Development of an Analytical Wall Function for Bypass Transition

The objective of the present work is to propose an extended analytical wall function that is capable of predicting the bypass transition from laminar to turbulent flow. The algebraic γ transition model, the k−ω turbulence model and the analytical wall function are integrated together in this work to detect the transition onset and start the transition process. The present analytical wall function is validated with the experimental data, the Blasius solution and the law of the wall. With this analytical wall function, the transition onset in the skin friction coefficient is detected and the growth rate of transition is properly generated. The predicted mean velocity profiles are found to be in good agreement with the Blasius solution in the laminar flow, the experimental data in the transition zone and the law of the wall in the fully turbulent flow.

Investigation Into Boundary Layer Transition Using Wall-Resolved LES and Modeled Inflow Turbulence

The objective of the present paper is to investigate the transition scenario of the flow around a typical section of a wind turbine blade exposed to different levels of inflow turbulence. As a first step towards this objective, a rather low Reynolds number of Rec = 105 is studied at a fixed angle of attack but under five different turbulence intensities (TI) up to TI = 11.2 %. Using wall-resolved large-eddy simulations combined with an inflow procedure relying on synthetically generated turbulence and a source-term formulation for its injection within the computational domain, relevant flow features such as the separation bubble, inflectional instabilities and streaks can be investigated. The study shows that the transition scenario significantly changes with rising TI, where the influence of inflectional instabilities due to an adverse pressure gradient decreases, while the influence of streaks increases resulting in a shift from the classical scenario of natural transition to bypass transition. The primary instability mechanism in the separation bubble is found to be inflectional and its origin is traced back to the region upstream of the separation. Thus, the inviscid inflectional instability of the separated shear layer is an extension of the instability of the attached adverse pressure gradient boundary layer observed upstream. The boundary layer is evaluated to be receptive to external disturbances such that the initial energy within the boundary layer is proportional to the square of the turbulence intensity. Boundary layer streaks were found to influence the instantaneous separation location depending on their orientation. A varicose mode of instability is observed on the overlap of the leading edge of a high-speed streak with the trailing edge of a low-speed streak. The critical amplitude of this instability was analyzed to be about 32 % of the free-stream velocity.

Transition Prediction in Incompressible Boundary Layer with Finite-Amplitude Streaks

Modulating the boundary layer velocity profile is a very promising strategy for achieving transition delay and reducing the friction of the plate. By perturbing the flow with counter-rotating vortices that undergo transient, non-modal growth, streamwise-aligned streaks are formed inside the boundary layer, which have been proved (theoretical and experimentally) to be very robust flow structures. In this paper, we employ efficient numerical methods to perform a parametric stability investigation of the three-dimensional incompressible flat-plate boundary layer with finite-amplitude streaks. For this purpose, the Boundary Region Equations (BREs) are applied to solve the nonlinear downstream evolution of finite amplitude streaks. Regarding the stability analysis, the linear three-dimensional plane-marching Parabolized Stability Equations (PSEs) concept constitutes the best candidate for this task. Therefore, a thorough parametric study is presented, analyzing the instability characteristics with respect to critical conditions of the modified incompressible zero-pressure-gradient flat-plate boundary layer, by means of finite-amplitude linearly optimal and suboptimal disturbances or streaks. The parameter space is extended from low- to high- amplitude streaks, accurately documenting the transition delay for low-amplitude streaks and the amplitude threshold for streak shear layer instability or bypass transition, which drastically displaces the transition front upstream.

Freestream Turbulence Induced Boundary-Layer Transition in Low-Pressure Turbines

The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths towards a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behaviour of the low-pressure turbine blade. The computational set-up was designed to follow the experiments by Lengani & Simoni (2015). In the simulations, the flow past one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet as a superposition of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu=0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in case of low free-stream turbulence on the suction side, the Kelvin–Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Gortler instability.

Identification of a Pressure-Strain Correlation Based Bypass Transition Onset Marker

Temporally developing direct numerical simulations are performed for bypass transition flow with zero pressure gradient over a flat plate boundary layer for a range of free-stream turbulence intensities (Tu) of 1.4% to 6%. The objective is to understand the role of pressure-strain terms on bypass transition onset, and to propose and validate a phenomenological hypothesis for the identification of a robust transition onset marker for use in transition-sensitive Reynolds-averaged Navier-Stokes (RANS) simulations. Results show that transition initiates at a location where the slow pressure-strain term becomes more dominant than the rapid term in the pre-transitional boundary layer region. A simple transition onset marker based on one-point statistical quantities is derived from the scaling of the ratio of the slow and rapid pressure fluctuation source terms. The critical value of the marker is found to vary within a narrow range (+/- 3.2%), and satisfies previously identified criteria for a robust transition onset marker.

INVESTIGATION OF COMPRESSIBLE FLOW THROUGH A TURBINE BLADE CASCADE FOR VARIOUS TRANSONIC FLOW REGIMES

This paper deals with the numerical simulation of 2D transonic flow through the SE1050 turbine blade cascade at various flow conditions. The first one concerns the design operation with a zero incidence angle involved in the ERCOFTAC Database CFD-QNET and the second one with a +20° incidence angle corresponding to an off-design operation. Advanced mathematical models with two different models of the bypass transition to turbulence were applied for the simulation of different regimes of transonic flows as well as with attached and separated flows. Transition models proposed by Dick et al. [1] and by Menter and Smirnov [2] are based on transport equations for the intermittency coefficient. Numerical results were compared with experimental data based on the optical and pressure measurements.

From Bypass Transition to Flow Control and Data-Driven Turbulence Modeling: An Input–Output Viewpoint

Transient growth and resolvent analyses are routinely used to assess nonasymptotic properties of fluid flows. In particular, resolvent analysis can be interpreted as a special case of viewing flow dynamics as an open system in which free-stream turbulence, surface roughness, and other irregularities provide sources of input forcing. We offer a comprehensive summary of the tools that can be employed to probe the dynamics of fluctuations around a laminar or turbulent base flow in the presence of such stochastic or deterministic input forcing and describe how input–output techniques enhance resolvent analysis. Specifically, physical insights that may remain hidden in the resolvent analysis are gained by detailed examination of input–output responses between spatially localized body forces and selected linear combinations of state variables. This differentiating feature plays a key role in quantifying the importance of different mechanisms for bypass transition in wall-bounded shear flows and in explaining how turbulent jets generate noise. We highlight the utility of a stochastic framework, with white or colored inputs, in addressing a variety of open challenges including transition in complex fluids, flow control, and physics-aware data-driven turbulence modeling. Applications with temporally or spatially periodic base flows are discussed and future research directions are outlined.