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.

A large-scale control strategy for drag reduction in turbulent boundary layers

Abstract: Using direct numerical simulations of turbulent channel flow, we present a new method for skin friction reduction, enabling large-scale flow forcing without requiring instantaneous flow information. As proof-of-principle, x-independent forcing, with a z wavelength of 400 wall units and an amplitude of only 6% of the centerline velocity, produces a significant sustained drag reduction: 20% for imposed counterrotating streamwise vortices and 50% for colliding, z-directed wall jets. The drag reduction results from weakened longitudinal vortices near the wall, due to forcing-induced suppression of an underlying streak instability mechanism. In particular, the forcing significantly weakens the wall-normal vorticity ωy flanking lifted low-speed streaks, thereby arresting the streaks’ sinuous instability which directly generates new streamwise vortices in uncontrolled flows. These results suggest promising new drag reduction techniques, e.g., passive vortex generators or colliding spanwise jets from x-aligned sl...

Turbulent drag reduction by passive mechanisms

Abstract: In many situations involving flows of high Reynolds number (where inertial forces dominate over viscous forces), such as aircraft flight and the pipeline transportation of fuels, turbulent drag is an important factor limiting performance. This has led to an extensive search for both active and passive methods for drag reduction1. Here we report the results of a series of wind-tunnel experiments that demonstrate a passive means of effectively controlling turbulence in channel flow. Our approach involves the introduction of specified patterns of protrusions on the confining walls, which interact with the coherent, energy-bearing eddy structures in the wall region, and so influence the rate at which energy is dissipated in the turbulent flow. We show that relatively small changes in the arrangement of these protrusions can alter the response of the system from one of drag decrease to increased mixing (drag enhancement).

An analytical one-dimensional model of momentum transfer by vegetation of arbitrary structure

Abstract: An analyticalone-dimensional model of momentum transferby vegetation with variable foliage distribution,sheltering and drag coefficientis developed. The model relies on a simpleparameterization of the ratio of theabove-canopy friction velocity, u*, to thewind speed at the top of the canopy,u(h), to predict vegetation roughness length(z0) and displacement height(d) as functions of canopy height (h) and dragarea index. Model predictionsof d/h and z/h compare very favorably withobserved values.A model sensitivity analysis suggests that shelteringeffects for momentum transfertend to make canopies with non-uniform foliagedistribution resemble canopies withmore uniform foliage distribution and that anyinfluence wind speed has on d/hand z0/h is more likely to be related to theinfluence that wind speed may haveon u*/u(h) rather than the influence windspeed may have on the foliage dragcoefficient. Model results indicate that z0/hand d/h are sensitive to uncertaintiesin the numerical values of the model parameters,foliage density and distribution,sheltering effects and variations in drag coefficientwithin the canopy. In additionz0/h is also shown to be sensitive to thepresence or absence of the roughnesssublayer. Given the simplicity of the model it issuggested that it may be of usefor land surface parameterizations in large scalemodels.

Effect of compliant wall motion on turbulent boundary layers

Abstract: A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Wingbeat frequency and the body drag anomaly: wind-tunnel observations on a thrush nightingale (Luscinia luscinia) and a teal (Anas crecca)

Abstract: A teal (Anas crecca) and a thrush nightingale (Luscinia luscinia) were trained to fly in the Lund wind tunnel for periods of up to 3 and 16 h respectively. Both birds flew in steady flapping flight, with such regularity that their wingbeat frequencies could be determined by viewing them through a shutter stroboscope. When flying at a constant air speed, the teal's wingbeat frequency varied with the 0.364 power of the body mass and the thrush nightingale's varied with the 0.430 power. Both exponents differed from zero, but neither differed from the predicted value (0.5) at the 1 % level of significance. The teal continued to flap steadily as the tunnel tilt angle was varied from -1 ° (climb) to +6 ° (descent), while the wingbeat frequency declined progressively by about 11 %. In both birds, the plot of wingbeat frequency against air speed in level flight was U-shaped, with small but statistically significant curvature. We identified the minima of these curves with the minimum power speed (Vmp) and found that the values predicted for Vmp, using previously published default values for the required variables, were only about two-thirds of the observed minimum-frequency speeds. The discrepancy could be resolved if the body drag coefficients (CDb) of both birds were near 0.08, rather than near 0.40 as previously assumed. The previously published high values for body drag coefficients were derived from wind-tunnel measurements on frozen bird bodies, from which the wings had been removed, and had long been regarded as anomalous, as values below 0.01 are given in the engineering literature for streamlined bodies. We suggest that birds of any size that have well-streamlined bodies can achieve minimum body drag coefficients of around 0.05 if the feet can be fully retracted under the flank feathers. In such birds, field observations of flight speeds may need to be reinterpreted in the light of higher estimates of Vmp. Estimates of the effective lift:drag ratio and range can also be revised upwards. Birds that have large feet or trailing legs may have higher body drag coefficients. The original estimates of around CDb=0.4 could be correct for species, such as pelicans and large herons, that also have prominent heads. We see no evidence for any progressive reduction of body drag coefficient in the Reynolds number range covered by our experiments, that is 21 600­215 000 on the basis of body cross-sectional diameter.