Marco Placidi

Dr Marco Placidi


Lecturer in Experimental Fluid Mechanics
BEng, MEng, PhD
+44 (0)1483 684632
01 AB 02
Wednesday 09:00-11:00 am

Biography

My qualifications

2015
Ph.D. in Engineering Sciences
University of Southampton
2010
MEng. in Aeronautical Engineering
Sapienza University of Rome
2006
BEng. in Aerospace Engineering
Sapienza University of Rome

Affiliations and memberships

The Royal Aeronautical Society
MRAeS
American Physics Society
Early Career Member
American Institute of Aeronautics and Astronautics
Member

Research

Research interests

Research collaborations

My teaching

My publications

Publications

Vanderwel C., Placidi M., Ganapathisubramani B. (2017) Wind resource assessment in heterogeneous terrain, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2091) pp. 1-14 The Royal Society
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 j2 and j3 (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 j2 and j3
Placidi Marco, Atkin Chris J. (2016) On the effect of different step geometries on disturbance growth in 3D boundary layers, Proceedings of the 2016 RAeS Applied Aerodynamics Conference, 19 - 21 July 2016, Bristol, UK Royal Aeronautical Society
Experiments on the stability of a 3D boundary layer
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.
Placidi Marco, Atkin Chris J. (2018) Effect of imperfection geometry on the stability of 3d boundary layers, 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, ´*, 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.

Placidi M., Atkin C. (2017) Surface imperfections effect on the stability of 3D boundary layers, Proceedings of European Drag Reduction and Flow Control Meeting Villa Mondragone Monte Porzio Catone (Rome) April 3-6 2017 Springer
Placidi M., Ganapathisubramani B. (2018) Turbulent Flow Over Large Roughness Elements: Effect of Frontal and Plan Solidity on Turbulence Statistics and Structure, Boundary-Layer Meteorology 167 (1) pp. 99-121 Springer Verlag
Wind-tunnel experiments were carried out on fully-rough boundary layers with large roughness (´/hH10 ´/hH10, where h is the height of the roughness elements and ´ ´ 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 (» P »P) with variations in frontal density (» F »F), while the other six cases have varying » P »P for fixed » F »F. 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 » F »F, however, the agreement is worse for cases with varying » P »P. 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.
Placidi Marco, Ganapathisubramani Bharathram (2017) Turbulent wall-bounded flows over rigid- and flexible-rough beds, Proceedings of the 16th European Turbulence Conference, 21-24 August, 2016, Stockholm, Sweden European Mechanics Society
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 [4], contributes to the thriving of fish and corals [6], plays a
role in reducing coastal erosion [1] and drastically improves the water quality by producing oxygen [3]. 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, van Bokhorst Evelien, Atkin Christopher J. (2016) On the effect of discrete roughness on crossflow instability in very low turbulence environment, Proceedings of the 8th AIAA Flow Control Conference, AIAA AVIATION Forum American Institute of Aeronautics and Astronautics
Wind tunnel experiments were conducted in a low-turbulence environment (Tu 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 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.
Coelho Lazaro, Placidi Marco, Atkin Chris, Sun Zhengzhong (2016) Experimental Investigation of a Handley Page Triple Slotted Aerofoil, Proceedings of the 2016 RAeS Applied Aerodynamics Conference, 19 - 21 July 2016, Bristol, UK Royal Aeronautical Society
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.
Placidi Marco, Atkin C. J. (2016) Effect of different surface roughnesses on crossflow instability, DiPaRT 2016 Flight Physics Symposium The Airbus Flight Physics Distributed Partnership R&T
Placidi M., Ganapathisubramani B. (2015) Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers, Journal of Fluid Mechanics 782 pp. 541-566 Cambridge University Press (CUP)
Experiments were conducted in the fully rough regime on surfaces with large relative
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.

Additional publications