Session A7: Separation Flow Around Obstacles

Effects of freestream turbulence on the characteristics of separation and re-attachment in flow past obstacles.

We study the characteristics of separation and reattachment in the presence of freestream turbulence (FST) on flows on ribs. This two-dimensional obstacle represents a canonical geometry in industrial aerodynamics. It consists of a forward-facing step, FFS, followed by a backward-facing step, BFS. An experiment at Reynolds number 20000 based on rib height,H, was carried out. The rib was fully submerged in a boundary layer and the freestream was subjected to varying turbulence intensities:0.5, 3.5, 7.5 and 9.0\%. Three rib lengths of L/H=1,4 and 8 were tested. Particle Image Velocimetry measurements show that increasing freestream turbulence consistently decreases recirculation lengths both on top and in the wake of ribs. The shrinkage of recirculation bubbles is also dependent on rib length because of the interaction between FFS and BFS. Snapshot Proper Orthogonal Decomposition suggests that FST modifies the dominant motions in the flow. The shape and ranking of POD modes of velocity above short ribs (L/H=1) seems unaffected by FST until the 4th mode. In contrast, the longer ribs (L/H=4 and 8) produce different dominant modes for each FST intensity. This signifies the dominant motions in the flow are affected by FST which could explain the different recirculation characteristics.   

Surface obstacles in pulsatile flow

Flows past obstacles mounted on flat surfaces have been widely studied due to their ubiquity in nature and engineering. For nearly all of these studies, the freestream flow over the obstacle was steady, i.e. constant velocity unidirectional flow. Unsteady, pulsatile flows occur frequently in biology, geophysics, biomedical engineering, etc. Our study is aimed at extending the comprehensive knowledge base that exists for steady flows to considerably more complex pulsatile flows. Beyond the important practical applications, characterizing the vortex and wake dynamics of flows around surface obstacles embedded in pulsatile flows can provide insights into the underlying physics in all wake and junction flows. In this study, we experimentally investigated the wake of four canonical surface obstacles: hemisphere, cube, and circular cylinders with aspect ratio of 1:1 and 2:1. Phase-averaged PIV and hot-wire anemometry are used to characterize the dynamics of coherent structures in the wake and at the windward junction of the obstacles. Complex physics occur during the deceleration phase of the pulsatile inflow. We propose a framework for understanding these physics based on self-induced vortex propagation, similar to the phenomena exhibited by vortex rings.   

A different approach on the onset of separation in the flow around a circular cylinder.

The onset of separation in the flow around a cylinder is revisited with new insight. The goal of the research is to compute the smallest Reynolds number where the separation actual occurs rather than computing small eddies and extrapolating to the value of the Reynolds number where separation may occur. To this purpose, an accurate home made code is designed with Galerkin finite elements. The computational domain is chosen as the laboratory experiments by Taneda. It is found that in all six different choices of Taneda's diameters of the cylinders he used, separation is not observed for \(Re < 6.1 \). Actually, separation is computed in all of his six cases for \(Re = 6.14 \). Images of this smallest eddy are shown for the first time where all characteristics of eddies are recognisable (vortex centre, separation length etc). The vorticity of the flow is computed along the cylinder surface and it is shown that, at separation, vorticity changes sign. Byproducts of this research is the computation of the drag coefficient for Reynolds numbers starting from $1 \cdot 10^{-5}$ up to $40$. In addition, the separation angle (point where vorticity changes sign) is computed for \( 6.14 \leq Re \leq 40 \). This research aims to be the most thorough work done on that subject so far.   

Three-Dimensional, Laminar Flow Past a Short, Surface-Mounted Cylinder

The topology and evolution of three-dimensional flow past a cylinder of slenderness ratio $SR = 1$ mounted in a wind tunnel is examined for \( 0.1 \leq Re \leq 325 \) (based on the diameter of the cylinder) where steady-state solutions have been obtained. Direct numerical simulations were computed using an in-house parallel finite element code. Results indicate that symmetry breaking occurs at $Re = 1$, while the first prominent structure is a horseshoe vortex downstream from the cylinder. At $Re = 150$, two foci are observed, indicating the formation of two tornadolike vortices downstream. Concurrently, another horseshoe vortex is formed upstream from the cylinder. For higher Reynolds numbers, the flow downstream is segmented to upper and lower parts, whereas the topology of the flow on the solid boundaries remains unaltered. Pressure distributions show that pressure, the key physical parameter in the flow, decreases everywhere except immediately upstream from the cylinder. In addition, creation of critical points from saddle-node-type bifurcations occur when the streamwise component of the pressure gradient changes sign. Finally, at $Re = 325$, an additional horseshoe vorrtex is formed at the wake of the cylinder   

Flow past a rotating cylinder at high Reynolds number using PANS method

In the present study, high-Reynolds number flow past a rotating cylinder has been simulated using Partially-Averaged Navier-Stokes (PANS) method. The simulations are performed at Re $=$ 140000. The spin ratio of the cylinder, which is defined by the ratio of the circumferential speed of the cylinder to the free-stream speed, varies from a $=$ 0 to a $=$ 4. The resolved and the modeled physical scales have been compared with the corresponding LES data for better understanding of the efficacy of the PANS method. The comparison of PANS results with the LES results showed good agreement. It has been recognized that the PANS simulation is able to produce fairly acceptable results using even a coarse-mesh. It is recognized that the time-averaged flow statistics obtained using PANS and URANS simulations are approximately same. However the vortex structure is much better captured by the PANS method. With the increase in the spin ratio, decrease in the time-averaged drag and increase in the time-averaged lift force acting on the cylinder have been observed. The vortices in far wake region are displaced and deformed but those in the vicinity of the cylinder are stretched at the bottom and accumulated over the top of the cylinder.   

Laminar flow separation subject to control by zero-net-mass-flux jet

The flow around slender bodies at moderate Reynolds numbers often features a laminar separation bubble. Convective amplification of small-amplitude perturbations leads to the formation of two-dimensional large-scale vortices that are shed from the bubble. These perturbations can be triggered through a zero-net-mass-flux actuator in order to control the bubble size and shedding frequency. Using data from Navier-Stokes simulations for the flow around a canonical airfoil-like geometry, it is found that linear modes with intermediate frequencies exhibit strongest convective amplification caused by Kelvin-Helmholtz instability. Forcing at these frequencies is most effective. For low frequencies, the front part of the bubble still diminishes due to the interaction of a vortex that starts from the actuator with the wall. This vortex transiently amplifies downstream due to the Orr mechanism. Actuation at high frequencies leads to visible, amplified instability waves in the shear layer, but is not effective in reducing the size of the bubble.   

Experimental study on the flow around in-line array of spheres

In this study, we investigate the flow around a in-line array of spheres focusing on the interactions between the wakes flows. In a circulating water tunnel, 12.7 mm-diameter spheres have been aligned in line with the direction of flow, with each sphere held by a 0.1 mm thin stainless-steel wire. Considered Reynolds number for a single sphere is 1000 and the number of spheres is increased up to five with varying the distance between them, as well. To measure the flow field, we use dye visualization and PIV together, and the drag forces of each sphere are indirectly measured using two-dimensional optical micrometer. As the center-to-center distance increases, the wake instability in the gap between them is enhanced, and the axisymmetric structure of wake collapses and the turbulence levels becomes large. Based on this observation, flow structure around the sphere array is classified depending on the symmetricity, steadiness and turbulent intensity between spheres and the wake behind a following sphere. The drag on each sphere will be analyzed on the basis of this classification.   

Experimental and modeling study of the flow over a skewed bump

Three-dimensional separated flows can be very sensitive to geometry and inlet conditions, such that a small change in the geometry or the upstream boundary layer could cause the flow structure to change drastically. This study examines the geometric sensitivity of a skewed bump with axis ratio 4/3 by changing the angle of the bump with respect to the flow. The three-dimensional, three-component mean velocity field was acquired using Magnetic Resonance Velocimetry (MRV) for several bump angles. The flow is dominated by large coherent vortices in the wake. For a symmetric case, two counter-rotating vortices exist in the wake, but when the bump is skewed relative to the oncoming flow one vortex structure is much stronger and overwhelms the other vortex. A comparison to RANS simulations found that the RANS simulations predict the velocity fields with reasonable accuracy within the separation bubble, but are very inaccurate downstream of reattachment. Using a time-resolved MRV sequence, the shedding frequency of the wake was determined for two bump angles. Hot-wire anemometry confirmed the shedding frequencies found from the MRV data and observed that the shedding frequency is sensitive to the bump angle at low bump angles, but is insensitive at high bump angles.   

Optimization of air injection parameters toward optimum fuel saving effect for ships

Air lubrication method is the most promising commercial strategy for the frictional drag reduction of ocean going vessels. Air bubbles are injected through the array of holes or the slots installed onto the flat bottom surface of vessel and a sufficient supply of air is required to ensure the formation of stable air layer by the by the coalescence of the bubbles. The air layer drag reduction becomes economically meaningful when the power gain through the drag reduction exceeds the pumping power consumption. In this study, a model ship of 50k medium range tanker is employed to investigate air lubrication method. The experiments were conducted in the 100m long towing tank facility at the Pusan National University. To create the effective air lubrication with lower air flow rate, various configurations including the layout of injection holes, employment of side fences and static trim have been tested. In the preliminary series of model tests, the maximum 18.13{\%}(at 15kts) of reduction of model resistance was achieved. This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) through GCRC-SOP (Grant No. 2011-0030013).