Zhengzhong Sun

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.

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.

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....

Micro Vortex Generators for Boundary Layer Control: Principles and Applications

A review is given to the major developments in the boundary layer control by means of micro vortex generator (MVG). The appearance of MVG dates back to about three decades from the present review and this class of device rapidly spreads as an efficient and robust boundary layer control strategy for flows with separation in particular. Substantial successful applications of MVG have been achieved in subsonic flows and its application into supersonic flow is under progressive development, which requires the current status and capability of MVG to be reviewed. The fundamental aspects of MVG are first summarized, including its working principle and various geometries. Understanding of the fluid mechanics involved in the wake of a MVG device is of uppermost importance: the time-averaged and instantaneous wakes are discussed respectively. In the former, effects from the MVG geometry parameters are presented; while in the latter, focus is placed on the phenomenon associated with flow instability inside the wake....

Numerical and Experimental Investigations of the Flow behind a Supersonic Micro-Ramp

The flow past a micro-ramp immersed in a supersonic turbulent boundary layer was studied by means of experiments conducted with planar and tomographic PIV and numerical simulations with the implicit large eddy simulation (ILES) technique. The experimental data is mostly used to ascertain the validity of the numerical results. The time-averaged velocity distribution is compared considering the streamwise evolution of the streamwise velocity profile and the transverse velocity pattern induced by the pair of counter-rotating vortices. The comparison yields a satisfactory agreement both in terms of the momentum deficit recovery and of the rate at which the vortex pair is lifted off the surface. The flow behind the micro-ramp 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. This instability is visualized by PIV and is also retrieved in the ILES simulations. The resulting vortices are arch-shaped and induce localized high-speed arcs in the outer region and a deceleration within the wake associated to ejection of low-momentum fluid. The vortical structure of the wake is given specific attention. The streamwise evolution of time-averaged streamwise vortices is characterized in terms of peak vorticity and circulation, both of which exhibit a rapid decay ascribed to the destabilizing effect of the shear layer unsteady fluctuations. The coherent K-H vortices exhibited in the near-wake region undergo further instabilities such as pairing and ultimately breakdown, with abrupt loss of coherence beyond approximately 10 ramp heights. At about the same distance from the ramp trailing edge the streamwise vortices exhibit less coherence, which is ascribed to the interaction between the arc-vortices and the streamwise vortex pair in the later stage.

Numerical simulation of transitional flow on a wind turbine airfoil with RANS-based transition model

ABSTRACT This paper presents a numerical investigation of transitional flow on the wind turbine airfoil DU91-W2-250 with chord-based Reynolds number Rec = 1.0 × 106. The Reynolds-averaged Navier–Stokes based transition model using laminar kinetic energy concept, namely the k − kL − ω model, is employed to resolve the boundary layer transition. Some ambiguities for this model are discussed and it is further implemented into OpenFOAM-2.1.1. The k − kL − ω model is first validated through the chosen wind turbine airfoil at the angle of attack (AoA) of 6.24° against wind tunnel measurement, where lift and drag coefficients, surface pressure distribution and transition location are compared. In order to reveal the transitional flow on the airfoil, the mean boundary layer profiles in three zones, namely the laminar, transitional and fully turbulent regimes, are investigated. Observation of flow at the transition location identifies the laminar separation bubble. The AoA effect on boundary layer transition over wind turbine airfoil is also studied. Increasing the AoA from −3° to 10°, the laminar separation bubble moves upstream and reduces in size, which is in close agreement with wind tunnel measurement.

Modeling and optimization of the multichannel spark discharge

This paper reports a novel analytic model of this multichannel spark discharge, considering the delay time in the breakdown process, the electric transforming of the discharge channel from a capacitor to a resistor induced by the air breakdown, and the varying plasma resistance in the discharge process. The good agreement between the experimental and the simulated results validated the accuracy of this model. Based on this model, the influence of the circuit parameters on the maximum discharge channel number (MDCN) is investigated. Both the input voltage amplitude and the breakdown voltage threshold of each discharge channel play a critical role. With the increase of the input voltage and the decrease of the breakdown voltage, the MCDN increases almost linearly. With the increase of the discharge capacitance, the MDCN first rises and then remains almost constant. With the increase of the circuit inductance, the MDCN increases slowly but decreases quickly when the inductance increases over a certain value. There is an optimal value of the capacitor connected to the discharge channel corresponding to the MDCN. Finally, based on these results, to shorten the discharge time, a modified multichannel discharge circuit is developed and validated by the experiment. With only 6-kV input voltage, 31-channels discharge is achieved. The breakdown voltage of each electrode gap is larger than 3 kV. The modified discharge circuit is certain to be widely used in the PSJA flow control field.

Shock wave boundary layer interaction controlled by surface arc plasma actuators

An array of 16 surface arc plasma actuators (SAPAs) is employed to control the shock wave boundary layer interaction (SWBLI) at a 26° compression ramp in a Mach 2.0 flow. A new electrical circuit is used to actuate all 16 SAPAs. The electrical measurement reveals significant augmentation in peak current (200 A) and an energy deposition of 1.05 J, which are the nominal characteristics of the setup. The SAPA array is later applied for SWBLI control. The actuator array is placed upstream of the SWBLI and operates at four different frequencies, namely, 500 Hz, 1 kHz, 2 kHz, and 5 kHz. In the wind tunnel experiment, high-speed schlieren at 25 000 frames per second is used for flow visualization. The shock wave system is modified significantly by the controlling gas blobs (CGBs) or controlling gas bulbs (CGBUs) generated by SAPAs. The foot portion of the separation shock wave disappears, and the oblique shock wave bifurcates when the CGBs pass through the interaction region. The shock weakening effect is further verified through the rms of the schlieren intensity of the same phase.