Scale-by-scale energy budget in a turbulent boundary layer over a rough wall

Hot-wire velocity measurements are carried out in a turbulent boundary layer over a rough wall consisting of transverse circular rods, with a ratio of 8 between the spacing (w) of two consecutive rods and the rod height (k). The pressure distribution around the roughness element is used to accurately measure the mean friction velocity ($U_{\tau }$) and the error in the origin. It is found that $U_{\tau }$ remained practically constant in the streamwise direction suggesting that the boundary layer over this surface is evolving in a self-similar manner. This is further corroborated by the similarity observed at all scales of motion, in the region $0.2⩽y/\delta ⩽0.6$, as reflected in the constancy of Reynolds number ($R_{\lambda }$) based on Taylor’s microscale and the collapse of Kolmogorov normalized velocity spectra at all wavenumbers.A scale-by-scale budget for the second-order structure function $〈{(\delta u)}^2〉$ ($\delta u=u(x+r)-u(x)$, where u is the fluctuating streamwise velocity component and r is the longitudinal separation) is carried out to investigate the energy distribution amongst different scales in the boundary layer. It is found that while the small scales are controlled by the viscosity, intermediate scales over which the transfer of energy (or $〈{(\delta u)}^3〉$) is important are affected by mechanisms induced by the large-scale inhomogeneities in the flow, such as production, advection and turbulent diffusion. For example, there are non-negligible contributions from the large-scale inhomogeneity to the budget at scales of the order of $\lambda$, the Taylor microscale, in the region of the boundary layer extending from $y/\delta =0.2$ to 0.6 ($\delta$ is the boundary layer thickness).     

“Inviscid Finger” Instability in Regular Models of a Porous Medium

Displacement of a fluid from a porous medium is considered. The flow is assumed to be fast enough, i.e., the Reynolds number based on the characteristic pore size is large. If he driving fluid is less dense (for example, a gas), the interface is unstable. This instability is similar to the well–known viscous finger instability but the governing parameter is density instead of viscosity. The instability is demonstrated experimentally using two–dimensional models. In square lattices of perpendicular channels, noticeable branching of fingers is not observed, which is attributed to the anisotropy of such an artificial porous medium. A more ordinary pattern with finger branching is obtained in a two–dimensional layer of spheres, which appears to be more isotropic. A simple model describing flow in a square lattice is proposed. The initial stage of growth is considered, and the instability increment is estimated. A qualitative analysis of the nonlinear stage is performed.     

火场条件下相邻汽油罐油蒸汽泄漏及爆炸规律

油库发生火灾时,临近的油罐内汽油受热形成油蒸汽从呼吸阀泄出,泄漏的油蒸汽与空气混合易点燃,易引起燃烧爆炸事故。本文中以容积为5 000 m^3(22 m×13 m)的汽油罐泄漏油蒸汽为研究对象,以数值模拟为研究方法,研究了汽油蒸汽泄漏及爆炸规律。研究发现:在距地面高1 m的平面上,当无风且呼吸阀泄漏油蒸汽速率为0.25 m/s时,距该罐中心50 m以外就可视作安全区域;当呼吸阀泄漏的油蒸汽速率为0.25 m/s时,风速达到5.0 m/s及以上,不易积聚成爆炸油蒸汽;当无风时,呼吸阀泄漏油蒸汽的速率增大1个量级,油蒸汽积聚达到爆炸下限1/2所需时间缩短2个量级;当风速为3.0 m/s、呼吸阀泄漏油蒸汽速率为0.25 m/s、泄漏时间为200 s、着火点距罐壁1 m时,距点火源距离增大1个量级,超压峰值下降1~2个量级。