Prediction of pressure characteristics in setting chamber of 0.6m wind tunnel for supersonic testing

摘要

It is proposed to augment the NAL 0.6m wind tunnel with a variable Mach number flexible Nozzle (VMFN)to enhance the testing capability from transonic to supersonic Mach numbers(up to 4.0).In order to avoid the star stop loads that are inherent in blow down wind tunnels, it is proposed to start the tunnel at a low Mach number(say 1.0)and then increasing the Mach number by reducing the nozzle throat(maximum Mach number =4.0)by continuously flexing the nozzle walls; the reverse process is to be adopted while stopping in such an operation two important issues arise. Firstly, the setting chamber pressure should always be maintained above the minimum 'running ' pressure at any supersonic Mach number to avoid flow breakdown in the test section. Secondly,in order to maintain the free-stream dynamic pressure constant during the useful runtime and within desirable limits during the transition from Mach 1.0 to 4.0 and vice versa, the pressure regulating valve(PRV)must be operated in a closed-loop pressure control. In the problem is formulated based on assumptions of quasi-steady is entropic elations and a program is presented in C language to study the nature of variation of stagnation pressure in the settling chamber for various trajectories of the results predicated from the program and experimental data obtained at subsonic Mach numbers in the existing 0.6m wind tunnel are shown. Predictions for VMFN operation show that the setting chamber pressure rapidly builds up towards the value of storage tank pressure.When the VMFN nozzle throat reduces from Mach 1.0 to Mach 4.0 condition.Presumably due to constriction of the flow when the VMFN nozzle throat reduces from Mach 1.0 to 4.0 condition.Presumably due to constriction of the flow passage at the first throat.Likewise. the pressure rapidly falls when the VMFN reverses from Mach 4.0 to 1.0 condition by suitably controlling the initial opening and the trajectory of opening and closing of the PRV it is possible to ensure that the stagnation pressure in the setting chamber is always greater than the minimum (renning) pressure that is necessary for stable flow in the test section.However it is seen that during the transition from Mach 1.0 to 4.0 and vice versa the free-stream dynamic pressure overshots to relatively high values, which has significance on model and balance design for aerodynamic force and moment measurements at supersonic Mach numbers in the 0.6m wind tunnel