space, ESDU). At City, the work was predominantly in the field of flow instability and laminar/turbulent transition. He has recently (2018) joined the Department of Mechanical Engineering Sciences (Environmental Flow Research Centre) at the University of Surrey as a Lecturer in Experimental Fluid Mechanics.
Affiliations and memberships
- Environmental flows
- Urban dispersion
- Rough-wall boundary layers
- Laminar-flow control
- Wind power aerodynamics
- Traffic Modelling Group, Transport for London, UK
- Department of Meteorology, University of Reading, UK
- Mesoscale Modelling Group, Met Office, UK
- Central Research and Technology - Airbus Operation Limited
- Global Centre for Clean Air Research (GCARE), University of Surrey, UK
- Department of Mechanical Engineering & Aeronautics - City, University of London, UK
- Department of Mathematics - Imperial College London, UK
- Aerodynamics and Flight Mechanics Group - University of Southampton, UK
were performed in a very low turbulence wind tunnel (Tu L
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.
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, ´*, 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.
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
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 24¼m 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.
roughness height (h/´ H 0.1, where h is the roughness height and ´ 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, »F, at fixed plan solidity, »P,
and vice versa, for a total of twelve rough-wall cases. The results indicated that the
drag reaches a peak value »F H 0.21 for a constant »P = 0.27, while it monotonically
decreases for increasing values of »P for a fixed »F = 0.15. This is in contrast to
previous studies in the literature based on cube roughness which show a peak in drag
for both »F and »P 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.
Placidi, M. and Ganapathisubramani, Bharathram (2019). Velocity statistics for rough-wall turbulent boundary layer flow over LEGO roughness elements in different layouts. University of Southampton doi:10.5258/SOTON/D0829.