Hot-wire measurements in the boundary layer developing on a concave surface of 2.0 m radius of curvature in the presence of forced wavelength Görtler vortices have been conducted for a free-stream velocity of 3.0 m / s. The wavelengths of vortices were preset by vertical perturbation wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices that are similar to those found in the transitional flow field. It shows the occurrence of the second instability mode that is indicated by the formation of small horseshoe eddies generated between the two neighboring vortices traveling in the streamwise direction to form mushroom-like structures as a consequence of the nonlinear growth of the Görtler vortices. The breakdown of these structures before the boundary-layer flow becomes turbulent is also shown to qualitatively predict the start of the transition in the flow. The Görtler number where the start of the transition was predicted is found to be within the range of transitional Görtler numbers previously reported for naturally developed Görtler vortices. The average of the spanwise wavelength after being normalized by / u is comparable with the generally quoted value of 100 for turbulent boundary layers.

The current investigation is aimed to simulate the complex internal flow in a centrifugal pump impeller with six twisted blades by using a three-dimensional Navier-Stokes code with a standardk-εtwo-equation turbulence model. Different flow rates were specified at inlet boundary to predict the characteristics of the pump. A detailed analysis of the results at design load,Qdesign, and off-design conditions, Q = 0.43Qdesignand Q = 1.45Qdesign, is presented. From the numerical simulation, it shows that the impeller passage flow at design point is quite smooth and follows the curvature of the blade. However, flow separation is observed at the leading edge due to nontangential inflow condition. The flow pattern changed significantly inside the volute as well, with double vortical flow structures formed at cutwater and slowly evolved into a single vortical structure at the volute diffuser. For the pressure distribution, the pressure increases gradually along streamwise direction in the impeller passages. When the centrifugal pump is operating under off-design flow rate condition, unsteady flow developed in the impeller passage and the volute casing.

No abstract

Purpose -The purpose of this paper is to report on the flow past a porous square cylinder, implementing the stress jump treatments for the porous-fluid interface. Design/methodology/approach -The numerical method was developed for flows involving an interface between a homogenous fluid and a porous medium. It is based on the finite volume method with body-fitted and multi-block grids. The Brinkman-Forcheimmer extended model was used to govern the flow in the porous medium region. At its interface, a shear stress jump that includes the inertial effect was imposed, together with a continuity of normal stress. Findings -The present model is validated by comparing with those for the flow around a solid circular cylinder. Results for flow around porous square cylinder are presented with flow configurations for different Darcy number, 10 2 2 to 10 2 5 , porosity from 0.4 to 0.8, and Reynolds number 20 to 250. The flow develops from steady to unsteady periodic vortex shedding state. It was found that the stress jump interface condition can cause flow instability. The first coefficient b has a more noticeable effect whereas the second coefficient b 1 has very small effect, even for Re ¼ 200. The effects of the porosity, Darcy number, and Reynolds number on lift and drag coefficients, and the length of circulation zone or shedding period are studied. Originality/value -The present study implements the numerical method based on finite volume method with a collocated variable arrangement to treat the stress jump condition.

The linear and nonlinear developments of Görtler vortices were experimentally investigated by means of hot-wire anemometer measurement. The wavelengths of the vortices were preset to be the most amplified using thin perturbation wires. Three different vortex wavelengths of 12, 15, and 20 mm were considered. These wavelengths were preserved downstream which confirm the prediction of the most amplified wavelength of Görtler vortices. The onset of the nonlinear region occurs at about the same Görtler number of 5.0 for all the wavelengths considered. In this nonlinear region, the secondary instability is initiated near the boundary layer edge, and it develops further downstream. The maximum turbulent intensity increases as the secondary instability becomes dominant in the flow. In the transition region, however, it slightly decreases before drastically increasing due to the onset of turbulence. Three maxima of intense turbulence are found in the turbulent intensity contours in the nonlinear region, which indicate the occurrence of the so-called varicose and sinuous modes of the secondary instability. Comparison with the previous available results shows that all data of maximum disturbance amplitude obtained from the same experimental setup seem to lie on a single line when they are plotted against Görtler number, regardless of the values of free-stream velocity and concave surface radius of curvature. Smaller radius of curvature results in higher vortex growth rate in the linear region due to stronger centrifugal effect. However, the vortex growth rate seems to be unaffected by free-stream velocity. The normal position of maximum disturbance amplitude reaches the maximum point exactly at the onset of nonlinear region before it drastically drops as the secondary instability is overtaking the primary instability.