The fountain flow effects created by a pair of impinging gas jets
A comprehensive experimental study of fountain flow has been completed. The study was concerned with the characteristics and effects of the flow domain below an idealised small-scale model of the underside of the fuselage of a vertical take-off or landing aircraft with a pair of matched jets. The fountain flow was generated by the normal impact of the jets with a ground board and it in turn impacted with a rectangular blocking tray representing the undersurface of a fuselage. A range of test programmes were carried out. After having examined the fountain flow pressure field and conducted simple flow visualisation tests, a series of tests were undertaken on a range of blocking trays to determine the magnitude and variation of the normal forces on the trays due to the effect of impinging fountain flows. A few tray force tests were carried out with a heated compressed air supply to the jets' nozzles. Following these tests, programmes of pressure distribution tests were carried out on a 'long' tray and on a 'short' tray. Temperature distribution tests were conducted on a long tray, these tests were exploratory only and could form the basis for future experimental studies. The test facility was appraised for jets flowing at high subsonic velocities and found to be suitable but no further tests were undertaken. Finally use was made of computational fluid dynamics (CFD) theoretically to solve the governing Navier-Stokes fluid flow equations for domains which simulated the experimental studies just completed and analysed. The analyses of the data obtained from the test programmes revealed that fountain flow caused towards 20% of jet lift augmentation at maximum with good correlation between direct tray force measurements and integrated tray pressure measurements. A useful plot of the variation of tray force against tray height was obtained from moving ground board tests but the velocity of vertical movement of the board towards the nozzles and tray was found not to make a significant difference for the given range of velocities. The plot displayed a distinct peak when the gap between the tray and the ground board was about 2% jet nozzle diameters. At smaller gaps the tray force fell steeply to a suckdown value approaching 20% of jet lift reduction. A reasoned explanation of this phenomenon is presented. The tray force tests using hot jets indicated that slightly lower tray forces occurred than for jets at ambient temperature, everything else being equal. For the general configuration of the model it was concluded that hot gas tests were not necessary as tests at room temperature gave more useful results for design purposes. The CFD computations were particularly effective in obtaining a qualitative impression of the flow patterns in the blocked fountain flow domain. A useful insight was also gained into entrainment into the domain. The tray force computaions were less satisfactory, however, resulting in underestimations of about 30% although the general pattern of pressure distributions across and along the tray were quite similar experimentally determined patterns. Overall it was concluded that the use of small scale model tests had been shown to be effective for certain design purposes for full-scale aircraft and that a dual approach using CFD and model tests could be more rewarding than either the one or the other technique in isolation.