• Wind and Structures
• /
• 제37권1호
• /
• pp.15-23
• /
• 2023

The wake behavior of extended flat plate and serration in the trailing edge of S809 airfoil is presented in this experimental study using wind tunnel testing. The clustering of wind turbines in wind parks has recently been a pressing issue, due to the expected increase in power output and deciding the number of wind turbines to be installed. One of the prominent factors which influence the performance of the subsequent wind turbines is the downstream wake characteristics. A series of wind tunnel investigations were performed to assess the downstream near wake characteristics of the S809 airfoil at various angles of attack corresponding to the Reynolds Number Re = 2.02 × 105. These experimental results revealed the complex nature of the downstream near wake characteristics featuring substantial asymmetry arising out of the incoherent flow separations prevailing over the suction and the pressure sides of the airfoil. Based on the experimental results, it is found that the wake width and the downstream velocity ratio decrease with an increase in the angle of attack. Nonetheless, the dissipation length and downstream velocity ratio increases proportionally in the downstream direction. Additionally, attempts were made to understand the physical nature of the near wake characteristics at 1C, 2C, 3C and 4C downstream locations.

• 한국군사과학기술학회지
• /
• 제6권4호
• /
• pp.29-37
• /
• 2003

Using a wind tunnel testing, the aerodynamic load characteristics of an elliptic airfoil was described. The experimental data was obtained for angles of attack $-20^{\circ}$ to $+20^{\circ}$ with $2^{\circ}$ increments at a chord Reynolds number of $0.99{\times}105$ and $2.48{\times}105$. For each test case, chordwise suction pressure distributions and wake surveys were obtained. Static pressure measurements were made over a 10 sec averaging time at a 10 Hz sampling rate. For each case, wake survey was conducted with a pilot-static probe at 1.0c downstream from the trailing edge at very fine spacing to resolve the wake velocity deficit profile. As can be expected, suction pressure coefficient was increased with angle of attack. The normal force, CNmax, appeared peak value at the incidence angle of $12^{\circ}~14^{\circ}$, and the significant increase in profile drag at this range of angles of attack.

• Wind and Structures
• /
• 제26권5호
• /
• pp.317-329
• /
• 2018

Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an open-loop wind tunnel. A finite square prism with a width D = 15 mm and a height H = 7D was vertically mounted on a horizontal flat plate. The Reynolds number was varied from $6.5{\times}10^3$ to $28.5{\times}10^3$ and the incidence angle ${\alpha}$ was changed from $0^{\circ}$ to $45^{\circ}$. The ratio of boundary layer thickness to the prism height was about ${\delta}/H=7%$. The time-averaged velocity, turbulence intensity and the vortex shedding frequency were obtained through a single-component hotwire probe. Power spectrum of the streamwise velocity fluctuations revealed that the tip and base vortices shed at the same frequency as that ofspanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at ${\alpha}_{cr}=15^{\circ}$ which is equal to that reported for an infinite prism. There is a reduction in the size of the wake region along the height of the prism when moving away from the ground plane towards the free end.

• 대한조선학회논문집
• /
• 제58권1호
• /
• pp.10-16
• /
• 2021

This paper systematically investigates and correlates pressure fluctuation and nominal wake characteristics according to the angle of the vortex generators by introducing the angle adjustment method of the Vortex Generator (VG). The vortex generators are installed at the port and starboard of a model ship. The vortex generator performance test is executed on a model ship installed in the Large Cavitation Tunnel (LCT) and the angle of VG is freely controlled by a servo motor. The systematic test results for the vortex generator show that the well-designed VG is an effective appendage for reducing the pressure fluctuation level and shows the direction of VG's angular design optimization.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제13권1호
• /
• pp.65-74
• /
• 2021

A key factor that governs the wave interferences of a submerged body is the dimensionless Froude number. Computational Fluid Dynamics (CFD) is used to describe the resistance force coefficients and the generated waves of two SUBOFF submarine models. Grid independence studies are performed on two cases, totally and shallowly submerged cases, with four sets of computing meshes. The highest peaks are marked by red points at given wavelengths, a line is fitted to those points with a least-squares approximation, and the half wake angle at multiple Froude numbers is defined between the fitted line and the centerline of the free surface. The results show that when the depth of the target is 1.1D, constructive interferences occur at Fn = 0.3 and 0.5, while destructive interference occurs at Fn = 0.35 with distortion of the waveform. The half wake angle is less than 19.47° because of the interference between the bow and stern wave systems.

• 유체기계공업학회:학술대회논문집
• /
• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
• /
• pp.401-402
• /
• 2006

The turbulent flow around a sphere was investigated using two experimental techniques: smoke-wire flow visualization in wind tunnel at Re=5300, 11000 and PIV measurements in a circulating water channel. The smoke-wire visualization shows flow separation points near an azimuthal angle of $90^{\circ}$, recirculating flow, transition from laminar to turbulent shear layer, evolving vortex roll-up and fully turbulent eddies in the sphere wake. The mean velocity field measured using a PIV technique in x-y center plane demonstrates the detailed near-wake structure such as nearly symmetric recirculation region, two toroidal vortices, laminar separation, transition and turbulent eddies. The PIV measurements of turbulent wake in y-z planes show that a recirculating vortex pair dominates the near-wake region.

• 한국가시화정보학회:학술대회논문집
• /
• 한국가시화정보학회 2004년도 추계학술대회 논문집
• /
• pp.14-17
• /
• 2004

A purpose of this visual experiment is to investigate the effect of reduced frequency qualitatively by examining wake pattern change for insect flying motion. Insect is composed of two pair wing with forewing and hindwing, flying motion of insect is performed pitching and plunging so it makes a separation over the wings. The separation affects at the wake pattern and changed wake pattern has an influence on lift, drag and propulsion. This experiment is conducted by using a smoke wire technique and a camera is fixed at hindwing to take a photograph of wake. An electronic device is mounted below test section to find exact the mean positional angle of wing. The reduced frequency in experiment is 0.15, 0.3 and 0.45. We obtained the result which that reduced frequency is closely related to wake pattern that determines flight efficiency.

• 대한기계학회논문집
• /
• 제18권10호
• /
• pp.2664-2674
• /
• 1994

Drag, lift, and pitching moment measurements have been made on a range of slanted-base ogive cylinders, using the KANOMAX wind tunnel and balance system. Test Reynolds numbers(based on model maximum diameter) varied from $0.54{\times}10^{5}{\;}to{\;}1.56{\times}10^{5}$. Crossflow velocity maesurement was conducted by 5-hole pitot tube at $Re_{D}=1.46{\times}10^{5}$. For two base angle $({\theta}=30$ and 45 deg.), aerodynamic forces and moment were measured with increasing angle of attack(0~30 deg.). Two types of wake flow were observed, a quasisymetric turbulent closure or a longitudinal vortex flow. Aerodynamic characteristics differ dramatically between the two wake types. It was found that the drag, lift and pitching moment coefficients increased with increasing angle of attack.

• 융복합기술연구소 논문집
• /
• 제4권1호
• /
• pp.37-41
• /
• 2014

For the analysis of lifting surface at high angle of attack, a Nonlinear Vortex Lattice Method(NLVLM) was used. The NLVLM is intented to compute the interactions between lifting surfaces and separated vertical flow. The lifting surfaces are represented by a lattice of discrete vortex rings. And wakes are represented by families of non-lintersecting, semi-infinite vortex line segments. The image method also used to analyze the ground effect. It is found that vortex lines separated from lifting surfaces represent the separated flows successfully. Although the present method is applied for the rectangular wing and delta wing, extensions can be possible for the arbitrary lifting surfaces. The Present results show good agreement with experimental data.

• 대한기계학회논문집B
• /
• 제33권7호
• /
• pp.506-511
• /
• 2009

Insect flight is adapted to cope with each circumstance by controlling a variety of the parameters of wing motion in nature. Many researchers have struggled to solve the fundamental concept of insect flight, but it has not been solved yet clearly. In this study, to find the most effective flapping wing dynamics, we conducted to analyze CFD data on fixing some of the optimal parameters of wing motion such as stoke amplitude, flip duration and wing rotation type and then controlled the deviation angle by fabricating wing tip motion. Although all patterns have the similar value of lift coefficient and drag coefficient, pattern A(pear-shape type) indicates the highest lift coefficient and pattern H(pear-shape type) has the lowest lift coefficient among four wing tip motions and three deviation angles. This result suggest that the lift and drag coefficient depends on the angle of attack and the deviation angle combined, and it could be explained by delayed stall and wake capture effect.