Seung O. Park

Analysis of the flow and mass transfer processes for the incompressible flow past an open cavity with a laminar and a fully turbulent incoming boundary layer

The three-dimensional incompressible flow past a rectangular two-dimensional shallow cavity in a channel is investigated using large-eddy simulation (LES). The aspect ratio (length/depth) of the cavity is

Assessment of Predictive Capabilities of Detached Eddy Simulation to Simulate Flow and Mass Transport Past Open Cavities

L/D\,{=}\,2

A Study of Impeller-Diffuser-Volute Interaction in a Centrifugal Fan

and the Reynolds number defined with the cavity depth and the mean velocity in the upstream channel is 3360. The sensitivity of the flow around the cavity to the characteristics of the upstream flow is studied by considering two extreme cases: a developing laminar boundary layer upstream of the cavity and when the upstream flow is fully turbulent. The two simulations are compared in terms of the mean statistics and temporal physics of the flow, including the dynamics of the coherent structures in the region surrounding the cavity. For the laminar inflow case it is found that the flow becomes unstable but remains laminar as it is convected over the cavity. Due to the three-dimensional flow instabilities and the interaction of the jet-like flow inside the recirculation region with the separated shear layer, the spanwise vortices that are shed regularly from the leading cavity edge are disturbed in the spanwise direction and, as they approach the trailing-edge corner, break into an array of hairpin-like vortices that is convected downstream the cavity close to the channel bottom. In the fully turbulent inflow case in which the momentum thickness of the incoming boundary layer is much larger compared to the laminar inflow case, the jittering of the shear layer on top of the cavity by the incoming near-wall coherent structures strongly influences the formation and convection of the eddies inside the separated shear layer. The mass exchange between the cavity and the main channel is investigated by considering the ejection of a passive scalar that is introduced instantaneously inside the cavity. As expected, it is found that the ejection is faster when the incoming flow is turbulent due to the interaction between the turbulent eddies convected from upstream of the cavity with the separated shear layer and also to the increased diffusion induced by the broader range of scales that populate the cavity. In the turbulent case it is shown that the eddies convected from upstream of the cavity can play an important role in accelerating the extraction of high-concentration fluid from inside the cavity. For both laminar and turbulent inflow cases it is shown that the scalar ejection can be described using simple dead-zone theory models in which a single-valued global mass exchange coefficient can be used to describe the scalar mass decay inside cavity over the whole ejection process.

Measurements and Control of Particle Deposition Velocity on a Horizontal Wafer with Thermophoretic Effect

The three-dimensional (3D) incompressible flow past an open cavity in a channel is predicted using the Spalart-Almaras (SA) and the shear-stress-transport model (SST) based versions of detached eddy simulation (DES). The flow upstream of the cavity is fully turbulent. In the baseline case the length to depth (LID) ratio of the cavity is 2 and the Reynolds number Re D =3360. Unsteady RANS (URANS) is performed to better estimate the performance of DES using the same code and meshes employed in DES. The capabilities of DES and URANS to predict the mean flow, velocity spectra, Reynolds stresses, and the temporal decay of the mass of a passive contaminant introduced instantaneously inside the cavity are assessed based on comparisons with results from a well resolved large eddy simulation (LES) simulation of the same flow conducted on a very fine mesh and with experimental data. It is found that the SA-DES simulation with turbulent fluctuations at the inlet gives the best overall predictions for the flow statistics and mass exchange coefficient characterizing the decay of scalar mass inside the cavity. The presence of inflow fluctuations in DES is found to break the large coherence of the vortices shed in the separated shear layer that are present in the simulations with steady inflow conditions and to generate a wider range of 3D eddies inside the cavity, similar to LES. The predictions of the mean velocity field from URANS and DES are similar. However, URANS predictions show poorer agreement with LES and experiment compared to DES for the turbulence quantities. Additionally, simulations with a higher Reynolds number (Re D = 33,600) and with a larger length to depth ratio (L/D=4) are conducted to study the changes in the flow and shear-layer characteristics, and their influence on the ejection of the passive contaminant from the cavity.

MIXED CONVECTION FLOW IN CURVED ANNULAR DUCTS

This paper reports velocity measurement data in the interaction region between the impeller and vaned diffuser and the results of numerical flow simulation of the whole machine (impeller vaned diffuser and volute) of a single stage centrifugal fan. Two-dimensional instantaneous velocity measurement is done using particle image velocimetry (PIV). Numerical simulation of impeller-diffuser-volute interaction is performed using CFX-Tascflow commercial code. A frozen rotor simulation model is used for the steady calculation and a rotor-stator simulation model is used for the unsteady calculation using the steady results as an initial guess. The simulation results show that the separated flow regime near the diffuser hub extends to the volute. Comparison between the unsteady computation and those of measurement indicates that the rotor/stator model employed in the simulation predicts essential characteristics of unsteady flow in the centrifugal fan. However, quantitative agreement remains rather poor.

MEASUREMENT OF PARTICLE DEPOSITION VELOCITY TOWARD A HORIZONTAL SEMICONDUCTOR WAFER BY USING A WAFER SURFACE SCANNER

To investigate positive and negative thermophoretic effects for polystyrene latex (PSL) spheres of diameter between 0.3 and 0.8 μm, the average deposition velocity toward a horizontal wafer surface in vertical airflow is measured keeping the wafer surface temperature different from the surrounding air temperature. The temperature difference ranges from -10° to 4°C. The number of particles deposited on a wafer surface is obtained from the measurements by using a wafer surface scanner (PMS SAS-3600). Experimental data of particle deposition velocity are compared with those given by the prediction model to validate the model. Since thermophoresis changes greatly the particle deposition velocity, temperature difference necessary for the particle deposition velocity to remain under a given value is sought as a means to control the deposition velocity. The minimum temperature differences required to keep the average deposition velocity smaller than 10−4cm/s and 10−5 cm/s in a clean room environment are suggeste...

Laminar entrance flow in curved annular ducts

Abstract Mixed convection flows in concentric curved annular ducts with constant wall temperature boundary condition are studied numerically. The flow is assumed to be fully developed so as to maintain a constant streamwise pressure and temperature gradient. Key parameters for the flow are the radius ratio (ratio of the inner core radius to the outer pipe radius), the Dean number, and the Grashof number. Computations are carried out for flows of various radius ratio with Dean numbers in the range 0–900 and Grashof numbers of 12.5 and 12 500. The secondary flow patterns, the streamwise velocity profiles and the heat transfer coefficients are presented. Effects of the Dean number and the Grashof number on the flow pattern, on the friction ratio (ratio of the friction for a curved annular duct to that for a straight annular duct flow), and on the heat transfer property are discussed based on the computational results. It is found that both the friction ratio and the Nusselt number ratio are strong functions of the radius ratio and the Dean number when Gr = 12.5, while these ratios do not change much with the radius ratio and the Dean number when Gr = 12 500.

On the limiters of two-equation turbulence models

The average particle deposition velocity toward a horizontal semiconductor wafer in a vertical airflow was measured by a wafer surface scanner (PMS SAS-3600) to shorten the exposure time and hence to improve repeatability. Polystyrene latex (PSL) spheres with diameters between 0.2 and 1.0 μm were used. For the present experiment, convection, diffusion, and sedimentation comprise important agents of the deposition mechanism. The mean and standard deviation of average deposition velocities were obtained from more than 10 data sets for each PSL sphere size, and the deposition velocity distributions from the measurement data were compared to the theoretical distributions.

A compressible turbulence model for the pressure–strain correlation

Abstract Steady laminar flows in coiled annular ducts are investigated numerically. Numerical solutions are obtained by solving the incompressible Navier-Stokes equation with a SIMPLE type procedure for annular curved ducts of various radius ratio for a given Reynolds number. Effect of radius ratio on the flow development is given particular attention. Computational results indicate that the secondary flow in a half cross section (above or below the line of symmetry) for the case of moderate radius ratio is characterized by a pair of counter-rotating vortices; the flow in the core region is toward the outside bend and the flow near the inner and outer walls is toward the inside bend. However, when the radius ratio is very large, say greater than 0.8, the secondary flow is undirectional and is toward the inside bend, owing to the strong viscous effect. It is also found that the downstream flow development is greatly affected by the radius ratio. When the radius ratio is moderate, the centroid of the first moment of streamwise velocity lies on the outside half plane; when the radius ratio is very large, the centroid lies on the inside half plane. In contrast to the case of straight annular duct, the flow in a curved annular duct is not necessarily fully developed earlier when the radius ratio is larger owing to the complicated interaction between the viscous and the centrifugal forces.

Flow regimes of unsteady laminar flow past a slender elliptic cylinder at incidence

When two-equation turbulence models are used, unrealistically large values of turbulence variables can appear due to the infringement of a realizability condition or to numerical error. To cure this in practical calculations, various limiters on the source terms are often employed. In the present work, a mathematically correct bound for eddy viscosity is obtained from the realizability condition itself. From this, realizability bounds for several terms of model equations are given. The effects of various bounds including the present one, are investigated on the predictions of fundamental flows including simple shear flows, supersonic compression ramp flow and supersonic base flow. It is shown that the limiter affects the prediction very significantly.