Unsteady Shock Motion Research Articles

Leading-edge sweepback and shape effects on fin-induced fluctuating pressures

Measurements of the fluctuating wall pressure have been made on centerline upstream of blunt fins in a Mach 5 turbulent boundary layer. Leading-edge sweep considerably reduces the mean and rms pressure loading at the fin root, the extent of the region of unsteady separation shock motion (i.e., the intermittent region), and the separation length. The frequency of shock-induced pressure fluctuations in the intermittent region increases with leading-edge sweepback, while frequency spectra in the separated region are virtually unchanged. A strake at the root of an unswept fin has virtually no effect, whereas a swept blunted root fillet reduces the upstream influence and intermittent region lengths by 50%, and reduces the mean and rms pressure loading at the fin root by about 75% and 95%, respectively. Experiments using hemicylindrical, wedge-shaped, and flat leading edges show that separated-flow scales and root loading increase with increasing bluntness (i.e., wedge to hemicylinder to flat), while the intermittent-region length increases (in terms of fin thicknesses). The changes in flowfield unsteadiness are related to changes in separated-flow structure which alter the dynamics of the primary vortex and recirculation process.

On the mechanism of unsteady shock oscillation in shock wave/turbulent boundary layer interactions

An experimental investigation into the mechanism of shock wave oscillation in compression ramp-generated shock wave/turbulent boundary layer interactions is presented. Particular emphasis is focused upon documenting the respective roles played by both burst-sweep events in the turbulent boundary layer immediately upstream of the interaction and the downstream separated shear layer upon unsteady shock front motion. Unlike the majority of compression ramp experiments which involve bulk separation and large-scale shock motion, consideration is given here to comparatively “weak” interactions in which the streamwise spatial excursion of the shock front is always less than one boundary layer thickness. In this manner any shock motion due to upstream burst-sweep events should be more apparent in relation to that oscillation associated with the separated region. A discrete Hilbert transform-based conditional sampling technique is used to obtain wall pressure measurements conditioned to burst-sweep events. The conditional sampling technique forms the basis by which the instantaneous shock motion is conditioned to the occurrence of upstream bursting. The relationship between the separation bubble and shock motion is also explored in detail. The results of the experiments indicate that the separation bubble represents a first-order effect on shock oscillation. Although it is demonstrated theoretically that the burst-sweep cycle can also give rise to unsteady shock motion of much lower amplitude, the experiments clearly demonstrate that there is no discernible statistical relationship between burst events and spanwise coherent shock front motion.

Effect of periodic blowing on attached and separated supersonic turbulent boundary layers

Periodic blowing at frequencies up to 5 kHz was used to control the unsteadiness of two-dimensional shockwave/turbulent boundary-layer interactions. Two separate experiments were performed. In the first case, periodic blowing was introduced through a spanwise slot in the wall to produce an unsteady shock-wave/boundarylayer interaction boundary layer on the tunnel wall. In the second case, periodic blowing was introduced into the shock-induced separation bubble formed by a 24-deg compression corner interaction. The incoming flow conditions for both experiments were My. = 2.84, Rejl = 6.5 x 10 7/m, and 80 = 26 mm. Measurements of the fluctuating mass flux and wall pressure were made, and the unsteady flowfield was visualized through stroboscopic schlieren videography. The measurements were conditionally sampled based on the phase of the blowing cycle. The results suggest that (at least in this case) the naturally unsteady shock motion in the compression ramp interaction does not contribute significantly to the turbulence amplification, as had been previously thought. Instead, there is strong evidence to suggest that large-scale motions associated with the maxima in the angular momentum profiles in the downstream boundary layer are responsible for the large mixing observed.

Analytical and computational study of unsteady shock motion on hypersonic forebodies

This work examines the behavior of a hypersonic bow shock subject to unsteady surface motions. In particular, the relative motion of a hypersonic bow shock is examined, subject to normal surface oscillations, for a wide range of frequencies. There are two analytical limits of response: a low-frequency, quasisteady regime where shock motion follows the surface pertubations; and a high-frequency, compression-expansion range where the finite disturbances generated by surface motions cancel out before reaching the shock

Flow Unsteadiness by Weak Normal Shock Wave/Turbulent Boundary Layer Interaction in Internal Flow

The interaction of a weak normal shock wave with a turbulent boundary layer developing along the wall surface of a supersonic diffuser was investigated experimentally, where the flow Mach number was 1.14∼1.53 and the Reynolds number based upon boundary layer thickness was 1.35∼2×104. Detailed streamwise surface pressures were measured, and a number of data sets obtained by wall pressure transducers were statistically analyzed. The power spectral density function, the intermittency and the higher-order moments of the surface pressure fluctuations were taken. The objective of the present experiment was to investigate the behaviors of the oscillations caused by the normal shock wave/turbulent boundary layer interaction (STBI). The results show that the intermittent feature of the unsteady shock motion is similar to those in external flows and that the maximum standard deviations correspond to points where the intermittency factor approaches 0.5 and are significantly influenced by flow separation. Also the present study indicates that far downstream from the STBI region the amplitude of pressure fluctuations is nearly independent of the flow Mach number.

Turbulence structure in a shock wave/turbulent boundary-layer interaction

The boundary-layer turbulence structure in a two-dimensional 24-deg compression corner flow was investigated experimentally in the Princeton University 203 x 203-mm supersonic blowdown wind tunnel. The incoming conditions were M<» = 2.84, Re^/l = 6.5 x 10, and 60 = 26 mm. The results show that the maximum mass-flux turbulence intensity is amplified by a factor of about 5, whereas the mean mass flux increases by only 2 through compression. Probability density functions (pfd's) of the mass-flux fluctuations show that upstream the distributions are Gaussian-like and typical of a fully turbulent boundary layer. Behind the interaction, however, the distributions are bimodal, centering about a level indicative of the freestream mass flux and a lower level indicative of the mass flux near the wall. The unsteady shock motion does not appear to contribute signficantly to the turbulence amplification. It is suggested that the strong, bimodal mixing indicated in the pdf's is probably caused by the presence of large-scale motions associated with the instability of the inflectional velocity profiles observed downstream of the interaction. Space-time correlations of the mass flux and wall pressure were measured, as well as the level of intermittency of the boundary layer to further characterize the changes in the boundary-layer structure through the interaction.