Effect of a rotor wake on the local heat transfer on the forward half of a circular cylinder

Turbine rotor-stator wake dynamics was simulated by a spoked wheel rotating in annular flow, generating rotor wakes. Spanwise averaged circumferentially local heat transfer in the circular cylindrical leading edge region of a turbine airfoil was obtained. Reynolds numbers ranged from 35,000 to 175,000. Strouhal numbers ranged from 0.63 to 2.50. Wakes were generated by 2 sets of circular cylindrical bars, 1.59 and 3.18 mm in diameter. The rotor could be rotated either clockwise or counterclockwise. Grid turbulence was introduced upstream yielding freestream turbulence of 1.0 to 2.5% at the stator. Data represented an extensive body of local heat transfer coefficients, which can be used to model the leading edge region of a turbine airfoil. In the presence of rotor wakes, an asymmetry from the leeward to windward side was noted. Windward side levels were 30 to 40% higher than the corresponding leeward side

Direct numerical simulation of heat transfer from the stagnation region of a heated cylinder affected by an impinging wake

Copyright © 2011 Cambridge University Press.The effect of an incoming wake on the flow around and heat transfer from the stagnation region of a circular cylinder was studied using direct numerical simulations (DNSs). Four simulations were carried out at a Reynolds number (based on free-stream velocity and cylinder diameter D) of Re = 13200: one two-dimensional (baseline) simulation and three three-dimensional simulations. The three-dimensional simulations comprised a baseline simulation with a uniform incoming velocity field, a simulation in which realistic wake data - generated in a separate precursor DNS - were introduced at the inflow plane and, finally, a simulation in which the turbulent fluctuations were removed from the incoming wake in order to study the effect of the mean velocity deficit on the heat transfer in the stagnation region. In the simulation with realistic wake data, the incoming wake still exhibited the characteristic meandering behaviour of a near-wake. When approaching the regions immediately above and below the stagnation line of the cylinder, the vortical structures from the wake were found to be significantly stretched by the strongly accelerating wall-parallel (circumferential) flow into elongated vortex tubes that became increasingly aligned with the direction of flow. As the elongated streamwise vortical structures impinge on the stagnation region, on one side they transport cool fluid towards the heated cylinder, while on the other side hot fluid is transported away from the cylinder towards the free stream, thereby increasing the heat transfer. The DNS results are compared with various semi-empirical correlations for predicting the augmentation of heat transfer due to free-stream turbulence.German Research Foundatio

Flow modification around a circular cylinder applying splitter plates

A number of different studies were reviewed to investigate the functionality of splitter plates for the purpose of drag reduction and vortex elimination behind a circular cylinder. The studies were carried out numerically or experimentally in different combinations of Reynolds range, 2D or 3D dimensions, with intention of drag reduction, vortex suppression or both. Results were compared to discover the generalities of a splitter plate's applications and its performance in drag reduction and vortex control. The reduction of 12% up to 38.6% in drag coefficient suggests that all reviewed studies verified the effectiveness of upstream plate in drag reduction. Varied upstream plate's gap ratios (gap between the plate and cylinder) were tested and the optimum position was obtained. For the finite cylinder case, however, the studies discovered that the effectiveness of upstream plate decreased severely and thus, are barely considered as a drag reductive tool for shorter cylinders. Although downstream plate influences drag force, its prominent application is found to be vortex shedding elimination (up to 14.7%). The length ratio and gap ratio of downstream plate were varied in these studies and it was found that the length ratio was a more important factor compared with the gap ratio in the case of vortex suppression

Low velocity impact response of laminated textile coir-aramids/epoxy hybrid composites subjected to transverse penetration loading

The hybrid of natural and synthetic fibres in a composite system has gained interest in research field due to the environmental consciousness. Coir natural fibre has been found to have a high potential as impact resistance, in which brittle fibre helps to spread the impact energy over a wider area. The focus of the study is to determine if any improvement in impact response exists as a result of combining high performance Kevlar synthetic fibre and coir natural fibre in a specified laminated stacking sequence. In this research, quasi-static experiments were conducted at the speed of 1.25 mm/s, and low velocity impact tests were conducted using a 12.7 mm hemispherical impactor at the speed varying from 5 m/s to 17 m/s. Matlab software was used for curve fitting of the raw data, whereas ANOVA and DOE via Minitab software were employed for statistical examination to support the results. The area of the damaged composite was evaluated using the image analysis technique by Matlab image processing tool. The morphology analysis of the impact fractured surfaces was also observed by SEM. At the beginning stage, the optimum impact responses of coir epoxy composites subjected to different reinforcement architecture, composite manufacturing method, fabric modification and fabric density were determined. The findings showed that the impact responses could be effectively controlled by varying material parameters. Coir fibre in the form of woven structure with dense structure, treated using 6% of NaOH solution and manufactured by compression moulding method was found to be effective in absorbing and propagating impact energy. The respective coir was then used in hybrid laminates. Coir/Kevlar epoxy hybrid composites consisted of three interply laminates layers at six different stacking configurations were developed. It was observed that the hybrid composite (two coir layers and one Kevlar layer) had equivalent specific total energy absorption as 100% Kevlar laminate (at three layers). It can be concluded that the hybrid of coir/Kevlar-epoxy laminated composites had shown an improved in impact response. It also provides cost-effective materials and contributes towards green technology which will be of great benefit to the industry and community

Wind tunnel studies of circulation control elliptical airfoils

Effects of blown jets on the lift and drag of cambered elliptical airfoils are described. Performance changes due to a splitter plate attached to the lower surface of an elliptical airfoil near the trailing edge with and without blowing are indicated. Lift and drag characteristics of airfoils with two blown jets are compared with airfoils with single blowing jets. Airfoil designs that vary the location of a second jet relative to a fixed jet are described

The role of approach flow and blockage on local scour around circular cylinders with and without countermeasures

Local scour modelling has been established as an imperative tool in the understanding of local scour mechanisms and development of effective design methodologies for use in practice. However, there are limitations in physical scale modelling which must be fully understood in order to acquire useful experimental results to this end. In hydraulic modelling, facility constraints often result in dimensionless geometric parameters which are considerably altered from prototype conditions. Channel width b in a typical laboratory flume is limited, resulting in small values of channel aspect ratio AR (b/h, where h is flow depth). To further complicate matters, cylinder diameter D in local scour studies must be maximized to obtain a measurable scour pattern and maintain acceptably high relative coarseness D/d50 (where d50 is the median diameter of bed material). This results in cylinder sizes which pose a significant blockage to flow. While blockage effects have been explored for a cylinder mounted on a fixed bed, the effect of blockage ratio (D/b) for a cylinder in an erodible bed has only been explored through comparison of bed formations. In order to fully understand the effect of sidewall proximity on the flow field surrounding a cylinder under equilibrium of local scour, detailed velocity measurements are required. Furthermore, the effect of D/b on local scour must be isolated by holding all other scour-governing parameters constant. In order to achieve this, channel width b has been altered in the present investigation by movable flume sidewalls. In altering b, the channel aspect ratio AR is also affected. Particle Image Velocimetry (PIV) measurements have been undertaken in order to explore the effects of channel aspect ratio on open-channel flow over a porous bed. The effects of increasing vertical confinement(decreasing h) and horizontal confinement (decreasing b) are explored by comparison of vi mean and turbulence properties as well as third-order turbulent moments and quadrant analysis. The findings are then applied to an investigation of the role of D/b on the flow field mechanisms surrounding an emergent circular cylinder under equilibrium of clear-water scour. Changes in the distribution of time-averaged flow velocity, Reynolds shear stress and spanwise vorticity are presented and related to the bed formation. Increasing sidewall proximity is observed to confine the wake region and therefore influence the geometry of the dune downstream of the cylinder. The dune geometry subsequently affects the features in the surrounding flow field, and the streamwise velocity upstream of the cylinder is reduced as D/b increases. A primary objective of scour modelling is development of countermeasures for mitigation of the mechanisms which drive local scour. Lastly, an investigation of two types of scour countermeasures was carried out for flow around a submerged circular cylinder at equilibrium of clear-water scour. Based on fluid dynamic considerations, a vertical splitter plate and a horizontal base plate were chosen as potential flow-altering devices and the efficacy of these devices for scour mitigation are explored. PIV measurements also facilitate understanding of the methods by which each countermeasure affects the flow field mechanisms surrounding the cylinder, and the distribution of flow velocity and Reynolds shear stress are presented. The vertical splitter plate is found to affect the interaction between shear layers in the wake region, reducing the depth of scour downstream of the cylinder. The horizontal base plate is found to eliminate scour upstream of the cylinder altogether by protecting the bed around the sides of the cylinder where shear stress is maximized. Local scour modelling has been established as an imperative tool in the understanding of local scour mechanisms and development of effective design methodologies for use in practice. However, there are limitations in physical scale modelling which must be fully understood in order to acquire useful experimental results to this end. In hydraulic modelling, facility constraints often result in dimensionless geometric parameters which are considerably altered from prototype conditions. Channel width b in a typical laboratory flume is limited, resulting in small values of channel aspect ratio AR (b/h, where h is flow depth). To further complicate matters, cylinder diameter D in local scour studies must be maximized to obtain a measurable scour pattern and maintain acceptably high relative coarseness D/d50 (where d50 is the median diameter of bed material). This results in cylinder sizes which pose a significant blockage to flow. While blockage effects have been explored for a cylinder mounted on a fixed bed, the effect of blockage ratio (D/b) for a cylinder in an erodible bed has only been explored through comparison of bed formations. In order to fully understand the effect of sidewall proximity on the flow field surrounding a cylinder under equilibrium of local scour, detailed velocity measurements are required. Furthermore, the effect of D/b on local scour must be isolated by holding all other scour-governing parameters constant. In order to achieve this, channel width b has been altered in the present investigation by movable flume sidewalls. In altering b, the channel aspect ratio AR is also affected. Particle Image Velocimetry (PIV) measurements have been undertaken in order to explore the effects of channel aspect ratio on open-channel flow over a porous bed. The effects of increasing vertical confinement(decreasing h) and horizontal confinement (decreasing b) are explored by comparison of vi mean and turbulence properties as well as third-order turbulent moments and quadrant analysis. The findings are then applied to an investigation of the role of D/b on the flow field mechanisms surrounding an emergent circular cylinder under equilibrium of clear-water scour. Changes in the distribution of time-averaged flow velocity, Reynolds shear stress and spanwise vorticity are presented and related to the bed formation. Increasing sidewall proximity is observed to confine the wake region and therefore influence the geometry of the dune downstream of the cylinder. The dune geometry subsequently affects the features in the surrounding flow field, and the streamwise velocity upstream of the cylinder is reduced as D/b increases. A primary objective of scour modelling is development of countermeasures for mitigation of the mechanisms which drive local scour. Lastly, an investigation of two types of scour countermeasures was carried out for flow around a submerged circular cylinder at equilibrium of clear-water scour. Based on fluid dynamic considerations, a vertical splitter plate and a horizontal base plate were chosen as potential flow-altering devices and the efficacy of these devices for scour mitigation are explored. PIV measurements also facilitate understanding of the methods by which each countermeasure affects the flow field mechanisms surrounding the cylinder, and the distribution of flow velocity and Reynolds shear stress are presented. The vertical splitter plate is found to affect the interaction between shear layers in the wake region, reducing the depth of scour downstream of the cylinder. The horizontal base plate is found to eliminate scour upstream of the cylinder altogether by protecting the bed around the sides of the cylinder where shear stress is maximized

The Effect of a Splitter Plate on the Flow around a Surface-Mounted Finite Circular Cylinder

Splitter plates are passive flow control devices for reducing drag and suppressing vortex shedding from bluff bodies. Most studies of splitter plates involve the flow around an “infinite” circular cylinder, however, in the present study the flow around a surface-mounted finite-height circular cylinder, with a wake-mounted splitter plate, was studied experimentally in a low-speed wind tunnel using a force balance and single-component hot-wire anemometry. Four circular cylinders of aspect ratios AR = 9, 7, 5 and 3 were tested for a Reynolds number range of Re = 1.9×10^4 to 8.2×10^4. The splitter plates had lengths, relative to the cylinder diameter, of L/D = 1, 1.5, 2, 3, 5 and 7, thicknesses ranging from T/D = 0.10 and 0.15, and were the same height as the cylinder being tested. The cylinders were partially immersed in a flat-plate turbulent boundary layer, where the range of boundary layer thickness relative to the cylinder diameter was δ/D = 1.4 to 1.5. Measurements were made of the mean drag force coefficient, the Strouhal number at the mid-height position, and the Strouhal number and power spectra along the cylinder height. For all four finite circular cylinders, the splitter plates were effective at reducing the magnitude of the Strouhal number, and weakening or even suppressing vortex shedding, depending on the specific combination of AR and L/D. Compared to the case of an infinite circular cylinder, the splitter plate is less effective at reducing the mean drag force coefficient of a finite circular cylinder. The largest drag reduction was obtained for the cylinder of AR = 9 and splitter plates of L/D = 1 to 3, while negligible drag reduction occurred for the shorter cylinders

Influence of freestream turbulence on the near-field growth of a turbulent cylinder wake: Turbulent entrainment and wake meandering

he effect of freestream turbulence on the spreading of the near wake of a circular cylinder (<11 cylinder diameters from the rear face of the cylinder) is investigated through particle image velocimetry data. Different “flavors” of freestream turbulence, in which the turbulence intensity and integral length scale are independently varied, are subjected to the cylinder. The time-averaged spreading of the near wake is decoupled into the growth through entrainment of background fluid and the envelope of the spatial extent of the instantaneous wake due to wake “meandering,” induced by the presence of large-scale vortical coherent structures, i.e., the von Kármán vortex street. Unlike for the far-field of a turbulent wake, examined by Kankanwadi and Buxton [J. Fluid Mech. 905, A35 (2020)], it is shown that freestream turbulence enhances the entrainment rate into the wake in comparison to a nonturbulent background. Furthermore, both the turbulence intensity and the integral length scale of the background turbulence are important in this regard, further contrasting to the far wake where only the turbulence intensity is important. Additionally, wake meandering is enhanced by the presence of background turbulence, and here the integral length scale is the dominant parameter. Combining these findings yields the oft-reported result that background turbulence enhances the time-averaged near-wake growth rate. The influence of wake meandering is isolated by conducting similar experiments in which a splitter plate is mounted to the rear face of the cylinder, thereby eliminating the von Kármán vortex street. These results show that when large-scale vortices are not present within the turbulent wake then freestream turbulence actually suppresses the entrainment rate, relative to a nonturbulent background, in results that mirror Kankanwadi and Buxton [J. Fluid Mech. 905, A35 (2020)]. We therefore postulate that freestream turbulence has the effect to enhance large-scale entrainment via engulfment, but suppress entrainment via small-scale nibbling. Finally, we observe that the presence of freestream turbulence occasionally leads to a transient elimination of vortex shedding, an effect that is bound to have consequences on the instantaneous entrainment rate as outlined above

Vortical Flows Research Program of the Fluid Dynamics Research Branch

The research interests of the staff of the Fluid Dynamics Research Branch in the general area of vortex flows are summarized. A major factor in the development of enchanced maneuverability and reduced drag by aerodynamic means is the use of effective vortex control devices. The key to control is the use of emerging computational tools for predicting viscous fluid flow in close coordination with fundamental experiments. In fact, the extremely complex flow fields resulting from numerical solutions to boundary value problems based on the Navier-Stokes equations requires an intimate relationship between computation and experiment. The field of vortex flows is important in so many practical areas that a concerted effort in this area is justified. A brief background of the research activity undertaken is presented, including a proposed classification of the research areas. The classification makes a distinction between issues related to vortex formation and structure, and work on vortex interactions and evolution. Examples of current research results are provided, along with references where available. Based upon the current status of research and planning, speculation on future research directions of the group is also given

The Effect of a Wake-Mounted Splitter Plate on the Flow around a Surface-Mounted Finite-Height Square Prism.

The flow around a finite square prism has not been studied extensively when compared with an “infinite” (or two-dimensional) square prism. In the present study, the effect of a wake-mounted splitter plate on the flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of specific interest were the combined effects of the splitter plate length and the prism’s aspect ratio on the vortex shedding, mean drag force coefficient, and the mean wake. Four square prisms of aspect ratios AR = 9, 7, 5 and 3 were tested at a Reynolds number of Re = 7.4×104 and a boundary layer thickness of /D = 1.5. Splitter plate lengths of L/D = 1, 1.5, 2, 3, 5, and 7, were tested, with all plates having the same height as the prism. Measurements of the mean drag force were obtained with a force balance, and measurements of the vortex shedding frequency were obtained with a single-component hot-wire probe. A seven-hole pressure probe was used to measure the time-averaged wake velocity at a Reynolds number of Re = 3.7×104 for AR = 9 and 5 with splitter plates of lengths L/D = 1, 3, 5, and 7. These measurements were carried out to allow for a better understanding of how the splitter plate affects the mean wake of the finite prism. The results show that the splitter plate is a less effective drag-reduction, but more effective vortex-shedding-suppression, device for finite square prisms than it is for infinite square prisms. Significant reduction in the mean drag coefficient was realized only for short prisms (of AR ≤ 5) when long splitter plates (of L/D ≥ 5) were used. A splitter plate of length L/D = 3 was able to suppress vortex shedding for all aspect ratios tested. However, for square prisms of aspect ratios AR ≤ 7, the splitter plate is a less effective vortex-shedding-suppression device when compared to its use with finite circular cylinders, i.e. longer splitter plates are needed for vortex shedding suppression with square prisms. Wake measurements showed distinct wake velocity fields for the two prisms tested. For the prism of AR = 9, a strong downwash flow in the upper part of the wake became weaker towards the ground plane. For the prism of AR = 5, the downwash remained strong close to the ground plane. With splitter plates installed, the downwash became weaker for both prisms. The splitter plate was found to narrow the wake width, especially close to the ground plane, and led to the stretching of the streamwise vortex structures in the vertical direction, and increased entrainment towards the wake centreline in the cross-stream direction