Effects of surface shapes on properties of turbulent/non-turbulent interface in turbulent boundary layers

The effects of directional riblets surfaces on the turbulent/non-turbulent(T/NT) interface in turbulent boundary layers are experimentally investigated using two-dimensional time-resolved particle image velocimetry(PIV). The velocity field of streamwise-wall-normal plane for the smooth surface, converging and diverging riblets surfaces are measured. The interface is detected using the criterion of local kinetic energy. The statistical properties of interface height and conditional averaged velocity for different surfaces are analyzed. It is shown that, the converging and diverging riblets surfaces have little effect on the fractal dimension of the T/NT interface, but they cause the intermittency profile deviate from error function and the probability distribution of interface height deviate from Gaussian function. To be specific, the distribution of interface height for the converging riblets surface shows a positive skewness while it shows a negative skewness for the diverging riblets surface.Moreover, the conditional averaged streamwise velocity and spanwise vorticity across the interface are analyzed, and it is found that their self-similarities are preserved for different surfaces when normalized with respective friction velocity. The correlation analysis reveals that near-wall streamwise velocity fluctuation and interface height show a negative correlation.

A Four-Equation Eddy-Viscosity Approach for Modeling Bypass Transition

It is very important to predict the bypass transition in the simulation of flows through turbomachinery.This paper presents a four-equation eddy-viscosity turbulence transition model for prediction of bypass transition.It is based on the SST turbulence model and the laminar kinetic energy concept.A transport equation for the non-turbulent viscosity is proposed to predict the development of the laminar kinetic energy in the pre-transitional boundary layer flow which has been observed in experiments.The turbulence breakdown process is then captured with an intermittency transport equation in the transitional region.The performance of this new transition model is validated through the experimental cases of T3AM,T3A and T3B.Results in this paper show that the new transition model can reach good agreement in predicting bypass transition,and is compatible with modern CFD software by using local variables.

Evolution of turbulent boundary layer over a three-dimensional bump

A bump is typically used in the inlet system of an aircraft engine to compress the incoming airflow and to reduce boundary layer thickness developed over fuselage.In this work,the turbulent flow over a three-dimensional bump is experimentally studied.The bump model is mounted in a closed return wind tunnel operated at the nominal velocity 10 m/s,corresponding to a friction Reynolds number of 2300.The flow field upstream the bump,along the bump centerline and at two different spanwise planes is measured with Particle Image Velocimetry(PIV).It is observed that a favorable pressure gradient develops until the suction peak of the bump,and that the average turbulence intensity within boundary layer is attenuated due to this favorable pressure gradient.The boundary layer thickness identified by examining profiles of streamwise velocity decreases significantly along the bump.The wall-normal position of the Turbulent/Non-Turbulent Interface(TNTI)identified with the vorticity criterion is also observed to decrease along the bump.When studying the behavior of the boundary layer thickness at different spanwise positions,we found that it tends to be larger in planes away from the centreline,which suggests that the bump diverts the flow.