Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

New insights into large eddy simulation

This paper presents the results of a detailed hydrodynamic study of a row of inclined jets issuing into a crossflow with a density ratio of injectant to freestream of two. Laser Doppler anemometry was used to measure the vertical and streamwise components of velocity for a jet-to-freestream mass flux ratio of 0.5. Mean velocity components and turbulent Reynolds normal and shear stress components were measured at locations in a vertical plane along the centerline of the jet from 1 diameter upstream to 30 diameters downstream of the jet. The results, which have application to film cooling, give a quantitative picture of the entire flow field, from the approaching flow upstream of the jet, through the interaction region of the jet and freestream, to the relaxation region downstream where the flow field approaches that of a standard turbulent boundary layer.Copyright © 1989 by ASME

Effects of density ratio on the hydrodynamics of film cooling

Large-eddy simulation of film cooling flows at density gradients

A review of gas turbine effusion cooling studies

The spectra and correlations of the velocity fluctuations in turbulent channels, especially above the buffer layer, are analysed using new direct numerical simulations with friction Reynolds numbers up to Re at very large ones.

Scaling of the energy spectra of turbulent channels

In this paper, the combination of passive and active drag reduction techniques is investigated using Particle-Image Velocimetry (PIV) and Micro Particle-Tracking Velocimetry (µ-PTV). The impact of a riblet-structured surface, which represents the passive control technique, undergoing spanwise transversal wave motion, which defines the active flow control method, on the wall-shear stress distribution of a zero-pressure gradient turbulent boundary layer is analyzed. The PIV and μ-PTV measurements are conducted at two Reynolds numbers based on the momentum thickness of Re θ = 1200 and 2080, respectively. The results show that the transversal wave motion extends the drag reduction efficiency of the riblets.

Turbulent drag reduction by spanwise traveling ribbed surface waves

Evolution of jets effusing from inclined holes into crossflow

Active friction drag reduction by spanwise transversal traveling surface waves is investigated experimentally in a fully developed zero-pressure gradient (ZPG) turbulent boundary layer (TBL). The spanwise transversal traveling wave of an aluminum surface is generated by an electromagnetic actuator system. A parametric study focusing on the influence of the wave amplitude (A+) and wave period (T+) is performed to analyze the impact of the wave parameters on drag reduction. Within the range of the parameters investigated, the maximum local drag reduction of 4.5% is found at A+ = 11.8 and T+ = 110. Furthermore, the TBL flows above the wave crest and trough are investigated by phase-locked PIV and µ-PTV measurements. The results evidence that the drag reduction effect is not only enhanced by increasing the amplitude, but also by reducing the period in the range of the current parameters. The turbulence statistics show that the velocity fluctuations and the Reynolds shear stresses in the streamwise and in the wall-normal direction are damped by the traveling surface wave motion in the near-wall region. The outer velocity distribution deviates from the inner scaling based on the actuated friction velocity, i.e., it possesses a slight tendency of a varying slope in the log region. The phase-locked measurements of the velocity profiles above the crest and the trough show that only above the crest the inner scaling property is valid. Above the moving surface a non-zero spanwise secondary flow is induced. The quadrant decomposition of the turbulent productions shows that the sweep and ejection events are weakened.

Parametric investigation of friction drag reduction in turbulent flow over a flexible wall undergoing spanwise transversal traveling waves

The turbulent flow field of a film cooling flow is investigated using the particle-image velocimetry (PIV) technique. Cooling jets are injected from a multi-row hole configuration into a turbulent boundary layer flow of a flat plate in the presence of a zero and an adverse pressure gradient. The investigations focus on full-coverage film cooling. Therefore, the film cooling configuration consists of three staggered rows of holes with a lateral spacing of p/D = 3 and a streamwise row distance of l/D = 6. The inclined cooling holes feature a fan-shaped exit geometry with lateral and streamwise expansions. Jets of air and CO2 are injected separately at different blowing ratios into a boundary layer to examine the effects of the density ratio between coolant and mainstream on the mixing behavior and consequently, the cooling efficiency. For the zero pressure gradient case the measurement results indicate the different nature of the mixing process between the jets and the crossflow after the first, second, and third row. The mainstream velocity distributions evidence the growth of the boundary layer thickness at increasing row number. The interaction between the undisturbed boundary layer and first two rows leads to maximum values of turbulent kinetic energy. The presence of an adverse pressure gradient in the mainstream clearly intensifies the growth of the boundary layer thickness and increases the velocity fluctuations in the upper mixing zone. The measurements considering an increased density ratio show higher turbulence intensities in the shear zone between the jets and the main flow leading to a more pronounced mixing in this area. The results of the experimental measurements are used to validate numerical findings from a large-eddy simulation. This comparison shows a very good agreement for mean velocity distributions and velocity fluctuations.Copyright © 2010 by ASME

Particle-Image Velocimetry Measurements of Film Cooling in an Adverse Pressure Gradient Flow

The influence of a spanwise traveling transversal surface wave on the near-wall flow field of turbulent boundary layers is investigated by particle-image velocimetry (PIV) and micro-particle tracking velocimetry (μ-PTV). The experimental setup consists of a flat plate equipped with an insert to generate a transversal spanwise traveling wave of an aluminum surface. PIV and μ-PTV measurements are conducted for three Reynolds numbers based on the freestream velocity and momentum thickness immediately downstream of the actuated surface Re
 θ
  = 1200, 1660, and 2080. The transversal traveling wave is generated by a newly developed electromagnetic actuator system underneath the aluminum surface. Three amplitudes A = 0.25, 0.30, and 0.375 mm at a wave length of $$\lambda \, = \,160\,{\text{mm}}$$
 and a frequency of f = 81 Hz are investigated. The detailed analysis of the velocity profile shows the transversal traveling surface motion to redistribute the velocity in the viscous sublayer and in the logarithmic region of the turbulent boundary layer. The streamwise and wall-normal velocity fluctuations in the outer boundary layer are increased and the streamwise momentum in the near-wall regime is lowered. The drag reduction ratio (DR) due to the actuation is determined by the velocity gradient in the viscous sublayer. At the lowest Reynolds number the drag-reducing impact is proportional to the amplitude of the wave. That is, the higher the amplitude, the more pronounced the drag reduction resulting in a friction drag reduction up to 3.4 % compared to the non-actuated configuration.

Wilhelm Jessen

Papers

Turbulent drag reduction by spanwise traveling ribbed surface waves

Evolution of jets effusing from inclined holes into crossflow

Parametric investigation of friction drag reduction in turbulent flow over a flexible wall undergoing spanwise transversal traveling waves

Particle-Image Velocimetry Measurements of Film Cooling in an Adverse Pressure Gradient Flow

Experimental investigation of turbulent boundary layers over transversal moving surfaces