Choung Mook Lee

Numerical simulation of wind flow over hilly terrain

Wind flow over hilly terrain is simulated by solutions of the Reynolds-averaged Navier–Stokes equations without the hydrostatic approximation. The standard and RNG-based k–e models are used together with wall functions to account for surface roughness. The numerical model uses finite-volume discretization and boundary-fitted coordinates to resolve the terrain. Simulations were made for flow at four sites, namely, Coopers Ridge, Kettles Hill, Askervein Hill and Sirhowy Valley, for which field data are available. Comparisons with wind data show agreement with respect to the profiles of local wind magnitude and direction. The numerical model is therefore deemed suitable for reliable prediction of local-scale wind flow over hilly terrain with regions of flow separation.

An experimental and numerical study on the flow over two-dimensional hills

Abstract An experimental and numerical investigation on the flow over two-dimensional hilly terrain is presented. Experiments for single hills and continous double hills are performed in a boundary-layer wind tunnel, and mean velocity profiles, turbulence characteristics, and surface pressure distributions are measured. The numerical model developed for the present work is based on the finite-volume-method and the SIMPLEC algorithm with a non-orthogonal body-fitted grid system. Several turbulence models are tested for the validation of the prediction accuracy in separated flow cases. Comparisons of the mean velocity profiles and surface pressure distributions between the numerical predictions and the measurements show good agreement. The linear theory provides generally good prediction of speed-up characteristics at the hill top for the hill slope of 0.3, which is defined as the ratio of the hill height to the base length at the upwind mid-height of the hill. Flow separation occurs in the hill slope of 0.5, and the measured reattachment points are compared with the numerical prediction. The low-Reynolds-number model with an orthogonal grid is found to predict the separated flow better than the other turbulence models.

Quantitative visualization of flow inside an evaporating droplet using the ray tracing method

Liquid droplets possess many practically important applications and academically interesting issues. Accurate flow data are necessary to correlate the hydrodynamic characteristics with the physicochemical processes occurring inside a droplet. However, the refraction of light at the droplet surface makes it difficult to measure the flow field inside the droplet accurately. To resolve this problem, two correction methods based on the ray tracing technique are employed. One is the image mapping method and the other is the velocity mapping method. For this, a mapping function between the image plane and the object plane is derived. The two correction methods are applied to the flow inside evaporating droplets of different ethanol concentrations for measuring their velocity fields, using a PIV method. The results obtained with the two methods are nearly identical. The major differences between the original results and the corrected results are found in the locations of the vortex centres and the magnitude of velocity vectors. Between the two correction methods, the velocity mapping method is recommended, because it is more convenient and recovers a greater number of velocity vectors, compared with the image restoration method.

Prediction of oil boom performance in currents and waves

Abstract The threshold velocity of surface currents causing the entrainment failure for oil booms is investigated and an empirical formula for the threshold velocity is proposed. A theoretical prediction of the deformation of the skirt of an oil boom due to surface currents is made, and the results of the prediction are verified by laboratory experiments. The skirt deformation results in reduction of the effective boom draft which in turn degrades the effectiveness of the boom. The motion of oil booms excited by the ocean waves are predicted and verified by experiments. The degradation in the boom effectiveness due to the wave motion is discussed. The effectiveness of tandem booms in trapping the leaked oil is investigated and a method of predicting an optimum separation distance between the two booms is described.

Control of flows around a circular cylinder: suppression of oscillatory lift force

The flow past a circular cylinder under continuous and pulsed electromagnetic forces (or the Lorentz force) is investigated. The Lorentz force is applied locally on the cylinder surface in the region of 70–130° from the stagnation point along the cylinder circumference in both upper and lower surfaces. The experiment is conducted to obtain the force measurement and the visualization of the flow around the circular cylinder on which a circumferential Lorentz force tangential to the cylinder surface is applied. It was found that both continuous and pulsed Lorentz forces have a significant effect on the flow behaviors around and behind the cylinder as well as on the drag and lift forces acting on the cylinder. Natural seawater is used for the working fluid and the induction effect can be neglected due to the low flow speed and the Lorentz force is found to be linearly proportional to the applied magnetic field and electric current strength within the condition of the present investigation.

Steady streaming of viscous surface layer in waves

An investigation of the drift velocity induced by water waves of a contminated surface layer is carried out. The theoretical analysis is based on a thin boundary layer on the free surface. The results of the analysis reveal that the drift velocity of a viscous layer on the water surface is 7/4 times the Stokes prediction ofc(ak)2 wherec is the wave celerity andak the wave slope. The present experimental investigation confirms the validity of the theoretical prediction for the drift velocity for a lightly contaminated surface layer; however, for a heavily contaminated surface layer, the experimental results exceed the theoretical prediction. An investigation for a heavily contaminated layer is carried out assuming an inextensible surface layer. Thus, in the experiment, vinyl sheets are used to substitute the contaminated layers. By balancing the wave-induced mean thrust force with the mean drag force, the drift velocity is obtained and compared with the experimental results. Based on the theoretical and experimental analyses, formulae for predicting the drift velocities for laminar and turbulent flow conditions are proposed.

Effects of a uniform magnetic field on a growing or collapsing bubble in a weakly viscous conducting fluid

The effects of a uniform magnetic field on a growing or collapsing bubble are investigated. The governing equations for the volume and shape modes of oscillation are derived. To obtain the pressure correction due to the electromagnetic field, the perturbed flow by the electromagnetic force is analyzed with the aid of the domain perturbation method. The viscous effect is assumed to be confined to a thin layer adjacent to the bubble surface. It is shown that the electromagnetic field exerts a damping force on the volume mode so that both the growing and collapsing speeds of a bubble are reduced. The magnetic field also affects the shape mode by contributing to a forcing term. Due to the forcing term, the shape of a growing–collapsing bubble becomes unstable even in the case of no initial disturbance.

Pollutant dispersion over two-dimensional hilly terrain

Numerical prediction of the wind flow and pollutant dispersion over two-dimensional hilly terrain is presented. The wind tunnel experiments are conducted to validate the numerical results of the flow field. Measured mean velocity profiles, turbulence characteristics, and surface pressure distributions show good agreement with the numerical predictions. The hypothesis of Reynolds number independency for an atmospheric boundary layer flow over aerodynamically rough terrain is numerically confirmed. The linear theory provides generally good prediction of speed-up characteristics for gently sloped low hills. The effect of two-dimensional double hills on the dispersion of pollutants from continuously or temporally released line source of different emission heights and locations is also investigated. The ground-level concentrations are considerably reduced as emission heights are increased. The variances of ground-level concentration with respect to time from a temporally released source are strongly influenced by the flow separation.

Experimental Investigation of Flow Characteristics of a Magnetohydrodynamic ( MHD ) Duct of Fan-Shaped Cross Section

Experiments along with numerical calculations are carried out to investigate the effect of the geometry of the cross section on flow characteristics of a MHD propulsion duct. A fan-shaped cross-section MHD thruster duct is chosen for comparison with a previously investigated rectangular cross section. Measurement of the velocity field is made using LDV (Laser Doppler Velocimetry) system with a fiber-optic cable and pressure distribution is measured with static pressure holes at the bottom surface. Comparison with rectangular cross section shows that the velocity profile is significantly influenced by the geometry of cross section while the axial pressure distribution is not so affected.

Prediction of drift in a free surface

An analytical model including the boundary-layer effect is developed to find the steady drift induced by the non-linear effect of the surface-wave motion. The analytical result is compared with the experimental results obtained from a wave tank. Results show a qualitative agreement. A semi-empirical formula which predicts drift velocity of a contaminant layer on the free surface is introduced and its validity is investigated.