Prediction of drag reduction effect by streamwise traveling wave-like wall deformation in turbulent channel flow at practically high Reynolds numbers

A fully developed turbulent channel flow controlled by traveling wave-like wall deformation under a constant pressure gradient condition is studied numerically and theoretically. First, direct numerical simulation (DNS) at three different friction Reynolds numbers, ${\text{Re}}_{\tau }=90,$ 180, and 360, are performed to investigate the modification in turbulence statistics and their scaling. Unlike the previous study assuming a constant flow rate condition, suppression of the quasi-streamwise vortices is not observed in either drag decrease cases or drag increase cases. It is found in the drag reduction case, however, that the periodic component of the Reynolds shear stress (periodic RSS) is largely negative in the viscous sublayer and the buffer layer. For the maximum drag reduction case, the set of control parameters is found to be identical in wall units regardless of the Reynolds number, and the resulting mean velocity profiles are also observed to be approximately similar even with an additional case of ${\text{Re}}_{\tau }=720$. Based on this scaling, we propose a semi-empirical formula for the mean velocity profile modified by the present control. With this formula, about $20\%-25\%$ drag reduction effect is predicted even at practically high Reynolds numbers, ${\text{Re}}_{\tau }\sim {10}^5-{10}^6$.     

Acousto-mechanical behaviour of ex-vivo skin: Nonlinear and viscoelastic properties

The mechanical behaviour of skin is significant for some applications including dermatology, surgery, and impact biomechanics science. In this work, we have investigated the study of the acousto-mechanical viscoelastic properties of skin. For that, both tensile-relaxation and ultrasonic tests were conducted on porcine tissue samples in fibre directions. To understand the complex skin aging phenomena, we used strength tensile test correlated with the Nonlinear Time Reversal signal processing tool extension “TR-NEWS”. Uniaxial tensile tests were carried out at a strain rate of $5?{10}^{?3}\text{ mm}{\text{s}}^{?1}$ on skin using a load-relaxation-discharge load path with increasing amplitude and offset. This work is also under way to extend the frequency range of ultrasounds to 50 MHz. Digital Image Correlation was used for 2D strain measurement of the dermis. From this analysis, we conclude that fresh porcine skin should be modelled as a nonlinear viscoelastic material with strain-rate dependence. The obtained hysteresis loop shall be taken as significant skin damage.