Development of Hypersonic Quiet Tunnels

a = speed of sound e = integrated amplification ratio for a linear instability, usually from location of first instability to transition k = roughness height Mw = Mach number at the nozzle wall, at the boundarylayer edge M1 = Mach number in the freestream N factor = ln A=A0, the linear-theory amplification ratio from onset of instability A0 usually to measured transition A Pmean = mean pitot pressure Pt = stagnation or total pressure P0 = rms pitot-pressure fluctuations p = mean static pressure ~ p = rms static-pressure fluctuations Ree = Reynolds number based on conditions at the boundary-layer edge Rek = Reynolds number based on roughness height k and local conditions in the undisturbed laminar boundary layer at the height k Re x = quiet length Reynolds number based on freestream conditions and x Re = Reynolds number at transition (usually onset) based on momentum thickness and conditions at the boundary-layer edge Re1 = unit Reynolds number in the freestream; units given where used U = freestream velocity x = axial coordinate for 2-D nozzles, usually from the nozzle throat y = vertical coordinate, in the curvature plane for 2-D nozzles z = for 2-D nozzles, a horizontal coordinate perpendicular to the flat side walls for axisymmetric nozzles, an axial coordinate from the nozzle throat s = length along cone from the nosetip to the location in which nozzle wall radiated noise reaches the cone x = length along nozzle centerline from the onset of uniform flow to the location in which nozzle-wall radiated noise reaches the centerline Introduction C ONVENTIONAL hypersonic wind tunnels and shock tunnels suffer from high levels of freestream fluctuations, which are typically 1 to 2 orders of magnitude above flight levels. These freestream fluctuations are generally dominated by acoustic noise radiated from the turbulent boundary layers on the nozzle walls. Although the effects of the noise are often small, and so can be neglected, this noise often has a dramatic effect on laminar-turbulent transition on models, and it can have a significant effect on other phenomena as well. Quiet-flow wind tunnels provide uniform flow at supersonic and hypersonic speeds with low noise levels comparable to flight. They have been sought for more than 50 years [1]. Low-noise subsonic tunnels were essential to the discovery of the Tollmien–Schlichting waves that lead to low-speed transition on flat plates and many airfoils [2,3]. Low turbulence in subsonic tunnels is often essential to achieving flow conditions representative of flight. Transition is one factor that affects scaling from ground to flight, and tunnel noise was long known to be important for supersonic tunnels also [4]. Thus, the development of low-noise tunnels at supersonic and hypersonic speeds was a natural extension of earlier work. However, it has been much more difficult to develop comparable low-noise facilities at high speeds, for four main reasons. First, supersonic and hypersonic tunnels are much more expensive to build, modify, and operate, and so the inevitable resource limitations make progress much more difficult. Second, instrumentation for measurement of freestream fluctuations is also much more difficult and expensive at high speeds, due to the high pressures, high temperatures, and high disturbance frequencies associatedwith highspeed flow. Third, the dominant source of noise in most high-speed tunnels turns out to be acoustic waves radiated from the turbulent boundary layers on the nozzle walls, a phenomena that has proven to be very difficult to control. Fourth, the need for hypersonic quietflow is obvious only for certain vehicle designs that depend heavily on the location of transition, and the market for quiet tunnels is therefore often assessed as too small to justify the investment. A visual example of the noise radiated from a turbulent boundary layer is shown in Fig. 1, which shows a magnified portion of a shadowgraph obtained in the Naval Ordnance Lab ballistics range [5]. The sharp cone model is flying at Mach 4.3 near zero angle of attack, at a freestream Reynolds number of 3:2 10 ft 1