Junlin Yuan

Michigan State University | MSU · Department of Mechanical Engineering

The direct numerical simulation of an accelerating boundary layer over a rough wall has been carried out to investigate the coupling between the effects of roughness and strong freestream acceleration. While the favourable pressure gradient is sufficient to achieve quasi-laminarization on the smooth wall, on the rough wall the flow reversion is pre...

Large-eddy simulations are carried out in turbulent open-channel flows to determine the roughness function and the equivalent sand-grain roughness height, ks, over sand-grain roughness and different types of realistic roughness replicated from hydraulic turbine blades. A range of Reynolds numbers and mean roughness heights is chosen, leading to bot...

This study uses high-fidelity simulations (DNS or LES) and experimental datasets to analyse the effect of non-equilibrium streamwise mean pressure gradients (adverse or favourable), including attached and separated flows, on the statistics of boundary layer wall-pressure fluctuations. The datasets collected span a wide range of Reynolds numbers ($R...

This study uses high-fidelity simulations (DNS or LES) and experimental datasets to analyse the effect of non-equilibrium streamwise mean pressure gradients (adverse or favourable), including attached and separated flows, on the statistics of boundary layer wall-pressure fluctuations. The datasets collected span a wide range of Reynolds numbers (Re...

This work compares various existing rough-wall models on a large collection of rough surfaces with different characteristics and studies the potentials of these models in accommodating new datasets. We consider three empirical roughness correlations, two physics-based models, and one data-driven machine-learning model on 68 rough surfaces inside an...

Wall-pressure and velocity statistics in the turbulent boundary layer (TBL) on a cambered controlled-diffusion aerofoil at $8^{\circ }$ incidence, a Mach number of 0.25 and a chord-based Reynolds number ${Re}_c=1.5\times 10^{5}$ are analysed at four locations on the suction side with zero and adverse pressure gradients (ZPG and APG), characterised...

Direct numerical simulations were performed to characterize fully developed supersonic turbulent channel flows over isothermal rough walls. The effect of roughness was incorporated using a level-set/volume-of-fluid immersed boundary method. Turbulence statistics of five channel flows are compared, including one reference case with both walls smooth...

Wall roughness induces form-induced (or dispersive) velocity and pressure perturbations inside the roughness sublayer of a wall-bounded turbulent flow. This work discusses the role played by the form-induced velocity in influencing turbulence statistics and structure , using existing direct numerical simulation data of transient half channels in re...

The force partitioning method [Menon and Mittal, J. Fluid Mech. 918, R3 (2021)0022-112010.1017/jfm.2021.359] is employed to decompose and analyze the pressure-induced drag for turbulent flow over rough walls. The pressure drag force imposed by the rotation-dominated vortical regions (Q>0, where Q is the second invariant of the velocity gradient ten...

To understand how different bed morphological scales affect stream hyporheic exchange, pore-resolved direct numerical simulations (DNS) of half channels at a Re τ of 1580 on a porous symmetric dune with two different grain-scale roughnesses are conducted. The permeability Reynolds number is Re K = 2.5, representative of streamflows over sandy beds....

Plain Language Summary Riverine flows are turbulent flows with wide range of scales of motion in both space and time. Driven and modified by natural and anthropogenic factors, channel flows interact with local features (e.g., bars, ripples, and dunes), and smaller features down to the scale of individual sediment grains. The magnitude of exchange o...

Direct numerical simulations (DNSs) of an incompressible turbulent boundary layer on an airfoil (suction side) and that on a flat plate are compared to characterize the non-equilibrium turbulence and the effect of wall curvature on the flow. The two simulations effectively impose matching streamwise distributions of adverse pressure gradient (APG)...

Direct numerical simulations (DNS) of flows over an airfoil (suction side) and a flat plate are compared to characterize non-equilibrium turbulent boundary layers with wall curvature. The two simulations have matched adverse pressure gradient (APG) quantified by the acceleration parameter (K). For the airfoil flow, an existing DNS carried out by Wu...

The problem of Reynolds-averaged modeling of turbulent boundary-layer flow over surfaces of arbitrary roughness is studied using the results of direct numerical simulations of turbulent flow over many different rough surfaces. The complexity of flow in the roughness sublayer is such that the most effective general strategy appears to be to model it...

The dynamical effects of roughness geometry on the response of a half-height turbulent channel flow to an impulse acceleration are investigated using direct numerical simulations. Two rough surfaces different in the surface height spectrum are compared between themselves and with a smooth-wall baseline case. Both rough cases develop from a transiti...

This paper explores the use of a small-span direct numerical simulation for a transient, smooth-wall turbulent channel flow and then applies the small-span simulation to a transient channel flow with riblets. A flow configuration similar to that of S. He and M. Seddighi (J Fluid Mech. 2013;715:60–102) is used to study the impulse response of a half...

Direct numerical simulations were performed to characterize fully developed supersonic turbulent channel flows over isothermal rough walls. The effect of roughness was incorpo- rated using a level-set/volume-of-fluid immersed boundary method. Turbulence statistics of five channel flows are compared, including one reference case with both walls smoo...

This paper investigates a long-standing question about the effect of surface roughness on turbulent flow: what is the equivalent roughness sand-grain height for a given roughness topography? Deep Neural Network (DNN) and Gaussian Process Regression (GPR) machine learning approaches are used to develop a high-fidelity prediction approach of the Niku...

Since the early work of Johnston [Johnston, Halleent, and Lezius, J. Fluid Mech. 56, 533 (1972)], the mean flow scaling in a spanwise rotating channel has received much attention. While it is known that the mean velocity near the pressure, turbulent side follows a linear scaling U=2Ωy+C at high rotation speeds, the functional dependence of C on the...

The effects of bed roughness, isolated from that of bed permeability, on the vertical transport processes across the sediment-water interface (SWI) are not well understood. We compare the statistics and structure of the mean flow and turbulence in open-channel flows with a friction Reynolds number of 395 and a permeability Reynolds number of 2.6 ov...

This work addresses a long standing question about roughness: what is the equivalent sand-grain height, given the roughness topography? Deep Neural Network (DNN) and Gaussian Process Regression (GPR) machine learning approaches are used to develop a high-fidelity prediction approach of the Nikuradse (1933) equivalent sand-grain height $k_s$ for tur...

Turbulence structures in flow over three types of wall roughness: sand-grain, cube roughness and a realistic, multi-scale turbine-blade roughness, are compared to structures observed in flow over a smooth wall in open channel flow at Reτ=1000, using direct numerical simulations. Two-point velocity correlations, length scales, inclination angles, an...

A long-standing problem in turbulence modeling is that the Reynolds stress tensor alone is not necessarily sufficient to characterize the transient and non-equilibrium behaviors of turbulence under arbitrary mean deformation or frame rotation. A more complete single-point characterization of the flow can be obtained using the structure Dimensionali...

A long-standing problem in turbulence modeling is that the Reynolds stress tensor alone is not necessarily sufficient to characterize the transient and non-equilibrium behaviors of turbulence under arbitrary mean deformation or frame rotation. More complete flow characterizations include additional, single-point structure tensors, such as the dimen...

Single-point turbulence statistics are compared in the roughness sublayer (RSL) of turbulent open-channel flows over smooth wall and wall roughness with different textures using direct numerical simulations (DNS). The goal is to identify how the range of scales contained in a roughness topography affects the drag generation, momentum transfer, and...

An approach to modeling the effect of rough surfaces on turbulent boundary-layer flow is proposed and developed. It is based on the concept of a rough-wall eddy viscosity, in which the pressure and viscous drag forces which arise on account of flow past roughness elements are recast as an equivalent viscous shear force within the roughness sublayer...

Large eddy simulations of separated turbulent boundary layers over a flat plate were performed to study the effects of wall roughness. Rough-wall cases have a larger separation bubble with both earlier separation and later reattachment. Roughness leads to early flow reversal upstream of the real separation.

The physics of the roughness sublayer are studied by direct numerical simulations (DNS) of an open-channel flow with sandgrain roughness. A double-averaging (DA) approach is used to separate the spatial variations of the time-averaged quantities and the turbulent fluctuations. The spatial inhomogeneity of velocity and Reynolds stresses results in a...

This study reports the numerical predictions of flows over turbine blades, which include flow acceleration and deceleration. Two issues are addressed: (1) accurately predicting roughness effects, and (2) evaluating the performance of Reynolds-Averaged Navier-Stokes (RANS) simulations on moderately accelerating flows. For the present turbine surface...

Turbulent sink flows over smooth or rough walls with sand-grain roughness are studied using large-eddy and direct numerical simulations. Mild and strong levels of acceleration are applied, yielding a wide range of Reynolds number (Reθ = 372 - 2748) and cases close to the reverse-transitional state. Flow acceleration and roughness are shown to exert...

We present the results of direct and large-eddy simulations of spatially developing boundary layers subjected to favorable pressure gradient, strong enough to cause reversion of the flow towards a quasi-laminar state. The numerical results compare well with experimental data. Visualization of the flow structures shows the well-known stabilization o...

Rough-wall turbulent boundary layers subject to favourable pressure gradients (FPGs) and freestream acceleration are found in many engineering applications. On a smooth wall with a strong FPG the flow may revert to a quasi-laminar state. Roughness, on the other hand, enhances turbulent fluctuations near the wall. We investigate strongly acceleratin...

Turbulent boundary layers subject to a favorable pressure gradient (which induces freestream acceleration) are found in many engineering applications, such as airfoils or curved ducts. If the acceleration is sufficiently large, turbulence production decreases, and the flow reverts to a laminar or quasi-laminar state. Once the cause of relaminarizat...