Athanassios A. Dimas

Effect of bed dunes on spatial development of open-channel flow

The spatial development of turbulent, sub-critical open-channel flow over five identical dunes is studied by the numerical solution of the RANS equations utilizing theVOF free-surface formulation and the k–e or Spalart–Allmaras turbulence models. Results are presented for smooth and rough walls and several dune dimensions. One of the cases was also studied experimentally. The flow separation at each dune crest generates a recirculation in the dune lee-side and reattachment at a distance, which increases with increasing dune height and decreasing dune length. Turbulence does not fully develop over the dunes, while the majority of turbulent kinetic energy production takes place in the recirculation region. The spatially mean, free-surface level decreases in the flow direction over the dunes, while the free-surface amplitude increases with increasing dune height. The dune drag coefficient increases with increasing dune height, while the contribution of form resistance on the drag increases with increasing dune height and decreasing dune length.

Discrete Models for Seismic Analysis of Liquid Storage Tanks of Arbitrary Shape and Fill Height

A finite element method (FEM)-based formulation is developed for an effective computation of the eigenmode frequencies, the decomposition of total liquid mass into impulsive and convective parts, and the distribution of wall pressures due to sloshing in liquid storage tanks of arbitrary shape and fill height. The fluid motion is considered to be inviscid (slip wall condition) and linear (small free-surface steepness). The natural modal frequencies and shapes of the sloshing modes are computed, as a function of the tank fill height, on the basis of a conventional FEM modeling. These results form the basis for a convective-impulsive decomposition of the total liquid mass, at any fill height, for the first few (two or three at most) sloshing modes, which are by far the most important ones in comparison to all other higher modes. This results into a simple yet accurate and robust model of discrete masses and springs for the sloshing behavior. The methodology is validated through comparison studies involving vertical cylindrical tanks. Additionally, the application of the proposed methodology to conical tanks and to the seismic analysis of spherical tanks on a rigid or flexible supporting system is demonstrated and the results are compared to those obtained by rigorous FEM analyses.

Incipient breaking of steady waves in the presence of surface wakes

The effect of free-surface drift layers on the maximum height that a steady wave can attain without breaking is explored through experiments and numerical simulations. In the experiments, the waves are generated by towing a two-dimensional fully submerged hydrofoil at constant depth, speed and angle of attack. The drift layer is generated by towing a plastic sheet on the water surface ahead of the hydrofoil. It is found that the presence of this drift layer (free-surface wake) dramatically reduces the maximum non-breaking wave height and that this wave height correlates well with the surface drift velocity. In the simulations, the inviscid two-dimensional fully nonlinear Euler equations are solved numerically. Initially symmetric wave profiles are superimposed on a parallel drift layer whose mean flow characteristics match those in the experiments. It is found that for large enough initial wave amplitudes a bulge forms at the crest on the forward face of the wave and the vorticity fluctuations just under the surface in this region grow dramatically in time. This behaviour is taken as a criterion to indicate impending wave breaking

Flow Dynamics and Bed Resistance of Wave Propagation over Bed Ripples

The viscous, two-dimensional, free-surface flow induced by the propagation of nonlinear water waves over a rigid rippled bed was simulated numerically. The simulations were based on the numerical solution of the Navier-Stokes equations subject to fully nonlinear free-surface boundary conditions and appropriate bottom, inflow, and outflow boundary conditions. The equations were properly transformed so that the computational domain became time-independent. A hybrid scheme was used for the spatial discretization with finite differences in the streamwise direction and a pseudospectral approximation with Chebyshev polynomials in the vertical direction. A fractional time-step scheme was used for the temporal discretization. Over the rippled bed, the wave boundary layer thickness increased significantly, while vortex shedding at the ripple crest generated alternating circulation regions over the ripple trough. The velocity of the Eulerian drift profile was opposite to the direction of wave propagation far above the ripples, whereas close to the bed, its magnitude was influenced by the ripples up to a height of about six times the ripple height above the ripple crest. The amplitude of the wall shear stress on the ripples increased with increasing ripple steepness, whereas the amplitude of the corresponding friction drag force on a ripple was insensitive to this increase. The amplitude of the form drag force attributable to the dynamic pressure increased with increasing ripple steepness; therefore, the percentage of friction in the total drag force decreased with increasing ripple steepness. The period-averaged drag forces on a ripple were very weak, while the influence of form drag increased with increasing ripple steepness.

Large-eddy simulation of subcritical transition in an attachment-line boundary layer

Abstract The large-eddy simulation (LES) technique is applied to the subcritical transition to turbulence of a finite-amplitude instability in the attachment-line boundary layer of a swept wing. The three-dimensional swept Hiemenz solution is used to model the base laminar flow along the leading edge of a swept wing. The filtered Navier-Stokes equations are solved numerically using a localized dynamic eddy-viscosity model to parameterize the unresolved scales. Outflow conditions are imposed using the buffer-domain technique, and initial disturbances are introduced through a blowing/suction strip. The linear stage of transition is bypassed due to the finite-amplitude perturbation. The instability growth rate is found to be significantly higher than in previous two-dimensional results, due to the commonly-made Gortler-Hummerlin assumption that the perturbation has the same spanwise structure as the base flow; this assumption is not supported by our results, most probably due to the finite spanwise length of the blowing/suction strip. As the flow propagates downstream, energy is fed into the spanwise modes, and turbulence generated on the attachment line is transported to other spanwise locations by the mean motion in this direction. The shape factor on the attachment line matches the turbulent boundary layer estimates, while the turbulent mean-velocity profiles on the attachment line deviate from the universal logarithmic behavior due to the appearance of inflectional points, which make the flow susceptible to inviscid instabilities.

Effect of Cross-Flow Velocity at Forebay on Swirl in Pump Suction Pipe: Hydraulic Model of Seawater Intake at Aliveri Power Plant in Greece

The hydraulic performance of pumps in a cooling water intake is directly affected by the nonuniformity of the approach flow at each pump bay, which in turn is influenced by the strength of the cross-flow at the pumps’ common forebay. The effect of the cross-flow velocity at the forebay on the swirl angle in the pump suction pipes is investigated in a hydraulic model of the seawater intake at the Aliveri Power Plant in Greece. The particular intake features two pumps, and a total of 10 cases were examined based on differing values of water depth, number of pumps in operation, and pump flow rate. Velocity measurements at the forebay-dividing cross section were obtained by an acoustic Doppler velocimeter (ADV), while swirl angle in the suction pipe was measured by a vortimeter. A highly nonuniform velocity profile develops at the forebay, when one of the two cleaning channels is closed, and the swirl angle depends solely on the intake forebay geometry when the mean cross-flow velocity drops below a critical value.

Numerical Computation of Turbulence Development in Flow Over Sand Dunes

The spatial development of sub-critical, turbulent, open-channel flow over a bottom with five dunes is studied. The steady-state flow is described by the RANS equations utilizing the k−ɛ turbulence model. The free-surface treatment is based on the rigid-lid approximation method, while the numerical solution is based on a finite-volume discretization. The Froude number is Fr=0.3, while both smooth and rough bottom cases are considered. Mean velocity and turbulence numerical predictions are in good agreement to available experimental data and analytical expressions. Then, results are presented for the streamwise development of eddy viscosity, Reynolds stresses and turbulence production of the flow over bed with train of five fixed dunes with dune length to water depth ratio L/d = 5 and dune height to water depth ratio h/d = 0.25. Numerical results are compared to available experimental data and overall agreement is satisfactory. Turbulence is generated mostly by flow separation at the dune crest and does not reach an equilibrium state over the dunes.

Numerical Simulation of Oblique Wave Breaking and Wave-Induced Currents in the Surf Zone

In the present study, the three-dimensional, incompressible, turbulent, free-surface flow, developing by the propagation and breaking of nonlinear gravity waves over a constant-slope beach, is numerically simulated. The main objective is to investigate the flow structure in the surf zone as a result of the interaction between the longshore and the undertow current, induced by spilling wave breaking, oblique to the shoreline. The simulations are performed employing the so-called large-wave simulation (LWS) method coupled with a numerical solver for the Navier-Stokes equations. According to the employed LWS methodology, large velocity and free-surface scales are fully resolved, while the effect of subgrid scales is modeled by eddy-viscosity stresses, similar to large-eddy simulation (LES) methodology. In order to validate our model, the case of incoming Stokes waves with wavelength to inflow depth ratio λ/dI ≈ 6.6 and wave steepness H/λ ≈ 0.025, propagating normal to the shore over a bed of constant slope 1/35, is investigated. Our results are compared to published experimental measurements, and it is found that the LWS model predicts adequately the wave breaking parameters — breaking height and depth — and the distribution of the undertow current in the surf zone. Two cases of oblique breaking waves, with inflow angles φI = 20° and 30°, and all other parameters identical to that of the validation case, are considered. The gradual breaking of the refracted waves is captured, as well as the three-dimensional structure of the flow in the surf zone. LWS-predicted profiles of the undertow and the longshore current at several positions in the surf zone, are presented. It is indicated that the undertow prevails in the outer surf zone, while the longshore current becomes stronger in the inner surf zone and reaches its maximum magnitude close to the shore.Copyright

Large-Wave Simulation of Breaking Waves Over a Beach

Wave transformation and breaking on a beach are associated with important coastal processes like wave-generated currents, sediment transport and coastal erosion.

Groyne spacing role on the effective control of wall shear stress in open-channel flow

ABSTRACT The objective of an effective configuration for a series of groynes in an open channel is the attainment of large bed shear stress in the main channel for channel deepening and small sidewall shear stress in the groyne fields for bank protection with the lowest cost, i.e. with the largest spacing between groynes. A Reynolds-averaged Navier–Strokes, finite-volume, numerical model was used and it was validated against experimental data for the case of a single groyne. For series of groynes with uniform spacing between them, the most effective configuration was the one with spacing equal to six groyne lengths. For the range of parameters considered, a substantial improvement of the effectiveness was achieved by implementing a novel non-uniform configuration where the spacing between groynes was reduced to one and a half groyne lengths in the first four groyne fields, and remained equal to six groyne lengths in the subsequent ones.