Self-excitation of the motion of a structure has become a prominent aspect of engineering projects over recent years as designers are using materials at their limits, causing structures to become progressively lighter, more flexible and, therefore, prone to vibrate. Stimulated by the increasing interest in fluid–structure interaction (FSI) problems, this study investigated the instability and consequent FSI-induced self-excited oscillation of flexible structures in uniform flows at Reynolds numbers between $10$ and $1. 69\times 1{0}^{5} $. The investigations were performed in both water and a highly viscous syrup ($\nu = 1. 64\times 1{0}^{- 4} ~{\mathrm{m} }^{2} ~{\mathrm{s} }^{- 1} $) and considered three structures of different geometries. The results were conclusive in showing that the motion of the structure was characterized by a sequence of oscillation modes as a function of the characteristics of the structure and flow properties. In addition, it was possible to identify the self-excitation mechanisms as being of the instability-induced excitation (IIE) or movement-induced excitation (MIE) types. IIE was observed to be the most dominant mechanism of excitation at lower velocities and it was defined by a direct relation between the flow fluctuation and natural frequencies of the structure. For that reason, IIE was strongly determined by the geometry of the front body of the structure. At higher velocities, the amplitudes of the flow disturbances generated by the structure movement increased and excitations of the MIE type became predominant for all structures. The MIE mechanism was found to be weakly influenced by the shape of the structure but very sensitive to its dynamic characteristics and to the properties of the fluid, especially the Reynolds number.

We investigate the dynamics and rheology of elastic particles in a viscous extensional flow under Stokes flow conditions by means of the large deformation method developed by Gao, Hu & Ponte Castañeda (J. Fluid Mech., vol. 687, 2011, pp. 209–237). The particles are assumed to be homogeneous, incompressible and neo-Hookean solids. When subjected to extensional flow, an initially ellipsoidal (elliptical) elastic particle stretches and rotates simultaneously, tending to deform into a stable ellipsoidal shape with the initial major axis aligned with the extension direction. However, the steady-state solutions may not exist when the particle stiffness is lower than a certain critical value. By using the solution of a single particle, the macroscopic rheological properties are evaluated for a dilute suspension of elastic particles under extension. Similar to some polymer blends, softer particles lead to a larger extensional viscosity for the suspension.

We investigate the effect of a prescribed tangential velocity on the drag force on a circular cylinder in a spanwise uniform cross flow. Using a combination of theoretical and numerical techniques we make an attempt at determining the optimal tangential velocity profiles which will reduce the drag force acting on the cylindrical body while minimizing the net power consumption characterized through a non-dimensional power loss coefficient (${C}_{\mathit{PL}} $). A striking conclusion of our analysis is that the tangential velocity associated with the potential flow, which completely suppresses the drag force, is not optimal for both small and large, but finite Reynolds number. When inertial effects are negligible ($\mathit{Re}\ll 1$), theoretical analysis based on two-dimensional Oseen equations gives us the optimal tangential velocity profile which leads to energetically efficient drag reduction. Furthermore, in the limit of zero Reynolds number ($\mathit{Re}\ensuremath{\rightarrow} 0$), minimum power loss is achieved for a tangential velocity profile corresponding to a shear-free perfect slip boundary. At finite $\mathit{Re}$, results from numerical simulations indicate that perfect slip is not optimum and a further reduction in drag can be achieved for reduced power consumption. A gradual increase in the strength of a tangential velocity which involves only the first reflectionally symmetric mode leads to a monotonic reduction in drag and eventual thrust production. Simulations reveal the existence of an optimal strength for which the power consumption attains a minima. At a Reynolds number of 100, minimum value of the power loss coefficient (${C}_{\mathit{PL}} = 0. 37$) is obtained when the maximum in tangential surface velocity is about one and a half times the free stream uniform velocity corresponding to a percentage drag reduction of approximately 77 %; ${C}_{\mathit{PL}} = 0. 42$ and $0. 50$ for perfect slip and potential flow cases, respectively. Our results suggest that potential flow tangential velocity enables energetically efficient propulsion at all Reynolds numbers but optimal drag reduction only for $\mathit{Re}\ensuremath{\rightarrow} \infty $. The two-dimensional strategy of reducing drag while minimizing net power consumption is shown to be effective in three dimensions via numerical simulation of flow past an infinite circular cylinder at a Reynolds number of 300. Finally a strategy of reducing drag, suitable for practical implementation and amenable to experimental testing, through piecewise constant tangential velocities distributed along the cylinder periphery is proposed and analysed.

This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from ${\mathit{Re}}_{\theta } = 11\hspace{0.167em} 100$ to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.