The present paper is a wide review on AC surface dielectric barrier discharge (DBD) actuators applied to airflow control. Both electrical and mechanical characteristics of surface DBD are presented and discussed. The first half of the present paper gives the last results concerning typical single plate-to-plate surface DBDs supplied by a sine high voltage. The discharge current, the plasma extension and its morphology are firstly analyzed. Then, time-averaged and time-resolved measurements of the produced electrohydrodynamic force and of the resulting electric wind are commented. The second half of the paper concerns a partial list of approaches having demonstrated a significant modification in the discharge behavior and an increasing of its mechanical performances. Typically, single DBDs can produce mean force and electric wind velocity up to 1 mN/W and 7 m/s, respectively. With multi-DBD designs, velocity up to 11 m/s has been measured and force up to 350 mN/m.

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Electrical and mechanical characteristics of surface AC dielectric barrier discharge plasma actuators applied to airflow control

Face mask filters-textile, surgical, or respiratory-are widely used in an effort to limit the spread of airborne viral infections. Our understanding of the droplet dynamics around a face mask filter, including the droplet containment and leakage from and passing through the cover, is incomplete. We present a fluid dynamics study of the transmission of respiratory droplets through and around a face mask filter. By employing multiphase computational fluid dynamics in a fully coupled Eulerian-Lagrangian framework, we investigate the droplet dynamics induced by a mild coughing incident and examine the fluid dynamics phenomena affecting the mask efficiency. The model takes into account turbulent dispersion forces, droplet phase-change, evaporation, and breakup in addition to the droplet-droplet and droplet-air interactions. The model mimics real events by using data, which closely resemble cough experiments. The study shows that the criteria employed for assessing the face mask performance must be modified to take into account the penetration dynamics of airborne droplet transmission, the fluid dynamics leakage around the filter, and reduction of efficiency during cough cycles. A new criterion for calculating more accurately the mask efficiency by taking into account the penetration dynamics is proposed. We show that the use of masks will reduce the airborne droplet transmission and will also protect the wearer from the droplets expelled from other subjects. However, many droplets still spread around and away from the cover, cumulatively, during cough cycles. Therefore, the use of a mask does not provide complete protection, and social distancing remains important during a pandemic. The implications of the reduced mask efficiency and respiratory droplet transmission away from the mask are even more critical for healthcare workers. The results of this study provide evidence of droplet transmission prevention by face masks, which can guide their use and further improvement.

On respiratory droplets and face masks.

A complete set of V–T (vibration–translation) relaxation rates and of dissociation coefficients for the system O–O2 have been obtained by using quasiclassical trajectories on the Varandas and Pais potential energy surface. The results, averaged on a Boltzmann rotational distribution, cover the whole range of the vibrational ladder and are reproduced in closed form ready to be implemented in state-to-state kinetic models. The accuracy of the results has been tested by comparing them with available experimental and theoretical values (ASI-CAST project is acknowledged).

N-N2 state to state vibrational-relaxation and dissociation rates based on quasiclassical calculations

A rovibrational collisional model is developed to study energy transfer and dissociation of N2(1Σg+) molecules interacting with N(4Su) atoms in an ideal isochoric and isothermal chemical reactor. The system examined is a mixture of molecular nitrogen and a small amount of atomic nitrogen. This mixture, initially at room temperature, is heated by several thousands of degrees Kelvin, driving the system toward a strong non-equilibrium condition. The evolution of the population densities of each individual rovibrational level is explicitly determined via the numerical solution of the master equation for temperatures ranging from 5000 to 50 000 K. The reaction rate coefficients are taken from an ab initio database developed at NASA Ames Research Center. The macroscopic relaxation times, energy transfer rates, and dissociation rate coefficients are extracted from the solution of the master equation. The computed rotational-translational (RT) and vibrational-translational (VT) relaxation times are different at low heat bath temperatures (e.g., RT is about two orders of magnitude faster than VT at T = 5000 K), but they converge to a common limiting value at high temperature. This is contrary to the conventional interpretation of thermal relaxation in which translational and rotational relaxation timescales are assumed comparable with vibrational relaxation being considerable slower. Thus, this assumption is questionable under high temperature non-equilibrium conditions. The exchange reaction plays a very significant role in determining the dynamics of the population densities. The macroscopic energy transfer and dissociation rates are found to be slower when exchange processes are neglected. A macroscopic dissociation rate coefficient based on the quasi-stationary distribution, exhibits excellent agreement with experimental data of Appleton et al. [J. Chem. Phys. 48, 599–608 (1968)]10.1063/1.1668690. However, at higher temperatures, only about 50% of dissociation is found to take place under quasi-stationary state conditions. This suggest the necessity of explicitly including some rovibrational levels, when solving a global kinetic rate equation.

Rovibrational internal energy transfer and dissociation of N2(1Σg+)−N(4Su) system in hypersonic flows

Numerical simulations of axisymmetric flows over reentry configurations at hypersonic conditions using a Navier-Stokes solver are presented. The Navier-Stokes equations are modified using Park’s two-temperature model to account for thermochemical nonequilibrium and weak ionization eects. The finite-volume method is used to solve the set of dierential equations. The code has the capability to handle any mixture of hexahedra, tetrahedra, prisms and pyramids in 3D or triangles and quadrilaterals in 2D. The results in this paper only use quadrilaterals. Numerical fluxes between the cells are discretized using a modified Steger-Warming Flux Vector Splitting approach which has low dissipation and is appropriate to calculate boundary layers. A point or line implicit method is used to perform the time integration. Pressure, heat transfer rates and electron number density profiles are compared to available experimental and flight measurements.

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Numerical Simulation of Weakly Ionized Hypersonic Flow for Reentry Configurations

A generalized set of conservative equations for simulating the flowfield in a hypersonic weakly ionized gas flows is presented. Additional numerical and physical complexities associated with the plasma state are identified and discussed, including the influence of external and space charge fields and the nonequilibrium coupling of reactive and nonreactive collisions. A restricted set of equations is then employed to simulate and analyze the flowfield of an air plasma generated by associative ionization. With use of a seven-species air model, details of the plasma flowfields are presented and compared with available experiments. Refined estimates of dissociation products, which are the precursors to the ionization reaction, are obtained. Specific attention is given to limiting forms of the electron diffusion coefficient and the influence of vibration-dissociation coupling. Details of the chemical reactions in the flowfield influencing the elastic and inelastic collisional energy transfer are reported to highlight the more important species and reaction mechanisms.

Governing equations for weakly ionized plasma flowfields of aerospace vehicles

Numerical simulations are presented of a steady-state hypersonic flow past a hemisphere cylinder. Two types of models, one a lumped Landau-Teller vibrational relaxation model (Landau, L., and Teller, E.) and the other a discrete state kinetic relaxation model (DSKR), were used to study effects of vibration-dissociation coupling on the flow physics. The widely used Park's dissociation model was used as baseline for coupling vibration and dissociation processes (Park, C.). For a Mach 8.6 flow, both relaxation models matched experimental data. At Mach 11.18, however, the underprediction of shock-standoff distance by both relaxation models using Park's model for dissociation coupling provided the motivation to implement a new master equation-based (DSKR) depletion model. The new model was used to study the effect of dissociation on population depletion in the vibrational states of the nitrogen molecule. The new model helps explain the restricted success of Park's dissociation model in certain temperature ranges of hypersonic flow past a blunt body. In the range of 5000-15,000 K, the new model yielded a substantial rate reduction relative to Park's equilibrium rate at lower temperatures and a consistent value at the high end

Vibration-Dissociation Coupling Using Master Equations in Nonequilibrium Hypersonic Blunt-Body Flow

The generalized depletion equations, considering state-to-state kinetics of dissociating nitrogen, were solved to predict the extent of vibrational depletion for 4000 and 6000 K. Two different vibrational transition rate sets were used in the depletion analysis. One was based on the Schwartz–Slawsky–Herzfeld (SSH) theory and the other based on the analytical methods of Capitelli, Billing, Fisher, Doroshenko and co-workers. Transition rates based on the SSH theory predicted a lower depletion due to very high rates in the upper levels. With the objective of delineating the vibration–vibration (V–V) exchanges and vibration–translation (V–T) transfers in nonreactive, vibrationally excited gas dynamic flows, the flowfield simulations of blunt body and expanding nozzles were performed. For vibrational cooling cases (where vibrational temperature is greater than translational temperature) of flow past blunt body and the nozzle flows, the population distribution showed the Treanor-like distribution only at low tr...

William F. Bailey

Papers

Governing equations for weakly ionized plasma flowfields of aerospace vehicles

Vibration-Dissociation Coupling Using Master Equations in Nonequilibrium Hypersonic Blunt-Body Flow

Reactive and nonreactive vibrational energy exchanges in nonequilibrium hypersonic flows

Governing Equations for Weakly Ionized Plasma Flowfields of Aerospace Vehicles

Shock-Train Structure Resolved with Absorption Spectroscopy Part I: System Design and Validation