To investigate the intrinsic mechanism for mixing enhancement by variable density behaviour, a canonical variable density (VD) mixing extracted from a supersonic streamwise vortex protocol, shock bubble interaction (SBI), is numerically studied and compared with a counterpart of passive scalar (PS) mixing. It is meaningful to observe that the maximum concentration decays much faster in VD SBI than in PS SBI regardless of the shock Mach number (Ma = 1.22 ~ 4). The quasi-Lamb-Oseen type velocity distribution in the PS SBI is found by analyzing the azimuthal velocity that stretches the bubble. Meanwhile, for the VD SBI, an additional stretching enhanced by the secondary baroclinic vorticity (SBV) production contributes to the faster-mixing decay. The underlying mechanism of the SBV-enhanced stretching is further revealed through the density and velocity difference between the shocked light bubble and the heavy ambient air. By combining the SBV-accelerated stretching model and the initial shock compression, a novel mixing time estimation for VD SBI is theoretically proposed by solving the advection-diffusion equation under a deformation field of an axisymmetric vortex with the additional SBV induced azimuthal velocity. Based on the mixing time model, a mixing enhancement number defined by the ratio of VD and PS mixing time further reveals the contribution from the variable-density effect, which implies a better control of density distribution for mixing enhancement in a supersonic streamwise vortex.
This paper presents an experimental analysis of flame-induced enstrophy transport in premixed swirl combustion at Karlovitz numbers between 20-50. Such flames posses a large-scale pressure field that -- in addition to the pressure fields associated with small-scale turbulent vortices -- can interact with density gradients to produce baroclinic torque. Simultaneous tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain high-resolution velocity and progress-variable fields. This allows statistical evaluation of the various terms …
New exact spatially localized solutions of the (3 + 1) -dimensional nonlinear non-dissipative quasi-geostrophic potential vorticity equation for an exponential atmosphere
The oscillatory flows present in an inkjet printhead can lead to strong deformations of the air-liquid interface at the nozzle exit. Such deformations may lead to an inward directed air jet with bubble pinch-off and the subsequent entrainment of an air bubble, which is highly detrimental to the stability of inkjet printing. Understanding the mechanisms of bubble entrainment is therefore crucial in improving print stability. In the present work, we use ultrafast X-ray phase-contrast imaging and direct numerical simulations based on the Volume-of-Fluid method to study the mechanisms underlying t…
The Richtmyer-Meshkov (RM) instability has long been an interesting subject due to its fundamental significance in scientific research, as well as its crucial role in engineering applications. In this study, the contribution of shock Mach number on the evolution of the RM instability induced by a shock-accelerated square light bubble is investigated numerically. The square bubble is composed of helium gas and the surrounding (ambient) gas is nitrogen. Three cases of incident shock strength are considered: Ms = 1.21, 1.7, and 2.1. An explicit mixed-type modal discontinuous Galerkin scheme with …
The present study investigates the shock wave interactions involving stationary and moving wedges using a sharp interface immersed boundary method combined with a fifth order weighted essentially non oscillatory (WENO) scheme. Inspired by Schardins problem, which involves moving shock interaction with a finite triangular wedge, we study influences of incident shock Mach number and corner angle on the resulting flow physics in both stationary and moving conditions. The present study involves three incident shock Mach numbers (1.3, 1.9, 2.5) and three corner angles (60deg, 90deg, 120deg), while …
激波与球形密度不均匀体的相互作用通过斜压涡度的脉冲沉积导致涡环的发展。目前的流体动力学视频显示这种现象，并在威斯康星州激波管实验室（WiSTL）的9.2米，向下射击激波管进行了实验研究。该管具有方形内部横截面（0.25 m x 0.25 m），具有用于光学访问的多个熔融石英窗口。球形肥皂泡是由一个可伸缩的注射器产生的，并释放到自由落体200毫秒前的初始冲击加速。向下运动的M = 2.07的冲击波使气泡冲动地加速并从管端壁反射。反射的冲击波使气泡重新加速（再冲击），气泡现在已经发展成涡环，沉积了额外的涡量。在没有任何流动扰动的情况下，反射激波后面的流动是静止的。因此，任何观察到的涡环运动都是由于环流造成的。冲击涡环成像在12,500 fps的平面米氏散射。
Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters,…
