Huseyin Akilli

Vortex formation from a cylinder in shallow water

The near wake of a vertical, circular cylinder in shallow water is investigated using a combination of visualization marker and a technique of high-image-density particle image velocimetry. The formation of a large-scale Karman vortex involves upward ejection of fluid through its center, which eventual leads to a horizontal vortex that induces significant distortion of the free surface. The strength of this secondary vortex is, however, an order of magnitude smaller than that of the large-scale Karman vortex. Using global, instantaneous imaging, the relationship between patterns of streamline topology and vorticity is established for successive phases of the oscillation cycle, and in horizontal planes at and above the bed. The sequence of phase-referenced states shows that it is possible to identify topological characteristics, including the location of a specific critical (saddle) point that are consistent at elevations on and above the bed. In turn, the sequential development of topological streamline and vorticity patterns is related to time-averaged representations. At the bed, the time-averaged streamline topology downstream of the base of the cylinder takes on a form known as an owl face of the first kind, which was originally defined for a completely different exterior flow. Immediately adjacent to the base of the cylinder, an additional system of saddle points is located at either end of a nodal line. At locations above the bed, one of the two principal saddle points of the owl face of the first kind disappears and the principal foci are transformed from a stable to an unstable state. Furthermore, at progressive elevations above the bed, the degree of concentration of vorticity of the major, large-scale vortex successively decreases. In turn, the foregoing features are related to the patterns of Reynolds stress and velocity fluctuation at and above the bed.

Gas-solid flow behavior in a horizontal pipe after a 90° vertical-to-horizontal elbow

Abstract The characteristics of the particle flow in a horizontal pipe following a 90° vertical-to-horizontal elbow were investigated both numerically and experimentally. Laboratory experiments were conducted in a 0.154 m ID test section. The effects of air velocity, the ratio of air-to-solids mass flow rate, geometry of the elbow and inlet conditions on gas–solid flow patterns were investigated experimentally. Pulverized coal with a mean particle diameter of 50 μm was used as the solid material. Experiments were performed with conveying air velocities ranging from 15 to 30 m/s and air-to-solids mass flow rate ratios of 1 and 3, with elbows having bend radius to pipe diameter ratios of 1.5 and 3. Measurements of particle concentration and particle velocity were performed at various locations along the horizontal pipe using a fiber-optic probe which was traversed over the pipe cross-section of the pipe. It was observed that the strong rope created by the elbow disintegrates within an axial distance of 10 pipe diameters. Fully developed concentration and velocity profiles were obtained within approximately 30 pipe diameters from the elbow exit plane. The rope behavior was different for the two elbows studied ( R / D =1.5 and 3). The shapes of the fully developed profiles were found to be independent of inlet conditions. CFD simulations of gas–solid flow through 90° circular elbows were performed using the Lagrangian approach. The simulations were used to predict the location of the rope and its dispersion rate along the horizontal pipe after the elbow exit plane.

Control of Vortex Shedding of Circular Cylinder in Shallow Water Flow Using an Attached Splitter Plate

In the present work, passive control of vortex shedding behind a circular cylinder by splitter plates of various lengths attached on the cylinder base is experimentally investigated in shallow water flow. Detailed measurements of instantaneous and time-averaged flow data of wake flow region at a Reynolds number of Re=6300 were obtained by particle image velocimetry technique. The length of the splitter plate was varied from L∕D=0.2 to L∕D=2.4 in order to see the effect of the splitter plate length on the flow characteristics. Instantaneous and time-averaged flow data clearly indicate that the length of the splitter plate has a substantial effect on the flow characteristics. The flow characteristics in the wake region of the circular cylinder sharply change up to the splitter plate length of L∕D=1.0. Above this plate length, small changes occur in the flow characteristics.

Dye Visualization of the Flow Structure over a Yawed Nonslender Delta Wing

S TUDIES of aerodynamic structures and behaviors of the nonslender delta wings are invariably essential to develop a method to control the development of the vortex breakdown as well as the development of vortices. Unsteady aerodynamics of nonslender delta wings, consisting of shear layer instabilities, the structure of vortices, the occurrence of breakdown, and fluid/structure interactions were extensively reviewed by Gursul et al. [1]. They emphasized the sensitivity of the vortical flow structures varying the angle of attack of the deltawing.Yavuz et al. [2] studied thevortical flow structure on a plane immediately adjacent to the surface of nonslender delta wing, 38:7 deg. Yaniktepe and Rockwell [3] performed experimental investigations on the flow structures at trailing-edge regions of diamondand lambda-type wings. In both wings, vortical flow structures in the crossflowplanes of trailing edge vary rapidly with the angles of attack . Sohn et al. [4] visually investigated the development and interaction of vortices in crossflow planes at various locations on the delta wing with leading edge extension (LEX) using micro water droplets and a laser beam sheet. The range of angle of attack was taken as 12 24 deg at yaw angles of 0, 5, and 10 deg. It was indicated that, by introducing yaw angle , the coiling, merging, and diffusion of thewing and LEX vortices increased on the windward side, whereas they became delayed significantly on the leeward side. Their study confirmed that the yaw angle had a profound effect on the vortex structures. Taylor and Gursul [5] visualized leading-edge vortices of a 50 deg sweep angle, having angles of attack as low as 2:5 deg. Gursul et al. [6] report that combat air vehicles (UCAVs) and micro air vehicles have particularly dominant vortical flows having low sweep angles (25–55 deg), and future UCAVs are expected to be highly maneuverable and highly flexible. Yaniktepe and Rockwell [7] aimed at investigating the unresolved concepts, which included averaged structure of shear layer from the leading edge of the wing, unsteady features of separated layer adjacent to the surface of the wing, and control of flow structure by leading-edge perturbations. Elkhoury and Rockwell [8] have investigated to provide various measurements of the visualized dye patterns, including the degree of interaction of vortices, the onset of vortex breakdown, and effective sweep angle of the wing root vortex, as a function of both Reynolds number and angle of attack . Elkhoury et al. [9] had investigated the Reynolds number dependence of the near-surface flow structure and topology on a representative UCAV planform. The present investigation focuses on the formation and development of leading-edge vortices, vortex breakdown, and threedimensional separationandstallof thecomplexanddisorganizedflow structure over the delta wing. The leading-edge sweep angle was 40 deg. The angle of attack was varied within the range of 7 17 deg and the yaw angle was varied within the range of 0 15 deg.

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

D ELTAwings have evolved over the years and are primarily used on many fighter aircraft. As these aircraft become more and more maneuverable, delta-wing vortex dynamics and the understanding of the physics of time-dependent unsteady flows have become substantially important [1]. Several variables affect the deltawing vortex dynamics. As indicated by Yaniktepe [2], some of these variables are angle of attack, leading-edge geometry, wing thickness, sweep angle, Reynolds number, and freestream conditions. Yaniktepe and Rockwell [3] investigated aerodynamics of the delta wing with a sweep angle of 38:7 for the value of Reynolds number based on the chord length C, which was maintained at Re 10. They reported that the nonslender delta wings exhibited more distinctive features than the slender delta wings, especially at a higher angle of attack as a result of the earlier onset of vortex breakdown, which are based on the time-averaged velocity and vorticity distributions in the crossflow plane. Canpolat et al. [4] observed the variation of flow structures on the delta-wing surface with a sweep angle of 40 as a function of the angle of attack and yaw angle , using the dye visualization technique. When the delta wing is under the effect of a yaw angle, the symmetrical flow structure deteriorates, and a vortex breakdown occurs earlier on the windward side of the delta wing, as compared with the leeward side. The main vortices in crossflow planes occur in the inner side close to the central axis of the delta wing. Many small-sized vortices are also evident next to themain rotating vortices. Yayla et al. [5] investigated the flow structure close to the surface of the nonslender diamond wing, both qualitatively and quantitatively, using the dye visualization and the stereoscopic particle image velocimetry (PIV) techniques. It was stated that, when the yaw angle is increased, the locations of vortex breakdowns approach thewing apex, but the other one moves toward the trailing edge. Goruney and Rockwell [6] investigated the near-surface flow structure and topology on a delta wing of low sweep angle having sinusoidal leading edges of varying amplitude and wavelength. Gursul et al. [7] reviewed unsteady aerodynamics of nonslender delta wings, covering topics of shear layer instabilities, structure of nonslender vortices, breakdown, maneuvering wings, and fluid/structure interactions. Yaniktepe and Rockwell [8] characterized the instantaneous and the time-averaged flow structure on the nonslender diamond and lambda planforms by using the PIV technique. Ozgoren et al. [9] investigated the structure of vortex breakdown and the effect in the surface of the wing of the separated flow region in the case of the high angle of attack over the slender delta wing. They declared that the high angle of attack rather affects the onset of vortex breakdown, spiral vortex structure, and separated flow region. Breitsamter [10] presented selected results from extensive experimental investigations on turbulent flowfields and unsteady surface pressures caused by leading-edge vortices, in particular, for vortex breakdown flow. Another important parameter for the delta wing is the yaw angle. The influence of sideslip on the flow about a sharp-edged biconvex delta wing of a unit aspect ratio was investigated by Verhaagen and Naarding [11] using flow visualization techniques as well as pressure and force balance measurements. It was observed that the yaw angle affects the structure of the leading-edge vortex, vortex breakdown, and formation of nonsteady flow structure substantially, which is generated after vortex breakdown. Sohn et al. [12] presented the development and interaction of vortices over a yawed delta wing with leading-edge extension (LEX) through offsurface flow visualization using microwater droplets and a laser beam sheet. By sideslip, the coiling, the merging, and the diffusion of the wing and LEX vortices increase on the windward side, whereas they are delayed significantly on the leeward side. Also, the migration behavior of vortices on the windward and leeward sides of the wing change considerably. A review of experimental data for delta wings under both steady and unsteady conditions was presented from a vortex dynamics point of view by Lee and Ho [13]. Conclusions were derived that vortices on the suction surface provide an important contribution to the lift of a delta wing, especially for the wings with large sweep-back angle. Delery [14] stated that, in three-dimensional flows, boundary-layer separation leads to the formation of vortices formed by the roll up of the viscous flow sheet, previously confined in a thin layer attached to the wall, which suddenly springs into the outer nondissipative flow. Comprehensive reviews of experimental and numerical works on vortex breakdown were reported by Leibovich [15,16], Escudier [17], and Visbal [18]. Sahin et al. [19] concluded that substantial retardation, or delay, in the onset of vortex breakdown, and thereby the development of largescale concentration of vorticity due to the helical mode of vortex breakdown, are attainable when the leading edge of the delta wing is perturbed at a natural frequency of vortex breakdown. They also found that upstream movement of the onset of vortex breakdown is attainable when the period of excitation frequency is sufficiently large. Akilli et al. [20] used the technique of PIV to characterize the alterations and structure of the leading-edge vortex formed from a

Vortex Breakdown-Edge Interaction: Consequence of Edge Oscillations

High-image-density particle image velocimetry is employed to determine the instantaneous and averaged features of distortion of vortex breakdown incident on a stationary and an oscillating leading edge. It is demonstrated that the onset of vortex breakdown can be advanced or retarded substantially, depending on the period of the edge oscillation relative to the inherent frequency of vortex breakdown. These features are interpreted with the aid of global representations of averaged and rms distributions of velocity, vorticity, and Reynolds stress, as well as a cinema sequence of instantaneous patterns of velocity and vorticity. Moreover, instantaneous, whole-field images in a cinema sequence allow evaluation of global representations of spectra and cross spectra, providing further insight into the central mechanisms that dictate the surface loading of the edge

Observation of the Vortical Flow over a Yawed Delta Wing

AbstractThe development and formation of the leading-edge vortices due to the change in the angle of attack, α, and yaw angle, θ, for a unique cross-flow plane at a dimensionless distance of x/C=0.8 from the apex of the stationary delta wing with a sweep angle of Λ=40° were observed using stereoscopic particle-image velocimetry (stereo-PIV). In addition, the experiments were conducted on three different cross-flow planes such as x/C=0.6, 0.8, and 1 using dye visualization to reveal the development of leading-edge vortices over the delta wing. The angle of attack was varied within the range of 7≤α≤17° and the yaw angle was varied within the range of 0≤θ≤8°. The vortical flow structure and loadings toward the wing surface due to the fluctuations and unsteadiness in the flow structure near the delta wing are investigated using time-averaged parameters such as streamlines, contours of vorticity distributions, Reynolds stress correlations, distributions of turbulent kinetic energy, vertical velocity, and RMS o...

Artificial neural network approaches for prediction of backwater through arched bridge constrictions

This paper presents the findings of laboratory model testing of arched bridge constrictions in a rectangular open channel flume whose bed slope was fixed at zero. Four different types of arched bridge models, namely single opening semi-circular arch (SOSC), multiple opening semi-circular arch (MOSC), single opening elliptic arch (SOE), and multiple opening elliptic arch (MOE), were used in the testing program. The normal crossing (@f=0), and five different skew angles (@f=10^o, 20^o, 30^o, 40^o, and 50^o) were tested for each type of arched bridge model. The main aim of this study is to develop a suitable model for estimating backwater through arched bridge constrictions with normal and skewed crossings. Therefore, different artificial neural network approaches, namely multi-layer perceptron (MLP), radial basis neural network (RBNN), generalized regression neural network (GRNN), and multi-linear and multi-nonlinear regression models, MLR and MNLR, respectively were used. Results of these experimental studies were compared with those obtained by the MLP, RBNN, GRNN, MLR, and MNLR approaches. The MLP produced more accurate predictions than those of the others.

Control of vortex breakdown by a transversely oriented wire

A small wire oriented orthogonally to the axis of the leading-edge vortex on a delta wing at high angle of attack generates substantial changes in the vortex structure, which is characterized using a technique of high-image-density particle image velocimetry. A wire having a diameter two orders of magnitude smaller than the diameter of the leading-edge vortex prior to the onset of vortex breakdown can substantially advance the onset of breakdown by as much as 15 vortex diameters. Depending upon the dimensionless diameter of the wire and wire location along the axis of the vortex, the onset of vortex breakdown can occur either upstream or downstream of the wire. Contours of constant velocity indicate that the rate of decrease of streamwise velocity along the centerline of the vortex is substantially enhanced, even for locations well upstream of the wire, relative to the case of vortex breakdown in the absence of a wire. Patterns of instantaneous vorticity in the presence of the wire typically exhibit a for...

Effect of inlet port on the flow in the cylinder of an internal combustion engine

Abstract In this study, the characteristics of flow emerging from the inlet of the intake port in the cylinder were investigated experimentally. A particle image velocimetry (PIV) technique was used to measure the velocity distribution in order to observe and analyse the flow behaviour. High-image-density PIV provided acquisition of patterns of instantaneous and averaged vorticity and velocity, revealing the detail of the flow characteristics in the cylinder cavity. With this measuring technique, it is possible to study the effect of intake valve geometry on the flow behaviours. The results showed that the flow structure changed substantially along the cylinder stroke due to the geometry of the intake valve port.