Showing papers by "Sergio Pirozzoli published in 2016"

Passive scalars in turbulent channel flow at high Reynolds number

Abstract: We study passive scalars in turbulent plane channels at computationally high Reynolds number, thus allowing us to observe previously unnoticed effects. The mean scalar profiles are found to obey a generalized logarithmic law which includes a linear correction term in the whole lower half-channel, and they follow a universal parabolic defect profile in the core region. This is consistent with recent findings regarding the mean velocity profiles in channel flow. The scalar variances also exhibit a near universal parabolic distribution in the core flow and hints of a sizeable log layer, unlike the velocity variances. The energy spectra highlight the formation of large scalar-bearing eddies with size proportional to the channel height which are caused by a local production excess over dissipation, and which are clearly visible in the flow visualizations. Close correspondence of the momentum and scalar eddies is observed, with the main difference being that the latter tend to form sharper gradients, which translates into higher scalar dissipation. Another notable Reynolds number effect is the decreased correlation of the passive scalar field with the vertical velocity field, which is traced to the reduced effectiveness of ejection events.

Mixed convection in turbulent channels with unstable stratification

Abstract: We study turbulent flows in planar channels with unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of asymptotic developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi--streamwise rollers occupying the full channel thickness with aspect--ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum. The mean values and the variances of the flow variables do not appear to follow Prandtl's scaling in the flow regime near free convection, except for the temperature and vertical velocity fluctuations, which are more affected by turbulent plumes. Nevertheless, we find that the Monin--Obukhov theory still yields a useful representation of the main flow features. In particular, the widely used Businger--Dyer relationships provide a convenient way of accounting for the bulk effects of shear and buoyancy, although individual profiles may vary widely from the alleged trends. Significant deviations are found in DNS with respect to the commonly used parametrization of the mean velocity in the light-wind regime, which may have important practical impact in models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulas for the heat flux and friction coefficients which can be used with about $10\%$ maximum error in a wide range of flow parameters.

An efficient semi-implicit solver for direct numerical simulation of compressible flows at all speeds

Abstract: We develop a semi-implicit algorithm for time-accurate simulation of the compressible Navier-Stokes equations, with special reference to wall-bounded flows. The method is based on linearization of the partial convective fluxes associated with acoustic waves, in such a way to suppress, or at least mitigate the acoustic time step limitation. Together with replacement of the total energy equation with the entropy transport equation, this approach avoids the inversion of block-banded matrices involved in classical methods, which is replaced by less demanding inversion of standard banded matrices. The method is extended to deal with implicit integration of viscous terms and to multiple space dimensions through approximate factorization, and used as a building block of third-order Runge-Kutta time stepping scheme. Numerical experiments are carried out for isotropic turbulence, plane channel flow, and flow in a square duct. All available data support higher computational efficiency than existing methods, and saving of resources ranging from $85\%$ under low-subsonic flow conditions, to about $50\%$ in supersonic flow.

On the Size of the Eddies in the Outer Turbulent Wall Layer: Evidence from Velocity Spectra

Abstract: The scaling of size of the momentum-bearing eddies in wall-parallel planes in the outer part of turbulent wall layers is analyzed, by examining spectra of the fluctuating velocities taken from direct numerical simulations and experiments. For all flows under scrutiny the normalized spectra highlight growth of the eddies size with the wall distance. The results indicate the capability of a modified mixing length (Pirozzoli, J Fluid Mech 702:521–532, 2012 [1]) of accounting with greater precision for the wall-normal variation of the size of the eddies bearing streamwise momentum. This observation can be explained by assuming that outer layer momentum streaks (superstructures) spread under the collective action of the other eddies, which impart a (nearly) uniform eddy diffusivity throughout the outer wall layer.

A high-fidelity solver for turbulent compressible flows on unstructured meshes

Abstract: We develop a high-fidelity numerical solver for the compressible Navier-Stokes equations, with the main aim of highlighting the predictive capabilities of low-diffusive numerics for flows in complex geometries. The space discretization of the convective terms in the Navier-Stokes equations relies on a robust energy-preserving numerical flux, and numerical diffusion inherited from the AUSM scheme is added limited to the vicinity of shock waves, or wherever spurious numerical oscillations are sensed. The solver is capable of conserving the total kinetic energy in the inviscid limit, and it bears sensibly less numerical diffusion than typical industrial solvers, with incurred greater predictive power, as demonstrated through a series of test cases including DNS, LES and URANS of turbulent flows. Simplicity of implementation in existing popular solvers such as OpenFOAM is also highlighted.

Optimized prefactored compact schemes for wave propagation phenomena

Abstract: A new family of prefactored cost-optimized schemes is developed to minimize the computational cost for a given level of error in linear wave propagation applications, such as aerodynamic sound propagation. This work extends the theory of Pirozzoli to the prefactored compact high-order schemes of Hixon, which are MacCormack type schemes that use discrete Padé approximations. An explicit multi-step Runge-Kutta scheme advances the states in time. Theoretical predictions for spatial and temporal error bounds are used to drive the optimization process. Theoretical comparisons of the cost-optimized schemes with a classical benchmark scheme are made. Then, two numerical experiments assess the computational efficiency of the costoptimised schemes for computational aeroacoustic applications. A polychromatic sinusoidal test-case verifies that the cost-optimized schemes perform according to the design highorder accuracy characteristics for this class of problems. For this test case, upwards of a 50% computational cost-saving at the design level of error is recorded. The final test case shows that the cost-optimized schemes can give substantial cost savings for problems where a fully broadband signal needs to be resolved.

Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions

Abstract: Direct numerical simulations are carried out to investigate the effect of the wall temperature on the behavior of oblique shock-wave/turbulent boundary layer interactions at freestream Mach number $2.28$ and shock angle of the wedge generator $\varphi = 8^{\circ}$. Five values of the wall-to-recovery-temperature ratio ($T_w/T_r$) are considered, corresponding to cold, adiabatic and hot wall thermal conditions. We show that the main effect of cooling is to decrease the characteristic scales of the interaction in terms of upstream influence and extent of the separation bubble. The opposite behavior is observed in the case of heating, that produces a marked dilatation of the interaction region. The distribution of the Stanton number shows that a strong amplification of the heat transfer occurs across the interaction, and the maximum values of thermal and dynamic loads are found in the case of cold wall. The analysis reveals that the fluctuating heat flux exhibits a strong intermittent behavior, characterized by scattered spots with extremely high values compared to the mean. Furthermore, the analogy between momentum and heat transfer, typical of compressible, wall-bounded, equilibrium turbulent flows does not apply for most part of the interaction domain. The pre-multiplied spectra of the wall heat flux do not show any evidence of the influence of the low-frequency shock motion, and the primary mechanism for the generation of peak heating is found to be linked with the turbulence amplification in the interaction region.