Change of Tunnel Conditions
In ETW the speed at which the various parameters are changed from one condition to another has been optimized to achieve high efficiency and cost effectiveness of the test.
The duration of a set point change during a run is very much dependent on the nature of the change, the dynamic behaviour of the tunnel, the response time and the accuracy of the control systems, the characteristics of the control inputs which activate the change and various limits defined by the structure of the tunnel, the operator and the client.
The primary control elements are the compressor, the liquid nitrogen injection, the gaseous nitrogen exhaust, the second throat and the supersonic nozzle.
- The compressor’s primary role is to provide a pressure ratio to drive the test gas through the tunnel, to compensate for the friction losses within the circuit and to keep the second throat or the supersonic nozzle, when they are active, in choked conditions (i.e. maintain local sonic flow). The control parameter is the rotational speed of the compressor.
- The liquid nitrogen injection compensates for the energy dissipated by the flow and thus can be used to control the temperature.
- The gaseous nitrogen exhaust controls the venting of nitrogen to the atmosphere. It combines with the compressor rotational speed and the liquid nitrogen mass flow to control the stagnation pressure of the tunnel.
- Up to Mach 0.7 (and up to Mach 1 if the second throat is kept fully open) the Mach number is primarily adjusted by the compressor speed.
- From Mach 0.7 to Mach 1 the second throat can be used to control the Mach number in the test section.
- Above Mach 1, the supersonic nozzle is the main control element for the Mach number.
The set point change for temperature in the tunnel circuit is by far the most complicated and most time consuming. A general rule for organizing the sequence of a test campaign with high accuracy requirements must therefore be, to change all other flow parameters at a constant temperature level, before changing the temperature. In order to save liquid nitrogen, this should be done whenever possible with increasing temperature levels, starting with the coldest condition, and spending sufficient time for thermal equilibrium.