Research Topics

  1. Experiments
    1. Experiments -- plasma/gas flow interactions
    2. Experiments -- hypersonic jets
    3. Experiments -- turbulence
    4. Experiments -- bluff body wakes
    5. Devices and Systems for experimental fluid dynamics
  2. Analytic Models for Fluid Dynamics
    1. Perturbation methods for the drag reduction problem
    2. Asymptotic expansions for the bluff-body wakes 
    3. Wake Instability at low Re numbers


Experiments -- plasma/gas flow interactions

These experiments deal with the interaction between non-thermal plasma and gas flows, where non-thermal plasma is created by DC or AC discharges. The possible plasma-gas flow interactions cover a wide field of research, virtually including flow control problems, free coupling of electrical and inertia forces and even plasma control by means of an airflow.

Powered multi-tip DBD on the leading edge of a NACA0015 airfoil
Separation control by corona discharge at low Reynolds numbers
interaction actuation

Electrohydrodynamic propulsion in atmosphere is a promising technology aimed to create thrust by exploiting the ionic wind generated by suitable devices.
It provides several advantages such as absence of moving parts and low noise production. Recent developments point out the importance of electrodes design to maximize thrust and reduce aerodynamic drag.

Propulsion system at test bench Photograph of the system sketched in the left panel                 
setup sketch

The plasma actuators are widely studied because of their ability to produce a ionic wind, that can be used for several flow control purposes. The shape of the electrodes seems to be a very important parameter: in particular, multiple tips on the edge of the active electrodes can improve the efficiency of the actuators.

Sketch of the gas flow generated by a multi-tip corona actuator
Sketch of the gas flow generated by a multi-tip DBD actuator

Flow control by means of DBD actuators creates local tridimensional flow even when the basic flow is bidimensional. When also the basic flow  is inherently tridimensional, the flow topology may remarkably differ from simplified plane representations obtained for instance by a longitudinal section close to the centerline.

Action of a multi-tip DBD actuator installed on the leading edge of a wind tunnel airfoil. The finite span creates a naturally 3D separation which is modified by plasma flow control.

The corona discharge is widely known, with numerous applications ranging from engineering to chemistry and  medicine.  The technology for creating it in a volume of gas is well established,  whereas the development of a corona discharge along a surface is difficult. Surface corona discharges can be stabilized by feeding an electrode with multiple tips by a suitable waveform.

Pulsed generator for surface corona devices Operating principle

Experiments -- hypersonic jets

Highly underexpanded jets

This research project is devoted to the study of hypersonic underexpanded and nearly matched jets. The flows considered here are important both in fluid dynamics and in astrophysics. The facilities are suitable for investigating the spatial evolution of a jet over long scales, in the order of hundreds of initial diameters. It is possible to set the jet-to-ambient density ratio and the jet Mach number independently of each other,  this is obtained using different gases for the jet and the ambient. Observations and measurements are performed by means of an electron gun (designed and built in this laboratory), that makes possible to get fluorescent plane sections of the jets, acquired as digital images. After suitable image processing, thanks to an original method purposely developed, each image becomes a measurement field where density and concentration maps of the different gases can be obtained: this gives information on jet substructures, compressible lateral mixing, and long term jet spreading.

Left panel: an argon jet flowing in an helium ambient, Mach number upstream of the normal shock > 30.
Right panel: a pair of Ar concentration curves along radial sections:  jet/ambient mixing is more effective after the normal shock

z measurements

Nearly matched jets at high Mach numbers

An experiment run in the same facilities as above is devoted to the study of nearly matched jets issuing from de Laval nozzles. These jets have Mach numbers (5 to 20) and jet/ambient density ratios (0.1 to 100) in the same range of the stellar jets known in astrophysics by direct observations and widely studied by means of numerical simulations.The lab tests show that many properties of the stellar jets can be explained in terms of pure fluid dynamics, since MHD and radiation effects are not included in this experiment. In particular, the jet morphology exhibits many known structures as bow and terminal shocks, backward flow cocoons and compression knots . 
The research program involves:  + Politecnico di Milano, + Universita' di Torino, + Politecnico di Torino, + Max-Planck-Institute for Dynamics and Self-Organization - Göttingen 

Electron beam visualization of a Xenon jet flowing in Argon ambient,
reconstruction obtained by juxtaposing adjacent frames of the same movie

( web quality, speed = realtime/1000 )
 Nominal Mach number = 15.
Gas velocity ~ 0.6 km/s.  
Jet/Ambient density ratio >> 1.
A video about this research topic
(italian version, english subtitles)

He jet in Xe ambient       Video    

Experiments -- turbulence

Drag reduction 

The ubiquity of turbulent flows, with their large frictional drag brought about by momentum diffusion, makes the reduction of such drag an objective of pervasive importance in many applications. A new method for forcing a turbulent flow, based on streamwise-traveling waves of spanwise velocity at the wall, is tested by means of an experiment, where a pipe was built with the capability of producing the required wall forcing. The drag reduction can reach 30% and more, with saved energy larger than the control energy by almost two orders of magnitude.

Experiment setup: a turbulent flow in a pipe is forced at the wall by a tangential velocity
 of the kind 'travelling wave' 
w = sin (kx-omega t)
Drag reduction DR (%) at Re=5000: a comparison between DNS results (grid surface) and experimental results
 (large dots fall above the surface, small dots fall below) as functions of frequency and wavenumber of the travelling wave
the pipe results map

Turbulent flow in helically coiled pipe

The study of  turbulent flow in helically coiled pipes is important both for its engineering applications and its scientific interest. Aim of this research project is to describe the turbulent flow in a helical pipe of circular cross-section, and in particular to study the secondary structures in turbulent conditions with an endoscopic Particle Image Velocimetry (PIV) technique.

Experiments -- bluff body wakes

A well known problem in wind engineering is the flow around two bodies in tandem configuration. This experimental investigation studies the important case of two flat plates at 90o  incidence. The experiment is carried out by means of smoke visualizations, hot wire (HWA) and laser doppler (LDA) anemometry.  Accurate measurements of the Strouhal number were obtained for plate separations ranging from 0.25 chords to 7.5 chords. Different flow regimes are described, depending on the distance between the plates, by exploiting the Strouhal number behaviour and the flow visualizations. A narrow interval of distances where two regimes alternatively appear is found; this intermittence is reported in detail by means of simultaneous visualizations, HWA and LDA measurements. Mean and instantaneous flows fields between the two plates and past the aft plate are determined by LDA for each configuration.

______________________ Graphical outline of the bistable flow  ___________________

Wake bistability

__________________ One-body mode measurements __________________
Vorticity and velocity fields

Vorticity/velocity fields
Visualization and velocity field
Visualization/Velocity field

Devices and Systems for experimental fluid dynamics

Many electronic devices and systems for the laboratory are designed by the author of these pages
(low and high voltage power supplies, DC and stepper motor controls, pressure sensor controllers, timers...)

    Example: a DC motor closed-loop velocity controller.
    Input: any velocity waveform, Vpp>1.5V;
    Output: 100W (up to 42Vpp for DC motors from 6 to 24V)
    (click on the figure to enlarge)

schematic diagram example

A pressure probe designed for boundary layer measurements around models not provided with pressure taps
is presented in


Analytic Models for Fluid Dynamics

Perturbation methods for the drag reduction problem

Some applications of the perturbation methods to the drag reduction technique described above can be found in

Asymptotic expansions for the bluff-body wakes 

These works report asymptotic expansion solutions of the momentum equation for the bluff-body wakes at low Re numbers,
particularly useful as basic flow for instability problems.

Wake Instability at low Re numbers

The reference flow under study is the cylinder wake at low Re numbers, altough the theory may be easily generalized to other wakes. The early works are based on weakly nonlinear models. The most recent make use of a multiscale approach where 1/Re is the small parameter: by introducing a spatio-temporal multiscaling into a linearized instability model, a hyerarchy of ODEs is obtained. The first one is a parametric Orr-Sommerfeld equation, the second one is an inhomogeneous equation that gives the non-parallel corrections to the eigenvalues found at the first step, and leads to a modulation equation for the instability wavetrains.The streamwise variation of the instability characteristics can then be deduced from the wave modulation,  as a function of the perturbing wave numbers. It is found that the non-parallel effects are not negligible, and that the absolute instability region in the  first wake is larger than what calculated from a parallel model. The dominant frequencies found here are in good agreement with the global modes of the wake known in literature.

 Top to bottom:
- analytic velocity field (blue streamlines) for the cylinder wake at Re=38
- local instability pulsation with and without non parallel effects
- local instability growth rate with and without non parallel effects

wake instability

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