Drag coefficient

About: Drag coefficient is a research topic. Over the lifetime, 14471 publications have been published within this topic receiving 303196 citations. The topic is also known as: drag factor.

Near-wall structure of turbulent boundary layer with spanwise-wall oscillation

Abstract: An investigation was carried out with an aim to better understand the mechanism of turbulent drag reduction with spanwise-wall oscillation, by carefully analyzing the experimental data of the near-wall structure of the boundary layer modified by the wall motion. It was found that the mean velocity gradient of the turbulent boundary layer was reduced close to the wall, and the logarithmic velocity profile shifted upwards by the wall oscillation. We argue that these changes in the mean velocity profile are mainly due to a negative spanwise vorticity created in the near-wall region of the boundary layer over the oscillating wall. The resultant near-wall velocity reduction seems to have weakened the near-wall turbulence activity by hampering the stretching of the quasistreamwise longitudinal vortices, leading to a reduction in skin-friction drag. Indeed, the signatures of the sweep events over the oscillating wall indicated that the duration and the strength were reduced by 78% and 64%, respectively. The reductions obtained by a 2D numerical model, incorporating the streamwise flow and associated stretching of the longitudinal vortical structure, were 47% for the sweep duration and 23% for the sweep strength, although these reductions were observed within the first cycle of wall oscillation. By phase averaging the conditionally sampled velocity data, we were able to show that the frequency of sweep events was reduced with a reduction in streamwise velocity when the negative spanwise vorticity is created by the wall oscillation in the near-wall region. It was also shown that the turbulent skin-friction reduction with spanwise-wall oscillation can be optimized with a nondimensional, spanwise wall velocity, and nearly 45% drag reduction can be obtained in the turbulent boundary layer at an optimum value of w+=15.

Mixing in flows down gentle slopes into stratified environments

Abstract: Observations of the flow of dense fluid into uniformly density-stratified environments down plane slopes with small inclination to the horizontal (≤ 20°) are described, and a quantitative model for such flows is presented. In these experiments the dense fluid is released at the top of the slope for a finite period of time. The resulting downslope gravity current, or downflow, has uniform thickness with a distinct upper boundary, until it approaches its level of neutral density where the fluid leaves the proximity of the slope. Turbulent transfers of mass and momentum occur across the upper boundary, causing a continuous loss of fluid from the downflow in most cases, and associated loss of momentum. The flow may be characterized by the local values of the Richardson number Ri, the Reynolds number Re (generally large), and of M = QN 3 /g' 2 , where Q is the (two-dimensional) volume flux, N the buoyancy frequency and g' the (negative) buoyancy of the dense fluid. The model for the downflow describes the turbulent transfers in terms of entrainment, detrainment and drag coefficients, E e , E d and k respectively, and the observations enable the determination of these coefficients in terms of the local values of M and Ri. The model may be regarded as an extension of that Ellison & Turner (1959) to stratified environments, describing the consequent substantial changes in mixing and distribution of the inflow. It permits the modelling of the bulk properties of these flows in geophysical situations, including shallow and deep flows in the ocean.

Local drag laws in dispersed two-phase flow

Abstract: The ability to predict the interfacial drag between phases is of considerable importance for analyzing a dispersed two-phase system undertransient conditions. The present report develops constitutive relations for the drag force for bubbly, droplet, and particulate flows by a unified method. Simple drag-similarity criteria and a mixture-viscosity model are introduced in the analysis. The present drag correlations cover all concentration ranges and wide Reynolds-number ranges, from the Stokes regime up to the Newton's regime or the churn-turbulent-flow regime.

Reynolds Stress and Turbulent Energy Production in a Tidal Channel

Abstract: A high-frequency (1.2 MHz) acoustic Doppler current profiler (ADCP) moored on the seabed has been used to observe the mean and turbulent flow components in a narrow tidally energetic channel over six tidal cycles at neap and spring tides. The Reynolds stress has been estimated from the difference in variance between the along-beam velocities of opposing acoustic beams with a correction for the sampling scheme and bin size. Shear stress was found to vary regularly with the predominantly semidiurnal tidal flow with the stresses on the spring ebb flow (up to 4.5 Pa) being generally greater than on the flood flow (<2 Pa) when the currents are weaker. The vertical structure approximated to linear stress profiles decreasing from maximum values near the bed to almost zero stresses just below the surface. The variation in the bed stress was well represented by a quadratic drag law, based on the depth-mean current, with an estimated drag coefficient of 2.6 ± 0.2 × 10−3. The production of turbulent kinetic...