arXiv subjects

B. Frohnapfel

Publications and source records attributed to B. Frohnapfel.

5 recordsLinked to original sources

arXiv2022

The effect of spanwise heterogeneous surfaces on mixed convection in turbulent channels

Turbulent mixed convection in channel flows with heterogeneous surfaces is studied using direct numerical simulations. The relative importance between buoyancy and shear effects, characterized by the bulk Richardson number $Ri_b$, is varied in order to cover the flow regimes of forced, mixed and natural convection, which are associated with different large-scale flow organization. The heterogeneous surface consists of streamwise-aligned ridges, which are known to induce secondary motion in case of forced convection. The large-scale streamwise rolls emerging under smooth-wall mixed convection conditions are significantly affected by the heterogeneous surfaces and their appearance is considerably reduced for dense ridge spacings. It is found that the formation of these rolls requires larger buoyancy forces than over smooth walls due to the additional drag induced by the ridges. Therefore, the transition from forced convection structures to rolls is delayed towards larger $Ri_b$ for spanwise heterogeneous surfaces. The influence of the heterogeneous surface on the flow organization of mixed convection is particularly pronounced in the roll-to-cell transition range, where ridges favor the transition to convective cells at significantly lower $Ri_b$. In addition, the convective cells are observed to align perpendicular to the ridges with decreasing ridge spacing. We attribute this reorganization to the fact that flow parallel to the ridges experience less drag than flow across the ridges, which is energetically more beneficial. Furthermore, we find that streamwise rolls exhibit very slow dynamics for $Ri_b=1$ and $Ri_b=3.2$ when the ridge spacing is in the order of the rolls' width. For these cases the up- and downdrafts of the rolls move slowly across the entire channel instead of being fixed in space as observed for the smooth-wall cases.

physics.flu-dyn↗

arXiv2021

Rearrangement of secondary flow over spanwise heterogeneous roughness

Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane open channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise-heterogeneous rough surface. To this end, while the statistical properties of roughness texture as well as the width and spacing of the rough stripes are kept constant, the elevation of the smooth stripes is systematically varied in different simulation cases. Utilizing this variation three configurations representing protruding, recessed and an intermediate type of roughness are analysed. In all cases secondary flows are present and the skin friction coefficients calculated for all the heterogeneous rough surfaces are meaningfully larger than what would result from the area-weighted average of those of homogeneous smooth and rough surfaces. This drag increase appears to be linked to the strength of the secondary flow. The rotational direction of the secondary motion is shown to depend on the relative surface elevation. The present results suggest that this rearrangement of the secondary flow is linked to the spatial distribution of the spanwise-wall-normal Reynolds stress component which carries opposing signs for protruding and recessed roughness.

physics.flu-dyn↗

arXiv2021

Tripping and laminar--turbulent transition: Implementation in RANS--EVM

Fundamental fluid--mechanics studies and many engineering developments are based on tripped cases. Therefore, it is essential for CFD simulations to replicate the same forced transition in spite of the availability of advanced transition modelling. In the last decade, both direct and large--eddy simulations (DNS and LES) include tripping methods in an effort to avoid the need for modeling the complex mechanisms associated with the natural transition process, which we would like to bring over to Reynolds--averaged Navier--Stokes (RANS) turbulence models. This paper investigates the necessity and applications of numerical tripping, despite of the developments in numerical modeling of natural transition. The second goal of this paper is to assess a technique to implement tripping in eddy-viscosity models (EVM) for RANS. A recent approach of turbulence generation, denoted as turbulence-injection method (kI), is evaluated and investigated through different test cases ranging from a turbulent boundary layer on a flat plate to the three dimensional(3D) flow over a wing section. The desired tripping is achieved at the target location and the simulation results compare favorably with the reference results (DNS, LES and measured data). With the application of the model, the challenging transition region can be minimized in a simulation, and consequently more reliable results are obtained.

physics.flu-dyn↗

arXiv2018

Assessment of skin-friction-reduction techniques on a turbulent wing section

The scope of the present project is to quantify the effects of uniform blowing and body-force damping on turbulent boundary layers subjected to a non-uniform adverse-pressure-gradient distribution. To this end, well-resolved large-eddy simulations are employed to describe the flow around the NACA4412 airfoil at moderate Reynolds number 200, 000 based on freestream velocity and chord length. In the present paper we focus on uniform blowing and the conference presentation will include a comparison with body-force damping applied in the same region. The inner-scaled profiles of the mean velocity and of selected components of the Reynolds-stress tensor are examined and compared with the uncontrolled cases. It is known that uniform blowing and adverse-pressure gradients share some similarities in their effect on the boundary layers, and our results will show that these effects are not independent. The behaviour of the skin-friction coefficient is analyzed through the FIK decomposition, and the impact of this control strategy on the aerodynamic efficiency of the airfoil is discussed.

physics.flu-dyn↗

arXiv2016

Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer

A numerical investigation of two locally applied drag reducing control schemes is carried out in the configuration of a spatially developing turbulent boundary layer (TBL). One control is designed to damp near-wall turbulence and the other induces constant mass flux in the wall-normal direction. Both control schemes yield similar local drag reduction rates within the control region. However, the flow development downstream of the control significantly differs: persistent drag reduction is found for the uniform blowing case whereas drag increase is found for the turbulence damping case. In order to account for this difference the formulation of a global drag reduction rate is suggested. It represents the reduction of the streamwise force exerted by the fluid on a finite length plate. Furthermore, it is shown that the far downstream development of the TBL after the control region can be described by a single quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a changed virtual origin. Based on this result, a simple model is developed that allows relating for the local drag reduction rate to the global one without the need of conducting expensive simulations or measurements far downstream of the control region.

physics.flu-dyn↗