Experimental Aerodynamics Division

In this study we focus on the effect of mean flow acceleration on the near wall structures within turbulent boundary layers. 2D PIV measurements in streamwise-wall normal and streamwise-spanwise planes have been performed upstream of and within a sink flow for inlet Re θ of 6326 and 3071, and at constant acceleration parameters of K=0.6×10 -6 and 1.1×10 -6 , respectively. Due to the imposed favorable pressure gradient (FPG), the Reynolds stresses normalized by the local freestream velocity decrease over the entire boundary layer. However, when scaled by the inlet freestream velocity, stresses increase close to the wall and decrease in the outer part of the boundary layer. This is caused by the confinement of the structures in the near-wall region in the accelerating flow. The weaker normalized strength of the vortical structures and the substantial negative wall-normal mean velocity in the FPG region are identified as the likely contributors to this trend. Data in the wall parallel planes dissecting the large scale structures shows their signatures in the form of "swirling" patterns and low speed streaks. For the higher Reynolds number flow, the high near-wall ∂U/∂y in the accelerating region decreases the angle of inclination of the large eddies. Consequently, their signature in the x-z plane is weaker and elongated in the streamwise direction. In the FPG region, the small scale structures tend to occur in streamwise aligned groups and almost all of them are observed in the low speed streaks, which are the regions of ejection induced by large eddies. Due to the lower turbulence levels in the outer parts, the high momentum regions, indicating sweep events, contain very few small scale structures. This distinction between the low and high momentum regions is much weaker in the zero pressure gradient (ZPG) area which has high turbulence levels in the outer layer.

Separating and reattaching flows are important in a large number of engineering configurations, which includes geometrical configurations in aircraft engines. The flow through a sudden expansion (backward-facing step) represents a conceptually simple case of this class of flows. The present study focuses on the effect of the expansion ratio (defined as the ratio of downstream height to upstream height) on the unsteady flow features in the reattachment region and further downstream.

This study focuses on the effects of mean (favourable) and large-scale fluctuating pressure gradients on boundary layer turbulence. Two-dimensional (2D) particle image velocimetry (PIV) measurements, some of which are time-resolved, have been performed upstream of and within a sink flow for two inlet Reynolds numbers, Re θ (x 1 ) = 3360 and 5285. The corresponding acceleration parameters, K, are 1.3 × 10 −6 and 0.6 × 10 −6 . The time-resolved data at Re θ (x 1 ) = 3360 enables us to calculate the instantaneous pressure distributions by integrating the planar projection of the fluid material acceleration. As expected, all the locally normalized Reynolds stresses in the favourable pressure gradient (FPG) boundary layer are lower than those in the zero pressure gradient (ZPG) domain. However, the un-scaled stresses in the FPG region increase close to the wall and decay in the outer layer, indicating slow diffusion of near-wall turbulence into the outer region. Indeed, newly generated vortical structures remain confined to the near-wall region. An approximate analysis shows that this trend is caused by higher values of the streamwise and wall-normal gradients of mean streamwise velocity, combined with a slightly weaker strength of vortices in the FPG region. In both boundary layers, adverse pressure gradient fluctuations are mostly associated with sweeps, as the fluid approaching the wall decelerates. Conversely, FPG fluctuations are more likely to accompany ejections. In the ZPG boundary layer, loss of momentum near the wall during periods of strong large-scale adverse pressure gradient fluctuations and sweeps causes a phenomenon resembling local 3D flow separation. It is followed by a growing region of ejection. The flow deceleration before separation causes elevated near-wall small-scale turbulence, while high wall-normal momentum transfer occurs in the ejection region underneath the sweeps. In the FPG boundary layer, the instantaneous near-wall large-scale pressure gradient rarely becomes positive, as the pressure gradient fluctuations are weaker than the mean FPG. As a result, the separation-like phenomenon is markedly less pronounced and the sweeps do not show elevated small-scale turbulence and momentum transfer underneath them. In both boundary layers, periods of acceleration accompanying large-scale ejections involve near-wall spanwise contraction, and a high wall-normal momentum flux at all elevations. In the ZPG boundary layer, although some of the ejections are preceded, and presumably initiated, by regions of adverse pressure gradients and sweeps upstream, others are not. Conversely, in the FPG boundary layer, there is no evidence of sweeps or adverse pressure gradients immediately upstream of ejections. Apparently, the mechanisms initiating these ejections are either different from those involving large-scale sweeps or occur far upstream of the peak in FPG fluctuations.

The present study focuses on the effect of mean (favorable) and large-scale fluctuating pressure gradients on boundary layer turbulence. Two-dimensional particle image velocimetry measurements, some of which are time-resolved, have been performed in multiple streamwise-wall-normal and wall-parallel planes upstream of and within a sink flow region, for two different inlet Reynolds numbers, Re θ (x 1)=3360 and 5285. The corresponding values of the acceleration parameter, K, are 1.3×10-6 and 0.6×10-6 , respectively. The time-resolved data at the lower Reynolds number enables us to calculate the instantaneous pressure distributions by integrating the planar projection of the material acceleration of the fluid. v Finally, I thank my parents and my sister, who have supported me through thick and thin. Words would fail to express my gratitude towards them.

We perform direct numerical simulations to study the effects of the finite Reynolds number and domain size on the decay law of Saffman turbulence. We observe that the invariant for Saffman turbulence, u 2 ℓ 3 , and non-dimensional dissipation coefficient, Cϵ = ϵ/(u 3 /ℓ), are sensitive to finite domain size; here, u is the rms velocity, ℓ is the integral length scale, and ϵ is the energy dissipation rate. Consequently, the exponent n in the decay law u 2 ∼ t −n for Saffman turbulence deviates from 6/5. Due to the finite Reynolds number and the domain size, Saffman turbulence decays at a faster rate (i.e., n > 6/5). However, the exponent n = 6/5 is more sensitive to the domain size than to the Reynolds number. From the simulations, we find that n remains close to 6/5 as long as R λ ≳ 10 and ℓ ≲ 0.3L box ; here, R λ is the Reynolds number based on the Taylor microscale and L box is the domain size. We also notice that n becomes slightly lower than 6/5 for a part of the decay period. Interestingly, this trend n < 6/5 is also observed earlier in freely decaying grid-generated turbulence.

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

This experimental study focuses on the effect of horizontal boundaries with pyramid-shaped roughness elements on the heat transfer in rotating Rayleigh–Bénard convection. It is shown that the Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in the case of smooth top and bottom surfaces, is unaffected by the roughness as long as the Ekman layer thickness $\unicode[STIX]{x1D6FF}_{E}$ is significantly larger than the roughness height $k$. As the rotation rate increases, and thus $\unicode[STIX]{x1D6FF}_{E}$ decreases, the roughness elements penetrate the radially inward flow in the interior of the Ekman boundary layer that feeds the columnar Ekman vortices. This perturbation generates additional thermal disturbances which are found to increase the heat transfer efficiency even further. However, when $\unicode[STIX]{x1D6FF}_{E}\approx k$, the Ekman boundary layer is strongly perturbed by the roughness elements and the Ekman pumping mechanism...

Submitted for the DFD15 Meeting of The American Physical Society Tuning transitions in rotating Rayleigh-Bénard convection 1 PRANAV JOSHI, RUDIE KUNNEN, HERMAN CLERCX, Technische Universiteit Eindhoven-Turbulent rotating Rayleigh-Bénard convection, depending on the system parameters, exhibits multiple flow states and transitions between them. The present experimental study aims to control the transitions between the flow regimes, and hence the system heat transfer characteristics, by introducing particles in the flow. We inject near-neutrally buoyant silver coated hollow ceramic spheres (∼100 micron diameter) and measure the system response, i.e. the Nusselt number, at different particle concentrations and rotation rates. Both for rotating and non-rotating cases, most of the particles settle on the top and bottom plates in a few hours following injection. This rapid settling may be a result of "trapping" of particles in the laminar boundary layers at the horizontal walls. These particle layers on the heat-transfer surfaces reduce their effective conductivity, and consequently, lower the heat transfer rate. We calculate the effective system parameters by estimating, and accounting for, the temperature drop across the particle layers. Preliminary analysis suggests that the thermal resistance of the particle layers may affect the flow structure and delay the transition to the "geostrophic" regime.

Submitted for the DFD15 Meeting of The American Physical Society Tuning transitions in rotating Rayleigh-Bénard convection 1 PRANAV JOSHI, RUDIE KUNNEN, HERMAN CLERCX, Technische Universiteit Eindhoven-Turbulent rotating Rayleigh-Bénard convection, depending on the system parameters, exhibits multiple flow states and transitions between them. The present experimental study aims to control the transitions between the flow regimes, and hence the system heat transfer characteristics, by introducing particles in the flow. We inject near-neutrally buoyant silver coated hollow ceramic spheres (∼100 micron diameter) and measure the system response, i.e. the Nusselt number, at different particle concentrations and rotation rates. Both for rotating and non-rotating cases, most of the particles settle on the top and bottom plates in a few hours following injection. This rapid settling may be a result of "trapping" of particles in the laminar boundary layers at the horizontal walls. These particle layers on the heat-transfer surfaces reduce their effective conductivity, and consequently, lower the heat transfer rate. We calculate the effective system parameters by estimating, and accounting for, the temperature drop across the particle layers. Preliminary analysis suggests that the thermal resistance of the particle layers may affect the flow structure and delay the transition to the "geostrophic" regime.

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: