Sushanta Dutta

Effects of Delta-Shaped Obstacles on the Thermal Performance of Solar Air Heater

An experimental investigation has been carried out to study the effect of delta-shaped obstacles mounted on the absorber surface of an air heater duct with an aspect ratio 6 : 1 resembling the conditions close to solar air heaters. This study encompassed the Reynolds number (Re) ranging from 3400 to 27600, longitudinal pitch of the obstacle ( P l / e ) varied from 3/2 to 11/2, and relative obstacle height (e/H) varied from 0.25 to 0.75. The relative obstacle transverse pitch ( P t / b ) = 7 / 3 and the angle of attack of flow on obstacle = 90° are kept constant during the whole experimentation. By comparing the heat transfer data obtained from the obstacle-mounted duct with that of smooth duct under similar geometrical and flow conditions, the obstacle-mounted duct enhances the heat transfer by 3.6-times at Re = 7276.82, P l /e= 3 / 2 , and e / H = 0.75 .

Heat transfer augmentation in solar air heater using delta-shaped obstacles mounted on the absorber plate

An experimental investigation has been carried out to study the heat transfer augmentations by using delta-shaped obstacles mounted on the absorber surface of an air heater duct with an aspect ratio 6:1 resembling the conditions close to solar air heaters. The experiment encompasses the Reynolds number ranging from 3400 to 28,000, obstacle longitudinal pitch (P l/e) varying from to and relative obstacle height (e/H) varying from 0.50 to 0.75, while relative obstacle transverse pitch and the angle of attack of flow on obstacle=90° are kept constant during the whole experimentation. The maximum heat transfer augmented by using an obstacle-mounted duct was 3.6 times higher compared with a smooth duct under similar geometrical and flow conditions. A numerical analysis using FLUENT software has also been done for further observation of heat transfer behaviour near the obstacles.

Sensitivity of a Square Cylinder Wake to Forced Oscillations

The wake of a square cylinder at zero angle of incidence oscillating inline with the incoming stream has been experimentally studied. Measurement data are reported for Reynolds numbers of 170 and 355. The cylinder aspect ratio is set equal to 28 and a limited study at an aspect ratio of 16 has been carried out. The frequency of oscillation is varied around the Strouhal frequency of a stationary cylinder, and the amplitude of oscillation is 10–30% of the cylinder size. Spatial and temporal flow fields in the cylinder wake have been studied using particle image velocimetry and hot-wire anemometry, the former providing flow visualization images as well. A strong effect of forcing frequency is clearly seen in the near wake. With an increase in frequency, the recirculation length substantially reduces and diminishes the time-averaged drag coefficient. The timeaveraged vorticity contours show that the large-scale vortices move closer to the cylinder. The rms values of velocity fluctuations increase in magnitude and cluster around the cylinder as well. The production of turbulent kinetic energy shows a similar trend as that of spanwise vorticity with the former showing greater asymmetry at both sides of the cylinder centerline. The instantaneous vorticity contours show that the length of the shear layer at separation decreases with increasing frequency. The effect of amplitude of oscillation on the flow details has been studied when the forcing frequency is kept equal to the vortex-shedding frequency of the stationary cylinder. An increase in amplitude diminishes the time-averaged drag coefficient. The peak value of rms velocity increases, and its location moves upstream. The length of the recirculation bubble decreases with amplitude. The reduction in drag coefficient with frequency and amplitude is broadly reproduced in experiments with the cylinder of lower aspect ratio. DOI: 10.1115/1.2742736

Experimental Investigation of Flow Over a Transversely Oscillating Square Cylinder at Intermediate Reynolds Number

This paper presents an experimental study of flow over a square cylinder oscillating in transverse direction. The Reynolds number selected for present study is 485. Limited study has also been made for two other Reynolds numbers, namely, 295 and 775. The objective of the present study is to modify the near-wake flow structure using actuation of the cylinder for possible reduction in drag force. Transverse oscillations to the cylinder are provided using electromagnetic actuators. The flow field is investigated using two-dimensional (2D)-particle image velocimetry (PIV) system, hotwire anemometer (HWA), as well as flow visualization techniques. The effect of oscillation frequency and the amplitude on parameters like Strouhal number, drag coefficient, recirculation length, power spectrum, and Reynolds stress are studied. It is observed that the recirculation length is reduced significantly with increase in forcing frequency, and consequently drag coefficient is also reduced. For a constant forcing frequency, the vortex strength is reduced with the increase in the amplitude. Further, variation of instantaneous spanwise vorticity shows that separated shear length decreases with increase in forcing frequency. As a result, vortices are moved closer to the cylinder. These phenomena affect the forces acting on the cylinder. Lock-on is also observed at a frequency close to the vortex shedding frequency of the stationary cylinder.

Investigation of Flow Interference Between Two Inline Square Cylinders With Different Spacing Ratios in Subcritical Reynolds Number Regime

When two cylinders are in proximity with each other the vortex shedding of one cylinder are influenced by the other. The flow field becomes even more complex when one cylinder subjected to an oscillation. In present study flow around two inline square cylinders of aspect ratio 50 at different spacing ratio is investigated experimentally. Experiments are performed for two identical square cylinders of 6 mm diameter at Reynolds number 500 in a low speed wind tunnel using Particle image velocimetry (PIV) and hotwire anemometer (HWA). The upstream cylinder is given predetermined oscillation and the gap between the two cylinders varied over wide range. Present study focused on effect of forcing frequency and spacing between centers of two cylinders (s/D = 1.5–5.0). The upstream cylinder is oscillated in transverse direction at f/f0 = 0 to 2 and the effect of this oscillation is observed behind the downstream stationary cylinder at different spacing of the cylinders by hotwire.

An Experimental Study on Flow Past an Equilateral Triangular Prism at Intermediate Reynolds Number and the Effect of its Orientation

This paper explores the effect of two orientations (0° and 180°) on near wake region of an equilateral triangular prism at intermediate Reynolds number. The paper also investigates the effect of Reynolds number on the near wake region at intermediate regime. Two orientations are defined such that the apex of the equilateral triangle is pointing upstream and downstream respectively. The flow field behind a triangular prism is different in these two orientations. When apex angle points upstream it corresponds to a fore-body attached to a flat plate while when apex angle points downstream the flat face acts like a flat plate with an after body attached to it. This after-body penetrates the recirculation zone formed behind the flat plate. These two effects are investigated for various Reynolds numbers in intermediate regime. Factors affecting drag coefficient and Strouhal number are different in both cases. In first case the fore-body changes the flow before it gets separated in two free shear layers. In other words, the fore-body affects the free shear layers formed and hence the drag. In the second case, the two shear layers are formed first then their interaction is modified by the after-body in the near wake region behind the prism. Particle Image Velocimetry (PIV) and Hotwire anemometry is being used in present study. Detailed flow field is investigated in terms of velocity magnitude, stream traces, vorticity contours, centerline recovery, power spectra, velocity profiles, Strouhal number and drag coefficient.Copyright