Articles written in Sadhana

    • Three-dimensional flow separations on a rolling sphere


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      Experiments are conducted to study the flow separations on a sphere rolling on an inclined plane submerged in water. These experiments are performed at Reynolds numbers Re between 1350 and 1550. The experiments show that the flow separations on the surface of a rolling sphere can be organized into four distinctregions: (i) region of primary separation-I on the front upper sphere and extending below the poles, (ii) region of viscous blockage at the crevice surrounding the point of contact and shear layer separation ahead and at the sides of the viscous blockage, (iii) region of primary separation-II on the rear lower sphere and (iv) secondary separations on the rear upper sphere surface. The ratio of the width of the viscous blockage to the diameter of the sphere is found to be 0.4. Primary separation-I surface is symmetrical about the equatorial plane of the rollingsphere. Primary separation-II from the rear lower sphere surface is asymmetrical about the equator and eddies are shed alternately on either side of the equator from this separation surface. These lower eddies are energetic and dominate the dynamics of the wake. The upper eddy shedding from the primary separation-I surface and the lower eddy shedding from the primary separation-II surface are synchronized.

    • Actively and passively aspirated temperature sensors in a windless environment like greenhouses


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      Windless environments are prevalent in greenhouses, where precise temperature control is critical to the health of plants and errors in temperature measurement must be avoided. A mathematical model based on heat balance is used to estimate the error in temperature sensors under different conditions and sensorgeometries. The model is then applied to two geometries and solved numerically. The sensors used are a T-type thermocouple and a Sensirion SHT 75. Experiments are carried out in a greenhouse prototype to replicateconditions in a greenhouse and validate the error model. Active ventilation of the sensors is provided by a fan and passive ventilation is provided using a model built on the concept of a ‘solar chimney’. It is shown that even small flow rates can decrease the error shown by unventilated sensors by as much as 10°C.

    • Gust enhancement using jet at the trailing edge of a pair of pitching foils


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      A fluid dynamics numerical simulation is carried to study the gust characteristics produced downstream of a pair of sinusoidally pitching two-dimensional thin foils with a certain lateral spacing and placed in a freestream flow. The flow was simulated for Reynolds number (Re) = 1.6 X 105, the airfoil pitching angle =6.9°, and a range of reduced frequencies (0.54 ≤ k ≤ 1.2). Between the two (undulating) wakes of the two foils, the flow is irrotational and unsteady that can be used to study gust response of, for example, micro-air vehicles (MAVS). The maximum gust velocity that can be generated by this technique is, however, limited to (5–7) % of the mean flow velocity. To enhance the gust intensity, a new method is proposed: introduction of a jet at the trailing edge of each pitching foil. Results show that the addition of the trailing jet enhances the gust intensity by2–7 times for the Re, jet velocity, and the range of reduced frequencies considered. This technique provides a simple, economical, and controlled way to study gust response of MAVs and aircraft in wind tunnels.

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