1. Multiscale Thermofluid Studies Shahram Pouya – Turbulent Mixing and Unsteady Aerodynamics Laboratory , Mechanical Engineering
Develop novel optical measurement techniques to address unique challenges of fluid mechanics problem.
Single Quantum Dot Velocimetry
Molecular Tagging Velocimetry/Thermometry
Application of femtosecond lasers
Fluid Vorticity measurement using laser beams with OAM (Orbital Angular Momentum)
Experimental study of various fundamental and applied fluid mechanics problems from micro to macro scales.
Study of slip/no-slip boundary condition in microchannels.
Surface roughness and slug flow in microreactors.
Diffusion of nano-particles in confined geometries.
Characterization of flow in NASA ZBOT experiment.
Molecular Tagging in air
Flow disturbance induced by femtosecond laser pulses.
Collaborations
Prof. Dantus. Prof. Blanchard (Chemistry)
Prof. Koochesfahani, Prof. Naguib, Dr. Yeom (Mechanical Eng.)
Single QD Velocimetry
MTV with femtosecond excitation
Vorticity measurement using OAM beam
NASA ZBOT
Slug flow in microreactors

2. Single Quantum Dot (QD) Velocimetry
Using Evanescent field illumination to excite and image single (6 nm core size) Quantum Dots within nm of the surface.
Individual QDs are tracked in time to generate an ensemble of displacement/velocity data.
Extracting statistical quantities is not trivial
Exposure effect
Bias diffusion near wall
Using correction factors based on Brownian diffusion simulation, a difference in slip length of 31 nm was measured between hydrophilic and hydrophobic channels.
ICCD camera
Quartz microchannel
(200 μm gap)
Syringe pump
0.5 mL/min
( =5.2 mm/s, Re=202)
θ > θc
Evanescent field
More info: Pouya et al., EIF Pouya et al. APS-DFD Pouya et al., JFM 2008

3. Vorticity measurement using OAM beams
Vorticity is a fundamental parameter in fluid mechanics yet no direct measurement of it is available.
Direct non-intrusive measurement of vorticity.
Relies on Rotational Doppler Effect to measure the angular velocity of a rotating object (similar to LDV used for measuring translational velocity).
An OAM beam imposes a spatial modulation of the phase front.
Successful measurements were demonstrated for a simple flow (solid body rotation) using single 100 micron particles.
Detector
DOF ~ 3 mm
Dichroic mirror
488nm Laser beam
55 mm
Time series and power spectrum of collected signal for two different rotation frequencies of the cylindrical container
Ryabtsev et al., Optics Express 2016

4. Velocimetry and Thermometry in NASA ZBOT
ZBOT is a microgravity experiment being developed for operation on the ISS (as a model for cryogenic storage for Mars missions).
It examines an innovative ventless means of controlling tank pressure through active heat removal and forced liquid jet mixing.
It uses a simulant-fluid (perfluoropentane or PnP) with low boiling temperature.
There has been issues with using particle based techniques (PIV), hence the interest for molecular based methods.
We are developing molecular based methods that can be used for this experiment
Very few molecular tracers into PnP!
Identified Coumarin 153 as a dye that dissolves.
Using expertise of Prof. Blanchard’s lab identified fluorous group that help C153 going into PnP
Synthesized QDs with similar group attachments to make them soluble !
Modified QDs have been shown to have good temperature sensitivity
Velocimetry based on MTV with photobleaching appear possible.
T=23 °C
Emission intensity of QDs is temperature dependent
T=47 °C
Laser sheet used for interrogation of tagged region
Laser beam used for molecular tagging by photobleaching
Thermocouple

5. MTV/MTT using Femtosecond excitation
Using femtosecond pulse of ultrafast infrared lasers for multi-photon resonant excitation of the water-soluble phosphorescent triplex for MTV/MTT measurement.
Multi-photon excitation alleviates one of the constraints in the use of MTV/MTT, the need for short wavelength UV lasers to create the molecular tags and UV optical access in flow facilities.
It also offers the opportunity for MTV imaging at high repetition rates.
Ambient Co-Flow
T~ 26 °C
Cold Jet Flow ~ 15 exit
Re ~ 400
Femtosecond Laser 400 nm
d=2 mm ID
50 mm
2.2 mm
3.8 mm
140µs after laser pulse
4.2 ms delay after first image