Lab

Bharathram Ganapathisubramani's Lab

Featured projects (1)

Project
In this project, funded by the European Commission under H2020 Grant ID 769237, we are developing innovative experimental techniques and novel data analysis strategies for the experimental characterization of aeroelastic problems and fluid-structure interactions.

Featured research (11)

Mean-flow measurements of turbulent boundary layers over porous walls (permeable and rough) with varying pore size (s), permeability (K) and thickness (h) are presented across a wide range of friction Reynolds numbers (Reτ≈2000–18000) and permeability based Reynolds numbers (ReK≈1.5–60). The mean wall shear stress was determined using a floating element drag balance and the boundary layer profiles were acquired using hot-wire anemometry. Substrate permeability is shown to increase the magnitude of the mean velocity deficit. The use of a modified indicator function, assuming “universal” values for von Karman constant (κ=0.39) supports previous results where a strongly modified logarithmic region was observed. The indicator function was also used to estimate the zero-plane displacement (yd), the roughness function (ΔU+), and equivalent sandgrain roughness (ks). At high Reynolds numbers, the roughness function data collapses on to the Nikuradse's fully rough asymptote. However, at low roughness Reynolds numbers (ks+<100), we observe the flow to be transitionally rough, evolving with Nikuradse-type behavior. The equivalent sandgrain roughness ks for each substrate appears to include roughness and permeability contributions. These two contributions can be separated using data obtained from the same substrates with different thickness. This may allow us to model the porous wall as a combination of rough and permeable wall.
In this work, we describe the impact of aspect ratio (AR) on the performance of optimally phased, identical flapping flippers in a tandem configuration. Three-dimensional simulations are performed for seven sets of single and tandem finite foils at a moderate Reynolds number, with thrust producing, heave-to-pitch coupled kinematics. Increasing slenderness (or aspect ratio - AR) is found to improve thrust coefficients and thrust augmentation but the benefits level off towards higher values of AR. On the other hand, the propulsive efficiency shows no significant change with increasing AR, while the hind foil outperforms the single by a small margin. Further analysis of the spanwise development and propagation of vortical structures allows us to gain some insights on the mechanisms of these wake interactions and provide valuable information for the design of novel biomimetic propulsion systems.
The motion of thin curved falling particles is ubiquitous in both nature and industry but is not yet widely examined. Here, we describe an experimental study on the dynamics of thin cylindrical shells resembling broken bottle fragments settling through quiescent fluid and homogeneous anisotropic turbulence. The particles have Archimedes numbers based on the mean descent velocity 0.75 × 10 4 Ar 2.75 × 10 4. Turbulence reaching a Reynolds number of Re λ ≈ 100 is generated in a water tank using random jet arrays mounted in a coplanar configuration. After the flow becomes statistically stationary, a particle is released and its three-dimensional motion is recorded using two orthogonally positioned high-speed cameras. We propose a simple pendulum model that accurately captures the velocity fluctuations of the particles in still fluid and find that differences in the falling style might be explained by a closer alignment between the particle's pitch angle and its velocity vector. By comparing the trajectories under background turbulence with the quiescent fluid cases, we measure a decrease in the mean descent velocity in turbulence for the conditions tested. We also study the secondary motion of the particles and identify descent events that are unique to turbulence such as 'long gliding' and 'rapid rotation' events. Lastly, we show an increase in the radial dispersion of the particles under background turbulence and correlate the time scale of descent events with the local settling velocity.
Free-stream turbulence characteristics play an important role in the mechanisms of power harvesting for wind turbines. Acquisitions of power and thrust from a model wind turbine of diameter 0:18m have been carried out in a wind tunnel for a wide range of turbulent base flows, with varying free-stream turbulence intensity in the range between 3% and 16% and integral time scale spanning from 0:1 to 10 times the turbine rotation period. The results demonstrate that power is significantly affected both by the inflow turbulence scales and its intensity, while thrust is scarcely affcted by free-stream turbulence. Fluctuations in the generated torques are also measured, with their behaviour dominated by the free-stream turbulence scale, and only moderately affected by turbulence intensity. The frequency response of thrust fluctuations has been measured for a selected subset of operating conditions, demonstrating that the turbine thrust is unaffected by high-frequency components in the in flow. Conclusions are drawn on the necessity to match both turbulence intensity and base flow frequency content in wind-tunnel studies if realistic results are to be obtained from small-scale studies.

Lab head

Bharathram Ganapathisubramani
Department
  • Aerodynamics and Flight Mechanics Group,

Members (12)

Renan Soares
  • University of Southampton
Luis Blay
  • Bardehle Pagenberg
Prateek Jaiswal
  • University of Southampton
Girish K. Jankee
  • Norwegian University of Science and Technology
Manuel Ferreira
  • University of Southampton
Claudia Nicolai
  • University of Southampton
Nikolaos S. Lagopoulos
  • Dolprop Industries AB
Takfarinas Medjnoun
  • University of Southampton