The flow around six in-line square cylinders has been studied numerically and experimentally for 0.5 s/d 10.0 and 80 Re 320, where s is the surface-tosurface distance between two cylinders, d is the size of the cylinder and Re is the Reynolds number. The effect of spacing on the flow regimes is initially studied numerically at Re = 100 for which a synchronous flow regime is observed for 0.5 s/d 1.1, while quasi-periodic-I, quasi-periodic-II and chaotic regimes occur between 1.2 s/d 1.3, 1.4 s/d 5.0 and 6.0 s/d 10.0, respectively. These regimes have been confirmed via particle-image-velocimetry-based experiments. A flow regime map is proposed as a function of spacing and Reynolds number. The flow is predominantly quasi-periodic-II or chaotic at higher Reynolds numbers. The quasi-periodic and chaotic nature of the flow is due to the wake interference effect of the upstream cylinders which becomes more severe at higher Reynolds numbers. The appearance of flow regimes is opposite to that for a row of cylinders. The Strouhal number for vortex shedding is the same for all the cylinders, especially for synchronous and quasi-periodic-I flow regimes. The mean drag (C Dmean ) experienced by the cylinders is less than that for an isolated cylinder, irrespective of the spacing. The first cylinder is relatively insensitive to the presence of downstream cylinders and the C Dmean is almost constant at 1.2. The C Dmean for the second and third cylinders may be negative, with the value of C Dmean increasing monotonically with spacing. The changes in root mean square lift coefficient are consistent with changes in C Dmean . Interestingly, the instantaneous lift force can be larger than the instantaneous drag force on the cylinders. These results should help improve understanding of flow around multiple bluff bodies.

Flow across a row of identical square cylinders placed side-by-side has been found to show interesting flow patterns which have complex characteristics depending upon the spacing (s/d) between the cylinders and the Reynolds number (Re). The combined effects of cylinder spacing and Reynolds number on the flow across a row of cylinders are numerically studied for 30≤Re≤140 and 1.0≤s/d≤4.0, where s is the surface-to-surface distance between two cylinders and d is the size of cylinder. It is found that the critical Reynolds number for the onset of vortex shedding increases with increase in gap ratio. The Reynolds number is found to have a strong effect on the flow especially at s/d=3.0,4.0. Secondary frequency in the signal for lift and drag coefficients significantly contributes to the forces experienced by the cylinders. It is observed that at s/d=3.0,4.0 the secondary frequency disappears at larger Reynolds number and the primary frequency dominates the flow. This means that the interaction of the wakes behind the cylinders at these gap ratios weakens with an increase in the Reynolds number. It is proposed that wake interaction is strongly influenced by the jets in the gap region, the nature of which alters with spacing and Reynolds number. This is confirmed by computing the average wake size as a function of Reynolds number. Based on this, two critical gap ratios, 2.0 and 4.0 for the range of Reynolds number under consideration are proposed. These gap ratios separate synchronous, quasiperiodic-I and quasiperiodic-II flow regimes depending on the Reynolds number. The mechanism of wake interaction has been studied to bring out these critical gap ratios.

This paper reports the effect of different adiabatic lengths on the thermal performance of loop thermosyphon with different filling ratios (FRs) and heat inputs. The carbon steel thermosyphon loop for three different adiabatic lengths is used in this analysis. The loop with plate-type, forced air-cooled condenser, along with two vertical inline evaporators, is filled and tested with distilled water. The transient and steady-state analyses are carried out to understand the thermal behavior of the loop. The thermal resistance is found to be lowest at 800-mm adiabatic length and 60% FR. The geyser boiling phenomenon is also noticed in the present thermosyphon loop. The period of oscillation and temperature fluctuations in the evaporator and condenser increase with the adiabatic length. The geyser boiling phenomenon may disappear at a very small adiabatic length of the loop thermosyphon with forced air-cooled condenser. This paper proposes a mathematical model for the loop thermosyphon in terms of the Nusselt number, the Reynolds number, the Prandtl number, and the adiabatic length-todiameter ratio, and the comparative study shows that proposed model validates the experimental results. Also, it is found that the adiabatic length-to-diameter ratio inversely varies with the Nusselt number.

A numerical study of flow across a row of transversely oscillating square cylinders (of diameter d) has been undertaken using the lattice Boltzmann method, for a better understanding of fluid–structure interaction problems. The effects of cylinder oscillation frequency ratio (fe/fo, where fe is the cylinder oscillation frequency and fo is the corresponding vortex shedding frequency for stationary row of cylinders), amplitude ratio (A/d), non-dimensional spacing between the cylinders (s/d) and Reynolds number (Re) on ensuing flow regimes and flow parameters have been studied to understand the flow physics. Six different flow regimes observed in this study are the quasi-periodic non-lock-on-I, synchronous lock-on, quasi-periodic lock-on, quasi-periodic non-lock-on-II, synchronous non-lock-on and chaotic non-lock-on. It is observed that the range of the lock-on regime depends upon the relative dominance of incoming flow and cylinder motion. Although the lock-on regime in the case of Re = 80, s/d = 4 and A/d = 0.2 is substantially larger as compared to that for a single oscillating cylinder, the range of the lock-on regime shrinks with a reduction in the cylinder spacing, increase in the Reynolds number or decrease in the oscillation amplitude. It is also observed that the wake interaction behind the cylinders weakens with an increase in fe/fo, Re, A/d or s/d, leading to the formation of independent wakes and synchronous nature of the flow. For fe/fo ≥ 1.2, independent and intact oscillating wakes are noted and an additional frequency (wake oscillation frequency) is obtained in the time series of the lift coefficient. Although it was expected that the complexity in the wake interaction would increase with cylinder oscillation or amplitude ratio, an opposite effect (that is, formation of independent wakes) is noted from the results.

The mathematical model for determination of auxiliary energy required to maintain set air temperature in the drying chamber is proposed for cabinet type solar dryer.The model is developed considering the heat exchanges across the dryer control volume with solar radiation intensity and set temperature as input. The sole purpose is to minimize the auxiliaries. The computational fluid dynamics (CFD) technique is used to simulate the air flow inside the drying chamber for different operating conditions.The simulations results for the temperature are very close to the results from mathematical model. The dryer configuration discussed in illustrative example found to have minimum auxiliary consumption at 45 C with total auxiliary expense of 97.3 kWh per day. When applied to full calendar year, the optimum temperature is found to be function of ambient temperature and solar radiation intensity. The results obtained from mathematical model are found to be in well agreement with simulation results. The simulation results provide a region with 1.2 m to 1.6 m on Xordinate, 0.8 m to 1.0 m on Y-ordinate and 0.7 m to 1.2 m and 3.7 m to 4.3 m on Zordinate where average temperature can be sensed. It helps in modulating the auxiliary input/s in integration with solar heat.