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.

Review of Hydraulic Roughness Scales in the Fully Rough Regime

Abstract: A review of predictive methods used to determine the frictional drag on a rough surface is presented. These methods utilize a wide range of roughness scales, including roughness height, pitch, density, and shape parameters. Most of these scales were developed for regular roughness, limiting their applicability to predict the drag for many engineering flows. A new correlation is proposed to estimate the frictional drag for a surface covered with three-dimensional, irregular roughness in the fully rough regime. The correlation relies solely on a measurement of the surface roughness profile and builds on previous work utilizing moments of the surface statistics. A relationship is given for the equivalent sandgrain roughness height as a function of the root-mean-square roughness height and the skewness of the roughness probability density function. Boundary layer similarity scaling then allows the overall frictional drag coefficient to be determined as a function of the ratio of the equivalent sandgrain roughness height to length of the surface. DOI: 10.1115/1.4001492

On Drag of Evaporating Liquid Droplets

Abstract: —Drag data of water, methanol, heptane and benzene droplets are reported here. This data together with the data of Eisenklam et al. cover the entire range of Reynolds numbers from 1 to 2000 and mass transfer numbers from 0 to 3. The present study shows that the drag coefficients as a function of Reynolds numbers correlate well with the “standard drag” curve provided the characteristic density is the free stream density and the characteristic viscosity coefficient is μ r (Tr ). The basis for the choice of these two characteristic properties is discussed. The present correlation is insensitive to the mass transfer number. This suggests that mass efflux has little effect on drag of evaporating droplets. Present study indicates that for the determination of the drag coefficient of any evaporative droplet at quasisteady state, one needs only to know the wet bulb temperature as a function of free stream temperature. This information is sufficient to calculate μ r (Tr ). The “standard drag” curve can th...

Ion drag force in complex plasmas.

Abstract: The problem of calculating the ion drag force in complex plasmas is considered. It is shown that the standard theory of Coulomb scattering usually fails for the ion-dust elastic collisions. A simple approach to extend this theory is proposed. This leads to a considerable enhancement in the ion-dust elastic scattering cross section and, hence, increases the ion drag force in comparison with the previous analytical results. Analysis shows that the ion drag usually exceeds the electrostatic force in the limit of weak electric field. We suggest that this is the cause of the central "void" observed in microgravity complex plasma experiments.

Drag reduction by riblets

Abstract: The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A + is a better characterization of this breakdown than the riblet spacing, with an optimum A + 1/2 ≈ 11. It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y + ≈ 30, with typical streamwise wavelengths l + ≈ 150. They are connected with a Kelvin–Helmholtzlike instability of the mean velocity profile, also found in flows over plant canopies and other surfaces with transpiration. A simplified stability model for the ribbed surface approximately accounts for the scaling of the viscous breakdown with A + .