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.

The structure of the axisymmetric high‐Reynolds number flow around an ellipsoidal bubble of fixed shape

Abstract: The structure of the flow around an oblate ellipsoidal bubble of fixed shape is studied by means of direct numerical simulation for Reynolds numbers Re up to 103. In agreement with a previous study by Dandy and Leal [Phys. Fluids 29, 1360 (1986)] the computations demonstrate that if the bubble aspect ratio χ is high enough a standing eddy can exist at the rear of the bubble in an intermediate range of Re. This eddy disappears beyond a certain Reynolds number and it is shown that its existence is governed by the competition between accumulation and evacuation of the vorticity in the flow. The range of Re where the eddy exists increases very rapidly with χ meaning that this structure is certainly present in many experimental situations. The evolution of the drag coefficient with Re reveals that the oblateness has a dramatic influence on the minimum value of Re beyond which Moore’s theory [J. Fluid Mech. 23, 749 (1965)] can be used to predict the rise velocity of a bubble of fixed shape. In contrast, owing to the shape of the vorticity distribution at the surface of the bubble, no noticeable influence of the standing eddy on the drag is found. A quantitative comparison between the present results and those of previous authors shows that the computational description of the boundary layer around curved free surfaces is not a trivial matter since a strong influence of the numerical method is observed.

Recent Mathematical Models for Turbulent Flow in Saturated Rigid Porous Media

Abstract: Turbulence models proposed for flow through permeable structures depend on the order of application of time and volume average operators. Two developed methodologies, following the two orders of integration, lead to different governing equations for the statistical quantities. The flow turbulence kinetic energy resulting in each case is different. This paper reviews recently published mathematical models developed for such flows. The concept of double decomposition is discussed and models are classified in terms of the order of application of time and volume averaging operators, among other peculiarities. A total of four major classes of models are identified and a general discussion on their main characteristics is carried out. Proposed equations for turbulence kinetic energy following time-space and space-time integration sequences are derived and similar terms are compared. Treatment of the drag coefficient and closure of the interfacial surface integrals are discussed

Enhanced drag of a sphere settling in a stratified fluid at small Reynolds numbers

Abstract: We present a combined experimental and numerical investigation of a sphere settling in a linearly stratified fluid at small Reynolds numbers. Using time-lapse photography and numerical modelling, we observed and quantified an increase in drag due to stratification. For a salt stratification, the normalized added drag coefficient scales as Ri 0.51 , where Ri=a 3 N 2 /(νU) is the viscous Richardson number, a the particle radius, U its speed, ν the kinematic fluid viscosity and N the buoyancy frequency. Microscale synthetic schlieren revealed that a settling sphere draws lighter fluid downwards, resulting in a density wake extending tens of particle radii. Analysis of the flow and density fields shows that the added drag results from the buoyancy of the fluid in a region of size (ν/N) 1/2 surrounding the sphere, while the bulk of the wake does not influence drag. A scaling argument is provided to rationalize the observations. The enhanced drag can increase settling times in natural aquatic environments, affecting retention of particles at density interfaces and vertical fluxes of organic matter.

Pressure Forces on a Circular Cylinder in Cross Flow

Abstract: Mean pressure forces have been measured at Reynolds numbers from about 100 to 3 × 105 (fluctuating pressure forces from about 650). In the range Re = 103 − 104 there was a change in the sectional r.m.s. lift coefficient by about one order of magnitude while the corresponding change in the mean pressure drag coefficient was 20%.

Turbulent diffusion in the wake of a bluntnosed body at hypersonic speeds

Abstract: : The structure of the wake behind a blunt-nosed body at hypersonic speeds is described, and simplified representations of the outer and inner wakes are introduced. The boundary between these two regions is supposed to be a sharp front, and the growth of the inner wake depends only on the gradient and value of the enthalpy at this front. Two limiting cases of the behavior of the turbulent diffusivity are studied: (1) locally similar turbulence, in which the flow at each station behaves like a slice of a low speed self-similar turbulent wake, so that the diffusivity is proportional to the local momentum defect or drag contained in the inner wake; (2) frozen diffusivity, in which the turbulent diffusivity depends only on the initial value of the drag coefficient for the inner wake at the neck. Specific examples of the growth of the turbulent inner wake are calculated for comparison with measurements made in ballistic ranges. A typical reentry example is also computed at M = 22 and an altitude of 100,000 feet, in order to illustrate the behavior of the enthalpy distribution, and the behavior of the profiles of electron density in the wake in the two limiting cases of thermodynamic equilibrium and pure diffusion (zero recombination).