• Title/Summary/Keyword: turbulent velocity intensity

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Computational Simulations of Turbulent Wake Behind a Pre-Swirl Duct Using a Hybrid Turbulence Model with High Fidelity (하이브리드 난류 모델을 이용한 전류고정덕트 후류의 고정도 수치 해석)

  • Kang, Min Jae;Jung, Jae Hwan;Cho, Seok Kyu;Hur, Jea-Wook;Kim, Sanghyeon;Lee, Sang Bong
    • Journal of the Society of Naval Architects of Korea
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    • v.59 no.3
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    • pp.141-148
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    • 2022
  • A hybrid turbulence model has developed by combining a sub-grid scale model using dynamic k equation in LES with k-𝜔 SST model of RANS equation. To ascertain potential applicability of the hybrid turbulence model, fully developed turbulent channel flows at Re𝜏=180 have been simulated of which computational domain has a top wall with coarse cells and a bottom wall with fine cells. The streamwise mean velocity and turbulent intensity profiles showed a good agreement with DNS data when using the hybrid model rather than using a single model in k-𝜔 SST or dynamic k equation models. Computational simulations of turbulent flows around KVLCC2 with a pre-swirl duct have been mainly performed using the hybrid turbulence model. Compared to the results obtained from RANS simulation with k-𝜔 SST model as well as LES with dynamic k equation SGS model, turbulent wakes of the duct in the present simulation using the hybrid turbulence model were very similar to that of LES. Also, the resistances acting on hull, rudder and duct in hybrid turbulence model were similar to those in RANS simulation whereas the viscous forces acting on the hull in LES had a significant error due to coarse cells inappropriate to the sub-grid scale model.

Effect of Particle Loading Ratio and Orifice Exit Velocity on a Particle-Laden Jet

  • Paik, Kyong-Yup;Yoon, Jung-Soo;Hwang, Jeong-Jae;Chung, Jae-Mook;Bouvet, Nicolas;Yoon, Young-Bin
    • International Journal of Aeronautical and Space Sciences
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    • v.12 no.3
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    • pp.296-304
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    • 2011
  • In order to design a shear coaxial injector of solid particles with water, basic experiments on a particle laden jet are necessary. The purpose of the present study is to understand the effect of particle loading ratio on the particle spray characteristics (i.e. spreading angle, distribution of particle number density, velocity profiles, and particle developing region length). Hydro-reactive Al2O3 particles with a primary particle diameter of 35~50 ${\mu}m$ are used in this experiment. An automated particle feeder was designed to supply constant particle mass flowrates. Air is used as the carrier gas. To determine the air velocity at the orifice exit, tracers (aluminum oxide, 0.5~2 ${\mu}m$ primary diameter) are also supplied by a tracer feeder. A plain orifice type injector with 3 mm diameter, and 20 mm length was adopted. Particle image velocimetry is used to measure the mean and fluctuating velocity components along the axial and radial directions.

Characteristics of Shear Layer Vortices in Crossflow Jets According to the Inlet Conditions (초기조건변화에 따른 횡단류 제트 유동의 전단층와류 거동 특성)

  • Kim, Gyeong-Cheon;Kim, Sang-Gi;Yun, Sang-Yeol;Lee, Seok-Ho
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.26 no.3
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    • pp.394-401
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    • 2002
  • The instantaneous flow characteristics of a round jet issuing normally into a crossflow has been studied using a flow visualization technique and particle image velocimetry. The effects of parameters such as jet inflow profile and turbulence intensity of the jet are evaluated for various Reynolds numbers in range between 735 and 3150, which are based on the crossflow velocity and jet-pipe diameter. The jet-to-crossflow velocity ratio is fixed at the value of 3.3. Instantaneous later tomographic images of the symmetry plane of the crossflow jet show that there exist very different natures in the flow structures of the near-field of the jet even though the velocity ratio is same. It is found that when the turbulence intensity of jet is elevated, the shear layer becomes much thicker due to the strong entrainment of the ambient fluid by turbulent interaction between the jet and crossflow. The detailed characteristics of instantaneous velocity and vorticity fields are presented to illustrate the effects of the above parameters on the vertical structures of the crossflow jet.

Effect of Swirl Angle on the Atomization Characteristics in Two-Fluid Nozzle with Dual Air Supplying System (이중공기공급 이유체노즐의 선회각 변화에 따른 분무특성)

  • Kim, E.S.;Kang, S.M.;Choi, Y.J.;Kim, D.J.;Lee, J.K.;Rho, B.J.
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03b
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    • pp.54-60
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    • 2008
  • The atomization characteristics of the dual air supplying twin-fluid nozzle were investigated experimentally using PIV and PDA systems. The two-fluid nozzle is composed of three main parts: the feeding injector to supply fluid that is controlled by a PWM (pulse-width modulation) mode, the adaptor as a device with the ports for supplying the carrier and assist air and the main nozzle to produce the spray. The main nozzle has the swirl tip with four equally spaced tangential slots, which give the injecting fluid an angular momentum. The angle of the swirl tip varied with 0$^{\circ}$ 30$^{\circ}$, 60$^{\circ}$ and 90$^{\circ}$, and the ratios of carrier air to assist air and ALR(total air to liquid) were 0.55 and 1.23, respectively. The macroscopic behavior of the spray was investigated using PIV system, and the mean velocity, turbulent intensity and SMD distributions of the sprays were measured using PDA system. As the results, the mean axial velocity at the spray centerline decrease with the increase of the swirl angle. The turbulent intensities of the axial and radial velocity were increased with the increase of the swirl angle. The mean SMD (Sauter mean diameter) of the radial direction along the axial distance shows the lowest value at the swirl angle of 60$^{\circ}$.

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Comparison of Velocity Fields of Wake behind a Propeller Using 2D PIV and stereoscopic PIV (2D PIV와 stereoscopic PIV 기법으로 측정한 프로펠러 후류의 속도장 비교 연구)

  • Paik Bu-Geun;Lee Sang-Joon
    • 한국가시화정보학회:학술대회논문집
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    • 2002.11a
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    • pp.23-26
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    • 2002
  • The phase-averaged velocity fields of 3 dimensional turbulent wake behind a marine propeller measured by 2D PIV and stereoscopic PIV(SPIV) were compared directly. In-plane velocity fields obtained from the consecutive particle images captured by one camera in 2D PIV have perspective errors due to out-of-plane motion. However, the perspective errors can be removed by measuring three component velocity fields using SPIV method with two cameras. It is also necessary to measure three components velocity fields for the investigation of complicated near-wake behind the propeller for the suitable propeller design. 400 instantaneous velocity fields were measured for each of four different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}C\;and\;54^{\circ}$. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the downstream region. The phase-averaged velocity fields show the viscous wake developed along the blade surfaces and tip vortices were formed periodically. The perspective errors caused by the out-of-plane motion was estimated by the comparison of 2D PIV and SPIV results. The difference in the axial mean velocity fields measured by both techniques are nearly proportional to the mean out-of-plane velocity component which has large values in the regions of the tip and trailing vortices. The axial turbulence intensity measured by 2D PIV was overestimated since the out-of-plane velocity fluctuations influence the in-plane velocity vectors and increase the in-plane turbulence intensities.

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A Study on Applicability of Coagulant Mixer and Flow Analysis of the Non-powered Vortex Mixer using CFD (전산유체역학(CFD)을 이용한 무동력 와류 혼화장치의 유동해석 및 응집제 혼화장치 적용 가능성 연구)

  • Kim, Soo Yeon;Chae, Jong Seong;Kim, Sin Young;Zhang, Meng Yu;Ohm, Tea In
    • Journal of Korean Society of Environmental Engineers
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    • v.39 no.12
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    • pp.706-713
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    • 2017
  • This study compared and analyzed the water treatment efficiency and the applicability of water treatment plant using the existing Mechanical Rapid-Mixer by introducing the Non-powered Vortex Mixer to the domestic water treatment plant. For this study, fluid flow characteristics and head loss of a Non-powered Vortex Mixer are calculated by Computational Fluid Dynamics (CFD)respectively. The head loss rate inside the mixer was 11.30% when the inflow velocity was 0.5 m/sec, 16.27% at 0.6 m/sec and 21.44% at 0.7 m/sec, the head loss rapidly increased at the optimal velocity of 0.5 m/sec. For the inflow velocity of 0.5 m/sec, the turbulent intensity at the inlet was 2.37% and at the outlet was 7.83%, so there was sufficient mixing strength for the particulate matter and the coagulant. The result of the water quality of the treatment plants with the inflow velocity of 0.38 m/sec that was operated in three years after replacing all 12 units of the existing Rapid-Mixer with the Non-powered Mixer met the standards. Hence, it is possible to reduce the energy consumption of 64,143 ~ 65,306 kWh/year since the Rapid-Mixer is replaced by the Non-powered Vortex Mixer.

Heat Transfer Characteristics on Impingement Surface with Control of Axisymmetric Jet ( 2 ) - With Acoustic Excitation - (원형제트출구 전단류 조절에 따른 제트충돌면에서의 열전달 특성 ( 2 ) - 음향여기된 제트 -)

  • Hwang, Sang-Dong;Lee, Chang-Ho;Cho, Hyung-Hee
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.24 no.3
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    • pp.373-381
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    • 2000
  • The flow and heat transfer characteristics on the impingement surface can be controlled by the change of vortex with the acoustic excitation, because the flow characteristics of an impinging jet are affected strongly by the vortices formed at the jet exit. To investigate the effects of acoustic excitation, we measured the velocity, turbulent intensity distributions for the free jet and local heat transfer coefficients on a impingement surface. As the acoustic excitation, subharmonic frequency of excited frequency plays an important role to the control of the jet flow. If the vortex pairings are promoted by the acoustic excitation, turbulence intensity of the jet flow is increased quickly. On the other hand if the vortex pairings are suppressed, the jet flow has low turbulence intensity at the center of the jet. Therefore, the low heat transfer rates are obtained on the impingement plate for a small nozzle-to-plate distance. However, it has high heat transfer rates at a large distance between the nozzle and plate due to the increasing of potential-core length.

Performance Characteristics of a V-type Probe Developed for Wall Vorticity Measurement (벽와도 측정을 위하여 개발된 V형 열선 프로브의 성능특성)

  • Kim, Seong-Uk;Ryu, Sang-Jin;Yu, Jeong-Yeol
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.25 no.4
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    • pp.514-522
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    • 2001
  • In order to investigate the relation between wall vorticity and streamwise velocity fluctuations in a turbulent boundary layer, a wall vorticity probe has been developed, which consists of two hot-wires on the wall aligned in V configuration. Although the measured intensity of spanwise wall vorticity fluctuations is somewhat lower than previous results, the intensity of streamwise wall vorticity fluctuations is in good agreement with them. It has been shown that the measured intensity of spanwise wall vorticity fluctuations is affected by transverse length of the wall vorticity probe. Instantaneous streamwise and spanwise wall vorticity fluctuations are compared with the results of DNS. Probability density function of spanwise wall vorticity fluctuations shows good agreement with previous results and is different from that of streamwise wall vorticity fluctuations. Energy spectrum of streamwisw wall vorticity fluctuations is lower than that of spanwise wall vorticity fluctuations in low frequency region.

Investigation of vortex core identification method for wind turbine wake (터빈 후류를 관찰하기 위한 와류 코어 식별 기법 연구)

  • Ko, Seungchul;Na, Jisung;Lee, Joon Sang
    • Journal of the Korean Society of Visualization
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    • v.15 no.1
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    • pp.19-24
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    • 2017
  • In this study, we conduct a numerical experiment of the single 5MW NREL wind turbine and compare the performance of various vortex core identification for the wake behind the wind turbine. In the kinetic analysis of wind turbine, 20% velocity deficit at 200 s is observed, showing wake which contains tip vortex near blade tip and rotor vortex at the center of the wind turbine. Time series of velocity and turbulent intensity show numerical simulation converge to a quasi-steady state near 200 s. In the comparison between methods for vortex identification, ${\lambda}_2$-method has good performance in terms of tip vortex, rotor vortex, vortex during its cascade process compared to vorticity magnitude criteria, ${\Delta}$-method. We conclude that ${\lambda}_2$-method is suitable for vortex identification method for wake visualization.

Wind-tunnel simulations of the suburban ABL and comparison with international standards

  • Kozmar, Hrvoje
    • Wind and Structures
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    • v.14 no.1
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    • pp.15-34
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    • 2011
  • Three wind-tunnel simulations of the atmospheric boundary layer (ABL) flow in suburban country exposure were generated for length scale factors 1:400, 1:250 and 1:220 to investigate scale effects in wind-tunnel simulations of the suburban ABL, to address recommended wind characteristics for suburban exposures reported in international standards, and to test redesigned experimental hardware. Investigated parameters are mean velocity, turbulence intensity, turbulent Reynolds shear stress, integral length scale of turbulence and power spectral density of velocity fluctuations. Experimental results indicate it is possible to reproduce suburban natural winds in the wind tunnel at different length scales without significant influence of the simulation length scale on airflow characteristics. However, in the wind tunnel it was not possible to reproduce two characteristic phenomena observed in full-scale: dependence of integral length scales on reference wind velocity and a linear increase in integral length scales with height. Furthermore, in international standards there is a considerable scatter of recommended values for suburban wind characteristics. In particular, recommended integral length scales in ESDU 85020 (1985) are significantly larger than in other international standards. Truncated vortex generators applied in this study proved to be successful in part-depth suburban ABL wind-tunnel simulation that yield a novel methodology in studies on wind effects on structures and air pollution dispersion.