S. Obi

Experimental study on the statistics of wall shear stress in turbulent channel flows

Measurements of local wall shear stress have been undertaken using a laser gradient meter that enables direct evaluation of the wall velocity gradient with high spatial resolution. Mean values as well as the statistical parameters of fluctuating wall shear stress, such as third and fourth moments, have been obtained in fully developed air flow in a two-dimensional channel at moderate Reynolds numbers. Agreement with the available direct numerical simulation (DNS) and experimental results is satisfactory. The dependence of the turbulence statistics on the Reynolds number is masked by the experimental ambiguity. Although the linear near-wall velocity profile is the principal requirement for the application of the present system, it is found that the measurement of the mean velocity gradient is less influenced by the nonlinearity of the velocity profile more distant from the wall than are the turbulence statistics.

Second-Moment Calculation Procedure for Turbulent Flows with Collocated Variable Arrangement

The paper presents a finite-volume calculation procedure using second-moment turbulence closure. A special interpolation technique is employed in connection with a collocated variable arrangement to avoid oscillatory solutions that might otherwise result from inappropriate coupling of the mean velocity, pressure, and Reynolds stress fields. The apparent diffusion fluxes arising from the interpolation procedure ensure numerical stability of the iterative solution process. The application of the second-moment closure model to backward-facing step flows yields slightly improved results as compared with k-e model predictions. Shortcomings of the current second-moment model are an overproportionally large damping of normal stresses due to inadequacies in the modeling of the pressure-strain terms and the predicted behavior in the reattachment region.

Turbulent separation control in a plane asymmetric diffuser by periodic perturbation

The influence of the artificial perturbation on the turbulent separating flow in the two-dimensional asymmetric diffuser is experimentally investigated. The perturbation is accomplished by periodically alternating the flow suction and injection through a spanwise slit on the diffuser wall. The improvement of pressure recovery performance and the reduction of the separation zone is significant, but only within the optimum frequency range. The measurements of ensemble-averaged velocity distribution has indicated the organized oscillatory fluid motion in the optimum frequency range, that is considered to be responsible to the enhancement of momentum transport across the diffuser and hence the reduction of separation length.

Heat transfer experiments in rotating boundary layer flow

Abstract The influence of Coriolis force on heat transfer in a rotating transitional boundary layer has been experimentally investigated. The experiments have been conducted for local Gortler numbers up to 150. Heat transfer measurements have been performed for a flat plate with nearly uniform heat flux applied to the surface, where the temperature was measured by the thermochromic liquid crystal method. The results indicate that heat transfer is enhanced when Coriolis force acts towards the wall, i.e., on the pressure surface. The velocity measurements under equivalent conditions show that Coriolis instability induces counter-rotating longitudinal vortices which augment the lateral transport of the fluid on the pressure surface. On the other hand, the heat transfer on the suction surface remains at the same level as compared to the case without system rotation. As a consequence, the heat transfer coefficient on the pressure surface is 1.8 times higher than that measured on the suction surface when averaged over the measured surface.

Prediction of Strongly Swirling Flows in Quarl Expansions with a Non–Orthogonal Finite–Volume Method and a Second Moment Turbulence Closure

A prediction method for strongly swirling flows with a co–located variable arrangement and a non–orthogonal coordinate system is presented. Turbulence modelling is done with a Reynolds stress closure, including all six stress components that appear in a swirling axially symmetric flow. The method is applied to three swirling flows with swirl number varying from 0.45 to 1.2. Two of the cases comprise flow expansion through quarls. Comparisons of the results with calculations performed with a standard k – e model turbulence closure and experimental data show significant improvements with the Reynolds stress model for the flows with the quarl expansions. The so-called “subcritical” flow character present in the flows with the higher swirl numbers is clearly reflected by the Reynolds stress turbulence model.

Experimental and computational study of turbulent heat transfer characteristics in serrated channel flow

A two-dimensional (2-D) channel with a serrated wall is proposed as a device for heat transfer augmentation. Measurements of the flow velocity in the whole field as well as of the heat transfer coefficient along the wall are undertaken for two different channel heights. The same flow fields are calculated using a second-moment closure and the standard k-e model. The flow field containing successive separation and reattachment partly resembles the conventional backward-facing step flow, although the extremely high turbulence level found in the present configuration indicates a promising performance of the serrated channel as a heat transfer promoter.

Finite-volume computation of merging parallel channel flows by a second-moment turbulence closure model

Abstract The mixing process of turbulent shear layer developing in a plane two-dimensional channel has been numerically investigated. Under the condition of various initial shear rates as accomplished by varying the mass flow rate of the two oncoming parallel channel flows, the performance of a second-moment closure model and the k - ϵ model has been examined in terms of representing the overall streamwise flow development as well as the profiles of individual variables. It is shown that the difference between the two models is largely attributable to the evaluation of the production rate of turbulent kinetic energy, the second-moment closure model ensuring a slightly more realistic simulation of the flows out of the equilibrium state of turbulent kinetic energy.