Near-wall Boundary Research Articles

Study on bubble pulsation process of underwater explosion between parallel plates with various distances

This paper presents a numerical and experimental study of the dynamic behavior of underwater explosion bubbles between two parallel plates at different detonation distances. To simulate the strong nonlinear interaction between cavitation bubbles and the near-wall boundary, the coupled Eulerian-Lagrangian (CEL) method is used. The evolution of bubble morphology in the fluid domain is observed simultaneously, revealing the flow inside the bubble and the pulsation process under various visual angles that are difficult to observe in the experiment. In the experimental system, micro-equivalent explosives are used to generate shock waves, and the shock wave load during bubble pulsation and wall pressure during collapse phase are measured. High-speed cameras capture the bubble morphology evolution process and dynamic behavior at different detonation distances, which are then compared with the numerical results to verify their accuracy.The study focuses on the bubble morphology evolution process with dimensionless distance φ ranging from 0.42 to 1.71, and the collapse modes of bubbles in different forms are discussed in detail. The pulsation process of bubbles in the free field is verified, and it is found that when the distance between the two plates is less than the maximum radius of the bubble, the bubble collapses in the center between the two plates as the dimensionless distance r increases. Moreover, the Locating column has a large interference on the bubble movement. The bubble surface is divided into five layers of ring ligament with obvious characteristics. This paper investigates the dynamic behaviour of cavitation bubbles between two parallel plates at different plate spacings. The bubble undergoes a series of shape changes, from a columnar cake to a multi-layer ring with obvious edge tomography, and finally collapses into a number of minor bubbles, producing a cavitation bubble band connecting the two cavitation layer ligaments. When the plate spacing is between 1 and 1.5 times the maximum bubble radius, the bubble edge contacts the target plate, causing it to change from a “Hamburg” shape to an hourglass shape, tear into two sub-bubbles, and then produce two bubble jets pointing at opposite target plates. The vortex at the edge of the bubble influences the bubble's shape, resulting in an obvious corner and the formation of an annular bubble. When the plate spacing is greater than 1.5 times the maximum bubble radius, the bubble is torn into two sub-bubbles, and the bubble edge is smooth and does not contact the target plate during expansion. The numerical model accurately simulates the bubble pulsation and jet formation process, which is validated by experimental results. However, the experimental system has limitations in terms of image interval and viewing angle, which can be addressed by using the numerical model to simulate the bubble pulsation process at a higher resolution and a three-dimensional perspective. This study provides valuable insights into the strong nonlinear interaction between bubbles and target plates at different detonation distances. The accuracy of the HPB measurement system is also verified, and the numerical method used in this study is effectively simulates the pulsation characteristics of cavitation bubbles, including the formation of jets and annular bubbles. This work lays a solid foundation for further studies on cavitation bubble dynamics and their interaction with target plates, which has important implications for the design and analysis of underwater structures subjected to explosive loads.

Formulation of wall boundary conditions in turbulent flow computations on unstructured meshes

Features of the formulation and numerical implementation of wall boundary conditions in turbulent flow computations on unstructured meshes are discussed. A method is proposed for implementing weak wall boundary conditions for a finite-volume discretization of the Reynolds-averaged Navier-Stokes equations on unstructured meshes. The capabilities of the approach are demonstrated in several gasdynamic simulations in comparison with the method of near-wall functions. The influence of the near-wall resolution on the accuracy of the computations is analyzed, and the grid dependence of the solution is compared in the case of the near-wall function method and weak boundary conditions.

Evaluation of Eddy Viscosity Models in Predicting Free- Stream Turbulence Penetration

Turbulence schemes have long been developed and examined for their accuracy and stability in a variety of environments. While many industrial flows are highly turbulent, models have rarely been tested to explore whether their accuracy withstands such augmented free-stream turbulence intensity or declines to an erroneous solution. In the present study, the turbulence intensity of an air flow stream, moving parallel to a flat plate is augmented by the means of locating a grid screen at a point at which Rex=2.5×10 5 and the effect on the flow and the near-wall boundary is studied. At this cross section, the turbulence intensity is augmented from 0.4% to 6.6% to flow downstream. Wind tunnel measurements provide reference bases to validate the numerical results for velocity fluctuations in the main stream and at the near-wall. Numerically, four of the most popular turbulence models are examined, namely the oneequation Spalart-Almaras, the two equation Standard   k , the two equation Shear Stress Transport and the anisotropy multi equation Reynolds Stress Models (RSM). The resulting solutions for the domain are compared to experimental measurements and then the results are discussed. The conclusion is made that, despite the accuracy that these turbulence models are believed to have, even for some difficult flow field, they fail to handle high intensity turbulence flows. Turbulence models provide a better approach in experiments when the turbulence intensity is at about 2% and/or when the Reynolds number is high.

Near-wall modelling in computations of turbulent flows on unstructured grids

The questions related to the formulation and numerical realization of boundary conditions on a wall at the computation of turbulent flows on unstructured grids are considered. A technique is proposed for realization of weak boundary conditions assuming a non-zero value of the tangent velocity on the wall at a discretization of the Reynolds-averaged Navier — Stokes equations by the control volume method. The capabilities of the developed approach are demonstrated by the example of computing the flow in the inter-blade channel of a low-velocity compressor. The influence of the near-wall grid step on the accuracy of computations, in particular, the pressure distribution near the profile trailing edge is shown, and the solution grid dependence is investigated when using the method of near-wall functions and weak boundary conditions.

Investigation of organized structures in the near-wall turbulent boundary layer zone on a plastically deformable surface

Investigation of organized structures in the near-wall turbulent boundary layer zone on a plastically deformable surface

Turbulent boundary layer of polymer solutions with a flow-retarding pressure gradient

A description of the mean velocity distribution in the near-wall turbulent boundary layer of polymer solutions with a flow-retarding pressure gradient is given by using dimensional analysis and similarity.