The laminar vortex shedding of airflow behind a circular cylinder with different heating temperatures was experimentally investigated with emphasis on the relationship of wake frequency and the Reynolds number. A new method to generate the two-dimensional parallel vortex shedding for the heated cylinder was developed and tested. An “effective Reynolds number” that employs a kinematic viscosity computed from an “effective temperature” is used to account for the temperature effects on the vortex shedding frequency. The present result shows that the frequency data could be successfully collapsed with the effective temperature computed by Teff=T∞+0.28(TW−T∞) for a wide range of cylinder temperatures, T∞ and TW being the free-stream temperature and cylinder surface temperature, respectively. Moreover, the relationship between Strouhal number and effective Reynolds number was found to be “universal.” The physical interpretation of Teff and the applicable region of the St–Reeff curve are discussed.
The “hybrid synthetic jet” combines the zero-net-mass-flux synthetic jet and fluidic pumping through a valveless pump. No fluid is supplied from an external source (blower or compressor). Hot-wire experiments on a demonstration model confirm that the hybrid synthetic jet exhibits a higher extrusion volume flow rate than the ordinary synthetic jet. For the tested configuration, the increase is by 25% at the 1.30 ratio of the extrusion and suction volume fluxes.
An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble.
D
A synthetic jet (SJ) is a fluid flow that is created from an oscillatory process of suction and blowing. A hybrid synthetic jet (HSJ) combines this principle with fluidic pumping through a valveless pump. The present study addresses round HSJs issuing into quiescent surroundings from an actuator orifice 8 mm in diameter. For comparison purposes, a common (zero-net-mass-flux) SJ is used. The working fluid is air, and the maximum Reynolds numbers are 11,000 and 9,000 for HSJs and SJs, respectively. The following five experimental methods are employed: flow visualization using a smoke wire technique, velocity measurements using a hot-wire anemometer, velocity measurements using a Pitot tube, impedance measurements of the actuators, and measurements of the jet momentum using precision scales. Flow visualization demonstrates phase-locked flow fields. The first resonance frequencies are theoretically derived to be 79 and 98 Hz for an SJ and HSJ, respectively. These values are confirmed by all of the experimental methods used. The results demonstrate the advantages of HSJs. The tested HSJ achieves a 24 % higher pumped volume flow rate in comparison to the SJ at a maximum volumetric efficiency of 33 %. Moreover, the overall energy efficiency of the HSJ actuator is 1.8 times higher than that of the SJ actuator. These promising HSJ features, including significantly higher efficiencies, can be useful for various heat transfer applications such as the cooling of highly loaded electronic devices.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.