In this paper, optimal control of Vortex shedding behind square cylinder in laminar flow regime (Re=200) has been investigated. When Navier-Stokes equations are used as state equations, the discretization of the optimality system leads to large scale optimization problems that represent a tremendous computational task. In order to reduce the number of state variables during the optimization process, a Reduced-Order Model (ROM) based on POD modes is derived to be used as state equations. Then, these equations are modified by introducing a control Function to ROM in order to gain equations which are suitable for optimal control. Optimal trajectory for control input has been found by employment of Quasi-linearization which is one of the numerical methods for solving optimal control.  

The unstable flow with rotating-stall-like (RS) effects in a rotor-cascade of an axial compressor was numerically investigated. The RS was captured with the reduction in mass flow rate and increasing of exit static pressure with respect to design operating condition of the single rotor. The oscillatory velocity traces during the stall propagation showed that the RS vortices repeat periodically, and the mass flow rate was highly affected by the blockage areas made by stall vortices. The results also showed that large scale vortices highly affects on the generation and growth of the new vortices. An unsteady two-dimensional finite-volume solver was employed for the numerical study which was developed based on Van Leer’s flux splitting algorithm in conjunction with TVD limiters and the κ-ε turbulence model was also employed. The good agreement of the computed mass flow rate with the experimental results validates the numerical study.

The dynamics of two-dimensional Vortex shedding from a rotating cluster of three cylinders was investigated using Computational Fluid Dynamics (CFD) and Dynamic Mode Decomposition (DMD). The cluster was formed from three circles with equal diameters in mutual contact and allowed to rotate about an axis passing through the cluster centroid. While immersed in an incompressible fluid with Reynolds number of 100, the cluster was allowed to rotate at non-dimensionalised rotation rates (Ω) between 0 and 1. The rotation rates were non-dimensionalised using the free-stream velocity and the cluster characteristic diameter, the latter being equal to the diameter of the circle circumscribing the cluster. CFD simulations were performed using StarCCM+. Dynamic Mode Decomposition based on the two-dimensional vorticity field was used to decompose the field into its fundamental mode-shapes. It was then possible to relate the mode-shapes to lift and drag. Transverse and longitudinal mode-shapes corresponded to lift and drag, respectively. Lift–drag polars showed a more complex pattern dependent on Ω in which the flow fields could be classified into three regimes: Ω less than 0. 3, greater than 0. 5, and between 0. 3 and 0. 5. In general, the polars formed open curves in contrast to those of static cylinders, which were closed. However, some cases, such as Ω = 0. 01, 0. 22, and 0. 28, formed closed curves. Whether a lift-drag polar was closed or open was deduced to be determined by the ratio of Strouhal numbers calculated using lift and drag time series, with closed curves forming when the ratio is an integer.

The present paper aims to study the lift forces acting on a cylinder oscillating in still water from spectral point of view. In the previous researches, Vortex shedding frequency has been related to the fundamental lift frequency. Here, the wavelet analysis is used as a relatively new concept in spectral analyses. The simultaneous time-frequency representations of the lift forces are investigated to localize the flow induced transitory characteristics. The peaks in the wavelet coefficients attributed to Vortex shedding are studied. The abrupt changes in the lift forces have also been studied by discrete wavelet decomposition. Small spikes have been observed in the results due to Vortex shedding. Wavelet analysis is considered as an efficient alternative method to predict Vortex shedding for larger Keulegan-Carpenter numbers (KC). Two different gap-to-diameter ratios, 0. 1 and 1. 0, are considered to account for the effect of bed proximity. Regular Vortex shedding is suppressed for lower gap ratios,this fact is confirmed by wavelet analysis as well. The KC numbers in the present study are in the range of 15 to 40. The flow is in the subcritical regime with Reynolds number in the range of 9500-26000. The cylinder and the plane bed are both smooth.

In open channels, when water flows through obstacles, boundary layer on upstream and the separation of streamlines on downstream of the obstacles and therefore Vortex shedding will occur. The overlap of Vortex wave reflecting from the walls of channels may produce surface waves which are perpendicular to the direction of the flow. When the Vortex frequency of obstacles and flow natural frequency become equal, resonance will occur. The purpose of current study was to determine the relative amplitude of transverse waves in submerged flow while resonance occurs. The submerge processing started from 0% into max percent while the waves are visible. This research included 71 tests in different prism obstacles with two style of water strike toward edge and face of the obstacles and in two different inline and staggered arrangements in two modes of transverse waves. The obstacle distances were 120 millimeters from each other. The maximum relative amplitude of the flow depth was 0.2 for mode one and 0.14 for the second modes of transverse wave. By using software and statistical analysis, a formula was suggested for determining the maximum relative amplitude of the flow depth. Finally, the data determined by suggested formula were compared with the laboratory data.

In this paper, flow control ability with suction over a circular cylinder has been numerically investigated in order to drag reduction and elimination of unsteadiness and the resultant vibrations. The efficiency of this flow control method is dependent to different parameters. Some of these parameters are dependent to geometry and the rest are dependent to flow characteristics. For this purpose, the geometric parameters of width and position of suction slat and also the suction flow velocity are studied in this investigation. 2D flow physics around the cylinder have been simulated in Re=3. 3*104 with K-ε (RNG) turbulence model. The obtained results show the elimination of Von Karman vortices under sufficient suction. Suction causes vacuum in viscous sublayer and elimination of low momentum regions via momentum transfer from outer layers, and consequently postpone the separation. The results indicate that pressure drag is decreased, skin friction drag is increased and total drag is decreased about 55%.