Mohammad K. Shah

Favorable pressure gradient turbulent flow over straight and inclined ribs on both channel walls

This paper reports on experimental study of turbulent flows over straight and inclined transverse ribs of square and triangular cross sections attached to the bottom and top walls of an asymmetric converging channel. The pitch-to-height ratio of the ribs was 10. A particle image velocimetry technique was used to conduct extensive velocity measurements at channel midspan and in planes close to the leading and trailing edges of the inclined ribs. From these measurements, spatial averaged profiles of the mean velocity and higher order statistics were obtained to study the effects of rib geometry, pressure gradient, spanwise plane, and rib inclination on the flow characteristics. The results show that rib geometry has no significant effects on the mean flow and turbulent quantities. The roughness effects produced by the straight ribs outweighed pressure gradient effects in the inner region of the flow. As a result, the skin friction coefficient is nearly independent of pressure gradient. The Reynolds shear st...

PIV Investigation of Flow over a Transverse Square Rib in Pressure Gradients

This paper reports on experiments conducted to study the characteristics of separated and reattached turbulent flows experiencing adverse and favorable pressure gradients. The adverse and favorable pressure gradients were produced, respectively, in asymmetric diverging and converging channels, and the flow separation was caused by transverse square rib attached to the lower wall of the channels. A high-resolution particle image velocimetry was used to conduct detailed velocity measurements in the separated and redevelopment regions. Reference measurements were also conducted in a parallel-walled channel for which the pressure gradient is small compared to those produced in the diverging and converging for comparison. From these measurements, iso-contour plots of the mean velocity, spanwise vorticity and turbulence production in the vicinity of the rib were reported. The profiles of the mean velocities and vorticity, Reynolds stresses, triple correlations, production terms in the transport equations for the turbulent kinetic energy and Reynolds shear stress, mixing length and eddy viscosity were obtained in the separated and redevelopment regions as far downstream as 120 rib heights.

Structure of turbulent flow over 90° and 45° transverse ribs

This article reports on an experimental study of turbulent flows over periodic arrays of transverse square ribs attached to the bottom and top walls of an asymmetric converging channel preceded by an upstream parallel section. In the first and second series of experiments, the ribs on both walls were, respectively, arranged perpendicular and then at an angle of 45° to the sidewalls of the channel in a non-staggered configuration. A high-resolution particle image velocimetry system was used to conduct detailed velocity measurements in the upstream parallel and converging sections of the channel. For the 45° ribs, measurements were made at three different streamwise planes to capture the secondary motions produced by the inclined ribs. From these measurements, profiles and contours of the mean velocities, one-point turbulent statistics including the Reynolds stresses, Reynolds stress ratios, triple velocity correlations, and two-point correlation functions were obtained. These results, together with reference smooth-wall measurements in the fully developed section of a plane channel, are used to document the effects of rib roughness, pressure gradient, rib inclination, and spanwise location of the mean flow and turbulent statistics.

Flow Relaxation Past a Transverse Square Rib in Pressure Gradients

An experimental investigation is undertaken to study the salient features of separated and reattached flows over a transverse square rib in an asymmetric diverging and converging channel. To facilitate comparison, experiments were also performed in a parallel-walled channel for which the pressure gradient is nearly zero. The transverse square rib was located 1000 mm from the inlet of a given channel (at the inlet to the convergence/divergence section). A high-resolution particle image velocimetry was used to conduct the velocity measurements. From these measurements, isocontour plots of mean velocity, Reynolds shear stress, and spanwise vorticity in the vicinity of the rib were reported. The profiles of mean velocities, Reynolds stresses, triple products, and some of the terms in the transport equations for turbulent kinetic energy and Reynolds shear stress are reported in the separated and redevelopment regions as far downstream as 200 step heights.

Turbulent Flow over Transverse Ribs at 90 o and 45 o to Channel Side-Walls (Invited)

This paper documents the salient features of favorable pressure gradient turbulent flow over transverse square ribs arranged at 90 o and then at 45 o to the side walls of the test section. The favorable pressure gradient was produced in an asymmetric converging channel that comprised of a straight top wall and a 3 o ramp fitted to a section of the bottom wall of the channel. The transverse ribs were glued onto both the top and bottom walls and the pitch-to-height ratio of the ribs was 10. A particle image velocimetry technique was employed to obtain detailed measurements over the ribs within the converging section and upstream of the converging section where the top and bottom walls of the channel were parallel. These measurements were made in the streamwise-transverse planes located at channel mid-span for the 90 o and 45 o ribs and also close to the trailing edge of the 45 o ribs. The iso-contours and profiles of the mean velocities, Reynolds stresses and production terms in the transport equations for the turbulent kinetic energy and Reynolds stress were obtained to study the effects of pressure gradient and rib inclination to the flow characteristics.

PIV Study of Reattaching Flow Downstream of a Rib in an Asymmetric Diverging Channel

An experimental investigation is undertaken to study the salient features of separated and reattached flows in an asymmetric diverging channel. The channel comprised a straight flat floor and a curved roof that was preceded and followed by straight parallel walls. A transverse square rib was used to create separation inside the channels. Three different rib locations (x′/h = -25, 0 and 25) were examined at Reynolds number based on step height (Reh) of 2760. A particle image velocimetry technique (PIV) was employed to obtain detailed measurements downstream of the rib as well as at the upstream location. From these measurements, the mean velocity and higher order turbulent quantities are reported.