Session M14: Jets: General

Comparing unsteady and steady jets issued into a flat plate laminar boundary layer

Jets have been widely used for flow control applications, due to their ability to enhance mixing and control separation, but it is unclear the role jet steadiness plays in flow control effectiveness.  Here we compare unsteady (synthetic) and steady rectangular jets issued into a laminar boundary layer with special focus on the flow features and statistics.  The compared jets had matched velocities, geometry, and orientation.  The effects of varying pitch (the angle between the jet exit centerline and the wind tunnel floor) and skew (rotation about the jet centerline) of the jet orifice are highlighted.  We used stereoscopic particle image velocimetry to capture spanwise planes upstream and downstream of the orifice.  The vortices produced by synthetic jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns.  Exploring the time-- and phase--averaged vorticity transport equation reveals that the time varying vorticity term is the reason for the enhanced vortex structure.  In addition, we compare and contrast steady and unsteady jets through measures of flow control effectiveness---including mixing, boundary layer momentum addition, and jet penetration.   

Dynamics of a plane pulsed jet

Pulsed jets are now extensively used in a wide variety of engineering devices for advanced aerodynamic applications, namely drag and lift control, heat transfer, vortex generation, noise reduction and mixing. Despite their efficacious use in so many applications, a clear understanding of the fundamental physics of an isolated pulsed jet has not been addressed yet. A plane pulsed free jet in the Reynolds number range of 50 – 500, and Strouhal number range of 0.01-0.5 was studied with DNS. Various lip configurations were changed in the nozzle to produce different vortical structures. With a zero net inflow mass flux, the pulsed jet, generates a downstream mean momentum flux which is extremely sensitive to the inflow conditions and nozzle geometry. Interestingly, with a constant Reynolds number, increasing the Strouhal number enhances the entrainment rate, presumably due to vortex pairing occurring closer to the inlet. Additionally, the mean momentum flux shows to increase downstream. We are addressing critical questions that will help to explain complex 3D configurations (e.g. synthetic jets) and their applications in flow control.   

On the generation of steady fountains by high intensity focused ultrasonic: flow field and simple formulation

Laboratory experiments were conducted to investigate the formation of fountains at a quiescent water-air interface with high-intensity focused ultrasound. Particle image velocimetry was used to characterize the flow field within and below the vicinity of the ultrasonic focal spot. We studied two types of ultrasonic transducers with different wave frequencies. Three fountain formation regimes were induced by varying the pressure level of the two transducers, namely weak, intermediate, and high-forced fountains. Between different types, the fountain height underwent a step-change in response to increases in acoustic pressure. A force balance using the flow field data shows that the magnitude of axial momentum flux is an order of magnitude lower than that of gravity and surface tension, indicating that the dominant driving force for the fountain generation is the acoustic pressure. We propose a basic model for the stable fountain shape considering the acoustic pressure, gravity, surface tension, and axial momentum. Our model neglects viscous force, which precludes capturing the intermediate fountain surface curvature; however, it predicts the geometry of the weak and intermediate steady fountains.   

The Counter-Current Shear Layer Analogy for Transverse Jets of Differing Geometries

This experimental study investigates upstream shear layer (USL) instabilities of a jet in crossflow for a variety of jet injector geometries. Prior work demonstrates that an improved analogy of a counter-current shear layer (CCSL), applied to the USL of a flush-nozzle injected equidensity transverse jet, can approximate whether the USL is convectively or absolutely unstable. The extension of this analogy to alternative jet injector geometries, including an elevated nozzle, tabbed flush nozzle, coaxial nozzle-generated jet, and flush round pipe, is explored. USL spectral measurements and PIV-based POD are used to determine the state of the USL instability. POD mode coefficient plots demonstrate qualitative consistency with spectral measurements in denoting convective or absolute instabilities. Velocity profiles and momentum thicknesses evaluated at the leading edge of the jet reveal good agreement between USL transition and CCSL critical velocity ratios predicted from theory. The CCSL analogy is found to be very robust in predicting USL transition, though it begins to fail for flush pipe-generated jets with a parabolic velocity profile and a diminished resemblance to counter-current shear layer phenomena.

Supported by AFOSR Grant FA9550-19-1-0191, NSF Grant CBET-1933310, and CAPES   

Optimized eddy-viscosity models for coherent structures in turbulent jets

The inclusion of eddy-viscosity models in resolvent analyses of turbulent jets gives dominant modes that compare favorably to observed structures. In previous studies only a limited region of the flow parameter space has been examined. In this study, we investigate the effects of different azimuthal modes and regions of the jet that had not been considered previously. We determine an optimal eddy-viscosity field by applying a Lagrangian optimization framework that maximizes the projection between resolvent analysis and spectral proper orthogonal decomposition (SPOD) modes determined from a large-eddy-simulation database for a round Mach 0.4 jet. We find that while the optimal eddy viscosity substantially improves the agreement between the resolvent and SPOD modes for all frequencies investigated (i.e., low to moderate frequencies), the improvements are inferior to those of the axisymmetric modes. As the first and second helical modes contain the most energetic structures at these frequencies, determination of a more effective turbulence model is important for predictive models. We investigate some alternatives suggested by the data.   

Modeling Turbulent Jets Using Turbulence Models Developed for Hydrodynamic Instability-Induced Turbulent Mixing

The hydrodynamics in inertial confinement fusion (ICF) capsules imploded by laser energy is very complex, involving both weakly-nonlinear Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instability growth and potentially turbulent mixing. In addition to these instabilities, other flow features are present such as jets produced from fill tubes and other structural elements in the targets. Thus far, the emphasis of turbulence modeling in ICF capsules has been on hydrodynamic instability-induced mixing: the models have been optimized and calibrated to predict canonical RT and RM flows. This raises the question of whether such models can also reasonably predict turbulent jets interacting with mixing layers. Using numerical solutions to the self-similar incompressible turbulent jet equations with coefficients used in K–ε and K–L-based RT and RM models, it is shown that these models cannot adequately predict key properties of such jets. An explanation for this is provided and further discussed.   

Visualization of the flow generated by human exhalation during jet ventilation using background-oriented schlieren (BOS)

Laryngeal surgery necessitates the use of jet ventilation, which can expel airborne viral pathogens from the patient's lungs into the operating room, placing the medical team at risk. In the present study, we use background-oriented schlieren (BOS) imaging to visualize the flow generated by the exhalations of a manikin under jet ventilation with several different ventilation settings. BOS has several advantages that make it well suited to visualize human-scale flows in a medical setting, which are leveraged in this work. We present high-speed image data using a large field-of-view BOS system for this case study and discuss some practical aspects of BOS imaging in general. The ability to visualize the flow allows us to evaluate the risk of airborne pathogens as a function of jet ventilation settings, and opens avenues for new strategies to minimize the chance of contamination for medical staff. Additionally, the successful demonstration of BOS in this context suggests future opportunities for the application of BOS imaging in other healthcare-related settings.   

Application of spectral proper orthogonal decomposition and resolvent analysis to periodically forced turbulent jets

While passive and active control strategies have been applied to turbulent jets to reduce their radiated sound, the mechanisms by which the forcing alters the turbulence and far-field sound are poorly understood. We perform large-eddy simulations of axisymmetric turbulent jets subjected to axisymmetric periodic forcing at multiple frequencies and amplitudes. Spectral proper orthogonal decomposition is used to study the effect of the forcing on the turbulence spectrum. Low-frequency forcing, St_f=0.3, while producing highly energetic tonal structures, has a limited effect upon the underlying turbulence spectrum of the jet and the most energetic modes. High levels of forcing, 1% of the jet velocity, are required to achieve a small change to the turbulent mean flow and a minor shift in the turbulence spectrum. The changes in the spectrum and the shift in the modes are predicted well via the resolvent analysis performed on the new turbulent mean flow. This suggests that there is little nonlinear interaction between the phase-locked structures and the natural turbulence. High-frequency forcing, St_f=1.5, shows similarly linear behavior except for around St≈0.75, which appears to be associated with vortex pairings triggered by the natural turbulence.   

Pattern of multiphase magnetohydrodynamic jet driven by Lorentz force

 The Lorentz force generated by perpendicularly placed magnetic field and electric field displacing conductive saltwater, chemically produced gases (oxygen and hydrogen) and solid precipitates (aluminium hydroxide) by the associated reaction forms typical three-phase MHD jet flows. Taking advantage of the bright gases, the emergence of jet flow is studied by the bubbly flow. Based on the control parameters, such as magnetic field strength, input current strength, geometry of the experimental apparatus, and fluid properties, a Lorentz-force based Reynolds number Re_L is proposed to categorize the flow regime from laminar to turbulence. For jets of lower Re_L, the hydrogen/saltwater interface appears apparently more unstable, because of lighter molecular weight and more chemically produced amount, such that typical Kelvin-Helmholtz instability is observed. The distinct behaviors of oxygen and hydrogen is indistinguishable for sufficiently high Re_L due to strong dispersive mixing.  Turbulent jet flow evolves once the Re_L exceeds critical value. Appropriateness of the Re_L is verified both by experiments associated with varied control parameters,  corresponding numerical simulation and relevant quantitative measures.   

4D Characterization of the swirling gas jet flow field in a coaxial two-fluid atomizer

The dynamics of the swirling gas jet in the near field of a coaxial two-fluid atomizer play a key role in the development of interfacial instabilities, the liquid break-up process, and spray characteristics. Due to the overwhelming momentum from the gas, and its turbulent state, the instabilities exhibit a wide range of spatial and temporal scales. Moreover, the addition of angular momentum in the gas dramatically influences the spray, including droplet formation and dispersion. This study investigates the near-field velocity field of the gas-only annular jet from the atomizer using 3D-PTV. As the liquid injection cross-sectional area and momentum are both negligible, the gas-only annular accurately represents the spray flow field. Momentum ratios M=5-174 and the swirl ratios between SR=0-1 are studied, with the mean and turbulence velocity statistics measured with high resolution. The near-field wake region grows with increasing swirl ratio, and the azimuthal velocity peaks for a value of the critical swirl number that is a function of the gas Reynolds number. Robust PCA is conducted to extract dominant flow structures, including the axial shear layer and helical vortices. Turbulent intensity depends on the momentum and swirl ratio in response to the instabilities triggered.