In this review, we present control methods for flow over a bluff body such as circular cylinder, 2D with blunt trailing edge, and sphere. We introduce recent major achievements in bluff-body controls 3D forcing, active feedback control, based on local global instability, synthetic jet. then classify the boundary-layer direct-wake modifications discuss important features associated these controls. Finally, some other issues Reynolds-number dependence, lowest possible drag by efficiency.

The objective of this study is to explore concepts for active control turbulent boundary layers leading skin-friction reduction using the direct numerical simulation technique. Significant drag achieved when surface condition modified suppress dynamically significant coherent structures present in wall region. accompanied by intensity wall-layer and reductions magnitude Reynolds shear stress throughout flow. apparent outward shift turbulence statistics controlled flows indicates a displaced...

Resolution requirements for large eddy simulation (LES), estimated by Chapman [AIAA J. 17, 1293 (1979)], are modified using accurate formulae high Reynolds number boundary layer flow. The new estimates indicate that the of grid points (N) required wall-modeled LES is proportional to ReLx, but a wall-resolving requires ÑReLx13/7, where Lx flat-plate length in streamwise direction. On other hand, direct numerical simulation, resolving Kolmogorov scale, ÑReLx37/14.

Direct numerical simulations of turbulent flows over riblet-mounted surfaces are performed to educe the mechanism drag reduction by riblets. The computed on riblet is in good agreement with existing experimental data. mean-velocity profiles show upward and downward shifts log–law for drag-decreasing drag-increasing cases, respectively. Turbulence statistics above riblets compared those a flat plate. Differences profile turbulence quantities found be limited inner region boundary layer....

Laminar vortex shedding behind a circular cylinder and its control using splitter plates attached to the are simulated. The completely disappears when length of plate is larger than critical length, this found be proportional Reynolds number. Strouhal number rapidly decreasing with increased until (l) nearly same as diameter (d). On other hand, at 1<l/d<2, shows two different behaviors for numbers investigated. net drag significantly reduced by plate, there exists an optimum...

The three-dimensional development of a plane free shear layer subjected to small sinusoidal perturbations periodically placed along the span is experimentally studied. Both laser induced fluorescence and direct interface visualization are used monitor between two fluids. different flow stabilities obtained through analysis temporal spatial evolution separating streams. It shown that characteristic time growth two-dimensional instability much shorter than instability. primary Kelvin-Helmholtz...

The present study numerically investigates two-dimensional laminar flow past a circular cylinder rotating with constant angular velocity, for the purpose of controlling vortex shedding and understanding underlying mechanism. Numerical simulations are performed flows Re=60, 100, 160 in range 0⩽α⩽2.5, where α is circumferential speed at surface normalized by free-stream velocity. Results show that rotation can suppress effectively. Vortex exists low rotational speeds completely disappears...

Turbulent drag reduction by polymer additives in a channel is investigated using direct numerical simulation. The dilute solution expressed with an Oldroyd-B model that shows linear elastic behaviour. Simulations are carried out changing the Weissenberg number at Reynolds numbers of 4000 and 20 000 based on bulk velocity height. onset criterion for predicted present study good agreement previous theoretical experimental studies. In addition, flow statistics such as r.m.s. fluctuations also...

Turbulent heat transfer to CO2 at supercritical pressure flowing in heated vertical tubes is investigated using direct numerical simulation the inlet Reynolds number Re0=5400, which based on bulk velocity and tube diameter. Temperature range within flow field covers pseudocritical region, where very significant fluid property variations are involved. Both upward downward flows considered. The wall temperature distribution shows well-known deterioration characterized by localized peak flows,...

Mathematical methods of control theory are applied to the problem fluid flow with long-range objective developing effective for turbulent flows. The procedure how cast controlling turbulence into a in optimal is presented using model problems through formalism and language theory. Then we present suboptimal feedback general stationary time-dependent calculus variations adjoint state gradient algorithms. This stochastic Burgers equation. Two types controls investigated: distributed boundary...

In the present study, we apply a distributed (i.e., spatially varying) forcing to flow over circular cylinder for drag reduction. The is realized by blowing and suction from slots located at upper lower surfaces of cylinder. profile each slot sinusoidal in spanwise direction but steady time. We consider two different phase differences between blowing/suction profiles: zero (in-phase forcing) π (out-of-phase forcing). Reynolds numbers considered are 40 3900 covering various regimes For all...

In this Letter we present a detailed mechanism of drag reduction by dimples on sphere such as golf-ball measuring the streamwise velocity above dimpled surface. Dimples cause local flow separation and trigger shear layer instability along separating layer, resulting in generation large turbulence intensity. With increased turbulence, reattaches to surface with high momentum near wall overcomes strong adverse pressure gradient formed rear As result, delay main reduce significantly. The study...

A database obtained by direct numerical simulation of turbulent channel flow was used to compute the three-dimensional frequency/wave-number spectrum wall-pressure fluctuations. The deduce scaling laws for pressure fluctuations and evaluate similarity form power spectrum. convection velocity as a function frequency, wave number, spatial temporal separations calculated compared with experimental data. problem artificial ‘‘acoustics’’ in incompressible flows is discussed.

Two simple feedback control laws for drag reduction are derived by applying a suboptimal theory to turbulent channel flow. These new require pressure or shear-stress information only at the wall, and when applied flow Re τ =110, they result in 16–22% skin-friction drag. More practical requiring local distribution of wall one component shear stress also shown work equally well.

Databases from direct numerical simulations of fully developed turbulent channel flow were used to examine the relation between skin-friction on wall and streamwise vortices observed near wall. It is shown that shear rate correlates with maximum correlation occurs downstream lateral displacement location measurement. Conditionally-averaged statistics taken high regions indicate higher values are associated located closer Visual studies time evolution near-wall also footprints can be...

Direct numerical simulation of a fully developed turbulent channel flow has been carried out at three Reynolds numbers, 180, 395, and 640, based on the friction velocity half width, in order to investigate very large-scale structures their effects wall shear-stress fluctuations. It is shown that exist outer layer they certainly contribute inner high number. Moreover, it revealed even fluctuations number, which are essentially associated with layer.

We present an overview of the aerodynamics heavy vehicles, such as tractor-trailers, high-speed trains, and buses. introduce three-dimensional flow structures around simplified model vehicles discuss flow-control devices used for drag reduction. Finally, we suggest important unsteady to investigate enhancement aerodynamic performance future directions experimental numerical approaches.

Large eddy simulations of turbulent flow over a sphere are conducted at subcritical Reynolds numbers (Re=3700 and 104) based on the freestream velocity diameter. At Re=3700, separating shear layer persists downstream to form cylindrical vortex sheet its instability becomes manifest x≈2d. The right behind contains only few vortices. On other hand, Re=104, shear-layer occurs in rings, near wake. Therefore, size recirculation region is smaller wake recovers more quickly than Re=3700. both...