Energy Transfer In Flow Research Articles

Effects of Surface Waviness on Fan Blade Boundary Layer Transition and Profile Loss—Part I: Methodology and Computational Results

AbstractThis two-part paper describes a new approach to determine the effect of surface waviness, arising from manufacture of composite fan blades, on transition onset location movement and hence fan profile losses. The approach includes analysis and computations of unsteady disturbances in boundary layers over a wavy surface, assessed and supported by wind tunnel measurements of these disturbances and the transition location. An integrated framework is developed for analysis of surface waviness effects on natural transition. The framework, referred to as the extended eN method, traces the evolution of disturbance energy transfer in flow over a wavy surface, from external acoustic noise through exponential growth of Tollmien–Schlichting (TS) waves, to the start and end of the transition process. The computational results show that surface waviness affects the transition onset location due to the interaction between the surface waviness and the TS boundary layer instability and that the interaction is strongest when the geometric and TS wavelengths match. The condition at which this occurs, and the initial amplitude of the boundary layer disturbances that grow to create the transition onset is maximized, is called receptivity amplification. The results provide first-of-a-kind descriptions of the mechanism for the changes in transition onset location as well as quantitative calculations for the effects of surface waviness on fan performance due to changes in surface wavelength, surface wave amplitude, and the location at which the waviness is initiated on the fan blade.

3D numerical investigation of energy transfer and loss of cavitation flow in perforated plates

ABSTRACT Perforated plates are prevalently used in various integrated energy systems for flow restriction, vibration, and noise reduction. The characteristics of flow energy transfer and loss in perforated plates have a significant impact on its piping systems. The existing design and previous studies largely paid little attention to the characteristics of flow energy variation, resulting in severe performance issues and energy consumption. In this study, the characteristics of energy transfer and loss of cavitation flow with various pressure ratios in single and multi-stage perforated plates were numerically studied. The parameters of energy ratio and energy gradient were developed to effectively evaluate the degree and rate of flow energy variation. Results showed that the contribution to energy loss generated from orifice outlet was relatively larger than the inlet and that the degree and rate of energy variation deceased with the increasing stages during the flow in multi-stage models. Additionally, increased stages of perforated plate resulted in a multi-stage stepped energy drop in throttling orifices, and lead to obvious suppression of the cavitation. This work intends to provide references to cavitation control and energy consumption optimization in piping systems using a perforated plate.

The influence of near-wall density and viscosity gradients on turbulence in channel flows

The influence of near-wall density and viscosity gradients on near-wall turbulence in a channel is studied by means of direct numerical simulation of the low-Mach-number approximation of the Navier–Stokes equations. Different constitutive relations for density $\unicode[STIX]{x1D70C}$ and viscosity $\unicode[STIX]{x1D707}$ as a function of temperature are used in order to mimic a wide range of fluid behaviours and to develop a generalised framework for studying turbulence modulations in variable-property flows. Instead of scaling the velocity solely based on local density, as done for the van Driest transformation, we derive an extension of the scaling that is based on gradients of the semilocal Reynolds number, defined as $Re_{\unicode[STIX]{x1D70F}}^{\star }\equiv Re_{\unicode[STIX]{x1D70F}}\sqrt{(\overline{\unicode[STIX]{x1D70C}}/\overline{\unicode[STIX]{x1D70C}}_{w})}/(\overline{\unicode[STIX]{x1D707}}/\overline{\unicode[STIX]{x1D707}}_{w})$ (the bar and subscript $w$ denote Reynolds averaging and wall value respectively, while $Re_{\unicode[STIX]{x1D70F}}$ is the friction Reynolds number based on wall values). This extension of the van Driest transformation is able to collapse velocity profiles for flows with near-wall property gradients as a function of the semilocal wall coordinate. However, flow quantities like mixing length, turbulence anisotropy and turbulent vorticity fluctuations do not show a universal scaling very close to the wall. This is attributed to turbulence modulations, which play a crucial role in the evolution of turbulent structures and turbulence energy transfer. We therefore investigate the characteristics of streamwise velocity streaks and quasistreamwise vortices and find that, similarly to turbulence statistics, the turbulent structures are also strongly governed by $Re_{\unicode[STIX]{x1D70F}}^{\star }$ profiles and that their dependence on individual density and viscosity profiles is minor. Flows with near-wall gradients in $Re_{\unicode[STIX]{x1D70F}}^{\star }$ ($\text{d}Re_{\unicode[STIX]{x1D70F}}^{\star }/\text{d}y\neq 0$) show significant changes in inclination and tilting angles of quasistreamwise vortices. These structural changes are responsible for the observed modulation of the Reynolds stress generation mechanism and the inter-component energy transfer in flows with strong near-wall $Re_{\unicode[STIX]{x1D70F}}^{\star }$ gradients.

On energy transfer in flow around a row of transversely oscillating square cylinders at low Reynolds number

In this paper, the effects of cylinder spacing, cylinder oscillation frequency, amplitude of cylinder oscillations and Reynolds number on the ensuing flow regimes and energy transition for flow across a row of transversely oscillating cylinders have been studied numerically using the lattice Boltzmann method. The lift and drag coefficient signals are analyzed in detail for finding the extent of lock-on regime and wake interaction mechanism at different spacings. It is noticed that the magnitude of the mean drag coefficient is large at small spacings, which is consistent with a strong wake interaction at small spacings. The effect of wake interaction can also be noticed from the non-monotonic variation of rms lift. The average energy transfer per cylinder oscillation cycle is large when the cylinders oscillate with a frequency near to the natural vortex shedding frequency. The direction of energy transfer changes between positive and negative values with small changes in the cylinder oscillation frequency, suggesting that the direction of energy transfer is very sensitive to this parameter. It is shown that the instantaneous lift coefficient and the cylinder velocity govern the energy transfer from or to the fluid. While the different parameters affect the flow regimes, the cylinder oscillation frequency primarily governs the energy transfer.

Energy analysis of turbulent channel flow using biorthogonal wavelets

Turbulent flows exhibit many different length and time scales. Hence, it is important to study energy transfer between the scales, in order to gain deeper insight into turbulence. Wavelets offer potential for the analysis of the energy transfer in a turbulent flow. This is mainly due to their locality and scalability property. The present work involves a wavelet decomposition of the terms in the turbulent kinetic energy transport equation of a fully developed channel flow and a study of the behavior of the important terms. A detailed analysis of the energy transfer term is performed. An attempt is made to identify some well-known structures in the flow at different scales of decomposition. The dynamics of these coherent structures is examined based on their contribution to energy transfer in the flow at discrete scales. This is performed by correlating the velocity and energy transfer in the flow. To analyze energy transfer due to spatial structures, we utilize the concept of “triad interaction” in wavelet space. The study involves computing all possible combinations of wavelet modes and finding the most energetic ones. We investigate these interactions to capture the dynamics of the local structures and their contribution to the energy transfer.