Kyoungsik Chang

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

Wake Characteristics of Vane-Type Vortex Generators in a Flat Plate Laminar Boundary Layer

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.

Methodology for system-level analysis of a fan–motor design for a vacuum cleaner

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.

ANALYSIS OF MECHANISMS OF CONTAMINANT REMOVAL FROM A BOTTOM RIVER CAVITY

Experimental and numerical investigations were conducted to identify the wake characteristics downstream of two vanetype vortex generators over laminar flat plate boundary layer. Experimental study was carried out by using the stereoscopic particle image velocimetry. To describe the flow field around the vortex generator in detail, numerical study was performed. We considered two different planform shapes of vortex generator: triangular and rectangular shape. The height of the generator was chosen to be about the boundary layer thickness at the position of its installation. Two different lengths of the generator were chosen: two and five times the height. Wake measurements were carried out at three angles of attack for each configuration. Wake characteristics for each case such as overall vortical structure, vorticity distribution, and location of vortex center with downstream distance were obtained from the PIV data. Wake characteristics, as expected, were found to vary strongly with the geometry and angle of attack so that no general tendency could be deduced. Causes of this irregular tendency were explained by using the results of the numerical simulation.

Numerical investigation of flow and turbulence structure through and around a circular array of rigid cylinders

In the present study, a methodology for conducting a system-level analysis of a fan–motor assembly in a vacuum cleaner is presented. This system consisted of three components, a fan, motor, and the flow resistance of the motor, or of the vacuum cleaner. The combined characteristics of the fan and the motor were obtained from torque matching at a constant throttling condition, and a pressure drop was implemented under a constant flow rate to account for the flow resistance. By combining these two steps, the performance characteristics of the fan–motor assembly and the vacuum cleaner system could be predicted over the whole range of operation, based on the characteristics of each component. The predicted performance for power, flow rate, pressure, and efficiency using the present method agreed well with the experimental results obtained for an equivalent system, within 2% difference at best efficiency point. Three models of the fan–motor assembly (S1, S2, and S3) were analyzed at the component level, and the decrease in efficiency produced by flow resistance was estimated to be 1% (S1 and S3 models) or 4.7% (S2 model) using the present method. The characteristics of the fan, extracted from those of the fan–motor assembly, were used for validating the computational fluid dynamics. The computational fluid dynamics results of this study predicted higher efficiency due to simplification of the geometry, but an accurate prediction of best efficiency point location was obtained. The proposed method is also applicable for detecting system leakage and identifying system resistance without direct measurement.

The purging of a neutrally buoyant or a dense miscible contaminant from a rectangular cavity

The mechanisms responsible for contaminant removal from a bottom rectangular cavity in a channel are investigated using fully three-dimensional (3D) Large Eddy Simulation (LES) simulations. The aspect ratio (length/depth) of the 2D cavity is L/D=2 and the neutrally buoyant contaminant (passive scalar) is introduced instantaneously inside the cavity once the flow has developed. The flow upstream the cavity is fully turbulent. The large scale coherent structures in the shear layer region induce large scale pressure fluctuations at the trailing cavity edge and convection of patches of vorticity inside the cavity, parallel to the trailing edge. These patches modulate the intensity of the jet like flow that develops along the trailing edge and bottom cavity walls. It is shown that the eddies convected from upstream of the cavity can play an important role in accelerating the extraction of contaminant from inside the cavity. It is found that the contaminant ejection can be described using simple 1D dead-zone theory models in which a single-valued global mass exchange coefficient can be used to describe the contaminant mass decay inside the cavity over the whole extent of the ejection process.