Transverse Wall Motion Research Articles

Drag Reduction of Turbulent Boundary Layers by Travelling and Non-Travelling Waves of Spanwise Wall Oscillations

Turbulence control in the form of a streamwise travelling wave of transverse wall motion was studied numerically by employing direct numerical simulations (DNS). Both total and phase averaging were utilised to examine the statistical behaviour of the turbulence affected by the wall forcing, with a focus on the skin friction. Comparison with results from pure temporal and spatial wall forcing are conducted, and a compilation of data is used to explore analogies with drag-reduced channel flow.

Friction-Drag Reduction by Transverse Wall Motion – A Review

ABSTRACTThe quest for drag reduction is driven by environmental concerns, in general, and the need to reduce fuel consumption in transport applications, in particular. Turbulent friction is especially important in civil aviation, accounting for over 50% of the total drag in cruise. In this context, spatially and/or temporally varying in-plane wall motion, while undoubtedly difficult to implement in practice, has attracted major interest, because of the large drag-reduction margins it yields. It is also a forcing method that is of fundamental interest, as it provokes intriguing interactions between the spanwise Stokes layer induced by the wall motion and the near-wall turbulence-regeneration mechanisms. This article provides a relatively brief, ‘entry-level’, review of research in this area, principally over the past two decades. While far from being exhaustive, the review conveys a reasonably detailed picture of some major physical issues as well as of the outcome of the most important computational and experimental studies. Particular emphasis is placed on the question of how results obtained in idealised laboratory conditions and by simulation at relatively low Reynolds-number values pertain to high values typical of high-speed transport.

Direct imaging of the interaction between spin wave and transverse wall in magnetic nanostrip

By micromagnetic simulations, the dynamical interaction between spin wave (SW) and a transverse wall (TW) in a magnetic nanostrip is studied. We find the dynamical interaction can be directly demonstrated by SW-induced TW oscillation, which can be obtained by calculating the total magnetic moment within the area of TW as a function of time. Two cases of the initial TW, in equilibrium state and in metastable state, are investigated and compared. Before SW reaches TW, the metastable TW oscillates naturally with a constant frequency, whereas the equilibrium TW does not oscillate. After SW acts on TW, both the metastable TW and the equilibrium TW will oscillate with a frequency that always equal to the frequency of the applied SW. The amplitude of the SW-induced TW oscillation for both the metastable case and the equilibrium case strongly depends on the frequency of the applied SW. Through tuning the frequency of the applied SW, we confirm that the natural oscillation of the metastable TW, which is independent of the applied SW, will not affect the amplitude of the SW-induced TW oscillation and the velocity of the TW motion in compare with those of the equilibrium TW. Interestingly, the frequency-response curves of the SW-induced TW oscillations display multiple resonance peaks. Moreover, we find the frequency-response curves of the SW-induced TW oscillation, SW reflection coefficient and TW velocity driven by SW share the same multiple-resonance property. It may suggest the SW-induced TW oscillation in the domain wall plays an important role in the SW-driven TW motion.

Pinning Effect on Current-Induced Domain Wall Motion in Nanostrip

AbstractPinning effect on current-induced magnetic transverse domain wall dynamics in nanostrip is studied for its potential application to new magnetic memory devices. In this study, we carry out a series of calculations by solving generalized Landau-Lifshitz equation involving a current spin transfer torque in one and two dimensional models. The critical current for the transverse wall depinning in nanostrip depends on the size of artificial rectangular defects on the edges of nanostrip. We show that there is intrinsic pinning potential for a defect such that the transverse wall oscillates damply around the pinning site with an intrinsic frequency if the applied current is below critical value. The amplification of the transverse wall oscillation for both displacement and maximum value of m3 is significant by applying AC current and current pulses with appropriate frequency. We show that for given pinning potential, the oscillation amplitude as a function of the frequency of the AC current behaves like a Gaussian distribution in our numerical study, which is helpful to reduce strength of current to drive the transverse wall motion.

Magnetic domain-wall motion by propagating spin waves

We found by micromagnetic simulations that the motion of a transverse wall (TW)–type domain wall in magnetic thin-film nanostripes can be manipulated via interaction with spin waves (SWs) propagating through the TW. The velocity of the TW motion can be controlled by changes of the frequency and amplitude of the propagating SWs. Moreover, the TW motion is efficiently driven by specific SW frequencies that coincide with the resonant frequencies of the local modes existing inside the TW structure. The use of propagating SWs, whose frequencies are tuned to those of the intrinsic TW modes, is an alternative approach for controlling TW motion in nanostripes.

Prediction of a Shear-Driven Three-Dimensional Turbulent Boundary Layer.

A three-dimensional turbulent boundary layer formed when an initially collateral boundary layer encounters transverse wall motion is calculated using a second-moment closure applicable up to a wall. The numerical solutions are extensively compared with an experiment by Lohmann, including the distributions of all the Reynolds stress components. In the inner layer, the turbulence model produces reasonable shear stress components parallel to the wall. This leads to a good prediction of the magnitude and direction of the wall shear stress. In the outer layer, however, a very rapid growth of the transverse shear stress component found in the experiment is not reproduced by the model. A modification of the redistribution model whose effect is limited to the outer layer in developing flows would be desirable.