Motion of solid object in fluid is studied in moving frame of reference. Navier-Stokes (N-S) equations in a non-inertial frame of reference is coupled with the equations of linear and angular momentums for the solid object. It is shown that the numerical implementation for solving the non-inertial N-S equations is equivalent to that of the arbitrary Lagrangian-Eulerian (ALE) method. The simulation results by the two methods show reasonable agreements for a 2-D flow field including a cylinder. In the present work, a generalised convective boundary condition is constructed for non-inertial frame of reference. Also, an appropriate boundary condition is proposed for fluid phase based on a least square adjustment (LSA) method. It is shown that the LSA is essential for the non-inertial study fluid-solid interaction. Also, the non-inertial N-S equation is applied to a problem of a concave object falling in a fluid. It is shown that traditional time-advancement schemes undermine the ortho-normality of the rotation matrix, resulting in non-physical deformation of the object. However, our time-advancement procedure with quaternion is found to be robust and effective even under a violent manoeuvre of the object.
Ultrasound velocity profiler (UVP) is applied to measurements of a horizontal turbulent bubbly channel flow to ascertain the mechanism of bubble-induced frictional drag reduction. Typical parameter regimes of the target flow are Re number of 0.6-6.0 x104, void fraction of 0-3%, and bubble diameter of 10-50mm. Since the UVP system only outputs velocity profiles on an ultrasound beam inside a liquid phase, a signal processing method for raw velocity data is proposed and used to detect the bubble interface. A conditional averaged liquid phase velocity profile that excludes gas phase data, has shown for the first time the structure of a turbulent boundary layer altered by large buoyant bubbles sliding along the channel wall.
Using a direct drag balance measurement for the local wall shear stress, self-preserving development of a turbulent boundary layer was achieved experimentally over a d-type rough surface without pressure gradients. The wall shear stress and mean velocity measurements confirmed the requirements for exact self-similarity and highly similar Reynolds stress profiles within the Reynolds number range for a constant skin friction coefficient. Under the condition that the boundary layer flow is completely independent of Reynolds number, the effect of wall roughness was investigated with respect to the similarity laws for the wall layer as well as the outer layer. Experimental observation reveals the wall similarity ∂U/∂y = uτ/ky to be applicable to the present rough wall boundary layer remaining the accepted value of Kármán constant to be κ= 0.41. Otherwise, investigation of the wake strength in the mean velocity and Reynolds stress profiles reveals that the wall roughness does affect the outer layer structure. Reynolds stress measurements indicate that the primary effect of wall roughness on turbulence properties is in the component normal to the wall.
An LES was carried out to clarify the mechanism of decaying swirl in a rod-bundle. In our LES, an immersed boundary method was used to treat this complex wall boundary in the Cartesian grid system. A consistent immersed boundary method and a one-equation dynamic SGS model were introduced for an accurate simulation of no-slip wall condition. The Reynolds number based on the bulk velocity, the rod pitch and the kinematic viscosity was approximately 4100. Our computational results were compared well with experimental and computational data obtained in other studies. The results were able to represent the effect of flow geometry: the flow around the mixing-vanes caused the swirl with a large-scale fluctuation enhancing heat transfer; a vortex enhancing enthalpy mixing between channels was produced in the rod gap; this developing vortex and the decreasing wake promoted the decay of the swirl more strongly than in a pipe. The LES technique introduced in this study is useful for designing the spacer by predicting the effect of flow control.
Turbulent reactive jet in liquid has been simulated by the “semi-empirical” Lagrangian probability density function (PDF) method, by which the velocity field consistent to the specified moments of the Eulerian PDF of velocity can be reproduced. The chemical reaction treated in this study is a second-order irreversible reaction, R + B → S, where a solution of species R is ejected from the nozzle into the main stream that includes species B. The Lagrangian velocities of the stochastic particles are modeled by a generalized Langevin equation in the cylindrical coordinate system. This model is constructed to satisfy the consistency condition of a velocity field and the thermodynamic constraint. For the molecular mixing of reactive scalar concentrations, the most fundamental IEM (interaction by exchange with the mean) was adopted. The simulation results are compared with the experimental data. It is found that the velocity field simulated by the present model satisfies the consistency condition up to the second-order moment and the thermodynamic constraint. The simulated reactive scalar concentration field shows almost the same distribution, on the whole, as the experimental data. Therefore, the models used in this study are useful for a turbulent reactive jet.