| The results of a study investigating the effects of an imposed pressure gradient on the viscous wall region of a turbulent flow are presented. Only imposed pressure gradients homogeneous in the streamwise direction are considered.;Measurements using electrochemical probes are presented of the variation of the wall shear stress, the turbulent fluctuations of the wall shear stress, and the transverse correlation coefficients of the streamwise velocity gradient at the wall that are caused by the imposition of a sinusoidal pressure gradient on the flow of a turbulent fluid through a two inch pipe. The correlation coefficient measurements are used to provide information about the variation of the streak spacing, (lamda)('+). Results are presented for pulsations having a constant centerline variation of 10% and dimensionless frequencies, (omega)('+), ranging from 0.0012 to 0.0915. At low (omega)('+) values, the variation of (lamda)('+) is roughly 10%. The scaling relations developed for constant pressure gradients are found to be also applicable for pulsating turbulent flow at low (omega)('+) values. At higher (omega)('+) values, the turbulence is no longer able to follow the imposed pressure gradient and the amplitude of the variation of the turbulence is close to zero. However, at (omega)('+) values larger than the bursting frequency, a highly nonlinear large amplitude variation is observed in the (lamda)('+) measurements. This variation cannot be explained by present theories of turbulence.;A computational model that neglects streamwise derivatives is used to study imposed pressure gradients that are constant in time. The shear stress at the edge of the viscous wall region, and not the wall shear stress, is shown to be the proper shear stress to use in scaling the coherent eddy structures responsible for the production of the turbulent kinetic in the viscous wall region. Arguments are presented that show the minimum P(,e)('+) value at the edge of the viscous wall region, where P(,e)('+) is defined by (tau)('+) = 1 + P(,e)('+)y('+), corresponds to the point of imminent relaminarization of a turbulent flow. It is suggested that the logarithmic region disappears at this minimum value. |
