Research

Professor Hamed’s research focuses on wall-bounded turbulence, environmental flows, renewable energy and wind farm flow dynamics, flow control and drag reduction, roughness-induced transition to turbulence, convective heat transfer, and flow measurement techniques.

Below are some of his recent research projects.

 

Effects of localized blowing on rough-wall turbulent boundary layers:

The 2D k-type roughness with and without the localized flow control through spanwise jets.

Planar particle image velocimetry was utilized to investigate the effects of localized blowing (injection) on the turbulent boundary layer over two-dimensional k-type roughness. In addition to a baseline no-injection case, localized blowing through five small spanwise jets was considered at two rates and for two injection locations. The overall flow organization is maintained across the five considered cases (baseline and four injection cases); however, the localized blowing alters the time-averaged streamwise velocity, boundary layer characteristics, Reynolds shear stress, and in-plane turbulent kinetic energy. The blowing-induced deviations from the baseline case extend far within the boundary layer; however, two-point correlations and proper orthogonal decomposition analyses provide evidence for a similar turbulence structure far above the roughness, despite the blowing-induced deviations in the aforementioned flow quantities.

Access the article here.

 

Turbulent flow characterization within canopy gaps:

The particle image velocimetry setup used for flow measurements within short canopy gaps.

The effects of a local heterogeneity in the form of a gap within canopies resembling aquatic vegetation were experimentally investigated in a water channel. Specifically, the turbulent flow within short canopy gaps with lengths L/h = 0.5, 1, 2, 3, and 4, where h denotes the canopy height, was experimentally investigated using planar particle image velocimetry. A thorough examination of the time-averaged streamwise and wall-normal velocities, shear layer growth, turbulent kinetic energy, Reynolds shear stress, and two-point correlations suggests a classification of the flow into two regimes: a skimming flow regime and a shear layer growth regime. In contrast with the skimming flow regime, the shear layer and enhanced turbulence exhibit deep penetration within the gaps and promote mixing in the shear layer growth regime.

Access the article here.

Effects of sheltering by an upstream roughness element on the wake of a downstream element:

Schematic of the roughness element setup employed to study the effects of sheltering.

Planar particle image velocimetry (PIV) was employed to investigate the effects of sheltering  (shielding) induced by an upstream cylinder on the wake of a downstream one. As shown in the schematic above, various spacings and height ratios were considered, and all elements were immersed within a turbulent boundary layer. Flow features in the wake, including the downwash, upwash, recirculation zone, velocity deficit, Reynolds shear stress, and turbulent kinetic energy (TKE), are dependent on the degree of sheltering, which is reliant on both the streamwise spacing and height ratio. Overall, sheltering results in a reduction in the downwash and size of the recirculation zone past the downstream cylinder. The magnitude and spatial distribution of the Reynolds shear stress and TKE varied significantly from those past the isolated cylinder. Depending on the streamwise spacing and height ratio, the presence of an upstream cylinder has the potential to enhance or reduce the Reynolds shear stress past the downstream cylinder. Quadrant analysis and proper orthogonal decomposition (POD) are used to quantify changes in the boundary layer turbulence structure due to the cylinder(s).

Access the articles here and here.

Vortical structures in the near wake of a wall-mounted tab:

Hairpin-like vortices and a counter-rotating vortex pair in the wake of a wall-mounted tab.

Vortex generators in the form of static, wall-mounted tabs are often used to promote mixing in convective heat transfer and external flow applications. In this project the vortical structures formed in the near wake of tabs with various geometries were experimentally investigated using 3D particle image velocimetry. The results highlight the structure of the dominant vortices and their interaction as well as the turbulence statistics and production in the near wake.

Access the article here.

 

Transition to turbulence over large-scale topography:

Observation of the onset of velocity fluctuation in a boundary layer flow over 2D large-scale wavy walls.

The transition to turbulence of a laminar flow over large-scale 2D and 3D wavy walls was experimentally studied using particle image velocimetry in a refractive-index-matching channel. The wavy walls occupied a significant portion of the boundary layer (up to 60% of the boundary layer thickness) and resulted in significant flow accelerations and deceleration regions. It is found that the transition is significantly delayed for the 3D case due to periodic topography-driven spanwise flows.

Access the article here.

 

Turbulent flow over a canopy in shallow submergence:

The particle image velocimetry setup used for flow measurements above and within the canopy. Measurements within the canopy were enabled by the use of refractive-index matching.

The turbulent flow over canopies resembling river vegetation was investigated to examine the impact of element height heterogeneity. The mean flow, turbulent kinetic energy, and Reynolds shear stress over a heterogeneous model were contrasted to those over a homogeneous one. The results indicated enhanced turbulence levels for the heterogeneous case and suggested greater vertical turbulent exchange at the interface.

Access the article here.

 

Wind Energy:

 

The impact of winglets on the wake was experimentally studied by comparing the wake of a standard model turbine with that of a wingletted one.

With the goal of enhancing power production from wind turbines and wind farms, alternations of wind turbine design have been investigated, including the addition of winglets as well as the use of windbreaks upstream of a wind turbine.

Access the article on winglests here.

Access the article on enhancing power production with windbreaks here.

 

Visit the publication page for more on Professor Hamed’s research.

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