Research

Molecular Energy Transfer Processes in Non-equilibrium Hypersonic Flows (Non-Equilibrium Thermodynamics Laboratory)

Advanced, laser-based optical diagnostics, such Coherent Raman Anti Stokes Spectroscopy (CARS), are employed to study and characterize high-speed flows with a high degree of vibrational non-equilibrium (Tv > T). The ultimate goal is to affect such flows by controlling the vibrational relaxation process and release of thermal energy at desired locations.

1. Optical Diagnostics

CARS setup for the supersonic non-equilibrium tunnel at NET

The primary method of measuring the vibrational and rotational temperatures in this project was CARS diagnostics. An in-house built broadband dye laser pumped by a commercial Nd:YAG laser was used to produce the Stokes beam. To ascertain T and Tv immediately downstream of the pulser-sustainer discharge, the pump and Stokes beams were guided through an optically translucent section of the tunnel plenum. Measurements showed a relatively high degree of vibrational non-equilibrium (T ˜ 400 K, Tv ˜ 1300 K) in the tunnel plenum at 300 Torr.  Measurements in the supersonic flow where the pressure is significantly lower ( ˜ 1.5 – 2 Torr) will be much more challenging.

CARS Setup Schematic

Vibrational relaxation in the tunnel plenum is characterized by a dual-pump CARS system, which allows measurements of N2 vibrational temperature and N2-CO2 mixture ratio. 30% of the second harmonic output from a Nd:YAG laser is split to pump a home-built broadband dye laser (output spectral pro le of 10 nm FWHM around 607 nm). This broadband dye laser employs a side-pumped oscillator cell and a Bethune pre-ampli er cell. The bore diameters of the pre-ampli er and ampli er are 2.6 mmand 4.0 mm, respectively. 60% of the second harmonic output pumps a tunable narrowband dye laser to generate a second pump beam at 561 nm. The rest is used as a rst pump beam of the CARS process. The pulse energies of the first pump, second pump, and the Stokes beams are 25 mJ, 12 mJ, and 10 mJ, respectively. These beams are focused in the test section through an f=100 mm lens, with the probe volume length of up to 5 mm. The generated CARS signal is  then resolved by a f=300 mm spectrometer . More details can be found in Nishihara’s SciTech 2016 paper.

 

2. CO2 Injection for Vibrational Relaxation

Previous work at NETL had demonstrated thermal energy release in a M = 2.7 non-equilibrium flow via injecting various relaxer species into the flow. To test the possibility of obtaining the same result in a M = 5 flow, special nozzle inserts with injection ports were designed and built. While relaxation can be easily achieved with CO2 injection in the high-pressure plenum (see the spectra below), obtaining the same result would be challenging in the low-pressure supersonic section of the tunnel as the relaxation time constant and CARS signal strength are directly related to pressure.

CARS spectra before and after CO2 injection in the plenum
Excited Flow Spectra Demonstrating Vibrational Excitation w/ Discharge in the Tunnel Plenum

Physics and Excitation of Flow over a Rotorcraft Blade Using Nanosecond Dielectric Barrier Discharge Actuators (Gas Dynamics and Turbulence Laboratory)

1. Effects of Low and High Strouhal Number Flow Excitation

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Time-averaged normalized total velocity at α = 19° and Re = 500,000

In the first part of the study, the effects of exciting a separated flow at a low Strouhal number (Ste < 1) were explored by employing various diagnostic techniques such as surface pressure measurements, ensemble-averaged and phase-locked PIV. The data indicates that low-frequency excitation leads to the generation of large-scale coherent structures that harvest momentum from the free-stream flow. The periodic shedding of these structures leads to a phenomenon known as shear layer flapping which entails unsteadiness in the separation line and unsteady ejection of vorticity concentrations into the wake.

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Isolines of zero velocity plotted over normalized streamwise velocity and swirling strength maps for excited flows

Medium to high-frequency excitation(4 < Ste < O [10]), on the other hand, results in the generation of small-scale structures that perform the same function as their larger counterparts but do not harvest as much energy from the flow. A significant reduction in drag has been observed as a result. While the lift enhancement by the large-scale structures is slightly higher than the one generated by the smaller structures, no unsteady loads will be experienced by the airfoil as a result of high-frequency excitation which would be a critical factor in rotorcraft applications. Further details on this investigation can found in our recently published AIAA Journal article.

2019 AIAA J Article

 

2. Flow Three-dimensionality and Stall Cells

Earlier measurements had indicated a significant discrepancy between PIV and surface pressure data –collected at different spanwise locations- which suggested that the flow field might not be two-dimensional as previously had been assumed. Preliminary experiments with surface pressure paint also confirmed this hypothesis. Conclusive evidence of the existence of stall cells for both baseline and controlled cases has been obtained through the use of fluorescent surface oil flow visualization.

The effects of excitation frequency on the development of surface topology for baseline and excited cases

As suggested by the PIV data and confirmed by SOFV experiments and PSP measurements, high-frequency excitation significantly reduces the size of the stall cells and eliminates unsteadiness in separation line location. Efforts are under way to collect stereo-PIV data downstream of the airfoil trailing edge to characterise the vortex nodes that are present due to stall cells. The details of this work on stall cells can be found in our recently published Experiments in Fluids article.

2018 EiF Article

 

 

 

Maps of normalized streamwise velocity and fluorescent surface oil flow visualisation for baseline and excited flow field at St =10.73
Agreement between streamwise 2D-2C PIV and spanwise stereo PIV data for the baseline and excited flow field at St = 4.27
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PSP and PIV data indicating 3D flow features for low (left) and high (right) frequency excitation
Comparison between surface topology patterns obtained from fluorescent surface oil flow visualisation and PSP for low-frequency (left) and high-frequency (right) excited cases

2017 AIAA SciTech Presentation

3. Excitation Waveform and Energy Coupling to the Flow

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Compression waves generated as a result of energy coupling to the flow

Plasma actuators function primarily through coupling energy to the flow in order to excite instabilities and any improvement in energy conversion efficiency would lead to higher control authority. The pressure rise behind the compression waves generated by the discharge, as seen in Fig. 7, is believed to be correlated to the amount of energy coupled to the flow hence it can be used to track changes in the coupled energy as a result of modifications of excitation waveforms.

The effects of modulating a low pulse repetition rate excitation signal with high-frequency (1 and 2 kHz) carrier waveforms have been studied in this work. Preliminary data, acquired in quiescent flow conditions, indicate that the compression waves become stronger when a higher frequency carrier waveform is used. Wind tunnel experiments suggest that using a frequency modulated signal would lead to an increase in drag coefficient. Stronger vortices generated with frequency modulated excitation might be responsible for the observed effects.

2016 AIAA SciTech Presentation

2015 MAE Dept. Research Poster

AC-DBD Plasma Streamwise Vortex Generators for Separation Control (DANA Aerodynamics and Turbomachinery Laboratory)

2014 Aero Dept. Research Poster

2014 Surface Oil Flow Visualization Poster