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

Effects of Micro-Ramps on a Shock Wave/Turbulent Boundary Layer Interaction

Shock wave/turbulent boundary layer interactions (SWTBLIs) are a class of fluid dynamic phenomena that are prevalent in many applications, e.g. helicopter blades, supersonic inlets, missile and aircraft after bodies, etc. The interactions are an important source of drag and can cause turbulent and unsteady separation of the boundary layer, leading to increased heat fluxes and fluctuating pressure loads, which can be severe enough to cause structural damage and premature fatigue of aero-structures. By placing vortex generators (VGs) upstream of a SWTBLI, the detrimental effects of the shock induced separation can be diminished. One particular type of VG is the micro-ramp. Because of their wedge-like shape, micro-ramps are very robust and easy to apply compared to most other VGs. Recently, studies that made use of velocity data in wall-parallel planes have led to renewed insight in the role of variations in the incoming boundary layer on the interaction’s phenomenology. For this reason, stereo-particle image velocimetry (stereo-PIV) measurements in two wall-parallel planes have been performed to investigate the effect of two configurations of micro-ramps, a single row and a staggered array, on an oblique shock reflection on a flat plate at a freestream Mach number of 1.84. The micro-ramps had a height of 20% of the unperturbed boundary layer thickness and the measurement planes were located 0.1 and 0.6 boundary layer thicknesses away from the wall. Additionally, oil-flow patterns at the surface were obtained. The micro-ramps were observed to generate individual vortex pair packets downstream of their vertices that on the mean look like longitudinal streamwise vortex pairs. These structures perturb the incoming boundary layer in such a way, that on the mean low-speed regions exist downstream of the vertices and high-speed regions at intermediate locations. Downstream of low-speed regions in the incoming boundary layer the probability of reversed flow occurrence in the interaction region was found to be higher and the subsonic region to be longer in streamwise direction. These and other results have been used to construct a tentative conceptual model of the effect of micro-ramps on the boundary layer’s and interaction’s topology. Overall, the probability of the occurrence of reversed flow in the interaction region decreased by 20% and 30% downstream of the single row and the staggered configuration respectively. Both configurations of micro-ramps were found to stabilize the shock motion by reducing the length of its motion by about 20% in the lower measurement plane. Farther away from the wall only the single row configuration was found to be effective, leading to a spanwise averaged reduction of 30% in the upper measurement plane. Because of all these benefits, it is recommended to further develop micro-ramp control, whereby the conceptual flow model proposed in the present investigation can provide (part of) an initial framework in which this can be done.Aerospace EngineeringAerodynamic

Effects of micro-ramps on a shock wave/turbulent boundary layer interaction

Stereoscopic particle image velocimetry is used to investigate the effects of micro-ramp sub-boundary layer vortex generators, on an incident shock wave/boundary layer interaction at Mach 1.84. Single- and double-row arrangements of micro-ramps are considered. The micro-ramps have a height of 20% of the unperturbed boundary layer thickness and the measurement planes are located 0.1 and 0.6 boundary layer thicknesses from the wall. The micro-ramps generate packets of individual vortex pairs downstream of their vertices, which produce counter-rotating longitudinal streamwise vortex pairs in a time-averaged view. These structures induce a pronounced spanwise variation of the flow properties, namely the mixing across the boundary layer interface. The probability of reversed-flow occurrence is decreased by 20 and 30% for the single- and double-row configurations, respectively. Both configurations of micro-ramps stabilize the shock motion by reducing the length of its motion by about 20% in the lower measurement plane. The results are summarized by a conceptual model describing the boundary layer’s and interaction’s flow pattern under the effect of the micro-ramps.Aerospace Design, Integration and OperationsAerospace Engineerin

Determination of instantaneous pressure in a transonic base flow using four-pulse tomographic PIV

A tomographic four-pulse PIV system is used in a transonic axisymmetric base flow experiment at a nominal free stream Mach number of 0.7, with the objective to obtain flow acceleration and pressure data. The PIV system, consisting of two double-pulse lasers and twelve cameras, allows acquiring two velocity fields with time separations as small as 2.5 ?s. A performance assessment is carried out and provides a typical average error estimate below 0.025 U? (0.3 voxel). The ability to use these velocity measurements to determine instantaneous, volumetric pressure distributions is assessed. To provide insitu validation, fast-response pressure transducers are used concurrent with the PIV measurements. PIV was found to yield a mean pressure profile with a similar shape as the mean pressure profile obtained with transducers and as reported in literature. The levels of pressure fluctuation were however found to be substantially higher. From this comparison it is concluded that the present PIV-based pressure determination procedure requires improvements to obtain more reliable pressure data. Such improvements may be obtained by optimizing the time separation between consecutive velocity fields and by making use of the temporal coherence between consecutive particle images through more advanced processing algorithms.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Determination of instantaneous pressure in an axisymmetric base flow using time-resolved tomographic PIV

Quantification of mean and fluctuating surface loads is critical for the efficient design of aerospace structures. To measure surface pressure in experiments, wind tunnel models are typically equipped with pressure transducers, which offer high sampling rates and high sensitivity. In order to have a sufficient spatial sampling of the surface pressure such that the instantaneous surface loads can be determined, a large number of transducers is required. From a practical point of view, the installation of transducers can be costly and can pose significant challenges due to spatial limitations inside the wind tunnel model. An alternative for measuring pressure is PIV-based pressure determination [1]. In this approach, PIV data are used to determine the material acceleration, which is related to the local pressure gradient via the momentum equation. Whereas the mean pressure field can be obtained from a series of uncorrelated velocity fields (e.g. [2]), the availability of time-resolved PIV data allow for the determination of instantaneous pressure fields. Recent developments in PIV measurement capabilities, in particular tomographic PIV [3], have made this technique increasingly feasible and appealing. A particular advantage of the technique is that it provides simultaneous velocity and pressure data in the full flow field, thus enabling a better understanding of the relation between fluid dynamics and the corresponding pressure field. The ability of PIV to determine the material acceleration, from which the pressure can subsequently be obtained, has been the subject of extensive study. Using two or more velocity fields closely separated in time, the material acceleration can be determined using traditional Eulerian or Lagrangian formulations (see e.g. [1] for details). An improved estimate of the material acceleration may be obtained using fluid trajectory tracking (FTC) which correlates more than two consecutive reconstructions [4, 5]. The present study builds on these efforts by using time-resolved tomographic PIV to obtain instantaneous pressure distributions in a low-speed axisymmetric base flow. Results are compared to simultaneous unsteady pressure measurements using microphones and mean pressure measurements using static pressure sensors.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Experiments on a hot plume base flow interaction at Mach 2

A wind tunnel model containing a solid rocket motor was tested at Mach 2 to assess the feasibility of investigating the interaction between a hot plume and a high-speed outer stream. In addition to Schlieren visualisation, the feasibility of applying PIV was explored. Recorded particle images revealed that the hot plume scatters and reflects laser light, leading to a strong deterioration of the illumination conditions in regions within and near the plume. Suitable processing of the particle images could partly compensate for this and reliable velocity measurements could be obtained in directly illuminated regions. In regions near the base and below the plume, velocity measurements could still be obtained but were considered to be of lower quality. No reliable velocity measurements could be obtained within the plume itself.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin