arXiv subjects

Ferdinand F. J. Schrijer

Publications and source records attributed to Ferdinand F. J. Schrijer.

3 recordsLinked to original sources

arXiv2026

Spatially developing drag reduction across a finite oscillating wall

The spatial evolution of a turbulent boundary layer at $Re_τ= 652$ developing across a finite spanwise oscillating wall is investigated with normalised oscillation period of $T^+ \approx 101$ and amplitude $A^+ \approx 7$, yielding a drag reduction of 32\%. The spatial distribution of skin friction is estimated adopting a recently introduced near-wall data nonlinear regression method based on the arctangent function, which circumvents the requirement of high wall-normal resolution to capture the viscous sublayer. The method viability is assessed against reference DNS data, and the trends of measurement errors from experimental data are well reproduced. The local impact of the control action is examined by evaluating the spatial development of scalar fields of turbulence statistics as well as conditional quadrant analysis to link the onset and recovery transients to turbulence events. The boundary layer response is observed to follow an exponential decay law for the skin friction spatial evolution. The decay length-scale corresponds to $0.6$ boundary layer thicknesses $δ$ for the onset transient and $0.9δ$ for the recovery one. The same exponential fit applied to the Reynolds stresses yields a response within $0.5δ$, with the exception of the shear stress peak, that requires a significantly longer distance.

physics.flu-dyn↗

arXiv2024

Experimental assessment of square wave spatial spanwise forcing of a turbulent boundary layer

We present an experimental realisation of spatial spanwise forcing in a turbulent boundary layer flow, aimed at reducing the frictional drag. The forcing is achieved by a series of spanwise running belts, running in alternating spanwise direction, thereby generating a steady spatial square-wave forcing. SPIV in the streamwise-wall-normal plane is used to investigate the impact of actuation on the flow in terms of turbulence statistics, drag performance characteristics, and spanwise velocity profiles, for a non-dimensional wavelength of $λ_x^+ = 397$. We confirm that a significant flow control effect can be realised with this type of forcing. The scalar fields of the higher-order turbulence statistics show a strong attenuation of stresses and production of turbulence kinetic energy over the first belt already, followed by a more gradual decrease to a steady-state energy response over the second belt. The streamwise velocity in the near-wall region is reduced, indicative of a drag-reduced flow state. The profiles of the higher-order turbulence statistics are attenuated up to a wall-normal height of $y^+ \approx 100$, with a maximum streamwise stress reduction of 45% and a reduction of integral turbulence kinetic energy production of 39%, for a non-dimensional actuation amplitude of $A^+ = 12.7$. An extension of the classical laminar Stokes layer theory is introduced, to describe the non-sinusoidal boundary condition that corresponds to the current case. The spanwise velocity profiles show good agreement with this extended theoretical model. The drag reduction was estimated from a linear fit in the viscous sublayer in the range $2 \leq y^+\leq 5$. The results are found to be in good qualitative agreement with the numerical implementations of Viotti et al. (2009), matching the drag reduction trend with $A^+$, and reaching a maximum of 20%.

physics.flu-dyn↗

arXiv2023

Density Field Reconstruction of an Overexpanded Supersonic Jet using Tomographic Background-Oriented Schlieren

A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to aid in the visualization of compressible flows. An experimental setup was devised around a sub-scale rocket nozzle, in which four cameras were set up in a circular configuration with 30° angular spacing in azimuth. Measurements were taken of the overexpanded supersonic jet plume at various nozzle pressure ratios (NPR), corresponding to different flow regimes during the start-up and shut-down of rocket nozzles. Measurements were also performed for different camera parameters using different exposure times and f-stops in order to study the effect of measurement accuracy. Density gradients and subsequently two-dimensional line-of-sight integrated density fields for each of the camera projections are recovered from the index of refraction field by solving a Poisson equation. The results of this stage are then used to reconstruct two-dimensional slices of the (time-averaged) density field using a tomographic reconstruction algorithm employing the filtered back-projection and the simultaneous algebraic reconstruction technique. By stacking these two-dimensional slices, the (quasi-) three-dimensional density field is obtained. The accuracy of the implemented method with a relatively low number of sparse cameras is briefly assessed and basic flow features are extracted such as the shock spacing in the overexpanded jet plume.

physics.flu-dyn↗