scholarly journals The Effect of Boattail Angles on the Near-Wake Structure of Axisymmetric Afterbody Models at Low-Speed Condition

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
The Hung Tran
Keyword(s):  
Strouhal Number ◽  
Reynolds Shear Stress ◽  
Flow Behavior ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Turbulent Intensity ◽  
Near Wake ◽  
Wake Structure ◽  
Low Speed ◽  
Speed Condition

The effect of a boattail angle on the structure of the wake of an axisymmetric model was investigated at low-speed condition. Four conical boattail models with angles of 0° (blunt-based body), 10°, 16°, and 22° were selected for this study. The Reynolds number based on the diameter of the model was around 1.97×104. Particle image velocimetry (PIV) was used to measure the velocity of the wake flow. The time-averaged flow characteristics including the length of recirculation of the afterbody, turbulent intensity, and Reynolds shear stress were analyzed and compared among those boattail models. The experimental results showed that the length of recirculation decreases with increasing boattail angle to 16°. At a boattail angle above 16°, the flow was fully separated near the shoulder and near-wake structure was highly changed. The turbulent intensity at a boattail angle of 22° showed a similar level to that in the case of the blunt-based body. Flow behavior on boattail surface should be accounted as an important parameter affecting the wake width and drag of the model. Power spectral density and proper orthogonal decomposition (POD) analyses showed that a Strouhal number of StD=0.2 dominated for the boattail model up to 16°. The fully separated flow was dominated by a Strouhal number of StD=0.03−0.06, which was firstly presented in this study.

Near-Wake Flow of a V-Gutter With Slit Bleed (Data Bank Contribution)

1993 ◽  
Vol 115 (1) ◽  
pp. 13-20 ◽  
Author(s):  
Jing-Tang Yang ◽  
Go-Long Tsai
Keyword(s):  
Reverse Flow ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Data Bank ◽  
Wake Flow ◽  
Near Wake ◽  
Cold Flow ◽  
Flame Holder ◽  
Flow Through

The cold-flow characteristics of a v-shape flame holder with flow bleed from a slit located at the leading edge have been investigated. According to experimental evidence, a nonsymmetric wake structure is developed behind the symmetric slit v-gutter. The flow through the slit induces greater reverse flow and greater back pressure in the near wake. It also provokes more extensive transport across the shear layers and reduces both the turbulent intensity and the Reynolds shear stress of the wake flow. These results indicate that the slit v-gutter can have a better flame holding ability and less pressure loss compared with the traditional v-gutter. In view of fluid dynamics features, the slit v-gutter is indeed a potentially useful design of flame holder.

Optical-Flow Algorithm for Near-Wake Analysis of Axisymmetric Blunt-Based Body at Low-Speed Conditions

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
The Hung Tran ◽  
Lin Chen
Keyword(s):  
Optical Flow ◽  
Cross Correlation ◽  
Transient Flow ◽  
Wake Flow ◽  
Near Wake ◽  
Dominant Type ◽  
Low Speed ◽  
Flow Method ◽  
Optical Flow Method ◽  
Flow Algorithm

Abstract In this study, ability of an optical-flow algorithm in extracting wake structure of axisymmetric model was investigated. The initial data for optical-flow processing were obtained in low-speed conditions by particle image velocimetry method. The Reynolds number based on the model diameter was around ReD = 1.97 × 104 in this study. Both the time-averaged and transient flow characteristics of near-wake flow were illustrated and examined by the optical-flow analysis method proposed. The processing results of optical-flow method showed good agreement with conventional cross-correlation methods. The ability of optical-flow method to extract flow fields was, thereby, confirmed for blunt-based flow at low-speed conditions. This study showed that the antisymmetric flow behavior of the near wake is the dominant type at low-speed conditions. Differing to traditional methods and cross-correlation results, the optical-flow results showed a frequency at around StD = 0.015 of the near wake for the first time, which is connected to vortex shedding behavior of the wake flow.

Numerical analysis of bluff body wakes under periodic open-loop control

2013 ◽  
Vol 739 ◽  
pp. 94-123 ◽  
Author(s):  
Derwin J. Parkin ◽  
M. C. Thompson ◽  
J. Sheridan
Keyword(s):  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
Open Loop ◽  
Wake Flow ◽  
Dynamic Mode ◽  
Two Dimensional ◽  
Near Wake ◽  
Recirculation Bubble ◽  
Number Range

AbstractLarge eddy simulations at$Re= 23\hspace{0.167em} 000$are used to investigate the drag on a two-dimensional elongated cylinder caused by rear-edge periodic actuation, with particular focus on an optimum open-loop configuration. The 3.64 (length/thickness) aspect-ratio cylinder has a rectangular cross-section with rounded leading corners, representing the two-dimensional cross-section of the now genericAhmed-body geometry. The simulations show that the optimum drag reduction occurs in the forcing Strouhal number range of$0. 09\leq S{t}_{act} \leq 0. 135$, which is approximately half of the Strouhal number corresponding to shedding of von Kármán vortices into the wake for the natural case. This result agrees well with recent experiments of Henninget al. (Active Flow Control, vol. 95, 2007, pp. 369–390). A thorough transient wake analysis employing dynamic mode decomposition is conducted for all cases, with special attention paid to the Koopman modes of the wake flow and vortex progression downstream. Two modes are found to coexist in all cases, the superimposition of which recovers the majority of features observed in the flow. Symmetric vortex shedding in the near wake, which effectively extends the mean recirculation bubble, is shown to be the major mechanism in lowering the drag. This is associated with opposite-signed vortices reducing the influence of natural vortex shedding, resulting in an increase in the pressure in the near wake, while the characteristic wake antisymmetry returns further downstream. Lower-frequency actuation is shown to create larger near-wake symmetric vortices, which improves the effectiveness of this process.

Experimental Study on the Near Wake Behind Two Staggered Cylinders of Unequal Diameters

2012 ◽  
Author(s):  
Yangyang Gao ◽  
Xikun Wang ◽  
Soon Keat Tan
Keyword(s):  
Reynolds Stress ◽  
Incident Angle ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Near Wake ◽  
Particle Image Velocimetry Technique ◽  
Image Velocimetry ◽  
Spacing Ratio

The wake structure behind two staggered circular cylinders with unequal diameters was investigated experimentally using the particle image velocimetry technique (PIV). This investigation was focused on the variations of flow patterns in terms of incident angle at Reynolds number Re = 1200. Comparisons of the time-averaged flow field of two staggered cylinders with unequal diameters at different angles were made to elucidate the mean flow characteristics. The characteristics of Reynolds shear stress contours at different incident angles and spacing ratios were also investigated. The results showed that with increasing of incident angle, the scale of Reynolds stress contours behind the upstream cylinder becomes larger, as well as the effect of spacing ratio on Reynolds stress contours.

Comparison of Near Wake-Flow Structure Behind a Solid Cap With an Attached Bubble and a Solid Counterpart

2003 ◽  
Author(s):  
Hidekazu No ◽  
Michel Call ◽  
Akira T. Tokuhiro
Keyword(s):  
Flow Structure ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Equivalent Diameter ◽  
Velocity Measurements ◽  
Near Wake ◽  
Downward Flow ◽  
Air Bubble ◽  
Square Channel ◽  
Image Velocimetry

An experimental study was conducted on the flow structure in the near-wake of a hollow cap with an air bubble attached underneath and a solid object possessing a bubble-like shape. The objective of the study was to elucidate distinguishing wake flow characteristics of the capped bubble relative to the solid. The experiment was performed in a square channel, 80×80mm2 in cross section. The bubble and solid were separately suspended in downward flow of purified water. Both the capped bubble and the solid were ellipsoidal in shape (the cap was shaped to represent the front of an ellipsoidal bubble) and had an approximate volume of 0.8ml. The Reynolds number for the flow, based on the objects’ equivalent diameter and average downward flow velocity (U = 25cm/s), was Re ≅ 2800. Velocity measurements were taken using Particle Image Velocimetry. The obtained velocity data were analyzed to deduce vorticity, turbulent kinetic energy, production, and Reynolds stress. Graphic and numerical comparisons between the two cases were made. The results to date are discussed.

scholarly journals Turbulent Wake-Flow Characteristics in the Near Wake of Freely Flying Raptors: A Comparative Analysis Between an Owl and a Hawk

2020 ◽  
Vol 60 (5) ◽  
pp. 1109-1122 ◽  
Author(s):  
Krishnamoorthy Krishnan ◽  
Hadar Ben-Gida ◽  
Gareth Morgan ◽  
Gregory A Kopp ◽  
Christopher G Guglielmo ◽  
...  
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Flight Behavior ◽  
Aerodynamic Noise ◽  
Particle Image Velocimetry System ◽  
Near Wake ◽  
Time Resolved ◽  
Turbulent Characteristics

Synopsis Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris’s hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds’ flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds’ motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.

Near-Wake Characteristics and Acoustic Resonance Excitation of Crimped Spirally Finned Cylinders in Cross-Flow

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Vortex Shedding ◽  
Sound Pressure ◽  
Vortex Formation ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Diameter Ratio ◽  
Wake Flow ◽  
Near Wake ◽  
Velocity Deficit

This paper presents an experimental investigation of the near-wake flow characteristics for isolated crimped spirally finned cylinders in cross-flow and its influence on the generated sound pressure during flow-excited acoustic resonance. Four crimped spirally finned cylinders are investigated, which have pitch-to-root diameter ratio (p/Dr) ranging between 0.384 ≤ p/Dr ≤ 1. A new equivalent diameter equation (Dc) has been developed to better capture the vortex shedding frequency emanating from the crimped spirally finned cylinders. The addition of crimped spiral fins reduces the coherence of the vortex shedding process as compared to that of a bare cylinder. Moreover, the addition of crimped spiral fins causes an elongation in the vortex formation region, as well as induces a larger velocity deficit in the near-wake. Reduction in the pitch-to-diameter ratio (p/Dr) leads to a progressive increase in the strength and coherence of the vortex shedding process. It also results in a gradual reduction in the vortex formation length and velocity deficit. The near-wake flow characteristics of the crimped spirally finned cylinders inherently affect the sound pressure during flow-excited acoustic resonance. Furthermore, the helical fins impose an asymmetrical inclination of the acoustic particle velocity. This hinders the flow-acoustic coupling, leading to a weakened energy transfer between the flow and sound fields. The findings of this investigation provide better understanding of the complex flow-sound interaction mechanism from crimped spirally finned cylinders in heat exchanger tube bundle.

Flow and Dispersion Characteristics of a Stack-Issued Backward Inclined Jet in Crossflow

2016 ◽  
Vol 33 (6) ◽  
pp. 841-852 ◽  
Author(s):  
M. G. Khouygani ◽  
R.-F. Huang ◽  
C.-M. Hsu
Keyword(s):  
Shear Layer ◽  
Inclination Angle ◽  
Flow Characteristics ◽  
Flux Ratio ◽  
Wake Flow ◽  
Flow Structures ◽  
Wake Vortex ◽  
Near Wake ◽  
Jet In Crossflow ◽  
Inclination Angles

AbstractThe effects of backward inclination angle on flow characteristics and jet dispersion properties of a stack-issued jet in crossflow were studied by means of instantaneous and long-exposure photography, particle image velocimetry (PIV), and tracer-gas concentration detections at a Reynolds number of 2,400, a jet-to-crossflow momentum flux ratio of 1.0, and the backward inclination angles θ = 0° - 60°. Three characteristic flow patterns featured by different near-wake flow structures were found within the surveyed span of the backward inclination angle: low (θ ≤ 25°), mediate (25° < θ < 50°), and high (θ ≥ 50°). In the range of low backward inclination angle, mushroom vortices appeared in the upwind shear layer. Jet fluids were entrained into the jet- and tube-wakes because the near wake region was characterized by a jet-wake vortex and a downwash flow. In the range of mediate backward inclination angle, forward-rolling vortices were formed in the upwind shear layer. Jet fluids were entrained into the jet wake but not appearing in the tube wake because the near wake was characterized by an isolated tube wake and up-going flows. In the range of high backward inclination angle, small-sized forward-rolling vortices were observed in the upwind shear layer. Jet fluids were not observed in both the jet- and tube-wakes because all flows went forward without reversal or vortex, which was similar to that in a jet in co-flow. Large turbulence intensities occurred around the jet-wake vortex and along sides of the tube wake bifurcation line, therefore the mixing at the low backward inclination angles presented better properties than those at mediate and high backward inclination angles owing to the featured flow structures and turbulence intensities.

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