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.

Drag reduction by polymer additives in a turbulent channel flow

Abstract: Turbulent drag reduction by polymer additives in a channel is investigated using direct numerical simulation. The dilute polymer solution is expressed with an Oldroyd-B model that shows a linear elastic behaviour. Simulations are carried out by changing the Weissenberg number at the Reynolds numbers of 4000 and 20 000 based on the bulk velocity and channel height. The onset criterion for drag reduction predicted in the present study shows a good agreement with previous theoretical and experimental studies. In addition, the flow statistics such as the r.m.s. velocity fluctuations are also in good agreement with previous experimental observations. The onset mechanism of drag reduction is interpreted based on elastic theory, which is one of the most plausible hypotheses suggested in the past. The transport equations for the kinetic and elastic energy are derived for the first time. It is observed that the polymer stores the elastic energy from the flow very near the wall and then releases it there when the relaxation time is short, showing no drag reduction. However, when the relaxation time is long enough, the elastic energy stored in the very near-wall region is transported to and released in the buffer and log layers, showing a significant amount of drag reduction.

Drag reduction through self-similar bending of a flexible body

Abstract: The classical theory of high-speed flow1 predicts that a moving rigid object experiences a drag proportional to the square of its speed. However, this reasoning does not apply if the object in the flow is flexible, because its shape then becomes a function of its speed—for example, the rolling up of broad tree leaves in a stiff wind2. The reconfiguration of bodies by fluid forces is common in nature, and can result in a substantial drag reduction that is beneficial for many organisms3,4. Experimental studies of such flow–structure interactions5 generally lack a theoretical interpretation that unifies the body and flow mechanics. Here we use a flexible fibre immersed in a flowing soap film to measure the drag reduction that arises from bending of the fibre by the flow. Using a model that couples hydrodynamics to bending, we predict a reduced drag growth compared to the classical theory. The fibre undergoes a bending transition, producing shapes that are self-similar; for such configurations, the drag scales with the length of self-similarity, rather than the fibre profile width. These predictions are supported by our experimental data.

The importance of the forces acting on particles in turbulent flows

Abstract: The analysis of the importance of the forces that act over an ensemble of particles in a turbulent field has been carried out by using direct numerical simulation for a wide range of density ratios (2.65<ρ<2650). It has been observed that, compared to the Stokes drag, the added mass is always negligible, the pressure drag is relevant for density ratios O(1), and the Basset force is appreciable for the whole range investigated. However, the effect of these forces on the particle dispersion is about 1% for ρ∼1 as well as for large density ratios.

Mechanical Consequences of Size in Wave‐Swept Algae

Abstract: The intertidal zone of wave-swept rocky shores is characterized by high velocities and exceedingly rapid accelerations The resulting hydrodynamic forces (drag, lift, and the accelerational force) have been hypothesized both to set an upper limit to the size to which wave-swept organisms can grow and to establish an optimal size at which reproductive output is maximized This proposition has been applied previously to inter- tidal animals that grow isometrically, in which case the accelerational force is the primary scaling factor that constrains size In contrast, it has been thought that the size of wave- swept algae is limited by the interaction of drag alone with these plants' allometric pattern of growth Here we report on empirical measurements of drag and accelerational force in three common species of intertidal algae (Gigartina leptorhynchos, Pelvetiopsis limitata, and Iridaea flaccida) The drag coefficients for these species decrease with increased water velocity, as is typical for flexible organisms For two of these species, this decline in drag coefficient is dramatic, leading to small drag forces with concomitant low drag-induced mortality at plant sizes near those observed in the field However, all three species have surprisingly large inertia coefficients, suggesting that these plants experience large acceler- ational forces in surf-zone flows Preliminary calculations show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these algae This study further models the interplay between size-dependent survivorship and re- productive ability to predict the size at which reproductive output peaks This "optimal size" depends on the strength distribution and morphology of the algal species and on the flow regime characteristic of a particular site This study shows that the optimal size predicted for G leptorhynchos, calculated using velocities and accelerations typical of the moderately protected location where this species was collected, closely matches its observed mean size Similarly, the predicted optimal sizes of P limitata and I flaccida at the exposed site where these plants were sampled also match their mean observed sizes These data, although preliminary, suggest that mechanical factors (in particular the accelerational force) may be important in limiting the size of intertidal macroalgae and that attention solely to biological constraints may be inappropriate

Drag reduction of a submersible hull by electrolysis

Abstract: Viscous drag reduction of a fully-submerged body of revolution is obtained by creating hydrogen gas on the hull by electrolysis. The bubbles alter both the laminar and turbulent boundary-layer characteristics resulting in a significant reduction in the viscous drag on the 3-foot model up to a speed of 8.5 feet per second, the maximum test velocity. Results show that the amount of drag reduction depends on the ratio of the mass flow of water in the wake to the time-rate of hydrogen mass produced beneath the boundary-layer. The results presented herein are model results and should NOT be considered directly applicable to any existing prototype.