The effects of pulse frequency and duty cycle on the skin friction downstream of pulsed jet vortex generators in an adverse pressure gradient turbulent boundary layer

An investigation on the viability of pulsed jets as active vortex generator devices was conducted. The devices were installed and tested on an adverse pressure gradient turbulent boundary layer designed to simulate the suction side of a conventional aircraft wing. Both co-rotating and counter-rotating jet geometries were used. The duty cycle and frequency of pulsation were varied and their effects were investigated by measuring the skin friction gains at a predefined location (the location of the minimum skin friction for the un-actuated situation) on the adverse pressure gradient turbulent boundary layer. Pulsing the jets proved to be successful in increasing the wall skin friction and therefore potentially delaying separation. The improvements in wall shear stress were approximately proportional to the duty cycle. The frequency of jet pulsation was found to be important for attaining optimal gains, however no clear relationship between frequency and shear stress gain was observed. Phase averaged wall shear stress measurements far downstream of actuation indicate that quasi-steady structures are introduced by the vortex generators when actuating with a sufficiently high pulse frequency. In this situation interactions between successive structures produced by the jets were likely to be occurring.     

Comparison between d- and lower e-region electron density profiles and IRI-79

The D and E-region electron density profiles obtained by different techniques are compared with the IRI-79 model to see how they fit. The rocket data showed good agreement. However discrepancies between the observed and model values were found especially for solar zenith angle greater than 50 degrees.     

On the survivability and detectability of terrestrial meteorites on the moon

Materials blasted into space from the surface of early Earth may preserve a unique record of our planet's early surface environment. Armstrong et al. (2002) pointed out that such materials, in the form of terrestrial meteorites, may exist on the Moon and be of considerable astrobiological interest if biomarkers from early Earth are preserved within them. Here, we report results obtained via the AUTODYN hydrocode to calculate the peak pressures within terrestrial meteorites on the lunar surface to assess their likelihood of surviving the impact. Our results confirm the order-of-magnitude estimates of Armstrong et al. (2002) that substantial survivability is to be expected, especially in the case of relatively low velocity (ca. 2.5 km/s) or oblique (相似文献   

Probability of Detecting a Swerling I Targetet on Two Correlated Observations

Modern radars characterized by electronically steered beams, frequency agility, and range-ambiguous waveforms can use a processing technique whereby each new detection is followed by a second dwell to verify the initial detection. The second dwell is used to minimize false alarms and to resolve range and/or Doppler ambiguities. Under the assumption of a Swerling I target model, the target cross section remains constant when both dwells occur on the same scan and the same transmission frequency is used. Analytic expressions have been developed for the probability of detecting a Swerling I target on both of the correlated dwells (same target cross section). These expressions are used to calculate the probability of a second dwell detection given a first dwell detection and the probability of at least one detection on two dwells. These probabilities are compared with those of independent dwells (independent target cross sections) which result when two transmission frequencies of sufficient separation are utilized.