Precursors of backflow events and their relationship with the near-wall self-sustaining process

This study examines the precursors and consequences of rare backflow events at the wall using direct numerical simulation of turbulent pipe flow with a high spatiotemporal resolution. The results obtained from conditionally averaged fields reveal that the precursor of a backflow event is the asymmetric collision between a high- and a low-speed streak (LSS) associated with the sinuous mode of the streaks. As the collision occurs, a lifted shear layer with high local azimuthal enstrophy is formed at the trailing end of the LSS. Subsequently, a spanwise or an oblique vortex spontaneously arises. The dominant nonlinear mechanism by which this vortex is engendered is enstrophy intensification due to direct stretching of the lifted vorticity lines in the azimuthal direction. As time progresses, this vortex tilts and orientates towards the streamwise direction and, as its enstrophy increases, it induces the breakdown of the LSS located below it. Subsequently, this vortical structure advects as a quasi-streamwise vortex, as it tilts and stretches with time. As a result, it is shown that reverse flow events at the wall are the signature of the nonlinear mechanism of the self-sustaining process occurring at the near-wall region. Additionally, each backflow event has been tracked in space and time, showing that approximately 50 % of these events are followed by at least one additional vortex generation that gives rise to new backflow events. It is also found that up to a maximum of seven regenerations occur after a backflow event has appeared for the first time.

On mixing enhancement by secondary baroclinic vorticity in a shock–bubble interaction

To investigate the intrinsic mechanism for mixing enhancement by variable-density (VD) behaviour, a canonical VD mixing extracted from a supersonic streamwise vortex protocol, a shock–bubble interaction (SBI), is numerically studied and compared with a counterpart of passive-scalar (PS) mixing. It is meaningful to observe that the maximum concentration decays much faster in a VD SBI than in a PS SBI regardless of the shock Mach number ( $Ma=1.22 - 4$ ). The quasi-Lamb–Oseen-type velocity distribution in the PS SBI is found by analysing the azimuthal velocity that stretches the bubble. Meanwhile, for the VD SBI, an additional stretching enhanced by the secondary baroclinic vorticity (SBV) production contributes to the faster-mixing decay. The underlying mechanism of the SBV-enhanced stretching is further revealed through the density and velocity difference between the light shocked bubble and the heavy ambient air. By combining the SBV-accelerated stretching model and the initial shock compression, a novel mixing time estimation for VD SBI is theoretically proposed by solving the advection–diffusion equation under a deformation field of an axisymmetric vortex with the additional SBV-induced azimuthal velocity. Based on the mixing time model, a mixing enhancement number, defined by the ratio of VD and PS mixing time further, reveals the contribution from the VD effect, which implies a better control of the density distribution for mixing enhancement in a supersonic streamwise vortex.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

Designing vortices in pipe flow with topography-driven Langmuir circulation

We present direct numerical simulation of a mechanism for creating longitudinal vortices in pipe flow, compared with a model theory. By furnishing the pipe wall with a pattern of crossing waves, secondary flow in the form of streamwise vortex pairs is created. The mechanism, ‘CL1’, is kinematic and known from oceanography as a driver of Langmuir circulation. CL1 is strongest when the ‘wall wave’ vectors make an acute angle with the axis, $\varphi =10^{\circ }$ – $20^{\circ }$ , changes sign near $45^{\circ }$ and is weak and of opposite sign beyond this angle. A competing, dynamic mechanism driving secondary flow in the opposite sense is also observed, created by the azimuthally varying friction. Whereas at smaller angles ‘CL1’ prevails, the dynamic effect dominates when $\varphi \gtrsim 45^{\circ }$ , reversing the flow. Curiously, the circulation strength is a faster-than-linearly increasing function of Reynolds number for small $\varphi$ . We explore an analogy with Prandtl's secondary motion of the second kind in turbulence. A transport equation for average streamwise vorticity is derived, and we analyse it for three different crossing angles, $\varphi =18.6^{\circ }, 45^{\circ }$ and $60^{\circ }$ . Mean-vorticity production is organised in a ring-like structure with the two rings contributing to rotating flow in opposite senses. For the larger $\varphi$ , the inner ring decides the main swirling motion, whereas for $\varphi =18.6^{\circ }$ , outer-ring production dominates. For the larger angles, the outer ring is mainly driven by advection of vorticity and the inner by deformation (stretching) whereas, for $\varphi =18.6^{\circ }$ , both contribute approximately equally to production in the outer ring.

Inlet Flow Distortion in an Advanced Civil Transport Boundary Layer Ingesting Engine Installation

This paper presents first-of-a-kind measurements, and complementary computations, of the flow through the propulsion system of a boundary layer ingesting, twin-engine advanced civil transport aircraft configuration. The experiments were carried out in the NASA Langley 14- by 22-foot Subsonic Tunnel, using a 1:11 scale model of the D8 “double-bubble” aircraft with electric ducted fans providing propulsive power. Overall force and moment measurements and flow field surveys at the inlet and nozzle exit planes were obtained. The computations were carried out with the NASA OVERFLOW code. The measurements and computations were conducted for a range of aircraft angles of attack and propulsor powers representing operating points during the aircraft mission. Velocity and pressure distributions at the propulsor inlet and exit, and integral inlet distortion metrics, are presented to quantify the flow non-uniformity due to boundary layer ingestion. The distorted inflow exhibits qualitative and quantitative changes over the mission, from a unidirectional stratified stagnation pressure at cruise to a streamwise vortex structure at climb conditions. The computations capture these flow features and reveal the interactions between airframe and propulsor that create these three-dimensional flow variations.

Coherent Streamwise Vortex Structure of a Three-Dimensional Wall Jet

The dynamics of the coherent structures in a turbulent three-dimensional wall jet with an exit Reynolds number of 250,000 were investigated using the Snapshot Proper Orthogonal Decomposition (POD). A low-dimensional reconstruction using the first 10 POD modes indicates that the turbulent flow is dominated by streamwise vortex structures that grow in size and relative strength, and that are often accompanied by strong lateral sweeps of fluid across the wall. This causes an increase in the bulging and distortions of streamwise velocity contours as the flow evolves downstream. The instantaneous streamwise vorticity computed from the reconstructed instantaneous velocities has a high level of vorticity associated with these outer streamwise vortex structures, but often has a persistent pair of counter-rotating regions located close to the wall on either side of the jet centerline. A model of the coherent structures in the wall jet is presented. In this model, streamwise vortex structures are produced in the near-field by the breakdown of vortex rings formed at the jet outlet. Separate structures are associated with the near-wall streamwise vorticity. As the flow evolves downstream, the inner near-wall structures tilt outward, while the outer streamwise structures amalgamate to form larger streamwise asymmetric structures. In all cases, these streamwise vortex structures tend to cause large lateral velocity sweeps in the intermediate and far-field regions of the three-dimensional wall jet. Further, these structures meander laterally across the jet, causing a strongly intermittent jet flow.

Modification of DDES Based on SST kω Model for Tip Leakage Flow in Turbomachinery

Both LES and DDES are conducted in a low-Reynolds number tip leakage flow model. The DDES uses the SST kω model and employs the same grid with the LES, but the turbulence field diverges from the LES result. Referring to the comparison between LES and DDES, a modification of the zonal function in the DDES model is proposed, which enhances the dissipation of the modeled turbulence thus promote the transition to fully LES in the tip region when the mesh is fine enough. It can generate much finer vortex structure than the original model, including the primary streamwise vortex, induced vortices and the vortex fragments after breakdown. The modification fixes the underestimation of the vorticity and pressure drop at the formation stage of the tip leakage vortex, and generates more reasonable turbulence field and energy spectra. The modified model is introduced to a real rotor simulation at engineering Reynolds number. Compared with the original model on both mean flow field and turbulence field, the modified model shows favorable agreements with the measurements. The study also gives a practical example of using the tip leakage flow model in turbulence modeling.