低レイノルズ数遷移チャネル乱流場の線形過渡成長 (非一様乱流の数理)

Generation process of a large stripy pattern of localized turbulence in pressure-driven plane channel flow under a subcritical transitional regime is considered. The most amplified kinetic energy of linear Navier-Stokes equations was computed for each mode in terms of the streamwise and spanwise wavelengths, with applying an eddy viscosity as the nonliner effect. In the study, we focused on the generation state of a turbulence spot, where the localized turbulence would grow or remain in a form of oblique band, or the turbulent stripe. With considering the large-scale secondary flow around the spot or along the oblique band as a base flow, we found specific pairs of wavelengths that are more amplified by the existence of spanwise velocity component in the base flow. It implies that the spanwise flow around a turbulence spot makes flow unstable in the oblique direction, to cause a stripy pattern of turbulence region

Investigation of maximum velocity induced by body-force fields for simpler modeling of plasma actuators

The relation between the parameters of the body-force field generated by a plasma actuator and the maximum induced velocity in quiescent air is investigated by expressing the body-force distribution as the Gaussian function of the spatial coordinates. The aim of this study is to identify the dominant parameters for modeling of the body-force distribution. For that purpose, the parametric study using numerical simulations and dimensional analysis are conducted to derive the nondimensional key parameters. It is found that the nondimensional maximum induced velocity is determined by the Reynolds number calculated by three parameters: the total induced momentum per unit time, the height of the center of gravity of the body-force distribution, and the standard deviation from the center of gravity. In addition, the relation for the Gaussian body-force distribution turns out to be applicable to a conventional model, i.e, the Suzen model, even though the shapes of the distribution differ. Thus, we conclude that the three body-force parameters above are the key parameters for the maximum velocity induced by a plasma actuator

Dominant parameters for maximum velocity induced by body-force models for plasma actuators

This study investigates the relationship between body-force fields and maximum velocity induced in quiescent air for development of a simple body-force model of a plasma actuator. Numerical simulations are conducted with the body force near a wall. The spatial distribution and temporal variation of the body force are a Gaussian distribution and steady actuation, respectively. The dimensional analysis is performed to derive a reference velocity and Reynolds number based on the body-force distribution. It is found that the derived Reynolds number correlates well with the nondimensional maximum velocity induced in quiescent conditions when the center of the Gaussian distribution is fixed at the wall. Additionally, two flow regimes are identified in terms of the Reynolds number. Considering the variation of the center of gravity of force fields, another Reynolds number is defined by introducing a new reference length. The nondimensional maximum velocity is found to be scaled with the latter Reynolds number, i.e., the maximum induced velocity in quiescent conditions is determined from three key parameters of the force field: the total induced momentum per unit time, the height of the center of gravity, and the standard deviation from it. This scaling turns out to be applicable to existing body-force models of the plasma actuator, despite the force distributions different from the Gaussian distribution. Comparisons of velocity profiles with experimental data validate the results and show that the flow induced by a plasma actuator can be simulated with simple force distributions by adjustment of the key body-force parameters