The effect of Reynolds number on turbulent drag reduction by streamwise travelling waves

This paper exploits the turbulent flow control method using streamwise travelling waves (Quadrio et al. J. Fluid Mech., vol. 627, 2009, pp. 161–178) to study the effect of Reynolds number on turbulent skin-friction drag reduction. Direct numerical simulations (DNS) of a turbulent channel flow subjected to the streamwise travelling waves of spanwise wall velocity have been performed at Reynolds numbers ranging from Reτ=200 to 1600. To the best of the authors’ knowledge, this is the highest Reynolds number attempted with DNS for this type of flow control. The present DNS results confirm that the effectiveness of drag reduction deteriorates, and the maximum drag reduction achieved by travelling waves decreases significantly as the Reynolds number increases. The intensity of both the drag reduction and drag increase is reduced with the Reynolds number. Another important finding is that the value of the optimal control parameters changes, even in wall units, when the Reynolds number is increased. This trend is observed for the wall oscillation, stationary wave, and streamwise travelling wave cases. This implies that, when the control parameters used are close to optimal values found at a lower Reynolds number, the drag reduction deteriorates rapidly with increased Reynolds number. In this study, the effect of Reynolds number for the travelling wave is quantified using a scaling in the form Re−ατ. No universal constant is found for the scaling parameter α. Instead, the scaling parameter α has a wide range of values depending on the flow control conditions. Further Reynolds number scaling issues are discussed. Turbulent statistics are analysed to explain a weaker drag reduction observed at high Reynolds numbers. The changes in the Stokes layer and also the mean and root-mean-squared (r.m.s.) velocity with the Reynolds number are also reported. The Reynolds shear stress analysis suggests an interesting possibility of a finite drag reduction at very high Reynolds numbers

Capacity fade modelling of lithium-ion battery under cyclic loading conditions

A pseudo two-dimensional (P2D) electro-chemical lithium-ion battery model is presented in this paper to study the capacity fade under cyclic charge-discharge conditions. The Newman model [1,2] has been modified to include a continuous solvent reduction reaction responsible for the capacity fade and power fade. The temperature variation inside the cell is accurately predicted using a distributed thermal model coupled with the internal chemical heat generation. The model is further improved by linking the porosity variation with the electrolyte partial molar concentration, thereby proving a stronger coupling between the battery performance and the chemical properties of electrolyte. The solid electrolyte interface (SEI) layer growth is estimated for different cut-off voltages and charging current rates. The results show that the convective heat transfer coefficient as well as the porosity variation influences the SEI layer growth and the battery life significantly. The choice of an electrolyte decides the conductivity and partial molar concentration, which is found to have a strong influence on the capacity fade of the battery. The present battery model integrates all essential electro-chemical processes inside a lithium-ion battery under a strong implicit algorithm, proving a useful tool for computationally fast battery monitoring system

Formation-breakdown cycle of turbulent jets in a rotating fluid

Results of comprehensive particle image velocimetry measurements investigating the dynamics of turbulent jets in a rotating fluid are presented. It is observed that background system rotation induces a time-periodic formation–breakdown cycle of the jets. The flow dynamics associated with this process is studied in detail. It is found that the frequency of the cycle increases linearly with the background rotation rate. The data show that the onset of the breakdown phase and of the reformation phase of the cycle can be characterized in terms of a local Rossby number employing an internal velocity and a length scale of the jet. The critical values for this local Rossby number, for onset of breakdown and reformation, scale linearly with a global Rossby number based on the flow conditions at the source. The analysis of the experimental data suggests centrifugal instability as the potential origin of the formation–breakdown cycle

Numerical simulation of a confined cavitating gas bubble driven by ultrasound

This work investigates the flow disturbance generated by an ultrasonically-driven gas bubble confined in a narrow gap over one acoustic cycle. Here, we provide a more accurate representation of ultrasonic cleaning by implementing a Volume-of-Fluid model in OpenFOAM that simulates the ultrasound as a sinusoidally time-varying pressure boundary condition. A modified Rayleigh-Plesset equation is solved to select an acoustic forcing that instigates bubble collapse. Simulations reveal the interaction between the inflow from the acoustic forcing and the flow deflected by the confining walls intensifies the strength of the self-piercing micro-jet(s), and consequently of the unsteady boundary layer flow, compared to the traditional collapse near a single rigid wall. Depending on the gap height and the position of bubble inception inside the gap, three distinct collapse regimes involving dual-jets or directed-jets are identified, each resulting in a different shear-stress footprint on the confining boundaries. Plots of the spatio-temporal evolution of the shear flow (that is difficult to measure experimentally) reveal peak shear-stress magnitudes at collapse that are double those reported for an undriven laser-induced bubble in similar geometric confinement. This twofold increase is attributed to the ultrasonic signal driving the collapse. Surprisingly, in our simulations we have not encountered a transferred-jet regime previously observed for an unforced bubble collapsing in a similar configuration. This unexpected finding highlights the different physics involved in modelling acoustically-driven bubbles compared to the conventional laser-induced bubbles used in experiments

Computational investigation of prolonged airborne dispersion of novel coronavirus-laden droplets

We have performed highly accurate numerical simulations to investigate prolonged dispersion of novel coronavirus-laden droplets in classroom air. Approximately 10,900 virus-laden droplets were released into the air by a teacher coughing and tracked for 90 min by numerical simulations. The teacher was standing in front of multiple students in a classroom. To estimate viral transmission to the students, we considered the features of the novel coronavirus, such as the virus half-life. The simulation results revealed that there was a high risk of prolonged airborne transmission of virus-laden droplets when the outlet flow of the classroom ventilation was low (i.e., 4.3 and 8.6 cm/s). The rates of remaining airborne virus-laden droplets produced by the teacher coughing were 40% and 15% after 45 and 90 min, respectively. The results revealed that students can avoid exposure to the virus-laden droplets by keeping a large distance from the teacher (5.5 m), which is more than two times farther than the currently suggested social distancing rules. The results of this study provide guidelines to set a new protection plan in the classroom to prevent airborne transmission of virus-laden droplets to students

Numerical model for cough‐generated droplet dispersion on moving escalator with multiple passengers

To investigate the motion of virus‐laden droplets between moving passengers in line, we performed numerical simulations of the distribution of airborne droplets within a geometrically detailed model similar to an actual escalator. The left and right sides and the ceiling of the escalator model were surrounded by walls, assuming a subway used by many people every day with concern to virus‐laden droplets. Steps and handrails were incorporated in the model to faithfully compute the escalator‐specific flow field. The ascending and descending movements of the escalator were performed with 10 or 5 passengers standing at different boarding intervals. To resolve the unsteady airflow that is excited by a moving boundary consisting of passengers, steps, and handrails, the moving computational domain method based on the moving‐grid finite‐volume method was applied. On the basis of the consideration that the droplets were small enough, droplet dispersion was computed by solving the equation of virus‐laden droplet motion using a pre‐computed velocity field, in which the flow rate of a cough, diameter distribution, and evaporation of droplets are incorporated. The simulation resolved the detailed motion of droplets in flow, and therefore, we were able to evaluate the risk of viral adhesion to following passengers. As a result, we found that the ascending escalator had a higher risk of being exposed to virus‐laden droplets than the descending escalator. We also reported that the chance of viral droplet adhesion decreases as the distance from the infected person increases, emphasizing the importance of social distancing

Effectiveness of active flow control for turbulent skin friction drag reduction

The effectiveness of the opposition control method proposed by Choi et al. [J. Fluid Mech. 262, 75 (1994)] has been studied using direct numerical simulations. In this study, the effects of the amplitude and the phase of wall blowing and suction control input were considered separately. It is found that the amplitude of wall blowing and suction as well as the detection plane location played an important role in active control for skin-friction drag reduction. By changing the amplitude, a substantial drag reduction was achieved for all detection plane locations considered, and the efficiency of the opposition control was also improved. When the control was effective, the drag reduction was proportional to the wall blowing and suction strength. There existed a maximum wall blowing and suction strength, beyond which the opposition control became less effective or even unstable. Turbulence characteristics affected by various wall blowing and suction parameters were analyzed to understand the underlying mechanisms for drag reduction. The wall-normal velocity and vorticity fluctuations showed a strong correlation with drag reduction. © 2011 American Institute of Physic

Direct numerical simulation of a turbulent 90º bend pipe flow

Direct numerical simulation (DNS) has been performed for a spatially develop- ing 90 bend pipe ow to investigate the unsteady ow motions downstream of the bend. A recycling method is implemented to generate a fully-developed turbulent in ow condition. The Reynolds number of the pipe ow is ReD = 5300 and the bend curvature is = 0:4. A long straight pipe section (40D) is attached in the downstream of the bend to allow the ow to develop. Flow oscillations downstream of the bend are measured using several methods, and the corresponding oscillation frequencies are estimated. It is found that di erent characteristic frequencies are obtained from various ow measurements. The stagnation point movement and single-point velocity measurements may not be good measures to determine the swirl-switching frequency. The oscillations of the lateral pressure force on the pipe wall and half-sided mass ow rate are proposed to be a more unambiguous measure of the unsteady ow motions downstream of the bend

LES of a low velocity-ratio pitched and skewed jet in a flat-plate boundary layer

The jet in cross-flow (JICF) is a commonly studied flow in the context of boundary layer control. Particularly, pitched and skewed jets has potential for flow separation control. In this study, both the perpendicular and the pitched and skewed jets were considered to highlight the different flow structures. The formation of the vertical "shell" was clearly seen in a low velocity-ratio perpedicular JICF, while no such structure was observed in the pitched and skewed case. Instantaneous and time-averaged flow field were analysed in this paper