## About

237

Publications

29,133

Reads

**How we measure 'reads'**

A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more

19,865

Citations

Introduction

Additional affiliations

August 2008 - December 2012

January 1988 - August 2008

Education

August 1983 - January 1988

August 1981 - July 1983

September 1973 - July 1979

## Publications

Publications (237)

A spatially developing turbulent boundary layer subject to a space- and time-dependent pressure gradient is analysed via large-eddy simulation. The unsteadiness is prescribed by imposing an oscillating suction–blowing velocity profile at the top boundary of the computational domain. The alternating favourable and adverse pressure gradients cause th...

We use large-eddy simulations to investigate a spatially developing turbulent boundary layer with a space-and time-dependent pressure gradient. The flow oscillates between times in which a favorable pressure gradient is followed by an adverse one, a zero-pressure gradient, and a period in which the adverse pressure gradient is followed by a favorab...

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered, h/δo*=0.5 and 1.0 (where δo* is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examin...

View Video Presentation: https://doi.org/10.2514/6.2022-0695.vid Equivalent sand-grain roughness, and the related concept of wall similarity, are essential elements for the prediction of turbulent boundary-layer flows since resolution of rough wall geometry is not in most cases a practical proposition. While the validity of these concepts has been...

View Video Presentation: https://doi.org/10.2514/6.2022-1033.vid This paper discusses the modeling activity of the NATO-STO Research Task Group AVT-349. The aim of this group is to improve the understanding and modeling of boundary layers in the complex flow around water vehicles. As such, the focus is on incompressible, high-Reynolds-number flows...

We performed large-eddy simulations of the flow over an aerofoil to understand the effects of leading-edge roughness designed to mimic ice accretion. The roughness elements protrude outside the boundary layer, which, near the leading edge, is very thin; thus, the configuration does not represent a classical rough-wall boundary layer, but rather the...

This work investigates the accuracy of commonly used bed-load transport models when applied in combination with high-resolution Navier-Stokes solvers. Empirical bed-load models predict the transport rate of sediments based on the average bottom shear-stress, while eddy-resolving approaches allow for a space- and time-dependent description of the bo...

Reliability assessment of large-eddy simulation (LES) of turbulent flows requires consideration of errors due to shortcomings in the modeling of sub-filter scale dynamics and due to discretization of the governing filtered Navier-Stokes equations. The Integral Length-Scale Approximation (ILSA) model is a pioneering sub-filter parameterization that...

The large-eddy-simulation technique is introduced. The mathematical formulation is presented, and several examples are discussed. Some issues related to the resolution of the wall layer are reviewed. Challenges and possible future developments are addressed.

This paper will discuss the modeling activity of NATO-STO Research Task Group AVT-349. The aim of this group is to improve the understanding and modeling of boundary layers in the complex flow around water vehicles. As such, the focus is on incompressible, high-Reynolds-number flows that can be subject to non-equilibrium conditions such as strong p...

We have studied the transport of a passive scalar for passive scalar with Prandtl numbers near unity in a plane channel with rough walls. The study was carried out by direct numerical simulations of the Navier-Stokes equations; an immersed boundary method was used to model the roughness. The well-known departure from the Reynolds analogy, which pos...

Scalar transport occurs frequently in engineering and the natural sciences, in problems ranging from the transport of particles or pollutants in a flow to heat transfer in mechanical systems.

A new model for the unresolved stresses in large-eddy simulations was recently proposed by Piomelli et al. [J Fluid Mech 2015; 766:499–527] and Rouhi et al., [Phys Rev Fluids 2016; 1(4):0444011], in which the length scale is not related to the grid size, but determined based on turbulence properties. This model, the Integral Length-Scale Approximat...

Reliability assessment of large-eddy simulation (LES) of turbulent flows requires consideration of errors due to shortcomings in the modeling of sub-filter scale dynamics and due to discretization of the governing filtered Navier–Stokes equations. The Integral Length-Scale Approximation (ILSA) model is a pioneering sub-filter parameterization that...

Turbulence models based on the Spalart-Allmaras Detached-Eddy Simulation (DES) approach are used to compute the turbulent flow over a two-dimensional dune geometry. DES was developed for massively separated flows, but has been applied as a wall model to attached flows as well. In attached shear layers, however, the lack of resolved eddies in the re...

Turbulent flows over rough surfaces are ubiquitous in nature and in man-made devices. They present many unique aspects, as roughness alters the turbulence structure and the momentum and energy transfer mechanisms. Numerical simulations that resolve the roughness sublayer are computationally demanding: for an accurate calculation of a flow in realis...

Separating turbulent boundary layers over smooth and rough flat plates are studied by large-eddy simulations. A suction–blowing velocity distribution imposed at the top boundary of the computation domain produces an adverse-to-favourable pressure gradient and creates a closed separation bubble. The Reynolds number based on the momentum thickness an...

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.

This paper investigates the effects of surface roughness on the flow past a circular cylinder at subcritical to transcritical Reynolds numbers. Large eddy simulations of the flow for sand grain roughness of size k/D = 0.02 are performed (D is the cylinder diameter). Results show that surface roughness triggers the transition to turbulence in the bo...

When hovering over sandy terrain, the rotor of helicopters generates a downward jet that induces resuspension of dust and debris. We investigate the mechanisms that govern particle resuspension in such flow using an Eulerian–Lagrangian approach based on large-eddy simulation of turbulence. The wake generated by the helicopter is modelled as a verti...

The accuracy of turbulence models for the Reynolds-Averaged Navier-Stokes (RANS) equations in rough-wall flows is evaluated by comparing the model predictions with the data obtained from large-eddy simulations (LES). We have considered boundary layers in favourable and adverse pressure gradients mimicking those encountered in hydroturbines. We find...

This paper focuses on the effects of surface roughness in the flow past a circular cylinder at different Reynolds numbers. Large eddy simulations of the flow, from subcritical to transcritical Reynolds numbers and at relatively high equivalent sand grain roughness of k s /D = 0.02 are performed. In order to determine the effects of the surface roug...

In the present work large eddy simulations of the flow past a rough cylinder are performed at a Reynolds number of Re = 4.2 × 10⁵ and an equivalent sand-grain surface roughness height ks = 0.02D. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results are compare...

We present a modification of the integral length-scale approximation (ILSA) model originally proposed by Piomelli et al. [Piomelli et al., J. Fluid Mech. 766, 499 (2015)] and apply it to plane channel flow and a backward-facing step. In the ILSA models the length scale is expressed in terms of the integral length scale of turbulence and is determin...

We performed large-eddy simulations (LES) of forced impinging jets over smooth and rough surfaces, containing large-scale, azimuthal vortices generated by the enhanced primary instability in the jet shear layer. The interaction between these vortices and the turbulence in the wall jet that is formed downstream of the impingement region determines t...

Turbulent mixing in the ocean is often quantified using observations of kinetic energy dissipation (e.g., the Osborn model). However, because direct measurement of dissipation remains a challenge, it is commonly estimated from acoustic Doppler velocity data using methods such as inertial fitting or the structure function. These methods employ Kolmo...

We have performed simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering near the ground. This problem has considerable practical importance since, when the landing area is covered with sand or snow, the interaction between the helicopter wake, the rotor-tip vortices and the solid particles on...

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 of the flow over surfaces with alternating forest patches and clearings of different horizontal scale were performed, modelling the forest canopies as a horizontally homogeneous drag field. The objective was to extend previous works that studied the flow over sudden changes in aerodynamic roughness length occurring typically...

Large-eddy simulations of turbulent plane and radial wall-jets were conducted at different Reynolds numbers using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results were validated with the available experimental data in the literature. Plane and radial wall jets have one basic geometrical difference; the radial wall jet has one...

We performed a grid-refinement study in large-eddy simulations (LES) of heat transfer in a turbulent channel flow at Reτ = 400, at Prandtl numbers up to 300. Dirichlet and Neumann boundary conditions for the temperature field have been investigated. Grid convergence of mean temperature profiles is reached for fairly coarse grids. As expected, the v...

Large eddy simulations of turbulent radial and plane wall jets were performed at different Reynolds numbers using the Lagrangian dynamic eddy viscosity subgrid-scale model. The results were validated with experimental data available in the literature. Compared to the plane ones, the radial wall jets have an extra direction for expansion, which caus...

We evaluated the accuracy of the inertial dissipation method to estimate the rate of dissipation of turbulent kinetic energy within boundary layers by performing well-resolved numerical simulations of turbulent channel flows, and comparing the dissipation calculated directly from the data, with that deduced from the frequency spectra. The convectio...

We have carried out large-eddy simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering in ground effect. The azimuthal vortices are generated by sinusoidal forcing of the velocity at the jet exit. They strengthen while they are advected towards the ground; when they are close to the solid surfa...

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...

We present LES simulations of the flow over a wing
with sinusoidal leading edge undulations. The undulations
act as a passive flow control device, offering superior poststall
aerodynamic performance relative to the unmodified
wing with the same mean chord. The baseline case of a regular
wing is also presented for comparison. In all cases the
Reynol...

Since its introduction, in the early 1970s, large eddy simulations (LES) have advanced considerably, and their application is transitioning from the academic environment to industry. Several landmark developments can be identified over the past 40 years, such as the wall-resolved simulations of wall-bounded flows, the development of advanced models...

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...

Large-eddy simulations were carried out to study the effects of surface roughness on a plane wall-jet using the Lagrangian dynamic eddy-viscosity subgrid-scale model, at Re = 7500 (based on the jet bulk velocity and height). Results over both smooth and rough surfaces were validated by experimental data at the same Reynolds number. As the jet is in...

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...

Barchan dunes are bed forms found in many sedimentary environments with a limited supply of sediment, and may occur in isolation or in more complex dune fields. Barchans have a crescentic planform morphology with horns elongated in the downflow direction. To study flow over barchan dunes, we performed large eddy simulations in a channel with differ...

We performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article (Omidyeganeh &...

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...

The flow in a plane channel with two idealized stents (one
Λ-shaped, the other X-shaped) is studied numerically. A periodic
pressure gradient corresponding to one measured in the left anterior
descending coronary artery was used to drive the flow. Two Reynolds
numbers were examined, one (Re = 80) corresponding to resting
conditions, the other (Re =...

We performed numerical simulations of dissolved oxygen (DO) transfer from a turbulent flow, driven by periodic boundary-layer turbulence in the intermittent regime, to underlying DO-absorbing organic sediment layers. A uniform initial distribution of oxygen is left to decay (with no re-aeration) as the turbulent transport supplies the sediment with...

We performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18 900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune sha...

We have tested a dissolved oxygen (DO) transport model based on
large-eddy simulation (LES) of a transitional oscillatory flow observed
in the bottom boundary layer of Lake Alpnach, Switzerland. The
transition from a quasi-laminar to a fully turbulent state makes this
flow difficult to study with a Reynolds-averaged Navier-Stokes equation
(RANSE) m...

Cerebral aneurysms are a common cause of death and disability. Of all the cardiovascular diseases, aneurysms are perhaps the most strongly linked with the local fluid mechanic environment. Aside from early in vivo clinical work that hinted at the possibility of high-frequency intra-aneurysmal velocity oscillations, flow in cerebral aneurysms is mos...

We have investigated the mechanisms involved in dissolved oxygen (DO) transfer from a turbulent flow to an underlying organic sediment bed populated with DO-absorbing bacteria. Our numerical study relies on a previously developed and tested computational tool that couples a bio-geochemical model for the sediment layer and large-eddy simulation for...

We have developed a model for dissolved oxygen (DO) transfer from water
to underlying organic sediment beds. The model couples large-eddy
simulation (LES) of turbulent transport in the water column, a
biogeochemical model for DO transport and consumption in the sediment,
and Darcy's Law for the pore water-driven transport. The model
highlights the...

We developed and tested a simple technique to predict, for flow visualisation purposes only, the evolution of coherent structures in between two given realisations of a turbulent flow. Classic coherent-structure eduction methods are adopted, such as the Q-criterion, pressure fluctuations and contours of velocity fluctuations. The kinematics of the...

The influence of spanwise geometrical undulations of the leading edge of an infinite wing is investigated numerically at low Reynolds number, in the context of passive separation control and focusing on the physical mechanisms involved. Inspired by the tubercles of the humpback whale flippers, the wavy leading edge is modeled using a spanwise sinus...

The abdominal aortic aneurysm (AAA) is a significant cause of death and disability in the Western world and is the subject of many clinical and pathological studies. One of the most commonly used surrogates of the human AAA is the angiotensin II (Ang II) induced model used in mice. Despite the widespread use of this model, there is a lack of knowle...

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...

We performed large-eddy simulation of the flow over two- dimensional
large-scale river-bed irregularities called dunes at laboratory scale
(the Reynolds number based on the average channel height and mean
velocity is 18,900). The flow separates at the dune crest, generating a
shear layer that plays a crucial role in the transport of momentum and
en...

We studied the effects of the imposed waveform on the flow in an
idealized stented artery. Two stents were used in this study, both based
on commercially available geometries. The Navier-Stokes equations were
solved using a cartesian staggered code, and the stents were modelled
via an immersed-boundary method. The Reynolds number and Womersley
numb...

We performed large-eddy simulations of the flow over a typical
two-dimensional dune geometry at laboratory scale (the Reynolds number based on
the average channel height and mean velocity is 18,900) using the Lagrangian
dynamic eddy-viscosity subgrid-scale model. The flow separates at the dune
crest and reattaches downstream on the bed (at x=5.7h)....

The performance of some commonly used eddy-viscosity turbulence models has been evaluated using direct numerical simulation (DNS) and large-eddy simulation (LES) data. Two configurations have been tested, a two-dimensional boundary layer undergoing pressure-driven separation, and a square duct. The DNS and LES were used to assess the k - epsilon, z...

The fundamental assumption underlying large-eddy simulations (LES) is that the large, energy-carrying, eddies are resolved, while only the smaller eddies are modeled. An implication of this assumption is that the filter-width Δ, the length scale that separates the resolved from the unresolved eddies, should be a fraction of the integral scale, whic...

We performed large-eddy simulations of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results are validated by comparison with simulations and experiments in the litera...