G. Nasif

Characteristics of Flow Structures in the Wake of a Bed-Mounted Bluff Body in Shallow Open Channels

The characteristics of the flow structures observed in the wake of a bluff body mounted vertically on the bed and normal to the flow in a shallow open channel are investigated using detached eddy simulation (DES). The flow structures in the shallow wake are identified using the λ2-criterion. A distinctive feature in the time-averaged flow field, referred to as the owl face of the first kind, is observed. The position of this spiraling structure is stable at locations close to the bed, while its rotation sense switches from stable inward to unstable outward spiraling as it moves toward the free surface, where the bed friction becomes insignificant and the flow develops into a traditional two-dimensional (2D) wake. A three-dimensional (3D) structure resulting from a horizontally oriented secondary roll-up process is observed immediately downstream of the base of the bluff body in the center of the near-wake region. In addition to the horseshoe vortex, a new structure that wraps around the bluff body in the toe region is identified, referred to as a collar vortex. The presence of the coherent structures in the near-bed region is highlighted and their influence on the wake region is discussed.

Heat Transfer Due to an Impinging Jet in a Confined Space

A numerical investigation using unsteady three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations with the k-ω SST (shear stress transport) turbulent model was conducted to determine the flow and thermal characteristics of an unsubmerged axisymmetric oil jet in air, impinging normally on to a heated flat disk with finite radius, bounded by cylindrical walls kept at constant temperature. A 10 mm thick disk subjected to a high uniform heat flux was located at impingement distances ranging from 40 to 80 mm from the nozzle exit, for nozzle exit diameters of d = 1.0, 2.0, and 4.0 mm. The volume of fluid (VOF) method with a high-resolution interface-capturing (HRIC) scheme was implemented in STAR-CCM+. A new methodology was developed to predict the stagnation zone and local heat transfer coefficients. Contrary to previous research, it is shown that the radial extent of the stagnation zone is not fixed but depends on the gradient of radial velocity along the disk. The normalized local Nusselt number profile along the disk radius is found to be weakly dependent on Reynolds number for a given nozzle size. It is also shown that the local Nusselt number is not uniform in the stagnation region as reported by experimental studies but depends on the distribution of the near-wall radial velocity gradient. Using the computational results, new correlations to predict the dimensionless radial velocity gradient and Nusselt number have been developed. The present correlations are dimensionally balanced, eliminating a deficiency in earlier correlations noted in the literature.

Characteristics of Shallow Wakes in an Open Channel

A numerical investigation of shallow wake flow in an open channel has been conducted using Detached Eddy Simulation (DES) based on the three-dimensional unsteady Reynolds-Averaged Navier Stokes (RANS) equations with the k-ω SST (shear stress transport) turbulence model. The features of shallow wake flows are strongly dependent on the bed characteristics. Using the λ2 criterion, the important features are identified and the role of coherent structures in the near-bed region is highlighted. Furthermore, a unique feature in the time-averaged flow field referred to as the owl face of the first kind, which consists of a well-defined pair of foci and saddle points, is observed. A three-dimensional structure resulting from a secondary roll-up process, which is horizontally oriented, is observed immediately downstream of the base of the bluff body. Other vortical structures, i.e., a horseshoe vortex and a collar vortex, are found to occur around and next to the bluff body, respectively, in the toe region.Copyright

Simulation of Jet Impingement Heat Transfer

ABSTRACT A numerical investigation to determine flow and thermalcharacteristics of an unsubmerged axisymmetric oil jet impinging on aconfined flat surface with uniform heat flux has been undertaken.Large impingement length to nozzle diameter ratios were chosen in thesimulations. The volume of fluid (VOF) method utilizing a HighResolution Interface Capturing scheme (HRIC) was used to performthe two-phase (air-oil) simulations. The governing 3D Navier-Stokesequations and energy equation were numerically solved using a finitevolume discretization on an unstructured mesh. A new methodologywas developed to define the radial extent of the stagnation region andunderstand the variation of the heat transfer coefficient in this region.The normalized local Nusselt number profile was found to be slightlydependent on Reynolds number for a given nozzle size. Correlations topredict the dimensionless velocity gradient and the Nusselt number inthe stagnation region were established. NOMENCLATURE B Dimensionless velocity gradient