This invited article, presented at the 2012 APS-DFD meeting, provides a thoughtful explanation of tools and approaches for the design of flow control strategies for both stable and unstable flows.

Experimental results from two parallel shear flows of a highly shear-thinning viscoelastic polymer solution show that at low flow rates, the mean velocity profiles are steady and in agreement with analytical expectations. At higher flow rate, however, instability arises and the flow becomes weakly time dependent.

The anisotropic minimum dissipation (AMD) subfilter eddy-viscosity model for large-eddy simulations is tested, and results show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. In particular, the AMD model can accurately predict the expected surface-layer similarity profiles and power spectra for both velocity and scalar concentration.

From observations by grid deflection and by interferometry, the evaporating film is found to be nearly flat across the interior with a ridge at its edge, the ridge giving a large receding contact angle.

A careful experimental study of the dynamics of nodes and the distribution of the sizes of cusps at the border of liquid sheets is presented.

A moderately concentrated suspension of particles is sheared periodically in an experiment. Above a critical amplitude of shear, the diffusivities of the particles and of the fluid are found to increase significantly.

Two regimes of the flux scaling in axially-homogeneous turbulent convection are observed. In one, the scaling exponent is ½, similar to the one expected in the so called ‘ultimate regime’ of turbulent convection and in the other, it is 0.3, similar to the one observed in Rayleigh-Benard convection. However, the flow is different from the Rayleigh-Benard convection, in particular, very high fluxes and Reynolds numbers are obtained in this case. Results from different studies for three Prandtl numbers nearly collapse with the proposed unified flux scalings for the two regimes

Theorists propose a model for predicting and interpreting blood spatter patterns resulting from gunshot wounds. The atomization process, the trajectories of the backspatter drops of blood from the wound to the ground, the impact angle and the impact Weber number on the ground, as well as the distribution and location of blood stains and their shape and sizes are predicted.

Why is there a clockwise flow around bacterial swarms? Is it due to the flagellar filaments of cells extending outside the swarms? Using both known and new flow singularity solutions, researchers build an analytical model to understand the flows driven by flagella near boundaries and around swarms.

Confined 3D suspensions of spherical and non-Brownian particles show a remarkable rheological property: hydrodynamic interactions between spheres negatively contribute to the effective viscosity. This effect survives in 2D, and simulations help explain this surprising effect

Shearing colloidal suspensions in viscoelastic fluids exhibit clustering at relatively high shear rates. New experiments on fluids with thermoresponsive viscoelasticity suggest that this clustering can be driven by the perturbative effect of suspension anisotropy on hydrodynamic interactions.

Vehicles descending onto a planet’s atmosphere create shocks and radiative heat transfer. A theoretical study on the behavior of the excited electronic states of CO molecules sheds light on the determination of the radiative heat loads generated while attempting to land on Mars.

Nitrogen dissociation is studied using direct molecular simulation with a new potential-energy surface. Both $N_2$ - $N_2$ and $N$ - $N_2$ processes are simulated as they concurrently take place in an evolving nonequilibrium gas system. The coupling between vibrational excitation and dissociation is quantified leading to several new insights.

The Rayleigh-Taylor instability (RTI) is investigated using the direct simulation Monte Carlo method of molecular gas dynamics. Fully resolved two-dimensional simulations are performed to quantify the growth of flat and single-mode perturbed interfaces between two atmospheric-pressure monatomic gases as a function of the Atwood number and the gravitational acceleration. Future simulations on more extreme computational platforms will enable investigation of the RTI in greater detail.

A rigorous approach to include the effect of free-stream turbulence in the optimal control of streaks developing in an incompressible laminar boundary layer is presented. The control problem is formulated in wavenumber space and solved analytically. Under appropriate conditions, it is shown that the adjoint variables admit a self similar solution, whose scaling is different from that of the direct variables. The new scaling is derived analytically and confirmed numerically.

The flow of an electrical conductive fluid (e.g., liquid metal) past a localized magnetic field is affected by Lorentz forces. These Lorentz forces act as an obstacle to the flow and can lead to a variety of different complex flow structures. Direct numerical simulations are used to reconstruct experimental data and thereby enable detailed insight into the flow structures for different parameters.

A probabilistic cellular automation model for the evolution of turbulent spots and a physics-inspired model for the nucleation of spots show how the recent theoretical progress on transitional wall-bounded flows can be extended to the much wider class of spatially developing boundary-layer flows.

A clever isomorphism to particles reflected on surfaces of maxima of the pressure leads to an interesting physical interpretation of the transient dynamics of perturbations in constant shear flows both in 2D and 3D configurations. Moreover this allows to reconstruct the linearized Euler equations.

Sloshing in a hyperbolic container suspended as a bifilar pendulum is investigated. From a typical experimental initial condition the fluid and vessel motion can be found to be synchronous or asynchronous depending upon the length of the pendulum.

A differentially rotating spherical shell experiment is used to describe the onset of inertial modes and turbulence. It is found that for counter rotating inner and outer shell, a number of inertial modes is excited that show triadic resonance. Increasing the shear, the amount of subharmonic instabilities is increasing until the flow becomes turbulent at a critical counter rotation. The critical value scales with the Ekman number. The finding of a critical counter rotation and a supercritical turbulent flow gives new insight into the dynamics of planetary interiors.

Light-induced interface stresses can locally pinch and destabilize flowing capillary threads. Optical tunability allows the mechanisms of thread fragmentation in a confined environment to be revealed and provides a way to reversibly produce stable monodisperse droplet trains of controlled size, beyond the Rayleigh-Plateau stability limit

The liquid microchannels located at the contact between three bubbles in liquid foams exhibit peculiar inertial flows. Experiments and theory show that their relaxation can trigger traveling depression waves having the characteristics of solitons.

The formation and stability of viscous stratifications made between miscible fluids are experimentally investigated in square microchannels. The authors characterize the appearance and dynamics of traveling interfacial waves, which offer useful features for implementing in-line mixing procedures between highly viscous fluids in microfluidic systems.

Onsager’s cross-coupling relations are validated for gas flows in microgeometries, including ratchet surfaces used for the design of Knudsen pumps.

A careful study of induced-charge electrophoresis of a cylinder in a cylindrical pore reveals that the cylinder not only translate but also rotate. The discovery of the cylinder rotation is an important stepping stone for the development of micromotors.

Three AFM techniques are combined to investigate hydrodynamics around micro- and nanofibers. Comparing the experimental findings to a classical theoretical model reveals the potential of AFM for quantitative measurement of dissipation processes at the submicron scale.

The emerging field of plasmofluidics combines plasmonics and nanofluids as an efficient technological platform for manipulating nanoparticles and begetting indirect mixing in aqueous phases due to micro/nano vortices. In a theoretical study, conical metallic structures with small apex angles are considered, and the analytical dispersion relation under optimal operating conditions is derived. It is shown that nanoparticle control can be achieved in way that is similar to optical tweezers.

A spheroid rotating in a shear flow is known to exhibit complex dynamics. Here, the mechanisms behind the dynamics are explained using linear stability analysis, which enables us to present the first complete quantitative state-space in terms of fluid and particle inertia.

A numerical study of the flow over dispersed spherical particles proposes new correlations to determine the statistics of the aerodynamic forces acting on each particle.

A theoretical and computational study of the axial dispersion of colloidal suspensions confined in a parallel-plate channel with colloid diameters comparable to the channel width is presented.

Numerical simulations of the turbulent/nonturbulent interface in stably stratified turbulent mixing layers determine the statistical properties near the interface, such as the inner structure of the interface, the characteristic length scales and buoyancy Reynolds number, and the effects of buoyancy on the small-scale turbulence dynamics.

Theorists offer an alternate approach to the 1960s theory for gravity currents. The new approach uses vorticity conservation instead of energy conservation. The latest contribution extends previous studies to examine the evolution of an intrusion of moderate-density fluid passing horizontally into a stratified experiment.

A new eddy-viscosity model for large eddy simulation is used for calculations of plane channel flow and flow over a backward-facing step. This new model modifies the previously proposed integral scale of turbulence (ILSA) model to make it more universally applicable to complex problems.

Experiments on grid-generated turbulence reveal that vortex shedding energy along the centerline is lower for a fractal square grid than for a single-square grid and than for a single-square grid with splitter plates. A less intense vortex shedding enhances non-Gaussian behavior of the velocity fluctuations in the production region, and reduces the rate of streamwise growth of the integral length scale in the decay region.

Small-scale isotropy of turbulence is an assumption required for most theoretical descriptions. A combined experimental-numerical approach shows how buoyant forcing and rotation affects the small- and large-scale isotropy in thermally driven turbulence.

An array of model turbines with rotors is compared experimentally to an array of model turbines with stationary porous disks. The main discrepancy in the mean velocity components between the two cases is found in the out-of-plane component while the fluctuations of the out-of-plane component are found to play a dramatically different role in the vertical flux of mean kinetic energy. The study has wide implications on the use of the actuator disk model as a parametrization for a rotor in computational work.

Moment generating functions in wall-bounded flows exhibit power-law scaling in the log region. Interpreted with extended self-similarity, the extent of those scalings extends to the bulk region and the viscosity-affected near-wall region. New findings indicate that the Townsend attached eddy hypothesis can be used to model the flow beyond the log region if we allow the characteristic velocity scale to depend on the wall normal distance.

The influence of freestream turbulence on the transitional characteristics of a 2D square prism shear layer was considered experimentally. Bypass transition was observed to cause intermittent shear layer reattachment; this event has a cascading influence downstream, altering the formation of the von-Kármán street.

A numerical investigation of the wake of an axial flow miniature wind turbine is presented to elucidate the three-dimensional structure of the wake and the mechanisms controlling near and far wake instabilities.

The layer structure associated with the total kinetic energy for wall-bounded turbulent shear flows are examined using high-quality numerical simulation data.

Experimental and simulation data show that nonequilibrium scaling laws hold for turbulent wakes behind regular plates, thus preserving the same laws previously shown for wakes behind irregular (fractal-like/multiscale) plates.

Leading-edge vortex burst on a rotating flat plate wing was investigated experimentally using stereoscopic particle image velocimetry. Vortex burst was found to occur near the point of maximum lift production and was characterized by a concentration of opposite-sign vorticity entrainment and axial flow stagnation within the vortex core.

The interaction of a pair of quantized vortex rings moving in the same direction is investigated numerically. The rings can pass by each other or reconnect. If one of the rings is initially deformed, then small rings are often created. The results are compared to experiments on quantum turbulence.