Whereas the heat transfer mechanisms in steady impinging jets are well understood, the available knowledge of heat transfer to impinging synthetic jets remains inconsistent. This paper provides an objective comparison of the stagnation point heat transfer performance of axisymmetric impinging synthetic jets versus established steady jet correlations. Furthermore, a general correlation for the stagnation point Nusselt number is proposed including the effect of all appropriate scaling parameters: Reynolds number (500 ≤ Re ≤ 1500), jet-to-surface spacing (2 ≤ H/D ≤ 16) and stroke length (2 ≤ /D ≤ 40). Based on the ratio of stroke length to jet-to-surface spacing L L 0 0 /H, four heat transfer regimes are identified.

Impinging synthetic jets have been identified as a promising technique for cooling miniature surfaces like electronic packages. This study investigates the relation between the convective heat transfer characteristics and the impinging synthetic jet flow structure, for a small jet-to-surface spacing H/D = 2, dimensionless stroke length 1 < L 0 /D < 22, and Reynolds number 1000 < Re < 4300. The heat transfer measurements show evidence for a power law relationship between the Reynolds and Nusselt number for a constant stroke length. A critical stroke length L 0 /H = 2.5 has been identified. Using phase-resolved particle image velocimetry, vortex quantification is applied to elucidate the influence of the impinging vortex on the timeaveraged heat transfer distribution.

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-Impinging synthetic jets have excellent potential for energy-efficient local cooling in confined geometries. For a given geometry, synthetic jet flows are mainly characterised by the Reynolds number and the ratio of stroke length to a geometric length scale. The flow field of an impinging synthetic jet and the corresponding surface heat transfer distribution are strongly dependent on the dimensionless stroke length, yet few studies have investigated the flow field dependence for a wide range of stroke lengths. Therefore, the aim of this paper is to identify the various flow regimes as a function of stroke length. The experimental approach combines high speed particle image velocimetry and single point hot wire anemometry, and investigates an 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Purpose: The aim of this work was to evaluate the influence of crystal habit on the dissolution and in vitro antibacterial and anitiprotozoal activity of sulfadimidine:4-aminosalicylic acid cocrystals.Methods: Cocrystals were produced via milling or solvent mediated processes. In vitro dissolution was carried out in the flow-through apparatus, with shadowgraph imaging and mechanistic mathematical models used to observe and simulate particle dissolution. In vitro activity was tested using agar diffusion assays.Results: Cocrystallisation via milling produced small polyhedral crystals with antimicrobial activity significantly higher than sulfadimidine alone, consistent with a fast dissolution rate which was matched only by cocrystals which were milled following solvent evaporation.Cocrystallisation by solvent evaporation (ethanol, acetone) or spray drying produced flattened, plate-like or quasi-spherical cocrystals, respectively, with more hydrophobic surfaces and greater tendency to form aggregates in aqueous media, limiting both the dissolution rate and in vitro activity. Deviation from predicted dissolution profiles was attributable to aggregation behaviour, supported by observations from shadowgraph imaging. Conclusions: Aggregation behaviour during dissolution of cocrystals with different habitsaffected the dissolution rate, consistent with in vitro activity. Combining mechanistic models with shadowgraph imaging is a valuable approach for dissolution process analysis.

Synthetic jets are used for active flow control and enhanced heat transfer, and are typically generated by an orifice connected to a cavity with movable diaphragm actuator. Low-power operation is achieved by matching actuator and Helmholtz resonance frequencies. This brief communication presents an analytical model derived from simplified gas dynamics, for estimating the synthetic jet velocity and actuator deflection, based on a cavity pressure measurement. Model closure is provided by a damping force in the orifice, which agrees with established pressure loss correlations for steady flow through short ducts. The model is validated against experimental data obtained for an axisymmetric synthetic jet. The valid frequency range extends from zero, over the Helmholtz resonance frequency, up to a geometry-dependent limit frequency. This model presents a reference against which synthetic jet velocity can be calibrated. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2823560͔ Synthetic or zero-net mass flux jets are being studied in various fields of fluid dynamics, from active flow control to enhanced heat transfer. A synthetic jet is produced by the interaction of a train of vortices that form by successive ejection and suction of fluid across an orifice. The orifice flow is forced by periodic pressure variations, typically generated in a cavity with a movable diaphragm. Optimum efficiency is achieved by matching the mechanical resonance frequency of the actuator to the Helmholtz resonance frequency of the gas dynamic cavity-orifice system.An unconfined synthetic jet flow is characterized by two parameters: the stroke length L 0 / d and the Reynolds number, and U͑t͒ is the area-averaged orifice velocity, although sometimes 1 the centerline velocity is used instead.As such, setting the operating point of a synthetic jet requires knowledge of the jet velocity, which is only measurable using advanced methods such as laser-Doppler anemometry, particle image velocimetry, and hot-wire anemometry. Therefore, a calibration is typically performed to determine the relationship between the synthetic jet velocity and the actuator input voltage. However, this relationship is subject to actuator degradation and other external influences. To overcome this problem, some researchers 1,2 suggest a calibration of velocity versus cavity pressure instead.Smith and Glezer 1 indicate that for a pair of closely spaced synthetic jets, the velocity is influenced by the presence of the adjacent jet, particularly when the jets are driven out of phase. For that reason, they recommend a pressurevelocity calibration. Smith and Glezer 2 use a synthetic jet to deflect a primary steady jet. A pressure-velocity calibration is used to characterize the synthetic jet, recording several calibration curves for different frequencies. Smith and Glezer 2 note that changes in the primary flow affect the pressurevelocity relationship.Crittenden and Glezer 3 describe a compressible flow synthetic jet based on a piston-crank arrangement, characterizing the ...