F.F.J. Schrijer

The three-dimensional flow organization past a micro-ramp in a supersonic boundary layer

The three-dimensional instantaneous flow organization in the near wake of a micro-ramp interacting with a Mach 2.0 supersonic turbulent boundary layer is studied using tomographic particle image velocimetry. The mean flow reveals a wake with approximately circular cross section dominated by a pair of counter-rotating streamwise vortices generating a focused upwash motion at the symmetry plane. In the instantaneous flow organization a flow instability of Kelvin-Helmholtz (K-H) type is observed in the shear layer between the wake and outer flow. Intermittent arc-shaped vortices are visualized that locally accelerate the outer fluid and further decelerate the inner region. The streamwise vortex pair displays an undulating behavior. Their interaction with the K-H vortices considerably increases the overall complexity of the wake. It appears that the streamwise vortex filaments under the K-H vortex train approach each other due to the focused ejection activity resulting from the K-H vortex. The statistical properties of turbulent fluctuations yield maximum activity at the core of the streamwise vortex and within the upwash region, and the Reynolds stresses peak within the shear layer. The topological organization of the wake vortices is formulated through a conditional average over the vorticity field.

Flow control of an oblique shock wave reflection with micro-ramp vortex generators: Effects of location and size

This study investigates the influences of micro-ramp size and location on its effectiveness as a flow control device for oblique shock wave reflections. The effectiveness is measured in terms of the size of the shock-induced separation bubble and the reflected shock unsteadiness. Particle image velocimetry measurements were carried out on the interaction region and the mixing region between micro-ramp and interaction. The separation bubble is shown to be most sensitive to the momentum flux contained in the lower 43% of the incoming boundary layer. The momentum flux added to this region scales linearly with micro-ramp height and larger micro-ramps are shown to be more effective in stabilizing the interaction. Full boundary layer mixing is attained 5.7δ downstream of the micro-ramp and this forms a lower limit on the required distance between micro-ramp and the start of the interaction region. Typical reductions in the average separated area and the shock unsteadiness of 87% and 51%, respectively, were reco...

Decay of the supersonic turbulent wakes from micro-ramps

The wakes resulting from micro-ramps immersed in a supersonic turbulent boundary layer at Ma = 2.0 are investigated by means of particle image velocimetry. Two micro-ramps are investigated with height of 60% and 80% of the undisturbed boundary layer, respectively. The measurement domain is placed at the symmetry plane of the ramp and encompasses the range from 10 to 32 ramp heights downstream of the ramp. The decay of the flow field properties is evaluated in terms of time-averaged and root-mean-square (RMS) statistics. In the time-averaged flow field, the recovery from the imparted momentum deficit and the decay of upwash motion are analyzed. The RMS fluctuations of the velocity components exhibit strong anisotropy at the most upstream location and develop into a more isotropic regime downstream. The self-similarity properties of velocity components and fluctuation components along wall-normal direction are followed. The investigation of the unsteady large scale motion is carried out by means of snapshot analysis and by a statistical approach based on the spatial auto-correlation function. The Kelvin-Helmholtz (K-H) instability at the upper shear layer is observed to develop further with the onset of vortex pairing. The average distance between vortices is statistically estimated using the spatial auto-correlation. A marked transition with the wavelength increase is observed across the pairing regime. The K-H instability, initially observed only at the upper shear layer also begins to appear in the lower shear layer as soon as the wake is elevated sufficiently off the wall. The auto-correlation statistics confirm the coherence of counter-rotating vortices from the upper and lower sides, indicating the formation of vortex rings downstream of the pairing region.

PIV investigation of the 3D instantaneous flow organization behind a micro-ramp in a supersonic boundary layer

The flow field resulting from a single micro-ramp in a Ma=2.0 supersonic boundary layer is investigated using tomographic particle image velocimetry (Tomo-PIV). The measurements were carried out within two volumes behind the micro-ramp. Mean flow characteristics are analyzed, evidence of the streamwise vortex pair is given. In the instantaneous flow, a Kelvin-Helmholtz (K-H) instability developing in the wake of the element is revealed, and the K-H vortices are further identified. A conceptual model is provided to describe the instantaneous flow organization generated by a single micro-ramp.

Experiments on Hypersonic Boundary Layer Separation and Reattachment on a Blunted Cone-Flare using Quantitative InfraRed Thermography.

Transient heat transfer measurements have been carried out on a blunted coneflare model in a short duration hypersonic facility at Mach 9 using quantitative infrared thermography (QIRT). The surface temperature transient was measured using an infrared camera and the temperature data were successfully correlated to convective heat transfer using two distinctly different data reduction techniques. Measurements were performed at different Reynolds numbers to investigate the phenomenon of boundary layer separation and reattachment in the hypersonic flow regime. A smooth nose and a roughened one were used to investigate the effect of roughness induced boundary layer turbulence on flow separation and reattachment. For both configurations the length of the separated region was found to decrease with increasing Reynolds number. Along the flare a monotonic increase in heat transfer is measured. Both QIRT and schlieren visualizations suggest that flow transition occurs along the flare.

Experimental Investigation of the Supersonic Wake of a Reentry Capsule

The wake behind an Apollo shaped capsule is investigated in the framework of the ’afterbody heating’ topic in the RTO WG043 working group. Measurements are performed by means of schlieren, shadowgraphy and stereo particle image velocimetry (SPIV) and are used for CFD validation purposes. The model geometry is a scaled version of the AS-202. It was found from the shadowgraph results that the angle of attack where separation occurs decreases with increasing Mach number and is not influenced by the Reynolds number. Furthermore, it was possible to correlate the shear layer transition location using Re2,xt , which is the Reynolds number based on post normal shock conditions where the length scale is the flow path from stagnation point to transition point. SPIV measurements were obtained at Mach 2 and 0o and 25o angle of attack. For the 0o model, the wake was completely separated while for the 25o model the wake was partially separated and reattaches half way the model. Overall the PIV data return a quantitative three dimensional description of the velocity field around the capsule.

Low-frequency behavior of the turbulent axisymmetric near-wake

The turbulent wake past an axisymmetric body is investigated with time-resolved stereoscopic particle image velocimetry (PIV) at a Reynolds number ReD = 6.7 × 104 based on the object diameter. The azimuthal organization of the near-wake is studied at different locations downstream of the trailing edge. The time-averaged velocity field features a circular shear layer bounding a region of recirculating flow. Inspection of instantaneous PIV snapshots reveals azimuthal meandering of the reverse flow region with a significant radial offset with respect to the time-averaged position. The backflow meandering appears as the major contribution to the near-wake dynamics in proximity of the base, whereas closer to the rear-stagnation point, the shear layer fluctuations become important. For x/D ≤ 0.75, the time-history and probability distributions of the backflow centroid position allow to identify this motion with an irregular precession about the model symmetry axis occurring at time scales in the order of 103 D/...

Numerical and Experimental Investigations of the Supersonic Microramp Wake

The flow past a microramp immersed in a supersonic turbulent boundary layer is studied by means of numerical simulations with the implicit large-eddy simulation technique and experiments conducted with tomographic particle image velocimetry. The experimental data are mostly used to verify the validity of the numerical results by ample comparisons on the time-averaged velocity, turbulent statistics, and vortex intensity. Although some discrepancies are observed on the intensity of the upwash motion generated by the streamwise vortex pair, the rates of the recovery of momentum deficit and the decay of streamwise vortex pair intensity are found in good agreement. The instantaneous flow organization is inspected, making use of the flow realizations available from implicit large-eddy simulation. The flow behind the microramp exhibits significant large-scale unsteady fluctuations. Notably, the quasi-conical shear layer enclosing the wake is strongly undulated under the action of Kelvin–Helmholtz (K–H) vortices....

Investigation of Görtler vortices in a hypersonic double compression ramp flow by means of infrared thermography

An experimental investigation is performed on the occurrence of Görtler vortices in a hypersonic flow by means of infrared thermography. A double compression ramp model with varying second ramp angle is tested at Mach 7.5 in a hypersonic Ludwieg tube. Due to the concave curvature of the streamlines in the separated region, a centrifugal instability exists that is responsible for generating stream-wise vortices. In the investigation vortices are generated with varying span- wise wavelengths by means of a comb-shaped element near the nose of the model. Using infrared thermography the amplification of these vortices is measured so that a critical wavelength and Görtler number can be identified. It is found that the vortex growth rate is the highest for the largest second ramp angle. Furthermore it is found that the growth rate decreases with increasing Görtler number.

Afterbody Effects on Axisymmetric Base Flows

The influence of an afterbody on an axisymmetric wake flow is investigated with two-component particle image velocimetry at a Reynolds number of ReD=6.7·104 based on the model diameter. The reattachment distance is found to vary from two to four step heights as the afterbody diameter is increased. Measurements performed on equivalent planar backward-facing step geometries reveal that the reattachment length scales similarly to a planar backward-facing step when the afterbody to main-body diameter ratio approaches unity. The distribution of turbulent normal and shear stresses reveals a progressive inhibition of the radial fluctuations for increasing afterbody diameter, whereas a proper orthogonal decomposition analysis highlights a concurrent weakening of the shear-layer flapping mode. For d/D≤0.4, the frequency spectrum of the time coefficient of the first proper orthogonal decomposition mode appears associated with the wake flapping motion and peaks in the range of StD=10−3−10−2. This frequency is associ...